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Gs750tz Fouling Plugs-even After Much Work!!!

  • Thread starter Thread starter revenge143
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revenge143

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i have an gs750tz, with 9k miles on it. its a 750cc , 16 vavle, inline 4 cylinder engine. runs strongs, and fast. After about 30 miles, cylinder 3 fouls out with thick carbon. i believe its gas. I had the bike to 3 different shops, they all checked the compression and said cylinder 3 is a little low, but not low enough to signal a bad valve.
I had the carbs synced 2 yrs ago, bike ran awsome all summer, then started to foul that plug. this last summer i had them professionally rebuilt and synced, the mechanic said that he was very experienced with these bikes and after doing the work, he recommeded i run the factory plug in it ( i was previously running a hotter plug) well, once more, it ran flawlessly all summer, and around september, approx 200miles after the work, same problem with cylinder 3..............HELP!!!!!!!!!!!!!!!!!!!!!!!!!!
 
The lower compression is important and a coincidence in your case. Are you sure it's still within factory spec AND within 10% of the other three? What were the readings?
Have you checked/adjusted the valve clearances at that cylinder? This is part of basic tuning and must be done.
If the compression is still acceptable, have you tried swapping #2 plug lead with #3 to eliminate the possibility of a poor electrical connection? Have you also tried a new plug just to be sure?
What was done to the carb? Are the jets all factory Mikuni? Any chance a jet needle spacer was left out, etc? Any chance the float needle valve or float adjustment/operation is creating a high fuel level? Have you verified that the mixture screw is set correctly? Choke operation looks good? Was the choke plunger inspected for wear?
If the carbs were vacuum synched correctly, that wouldn't be the problem and carbs wouldn't lose their synch that soon.
 
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if you are not already doing so...run the bike on 'premium' gas...bobo
 
when i had the bike at the shops, i also took the factory service and repair manual. both mechanics said the compression in that cylinder is with in factory specs and with in 10% of the others... the carbs were rebuilt with factory Mikuni rebuild kits. the last mechanic said he looked over the entire bike after rebuilding the carbs and syncing them, he said it will be fine, should not foul any more plugs. he said he tested the electrical system, and the spark is enough on all cylinders. i asked if its possible that piston ring was bad, causing oil to hit the plug, he doubted it....
i have not had the head pulled off, both mechanics said my problem was carb related.
Also, i could not find a local motorcycle mechanic that would pull the head off. I have a full gasket set, 2 intake and 2 exhause vavles i purchased from ebay hoping to get a top end job done....

i agree that it is odd that the carbs loose their sync that soon.
I am close to just putting the bike up for sale with fresh plugs.. i paid $300 for it, and put appox $400 into it with all the parts and mechanics visits.... but it only has 9k miles....
 
I have seen oil control rings fail without losing compression....

I have seen oil control rings fail without losing compression....

Not on a GS motor but most pistons are of similar design, could happen.
But most likely something in the carb has been overlooked. Where are you located, I bet someone on here could help you with it.
If swapping ignition wires doesn't do it, you could try swapping the carbs around to see if the problem moves with the carb or stays with that cylinder.
Or just bring a supply of sparkplugs and don't worry to much about it, just go ride it and get the rest of your $300 worth.
 
I didn't say the carbs are loosing their synch, I said they shouldn't be out of synch so that shouldn't be a problem.
Could still be many things. I have a feeling you're going to get rid of it though.
As for swapping carbs, you can swap parts but not the entire carb bodies. Each one must go in its specific position.
 
Is the bike stock or do you have a header and/or pod air filters? If so, the person who rejetted may have gone too rich.
 
Are you saying that it takes 200 miles to foul the plug or after 200 miles the plug fouls quickly? If its fouled by running rich it will have a black sooty appearence. The black debris will look like powder. If its getting fouled by oil it will have a wet appearence and burnt debris. How much oil did the engine loose in the 200 miles? Does your model have a vacuum acutated petcock and its it attached to #3?
 
the entire bike is stock... its possible he could of gone too rich, but everyone i called here recommened this guy saying hes the one to talk to on old japanese bikes....
 
tkent02 said:
Not on a GS motor but most pistons are of similar design, could happen.
But most likely something in the carb has been overlooked. Where are you located, I bet someone on here could help you with it.
If swapping ignition wires doesn't do it, you could try swapping the carbs around to see if the problem moves with the carb or stays with that cylinder.
Or just bring a supply of sparkplugs and don't worry to much about it, just go ride it and get the rest of your $300 worth.
Click to expand...


everyone is telling me that its carb related since the carbs are fouling out with gas.... i will soon try to swipe ignition wires and let you know how that works... i am located in Cleveland, ohio.... i
 
KEITH KRAUSE said:
I didn't say the carbs are loosing their synch, I said they shouldn't be out of synch so that shouldn't be a problem.
Could still be many things. I have a feeling you're going to get rid of it though.
As for swapping carbs, you can swap parts but not the entire carb bodies. Each one must go in its specific position.
Click to expand...


ok sorry for the confusion, they were synced 2 yrs ago and ran fine until it started to foul the plug again... when i took it to a new mechanic this past summer he first tested the carbs and said they were way out of snyc.... i would love to keep it if i can get it running...
 
fgh said:
Are you saying that it takes 200 miles to foul the plug or after 200 miles the plug fouls quickly? If its fouled by running rich it will have a black sooty appearence. The black debris will look like powder. If its getting fouled by oil it will have a wet appearence and burnt debris. How much oil did the engine loose in the 200 miles? Does your model have a vacuum acutated petcock and its it attached to #3?
Click to expand...


after both repairs.. the first one being a simple carb sync, and the second being a full rebuild and sync, the bike ran great for about 200 miles, then it fouled plug 3... i installed a new plug, and was getting approx 30 miles to the plug.... i am not sure if it has vacuum acutated petcock... how do i tell.... thanks for your help...
 
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