Won’t start when hot

Hey everyone,

I have a 1985 XJ 700 Maxim X that I’ve been working on. I finally got it running but once it ran for about 5-10 minutes it died out and won’t restart. If I try again once the engine is cold it starts on the first crank.

Things to note:

It started from a cold start and idled around 1000-1500 rpms which I consider normal. After about 2-3 minutes it picked up to 2000-2500 rpms and stayed there for about 5 more minutes before returning to regular idle and quickly dying out afterwards.

I immediately checked spark to see if the coils were the issue. I had plenty of spark.

I believe it to be a fuel issue. I have fuel in the bowls and I turned the valve to prime and still couldn’t get it to fire.

My idea is that it’s getting too hot and cooking the fuel before it reaches the cylinders. Thoughts?

 

Have the valve clearances been checked?

 

Hot vs cold compression test.

 

Zach,
Can you fill us in on what you have done to the bike so far? What shape did you find it in, band find or sitting in the Preveuse Owners (PO) shed? The more info we get on what you have done to the bike will help us help you.

If the bike was setting for a few yrs you will need to pull the carbs and go through them, it is just the nature of having a carb motorcycle. Have you looked down inside the gas tank? Clean or rusty or the gas smells bad? Did the PO have a inline fuel filter you can see? If so is gas flowing through it when you turn the engine over?

Have you pulled the plugs and taken a look to see what color they are? Shine a light down to look at each piston and see if they are clean or full of carbon?

As k-moe states above, the valves need to be looked at to make sure each cylinder is getting the best compression it can. Doing the "X" valves is interesting but can be done. It will be costly depending how many of the valves are out of spec. Those little pencil erasure type shims fun.

 

So I just check compression.

Cold compression starting at cylinder 1 and working my way across: 60,55,15,65 psi

Hot compression after it shut off after running about 10 minutes: 60,70,50,60 psi

 

I bought the bike from a guy who said it lost spark after getting caught in the rain. What I found was a broken wire within the harness which was weird but an easy fix. After getting it running I noticed the issue I’m having. The tank was clean and since it starts and idles fine when cold I assumed the carbs were fine. My original thought was that it wasn’t pulling enough fuel at low rpms because the boots between the carbs and air box were in rough shape. I replaced those and figured it would be able to pull enough vacuum and run smoothly on the low end. But after I got it back together I let it idle while I cleaned up my tools and that’s when the issue started again.

 

+1 on the plug colour.
If the mix is super rich it may be akin to the fuel enrichment circuit being open.

 

Here’s two of the plugs, the other two are similar. Keep in mind they’re new so they might not be as dirty as the would normally be.

 

To address your other questions:

The bike is in decent condition, some cosmetic issues but no fluid leaks, looks pretty clean, 22,000 miles on it.

The PO threw parts at it I think when he tried to fix the spark issue. It has new plugs, plug wires, and coils.

No in-line filter for the fuel.

I’ll try to check the valve clearances this afternoon.

 

I have a similar issue, my 85 xj700 always cold starts no issue but if I shut off to fill up the tank it sometimes doesn't want to start again for a minute or two. Cranking sounds a little dry to me, maybe a fuel delivery issue?

 

Uhh so how do I fit a feeler gauge under the cam? There’s next to no room in there. Especially with the engine on the bike.

 

Those numbers are way too low. They are less that half of what they should be.
Did you open the throttle when testing? Either that or the valves are not closing.

 

Didn’t touch the throttle. I guess the valves aren’t closing.

 

You need angled, narrow profile feeler gauges, or you can bend a set of standard gauges.

At 22,000 miles the valves might still be in spec, but should be checked anyway.

The very low compression you're seing might be stuck rings, or more likely due to a faulty gauge.

 

Compression test:
Adjust valves
Throttle wide open
Air filter out

 

Ok, I’ll have to retry the compression test. I wasn’t aware of those details. However I’m not too convinced that it’ll make my compression jump to 100 or so psi which I assume is normal compression.

My job has me away from home for the week and my schedule is going to get a little busy soon, but I’ll keep you updated as I move forward.

Thanks everyone!

 

While you are at it, check the voltage at idle--it should be approx 12.5 volts. My XJ700 would sometimes be hard to re-start and it turned out that I was not getting enough voltage to crank the engine and fire the plugs. I was getting only about 11.2 volts at idle and less than 13 volts at about 3000 rpm. XJ550H suggested that I check my voltage, which I would have never thought of. I keep my bike on a battery tender, so it tended to start easily the first time and usually if it was off only for a minute or two for a gas stop. I replaced the brushes and now have 12.5 volts at idle and about 14 volts at 3000 rpm. The bike starts much easier and the restart problem has disappeared.

 

I found this write up on a generic shim under bucket adjustment procedure. Does anyone know if there’s anything specific to maxim x valves that I should be aware of that this leaves out?

https://www.google.com/amp/blog.pro-x.com/checking-and-adjusting-valves-on-your-shim-under-bucket-motorcycle?hs_amp=true

 

That's

basically the process, but there are several model-specific details mentioned in the article that are different for the X.

This thread should do ya.
maxim-x liquid cooled valve adjusment with pics.

 

I appreciate that. So the sprockets can’t stay on the cam shaft for removal?

 

Not unless you plan on breaking the chain and fitting a new one.

 

No

 

Well I was able to bring the bike into my home shop this weekend. After topping off the coolant and putting in a switch to manually run the fan, the bike stays cool enough and runs without issues.

 

So do you think the coolant top off or the manual swx fan did the trick? I know on the V65 Honda most owners either put a manual swx in or changed the thermostat to a lower number. I hope for you this is a long term fix.

Did you make sure to put the swx in line or is it a switched power for fan to be on. I just wanted to give you a heads up if you have it hot wired, you might forget to turn it off one day.

 

After a lot of trials, I’m fairly confident it was an overheating issue.

For now it’s inline. I’m considering looking around for a timer of sorts that lets the fan run for a few minutes after I shut it off and then turns off on its own.

 

Why not just get the next lower temp thermostat and that might do the trick. That way you would not have to worry about a swx and the bike would run cooler? I am just glad you got it worked out and you can ride for the rest of the season.

 

Those are low figures , your valve clearance should be checked if you have not done so yet. The other thing were all 4 plugs out and battery fully charged when you did compression test. The other thing put a little oil down plug hole and try compression test , reading should be a little higher. If still low I would rent another gauge see what it reads with another gauge.