How to adjust the deceleration valve?

echrisconnor · echrisconnor's Garage

How does one adjust the deceleration valve? This is, of course, the adjustable type on a '76. The one with the threaded tube on the top where the vacuum hose connects.

I've heard that by loosening the lock nut and screwing the shaft in, will make revs drop more quickly, and that screwing it out will make revs drop more slowly.

Is this all there is to it?

Chris

echrisconnor · echrisconnor's Garage

bump

CarreraS2 · 03-01-2005, 08:38 AM

I wouldn't adjust it, I'd just stick a bb in the line going into the top of it to totally disable it. It's a totally useless piece of added-on equipment. Once disabled, you'll lose the "sticky" idle drop.

Hladun · 03-01-2005, 08:41 AM

Probably the best way to do it is as follows. (I assume everthing else is working.)

When you quickly release the throttle, rpm should drop to about 1,200 to 1,400 rpm, hold for a few seconds and then settle to 1,000 rpm. You are correct on your afdjustments, screwing the nipple up increases the effect and in reduces it. Probably start with it screwed in because if it's out then the engine will have a high idle to start with and you'll get fooled.

echrisconnor · echrisconnor's Garage

The problem is that I get the immediate throttle drop such that it drops super quickly and often stalls. Maybe it's my mixture...

Hladun · 03-01-2005, 08:45 AM

Quote:

Originally posted by SoCal911SC
I wouldn't adjust it, I'd just stick a bb in the line going into the top of it to totally disable it. It's a totally useless piece of added-on equipment. Once disabled, you'll lose the "sticky" idle drop.

I disagree, I've driven CIS cars without the decel valve and they are prone to stalling when you take your foot off the accelerator.

CarreraS2 · 03-01-2005, 08:50 AM

Hmm, I've disabled the decel on every CIS car I've ever owned (I hate that sticky idle drop) and never had a stalling problem.

Lots of other people on the board have, too.

So, it definately CAN be done.

But if your car stalls with the decel valve disabled, then I suppose there are only 3 choices. (1) Live with the stall (not a good choice!), (2) keep the decel valve, or (3) disable the decel valve and fix the stall problem (any CIS car can run fine without the decel valve if adjusted correctly IMO. The decel valve is only an added-on piece of emission equipment not necessary for the injection system to operate properly).

sammyg2 · 03-01-2005, 09:47 AM

IIRC the decel valve should open at 17 or 18 hg" and close at around 15" hg, but I'll have to check the factory manuals to make sure. It depends on the engine, so not all decel valves are set the same. Different engines pull different vacuums because of cam profiles etc.

basically, they figure out what manifold vacuum your engine idles at, and set the decel valve to be closed at that point. Then, set it to open at a higher vacuum, which occurs as the engine is spinning higher than idle speed and the throttle is snapped shut. The deadband, or difference between the two is the tricky part.

sammyg2 · 03-01-2005, 10:40 AM

I just checked my factory manuals, I didn't find anything at all about adjusting the decel valve. All I found was check it, if bad, replace.

Paulporsche · 03-01-2005, 11:23 AM

echris,

Your mixture may be set too rich.

RoninLB · 03-01-2005, 12:27 PM

I disconnected it and set idle at 1k

echrisconnor · echrisconnor's Garage

I'll try dialing the mixture back a hair and see if that helps. If not, maybe I'll try threading the stud on the decel valve out a bit and see if that changes things.

So, is pretty much everyone saying just disconnect it? I already have abou the most minimalist CIS system around having removed the AAR & AAV completely. I now run on just the WUR, thermotime and CSV.

azasadny · azasadny's Garage

When I disconnect the decel velve, the idle is good, but I get backfires through the exhaust...

JP911 · JP911's Garage

I had the stall at idle problem and plugging the decel valve + a slight leaning of the mixture resolved the issue. I have since jettisoned my CIS in favor of carbs and life is soooo much better!

azasadny · azasadny's Garage

Jon,
I'm going to lean out my mixture this weekend and see how it goes. I really wish I could dump the CIS and replace it with carbs, but that isn't feasible right now (or any time soon)... I don't have the patience for CIS troubleshooting, it's driving me crazy! I'll get everything running just right, then try it the next day and it's no good again, just like I never made any adjustments. I need a "real" mechanic who knows these cars, but I can't get the car to one since it won't run...