Engine idling problems, revs out of control. Please help.

[jtmtv18]

i know this is lengthy, and i apologize, but i appreciate your time in helping me.
I have read the megamanual a dozen times trying to fix this problem and i cant seem to find anything. The engine starts but just revs out of control until i just kill it. I thought it was a vacuum leak, i filled the manifold with 5psi of pressure and sprayed everything with soapy water hoping to find a leak but no such luck. Then i thought it was that the butterfly valves on the throttle bodies weren't shutting all the way but they really all look closed. I dont understand how it can rev so high if there is really no air going into the engine, can it rev that high just with extra fuel added? Im using throttle bodies off of a 2001 gsxr600, there are pictures on the site here. The bike is a 1979 GS750E, 750cc inline for cylinder. it has two coils, one for cylinders 1,3 and the other for 2 and 4. Im wired correctly into the first coil, but i have no way to to tie both coils together. I went into advanced ignition settings and selected dual coils off one missing tooth, It now shows the correct rpm. The ignition system on this bike is a old mechanical points system. Other information that people may need.

Max power of 72hp is at 8500 RPM.
Max torque of 52 ft-lb of torque is at 7200 RPM.
Redline is at 9000 RPM
injectors are high impedance, and flow 21+ lb/hour at 42 psi, 100% duty.

All cylinders have a min compression of 130psi, i check regularly. But they vary alot, one cylinder has 150psi, the lowest is 130 psi.
My vacuum seems low, only drop to 60 at idle on the map sensor. Anyways to recap, My issue is the uncontrolable idle, no matter how i adjust the fuel it does not drop down. Again i truly appreciate everyones help, i have tried everything i can think of for the last few days.

i have included the datalog (if i did it right) for the idle, you will see the rpm drop from 4k almost to zero once or twice, thats where i cut the engine to keep the noise down. I also included the msq file. Im using tunerstudio only because my computer is a linux OS and tunerstudio supports linux.


~jesse

[Bernard Fife]

Jesse,

You have a vacuum leak. That could be external (a hose missing or unconnected, PCV unconnected or malfunctioning) or internal (a gasket backwards, a crack in something, an idle valve stuck open, throttle not closing, etc.). Nothing else other than a vacuum leak will do this (everything else slows the engine down, not speed it up - including too much fuel or too little fuel).

Fixing this will lower the idle speed and raise the vacuum kPa.

I usually start by:
- listening for any whistling sounds (small leaks are noisy, but an be hard to hear over a running engine if it is loud),
- feeling around with my fingers to see if I can plug any leaks or feel air rushing in,
- using a propane torch around gaskets, etc. so see if the added propane makes any difference (anywhere that air shouldn't be getting in).

Lance.

[jtmtv18]

I'll keep looking. Could you explain how to do the propane trick ?

[jtmtv18]

I found a crack on one of the cylinder intake boots. It's big, but is a leak on one cylinder a cause to rev it ?

[24c]

...Could you explain how to do the propane trick ?

I have been known to use a small propane cylinder...plumbers torch over here. Basically turn on the gas and waft it over the intake area, if you get a change in engine state, you should be able to find the air leak. Unfortunately, it's deemed a stupid exercise by most, as the gas could ignite with negative consequences, you just have to be sensible, or manage the risk. Soapy water is best, or a plastic tube to the ear to help focus on the whistling over all the other stuff going on.

It is quite a useful tool for really tricky leaks, I remember a pesky perished O ring on a Weber idle jet on a Fiat 850 coupe , that fooled a dyno centre in Luton years ago, and cost me a wodge of student grant money at the time.

...and yes a leak on one cylinder will do it...just put your hand over the other inlets and see what happens...besides if the air leak is downstream of the butterfly the engine can get as much air as it likes.