Hard hot start 82 xj750 maxim

So I have a small issue with my bike. If I say go to the gas station to get a drink and then come back out like 5 to 10 min later the bike is hard to start. I have to crank it a bit and open the throttle a little for it to try to fire. Once it starts it runs great throttle response is perfect And idles great. There is an fuel smell from the exhaust after it starts. And goes away once it is running. Bike has 2 into 1 exhaust on each side with factory jetting and air box with factory style filter. The valve adjustment was just done. 100 miles ago. I just purchased the bike early this spring and have Taken the carbs to church and the cylinder head has been off to fix an oil leak. So I haven't really done much ridding with it afraid to go somewhere and not be able to restart I ordered a carb sync tool going to check that was wondering if that could be the issue or if it's kinda normal for this type of issue

 

Did you us a colortune to set the idle mixture?
Did you do a running carb synch?

What you have happening is not entirely abnormal, but I usually only have that problem in 100ºF + weather.

 

I have the same bike and have always had trouble getting it to start hot. Fires up super easy when cold with choke on. Valves were recently checked, did a running sync a few years ago with same results. I now have a color tune that I will use once I get forks and brakes squared away.

You are not alone Zack Clark

 

I would check to make SURE that the valve clearances (particularly the exhaust valves) are in spec, when a hot engine has a no-start situation that can point to valves that are too tight to begin with and which get much "tighter" as the engine heats up.

A plugged gas cap vent is also a possibility, after a short bit of riding a vacuum lock forms within the tank and no fuel can flow. Pop the gas cap open for a moment and see if that solves the problem.

Otherwise, there are other troubleshooting steps that can take place, but these are the easiest possible problems to diagnose.

 

Valve Clearance Hot vs. Cold:

I have read on here on quite a few occasions that the valve clearances will decrease when the engine is hot. I used to think this way, but was convinced by a very good mechanic that the opposite is true. This was in reference to a Toyota 22RE engine (overhead cam with rockers) that specified setting the valves with the engine at full operating temp – a painful process.

The initial logic:

Initially, the thought was the expansion of the cam lobe and valve stem / rockers / bucket would reduce the clearances when hot.

The logic that he used to change my thought:

Yes it is true that the cam and valve stem / rocker / bucket expand when heated. However, the majority of the heating and consequently expansion occurs within the combustion chamber. Therefore, as the head combustion chamber expands moving closer to the piston and the super heated valve within the combustion chamber heat and expand the net result is an increase in valve clearances.

Thoughts?

And, just to add to the OP's concern I notice the same issue when starting hot. If I run into a store and come right back out the bike will fire right up with just a touch of the starter button. However, if when hot and it sits that 10 minutes or longer it is not hard to start, but does not fire with just a touch of the starter button and sometimes requires a bit of open throttle. Some have suggested priming will alleviate this suspecting gas is boiling out of the bowls.

 

it is understood that all the parts of the valve train expand when hot. if his valves are tight and his shims expand and the valve stem expands ect it can result in his valves not closing enough to seal leaving the motor with low to no compression.
the exhaust valves have a larger clearance because they get hotter than the intake.

It would take about an hour to adjust the valves and clean the gas cap vent so why not do it to eliminate it as the possible problem.

this is not a toyota. that may be what toyota wants for that motor but is not what yamaha wants for its motors

 

http://www.lcengineering.com/LCTechPages/ValveAdjustment.html

overhead rocker arms not overhead cams.

http://www.hcdmag.com/toyota-22re-engine/

when a car needs it's valves adjusted you typicaly drop the car off the night before the work is done. the car is put in the bay and sits until morning to be adjusted. PIA for the owner and and shop.

if you wanted to you could check the valves on a cold yamaha motor then go for a run and check the clearances hot to see how much it has changed and then do it hot from there on. Myself I do not want to deal with hot motor and oil. just to put a cold shim in the bucket then warm up the motor to be sure it is the correct setting.

 

@Rooster53
Your mechanic is correct about the Celica engine (I had one for several years, and did my own work), but the reason that valve clearances are set when hot on it is because it uses a cam follower on a rocker to actuate the valves via a threaded locknut adjuster. That information about why clearances loosen when the engine is hot is not accurate for a shim over bucket (or shim under bucket) system.

Rocker actuated = clearances loosening when hot.

Direct actuation via shims = clearances tightening when hot.

 

I can be a believer in that, I was just wondering what the logical thought process is as to what causes a tightening when hot, and wondering why the largest expansion would not still occur at the valve head and combustion chamber creating a net effect of loosening when hot.

I suppose there is a lot to consider in the design with the aluminum engine and different thermal expansion coefficients of the valve material and the aluminum head. It would almost seem like the greatest amount of valve tightening would occur as the engine warms as the exhaust valve would heat and expand before the head had reached operating temperature. However, noting that aluminum CTE is nearly twice that of the ferrous valve material it would seem the valve lash would increase when warm.

Plus if Len says it is so it has to be

 

Thanks for the tips guys just got my carb sync tool going to sync them up and see what that does and also check the fuel tank vent. I will check back in with results also all my valve shims were set to .007-.008 of an inch on exhaust and .005-.006 of an inch on the intake they are on the big side of the specs so I doubt think valve shim is a problem but I have been wrong before

 

They are actually right in spec. In is in. Anything else is out, and they have to be more than a little out to cause a hot starting problem.

 

So I got the carbs all synced up they were a little off got that all fixed up idles a little better haven't been able to ride it due to the weather but will see how it does

 

So what happened??

I have a similar issue but with some associated noise.... really curious what you found....

Thanks

 

Lots going on when talking about valve clearances hot vs cold : yes ali has a higher coe, but the head doesn't get anywhere near as hot as the exhaust valve.
Now, the valve head expanding would pull the valve out of the guide, opening the clearance, but the stem, which is relatively much longer, will axpand more, going the other way, then of couse the cyl head itself will expand, but as I just said it will be at lower temps.
The other real big issue is, how do you know how hot is hot? Weather, riding style, timing, will all affect the above.
Just do it cold, take some measurements when hot if you want, my money is that that the clearance will close by a couple of thou to three on the exhaust, less on the inlet. Shims and cam expansion will be nearly zero btw.
Oh, re your problem, my money is on float valves or valve leaking, allowing fuel levels to rise and flood at least one pot.
To prove this you would need to close off the fuel supply just before stopping the engine.

 

that was done in another thread someone measured hot vs cold

 

You can measure clearance cold or hot, doesn't matter, the only problem is that Yamaha engineers/designers/metallurgists specified what the COLD (70F) clearances should be, so you're on your own to guess/figure out what the HOT clearances should be..........and what "HOT" actually means.

 

Yup, that's it