another mystery from the FL dummie!

[jazzrat2000]

Hi, I have been getting suggestions from you all about my mystery high-revving XS750... today I finally had a chance to check the 750 a bit...I wanted to try a couple of things...I wanted to try spraying some carb spray around the carb boots when it was running to see if there was a a vacuum leak... and I wanted to see if it would increase revs on its own if I just let it sit there and idle... (for those not following my other thread, the bike gradually picks up revs as I drive it and it warms up... after 10 minutes of riding it idles at 4K!).

so, I started her up, and let her run... after 10 minutes it hadn't picked up at all from around 1100rpm, so I figured I'd try the carb spray... there was the slightest little blip on one of the carbs, probably nothing... but then, I found the darndest thing... I checked the exhaust pipes, and only 3 was hot, (this one has lots more bluing than the other 2)... the others were barely warm after 10 minutes of running! I put water on the pipes, on three it vaporized instantly, and on the others it just sat there, no steaming even... is it possible it is running on only one cylinder? I checked the plugs, 1 and 2 (especially 2) were a little damp, but not completely gunked up like I thought they might be... always a mystery, maybe I need to bring in a pro? this is the bike I drove back to FL from SC 3 weeks ago with no problems,70 mph the whole way...

[John]

Hey Doc

Have you been to this site yet?
http://www.yamaha-triples.org/ From the home page , click on 'Lounge', and then click on 'Forums'. These guys seem to know their triples. I joined up there recently to try to scare up some interest in the brake caliper pistons I'm having made.

[prometheus578]

Yeah, it'll probably run on one cylinder, with occasional help from the other ones as they start firing at higher RPMs. (I've had 4 cylinder bikes that run on 2)
Damp plugs: Could be an electrical prob with the coils putting out intermittantly, or could be carbs, putting out too much fuel and fouling the plugs.
In my limited experience, I've found that bikes that don't run on all cylinders at idle, but pick them up at higher RPM's have carb probs.

[jazzrat2000]

thanks, my guess is that I'll be making a trip to my local old bike guru at his shop this week...

[jazzrat2000]

PS I just posted on the triple site, too, thanks for the suggestion!

[thumperjsa]

I had one cylinder not firing on my xs1100 and it turned out to be a vacuum leak with the intake manifold. Since you tested that, then that can't be it.

You may be able to check if it is a fuel circuit problem by pulling off the air box and spray some wd-40 or more preferred starting fluid into the carb throat of the cylinders not firing. If you do that, it should start to get the pipe warm.

I don't know enough about the triples to know if there is a coil for each cylinder or if you can swap a plug wire with one belonging to another cylinder to see if it is an issue like you can with the 1100.

Seems odd it would idle ok on a triple running on one cylinder though.

[cueball]

try biker.net they have lots of stuff for triples

[Deni]

Tim,
Have you discounted the possibility of an intermittent electrical problem? Maybe a bad connection in the ignition system is keeping the other 2 plugs from firing until the RPMs get high or it warms up enough, then they start firing. Maybe whoever set the idle last set it with only the 1 plug firing, hence when the other plugs kicked in, it was idling way too high.

First, confirm that you are getting CONSISTENT sparks to all 3 plugs. I used my inductive timing light to find that I was only getting an intermittent spark to one plug. It turned out to be a loose cap at the end of the plug wire. Simple fix. The others were kind of loose. If you get consistent sparks to all the plugs no matter what the RPMs, then it probably is not electrical. If the readings jump around, then there you go, it's electrical. Start at one end of the wires and work your way to the other end of the system.

I used to have a 1977 XS750 in college (in Florida) in the early 80's and remember that keeping the 3 separate sets of points adjusted and balanced properly was a BEAR. Does yours have the 3 separate points? Maybe you have some fried points that kick back in at high RPMs.

Hope you get it running right soon. I still remember riding my 750 from Sarasota to Pittsburgh and back one summer (1981?) to visit my family. What a good time!
-Deni

[RS Thornton]

I had a problem with the coils on my 750 cutting in and out, it would run on 2 cylinders really sluggish, then the coil would cut back in and I would just about launch the thing into the back end of whatever was in front of me. Not familiar with the triples arrangement, my CB750 was similair to the Xs1100, 4 cylinder and 2 coils.

[jazzrat2000]

thanks again for the suggestions... I am a rookie, so I will have to get out the Haynes to see how to do what you suggest!