Engine surge
Thread Starter
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Tuned
Joined: Oct 2012
Posts: 371
Likes: 6
From: Slocum, TX
Engine surge
Recent new Holley carb and tune up. Suddenly, usually at a stop, the vacuum will fall off. Speed up and vacuum restored. I had to speed up the idle to keep it running from 10" to 18". Squeezed off all the hoses with out any effect. Plugged the vacuum hose to brake booster, no effect.
It will run steady and suddenly drop off. If I catch it and speed up the cycle will start again. On the road at 55, all is well with good vacuum.
Where should I look next!
It will run steady and suddenly drop off. If I catch it and speed up the cycle will start again. On the road at 55, all is well with good vacuum.
Where should I look next!
Post Fiend
Joined: Apr 2008
Posts: 8,261
Likes: 15
From: Big Sky Country
Was the vacuum advance switched from manifold (full) to ported (timed) during the carb swap/tune-up?
Thread Starter
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Tuned
Joined: Oct 2012
Posts: 371
Likes: 6
From: Slocum, TX
Vacuum advance
Thread Starter
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Tuned
Joined: Oct 2012
Posts: 371
Likes: 6
From: Slocum, TX
Carb tune
Post Fiend
Joined: Mar 2011
Posts: 8,501
Likes: 4
From: Lynchburg, VA
From your description, it kind of sounds like it's flooding. The next time it happens, pop the hood, pull off the air cleaner, and look down the throat of the carb with the engine still running. If it's flooding, you'll see fuel dripping down into the throat of the carb.
Laughing Gas
Joined: May 2006
Posts: 1,140
Likes: 13
From: Port Orchard Wa
Just curious how you know your losing vacuum. Do you have a vacuum gage in the cab? Where is it connected?
Based on your description, it sounds like you are connected to ported vacuum and therefore have no vac advance at idle. Otherwise a leaky vacuum advance canister could be you problem.
Based on your description, it sounds like you are connected to ported vacuum and therefore have no vac advance at idle. Otherwise a leaky vacuum advance canister could be you problem.
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Thread Starter
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Tuned
Joined: Oct 2012
Posts: 371
Likes: 6
From: Slocum, TX
Vacuum
Just curious how you know your losing vacuum. Do you have a vacuum gage in the cab? Where is it connected?
Based on your description, it sounds like you are connected to ported vacuum and therefore have no vac advance at idle. Otherwise a leaky vacuum advance canister could be you problem.
Based on your description, it sounds like you are connected to ported vacuum and therefore have no vac advance at idle. Otherwise a leaky vacuum advance canister could be you problem.
I guess it is off to the local mechanic.
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Thread Starter
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Tuned
Joined: Oct 2012
Posts: 371
Likes: 6
From: Slocum, TX
engine surge
From your description, it kind of sounds like it's flooding. The next time it happens, pop the hood, pull off the air cleaner, and look down the throat of the carb with the engine still running. If it's flooding, you'll see fuel dripping down into the throat of the carb.
Guess it is off to the local mechanic.
Post Fiend
Joined: Apr 2008
Posts: 8,261
Likes: 15
From: Big Sky Country
Thread Starter
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Tuned
Joined: Oct 2012
Posts: 371
Likes: 6
From: Slocum, TX
Vac advance
Post Fiend
Joined: Apr 2008
Posts: 8,261
Likes: 15
From: Big Sky Country
That's correct as well, your symptoms led me to believe maybe the gauge was sourcing its vacuum signal from a timed port.
At this point i'd start spraying around for vacuum leaks, check everywhere, hoses, intake, carb, booster, etc.
(EDIT: I just noticed you had a FE so it's possible to have a vacuum leak under the valve covers that cannot be found using the spray technique)
Thread Starter
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Tuned
Joined: Oct 2012
Posts: 371
Likes: 6
From: Slocum, TX
Engine surge
After sleeping on this, maybe I'm looking at vacuum being the problem but it is only a result of the engine speeding up and slowing down. When the engine starts cold, the choke is on and the idle is fast, vacuum high. After warm up is when the problem starts. Driving at 55 all is normal. At idle, the engine surges and almost dies then speeds up again. A rhythmic pattern. Sort of like getting gas then not.
Thread Starter
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Tuned
Joined: Oct 2012
Posts: 371
Likes: 6
From: Slocum, TX
You were 100 % correct. After back and forth with Holley and a $75 spacer they sold me to fix the problem which it did not, Holley took the carburetor back and found the floats were bad. They repaired the carburetor at no charge except for my $20 to send it to them and my out of pocket at the local garage.
Runs fine now. I could have bought another new carburetor!
Runs fine now. I could have bought another new carburetor!
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