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1973 Suzuki GT750K, Fuel Injected, GSXR suspension

Posted: Wed Dec 11, 2013 2:53 pm
by JoostGT
Hello everyone,

My name is Joost and I'm a 25 year old student from the Netherlands. I have something that might interest you ;)

It's a 1973 Suzuki GT750K which has been retrofitted with fuel injection and 1990 GSXR running gear/suspension (front fork and swing arm). This is all done by myself on a shoe string budget trying to find inventive solutions for otherwise expensive problems. I've recently blown a crank seal and am looking for information on how to improve a couple of things whilst the engine is apart. But since I totally agree with a proper introduction (and the age old, "pics or it didn't happen"...) I have translated my Dutch topic about this bike for you to enjoy.

Since translating all those posts is a lot of tedious work, I kind of skipped a few steps and some parts may not be very clear about what's really going on. It's also somewhat (sometimes not at all) chronological, so sorry for that in advance. I do believe however that this kind of information must be shared, so if you have any questions at all I will do my very best to answer them!

With that said, I will start posting the translated part. It begins with me buying the bike and ends with me towing it home...

Re: 1973 Suzuki GT750K, Fuel Injected, GSXR suspension

Posted: Wed Dec 11, 2013 2:54 pm
by JoostGT
It all started in 2008 in a old barn, the definition of a barn find:

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Not a whole lot later, it looked like this and was actually not that bad, it was worn, but oke. Image

The plan was to restore it and sell it on with a slight profit, just for the experience. Of course that didn’t happen…
It turned out to be very non-original (didn’t know a thing about GTs back then). It has a 73 frame, 74 air cleaner, 75 engine and then some. So wasn’t worth it to restore to original condition, so I thought I’d better keep it ;)
First problem were the original exhausts, they were very rotten.
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After a small test run in the middle of the night with only the stingers, I decided I needed mufflers. Did enjoy the sound though and made a small clip of it: http://www.youtube.com/watch?v=AO_s3tDe-J0" onclick="window.open(this.href);return false;

Then I bought this and it all changed again
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Thrown together
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Then for a while it looked like this. It had several modifications including a rear disc brake conversion using a GS750 hub and GT750 rim with new spokes.

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It all looked quite nice but it handled like a pig and everything was hurting whilst driving it. I had no fun in driving it so I parked it for a while to have a rethink.
Buying this:
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And this:
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All changed the course of the project drastically and the project enjoyed some renewed enthusiasm.

Here are some more pictures of the rebuild, it’s somewhat chronological, but I guess the pictures will do most of the talking ;)
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With the fairing
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Friend of mine who wanted to try it out ;)
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This concludes the first rebuild, now I will show you my plans with the throttle bodies and GSXR donor bike.
I had a spare engine and frame (actually, there was a complete white GT750 with the fairings I bought..) so I could modify and fit the throttlebodies on the workbench instead of dismantling the running GT. This was done to limit the downtime.
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Also, I’ve rebuild the 3 in 3 exhaust into a 3 into 1 exhaust. By this time I’ve realised the exhaust I made were terrible and had to go. They were too fat and ungainly and they also made placing the footrest impossible. So I chopped it up and made a 3 into 1. I know this isn’t good either, but it had to do for now. I’ll have to build myself a proper set some day ;)
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Then it was time to start fitting the GSXR front fork. I trial fitted it into my spare frame and it seemed to be a straight fit, only having to change the bearings (!). By using a selection of parts from either the GSXR, the GT and a head bearing set meant for a XS650 it went straight in! Luckily the GSXR front end already had a Lucas superbike conversion including a nice fat handle bar. With some generic lampholders it was looking quite good.
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Just a shame the rear tyre is so skinny, better change that as well then! The GSXR swingarm was an almost direct fit in the GT frame, don’t you just love LEGO?;) I had to fit new swingarm bearings and some bushes to compensate for a swingarm axle diameter difference but that was quickly sorted. Of course I had to make some dual shock mounts as well, but I have a tame welder tucked away in a chopper shop nearby so that wasn’t a problem either.
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The following photos show the progress during the conversion to fuel injection. This all happened in 1 week of vacation and I had a strict deadline to go camping with the bike some months later. It all started with a pile or parts
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Wideband exhaust sensor, coolant sensor with modified thermostat housing, air intake temperature sensor and a couple of relays. Also a picture showing the idle control valve mounted on the throttlebodies. Since this was giving me all sorts of trouble later on I removed it.
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Wiring harnass, plugs, sensors, all the electrical stuff.
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Stripped down GT, ready for its shot (get it? Fuel injection? Shot? Never mind…)
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Everytime I see this I notice how puny the frame is and how enormous the engine is, no wonder it’s such a “flexible” ride ;)
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Drilled a hole in the airfilter to place the intake air sensor.
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Swapped thermostat housings.
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Throttlebodies mounted, fuel pump and pressure regulator mounted on a small subframe.
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Just a quick fit to see how the side covers would sit.
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ImageRemoved the battery tray to make room for the new parts.
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I wanted to create a new battery tray so I could create some more room. So I cut up a random tray laying around so I could save the powdercoated stock one.
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So I could save this one.
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Removed the mountings and rubbers from this very rusted GT tray.
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Sorted!
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Then it was time to play with hoses and adaptors.
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Whilst I was at it, I also fixed the original loom and fitted a new starter relay.
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Nieuw stangetje voor bediening van de oliepomp
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Very ugly holder for the fuel pump, but it will have to do. This first fit is more of a trial run and proof of concept than a completed product…
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Fitted a plate on the battery tray so I could mount the ECU there.
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And then the wiring started!
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The wideband lambda sensor with nut that had to be welded into the exhaust. This is actually a axle nut from a Peugeot van I believe. But it was the correct thread so it was perfect.
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Fuel lines!
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Started wiring the relays and new fuse holder. The old single, glass fuse was not going to cut it with all the added power drains.
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Had to move the Newtronics ignition module to the rear mudguard.
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Stole the signal and switched power from the ignition unit to feed the ECU.
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Modified plugs
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Had to slot this cover since it was fighting me all the way. Now I can just remove it without messing with the coolant hose.
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This ugly lump is a 3 way split to feed the injectors. Since I absolutely did not want any moisture in here I used several coats of high quality isolation.
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More wiring…
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The connectors that came with the ECU were very small and terrible to use. The larger wires almost wouldn’t fit, but with some perseverance (and a power drill) they eventually went in. Wiring this plug took me almost all of the afternoon…
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Last one!
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LED are a good sign!
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Intensive care ;) Large car battery to keep the electrics going, laptop for getting the ECU all set up.
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It’s alive!
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Well then, what’s left? Mounting the wideband sensor and getting it to run of course!
Problems thus far were unsuppressed sparkplugs and caps, making the ECU very unhappy and constantly resettting. I also had a large amount of noise and feedback from the ignition coil making triggering for the injection almost impossible. But using an inbuilt filter allowed me dial out all the noise.

At the beginning of this week the GT ran on injection, but eventhough it didn’t sound half bad, I still had lots to do.
I used the signal from 1 coil to give the ECU a RPM signal. This did work, but there was 1 mayor drawback, due to the wiring of the ECU (I had every injector on a separate injector channel) it was firing every injector alternately instead of all together. This meant that it was firing the injectors once every 3 rotations!
To solve this, I tried to combine all 3 coil signals into 1 wire, effectively tripling the frequency. I used 3 diodes to prevent crosstalk, but sadly, this didn’t work and I let the magic blue smoke out  Luckily I only blew the diodes, but it was clear I had to find another way.
Learning from commercial RPM sensors, I tried winding a piece of wiring around each plug wire and feeding the ECU the combined signal from these “Coils”. I got a signal, but it was too small and the ECU just couldn’t make sense of it.
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Instead of mucking about, I decided I was going to do this the proper way. By using a Ford VR sensor, this is a crank position sensor which triggers on moving pieces of metal. When a trigger wheel moves past this sensor, it generates an AC signal. The ECU can then calculate the RPM and trigger moments from this AC signal. Using the stock points wheel fitted behind the ignition plate I could get 3 trigger events per crank rotation with this sensor.
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It should go somewhere here.
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After measuring, measuring and measuring some more, I grew a pair and drilled a hole in the engine cover.

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Which resulted in this

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I had to move the ignition plate outwards a bit to make room for the VR sensor. It’s tight, but works like a treat!
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Fitted including waterproof plug and ignition cover.

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One of the last things now was fitting the wideband sensor

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Drilled a 13mm hole and used a small grinder until the nut fitted. I welded a small piece of pipe on the nut to shield the sensor from the oil in the exhaust. I got this idea from a KOSO 2 stroke wideband bung adaptor, but I’ve got no idea if it would work.
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Almost there.

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Done

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Quick blast of black paint, just wanted it to look right.

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And mounted.
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Then I had a USB issue to solve. The ECU has a internal USB B connector but no way of getting to this when the case is closed. Stupid design flaw which I had to find a way around.
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Drilled a small hole, threaded the USB cable through and a bit of glue/caulk to make it “waterproof” again.
This meant I had to solder a USB cable though, which was quite a pain since the wires are so incredibly small, but I got there in the end.
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By this time I constantly thought I was done, but small jobs kept coming up. Like modifying the rod from the carbs to the oil injection pump. This had to be bent and a tab made on the throttlebodies.
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Not a very pretty solution, but it works ;)


Small progress was made, I had to solder in a jumper wire inside the ECU to activate the onboard wideband controller, but after this it worked fine. I could go for a run, check everything on the handlebar mounted tablet whilst logging all parameters so I could look back afterwards and adjust the fuel tables.
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I’ve had the chrome redone a while ago, but I still had to protect the inside from rust. So bolted it from the bike
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And painted them black using POR15.

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The front fender could also use some attention, since it was black and broken. So I removed it, and sanded it down.
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“Fixed” it, just using a soldering iron to prevent it from tear any further.
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Then, a quick white spray can job and it was good to go.
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I wanted to combine the original seating/peg position but I also had fitted a rear disc so I had to find a way to mount and use the rear master cylinder.
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By using this extra pivot point it works perfectly and with some black paint on it looks like it was meant to be (atleast to me ;))
Also fitted the plastic radiator shroud after a quick blast of some silver paint.

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Time to fit the new GSXR swingarm; Image
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This goes to show that Suzukis are like LEGOs, the sprocket carrier, sprocketbolts and sprocket are a direct swap! In fact, I needed to fit the GT carrier in the GSXR wheel to align the chain.

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The rear fender is now too narrow
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Of course the Hagon rear shocks could stay, so I fitted some fancy Marzocchis.
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To make sure that it al fitted and to check the stancy of the bike, I created/fitted this very ugly subframe to get the bike rolling. Don’t worry, I didn’t drive it like this ;)
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Re: 1973 Suzuki GT750K, Fuel Injected, GSXR suspension

Posted: Wed Dec 11, 2013 2:56 pm
by JoostGT
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Duo shock mounting points!
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I had to search high and low for a suitable piece of aluminum channel, but finally ended up with this.

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Some measuring and a piece of wire rod meant I could go find a welder.
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This guy is incredible, took him 1,5 hour of cleaning, heating, sanding and awful lot of work, but it was the best piece of aluminum welding I’ve seen in a long time.

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To mount the swingarm I had to use some new bearings but after that it went in without too much of a problem.

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Since I wanted to take the bike on a 1000km trip (in a couple of weeks after these pictures) I had to fit some sort of baggage system, since I had this case laying around it was quickly adapted and fitting to the GT. It’s pretty ugly, but I’ve never seen a good looking topcase ;)

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The chain alignment is pretty good, but it was still a very tight fit. The chain had to clearance itself a bit, but with that complete it works like a charm.
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2 Of many stops to tune the ECU. The wideband sensor no longer works by this time, I guess it didn’t like 2 stroke oil after all.
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At first, I had painted it white just to have it all 1 color and it looked “oke” but it could easily be better. So,
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Now it’s appropriately 70s orange/brown. I didn’t sand it very well because I thought it would be a quick and dirty job. Sadly, the paint turned out pretty good and it’s the prepwork that’s letting down the overall finish. Oh well, maybe next time, I much more of a technical instead of optical guy anyway.

This is the last post I made about the GT and you’ll soon find out why.
After my mad dash to get the bike ready, I got up and fitted my luggage. With two of my friends we started off towards Luxembourg, about 400kms. This was the longest continues journey I’ve done so far, so I was very excited but also a bit apprehensive.

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Over the course of 400kms we had 2 break downs. On the first bumpy road we came across a connector on the fuel pump worked itself loose, but this was very easily fixed and on we went. Then, about 200km later, after a short stop, I heard a loud squeaking coming from the alternator. Turned out the charging voltage was about 16v and combined with all the added electrics, it was terribly overheating. After letting it cool down and adjusting the mechanical voltage regulator we were on our way again.

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Finally we arrived at Kautenbach, Luxembourg! It averaged about 9L/100km which was expensive but also to be expected.

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The next day started of great, but turned sour quite quickly. I started to misfire and loose power after blasting up and down the hills a couple of times. Eventually I pulled over and pulled the plugs, a small spring was lodged in the plug which was obviously not meant to be there.

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At the time I had no idea what it was, so I disassembled the entire intake to determine whether the injection might have lost a spring somehow. But (and you’ve probably already guessed it!) the spring belonged to the crank seal between the left and center cylinder 
So I called a friend, put the GT on a trailer and went home.

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Everything I modified was perfect and the fuel injection even allowed me to drive on 1,5 cylinder. So that’s a great succes, just very sad that the internals gave up after only 1100km.

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Re: 1973 Suzuki GT750K, Fuel Injected, GSXR suspension

Posted: Wed Dec 11, 2013 3:01 pm
by JoostGT
Recently I've started the rebuild of the engine by stripping it all to bits and see what needs doing. Sadly, almost everything needs doing :( Pistons, cylinders, seals, main crank bearings, big-end, conrods, even transmission bearings didn't escape. Obviously the engine ran too hot, with too little oil or to little fuel or maybe even all 3 combined! But it never happened in my possession, the engine was probably as rough as the outside when I bought it so its actually a small miracle I made it all the way to Luxembourg ;)

I'm currently trying to find out the best way to get this engine back together and strangely enough it's starting to seem that actually bringing the crank to the UK and having it done over there is cheaper then finding a way here? More news when it happens :) Will see about some pics of the current carnage..

Re: 1973 Suzuki GT750K, Fuel Injected, GSXR suspension

Posted: Wed Dec 11, 2013 3:35 pm
by tz375
That's impressive work there Joost.

Are the throttle bodies and ECU from a Triumph triple and if so what software did you use to change the fuel tables? I am assuming that teh USB was a connection to a laptop to program the ECU.

Are those injectors large enough to fuel the bike at high RPMs. Calculations we did seemed to suggest that injectors that were big enough at the top would be too large at low speed.

Crank seals do fail on bikes that have been sitting for a while. That's normal.

Welcome

Re: 1973 Suzuki GT750K, Fuel Injected, GSXR suspension

Posted: Wed Dec 11, 2013 4:30 pm
by JoostGT
Yes the throttle bodies are from a Triumph daytona 1050 I believe and the ECU is a mega squirt derivative, so fully mappable. It is also capable of running ignition but I've already got a newtronics unit fitted and I think that getting the fuel right is enough of a challenge in 1 go ;)

The injectors should be enough, but only just. I'm running them on a slightly increased fuel pressure but I'm not (yet) maxing them out. Perhaps if I were to fit a proper exhaust and/or do some cylinder work I may max them. I seem to recall that there is a function in the ECU to limit voltage over the injectors at low rpms to decrease fuel flow some more, not the prettiest solution, but if it works?

I know that crank seals fail after sitting, it's just a shame it happened after 1000+ kms and in a different country. But somewhere in the back of my mind I've always knew I had to replace them at some time.

Re: 1973 Suzuki GT750K, Fuel Injected, GSXR suspension

Posted: Wed Dec 11, 2013 5:22 pm
by tz375
When I did the numbers a couple of years ago the target was 180HP at the crank at 8500. Not that such a thing is practical but I needed to scale it appropriately and then I ran into issues. I would have needed larger injectors and higher fuel pressure and then the low end was a problem with not being able to get short enough duty cycle. Maybe I should have changed to fire once per two revs or all three at once but every 3 revs or something like that at low load.

I like the idea of lower fuel pressure at low load to reduce fuel flow.

I didn't recognize that as a Megasquirt. I have not seen one in an OEM type package before. Usually they come in DIY boxes and do not look very professional. That one looks almost like an OEM Keihin container but without the resin.

I was looking at a SAGEM for a Triumph 955I with Tuneboy software, but a megasquirt might be a cheaper option. A MoTeC would be nice but a bit out of my price range. Maybe if I sell off some of the sets of carbs I have on the shelf, I could afford one. :roll:

Re: 1973 Suzuki GT750K, Fuel Injected, GSXR suspension

Posted: Wed Dec 11, 2013 6:07 pm
by oldjapanesebikes
I'm impressed all to heck ! :up: :up: Very nice work - and welcome ! 8)

Re: 1973 Suzuki GT750K, Fuel Injected, GSXR suspension

Posted: Wed Dec 11, 2013 6:13 pm
by Alan H
Lots of work gone on there. Got to admire your efforts.

Re: 1973 Suzuki GT750K, Fuel Injected, GSXR suspension

Posted: Thu Dec 12, 2013 8:31 am
by JoostGT
tz375 wrote:When I did the numbers a couple of years ago the target was 180HP at the crank at 8500.

I didn't recognize that as a Megasquirt. I have not seen one in an OEM type package before. Usually they come in DIY boxes and do not look very professional. That one looks almost like an OEM Keihin container but without the resin.

I was looking at a SAGEM for a Triumph 955I with Tuneboy software, but a megasquirt might be a cheaper option. A MoTeC would be nice but a bit out of my price range. Maybe if I sell off some of the sets of carbs I have on the shelf, I could afford one. :roll:
If I were to get only half of that HP number it would be VERY pleased, so I'm hoping that my injectors will be up to the job.

It's a megasquirt based KDFI unit, it's a SMD factory made circuitboard in a IP65 housing. Great concept and very complete, but the feedback from the seller and documentary available make it very user unfriendly. Most of my time was spend trying to figure out what the hell the thing wanted from me and solving production problems :roll:

If it were to do it again I would look very hard at a microsquirt unit. All enclosed, small, bike friendly with all the megasquirt support. A little less features, but I can live with that. I don't think I would pay for a big name like motec, even though their product might be superior in some ways.

Thanks for the welcomes, it's nice to find like minded people :)

Re: 1973 Suzuki GT750K, Fuel Injected, GSXR suspension

Posted: Thu Dec 12, 2013 10:08 am
by tz375
Ah so. That explains it. KDFI have a terrible reputation in the MS community for build quality and for "stealing" the MS design.

Suzukidave here was fitting a Microsquirt with R1 throttle bodies to his GSXR mounted GT750. Wiring that looked like a nightmare too.

We don't have any realistic expectations of getting that sort of HP. That was a number used to see if EFI would be a good optoion and needed to know if there was enough headroom to stretch it.

The main reason to fit EFI is to get bigger throttle bodies without losing smooth on road performance at lower speeds. My street GT makes around 80HP with BS40 carbs and Kevin "Hatchet" is somewhere in the 125-130 range on his drag bike on old school VM carbs. EFI should make for a sweeter ride.

Re: 1973 Suzuki GT750K, Fuel Injected, GSXR suspension

Posted: Thu Dec 12, 2013 2:10 pm
by JoostGT
Yes, KDFI is frowned upon in the MS world, must say I don't want to get involved in that discussion, I bought it for it's housing, on board wideband controller and smd construction.

I've read about both your bike and Hatchett's and they show very well that there is nothing wrong with carbs, it's just that I like the challenge of fitting EFI and hope to learn something from it that might be useful in the future. It's actually a good thing the GT doesn't have a oil pump or I might be tempted to fit a turbo...

Re: 1973 Suzuki GT750K, Fuel Injected, GSXR suspension

Posted: Thu Dec 12, 2013 4:07 pm
by tz375
I do like the KDFI surface mount and the enclosure is so much better than the hobbyist box that MS seem to end up in. That's why I didn't recognize at first. No point in getting into that discussion.

Well a GT does have an oil pump but not high volume/high pressure. I was reading a thread on another sire about a Honda 360. They have a bad rap for oiling failures, so the builder fitted an electric oil pump. All you need is a small oil tank and an electric pump and that turbo could be next.

There was at least one company selling turbo kits for two stroke snowmobiles a few years ago. I wonder what they used for an oil supply to keep the turbo bearings alive.

Re: 1973 Suzuki GT750K, Fuel Injected, GSXR suspension

Posted: Mon Dec 16, 2013 10:19 am
by JoostGT
So, to try to save some money, I digged up my spare engine and had a look at that gearbox. I had to scoop out dead spiders, dirt, old grease and some leaves, but there seemed to be a gearbox in there ;) After a thoroughly cleaning and then oiling this gearbox looks a lot better (including the bearings) than the one that was in there!

But of course, there is a problem. I cannot, for the life of me, find the colored dot on either the gearsets or the engine casings! So how am I to determine if I can swap the gears? I have a full gearbox and kickgear, so I thought about swapping them all, but what is the difference between the colored dots? Why can't I just swap them around?

I also managed to remove and disassemble both waterpumps and they seem to be in great order! The carbon seal as well as the bellows seem to be fine, just have to slap on a new bearing and it'll be fine.

Now I just have pick and choose from the enormous pile of parts I've gathered...

Re: 1973 Suzuki GT750K, Fuel Injected, GSXR suspension

Posted: Mon Dec 16, 2013 12:24 pm
by tz375
JoostGT wrote: Now I just have pick and choose from the enormous pile of parts I've gathered...
I am familiar with that problem.

BTW, later gearboxes have better gear ratios.