#1 plug fouls twice in 40 miles, a new recent issue...GT750J

[busa1300]

I have put on about 1800 miles on my GT750J barn find this past 9-10 months. After rebuilding carbs/new points/new carb boots-exhaust gaskets/cleaned tank-rebuilt petcock/antifreeze-trans oil......and so on. Bike had sat for probably 20 years before, so I went through all I could till it was running good.
120-125 compression in every cylinder, when I first checked them months ago, and the numbers are still the same today. But I have fouled out my #1 plug 2 times in about 40-50 miles now.... never an issue before. center plug looks perfect, and #3 looks a little dark. both end carbs have richer main jets as original, yet I hardly ever run my bike that hard to get into the main jet. Oiling should still be the same throughout all 3 cylinders....could my coil be going bad? points are all still at same gap and have not moved... not sure where to look for carb....what could plug up, but cause it to be rich all of a sudden?... Floats are all set right.
Where do I look?

[pearljam724]

Did you check operating condition or replace needle valves ? Perhaps that cylinders needle valve is allowing a little more gas to bypass than it should. Consider synching the carbs with a manometer as well. That one cylinder could be out of synch with the other two creating a richer condition. Bench synch them first if you don't have a manometer. See what that does to the plugs condition, then resort to the manometer if you need to buy or make one. As long as it is running fine, I wouldn't be over concerned from a slight rich condition on one plug. Unless, it consistently fouls it to the point the cylinder will not fire. Then yeah, you need to find the problem.

[busa1300]

Everything in carbs is new. Set the slides as in the shop manual, all open at the exact same time. Everything has been working great until just a week ago...fouling plugs to the point they are seriously misfiring...then no fire at all. Which is why I was wondering if coils would go bad in this manner slowly, or if they work and then stop working all together....?
I'll look at throttle cables again...maybe the one cap is coming loose, pulling the slide up..? not sure if that would cause any real issue like this though.
Could be fuel being pulled through a loose fitting choke plunger also....

[jabcb]

Coils can go bad with a number of different outcomes. Depends on the specific problem a coil has.
Could have a short to ground resulting in no spark at all.
Could have an intermittent temperature-related short, where its fine when cold but intermittently stops working when it gets hot.
A short could reduce the coils output so that it works ok some of the time.

The plug caps have resistors which also can go bad.

Good starting point is to measure primary & HT resistances for the three coils.

New aftermarket coils and plug caps are available for reasonable $$$.

Capacitors can also go bad. Also can have heat related problems.

How do the contact surfaces for the three sets of points compare?

[pearljam724]

Everything in carbs is new. Set the slides as in the shop manual, all open at the exact same time. Everything has been working great until just a week ago...fouling plugs to the point they are seriously misfiring...then no fire at all. Which is why I was wondering if coils would go bad in this manner slowly, or if they work and then stop working all together....?
I'll look at throttle cables again...maybe the one cap is coming loose, pulling the slide up..? not sure if that would cause any real issue like this though.
Could be fuel being pulled through a loose fitting choke plunger also....

Ok. All the carb parts are new, good. But, just because you bench synched the carbs, set slides. Doesn't necessarily mean that the carbs are in synch. Bench synching is a good starting point. But as cylinders, pistons, piston rings, etc. wear. That can cause carbs to be out of synch. Bench synching alone won't necessarily be accurate enough if those items are worn. That why it's better to use a manometer just to reassure that that they are infact in synch taking wear into account. It's not a definite solution to your problem. Only one of several things to look at.

[Old School]

I dont see the carbs being out of sync fouling plugs. Ive seen bikes way out of sync, they dont idle right and dont rev good. I would look for either a loss of spark or a flooding issue.

EDIT
One easy way to help trouble shoot and narrow things down would be to, have a new plug ready and ride the bike until it fouls and drops the cylinder. Have a fresh plug with you and change it as quickly as you can. If the bike takes right back off and continues to run with everything hot and up to temp, I would rule out something electrical breaking down and focus on the fuel system.

On the fuel side. If you are not seeing gas over flowing from the carb, I would rule out the needle and seat. Also does it have any effect or help to ride the bike harder? (keep it from loading up)

[busa1300]

When it fouled the first time, I rode it to a parts store and put a plug in it to get home. Bike felt good about a minute or so after it cleaned itself from all the unburned oil. Same the second time, as I had one ready just in case, and fired ok with a new plug.
No fuel leaking out of carb either..... Haven't tried running up in the higher rev range to clean it out, as I was concerned of doing more possible damage in some form.

[Old School]

I would say the ignition sounds to be ok.

It is strange why this happened all the sudden, you might double check the oil pump setting. Maybe even lean the carb (s) a little.

I cant imagine you hurting the engine by simply running it a little harder. (within reason)

[tz375]

Is the plug fouled with oil or gasoline? It is often hard to tell, but if the plug is oily, it may be a leaky check valve filling that crankcase with oil.

if it's the right plug it may also be a failing crank oil seal letting transmission oil in.

[Old School]

Good question Richard. That was next on my list. If the check valve in the intake (or crank) is stuck open, it could allow a more than normal amount of oil into that cylinder.

[seviston]

How has this turned out?

Have you eliminated the possibility of a sticking check valve or bad crank seal? Check to see if you are getting consistant spark at the fouling plug and corresponding points set. In the past I have had a single points/condensor set go bad and create a similar problem before switching to electronic ignition.