What to do next…

Hi all. I have a 1982 XJ750 Maxim that I picked up a few months ago. It was running when I bought it, although poorly, it was running on initial start. Shortly after purchasing it, I’ve had lots of trouble getting it started. All the valve clearances were out of spec and I made the proper shim adjustments to get them all in spec. I’ve cleaned the carburetors and do not know what to investigate/work on next. The bike will start very randomly after trying to start for 10+ Minutes and then will run poorly for a minute or so before dying at idle. When trying to start I hear lots of pops and misfires both in the carbs and at the exhaust. The RPMs also fluctuate without any throttle adjustment. I’ve had it sitting for about a week and see oil leaking from what appears to be the valve cover gasket, not the head gasket. Any thoughts on where I should start next? I want to do a compression test but it’s difficult to get a trusty number without the engine being warm and my starting issues are preventing that from happening

 

Make sure your battery is fully charged.
Do the compression test with the engine cold and throttle wide open and post your results.
Do you have the original fuse box? You need a blade type one, the original Yamaha ones the clips break.
Do you have a Haynes manual for your bike?
What did you do to clean the carburettors?

 

ohm out your ignition system
in this link you will find the specs
The Ultimate Relay, Switch, Sensor, and Diodes Guide

also there will be a guide on the charging system.

you want to do a voltage drop test measure voltage of battery then press starter button see what the voltage drops to. this is a good indicator of charge state or battery condition.

replacing the fuses if still glass is also a good idea, box with blade fuses or inline blade fuse holders make sure the main is rated for 20 to 30 amps.

you should also replace donuts and valve cover gasket. I start with the donuts check to see if they are hard, then the gasket

 

Have you tried to turn the idle knob up ? Agree with Franz how did you "clean" the carbs? The only real way is to pull carbs break the rack IE: remove carbs from the rails and split them apart. Do a full tear down remove all rubber parts and soak in carb cleaner ( my go to is Berrymans carb dip one gallon container) or boil the carb bodies for twenty to thirty minutes. Recommend ordering new butterfly seals from xj4ever, and the fuel rail interconnect O rings. A carb that is out of synch will cause popping and can be difficult to start , if you have access to a synch manifold the four vacuum gauge set up or my favorite the Morgan Pro synch or a homage mamomter with two baby bottles to check synch. You might bottom out mixture screws, and back out two in half turns . Just thought of another item to inspect look at the carb intake boots , are they cracked , and are the caps fully on the nipples ? These are a few items you might want to check.

 

If you replace the battery, go ahead and get a Battery Tender Junior (or similar) and attach the SAE connector to make it easy to keep that little bastard charged.

 

If it's popping back in the airbox at least one carb is weak. Have you tried adjusting mixtures? Have you vacuum synched them? No point doing anything else until you've tried this. If any of them doesn't respond to idle mixture screw adjustments you didn't clean them properly. Idle speed changes could be air leaks or the engine responding to varying mixture needs as it warms up - or both. Carbs carbs carbs - 90% of xj woes, beginners trap....

 

I’ve been having a similar issue with my 85 XJ700X Maxim. I thought the bike had to be warmed up in order to do the vacuum sync?
One of my issues was a vacuum leak, which I traced back to the petcock. A rebuild kit with a new diaphragm fixed the uncontrollable reving issue.

 

it does. you can dial it in a little when turning it over and do it when partially warmed up , best when done after a ride . it is going to cool down somw when you remove the tank

 

I don't see an issue here - you start it and nurse it up to reasonable temps on the enrichner, wind up the idle control knob to keep it idling - voila - it's hot enough to do a running synch. But I would be opening up the mixture screws to stop it popping as well. A near enough running synch, then slow the idle down, tweek the mixtures, repeat - probably by then it will be hot enough to need a fan blowing over it.
If you can't sop it popping, try to identify which one is popping, change the plug, if it doesn't cure it carbs off again.
Check the float heights first!
The less you rely on the pesky colourtune the better you will get at adjusting mixtures by ear, and the quicker you get it done, but there is an element of experience here.
If I still had a colourtune I might be tempted to use it to check how close I'd got the mixtures by ear.

 

I had no idea. I would have done it a long time ago.