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Old 10-05-2007, 04:07 AM
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SBC hesitation problem

Guys, I'm stumped here, HELP! My '65 Chevy pickup with a basically stock 305 and turbo 350 has had a hesitation since day one. Engine is a .030 over stock rebuild with a Rochester 2GV carb and HEI distributor. It starts and idles fine, but cold or warm, it hesitates under acceleration. Idle speed is 500 RPM and I set timing at 10 degrees static, but when I connected the vacuum line, timing jumped up to nearer 20 degrees. My distributor is connected to the "ported vacuum" connection which should have ZERO at idle, but it reads 20" of vacuum. There are only two vacuum connections on the carb and both read 20" at idle. What gives here? Help or ideas will be appreciated. Thanks Mike,

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Old 10-05-2007, 11:26 AM
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Don't have a clue about the vaccum. But with hesitation problems I always suspect the accelerator pump in the carb. When was the last time the carb was rebuilt? This thing can be repaired without even removing the carb. About the vacuum, I have never seen 20 inches of vacuum on an engine. But I'm no expert on that subject. Are you sure your gauge is accurate?....Doug
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Old 10-05-2007, 02:09 PM
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Quote:
Originally Posted by 1953 Homer
Guys, I'm stumped here, HELP! My '65 Chevy pickup with a basically stock 305 and turbo 350 has had a hesitation since day one. Engine is a .030 over stock rebuild with a Rochester 2GV carb and HEI distributor. It starts and idles fine, but cold or warm, it hesitates under acceleration. Idle speed is 500 RPM and I set timing at 10 degrees static, but when I connected the vacuum line, timing jumped up to nearer 20 degrees. My distributor is connected to the "ported vacuum" connection which should have ZERO at idle, but it reads 20" of vacuum. There are only two vacuum connections on the carb and both read 20" at idle. What gives here? Help or ideas will be appreciated. Thanks Mike,
It looks like you're getting to much opening of the throttle blades at idle. This is common when an engine ages and parts are no longer working as when the machine was young, it is also is a typical problem with long duration camshafts. What happens is you keep adjusting more throttle opening to keep the idle speed where it needs to be and at some point the throttle butterflies become open enough to where they are exposing the transfer slots between the idle and main circuits. This also can result in the butterflies exposing the vacuum advance timed ports in the throttle bore so the vacuum canister is putting tto much advance in too soon.

The solutions are those that allow you to close the butterflies such that proper idle speed is maintained without the transfer ports being exposed. Given a 1965 engine this can be anywhere from a good tuneup and carb rebuild to a major engine overhaul.

On the cheap side of repairs you also need to look at whether the accelerating pump is working properly and peek at the fuel level in the bowls.
I say this because 20 inches of vacuum is a lot and could indicate that your throttle butterflies are in the proper position.

Bogie
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Old 10-05-2007, 07:15 PM
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Quote:
Originally Posted by 1953 Homer
Guys, I'm stumped here, HELP! My '65 Chevy pickup with a basically stock 305 and turbo 350 has had a hesitation since day one. Engine is a .030 over stock rebuild with a Rochester 2GV carb and HEI distributor. It starts and idles fine, but cold or warm, it hesitates under acceleration. Idle speed is 500 RPM and I set timing at 10 degrees static, but when I connected the vacuum line, timing jumped up to nearer 20 degrees. My distributor is connected to the "ported vacuum" connection which should have ZERO at idle, but it reads 20" of vacuum. There are only two vacuum connections on the carb and both read 20" at idle. What gives here? Help or ideas will be appreciated. Thanks Mike,
Just for the heck of it disconnect and plug the vacuum advance hose and see if the hesitation goes away.
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Old 10-06-2007, 10:15 AM
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Do this: Open the throttle by hand and watch to see how far it opens before it starts to move the accelerator pump rod. It might need to be adjusted to kick in sooner.
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