Plugged Decel valve: Negative side effects?
 

I plugged the Decel valve on my 1978 SC yesterday and I am really pleased with the improved throttle response.

However, I was wondering if there are any concerns/problems with this as the exhaust pops and rumbles a lot more (which is a good thing for me :D )

Is there a risk of having a backfire? (I do have a popoff valve in my airbox)

Your decel valve may not have been adjusted right. :confused:

I love the adjustable Decel valves on the '78-'79 US and Euro Sc's versus the fixed ones on the later US SC's because it allows to adjust the "hang time".

With the Decel valves plugged, the sudden rapid closure of the butterfly in the TB causes more oil being sucked into the combustion chamber through the rings and valve guides than would happen with correct adjustment and function of the valve.

You may love the bling of rumbling do to the mixture being screwed up but all it does make your oil prematurely dirty . :(

How does one go about plugging the Decel valve? Any downside to this?

The decel is a smog device.

Can you elaborate? Why does the rumbling and popping of the exhaust upon lifting off of the throttle mean that the mixture is screwed up? My car does that upon virtually every sudden lift off. I never knew it to be a sign of anything bad.

Can someone please post a pic of this device in the 'plugged' state? Is it true you can use a golf tee to do so?

Thanks in advance,
Dan

I plugged mine off when it hung up occasionally at a high idle, like sometimes 2k rpms

no ill effects and no cold start popping and this was on a more primitive system than SCs

I used a machine screw covered in Permatex, always soft, gasket cement

check pelican parts with your model year for a pic

You don't plug the decel device, you plug the vacuum line supplying it. IIRC it also supplies vacuum to the cruise control on some models.

There is no down side on removal/disabling it. All it does is reduce unburned hydrocarbon emissions on rapid decel as in "blipping the throttle" while not in gear,

I am going to try that as well. Just to confirm, if you loosen the lock nut, how should you adjust the valve to have more/less "hang time"?

Can you elaborate on this please? I don't see the link between the mixture, dirty oil and the plugging of the decel valve ...

Any engine works best with correct mixture (google Stoich).
When you open the throttle plate, cylinders suck in mixture.
Now imagine an engine running at 4-6000 RPM and you suddenly close the throttle effectively cutting off the mixture (air and fuel) coming in.
However, the engine is still revving at that point and the pistons going from TDC to BDC create a vacuum inside the cylinders. Since the throttle is closed, this vacuum tends to suck oil through rings and valve guides-seals, especially if the rings and valve guides-seals are worn.

Result: oil gets dirty one way or another. :)

The typical rumble and popping you hear when "lifting" the throttle, both, on street and race cars, is the sudden change in Stoich for the mixture.

The Decel Valve is essentially a devise to limit this action and was added mostly for emission regulations but I like the adjustable type for reasons mentioned.

People disconnect it without problems but I believe it speeds up oil contamination.

Do what makes you feel good. SmileWavy

Took her for a ride this evening and I must admit it sounds so much better :cool:

Fast drop in RPM feels also sportier while driving, it always annoyed me earlier to have the delay when going back to idle...


Any other opinions on the downside of plugging the decel valve? Those who have done it experience the exhaust "rumbling" as well?

Any other opinions / can it harm?

Other than fouling the combustion chamber, plugs etc. unnecessarily by sucking in oil that one doesn't want there plus getting the engine oil dirty faster, no harm. :(

A trade-off for "sporty" driving. :D

If you post a picture of your Decel Valve, I could help you with correct connection.
But if you don't want to use it at all, so be it.

The Golf-T-solution sounds good. :rolleyes: