Author Topic: Airhorn vent  (Read 105 times)

Offline mcx

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Airhorn vent
« on: June 09, 2019, 09:36:51 AM »
Cliff...what was the last year the qjet used the airhorn rubber  vent at the front of the carb,
 which has  small linkage attached to accel pump arm?  I have a 7042211 which has this feature but I feel it might be a mismatch of parts. Thx, Mike

Offline Cliff Ruggles

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Re: Airhorn vent
« Reply #1 on: June 10, 2019, 04:57:33 AM »
It was not used in 1972 and started to disappear by the end of the 1969 production run.

You'll find it on some 1970 models but can't say I've seen anything 1971 or later that used it outside of a few Service Replacement carbs.

I'll add here that the 1972 airhorn also used the large MAB's and most of the vented ones used much smaller MAB's so for sure it would not work well without recalibration if they are still in place.......Cliff

Offline mcx

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Re: Airhorn vent
« Reply #2 on: June 10, 2019, 10:40:24 AM »
Thank you for the info, I cross checked a 7042210 and it has .120" mains and sure enough this mismatch 7042211 has the much smaller bushing type main bleeds...I even rechecked a 7045229(1975 Chevy) and it has the huge mains in her, like you are saying.
...could the 7042211 Mains just be carefully drilled to match .120" and call it a day? Mike

Offline mcx

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Re: Airhorn vent
« Reply #3 on: June 10, 2019, 01:11:52 PM »
I was looking at the two airhorns , it seems the only other difference between the  pre 70 rubber vent type lid with the small bushed type main bleed and the later  post 70 non rubber version with the larger main bleeds is the air path through the airhorn idle air bypass...a pre 70 airhorn to main body gasket would be needed for it function,correct? Drill out the main to .120"and swap out to correct gasket?

Offline Cliff Ruggles

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Re: Airhorn vent
« Reply #4 on: June 10, 2019, 04:54:50 PM »
I would not drill out the MAB's without knowing what else has been done to the carb?

It may be a "reman", or other parts swapped in conjunction with the incorrect top.

If the airhorn isn't "notched" for bypass air the notches can be added.  Far better than trying to use a notched gasket as they will often "whistle" with airhorns that aren't notched as the air passes thru the small flat openings......Cliff

Offline mcx

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Re: Airhorn vent
« Reply #5 on: June 11, 2019, 08:31:16 AM »
Cliff...I checked the 7042211 base plate numbers against another 72 7042210 and it's the same stamping/serial number...I also matched the two main body and  checked the specs and they are the same jets/rods/bleedsize/ect... So it's definitely isolated to a incorrect pre 70 airhorn...Cliff, do you see any issue with using a pre 70 non- notched gasket and let the idle air bypass flow as is.? The pre 70 idle air bypass takes a slightly different path through its airhorn, but its fed identical.

Offline Cliff Ruggles

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Re: Airhorn vent
« Reply #6 on: June 12, 2019, 04:48:54 AM »
It's probably 1968 or earlier as many Chevy units didn't use bypass air so the airhorns aren't notched.

Just take a die grinder and gently notch the airhorn and use the later gaskets, they aren't fussy and I do it all the time here. 

As far as calibration I'd either install the small bleeds in the MAB locations in the main casting and calibrate it like a pre-1970 or take them out and run all four as large MAB's and calibrate it like a 1970-up unit.

The mixing/matching thing just doesn't work for me and it didn't work for Edelbrock either as they left the small MAB's out of the main body on their early style units but put them in the airhorn.  I install a set in the main body on every one we do here and it gets them right up to par every time......FWIW......Cliff

Offline mcx

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Re: Airhorn vent
« Reply #7 on: June 12, 2019, 06:01:42 AM »
Thank you Sir...very much appreciated.