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PostPosted: Wed May 25, 2016 5:14 pm 
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Turbo EFI
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Still trying to get my art car roadworthy. It's a 64 Dart with a 225 and a Holley 1920. I believe the carb was a remanufactured unit (I know...), but I got it from the same friend who I got the engine from, and it was reportedly running fine before we pulled the engine from the donor car a year and a half ago.

The engine behaves like it has a big vacuum leak. When hot, it will only stay running with the choke engaged. It won't run at all below ~1500 RPM, and the required amount of choke to keep it running drops off with higher RPM. I checked everywhere with a can of starting fluid and can't find a leak, including carb base, vacuum lines, and intake manifold to head connection. Blocking off the PCV line helps a little, but not enough. Either I'm missing a big source of unmetered air, or its just not getting enough fuel in the main circuit. I took the bowl off, and everything looked clean. No obvious gunk. The main jet was clear. I tried stepping up two jet sizes and it didn't make much difference, if any. There may be a clog somewhere deeper in the carb, but who knows where. I installed a new fuel filter before firing it up the first time, but I'm not sure how old the gas in the tank is.

The kicker is that it was behaving fine in the garage yesterday (in neutral). Then when I was idling it today to check the trans fluid, it suddenly died and started exhibiting the above symptoms upon restart. The only thing I know that has changed in the last day or so is the exhaust manifold choke stove broke, but I'm not sure if it was even working properly before. I don't recall the engine running with very much choke engagement before that. You don't just develop a huge vacuum leak that suddenly, and I can't find a vacuum leak anyway, so I'm thinking something in the main fuel circuit became partially clogged.

I was planning to take this thing on a road trip this weekend. Anything else I should check before tearing this carb apart?

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PostPosted: Wed May 25, 2016 5:41 pm 
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SpaceFrank wrote:
It won't run at all below ~1500 RPM, and the required amount of choke to keep it running drops off with higher RPM. (…) Either I'm missing a big source of unmetered air, or its just not getting enough fuel in the main circuit


The main circuit sounds fine; it's the idle circuit you should be suspecting.

Quote:
I took the bowl off


Hope you've got a new bowl gasket to reinstall it with, otherwise odds are excellent of a fuel leak right over the hot manifolds. While the carb is open, take off the upsized main jet and put the other one back in.

Once the bowl is back on, do these things:

-look down the carb and find the two holes, one small and one real small, facing the sky next to the front wall of the carb throat. Blast each hole with carburetor cleaner -- put the straw directly on the hole and spray for several seconds.

-Manually put the fast-idle cam in place and tighten the fast-idle screw so it'll hold the throttle open enough for the engine to run at a very fast idle. Start it. Close the choke as much as you can without killing the engine. With the engine running, remove the idle mixture needle. You may need a quick hand on the choke to keep the engine running once the needle's out. Blast the carburetor cleaner in through the idle mixture needle hole. Keep spraying and spraying; you may have to spray in bursts to keep from stalling the engine. The idea is to flush the trash out of the idle circuit. When you're done (or you run out of carb spray), shut off the engine. Reinstlal the mixture needle, lightly(!) seat it, back it off three full turns and then turn it back in 1/2 turn. Un-tighten the fast idle screw back to where it belongs, open the choke, and drop the fast idle cam out. Now see if it'll idle. If so, adjust the idle mixture and speed to where they should be.

(Use a good brand of carb spray; I prefer Berryman's Chemtool B12. Also, give a few shots around the idle mixture needle before removing it so you won't be blowing trash into the idle circuit.)

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PostPosted: Thu May 26, 2016 11:26 am 
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Turbo EFI
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I don't know why I didn't suspect the idle circuit. Did I mention I got 2 hours of sleep the night before I wrote this? :oops:

Thanks for the guidance on cleaning, Dan. I'll pick up a can of B-12 on my way home from work.

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PostPosted: Thu May 26, 2016 8:07 pm 
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Well, that did it. Thanks! It runs fine now.

I swapped the other jet back in, but the bowl gasket appears to be in good shape and isn't leaking yet. I went ahead and ordered one just in case.

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PostPosted: Thu May 26, 2016 8:08 pm 
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Hoorah for easy fixes! Bring that can of B12 along with you.

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PostPosted: Fri Jun 10, 2016 10:13 pm 
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I recently rebuilt my 1920 carb. Before I did it I didn't have a vacuum leak. Now it looks like I have one where the throttle shaft goes into the body. After I rebuilt the carb I also noticed that I have an oring that wasnt in the kit and did not get replaced. It looks like a small rubber band. Any idea where it goes? Are my two problems related?


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PostPosted: Tue Apr 12, 2022 6:30 am 
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Turbo EFI
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It's been almost 6 years since Uncle Dan helped me un-clog the idle circuit on a Holley 1920. Now I think I actually am having a problem with the main circuit, on the exact same carb, in the exact same car (but on a different engine, for reasons too complicated too explain here). Time is a non-Euclidean circle.

Long story kinda short, in the intervening years I've been using this car for parade duty and not much else. I recall it running hot in the summer of 2019 when I drove it in a July 4th parade. After that, it sat for over 2 years since there weren't a lot of parades during COVID. My friend who was storing the car for free moved to a new place last year, so I had to go collect the car, and it overheated pretty badly when driving it home on the highway.

While getting it ready for the Houston Art Car Parade the past couple weeks, I noticed it would hold temperature just fine as long as you cruised below about 35 mph (call it 1700 RPM in 3rd). It sounded a little rough during hard acceleration, but seemed to pull just fine. Above about 35 mph, the temp would climb without limit, but it would recover once you got down to 30 or so (although it would sputter and misfire while slowing down from highway speeds in gear). I went completely through the coolant system. Radiator professionally cleaned. Water pump inspected to confirm impeller wasn't spinning on the shaft. Block flushed multiple times. Whole system soaked with Evaporust overnight. None of it made a difference in the cooling behavior.

I moved on to mechanical considerations. Verified timing mark on the balancer with a piston stop tool, and measured initial timing around 8° BTDC. Total mechanical timing was only about 21°, which makes me think one of the advance weights might actually be hanging up; I can't imagine I welded up the slots on this dizzy. Compression test results were fine. Then I took a hard look at all the spark plugs (ZFR5N) and decided they looked awfully light tan, which makes me suspect it's actually a fueling problem. Based on prior experience, 1700ish RPM would be right around where the main circuit starts doing most of the work.

I pulled the carb bowl off, removed the main jet (it's a 58), and cleaned everything with spray carb cleaner. Spraying directly into the hole where the main jet goes seemed to just deflect a lot of spray back toward me. I put the bowl back on, disconnected the main fuel line from the pump, and ran a line to the carb from a funnel filled with a mix of ethanol-free 93 and Berryman's B12. Ran it for a while, goosing the throttle, until the funnel was just about empty, and then let it sit overnight. Hooked everything back up in the morning, and it made no effect on the overheating behavior. I should perhaps mention here that the accelerator pump appears to be working correctly.

At this point I'm ready to just order a rebuild kit and tear this carb down completely. Anyone have any other suggestions?

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
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PostPosted: Wed Apr 13, 2022 8:44 pm 
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Eventually the Holley 1920's main metering block grows clogged with a kind of metal mould—powdery corrosion that cannot be cleaned out or otherwise remedied. Especially "remanufactured" carburetors that have been abusively cleaned so as to strip off the anticorrosion surface treatment of the carburetor body and metering block. Yours may or may not have reached that point, but the odds lean (as it were) a little against success by tossing in a gasket kit. Nevertheless, carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download.

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PostPosted: Sat Apr 16, 2022 11:31 am 
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Turbo EFI
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Interesting. Is there a good way to test whether a metering block is clogged in this manner? I have another spare 1920 that I could check out if this one is borked.

Thanks for posting those links; I'll be reading through those before I tackle the rebuild.

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
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PostPosted: Sat Apr 16, 2022 4:58 pm 
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SpaceFrank wrote:
Interesting. Is there a good way to test whether a metering block is clogged in this manner?
Yeh: the carb makes problems no matter what you do to it.

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