Misfiring XJ750 "Doing My Head In......"

Hello everyone.
I've spent many hours looking over this website and just registered. I have a couple of questions for those much more knowledgeable than I.

I have an XJ750 that is running rough. I've pulled the carbs several times and cleaned thoroughly. Ran like a pig afterwards so I bench synched the carbs, cleaned the plugs and it ran pretty good for about 30km"s. Put it back in the shed, took it out a few days later and now it runs like crap again. Number two and three cylinders sometimes fire, sometimes they don't. Just pulled plug leads out whilst bike was running, pulled number 2 and it made no difference to the way it ran - pulled 1, 3 &4 and RPM's dropped, so they're OK. All plugs are pretty black but 2 & 3 are very black and sooty. So I'm not sure if this is a coil problem or a mixture problem (or both!) Compression is 120psi across all 4 cylinders.

Questions:

What is the correct order for doing the carbs? Float levels, sync then mixture?

Do valve clearances need to be done if compression is good and even (within 3psi) over 4 cylinders.

How do I check the coil? I've run an ohmeter on the leads and they check out OK. I've checked the coil as per the Haynes manual and that "seems" OK.

Where do I get a "colortune" plug in Australia? No-one seems to have heard of them!

When all the planets align she runs really well but more often than not she is running really rough.


Sorry for long post but any feedback appreciated.

 

I had a funny miss fire with one of my xj900's. It turned out to be a small amount of crud blocking the pilot air jet. On the 900's this jet is pressed into the opening of the carb (air cleaner side) and cannot be removed. Before i sourced the problem I had cleaned the carbs to spotless countless times, and blew through the hole with the compressor. searched for it electrically, still nothing. It wasn't until i attempted to push a small drill bit through (where talking tiny here) that there was a pop! and bingo, no more blockage. Two carbs where like this the other two the drill bit just slide through. so i used these "clear" jet holes to determine the right sized bit and pushed it through the others.

After that no more miss!

The funny thing is there was no sign of rich or lean mixtures on the plugs at all!

Where in Aus are ya?

 

Sounds like loose junk floating around in your carbs. Clean those carbs like its a surgical instrument and install a fuel filter. Remember bench sync is just to get you close. You need to do a carb syc with the tool and vacuum gauge or gauges. good luck

 

Hi Trihard, welcome, how clean is your tank?

 

Thanks guys.

Dunsborough Western Australia.

I had a leaky tank the PO didn't tell me about so I put a sealing kit thru it which seemed to work. I have also installed an in-line filter. I'll pull the carbs again and have another look. I aim to purchase a vacuum guage set soon to go with the YICS tool I have and try to give them a decent balance.

Do I do this before or after fiddling with the mixture screws or doesn't it matter? Is this more likely to be the carbs or ignition. I seem to get an OK spark on all plugs but maybe it craps out under load.

 

Welcome Trihard,

What do your plugs look like after you ride it? Aside from a good sync, make sure all your jets and ports are clean. And make sure you put all the jets back in the right places.

Wrong placed jets messed up one of my friend's bikes pretty good. 8O

 

I still say you have c**p floating around in there.
BTW. I had a house in Halls Head, Mandurah & one of my daughters got married at Elephant Rocks, Nr Margaret River. Wiz.

 

Why are you fiddling with the mixtures? have you been fiddling with the mixtures? Get you bike running ok (not missing) and then look at mixtures last.

Try and get a manometer rather than vac gauges. and if doing it by vac gauges only use one. There are to many incosistancies with gauges to get any exact figure across all cylinders if using more than one. And doing it with one sucks.

There is a thread some where on here of one you can make (manometer) for virtually nothing out of some bottles and stuff. Ive got a proper one but it is old, and my old mans, and have no idea where you get one these days. But the thing is fantastic, and precise, too precise

 

I think bottles (similar to manometers) work better than vac gauges. Look in my gallery for the bottles I made.

You have to suspect everything and start eliminating suspects.

I cleaned my carbs and bench synced them per RickCo's instructions. The bike ran like crap until I did the bottle synch. There was a horrible imbalance between #3 and #4. The bottles tell you the instant you have the balance perfect.