750 running problems when hot

Hi folks. Just got a 91 vfr750 on Friday. Bike wasn't used for a couple of years, but did get started regularly. Tank pretty much empty when I got it (which is good because fuel was old), so I filled with a few gallons of fresh fuel. Bike runs great when cold, but when it's up to operating temprature it's very rough. It stutters all the time if I try to accelerate, less so on a constant throttle but it's still there. Also if I come to a halt bike idles very rough and frequentystalls. When it stalls it will restart right away, but stall again after a few seconds running. When accelerating it splutters until I back off throttle, none of this happens for first 10 mins as the bike warms up from cold. I've drained float bowls a couple of times, and emptied the fuel filter, which hasn't made a difference. Should I drain and refill the tank, because I haven't done that yet. At first I thought it must be a spark plug failing (because I thought that if it only happens when hot it must be an electrical component failing), but now I'm thinking it's fuel related, but why is it only happening when bike gets to running temprature. Any suggestions as to what I should check now. I'm thinking I should defintely drain the tank first.:redface: Look forward to hearing from you. Am desperate to do a long run on the bike since we're enjoying good weather here in the UK at the moment. Bill. [PS. Love the sound of the gear driven cams....]

 

I wonder if something is opening up a vacuum leak when it gets hot. My first suspects would be the carb insulators. You might try the spray-carb-cleaner method to locate it. I've found this doesn't always find a leak, so don't rule it out right away.

Good luck!

 

Hi Jamie, much appreciate the reply. Also posted the same message to the uk group at "oracle biker central" and one of guys suggested cleaing the carbs as he has similar symptoms in his 750 after it was laid up for a couple of years. So I'm going to try cleaning the carbs after dismantling them. As you say it could be a vacum leak however I'll clean the carbs first, and see how it goes, and while I'm there I'll look for leaks.

Thanks again Jamie:smile:

 

Huh, my experiences are that clogged carbs only cause problems when cold. That's when you need more fuel and engines are more finicky. When engines warm up they tend to be more forgiving with not-so-perfect mixtures. That said, getting/keeping the carbs clean is probably a good idea.

Good luck!

 

I experienced the same problems when I got my VFR 750 out of storage after many years. In my case, there was quite a bit of lacquer (from evaporated fuel) which accumulated in the bottom of the gas tank. Upon refueling, the lacquer dissolved and made its way into the carbs.

Here's where it got really messy. Overnight, the lacquer solidified inside parts of the carb throats. The clue was that my throttle was stuck shut the next morning. I was able to open it with a bit more force, but then I knew there was something gummed-up inside the carbs. So, I removed the air cleaner case and saw the lacquer had built up on the throttle valves and the vacuum pistons. I dissolved the lacquer by spraying some light oil on it.

The part which caused the running problems was that the vacuum piston for the #1 cylinder was stuck shut. I pushed it loose with my finger and cleaned it with oil. That fixed all of my problems. It still had to run through a couple more tanks of gas before all of the lacquer was gone. But now, it is running perfectly.

So, be sure to check that your vacuum pistons move freely. You should be able to easily retract each of them by pushing with your finger. If they don't move freely, check for lacquer build-up.

Hope that info helps you !

 

Useful reply, Aux !!


WELCOME to the madhouse.

:welcome:

 

Turns out it was duff spark plugs, only found out after cleaning carbs, which made very little difference. Put new ones in and it's running great.

thanks for all the advice, much appreciated.

Bill.