Dashpot question
 

I did a search to try to learn a little more about my backfiring. The car runs strong and I get perfect boost, etc.
I get backfiring when I accelerate hard and then let off of the throttle. The backfiring occurs when I let off the gas. Relavant mods are: Pettit Intake, DP, High flow cat, Racing Beat exhaust.
When I had it at KD Rotary a few months back, the tech diagnosed some things for me. He wrote:

Thermal Wax Valve is slow to release ($99.55)
Dashpot at throttle body stuck closed ($153.73)
ISC Valve is slow to operate / Idle up for loads is very slow ($416.85)

My search on this site tells me that the backfiring is related to the fuel getting cut immediately as I let off the pedal - rather than gradually cutting back. I think this is done by the dashpot. Am I correct?

BTW: I did not have them do any of those jobs I listed. They still need to be fixed (I guess). Skip told me that these things are not going to affect the health of the car - it's basically nuisance stuff. Damn - that ISC Valve job is expensive!

Thanks for any feedback.

Dave

I'm no expert but as I understand, the dashpot help control throttle when you back off the gas and you can easily take it off and check the compression switch for proper movement. Some folks do have trouble with the ISC but this usually comes up with cars stalling on deccel. Lot's of us have some backfire during idle and or when letting off the gas, especially after boosting. Just a process of rotary engines maybe and with different exhaust components.

Dashpot is easy to check/adjust and replace if needed. If you move the throttle away from it, and the white "button" stays in, you need to replace it.

The dashpot mainly helps in idle recovery. When you let off the gas, the RPM's come down really quick. The dashpot "catches" the throttle and eases it into the idle position. Without it, the idle can drop really low after pushing the clutch in and possibly stall. That's really the only purpose it serves.

BTW, just bought one new from Malloy Mazda - it's $85. A ripoff, but that's a much better price than what you're looking at :). Takes a 17mm wrench and about 1 minute to replace, too.

The ISC typically just gets clogged up with junk - cleaning it out with some carb cleaner and working it a few times with 12v usually does the trick.

Dale

Dashpot is mainly to prevent from backfiring, it does make sense what you're sayng though, but mazda says it's there to prevent from quick closing up the trottle, so the fuel that's left in the combustion chamber at decel. gets some more air and complete a proper combustion, otherwise you will have some backfiring.
Mostly all todays cars have one, even automatic cars...


My car started backfiring alot I mean alot almoast at every decel., after I installed a DP and CB while I still had the stock center cat, I tried installing a new dashpot ($100.00 in the pit) months later I removed the center cat and the backfiring went away.



.

Rotaries have a tendency to pop and afterburn a little. Even brand-new RX-8's with exhaust will get some pop and burble on deceleration.

Dale

That's true...

I was playing with my throttle, and that part that touches the white stick BARELY skins it when the throttle is fully closed. I pushed the rod in and it came out just like a shock. Thats good right? Dale, should I just loosen the nut and screw the dashpot in a little more so the rod comes into contact more with the throttle?

Jeremy

Before you adjust the dashpot, make sure the engine is fully warmed up. The thermowax has to be disengaged so the throttle has its full range of movement.

You can experiment with adjusting it - you won't hurt anything. I think there's an adjustment procedure in the shop manual as well. Just loosen the 17mm nut and screw the dashpot in and out.

But, it's typically set properly from the factory, unless someone's messed with it and adjusted it improperly in the past. In theory, the only time you should have to mess with it is if it fails and you need a new one.

Dale

The shop manual suggests that the throttle must not touch the dashpot after 2800rpm(+-200rpm).

What is the ISC?

Idle Speed Control. It bypasses air around the closed throttle plates to set the idle speed.

Dale

your dashpot seems to be working correctly, but how far out does the white plunger come? I had this exact same problem with similar mods, new dashpot fixed it. (actually i just found a TB for sale that had a working dashpot (have the seller measure it) and bought it for like $50. Take off the dashpot, and sell the TB or keep it for polishing practice :bigthumb: )

Once you have a working dashpot have a friend hold the car at 2800rpms, adjust the dashpot until the white plunger is just touching the TB cam, tighten the lock nut.

Ah interesting...thanks Dale - I never knew it existed lol

does anyone have a picture of where this white rod is????

this is the wrong angle but it's the only pic i have. At the other end of this piece is where the white (read yellowed) plastic plunger sticks out.

just buy a new TB on e-bay for $25-40 and install it and call it a day.. if some sensor's are bad, you still have yours to make the best set out of ;)

just my opinion though :)

problem is a lot of dashpots don't work anymore

This thread keeps coming back :)
Is the actual dashpot an expensive part?

Yes very. $100+ even from Malloy I believe.

:)

lol, oops