Engine Undulating

[DJThom]

Hello All.

I've been enjoying the Elan for the past couple of seasons that it's been finished and recently took it on the first longer trip that involved highway driving.

Around 5000 RPM it feels like the engine is undulating although the tachometer is perfectly steady. If I accelerate there is lots of power that comes on instantly and smoothly. If I am accelerating gently or hard I have smooth power right through the range. It only happens if I'm cruising. I experimented and found that it happens regardless of what gear I'm in.

As always, suggestions for the inexperienced are welcome and appreciated.

[robertverhey]

It isn't those rotoflex donuts setting up some sort of harmonic vibration is it?

[DJThom]

I have cv joints, but very good suggestion. It's not a vibration type of sensation, but a surging/relaxing/surging/relaxing feeling

[john.p.clegg]

I'm not sure of this but it sounds like "lean surge"...

John

[CBUEB1771]

I'm not sure of this but it sounds like "lean surge"...

John

I think John is on to it. You didn't mention of this happens at light to mid throttle openings however I suspect it is the case as you mention you have good power when you open the throttle to accelerate. From time to time I have this condition in my +2 and it is always resolved by finding a broken Thackeray washer or a bad o-ring in the carburettor mounting. At high engine speeds and light throttle openings you have a lot of inlet manifold depression and air will be sucked in past a perished or not properly compressed o-ring. This gives a lean condition in one or more cylinders and produces a surging effect.

[CBUEB1771]

DJThom - Did you resolve the undulation problem?

[DJThom]

Unfortunatlely I have not had time. I've been traveling with work to Japan and Washington and now it's gotten cold and dark by the time I get home from work...

I will report back most definitely, even if it is in the spring!

[ill_will]

you could check if it's from lean running by pulling some plugs? drive at the conditions where it's happening for a couple of minutes then turn off the ignition and coast to a stop. white rather than tan plugs indicate lean. important to cut the ignition at the conditions you are trying to check, obviously.

On another track, I did have a similar surging problem like this in an Alfa 1750 GTV, which was because of overly rich running at low throttle openings. Plugs were very sooty, mileage was awful. Horrible surging, very hard to drive smoothly, particularly when doing small increases/decreases in speed. Wideband 02 sensor confirmed this, and going down an idle jet size solved it nicely. It pulled very well though at full throttle.

hope this helps.

will