What causes revs to drop low, then up to normal on decel ?

DCred911SC · 07-15-2010, 07:25 AM

what would be the main culprit of why the revs would drop to about 600 rpm then up to a steady idle of about 900 rpm when the engine spins down on decel. It doesn't stall, it's literally a momentary thing, and always during the warm up period. Doesn't do it when fully warm, only after about 5 mins of running up until fully warm.

any thoughts ?...cars a Euro 82SC

thanks

David

ossiblue · 07-15-2010, 07:41 AM

My thoughts only. Is your decel valve working or adjusted properly? The decel valve is designed to gradually let the rpm's settle down to idle when the accelerator is suddenly released. If the valve is plugged (many do this on purpose) the engine will drop rpm's immediately. What you are experiencing, when the engine is still not completely warm, could be from the rapid drop in rpm coupled with the overly rich mixture during warm up. Once the mix leans out when the engine is warm, the problem goes away. Just a thought to get you started.

DCred911SC · 07-15-2010, 08:02 AM

Makes sense.

Have been adjusting the mix very slightly and found that I can get it to improve by leaning it out, but it runs and idles not so smooth. Definitely runs better overall when richened but it doesn't take much before it stalls on the decel.

ossiblue · 07-15-2010, 08:10 AM

Quote:

Originally Posted by DCred911SC

Makes sense.

Have been adjusting the mix very slightly and found that I can get it to improve by leaning it out, but it runs and idles not so smooth. Definitely runs better overall when richened but it doesn't take much before it stalls on the decel.

If you've been adjusting the mix at the fuel distributor (the 3mm allen head bolt), you could be making more work for yourself.

Just to clarify, from your original post it sounds like the problem goes away when the engine is fully warmed (fuel mix is leaner due to the wur.) The overly rich mix I referred to was the one caused by the wur in its cold pressure state, which is normal. Typically, once the mixture screw is set at the fuel distributor, it doesn't need to be adjusted--or if it does, it is the last in a series of corrections to be made.

DCred911SC · 07-15-2010, 08:29 AM

I agree. I've been moving it very slightly, just to see the effect, but I know where the original position is (I've also got a gas tester so I can reset if it gets too far out). The WUR is new, so I'm assuming (!) it's delivering the correct pressures.
Originally thought it could be a vac leak, but from what you're explaining, it sounds more like a rich condition.

ossiblue · 07-15-2010, 04:51 PM

Quote:

Originally Posted by DCred911SC

I agree. I've been moving it very slightly, just to see the effect, but I know where the original position is (I've also got a gas tester so I can reset if it gets too far out). The WUR is new, so I'm assuming (!) it's delivering the correct pressures.
Originally thought it could be a vac leak, but from what you're explaining, it sounds more like a rich condition.

From your original post, it does sound like the wur is in proper specs. A too low cold pressure would cause the idle to hunt--going back and forth from low to high. You said that the rpm drops low then steadily moves up to proper speed. That doesn't strike me as a problem with the wur so my first thought was of a too rapid drop in rpm for the rich, cold mixture to recover. It'd never hurt to check the cold pressure since it's easy enough.

BTW, I have no experience with CIS using O2 sensors so this may be a factor as well and someone with knowledge can help you out.

DCred911SC · 07-16-2010, 04:12 AM

Maybe didn't quite explain the rev drop very well. It doesn't steadily move up to proper speed, it literally goes straight back there, a bit like a bounce up. For example, if I change gear but hold the clutch in such that the engine can spin down to idle, the tach needle drops past the normal the 900 rpm to about 5-600 rpm, but immediately bounces back up to 900 rpm. It literally happens in a moment, a bit like an overshoot then straight back to where it should be.

Strange

ossiblue · 07-16-2010, 05:43 PM

Quote:

Originally Posted by DCred911SC

Maybe didn't quite explain the rev drop very well. It doesn't steadily move up to proper speed, it literally goes straight back there, a bit like a bounce up. For example, if I change gear but hold the clutch in such that the engine can spin down to idle, the tach needle drops past the normal the 900 rpm to about 5-600 rpm, but immediately bounces back up to 900 rpm. It literally happens in a moment, a bit like an overshoot then straight back to where it should be.

Strange

Your latest clarification makes me think, again, it's the decel valve--if operating properly, it will not allow the rpm's to drop rapidly. As mentioned before, many run their cars with the valve blocked so the rpm's drop immediately but I don't know if that also causes symptoms like yours. Hopefully, someone who doesn't use the decel valve will respond as to what happens when they let off the accelerator.

BTW, have you checked your decel valve? The line entering the top, do you feel vacuum?