help with diagnosis, please

[tz89]

My brother in Wisconsin is having trouble figuring out his 91 VR. He's paid up here but I have more time.

 

He reported sluggish performance at speed. I had him run his bike cold for a minute and #3 didn't warm up. He pulled off his air box and diaphragm cover. The carbs looked clean, was probably cleaned by the dealer before he sold it, but he sprayed it down with carb cleaner anyway, and got it to spray up through the jets. He pulled the plug, grounded it against the case and reported seeing it spark. So he put it back together.

 

It ran rougher than before, which sounded like a new air leak but he hasn't gone back to check. The plug wire on #3 had a cap that was pretty chewed up. He's thinking of replacing all the plug wires. I've suggested his next step is to get his ohm meter out to test resistances of his ignition circuit, but other than saying that I don't know how to do it.

 

Any other diagnostic suggestions would be welcomed, as well as a pointer to resistance testing info. I've found a few scattered but not a definitive write up. Thanks!

 

Tom

[GaryZ]

Swap spark plugs around to see if the 'dead' cylinder changes location.

 

Run the bike around the block and check the temperature of the head-pipes. I use a temperature gun from Harbor Freight. If the pipes all seem to be close to the same temperature, the problem could be a clogged low-speed circuit. I successfully shot carb cleaner directly into the low-speed circuit by removing the adjuster screw and spraying straight into the jet.

 

Add about 6 ounces of Sea Foam to the tank and run the bike long enough to get it into the carbs. Let sit for 24 hours and re-check.

 

 

[Venturous Randy]

Go to a bike shop and get four new caps and cut about a 1/4 inch off the wires and screw the caps back on. These caps have resistors in them that corrode and will kill the fire.

RandyA

[twigg]

My brother in Wisconsin is having trouble figuring out his 91 VR. He's paid up here but I have more time.

 

He reported sluggish performance at speed. I had him run his bike cold for a minute and #3 didn't warm up. He pulled off his air box and diaphragm cover. The carbs looked clean, was probably cleaned by the dealer before he sold it, but he sprayed it down with carb cleaner anyway, and got it to spray up through the jets. He pulled the plug, grounded it against the case and reported seeing it spark. So he put it back together.

 

It ran rougher than before, which sounded like a new air leak but he hasn't gone back to check. The plug wire on #3 had a cap that was pretty chewed up. He's thinking of replacing all the plug wires. I've suggested his next step is to get his ohm meter out to test resistances of his ignition circuit, but other than saying that I don't know how to do it.

 

Any other diagnostic suggestions would be welcomed, as well as a pointer to resistance testing info. I've found a few scattered but not a definitive write up. Thanks!

 

Tom

 

Stick a timing light on the offending cylinder to see it it is sparking or not. If not, fix it.

 

If is is sparking, and the header is cold, then it is gas ... and those symptoms perfectly describe a blocked pilot jet which cannot be seen or fixed by cleaning the carbs. The jetblock has to be removed to get at the pilot jet.

 

Just been there and done that, this week.

 

One further test he can do ..... Run the bike hard enough that the gas is coming from the needle jet and main jet. That means getting over half throttle. If the header heats up then, you confirmed the pilot jet issue.

 

Pilot jets are critical to performance. Even at speed the throttle is less than 1/4 open most of the time. If that jet is blocked, the cylinder will run only intermittently, even at highway speeds.