Throttle Return Dashpot

[FastWoman]

No, my BCDD doesn't work anymore. I should just remove it. Many people remove the thing and just fashion a plate to cover the hole.

[TomoHawk]

I remember the vacuum advance dashpots on the distributors for older (muscle) cars moved easily if you sucked on the hose, unlike the stock L28. OTOH, I'm thinking now that an oil-filled one for the throttle might be so good as as it would probably give resistance in both directions.

I was trying to figure out how to get the BCDD off but it looks impossible to get your hand up there to reach the screws. You could still cover it and maybe screw in a few nipples for vacuum taps?

[argniest]

can you post of picture of your dashpot, especially if its from a 78 (or any z that would have simlar one to a 78). Im curious about this device, because a mechanic friend of mine was describing something like this dashpot. I didnt know until later, that what he was talking about was the dashpot ;-) And now here ya all are talking about it.

I believe I know where my dashpot is at, and it actually seems in OK shape. If there is supposed to be a vacuum line connected to the white nipple, well there isnt on my z car.

I have beeen told by a z mechanic to take if off and throw it away. Im not ready to give up on it yet.

[FastWoman]

Argneist, no need for a picture. You're already describing the doobob in question -- a round thingie that mounts to the front of the throttle body. It has a white plastic nipple that is not supposed to have any vacuum line on it, appearances to the contrary.

Check yours to see if it works OK. Just open your throttle by hand. You should see the protrusion on the dashpot move outwards. Then let the throttle close. The protrusion should be pushed back in by the lever on the throttle. The dashpot should slow the throttle closure right near the end of travel. If the dashpot is frozen or moves roughly, you might be able to get it working again by squirting WD-40 inside and working the thing. Dunno. That worked for my '75 Z back in the 80's.

[argniest]

There is a metal sliding portion inside the dashpot that still moves in and out when I pushed it up, then it just came back down on its own. Although it seems to be positioned so it doesnt effect the throttle/linkage at all. Im hopefully going to see a mechanic friend of mine this weekend. He was one of the people who was talking about something like the dashpot, although he called it something else. So we will take a look at it, and find out if its really working or not. he knows about them, and what to do.

He was describing something that helps keep the throttle from slamming shut, I think. Or maybe to hold it open....ahhh heck I cant remember.

The reason I am bringing it up, is even though my engine sounds good, the AFR's are looking good, no more lag/surge in higher rpm's, the drivibility is great, and my mpgs are 18 to 25 mpg from town to highway.......I have developed a backfire(at least I think that is what it is), when shifting gears. Now depending on how I shift, can make the backfire happen or not. And I am pretty sure it does it, even when the engine is just warming up but not hot

I guess my friend was saying if this dashpot wasnt doing its job, it could be causing my backfire...or I could be running too lean. But I mean, I dont know if that can be possible. I am getting 13.9 to 14.3 range on the AFR's when crusing around in several gears, and even at 65/70 mph I was getting those numbers. That doesnt seem to lean to me. But maybe there is some effect when shifiting, that is causing a quick lean condition...

ohhhhhhh well, its a small complaint and I hope he can help me figure it out, and maybe this dashpot is part of the problem or solution. I was told to look for leaks in exhaust system, I hooked up fog machine and didnt see anything coming out anywhere from exhaust. But I am going to stop by another friend who has a real smoke machine at his garage, and see if that shows any leaks in exhaust system. I guess the thinking is, that if I had a leaky exhaust it could be causing this popping when shifting in certain ways.

It never did this until I hooked up the TVS, and if I disconnect TVS is doesnt do it, and never did it before that I ever heard. Just another mystery to solve

But maybe the dashpot will fix it. We shaaaaaaaaalll see.

Edited July 20, 2011 by argniest

[FastWoman]

Those dashpots would cause the throttle to hang open if they started getting crusty inside, so sometimes they were backed off enough to do no harm.

I've not heard of a malfunctioning dashpot causing backfire before. However, I do know that a really steep RPM drop from a really high RPM can result in afterfire (out the exhaust, not the intake) as you cross that 2500 RPM boundary. What happens is that fuel is cut until you reach 2500, and then it is reintroduced. Then SOMEHOW you get some unburnt fuel reaching the hot exhaust, and .... pop! At least that's why MY engine does if I rev really high and don't ease the speed back down.

I confess I don't know what a TVS is. I'm guessing one of your diagnostic toys?

[cozye]

You guys need to check into the BCDD operation. When functioning correctly, it eases the throttle down and it eliminates the popping on deceleration. Fortunately mine works well, although I did have to adjust it some to keep the motor from hanging at higher rpms when stopping. My car does not have the dashpot, I'm not sure if it ever did although the bracket for it is there. I think if you have a working BCDD, both your problems might be solved. If it isn't working or the diaphragm in it is busted then part is nla and the common workaround is to delete it.

[FastWoman]

Yeah, but deleting the BCDD wouldn't resolve the problem. It would only clean up the engine compartment.

I thought it might be interesting to install an electric air valve bypassing the throttle. It could be actuated when the A/C compressor is running (eliminating the hard-to-adjust A/C high idle doobob), when the engine is cold (eliminating the unreliable air regulator), and for a couple of seconds when the throttle slams shut (eliminating the idle sag).

Edited July 20, 2011 by FastWoman

[cozye]

Yeah, I know deleting wouldn't resolve it. Properly adjusting would. There is a screw hidden under a rubber cover on the bottom of it. If you disconnect the wire to simulate going over 10 mph, then rev the car by hand, you can adjust it by ear until the rpm drop is gradual. The BCDD is letting air bypass in as you described. Not sure it would work for the ac idea though

[FastWoman]

I'll try that. Thanks. I think mine is completely dead, though. I tried working on it when I had the throttle off, but I couldn't budge any of the screws. Hopelessly corroded. I said, "Oh well..." and moved on.

[cozye]

I'll try that. Thanks. I think mine is completely dead, though. I tried working on it when I had the throttle off, but I couldn't budge any of the screws. Hopelessly corroded. I said, "Oh well..." and moved on.

Yes, the screws were very tight on mine as well, however the adjustment screw was very easy to turn once I discovered it under that rubber plug. I did eventually dissasemble mine and verified that my diaphragm was in good shape. Look right on the bottom of the thing for that rubber plug, right in the center. You might get lucky

[Jorgesan]

I need to adjust my bcdd . Any more advice?