GT380 - Centre downpipe cooler than the outers

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badgereng
On the street
Posts: 33
Joined: Fri Oct 01, 2021 6:14 am
Country: England
Suzuki 2-Strokes: GT380L - 1974
Location: North East UK

GT380 - Centre downpipe cooler than the outers

Post by badgereng »

Hi guys and thanks for letting me join this fantastic community. I have recently bought a 1974 US Suzuki GT380 that was imported into the UK a number of years ago and then registered in the UK on an ‘M’ plate.
It’s no show pony but is in decent condition for its age with almost everything but the tyres looking original or at least NOS.

I went for a 3-cyl 2 stroke triple as part of my retirement present as its just about the only inline engine format I have never owned, up to and including a CBX six in the mid 80’s. Just missing something with a V config…. maybe next one eh.

I live between Durham and Sunderland but originally made in Hull and have been riding bikes since I was 13 ish with C50’s & Raleigh Runabouts as track-bikes, then starting life on a FS1E once I was able to hit the road legally.
All this bike stuff got me into anything mechanical and I did my apprenticeship as a maintenance fitter then progressing to mechanical & process engineering specialising in high-speed production lines for personal care products over the decades.
My current bike is a 2017 Kawasaki Z1000SX that my wife and I use for bi-annual European tours and the odd world endurance race in France. Currently spending the early days of my retirement planning 2022's 3 week trip starting from Santander and ending in Amsterdam.

Anyway, that’s me…… Now for the Suzi!

I’ve had a good look thru the forum and have been unable to find the answer I’m looking for so starting my first thread and hopefully getting some feedback. I have a feeling it won’t be my last thread.

The bike fires up first or second kick everytime so that’s a good start eh! It runs OK if a little sluggish and smokes heavily when first started, however the smoke does usually clear after a decent run out. Oil pump timing mark looks synchro’d with the carb slide indicator, so exhaust draining & cleaning is part of the jobs to do list.

Anyway, the question I have relates to the centre pot / exhaust. The middle downpipe does not heat up anywhere near as quick as the two outer cylinders. After 30 seconds or so, it’s still possible to ‘dab touch’ the downpipe whereas the two outers would be melting skin.
Is this normal? Does anyone else have the same symptom / problem… or is it natural due to the centre pot exhaust gases being divided into two outlets and possibly more free flowing. I guess I don’t know for sure if I have a problem or not. Over time, the centre downpipe does become too hot to touch as you’d expect.

I’ve removed the plugs, cleaned and checked gaps etc. They were pretty clean, no hint of oil fouling and a decent colour. I swapped the plugs around to see if the problem shifted and it didn’t. I also swapped the condensers later to try and shift the problem…. If I have one. Again, the centre downpipe remained much cooler.

I’ve a few more things to try on the ignition side before venturing into fuelling but before I stop doing the easier stuff, I wanted to get some feedback if this is just normal or not.

Thanks for taking the time to read and hopefully feedback.

Cheers

Pete
FS1E
RD250 E
CB750 K6
CBX B
CB900 FD
CBR1000 FD
CBR600 FV
CBR600 F1
CBR600 RR
FZ1-S Fazer
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Z1000 SX Tourer
GT380L

Current - 1981 Honda CX500 & 2023 Kawasaki Z1000SX Ninja
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Alan H
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Suzuki 2-Strokes: 4 x GT550s - J, M, A, B.
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Re: GT380 - Centre downpipe cooler than the outers

Post by Alan H »

Welcome Badgereng. Sounds like you have blocked pilot jets in the centre carb. Basically, remove the tank for easy access and loosen the bolts to the frame that hold the airfilter box. Slacken the airbox to carb 'jubilee' clips and waggle the airbox out. If you are a real engineer you will already know the correct swearwords to assist, and if you don't just make some up.
Take the two throttle cables off the carb butterfly shaped linkage and mark which one is which - one is to open the carbs, t'other is to close them. Take the jubilee clips off the carbs to barrel connectors and remove the carbs. If you are good with carbs, strip each one separately (if one is partially blocked, t'others might be a bit too) and write down how many turns each screw takes to go in or out and blow through every orifice with carb cleaner (Toolstation is good and not expensive, but get 2 or you will run out - trust me on that one!) Remantling is the opposite of dismantling of course, but make sure all the carbs start to lift at exactly the same time using the adjusters on each slide actuator. Please be gentle with the clamps as they are made out of congealed snot cunningly disguised as aluminium and break VERY easily if you are the slightest bit ham fisted.
If you are less than happy with stripping and remantling carbs, send them to Baz Wilkes (details will be sent to you by email) and let him do them. He's done several carb sets for me over the last 10 years or so on 4 550s (+ a couple of spare sets), plus a Kettle. They come back looking better than new.
Think of how stupid the average person is, then realise that half of them are more stupid than that.
badgereng
On the street
Posts: 33
Joined: Fri Oct 01, 2021 6:14 am
Country: England
Suzuki 2-Strokes: GT380L - 1974
Location: North East UK

Re: GT380 - Centre downpipe cooler than the outers

Post by badgereng »

Hi Alan, thanks for the reply and pointing me in the fuel direction and also the email with additional info.... I now have another item going onto my 'to do list'

Due to the nature of the work I did and the travel involved, I'm now able to swear in quite a few different languages and it sounds like that will come in handy when I get started on the carbs.

Once done, i'll let you know how it went.

Thanks again :up:
FS1E
RD250 E
CB750 K6
CBX B
CB900 FD
CBR1000 FD
CBR600 FV
CBR600 F1
CBR600 RR
FZ1-S Fazer
Z1000 SX

Z1000 SX Tourer
GT380L

Current - 1981 Honda CX500 & 2023 Kawasaki Z1000SX Ninja
badgereng
On the street
Posts: 33
Joined: Fri Oct 01, 2021 6:14 am
Country: England
Suzuki 2-Strokes: GT380L - 1974
Location: North East UK

Re: GT380 - Centre downpipe cooler than the outers

Post by badgereng »

As promised, a bit of an update(ish)

Carbs have just come back from Baz Wilkes and as you said, they look better than new.

The bike itself is in a squillion pieces with parts away for vapour blasting, chroming, crank re-building and reboring etc etc so it’s going to be a while before stripping and cleaning is completed….. and re-assembly is started and completed. Hopefully all the work makes that particular problem go away.

The bike was also a very heavy smoker but it only had short 10 mile runs and garage starts whilst in my possession as my time home was short between contracts and subsequent travel.

However, now retired…. and I’ve just got round to de rusting, and de carbonising the pipes, it’s very obvious how much more oil there is in both the pipes that share the centre pot in comparison to the outer pots. Maybe the poor combustion was not just on start up / pilot as you indicated Alan but was also iffy through the rev range. I could sell a few barrels back to Aramco that’s for sure!

Time will tell I guess.

Cheers

Pete
FS1E
RD250 E
CB750 K6
CBX B
CB900 FD
CBR1000 FD
CBR600 FV
CBR600 F1
CBR600 RR
FZ1-S Fazer
Z1000 SX

Z1000 SX Tourer
GT380L

Current - 1981 Honda CX500 & 2023 Kawasaki Z1000SX Ninja
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Alan H
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Posts: 3160
Joined: Thu Feb 16, 2012 11:50 am
Country: England
Suzuki 2-Strokes: 4 x GT550s - J, M, A, B.
Location: The Republic of South Yorkshire

Re: GT380 - Centre downpipe cooler than the outers

Post by Alan H »

Best way that we used to clear exhausts was to remove baffles, and stand the pipe up against a wall inlet end up. Squirt some petrol down the pipe and chuck matches at the bottom end.
Watch it until the smoke clears!!! Chuck the baffles with padding removed on a bonfire until cherry red and smoke free.
Think of how stupid the average person is, then realise that half of them are more stupid than that.
badgereng
On the street
Posts: 33
Joined: Fri Oct 01, 2021 6:14 am
Country: England
Suzuki 2-Strokes: GT380L - 1974
Location: North East UK

Re: GT380 - Centre downpipe cooler than the outers

Post by badgereng »

Update time on the 380 rebuild and centre downpipe cooler than outers!

As mentioned in the first post of the thread, the bike started well and ran OK...

If only it stopped as well as it started. That front brake frightens the life out of me :shock: The only obvious fault with the bike after buying was the centre downpipe was initially always cooler than the outers and it smoked like a good 'un.

Forum advice was given, taken and actioned as part of a complete bike strip and rebuild. Pretty much the only nuts, bolts and screws that have not been apart are in the speedo / tacho assembly and the spokes.

Carbs to Baz, Crank to Applebee's (Engineers, not the restaurant chain) & Rebore via Armstrongs in Newcastle. Dismantle and rebuild by yours truly.

About 3 weeks ago the bike was ready to fill with fuel and give it a start. Primed and then kicked over without plugs until I could smell fuel in the barrels then put the plugs back in. 2 x kicks later and its running... what a relief.
Centre downpipe is freezing and the two outers are warming so turned it off pretty quickly (Maybe too quickly and fuel starvation could have been the reason) Took out the centre plug, ran it up again to check for a spark and there it was. Decent spark from the new B8ES's.
Popped the plug back in, 2 more kicks and its up and running again. This time the centre pot is warming at same rate as the others. Answers on a postcard please :D

Only done about 10 miles (2 x 5miles) on the bike since as I'm just waiting for the air filter end caps to come back from Chromespray then it'll get a better test ride. However, after the test rides, all the pipes feel evenly warm and the smoking is ''normal'' whatever you class as normal. In my case its significantly less than it was. As the pipes fill with oil over time, that may change.... however, for now.... :clap:

So, no definitive answer I'm afraid as too many variables changed during the rebuild. Just trying to bring some closure to the thread.

Pete
FS1E
RD250 E
CB750 K6
CBX B
CB900 FD
CBR1000 FD
CBR600 FV
CBR600 F1
CBR600 RR
FZ1-S Fazer
Z1000 SX

Z1000 SX Tourer
GT380L

Current - 1981 Honda CX500 & 2023 Kawasaki Z1000SX Ninja
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jabcb
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Suzuki 2-Strokes: 69 T350 thru 75 GT750
Location: southwestern Pennsylvania

Re: GT380 - Centre downpipe cooler than the outers

Post by jabcb »

The 72 GT380 has a seriously-undersized front drum brake.

Disk brake on later model GT380s is OK but has delayed performance when it’s wet. A common problem back when the manufacturers were switching to disk brakes.

Your brake may need overhauled, especially if it sat unused for an extended period.
Rebuild the master cylinder & caliper. For best performance use new modern pads & stainless steel brake line.

For improved wet performance, upgrade to EBC disk & pads. EBC disk VMD3014 are lighter & look a lot better than the MD3014.
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 & 75
GT380 72
T500 69 project & 73 project
T350 69 & 71
Honda 85 CB650SC & 86 CB700SC
09 Triumph Bonneville SE
User avatar
Alan H
Moto GP
Posts: 3160
Joined: Thu Feb 16, 2012 11:50 am
Country: England
Suzuki 2-Strokes: 4 x GT550s - J, M, A, B.
Location: The Republic of South Yorkshire

Re: GT380 - Centre downpipe cooler than the outers

Post by Alan H »

Suzuki's 'upgrade' to the original 380J front brake was to fit Ferodo green racing linings. I went with my mate to the Ferodo factory at Chapel-en-le-Frith near Buxton where they stripped the brake, fitted new relined shoes that were lathe skimmed to match the drum. What a difference! His was supposedly the first one done. When I got my 550J in August '72 , Eddie Crooks did the same with my 4ls front bake and it would outstop any disc setup at the time.
I daresay if you contact Ferodo they might be able to help.
Think of how stupid the average person is, then realise that half of them are more stupid than that.
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jabcb
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Suzuki 2-Strokes: 69 T350 thru 75 GT750
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Re: GT380 - Centre downpipe cooler than the outers

Post by jabcb »

Vintage Brake in the US upgades these old brakes with Ferodo relined shoes & skimming:
http://www.vintagebrake.com/index.html


My GT380J stops quite nicely with a Suzuki 4LS.
I bought a 4LS on eBay years ago before the prices went crazy. More recently bought the GT380J. Looked like it might fit, so I test fit the 4LS when the GT380 was on the lift. Fit perfectly, so I rebuilt the 4LS with new Vesrah shoes.

Suzuki used a similar 4LS with the same shoes (VB-331) on some more recent bike. As a result, new 4LS shoes are usually available on eBay & elsewhere.
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 & 75
GT380 72
T500 69 project & 73 project
T350 69 & 71
Honda 85 CB650SC & 86 CB700SC
09 Triumph Bonneville SE
badgereng
On the street
Posts: 33
Joined: Fri Oct 01, 2021 6:14 am
Country: England
Suzuki 2-Strokes: GT380L - 1974
Location: North East UK

Re: GT380 - Centre downpipe cooler than the outers

Post by badgereng »

Thanks for the info gents. The bike is a 74 US Sebring and has a front disc.

On the rebuild it got a new reservoir, master cylinder seal kit and piston rebuild kit, 2 of the 3 hoses replaced, completely stripped and cleaned brake caliper with replacement seal kit and piston, bleed nipple, new pads and lashings of fresh brake fluid.
The only things not replaced or spotlessly cleaned was the reservoir cap, the first hose out of the master cylinder and the disc itself.
Absolutely zero improvement over the way it worked previously.

% age improvement Vs cash spent makes it the biggest waste of money on the rebuild 😂

Pete
FS1E
RD250 E
CB750 K6
CBX B
CB900 FD
CBR1000 FD
CBR600 FV
CBR600 F1
CBR600 RR
FZ1-S Fazer
Z1000 SX

Z1000 SX Tourer
GT380L

Current - 1981 Honda CX500 & 2023 Kawasaki Z1000SX Ninja
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