1983 GS750: DynoJet Problems (Previous owner jetted): Looking for some insight on fix

[90'son-80'rider]

Hi Team,

Sending my best through the e-waves.

I have an 83' GS750E, Carbs are Mikuni BS32SS (Stock) and the P.O dynojetted the carbs. Realizing through my own experience and through reading online, dynojetting can be a nightmare.

On my bike specifically, Carb 3 (prime hose carb, second in from left) is NOT firing. It has spark (confirmed), put in new spark plugs. Run the bike and this exhaust is cold to touch. In addition, gasoline is leaking through the front header on this cylinder when running (photo attached).

My beginners mind went a couple places here. I thought float needle wasn't working and that may be causing an overflow of gas. Went in, float needle seemed jammed when inspecting it. Thought Booyah, here she is. Replaced float needle, cleaned all carbs on a basic level (Didn't want to touch dyno jet's as heard they are a pain) and put carbs back together.

Didn't get the fix. Still leaking. I also rigged up a prime hose shutoff and replaced fuel lines (there were clear plastic tubes on previously, were hard as rocks).

Then realized, I should have done a compression test first thing. Compression was alright when purchasing the bike but maybe something changed. Ran compression test.
Results
DRY: 95 in cylinders 3&4, 120 in 1&2.
Spot of Oil: 120-122 cylinder 3&4, 133-135 in 1&2.

Changed from when I bought. Was originally within 15 of each other. I see my piston rings likely need a replacing with a wet test bringing a better number (although still not great).

I can undertake that task with my service manual (thank you BikeCliff) and a bucketload of time (when I get it).

However, I'm still now stuck on these dang dyno carbs! With compression of 120, I should be able to get a fire but it seems so much gas is being jetted through it's literally drowning any spark and this gas has corroded the header gasket!

My question is this: Has anyone replaced one of their dyno jets with a Factory Stock Jet and had any success?

My thought is simply replacing the Dyno with stock parts (Thinking I need to replace jet needle and main jet), replacing the header gasket and seeing how it would turn out. I don't know enough about dynojets to know if this would work, hence my reach out here. Carb has PODS on and am worried about airflow but don't know all details here.


Excited to hear your insights here GSer's. Thanks for your time!


Kindest,
90's son.

DynoJet Install guide: https://www.dynojet.com/amfile/file/.../product/3556/

[Rich82GS750TZ]

That would be Carb/cylinder #2. Numbered 1234 L to R as you sit on the bike. There is no Prime hose carb. The port on the engine side of carb #2 is a vacuum port, should connect to the smaller of the 2 ports on the petcock. This confusion could be one cause of your issues.

see pg. 71,72 of this, the first of 2 CV carb rebuild tutorial on BikeCliff’s website for proper carb hose connections (all 4 of them)
Sorry, no answers, just want you to be clear. Carry on.

[tom203]

If all was well last July...I’d now suspect petcock failure letting fuel down vac line and flooding out #2 cylinder

[chris]

Hi, I would not worry about the dyno jet kit for now. If # 3 cyl. has spark and is not firing, I would guess the carb. is the culprit. It probably needs more than a basic cleaning. I would also confirm proper function of the petcock. Hope this helps. Good luck.

[salty_monk]

Dynojet kits may or may not cause your bike to run rich depending on how you set it & which Pipe and Air filter you are running. What they won't do is cause the problems you've described. Lets have a little more info is poss. I would suspect a full strip & clean of your carbs (you can catalog what is in them as far as Jets & needle settings at that point... we can help if you upload good pictures), replacing all the O rings, Intake boot O rings & checking the petcock for proper operation is a start. I would then check the valves for proper clearance.
Once you get it running, ride it for a few hundred miles then do you compression test again. If it's rings they may well be gummed up not sealing well & cycling the engine will free them up again

[90'son-80'rider]

Hi there Gents,

Thanks for all of your replies. They were very useful in finding the problem. Which was, as you guessed it, the petcock.

Took the carbs apart for a thorough cleaning. Was good to do this for my own understanding and the bike is now running much better because of it. Bikecliffs rebuild manual was very useful there, much appreciated.


I then took the petcock off the bike.
After some wrestling with the old thing, was able to get it apart and see the diaphragm had slowly eroded on the inside. No clean rip or tear, the rubber had worn down and could see the inner mesh of the diaphragm. Could tell something was up as when I first took it off as suction on the gas port without any vacuum attached brought fuel through.

Replaced with OEM (Very tough to find for these old ones), replaced o rings and intake boot o rings in carbs, new lines for petcock to carbs and she is running great.

So pleased.

Sending my best to each of you reading this, especially the guys who jumped in to help.
Hope everyone is healthy, riding, and squashing all the covid they can with their two wheels.

All the best.

90's son

Suzuki.jpg

[trevor]

Funny to read this...I have been dealing with the same issues on number two on my 83 750ed....realized it was the petcock and waiting for the new one.

[Carter Turk]

Cool the problem was resolved with help here, but I didn't see a mention of an oil change.

Seems fuel would be leaking past the rings on #2 into the engine.