XJ750 Loosing and Gaining Power Erratically

My 1983 XJ750 Maxim (approx 9,000 miles) is in excellent condition and has, for the most part, been running well since it was given to me 3 years ago. It had been sitting for some time before given to me and was rough to warm up but ran great when warm.

About 2 years ago as I was riding power went way down with little or no acceleration and would not top 40MPH. The tachometer bounced around erratically like crazy yet the RPMs, although low, were steady. After about 30 seconds it kicked back in to normal and the tach. also functioned as it should. NOW it functions poorly with very low power/acceleration and tach going crazy nearly all the time with only spurts of normal operation. So, it's been stored for 8 months. :cry:

I originally thought it was a fuel/air mix problem or carbs. After talking to two shops that were reluctant to look at it :| I talked to another shop who took the time to listen to the symptoms. This shop deduced, without looking at the machine, that it is likely an electrical problem :?: since I don't have problems starting it and the problem occurs after warmed up and in mid-operation. Yet they are also reluctant to work on it since it will be a process of elimination and may take many hours and parts to deduce the problem causing a huge repair bill.

I have very little mechanical knowledge but I have a couple friends who can tinker.

:arrow: Anyone else experience similar problems? :?: Where do I start? Who do I go to? Can you direct me to other posts here? :idea: All advice, suggestions and recommendations are welcome. Thank you :!:
-Mateo

P.S. It has been tuned up, oil changed, new air filter, new fuel filter, new spark plugs gapped correctly, fuses swapped from glass tubes to flat fuse box and a new battery.

 

I'm wondering if you have a cracked or otherwise defective ignition coil. If you have a manual (you should) and a VOM, there are tests you can do to check the coils. Maybe you could even tell by looking closely at the coil housings. Good luck

 

Thanks for the quick reply. I'll emphasize...I have LITTLE mechanical knowledge :wink: What is a VOM? At least I DO have a repair manual!

I'll add...The bike starts without problem. Could it start well in spite of a defective ignition coil?

 

Sure it could. Im kinda grasping at straws here, but a cracked coil could function normally at rest, then start acting up from movement & vibrations from riding. Im far from an expert, but this would be one thing to check. VOM= volt-ohms meter.
Your problem could be a lot of things-fuel related or electrical, but since you indicated the electric tach going crazy, Im leaning towards electrical. Keep posting, Im sure the members here can help you find & repair the problem. Good luck

 

Great....a place to start. Testing the ignition coil with a VOM looks like a pretty straight forward process. I'll give it a shot. Thanks!

 

Doubt it makes a difference but I realized my bike is 1982 Maxim...not an '83 as listed yesterday.

 

From what you are saying, regarding the tach jumping about, it does sound electrical. I had a similar problem to you. It sounds like the coils, or a coil. When I checked my problem, I found it to be one of the leads to the coil wasn't making contact properly. the metal spade hadn't gone on properly, so I fitted it correctly, and the problem disappeared.

 

Thanks for chiming in Smurf. Hopefully it will be the same for me. When you experienced a similar problem were you also loosing power? or just problems with the tach?

 

No my friend, I was losing power too. The nearest I can describe it to is, just as you start to run out of fuel, you open the throttle but there is no increase in power, just a burble of noise, release the throttle back and then it seems to be running properly.

I hope I've described that good enough for you

 

Hey I just thought of something else. Does your bike still have the original fuse block with the glass fuses? These bikes are notorious for having loose/broken spade clips that hold the fuse. Im wondering if theres a bad connection there with the main fuse. Anyways-you should upgrade to a blade style fuse holder in place of the old crappy one, just for good measure. Be worth your while to check it out-good luck

 

I'd agree with the coil diaganosis.

You can check the resistance of your coils using a multi-meter. Your manual will give you the specs.

If it is a coil issue, and is intermittent, then they might ohm out fine when you test them cold, but it's worth a look.

Inspect your coils for any cracks, and check your coil wires and spark plug caps too. You might just have a poor connection in the wires/plug caps that creates too much resistence when the engine is hot. If the tips of your plug wires are corroded, you can twist off the caps, snip 1/4 inch off the end, and twist the caps back on (this is the easiest fix, and should probably be attempted if your coils ohm out to spec).

You can always try a spare set of knows good coils, to eliminate the problem.

 

Talking about fuses (not meaning to hijack this thread), I went out on the bike on Saturday, I went to see someone, when I went back to the bike, I'd got electrics at all. My first thought was "blown master fuse", so went staright to it. What did I find.... A 13 amp plug fuse (for the mains) fitted!

What kind of plank was the person that put that in!!! Luckily, there was a proper 30 amp automotive sat in the spare space. I checked that and found it was ok, so swapped it over and the electrics came straight back.

Now the 30 amp master fuse is the round glass type, is it advisable to change the holder to the blade type, like the others, or is that one OK, as it's situated under the seat?

Advice welcome

 

The fuse that controls my headlight had actually melted a hole through the plastic fuse box. It became inoperable so I switched out the glass tube box for the flat blade fuses. Has worked well and dealing with less heat. There's an excellent "HOW TO" on that in this forum that I used to guide me in the process.