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fuel delivery? Ignition?
Not too shy to talk
Joined:
2010/12/17 5:59
From Romeo, Michigan
Posts: 21
Issues have been driving me bonkers!

Just put my rebuilt Rochester 4GC carb back on, car runs rough at high idle.
Its starts, idles fine, can take it around town up to about 35mph, and more acceleration, it bogs down...

I'm thinking there's various solutions, was hoping the carb was the issue. I did mess with the high idle screw and regular idle screw the day the car went to crap, it ran fine for about 20 miles.

Could there be a vacuum leak? what are the usual suspect places to check?
Is it ignition? wouldn't that affect starting and idle?

It seemed that if I mess with the high idle screw, it got a little better, is there a sweet spot? As of now, the car isn't kicking down to low idle because the screws are out of whack.

Any help is very appreciated!

Posted on: 7/12 5:55:34
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Re: fuel delivery? Ignition?
Forum Ambassador
Joined:
2007/4/20 17:54
From Fresno CA
Posts: 15115
Assuming there is a vacuum leak the location could depend on how the car is optioned and if any modifications have been done. Since you are working on a 4GC then presume were are talking a 55-6 so here are places to check.

If you have the power brakes then there is a large port with a check valve on the rear of the carb and a hose going across the firewall to a tee fitting. One hose drops down to the brake unit and another goes on to the reservoir mounted under the fender. A leak could be anywhere along that route including in the brake unit or the reservoir canister itself..

Assuming the car is stock, the wiper motor gets vacuum from two sources which are connected together by a balance valve located just below the wiper motor. A line from a smaller port near the carb goes a short distance along the right head toward the firewall where it connects to a port on the balance valve via a short length of hose.

The other vacuum source is from the oil pump mounted aux vacuum pump. That line exits the block about midpoint on the right side at a port just above the oil pan, goes thru a check valve and then along the block toward the firewall where it bends up and via another short length of hose connects to a second port on the balance valve.

The wiper motor connects to the large end of the balance valve via a short length of rubber hose. If the car has a windshield washer the vacuum supply for it comes off a port on the narrow end of the balance valve. A leak could be in any of the hoses. If there is no windshield washer that port on the balance valve should be capped off but the cap could be defective or missing allowing a leak to occur.

If the car has had one of the various oil pump mods or conversions the crankcase aux vacuum pump will be gone. Depending on how that work was done would determine if a vacuum leak would be possible in any of the abandoned tubing. The entire run should have been abandoned and the port at the balance valve blocked off but some owners elected for the stock look and the line was left connected. Possibly the port was blocked under the aux hose but in some cars the mod either relied on the check valve to prevent a vacuum leak or possibly the line was capped off at the end where it previously entered the block. Either location would need to be verified as not a source of a leak.

If the car had an electric wiper motor installed but still kept the windshield washer you would need to verify the old wiper port on the balance valve is blocked off. If washer is missing then all of that vacuum supply should be blocked off.

Posted on: 7/12 7:14:05
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Howard
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Re: fuel delivery? Ignition?
Not too shy to talk
Joined:
2010/12/17 5:59
From Romeo, Michigan
Posts: 21
Thank you! I kind of checked some of the lines, but wasn't sure about the oil pump line. The line closest to the firewall that goes to the reservoir seems ok. I'll go over the lines tonight

Posted on: 7/12 7:32:40
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Re: fuel delivery? Ignition?
Forum Ambassador
Joined:
2007/3/14 16:01
From New Jersey
Posts: 15400
Doing diagnostics and getting some facts is always better than just guessing and throwing parts at a problem. Instead of the speculation, how about just putting a vacuum gauge on it and seeing if you do or don't have a vacuum leak. If not, then you can use the same setup to adjust the idle mixture screws on the carburetor.

Then put a timing light on it and check the ignition timing, adjust if needed, and with the light still attached you can test to see if the centrifugal and vacuum ignition advance systems are functioning. Then reevaluate the performance with a road test and we can go further from there.

Posted on: 7/12 7:44:15
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Re: fuel delivery? Ignition?
Not too shy to talk
Joined:
2010/12/17 5:59
From Romeo, Michigan
Posts: 21
Thank you for the response, since I don't own a vacuum gauge or timing light, I was looking for some easy inspection areas before taking it to a mechanic and paying them for a possible simple fix.

Posted on: 7/12 7:57:12
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Re: fuel delivery? Ignition?
Forum Ambassador
Joined:
2007/3/14 16:01
From New Jersey
Posts: 15400
Timing light and vacuum gauge are among the very basic tools for auto maintenance, if you're in the hobby at all seriously, make the investment. Or go together with some other local enthusiasts and buy them collectively.

Posted on: 7/12 8:35:59
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Re: fuel delivery? Ignition?
Not too shy to talk
Joined:
2010/12/17 5:59
From Romeo, Michigan
Posts: 21
One little tiny piece of (carbon?) material that sits in the valley of the rotor came loose and was sitting in the bottom of the distributor. Local O'reilly's had a Standard brand that worked fine. Carb dialed in and car runs fantastic

Posted on: Yesterday 8:56:48
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