XJ750 Spark Failure

Hello,

I have a 1982 XJ750 Maxim with an odd failure that I can't figure out. I know no information from the previous owner, just have a signed title. When I first got the bike the jets that were installed were mixed between Canadian and American for the pilots and the mains. That tells me the previous person didn't know what he was doing. I installed the correct jets and set the pilot mixture screw 2.5 turns out. I installed a new air filter, intake manifold boots, and other rubber boots to ensure no air leaks. I bought a set of coils from a motorcycle junkyard that was in the spec in both the primary and secondary side (resistances). After measuring the float levels I noticed levels are slightly high however nothing too scary. I used the clear tub method.

So here's the problem the spark plug keeps failing for some reason. When I install new spark plugs the bike idles and revs nicely. However, after some like 5 minutes, the bike begins to have trouble idling and eventually dies. When I rev the engine up it seems to do fine but it eventually dies. When I install new spark plugs the bike runs great until it happens again about 5 minutes later. I did a compression test I read 150 +/- 10 PSI on all cylinders. Great right!? The odometer says 10K miles.

Also, the motor likes to smoke a lot through the mufflers. I haven't ridden the bike for an actual ride since I haven't registered it yet. It went through over two dozen heat cycles and I let the bike idle for over 10 minutes. And it always smokes a lot. It's either burning too much gas or burning oil. I am guessing from valve steam seals since the compression and low millage suggest so. Or at least I would hope so. Of course, new oil installed.

So I can only think of two reasons why the spark plugs are failing besides too much oil destroying them.

1) Since the float bowls are slightly high maybe just maybe there's too much gas in the pilot circuit. Hence why it smokes all the time. I'll lower the levels tomorrow.

2) Some kind of electronics might be malfunctioning. I have two sets of coils, A total of 4 coils. I at least know that's not the reason because of the resistance levels plus two complete sets. I am not familiar with the XJ series bikes but that is why I am here. But it shouldn't destroy the spark plugs. Sounds like it's either gas or oil. I am hoping it's gas. Really don't want to do valve seals. Also, I am not sure how long the bike was sitting for. I would assume for over 10 years with the milage and brake fluid condition.

What do you guys think? And what other suggestions do you have?

 

You should not have four coils the bike has a waste spark system so one coil operates spark plugs 1 and 4 and the other 2 and 3.

 

Make sure the sparkplugcaps are ok.. could save you A LOT of time and money.
It took me more then a year and A LOT of hassle/money/inconvenience before finding out it was one of the caps...

 

Welcome to the forum. As far as letting your bike sit and run, it might be a good idea to put a fan in front of the bike. The air fins on the heads need to have some air flow going past them to work.

You mentioned putting new plugs in and it works for 5 mins or so, how do the old plugs look? When they come out are the black, tan or white?

As far as having 4 coils, it that what you really mean or 4 individual plug wires going to each plug. As stated above, you should only have 2 coils on the bike.

Removing the plug caps and taking a look at the wire as the end of the plug wires is a good idea. If there is any green or crud at the end of the plug wires take care of that. One way is to take a little off the end and I mean a small amount. Make sure you don't short change your self so the plugs wires won't reach the plugs anymore.

Looking forward to see how the trouble shooting goes.

 

In my case the caps themselves were faulty as in the resistors within the cap.
The ignition(system) can only handle infinite ór some resistance, periodically and unpredictable, for so long.

 

Pics of the bad plugs please.
Number them as to the cylinder. When sitting on the bike #1 is on the left and #4 is on the right.
Use a neutral background (white preferably) and focus on the ground strap and the base of the threads.


What color is the smoke?

 

As for the coils I have extra set obviously two coils are installed on the bike at a time.

The smoke looks white-ish.

As for the spark plus two of them failed this morning cylinder 2 and 4. Touched both exhaust pipes and they were cold. So after replacing the spark plugs with known good ones she ran fine at idle. It looks like it’s running super rich. And it did smell like gas.

so it seems to be carburetor related. however I installed stock jetting. Or like what I was mentioning about to float level being slightly high. Anyways I’ll take the carbs apart again. Double check the float levels and all the jets

 

And even if he did it is easy to make the error of installing the air jets incorrectly as they are shown incorrectly in the FSM and if reversed will cause a rich condition.

Correct location courtesy of RickCoMatic:

 

Rooster53,

I believe that’s my issue. Since I was going off of the FSM. My dumpass always forgets to take pictures or notes when disassembling the carbs.

I’ll let you guys know hopefully later tonight.

 

White smoke is condensation/ dust being burnt off.

Both plugs are carbon fouled. Carbs, not oil.