Stinky AFB Follow Up

[Stuart F.]

To those of you who had participated in my previous threads about my heat soak problems with my AFB equipped 63 L-76 on which I have been running tests with two different carbs to try and reduce the stink in my attached garage after shut down;

Today I drove the car to my local ExxonMobil station and filled up with premium w/Max 2000 TEL additive. I have the 3721SB carb on now with Tomco Inlet valves and floats set down to 5/16" from the standard 9/32". I ran temp tests after I got home both running and after shut down. While running, there wasn't anything dramatic happening. After shut down, coolant passages in the manifold cooled down at an even pace. The fuel filter temp elevated about 20*, showing more temp closer to the carb. The carb itself elevated about 30 to 35 degrees. The Intake manifold also showed about a 20* temp elevation. The primary venturi main nozzles began to drip fuel, but not in a stream as before with the 3720SA carb which ran from the main nozzles and accelerator pump nozzles.

The only other change I made was to bend the fuel filter mounting bracket up slightly away from the water manifold (insignificant).

My conclusion so far is the heat or boiling/percolating fuel has nothing to do with the filter. In fact, as one of you suggested, my using a fuel hose in place of the steel fuel line from the filter to the carb may be a detriment as the steel may in fact draw off some of the heat from the carb instead of adding to it. I agree and will now reinstall the chrome steel line.

The top of the intake manifold is where the heat is coming from and it is heating up the carb significantly after shut down. I can only insulate the carb just so much w/o jacking it up until it hits the hood. I already have a sandwich of SS plates (2) and gaskets (2), and that hasn't helped much at all.

Tomorrow I will be changing my oil again and will remove the heat riser butterfly, which is already wired open, and replace it with an F.I. spacer. I have continued to see about 40* more temp in the R.H. manifold compared to the L.H. manifold even with it wired open, so I don't think that is as effective as it could be. Also, after I check my parts stock, I will pull my intake manifold to inspect the heat shield and replace it (the manifold) with either restrictor intake manifold gaskets or .050" SS shim stock (as suggested) to block/restrict the heat riser cross overs. I am certain this is the way to go now.

Thanks to all who have offered suggestions and advice. We'll get this solved yet.

Stu Fox

[Clem Z.]

Re: Stinky AFB Follow Up

To those of you who had participated in my previous threads about my heat soak problems with my AFB equipped 63 L-76 on which I have been running tests with two different carbs to try and reduce the stink in my attached garage after shut down;

Today I drove the car to my local ExxonMobil station and filled up with premium w/Max 2000 TEL additive. I have the 3721SB carb on now with Tomco Inlet valves and floats set down to 5/16" from the standard 9/32". I ran temp tests after I got home both running and after shut down. While running, there wasn't anything dramatic happening. After shut down, coolant passages in the manifold cooled down at an even pace. The fuel filter temp elevated about 20*, showing more temp closer to the carb. The carb itself elevated about 30 to 35 degrees. The Intake manifold also showed about a 20* temp elevation. The primary venturi main nozzles began to drip fuel, but not in a stream as before with the 3720SA carb which ran from the main nozzles and accelerator pump nozzles.

The only other change I made was to bend the fuel filter mounting bracket up slightly away from the water manifold (insignificant).

My conclusion so far is the heat or boiling/percolating fuel has nothing to do with the filter. In fact, as one of you suggested, my using a fuel hose in place of the steel fuel line from the filter to the carb may be a detriment as the steel may in fact draw off some of the heat from the carb instead of adding to it. I agree and will now reinstall the chrome steel line.

The top of the intake manifold is where the heat is coming from and it is heating up the carb significantly after shut down. I can only insulate the carb just so much w/o jacking it up until it hits the hood. I already have a sandwich of SS plates (2) and gaskets (2), and that hasn't helped much at all.

Tomorrow I will be changing my oil again and will remove the heat riser butterfly, which is already wired open, and replace it with an F.I. spacer. I have continued to see about 40* more temp in the R.H. manifold compared to the L.H. manifold even with it wired open, so I don't think that is as effective as it could be. Also, after I check my parts stock, I will pull my intake manifold to inspect the heat shield and replace it (the manifold) with either restrictor intake manifold gaskets or .050" SS shim stock (as suggested) to block/restrict the heat riser cross overs. I am certain this is the way to go now.

Thanks to all who have offered suggestions and advice. We'll get this solved yet.

Stu Fox

NOT .050 BUT .005 SS shim stock

[Stuart F.]

Re: Stinky AFB Follow Up

Clem;

Roger that. I didn't look at my notes.

Thanks;

Stu

[Stuart F.]

Re: Stinky AFB Follow Up

Just an interesting note in passing, as I continue my mods and experiments to reduce the fuel percolation problem; before I changed oil again yesterday and begin my heat riser changes, I put my "B" carb back on which features my most radical adjustments, i.e. floats (w/Tomco Valves) set at 5/16" (3/32" lower than stock adjustment of 7/32"). I then took a test drive and noticed an off idle hesitation/stumble that it never had before. Mid range and WOT worked fine (no starvation). Back home, I put the vacuum gauge on and found I could back out the idle screws another 1/2 turn and p/u an inch of vacuum (14"), but I still had the hesitation/stumble when I blipped the throttle. On closer examination I determined I no longer had any accelerator pump action. I dropped the pump adjustment down to 7/16" from standard 33/64" w/o any benefit.

Apparently, the 3/32" drop in float level is just enough to put the fuel level in the bowl just below the pump intake check ball valve - WOW! I had not changed anything else in the pump circuit, and I know the discharge check needle is in place and functions properly. The only thing that keeps this from being a major hesitation I suppose is the rich (63 shp) main jetting. The fuel begins to flow from the venturi nozzles almost instantly when you blip the throttle.

Sorry to bore you, but I thought it interesting as I never thought a carb could be so sensitive to small adjustments like this.

Stu Fox

[Clem Z.]

Re: Stinky AFB Follow Up

the lower the fuel level in the bowls the longer it takes for the fuel to be picked up by the venturi boosters. also make sure the idle transfer are open some because ITS take up the time it takes the fuel to get from the squirters to the intake because they are closer to the bottom of the carb. all carb systems must work together to get a smooth transition when you open the throttle

[Wayne K.]

Re: Stinky AFB Follow Up

Stu,

After many years of having fuel issues after shutdown and warped carb parts due to overheating of fuel and carb on my 67 L79 with a 3810 Holley I decided to replace the original with a new 3810 replacement. I didn't block off the exhaust transfer in the intake nor did I wire open or remove the heat riser as I live in a colder climate and both aid in warming up the engine. What I did do at the suggestion of someone that has more experience with these things than I was to make a spacer between the carb and the intake out of phenolic. The piece he gave me was either 3/8 or 1/4 inch thick. I used a gasket as a pattern and cut it out with a scroll saw fine tuning the size with different files. I smeared a thin coat of RTV on the intake and positioned the phenolic on the intake then put a regular gasket on top of the phenolic and then the carb. The height difference didn't bother any of the connections. After doing this I no longer had any boil overs from heat soak or experienced any warping issues. Just another idea that may help.

Wayne

[Stuart F.]

Re: Stinky AFB Follow Up

Thanks Wayne. I'm headed down thay same path myself. I know now that the percolation in my AFB is from the heat coming up through the manifold. I may yet close off the heat risers as I think I can live w/o them here in Florida. I have my heat riser butterfly wired open and intended to change it out to an F.I. spacer as I have all the parts. However, I don't expect to see much improvement there. As for the passages in the manifold, I may follow Clem's advice and block them off with .005" SS shimstock. What I'd really like to do is find a set of NOS F.I. intake manifold gaskets with the small oval holes in metal inserts that are part of the gaskets. I used to use them with much success, even up in Wisconsin. I also used stacks of fiber spacers (5 - 1/4" thick) under dual WCFB's on my drag racing engines and cut out the cross cuts to duplicate the manifold intake pattern.

I now have a sandwich of 2 L-75 SS plates and 2 gaskets on my 63, but it doesn't help much. I also have in my parts stock a laminated spacer which consists of 3 Aluminum plates and 2 gaskets which I may try using with 2 more new gaskets (top and bottom of laminate) to jack it up about 13/32". I tried this many years ago, but don't recall when and why, or whether the desired results were achieved. Probably not. I seem to recall having a vacuum leak with it and gave up on it. I think it was due to using the wrong top/bottom gaskets (perhaps w/exhaust passages like an
L-75). Then too, the areas around the hold down studs look compressed.

So, where did you get the phenolic material to make a spacer? Do you know of any existing spacer that I could adapt or modify to the AFB pattern?

Thanks again;

Stu Fox

[Stuart F.]

Re: Stinky AFB Follow Up

Clem;

Thanks for your AFB pointers regarding my latest observations. I suppose this may sound crazy and a bit contrary to normal thinking, but I recall that when I changed to the 236 (B28) VAC, I reset my ignition timing at the normal 12 degrees initial w/o vacuum advance, then connected the vacuum advance. Compared to with the original B20 VAC, my idle speed went up about 300 RPM. When I corrected back to the normal 750-800 RPM's, didn't I close off some of the ITS's? If so, would it have been enough to upset the transition? Remembering, the AFB on the L-76 application is unique to 62-63 applications only, and the 62 is a different model carb as well. My two units (3720SA and 3721SB) were never applied on an L-76. I have tried to duplicate an original 3461S with one or both in jetting, but maybe I am missing some other more subtle differences such as in the ITS characteristics.

Stu Fox

[Clem Z.]

Re: Stinky AFB Follow Up

Clem;

Thanks for your AFB pointers regarding my latest observations. I suppose this may sound crazy and a bit contrary to normal thinking, but I recall that when I changed to the 236 (B28) VAC, I reset my ignition timing at the normal 12 degrees initial w/o vacuum advance, then connected the vacuum advance. Compared to with the original B20 VAC, my idle speed went up about 300 RPM. When I corrected back to the normal 750-800 RPM's, didn't I close off some of the ITS's? If so, would it have been enough to upset the transition? Remembering, the AFB on the L-76 application is unique to 62-63 applications only, and the 62 is a different model carb as well. My two units (3720SA and 3721SB) were never applied on an L-76. I have tried to duplicate an original 3461S with one or both in jetting, but maybe I am missing some other more subtle differences such as in the ITS characteristics.

Stu Fox

take the carb off without changing the idle speed screw and see how much of the ITS are showing,it should form a "square" as long as it is wide.

[Wayne K.]

Re: Stinky AFB Follow Up

" So, where did you get the phenolic material to make a spacer? Do you know of any existing spacer that I could adapt or modify to the AFB pattern? "

The person that advised trying the phenolic cut a piece off some stock they had. He worked in a shop where they do vintage Ferrari restorations and the factory cars (some models) used this material between the carbs and intakes. Google the word phenolic and you should find some sources.

Wayne

[Clem Z.]

Re: Stinky AFB Follow Up

AFB carbed chevy engines came with a phenolic spacer under the carb. doc rebuild sells them

[Stuart F.]

Re: Stinky AFB Follow Up

Wayne;

Thanks. I'll see what I can find. I am leaning strongly in that direction.

Clem;

I did an inspection on my "A" carb that I last pulled off which was idling at 750-800 rpm, and using these old eyeballs, I'd have to say the ITS's are showing just slightly taller than wide (barely perceptible). A half turn on the idle set screw makes a lot of difference, and would also increase/decrease the idle speed about 200 to 300 rpm. So, I guess it could be said we're in the ballpark. It's not like we're dealing with a real wild racing cam here. Stll, I realize too that these adjustments are a lot more sensitive than I previously thought.

Thanks again. Appreciate your expert input. You've got me thinking.

Stu Fox

[Stuart F.]

Re: Stinky AFB Follow Up

Yes Clem. I believe those are for the L-75 engines (cast iron manifold with exhaust preheat chamber), but I think I can use one with the SS plate(s) to seal off that chamber effectively (similar to the L-75). Of course my manifold doesn't have the exhaust holes, so my concern would be for vacuum seal. It's worth a try. I'll give them a call tomorrow. Thanks.

Stu

[Clem Z.]

Re: Stinky AFB Follow Up

Yes Clem. I believe those are for the L-75 engines (cast iron manifold with exhaust preheat chamber), but I think I can use one with the SS plate(s) to seal off that chamber effectively (similar to the L-75). Of course my manifold doesn't have the exhaust holes, so my concern would be for vacuum seal. It's worth a try. I'll give them a call tomorrow. Thanks.

Stu

just put a carb to manifold gasket on top of the spacer also

[Howard M.]

Re: Stinky AFB Follow Up

FWIW. I installed a 5/16 phenolic spacer sandwiched between two regular gaskets under my 3814 (cal smog) Holly some 35 years ago. Never had a percolation problem in So Cal or here in Phoenix at 110*