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Old 01-31-2020, 08:26 AM   #1
marty in Ohio
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Default loping engine?

A friend's Model A has a problem with a loping engine. The car hasn't been run for several months, but was running O.K. last time. Now the engine idles fine but on giving it a little gas it tends to run fast and slow on it's own. When we first started it the idle adjust screw was out about 8 turns. We turned it all the way in and then backed it out about 1 1/2 turns. The only things we did was a brake job, water pump replacement, and manifold replacement, none of which should affect how it runs. Any suggestions would be appreciated.
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Old 01-31-2020, 09:01 AM   #2
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Default Re: loping engine?

Gunk in the carb? A couple months could be enough for the ethanol gas to go bad and do god knows what inside the carb. Might try putting a can of seafoam in the gas and driving it for a while to see if it gets better.
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Old 01-31-2020, 09:10 AM   #3
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Default Re: loping engine?

Check to make sure you don't have a manifold leak. Also check to see that the throttle linkage returns properly.
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Old 01-31-2020, 09:18 AM   #4
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Default Re: loping engine?

I agree with what the fellas said.
There may be a problem since the idle mixture screw was turned out so far. It should not be like that. [Its an air mixture screw, not fuel].
Also a manifold replacement can affect the way the engine runs.
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Old 01-31-2020, 09:31 AM   #5
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Default Re: loping engine?

Grab your distributor cam and try to move it side to side. If that bushing is worn, the cam can float and cause hunting.
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Old 01-31-2020, 09:46 AM   #6
AL in NY
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Default Re: loping engine?

Turn the gas off to the carburetor and then drain the gas out of the carburetor if you are not going to run the car for weeks or months. This will prevent carburetor problems in the future.
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Old 01-31-2020, 10:22 AM   #7
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Default Re: loping engine?

All the above plus the possibility of sluggish valves. A little Marvel Mystery Oil directly down the intake and four oz. per tank full of gas is the way to go.
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Old 01-31-2020, 10:30 AM   #8
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Default Re: loping engine?

If you have a spare carburetor, stick that on just to try and zero in on the source of the problem. Also before doing that spray some carb cleaner around the manifold gasket to see if there is a leak. I try to do simple things first to isolate the problem before messing around too much with things that aren't broken.
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Old 01-31-2020, 10:43 AM   #9
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Default Re: loping engine?

Changing the manifold was likely what affected things. This requires removal a fair chunk of the fuel system. A person stands a chance of several manifold pressure or fuel delivery discrepancies happening during that process. If the old manifold had some leakage before then the carburetor would have to be readjusted to compensate. If the current manifold has a leak then that is a problem. The engine will run lean which can cause hickups if the mixture can't not be compensated for by idle speed or mixture adjustments. Removing a carb can move the float around inside there and any crud that might be movable too. Disconnecting the choke control and associated mixture adjustment there could cause changes to that adjustment as well.
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Old 01-31-2020, 11:56 AM   #10
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Default Re: loping engine?

Its running rich !!! Close the GAV and see if that helps .
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Old 01-31-2020, 01:28 PM   #11
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Default Re: loping engine?

Thanks for all the suggestions. We will try each of these until we find out what is causing the problem. My friend said the plugs were wet when he checked them after a short run. We closed the GAV and then opened it "a bit" I know the GAV on my car likes about a quarter open.
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Old 01-31-2020, 02:39 PM   #12
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Default Re: loping engine?

Ah manifold replacement,gland rings installed? Move it outside and spray starting fluid on manifold with engine running if it speeds up manifold not seating properly.
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Old 01-31-2020, 03:44 PM   #13
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Default Re: loping engine?

Quote:
Originally Posted by rocket1 View Post
Ah manifold replacement,gland rings installed? Move it outside and spray starting fluid on manifold with engine running if it speeds up manifold not seating properly.
If you're spraying that stuff around the manifold area, it's easy to get a false result. It only takes a little overspray to get to the carburettor mouth and you have it entering the engine, causing it to change the way it is running.I use a piece of tubing (like an oversized vacuum cleaner hose) to make the carb draw air from well away from where I'm spraying. The engine is only idling so the hose doesn't have to be very big.
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Old 01-31-2020, 03:59 PM   #14
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Default Re: loping engine?

That is a great tip
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Old 01-31-2020, 06:14 PM   #15
marty in Ohio
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Default Re: loping engine?

Good thinking Synchro!
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Old 02-02-2020, 08:09 AM   #16
marty in Ohio
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Default Re: loping engine?

Thanks for all the help, guys. We've pretty much tracked it down to a bad gasket. A new one is to be shipped and we will install this week. Will update later.
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Old 02-04-2020, 11:15 PM   #17
Chuck Sea/Tac
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Default Re: loping engine?

I assume you meant manifold gasket? Did you machine or check the flatness/mating of the two manifolds? IE intake and exhaust. They have to be the same height or when you bolt them down it will act like a bad gasket, but really it’s one manifold can’t get pulled in tight to the block.
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Old 02-05-2020, 06:53 AM   #18
marty in Ohio
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Default Re: loping engine?

Yes, Chuck,
We had the manifolds machined to be certain they are flat. We are going to install them this morning and see what happens.
Thanks for the post.
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Old 02-05-2020, 05:54 PM   #19
marty in Ohio
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Default Re: loping engine?

Well, we put the manifolds back on all tight and sealed. We started the car and the loping was back. We tried a different carburetor and the engine ran smoothly. We took apart the original carb and couldn't find any wrong with it. Put it back on and the problem was back. This is a Zenith 2 and I guess it needs to be rebuilt. Any other suggestions?
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Old 02-05-2020, 06:17 PM   #20
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Default Re: loping engine?

Take it fully apart and make sure you can blow through every passage. Hit the flange of the carb and find the low spots. File until the whole surface is shiny. No low spots. New gaskets and try it again. Check the flatness on both carbs. Maybe one is flat and the other is not. Good luck.
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