[tom wurm]

Just got my car back on the road after a long frame off restoration. I noticed that at about 3500 rpm on acceleration I am getting a backfire/popping in the exhaust and miss. The car had the same problem before the engine and carb were rebuilt. I noticed when I was working on the Carb many months ago that there was quite a bit if play in the throttle valve shafts. I had the carb professionally rebuilt and the car runs really well when just putting along. I did verify that there is a vacuum leak at these points. I am running a stock 396/325 horse except for an Edelbrock performer cam, headers and Petronix ignitor points replacement. Are the loose throttle shafts causing the problem. If so what should I do to correct. I would prefer to keep this carb since it is the original. Thanks.

 

[1224]

Part of a professional rebuild should be to replace the throttle shaft bushings. Ask your rebuilder about it. Sounds like they forgot to do it.

Tino

 

[mike reeh]

i may be wrong but at that speed, what little air the throttle shafts will leak shouldnt affect the engine.. are you wide open when it happens? ideally, you should be running rich at that point if so, enough to make up for the leak.. but you never know.. when that happened to me, i had hesitation at real slow speeds just off idle where there was no jetting or adjustment that I could make.

you might try going to smaller rods in the carb so you're running a little richer, or a weaker power piston spring.

there is also the possibility that its your advance mechanism or even valve lash

Mike

 

[Tom Mobley]

tom, most likely you need to check to what total timing you have at that RPM. Between the initial, mechanical and vacuum advance you should have 45-50 deg. Did you disconnect the vac advance when you set the timing?

Throttle shaft leaks are irrelevent at any speed above idle because the small amount of air getting is completely lost in the mass of air going in through the throttles.

Tino, Q-jets don't have shaft bushings, they can be installed but it's a little bit of an operation. In my experience this is rarely a factor as long as the throttle return spring pulls back and down. If you put on a spring that pulls to the front you can get leaks below/behind the shaft.


Tom

 

[Gene Chas]

Tom, I just read Mr. Mobley's reply and he covered everything I was going to mention. Check your total timing at 3500 and I didn't recall that Q's had replacable bushings. Sure you can machine it and sleeve it, or you can find a donor base plate with tighter clearances. Hard to believe it would cause a problem at mid throttle, but, hey I'm still learnin'.

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