Post GS750 > 844 Build Plug Fowling

[sam000lee]

Hey folks, been a while since I've posted and I've got some background info for an issue I'm currently having so bear with me.

Back in 2022 I moved away for a year with my job. Right when I left my GS750 started running poorly, with smoke in the exhaust and poor, lurchy acceleration. When I pulled the plugs, the #3 was fowled. Replaced it, fowled again. I've gone through the bike and kept up the maintenance. I noticed there was some oil dripping out of one of the downpipe connectors and eventually came to the conclusion that the valve stem seal was failing on that cylinder.

Fast forward to earlier this year. I decided to rebuild the top end with a Wiseco 844 (69mm bore) kit. I got the cylinder bored out and the head bead blasted professionally, and also had them do a leak test on my cylinder head after replacing the valve stem seals (OEM Suzuki) and re-lapping in the valves. All 8 valves were sealing perfectly.

So I've just finished putting everything back together and the bike starts right up. I went through Wiseco's break in instructions. After finally riding it, I noticed that I'm still getting some surging at acceleration and the exhaust sounds blotchy. I pull the plugs again and once again the #3 plug is fowled. A couple days later, I replace it, start the bike again and notice that the #3 pipe isn't getting hot.

At this point I think there are three potential issues:

1. Something going on with that carb.
2. Poor spark
3. Valve stem seal is leaking still/again
4. Something else.

Today I fully disassembled the #3 and #4 carbs and ultrasonic cleaned them. Then I installed all of the jets and bits from the #4 into the #3 and vice versa to rule those out. I also switched the #2 and #3 spark plug wires to eliminate a bad plug wire. No luck. Bike still surges on acceleration, #3 cylinder doesn't get to temp, plug is fowled again. So I believe I've eliminated #1 and #2. I also did a compression check and all four cylinders are at ~130.

Here's a picture of the plug in question:





Some other info about the bike:

1. Dyna coils (Green) + Dyna electronic ignition
2. Coil relay mod, new battery, SH775
3. Delkavic Exhaust
4. Pilot jets=15, Mains=102, Needle is on the second setting (i.e. I raised the clip one position)
5. Just rebuilt the engine and did valve shim adjustments. Everything in spec there.

What am I missing? The only thing I can think of is that somehow the NEW valve stem seal that I installed is somehow also leaking oil on the same cylinder that made me do the rebuild to begin with, but that seems farfetched unless there's something wrong with the guide...


Thanks in advance for your help!

[unfocused]

Are you running the spark plug caps? You can take them apart and clean/test as needed.

[Rob S.]

Poor spark? My guess.

[sam000lee]

Hmm - thought I had ruled that out by switching out the #2 and #3 plug wires. If it were bad spark, the fowled plug would follow the wire to the #2 cylinder, but after flipping the wires it stays on #3. I did also swap the #1 and #3 plugs (i.e. to put a "good" plug in #3) with the same result. Similarly, if I pull the #3 wire with the engine running and a spare plug in it, it produces an extremely strong spark when grounded to the engine.

[sam000lee]

I swapped the #2 and #3 plug wires again just to double check and regardless of which wire is going to the #3 cylinder, that's the one that is not firing (pipe doesn't get warm on cold start). When I pull the plug wire it has a really strong spark arcing to the engine just between the cap or if I put a spare plug in. I'm almost positive it's getting a very strong spark.

Any other ideas to either test spark or for anything else that could be going wrong?


Interestingly, I'm finding that my issue is very similar to this persons:

https://oldskoolsuzuki.info/forums/topic/20399-cylinder-3-not-firing-78-gs750/page/2/

[dgates1]

I'd check for a sticking needle valve in carb #3. Mine was not firing on #2 and right after a carb clean using an ultrasonic and that what I found. Cleaned the seat and needle again and polished the seat with some rolled up cardboard. Runs on all 4 now.

[Rijko]

When I pull the plug wire it has a really strong spark arcing to the engine just between the cap or if I put a spare plug in. I'm almost positive it's getting a very strong spark.
Any other ideas to either test spark or for anything else that could be going wrong?

Got a timing light ?
Sometimes issues can be observed by irregular strobe flashes.
( compare #3 to the other cylinders )

Valve stem seals *can* be replaced without taking the head off.
It's fiddly, but i have done it in the past.

Did you change over the float from #3 also, or re-checked fuel level with the clear tube method ?
Surging is often caused by uneven fuel supply to the cylinders.
A black plug could point to a rich running cylinder, possibly caused by (ao) a sticky open float or one delivering way too much fuel because of the float level set too high.

The vacuum hose is connected to #3, right ?
Maybe try to unhook the vacuum hose from the tap and plug it.
Run it with the tap on PRIme and see if anything changes.
I have seen taps leaking through the vacuum hose into the carb and messing up mixture.

[tom203]

Since it’s happening quickly, sure sounds like a leaking petcock

[sam000lee]

Resolved!


It was clearly running overly rich and so I thought I'd see what happened if I removed the vacuum line to give it more air. After a quick ride around the block - it ran great, but when I returned there was gas all over my airbox. On further inspection, there's gas leaking out of the vacuum port on the petcock - it's been flooding the #3 carb and fowling the plug.

Ordering some parts then excited to finally get to rip this thing around. Thanks all for the help and great call Rijko and Tom203!

[Rijko]

You're welcome.
Great feeling when you find the issue isn't it

[rphillips]

I've read about this being a problem, fuel through vacuum line. I personally haven't seen it. Only problems I've seen was pet cock not sealing gas when vacuum was gone... Like he said great to find and know what problem is and how to fix. All that head scratching and trying to figure what's wrong can be a real pain... Congrats.