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08-13-2010, 02:12 PM | #1 |
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Most troublesome carb!
Greetings all,
I have a Zenith -2 Carb giving me trouble. The darn carb causes any engine I put it on to backfire upon quick let-off of the accelerator. Over the years, I have rebuilt many carbs and always flow test the jets, clean out the passages, follow Steve Parteger's booklet guidelines and refer back to Moler's book and the Model-A.org info on line when necessary. The jet settings on the carb giving me trouble are: 168ml/mn on the main jet flow, 142ml/mn on the compensator jet, 44ml/mn on the idle, 170ml/mn on the cap jet and the float is set to provide the correct fuel level height when measured with an outside sight level tube. The carb idles great, the mid range power is smooth and strong. The top end power is good. The GAV adjustment (closed to 2-3 turns open) causes no change in the backfiring problem. The GAV causes the typical reactions to adjustment when the engine is cold at start-up but limited change once the engine is warm. This would typically indicate a lean condition. My next step is to oversize the main jet to see if it will correct the problem but.... With the jets I have in it...it should not be lean unless something else is entering into the picture. Slightly choking the engine is real difficult to do when driving and letting off of the gas quickly...I can't tell if it helps much. I have two Model A's with very good running carbs. When I switch their carbs to this particular owner's car the car will not backfire. Likewise when I switch this offensive carb to one of my vehicles, it will cause backfiring on my A's. The problem must be in the carb...but what am I missing? Thanks in advance for any help offered! Is there such a thing as a "devil" carb? Good Day! Dave in MN Last edited by Dave in MN; 08-16-2010 at 08:30 AM. |
08-13-2010, 02:36 PM | #2 |
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Re: Most troublesome carb!
Just a thought, but could it be a crack in the housing? especially the upper housing. It must be drawing air somewhere its not suppose to. This would cause a lean condition that couldn't be tuned out. Good luck.
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08-13-2010, 02:52 PM | #3 |
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Re: Most troublesome carb!
my second guess was a crack in the housing also....First guess... if it came off Bob A*****'s then it IS possessed.
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08-13-2010, 04:47 PM | #4 |
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Re: Most troublesome carb!
I did check the carb over pretty good...and I see nothing suspect. I even completely stripped it and soaked it a second time in carb cleaner, glass beadblasted and re-painted it. In other words, I looked at the bare casting also.
I'll look it over again for a crack that might open up as the carb gets hot. Thanks! Anyone else? Dave in MN Preacher...not Bob's carb...his was actually pretty good. It's Joe C***A's from Stillwater... Last edited by Dave in MN; 08-13-2010 at 05:01 PM. |
08-13-2010, 05:12 PM | #5 |
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Re: Most troublesome carb!
Lack of response from the GAV when hot might indicate that this circuit is restricted when hot. I cannot think of anything that would cause this. I don't have a Zenith on a car right now, so I can't check to see how sensitive the GAV control should be when hot. Do the "good" carburetors that you tested have GAV response when hot?
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08-13-2010, 06:01 PM | #6 |
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Re: Most troublesome carb!
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08-13-2010, 06:39 PM | #7 |
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Re: Most troublesome carb!
How about a warped flange sucking air where it bolts to manifold. Try a flat file across it and see what you have.
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08-13-2010, 09:02 PM | #8 |
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Re: Most troublesome carb!
I try to get the main jet around 159, comp 151, cap 173, idle 44-48. Rex Reheis says to set the float 1-5/16" form the top surface to the solder seam on the float, this is what I have done for years of carburetor rebuilding.
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08-13-2010, 09:27 PM | #9 |
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Re: Most troublesome carb!
Hi, What is the size of the venthole in the fuel bowl ???
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08-13-2010, 09:29 PM | #10 |
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Re: Most troublesome carb!
No gasket on the idle jet!!! some people put one on! not suppose to have one!
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08-14-2010, 12:06 AM | #11 |
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Re: Most troublesome carb!
Have you checked the position of the throttle shaft in relation to the idle ports in the throttle bore? Maybe the factory drilled one or the other in the wrong place. Compare your other carbs for how far the throttle plate is open at idle. If the problem child is significantly different, investigate further.
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08-14-2010, 07:14 AM | #12 |
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Re: Most troublesome carb!
Put an o ring on the throttle shaft (outside away from the engine) so that you for sure seal up the throttle shaft from sucking air.
Also sounds like an idle circuit TOO LEAN condition to me. Try two gaskets where it connects to the manifold. Also might try an o ring under the spring of the idle jet adjusting screw to make sure there is no air leaking there as well. Let us know what it eventually was...as you will find it. Lastly, try switching top and bottom halves of your known to be good carbs with this one and see if the problem follows bottom or top half of the offending carb. Dennis L Oberer Green Bay WI |
08-14-2010, 08:39 AM | #13 |
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Re: Most troublesome carb!
If it were mine I'd be looking for a vacuum leak, Manafold to block or maybe the vacuum line to the wiper, if you have one.
just my W.A.G. JB |
08-14-2010, 12:06 PM | #14 |
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Re: Most troublesome carb!
OR, just let it bring out the kid in you. A high point of driving a Model A back in the day was to go down a hill with the GAV shut and spark lever all the way up and listen to it pop and bang! I remember a classmate impressed with a buddies' A because "it popped 40 times going down a hill!".
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08-15-2010, 05:02 PM | #15 |
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Re: Most troublesome carb!
Go back and read again what MikeK said.
Steve |
08-16-2010, 07:52 AM | #16 |
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Re: Most troublesome carb!
Thanks Guys...
I'll go back and try again...paying particular attention to the idle circuit and the also check the carb/carb gasket @ the 1/2 line. I'll let you know what I find. Dave in MN |
08-16-2010, 10:01 AM | #17 |
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Re: Most troublesome carb!
Dave - there are good suggestions listed above, but there is one I don't see. Have you checked the length of the jets? If they are not of the correct height, then float adjustment won't be right.
Just a thought.
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08-16-2010, 11:43 AM | #18 | |
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Re: Most troublesome carb!
Quote:
I am going to try fresh gaskets at the carb 1/2s and refresh the gasket at the manifold to carb tonight. Thanks Ron, Dave in MN |
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08-16-2010, 08:36 PM | #19 |
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Re: Most troublesome carb!
When you have it off look at the throttle valve disc, sometimes the edges near the throttle shaft wear , more flow there, less flow over the idle port.
Hold it up to light and check how well it fits the bore. Having a torn gasket hasn't made any difference to how my car runs --it is broke in several places some stuck to the top-some to the bottom --i just wiggle it so the breaks match up then tighten the bolt ---i have a new gasket ---somewhere |
08-17-2010, 07:30 AM | #20 |
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Most troublesome carb! Some improvement....
Greetings all,
Thanks for all the suggestions so far. Last night I re-checked all of the following: installed jet height in the lower body: main @ 3/8" & cap 7/16" below parting line, throttle shaft fit was tight, almost 100% sealed at throttle vane, idle port opening looked good but it is one of the larger (opening size) versions. I did get considerable improvement when I placed a 56ml/mn idle jet in the carb. The idle air ajustment settled in at 1.5 turns out from the seat. This flow is about 16% richer than the maximum spec in Pargater's booklet. My next step is to move up to about 60-64ml/mn flow on the idle jet and re-test. I should have tried a larger idle jet earlier...I was stuck on not going out of the suggested range of flow with the jets. At this point, I am about 10ml/mn rich on the combined flow of the compensator and main jets. I will bring those back into spec and keep the idle jet over spec by 16-33%. I will post again and let you know the results. Dave in MN Last edited by Dave in MN; 08-23-2010 at 09:54 AM. |
08-17-2010, 08:48 PM | #21 |
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Re: Most troublesome carb!
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!
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08-18-2010, 11:27 AM | #22 | |
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Re: Most troublesome carb!
Quote:
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 |
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08-18-2010, 06:39 PM | #23 |
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Re: Most troublesome carb!
Dave: Can you tell me the name of books you were reading on the idle port openings? Thank you. Dick H.
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08-20-2010, 07:56 AM | #24 | |
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Re: Most troublesome carb!
Quote:
Idle ports.pdf Good Day! Dave in MN |
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08-20-2010, 08:02 AM | #25 | |
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Re: Most troublesome carb!
Quote:
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 Last edited by Dave in MN; 08-20-2010 at 08:09 AM. |
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08-21-2010, 06:41 PM | #26 |
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Re: Most troublesome carb!
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08-23-2010, 08:12 AM | #27 |
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Re: Most troublesome carb!
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 Last edited by Dave in MN; 08-26-2010 at 11:04 AM. |
08-23-2010, 09:16 AM | #28 |
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Re: Most troublesome carb!
find yourself a Zenith 1 ,or 3, or a Tillotson X, or XF, Zenith 2s have a questionable reputation.
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08-23-2010, 11:04 AM | #29 |
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Re: Most troublesome carb!
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 |
08-23-2010, 02:31 PM | #30 |
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Re: Most troublesome carb!
[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 |
08-23-2010, 05:55 PM | #31 |
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Re: Most troublesome carb!
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 |
08-23-2010, 06:28 PM | #32 |
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Re: Most troublesome carb!
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 |
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