Popping out the exhaust....

c.ruber · Posted: Sun Apr 13, 2014 11:09 am Post subject: Popping out the exhaust....

I'm running a 2276 with IDAs. I've been having an annoying popping put of the exhaust for some time. It usually comes on at about 3200 to 4000 rpm. I've changed the plugs and they are dark. My jetting is 37mm Venturi's, 155 mains, F11 emulsion tubes, 120 idle jet holders, 60 idle jet and 200 air correction. Could I be experiencing vacuum leaks? Or is it just plain jetting?
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Danwvw · Posted: Sun Apr 13, 2014 11:16 am Post subject:

Could be the cross over between the Idle circuits and the mains is not happening quite right. (If this engine is in your type 3)Those manifolds on the type 3's are so short there is little room for error in the dual carburetor set-ups. Perhaps a little smaller on the air correction jets!
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Cptn. Calzone · Posted: Sun Apr 13, 2014 11:31 am Post subject: popping

Check exhaust for leaks and manifolds/carbs vacume leaks
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c.ruber · Posted: Sun Apr 13, 2014 11:55 am Post subject:

This is in my type1. I'm thinking the progression circuit. It has been suggested to me by a reputable source to increase the idle jet holder from 120 to 130 by drilling it. That should lean out the main circuit... Right???
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Cptn. Calzone · Posted: Sun Apr 13, 2014 12:10 pm Post subject: jets

Just change the jet then no guesswork
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67 912 Porsche Quintilla
67 Single Cab aka my work horse

Fred Winterburn · Posted: Sun Apr 13, 2014 3:15 pm Post subject: Re: Popping out the exhaust....

Just a thought. Are you certain the noise is coming from the exhaust? I have a car (not a VW) with a badly worn camshaft that has one exhaust lobe so badly worn that it doesn't allow all of the exhaust to escape in that cylinder on the exhaust stroke. When the intake valve opens, the remaining exhaust blasts out the throat of the carburetor. It barks and pops just like a Harley Davidson and the phenomenon begins at about 3000rpm. Worth checking if you have only been hearing it by listening from the driver's seat. It's a shot in the dark, but another possibility. Good luck, Fred

c.ruber · Posted: Sun Apr 13, 2014 4:44 pm Post subject:

Positive out the exhaust.
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Danwvw · Posted: Sun Apr 13, 2014 10:50 pm Post subject:

Is your car running on all 4 cylinders?
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1960 Beetle And 1679cc DP W-100 & Dual Zeniths!

tkelley · Posted: Mon Apr 14, 2014 5:33 am Post subject:

I would also check valve lash

Tom
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c.ruber · Posted: Mon Apr 14, 2014 8:22 am Post subject:

I've adjusted the valves with no change to the problem. The engine has an intermittent miss when cold. It seems to smooth out when its warm. The popping comes on at about 2800 to 3200 rpm.
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Dr OnHolliday · Posted: Mon Apr 14, 2014 8:40 am Post subject:

Bent and/or sticking valve?
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Fred Winterburn · Posted: Mon Apr 14, 2014 8:47 pm Post subject:

Alright then, Another shot in the dark. Could your dizzy rotor phasing be out of whack? Ie, could your rotor be pointing slightly toward the wrong cylinder when vacuum advance comes on strong? You could check the distributor cap alignment or anything really out of whack with the vacuum advance plate as that will definitely affect rotor phasing. It might only show up as a miss at idle, but become more pronounced under load even if under load there is less vacuum. Also, with increased rpm, and a standard ignition, the ignition voltage will be higher with the alternator or generator turning faster (given that voltage regulators aren't perfect) and cross-fire in the distributor would be worse. Also, with increased cylinder pressure under load it would be easier to cross-fire in the distributor to a lower compression cylinder. (Enough rambling Fred) Suggest check rotor phasing which is directly affected by the vacuum advance plate, or cap placement (rotor phasing is not affected by centrifugal advance by the way) Fred

modok · Posted: Mon Apr 14, 2014 9:44 pm Post subject:

Any one cylinder too rich OR too lean OR out of synch carbs will cause an intermittent miss that will cause such a backfire.

It could be anything but if he hasn't done the work to get the carbs dialed-in then it's probably the carbs.

c.ruber · Posted: Tue Apr 15, 2014 9:39 am Post subject:

Thanks modok, I'm thinking that I have debris in the carb possibly blocking the progression ports. I may not get to work on it until the weekend. I thought it was odd that when I was adjusting the idle mixture there was one cylinder that didn't change like the others. I'll start my search there.
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c.ruber · Posted: Fri Apr 18, 2014 5:04 pm Post subject:

Today I had a chance to work on the engine. I removed both carbs and gave them a thorough cleaning inside and out. I removed the idle jets and installed 55s. I also replaced the idle adjustment screws. After reinstalling the carbs and adjusting the idle mixture, I still have popping out the exhaust from about 2000 to 4500 rpm. My elevation is roughly 650 feet above sea level. I think I'm still running rich. I have also noticed the accelerator nozzles have the IDA drip. That may be a problem to address as well. Any ideas are welcome...
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66brm · Posted: Sat Apr 19, 2014 4:19 am Post subject:

Fuel pressure and float heights?
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c.ruber · Posted: Sat Apr 19, 2014 1:33 pm Post subject:

I've adjusted the fuel pressure to 3psi. I'm thinking a bad spark plug or manifold gasket. I installed one step down on the idle jet and no change in the idle miss and acceleration popping. Why are the damn things so hard to tune?
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c.ruber · Posted: Sun Apr 20, 2014 1:07 pm Post subject:

Today I adjusted the timing. Great difference in the way the engine idles. I'm sitting right about 8 to 9 degrees BTDC. My total advance is about 40 to 42 degrees. The popping at idle is gone and it accelerates better. Adjusting the idle mixture was only giving small differences in the way the engine idled so I left it where it idled smoothest. There is still backfiring while in the mid to upper rpm range. I may be running rich up there. I'll try a smaller main jet soon.
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[email protected] · Posted: Sun Apr 20, 2014 2:38 pm Post subject:

They are not hard to tune, you are just still learning. To me it sounds like you still have multiple issues you are sorting out. So of course the resolution of ONE isn't going to be the Holy Grail.

You have to make sure you don't have any drip first. And make sure timing and ignition are spot on. And no exhaust leaks (95% of exhaust popping are due to exhaust leaks!).

Also, I recommend instead of going down on the idle fuel, go up on the idle air, to 130.
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c.ruber · Posted: Sun Apr 20, 2014 6:47 pm Post subject:

Thanks John, I took your advise and opened up the idle jet holder. Any advise on what to do about the 3/4 side carb dripping? I removed the linkage so with that out of the equation could the throttle shaft be twisted? Also do you think my total timing advance be too much? Thanks Doug.
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