Most troublesome carb!

[Carb Guy]

What do you mean by "a larger idle port opening" They are all the same, sounds like someone in the past drilled the passage out and screwed up the port opening. This could be your whole problem. This passage should NEVER be drilled out!

[Dave in MN]

Originally Posted by Carb Guy

What do you mean by "a larger idle port opening" They are all the same, sounds like someone in the past drilled the passage out and screwed up the port opening. This could be your whole problem. This passage should NEVER be drilled out!

Thank you for confirming that the port should not be drilled. The opening has not been drilled out.
Now don't quote me on this but I was just reviewing a couple carb books last night and I think I noticed there were 3 distinct designs of this idle mixture opening. The design in this carb top is the design that matched one of the larger area openings.

Side note: This past winter, I was helping a group of friends rebuild some carbs on a Saturday morning and one gent made the mistake of drilling out the idle mixture opening we are speaking of. I thought the carb top was "toast" but he finished putting it together and tried it on his car. He said it works fine... This one time is my only experience with this happening so I can't speak to whether drilling out the idle mixture opening is often fatal to the opperation of the carb.

BTW: I still have not solved the problem with the carb but I have not given up!
Thanks,
Dave in MN

[Carb Guy]

Dave: Can you tell me the name of books you were reading on the idle port openings? Thank you. Dick H.

[Dave in MN]

Originally Posted by Carb Guy

What do you mean by "a larger idle port opening" They are all the same, sounds like someone in the past drilled the passage out and screwed up the port opening. This could be your whole problem. This passage should NEVER be drilled out!

Here is the info on the idle ports from Steve Pargeter's Version 7a booklet.
Idle ports.pdf
Good Day!
Dave in MN

[Dave in MN]

Originally Posted by Carb Guy

Dave: Can you tell me the name of books you were reading on the idle port openings? Thank you. Dick H.

One book: Steve Pargeter's ZENITH MODEL "A" CARBURETOR RESTORATION GUIDELINES Version 7a

Check back to your previous request on ports...I have attached a pdf of the idle port info.

Pargeter's book is a very good reference for carb restoration/tuning. It is my most used reference source when setting up carbs. I also refer to books by: Paul Moller, Gordon Biggar (Rex's methods) and the carb info on Model A .org.

The carb I am having trouble with has the "Port 1" shape. Since I started this post, I have spent an additional 4 hours tweaking the jets, and adjusting other items with no improvement to the backfiring problem. I will replace the "good looking" throttle vane with a new one this evening. I am about ready to junk the carb body and start with a different core.
Frustrated in MN!

Good Day!
Dave in MN

[Carb Guy]

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Thank you Dave. I don't have Steve Pargeter's book. I am pretty much a Rex R. man. He has done thousands of them, and his methods have worked real well for me. Send me your carb and I will take a look at it and run it on my car. Of all the carbs I have rebuilt over the years, I don't remember seeing the other 2 port openings. Never to old to learn something new. Thank you. Dick H.

[Dave in MN]

Success.....The carb has been brought into normal opperating ranges.

Here are the last four things I tried and each of them improved the operation of the carb:
1) Plug gap was varying by .003" Adjusted to .032 (Still backfired upon quick let-off of throttle between 35-50 mph when the engine was warm but idle and midrange operation were better.)
2) The thread fit between the idle air needle and carb body was loose causing excessive air. Pipe dope was applied to the threads of the needle and the idle fell into perfect range with the correct size idle jet. (Still backfired but it was less.) This correction reduced the idle jet from 52ml/mn to 46ml/mn (about 12% reduction)
3) Replacement of the throttle vane. The throttle vane was within normal tolerances based upon my past experiences. With the effort to eliminate air/gas mixture from passing by and eventually into the muffler, I swapped out the vane with other used vanes. I found one that was a near perfect fit to the carb body. (Still backfired but it was better.) Correcting these three problems brought the adjustment of the carb back into normal limits of all jet sizes.
4) The muffler had 3 areas of rust-out with small pin holes. The muffler would leak the exhaust gas out or let air in these areas and cause a backfire. Installing a different muffler corrected the excessive backfire condition. What the Heck...
The problems with the carb were multiple...but the main problem I was chasing was outside of the carb and was caused by a defective muffler.
I learned from struggling with this carb. Now if I can just remember it all next time.
Thanks to all who posted ideas to help.
Good Day!
Dave in MN

[sunnyorm]

find yourself a Zenith 1 ,or 3, or a Tillotson X, or XF, Zenith 2s have a questionable reputation.

[Ron in Quincy]

No one mentioned to check the type of secondary well; there are 3 types; I prefer type 3 as its undercut at the location of the tip of the compensator jet. Also check the small hole at the top of the secondary well to be sure its open. Also be sure the air idle adjustment needle completly closes off when the needle is seated; some of the replacement needle tips are too short and don't reach the seat.

Ron

[Dave in MN]

[QUOTE=Ron in Quincy;66322]No one mentioned to check the type of secondary well; there are 3 types; I prefer type 3 as its undercut at the location of the tip of the compensator jet. Also check the small hole at the top of the secondary well to be sure its open. Also be sure the air idle adjustment needle completly closes off when the needle is seated; some of the replacement needle tips are too short and don't reach the seat.

I do not know which type well was installed but the openings on the secondary well were open. I had soaked the well in carb cleaner and then blew it out with high pressure air and observed the openings to be clear.
The needle to seat seal was checked and it was good...the carb body had a replaceable seat installed.
Ron, From your experience, what benefit would the type 3 secondary well with its undercut have in the average performance of a carb over the others? Thanks...
Good Day!
Dave in MN

[Ron in Quincy]

I always check the length of the comp. jet, find originals are very short, if too long the tip of the comp. jet will come up against the secondary well and restict the fuel flow.

I don't know about others, but I set my float level at 1" to the float seam from the upper housing surface with the housing inverted.

I also prefer to use original jets; have found they even vary in the size. After checking the orface to be sure it isn't damaged, I check the opening and measure it by thousands; I don't flow test jets. Over the years I have developed a combination that seems to work well; idles Ok, excellerates with no hesitation, and the engine doesn't die when making a fast stop.

To measure the jets, I use torch tip cleaners which I have pre -measured the smooth tip end so I don't damage the orface.
What I use:

Idle Jet .021
Compensating Jet .037
Main Jet .037
Cap Jet .038 or .039

Ron

[31coupeman]

Don't be surprised at all if you need to run richer on the fuel. Many people I know have found that the ethinal fuel mix demands this change in order to run right.

I dought very much it is the gasket at the split halfs.

The idea suggested of swaping out one half of the carb at a time is a good one to isolate the problem to the top or bottom half.

If all else fails get it magnifluxed for cracks