GT750 CV to VM conversion NO START problem.

[mfrias2nd]

Hey all,

First off thanks to all those who offered tips and advise for this setup in my previous post...

I've completed the conversion to the vm carb setup. I have 117.5 - 120 - 117.5 mains and 45 - 47.5 - 45 pilots. with the needle in the middle position. The bike will only start if I cover the carb intakes almost completely. Once the bike starts it seems to run alright up the rpm range, maybe a little sluggish in the lower rpm's but I need to diagnose the no start. Any advise guys? move the needle? too little fuel to air? reduce pilot jet size?

any help would be appreciated!

THanks!

[GTandcbr]

Are the carbs clean? It sounds to me like you have a problem with the starter circuit. First make sure the choke pistons are lifting and move freely in the bores. Then remove the float bowls and check that the brass tube which sticks up is clear. It draws fuel through the little jet at the base. These easily get blocked with silt from the gas tank. Inside the hole there is a non-removeable jet. This can be cleaned by spraying carb cleaner and blowing out with compressed air if you have it. Lastly the choke tube has a number of little holes near to the top these should be cleaned to as the mix air with the gas being drawn up to cause it to froth thereby assisting the flow.
One final thing. When starting the motor dont touch the throttle as lifting the throttle valves even a little renders the starter circuit useless.

[tz375]

You fitted 72/3 carbs to a 73 cylinder and added pods. Stock jetting is 102.5/100/102.5 mains and #30 pilots so it should be running rich. The poor starting sounds like air leaks or partially blocked passages in the carbs or possibly low fuel levels.


Do you have stock 188 series P3/P3/P4 needle jets and Large round jets or something else?

[jabcb]

You might have the throttle stop screws turned in too far or no slack in the throttle cables. This could keep the slides open too much and make the choke less effective than it should be.

[mfrias2nd]

GTandcbr, I have jemco chambers equipped and from what I understand this is a good baseline.

117.5 - 120 - 117.5 mains and 45 - 47.5 - 45 pilots

if I'm incorrect or you all have a better baseline I'm open to suggestions.

I'm going to pull the carbs off, re-clean and start over.

[tz375]

Center main is smaller with stock OEM pipes and usually the same as the outers with chambers.

I would expect to start with 110-110-110 mains and 32.5 or 35 pilots.

Check that the slides are fully closed as suggested and if they are, strip and clean the float bowls. The starter jet is a tiny thing and it hides in the bottom of the float bowl where it gets clogged. They take some work to clean properly.

While the carbs are off, check that the slow jet passages are all clear. Spray cleaner or WD40 through the drillings on one carb and repeat on the other two to compare one circuit at a time. Check teh pilot jet outlet as well as the bypass circuit which is just in front of the slide.

[mfrias2nd]

Findings,

I pulled off the carbs and they were all very clean no clogs. I checked float levels and they're fine. However, I did find my mains are 125 across the board and 30 pilot. Again, I have jemcos.... does this jetting seem off? Also slides are all in sync and almost completely closed when off throttle.

I did, however, find a lot of black oil build up in the left most chamber, and a little in the middle and none in the right. Bike leans to the left. I am aware of worn seals on the bike but it's always ran before. Plugs were very oily. I just don't know if this was 2 stroke oil or transmission oil. It was already cooked and black. This could have been my no start issue. However, I before I install the carbs again can anyone chime in on the correct jetting? again I have as follows...

125 mains
30 pilots
Needle in the middle pos.
Pod filters (unfortunately until I get correct airbox)
Crystal clean carbs.

[Mgmark]

Are you using the electric starter or kicking it? My 750 wouldn’t start on the button but starts reasonably well with the kicker. I run 34mm VM carbs and Jemco pipes. Also, I changed the chokes from the stock cables to the lever type.

Mark

[mfrias2nd]

Electric start only Mark. I'll give the old kick a try.

I took the carbs off to clean them and found they were super clean. But at 30's i think my pilots are too small for the pod filters with jemcos. I've been seeing 47.5 on the forum. Anyone care to correct me?

[tz375]

47.5 is on CV carbs.

Try 35's and see how it runs.

[jabcb]

My gray 75 GT750 has VM carbs, K&N pods & Jemco 3-into-1. The PO’s documentation included some info about porting but I don’t know what he actually did to the ports.

Had a shop do the work. Jetting was something like +2 on the mains & +1 on the pilots. So it should be 120 main & 35 pilot.

[mfrias2nd]

UPDATE: I moved up to a 35 Pilot and I got it to start on its own, barely. However, I realized that my carbs, which I obtained thinking they were gt750 carbs are what appear to be Kawasaki carbs :facepalm: due to the top cap and bowl designs I compared to with some online pics. Also the "fuel resistant" bowl repair I did isn't holding up and clogged the pilots with JB weld. :2nd facepalm:

Question:

Before I get into repairing or finding a matching hard to find bowl should I just try and locate some original suzuki GT750 Vm32's? does anyone know if my tuning problem is because these carbs are Kawasaki triple carbs and not GT750 carbs??? will Kawasaki Triple carbs work on a GT750? they measure to 32mm.

Also, they're printed with 310R 310 M and 310L in case you guys know what those are.

[jabcb]

Suzuki part numbers are 5 digits + a dash + 5 digits. The first 5 digits define the type of part & the second 5 digits are unique for each version of that part.
Suzuki stamped parts like carbs with the second 5 digits.

The early GT750 carbs have part numbers 1320X-3101Y. Where X = 1, 2 or 3 depending on the cylinder. They had several versions of the part so it ranged from Y=1 to = 5 (I think). They added an L, C or R to tell you which cylinder it fit.
For example, the final version of the left carb was stamped with “31015L”.

Kawasaki has a different part numbering scheme & likely did something similar.


JBWeld needs a good bit of time to fully cure. Perhaps you didn't give it enough time before exposing it to gas.

[tz375]

Those are a set from a 72/73 GT750. 310 is the model code and L M R are obviously left middle and right.

Somewhere here I have a set of 310-13 that someone had fitted to a later model to replace the BS40s, and some 310-12(followed by L M or R).

Get new pilot jets. They are VM22/210 style. Do not use BS40 type pilot jets. They look almost the same but are not.

If the JB weld got into the jets, it may also be partially blocking the tiny holes in the carb body that the fuel passes through.

[mfrias2nd]

UPDATE fellas....

Thank you for the mikuni help btw. I'm just going along with the carbs I have.

I've got needle in mid position, 125 mains and 35 pilots now. Started up first try while WOT. Hasn't started since. =(

I've torn down the carbs again and found some of the JB weld I used to repair a pin hole on the mid bowl has crumbled and clogged the pilot. - Cleaned. other carbs were clean.

I keep fowling plugs with black oil. So instead of repeating the same problems I decided to pull the SRIS apart and see what drips out. - L flowed like a fountain M - dripped a couple drops R - clean as a whistle. Oil that came out was BLACK. I didn't do the last rebuild or SRIS maintenance on the bike but I noticed that the lines went straight up. IE. Left output went into Left input, middle to middle and right to right. They didn't cross over like the manual states it should. I also want to point out that I suspected leaky crank seals so before I installed the round slides I drained the gearbox. This oil in the crank HAS to be 2 stroke oil right?!? if so why does it accumulate in the left so badly? because of the SRIS lines? This could be my entire starting problem all along. I'll know again once the SRIS lines are replaced and correctly routed.