1978 GS750 Backfire & Soot on Spark Plugs

[peterpressure]

I just acquired a 1978 GS750 with 50k miles on the engine and body. It sat in someones garage for the winter (typical) but is in otherwise great condition, no oil on forks, new paint job, new points and everything seems to work!

I cleaned the Carbs (they already looked really clean), replaced the battery and replaced the points and plugs. The plugs I took out were black and covered with soot. After putting new plugs and points in, replacing the Gas, cleaning the carbs (I put the jets back to the position they were set too prior to cleaning) and replacing the battery and following the starting tips & tricks (blowing into carb vent)! it starts right up with the choke on, then I let it warm up and turn the choke off. After letting it run for 5-10 minutes it purrs nicely, but if I put any load on the bike, it bikefires a bit and after turning the bike off the plugs look dirty again. NOT wet, just black and covered in soot.

What is not stock and was on the bike when I picked it up was the 4 Mikuni carbs, K&N pods and 4 to 1 Vance and Hines exhaust. I have no idea whether its been re jetted after the aformentioned mods were done, but considering it seems to run Rich, not lean, I assume something has been done. (Correct me if I am wrong in my assumptions!)

Finally my question, Do you agree that the current problem with the bike is that the Air and Fuel pilot screws and or Floats are not adjusted properly and that all 4 cylinders are running way too rich?

Keith Krause had in the following post outlined the proper settings: http://www.thegsresources.com/_forum...64&postcount=6

Basics.
Be sure carbs are clean and float levels are good and all inner o-rings are good.
I suggest replacing the manifold o-rings at this time.
Valve clearances and ignition timing must be correct.
Pods oiled correctly (if oil used).
All electrical connections clean/good plugs/gapped correctly/battery good.
Now you're ready to re-jet.
For pods only(?), I'd try something close to 100 mains. Stock is 80, so I think 95 would be the minimum and 100 the maximum needed. Hard to guess the first time when running pods with a stock exhaust because they're not a good flow match. Test at full throttle.
Pods will require you to raise the jet needles AT LEAST one position richer. I believe your stock jet needle position is 3 (middle position). Go 1 lower and test at 1/3 throttle. Every time you change the needles, you must re-bench synch the slides at the very least. This should be followed by a vacuum tool synch for accurate test results.
Try an additional 1/2 turn richer (CCW) on the pilot fuel screws (underneath). The factory generally sets them about 1 turn out, so I'd try 1 1/4 to 1 1/2. Fine tuning will probably be necessary. Test the pilot circuit at minimal throttle.
Try initially setting the side air screws at 1 1/2 to 1 3/4 turns out.
REMOVE the two floatbowl vent lines and leave the ports open when running pods.
After all the above is done, warm up and adjust the side air screws using the highest rpm method.

Does anyone disagree that if I follow his outlined procedure I should be able to correct the problem?

Here is the beauty! Also, I do not think I have the small petcock nipple hooked up to the proper vaccuum nipple on the back of the #2 carb, anyone have a pic of where it goes?

[earlfor]

The petcock vacuum fitting on my 79 GS750E was on the left side of the number THREE carb. The fitting on the #2 carb is a vent. I am guessing that
if you have the petcock vacuum connected to the #2 carb vent and the bike runs, the petcock is likely set to the prime position, which may partly explain your sooty plugs. I would check valve clearances though before doing anything beyond correcting the vacuum line connection and petcock setting.

Earl

[vwdoctor3]

check valve clearance.
what does your spark look like ?
ohm out your coils. should be around 3.00 ohms
did you play with the fuel screws on the bottom of the carbs ? vm26 carbs
are the condensors good ?
charging system good & getting good voltage to coils.
what are the pilot & main jets #s ? did you put needle jets back in middle position ?

just about all of these old 750's I get have low output on the coils and someone has played with the fuel screws on the carbs. if the valve clearance and timing looks good just turn your fuel screws in on the bottom of the carbs an 1/8 to 1/4 turn and see what the plugs look like then. if these are a set of vm26 carbs they are a bear to dial in if someone messed with the fuel screws.

good luck, doc.....

[KEITH KRAUSE]

Peter, those settings you posted are suggested for a 550. They also are suggested for different mods than you have. They won't work for you.
I'll try to reply again later tonight if you still need help with jetting.

[peterpressure]

Thanks for pointing that out Keith!

I have the bike set to On, not prime, figured that out the hard way!
Petcock works fine,

I am not 100% sure how to check my valve clearances, perhaps I should take it to a professional, I just really had the crazy notion the carbs had to be adjusted some, since they all look to be at different settings currently, which surely is wrong.

I will check the ohm out on the coils next!
Then try to adjust the carb jets per someones suggestion? (thanks in advance keith!! ), and if all else fails bring it to a pro to check the valve clearances! Yes I realize this is all in reverse from what you recommend...

Thanks for the help! I am sure I will be posting again, and again... har har...

[KEITH KRAUSE]

I couldn't reply last night, sorry.
I just read this as I'm off to work. I'll try to get back and suggest some jetting tonight.

[KEITH KRAUSE]

Ok. Got a little time finally.
First off, the petcock vacuum line goes to carb 3. The other two brass nipples at carbs 2 and 4 are floatbowl vents. The vent lines should be removed when running pods and the ports left open.
Sounds like you're aware that you really should check/adjust the valve clearances and set point gap/advance timing, and bench synch followed by vacuum synch BEFORE accurately re-jetting. If it's SO rich that testing/tuning is difficult, you should at least check the valves, statically set the timing (hand turn crank with advancer held in full advance while you watch for the plug to spark), and tell me what the current jetting is so I know what's in there and I can suggest something for the jet needles, etc, before you put the carbs together.
Now, you said the bike has K&N pods and V&H pipe. That's fine. But I believe you also said the carbs aren't stock?? Do you mean just the jetting changes? If you have different carbs than the stock VM 26, I need to know. I can't really see from the pics. I'll assume the carbs are VM 26.
The other thought is if a PO has installed a jet kit. This is really important in making jetting suggestions. Do the jet needles have 5 or 6 e-clip positions? What size mains, etc? Since it appears very rich, the jet needles, etc, most likely need to be adjusted and the carbs taken apart anyway. Float levels, etc, need to be verified. I know it sounds like a lot of work, but I don't know exactly what we have to work with and you can't just assume the floats, etc, are set right. Floats should be right at .94". Pilot fuel screws (underneath) are generally 1 1/4 to 1 1/2 turns out as a starting point for your mods. Side air screws are generally 1 1/2 to 1 3/4 out after fine tuning.
As for mains, jet needle positions, and pilot jets, I need to know if you have stock jet needles or possibly aftermarket needles before suggesting something.

[KEITH KRAUSE]

Oh, and as others said, be sure the spark is good quality and not weak. Is it reasonably bluish and not orange? It's basic maintanance to check/clean up the plug caps, good condition plug leads, all electrical connections must be clean and tight, good battery, points set squarely and no pitting, correct timing, etc. Verify correct ohms at primary/secondary sides of coils.
You don't want to confuse weak spark for rich jetting.

[vwdoctor3]

You don't want to confuse weak spark for rich jetting.[/quote]


yeah I chased a set of carbs around for about 2 months the spark looked good but would weaken under load and then I updated my coils & points to a dyna ignition. now no more sootie/fouled plugs.