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Re: Stinky 63 340hp
John;
Hope you all have a good fall show at Carlisle. As you fellows in the north are getting ready to hybernate, we're just beginning to appreciate why we chose to live here - 80's in the day, 60's at night, and never, never put the top up cause there won't be any rain either. I'll be putting 90% of my annual mileage on the Vette these next 6 months or so.
Popped my "A" carb back on this morning with a sandwich of SS base plates and gaskets in an attempt to stop the crappy fuel boil problem. So far, after a test drive, it came through the shut down heat soak OK (no boil). I can still smell gas in the garage, but it may be left over residual from testing the last few days. My "A" carb has standard needle & seats, but that doesn't mean I"m giving up on my "B" carb with the Tomco valves. It is a better (less worn) piece and I want to make it work. It also has the slightly richer Idle Speed Jetting of .037" vs. .035" of the "A" carb which my car seems to like. All the other jets are the same now for both ,and I swap the 16-389 metering rods back and forth for the slightly richer power step over the other 16-204's that I have.
We're having a show over at Old Town this coming Saturday night (Octobervettefest), and I want to get it running well enough to go there with my son and his 78 P.C.. It's not too often we can park them together at a show as we are usually separated by C-2 and C-3. This will be fun.
Stu FoxComment
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Re: Stinky 63 340hp
Though I only use this vehicle on occasion now I have the stinky problem too with my other been-around-a-long-time vehicle - a '69 GMC truck. I've scratched my head over this one too.
The float level has been checked, and rechecked for proper height as per specification .. . The carb has been rebuilt twice (new needles/seats, etc.) .. The problem with fuel dripping out of the venturi's when the engine in shut down continues. It's to the point now where fuel flows out through the throttle butterfly shaft..............and on to the intake manifold. Not GOOD! ..
The next thing to try is to lower the float level a bit and see if that might cure the problem. Hopefully, drivability won't be affected. If this doesn't work I'm thinking of installing a manual inline fuel shut-off valve controlled by a manual choke cable run inside the cab. The idea is to shut off fuel flow just enough when close to the destination to let the engine run and burn off carb bowl fuel, only, without being 'resupplied' ..Last edited by John G.; September 30, 2008, 04:50 PM.Comment
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Re: Stinky 63 340hp
Bill;
My sandwich is 2 SS plates and 2 gaskets. The nice SS plate is from a 300hp L-75 and is of quality to use directly on the base of the carb. The second SS plate is an aftermarket piece, and both gaskets are 4 hole w/o any cross cuts. The L-76 manifold has a front to rear cross cut on the R.H. bank, probably to pack up more volume between those cylinders. I have not seen any evidence of a lean condition on that bank so I did not try to maintain the cross cut pattern in my sandwich.
As stated before, I am running the floats down from the gasket @ 5/16" instead of the standard spec of 7/32" (+3/32"). Drop dimension remains @ 23/32" which is standard. I have only tried this (tested) this on my "B" carb equipped w/ Tomco inlet valves which pass a greater volume of makeup fuel than the standard needle & seats. I am now testing my "A" carb with the lower float setting using standard n/s's.
I am still looking for a syringe that I could use to suck the fuel out of the bowls at shut down, and prime the bowls at start up. It almost has to be like a battery tester with a very small hose to enter the vent tubes. If any one has seen such an item, please let me know where I might purchase one. That, needless to say, would be the last course of action. Beyond that, the only thing to do would be to remove the carb at each shut down, Ha! I'm getting pretty quick at doing that.
Stu FoxComment
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Re: Stinky 63 340hp
Stu,
Have you given any thought to trying the spacer then the stainless shield off a 300HP?
I know you have been through this but do you have the inital timing at 12-16*
Also, when you overhauled the carb did you run a wire through the air bleeds? I think if these get stopped up fuel will siphen up and out with expansion.Comment
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Re: Stinky 63 340hp
Stu,
A good permanent solution to the smell problem is a vent fan in the garage, like they put in bathrooms.
But fix the car first
Having fun,Jerry Fuccillo
1967 327/300 Convertible since 1968Comment
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Re: Stinky 63 340hp
I attempted this decades ago. The shield is of no use because the SHP manifold has no "heat slot" as on the cast iron 300 HP manifold.
The iinsulating spacer raises the carb, so the fuel pipe from the filter outlet to the carb inlet won't work. You'd have to fab a new pipe to proper dimensions, get it chrome plated...
Next I blocked the heat riser passages - both sides, but that was no good because the engine would stumble badly on start from a dead stop until the engine had been operating for at least 20-30 minutes around town, so that was not the answer.
Though I have not tried this, what I recommed now is blocking just one side of the heat riser - either side - doesn't make any difference which side. This creates a dead end exhaust passage under the carb. It allows some heat below the plenum to promote good fuel atomization, but not as much as if the passage is completely open side to side.
I might be the way to go in a warm climate when there are fuel percolation problems.
Remember that if the heat riser passage is blocked, even just one side the heat riser valve must be securely wired full open or replaced with the FI spacer. A closed heat riser valve and blocked heat riser package means you have completely blocked off the right side exhaust.
DukeLast edited by Duke W.; September 30, 2008, 06:22 PM.Comment
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Re: Stinky 63 340hp
Tim and Jerry;
Thanks for your responses.
- I run my initial timing @ 12*, and use a 236 VAC and a modified (quickened) mechanical advance curve.
- When I overhauled both carbs (3720SA ["A"] and the 3721SB [B]), I soaked them in my Gunk can, sprayed cleaner in all small passages, ran wires through the air bleeds, then blew them out/dry w/compressed air.
Note; I only test w/one at a time.
It appears to be a boiling fuel problem, primarily heat soak through the
L-76 aluminum manifold. Running a fuel hose from the filter has made no temperature difference, nor has going to an electric choke w/o the heated air tube.
My "sandwich" does not include a spacer because -1) I don't have one, and 2) the L-76 manifold is a slight "hi-riser" and I had tried a 3/8" spacer before years ago and the hood was popping open (can you say clearance problem, Ha!).
I may, just for the sake of study, try to re-create a worse case scenario where I am actually getting fuel flow puking out of the venturi's after shut down, then do temperature tests with my I.R. gun to see what the carb is seeing, the adjacent manifold areas, and the fuel filter. I should have done this before, just like the good old engineering lab test days at EMD.
We'll beat the 10 percenters yet (ethynol)!!
Stu foxComment
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Re: Stinky 63 340hp
The spacer is a good possible solution. Anything that will stop heat transfer by convection from the manifold to the carburetor will help.Stu,
Have you given any thought to trying the spacer then the stainless shield off a 300HP?
I know you have been through this but do you have the inital timing at 12-16*
Also, when you overhauled the carb did you run a wire through the air bleeds? I think if these get stopped up fuel will siphen up and out with expansion.
When the engine is running, the air passing through the carburetor will have a cooling effect on it. As soon as the engine is shut down, heat travels by convection into the carburetor and raises the bowl temperature.
Heat will also rise from the hot manifold by radiation so the non metal spacer won't eliminate all of the the heat transfer but hopefully it will reduce it enough to stop the problem.
If you remove the carburetor top fast enough on a hot engine, you can actually see the bubbles forming in the fuel.
It would be interesting to see if temp of a carburetor increase after hot shut down using an IR gun.Last edited by Michael H.; September 30, 2008, 06:33 PM.Comment
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Re: Stinky 63 340hp
Duke;
I'm just about ready to get out my tin snips to make me a coffee can block off for one side (or both) heat riser.
I am able to play with the sandwich under the carb a little bit because I am using a hose from the filter to the carb during my test phases. I also removed the choke heat tube and use an electric choke for testing as well. As I stated, so far I see no temperture difference at the carb, so neither "steel tube" was responsible for conducting heat to the carb individually or collectively - in fact, I was told by one member that the tubes might actually work to draw off heat from the carb instead. He has a lot more faith in the Chevrolet Engineers than do I - the same guys that gave us the 63 PCV system w/o a flame arrester that caused my crankcase explosion. I took my "Q" from the changes made to the 64 L-76 plumbing. Then there is the 355 VAC situation too.
Stu FoxComment
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Re: Stinky 63 340hp
Also Stu,
I forgot this. When you pull into the garage after a hot run, and maybe during your test, pull your gas cap to see that your gas tank is not holding pressure.
I have found that sometimes the valve on the vented cap will stick in the venting out. With the heat from the mufflers, which are close to the fuel line and tank, you could get some pressure in the tank which would force fuel into the carb after shutdown, and you get that drip, drip, drip into the air horn.
Just my theory, but it could also be just the heat from the engine rising to the top after shutdown and cooking the fuel in the carburetor (like my Grappa still).Jerry Fuccillo
1967 327/300 Convertible since 1968Comment
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Re: Stinky 63 340hp
Jerry;
I am currently using a replacement cap with the PRV (just bought and applied a few weeks ago). Prior to that, I had on the GM Chrome locking cap that uses the "nickle" key and a simple vent hole. My original cap (which I still have) also has just the simple vent hole. I have not seen any differences, but thanks for the suggestion. I'll keep an eye on it.
StuComment
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Re: Stinky 63 340hp
Michael;
As mentioned, I will really get into this now with my I.R. gun so I can find out what is happening. I'll keep you posted.
GO SOX!!
Stu FoxComment
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Re: Stinky 63 340hp
Use SS to make the shim - as thin as about .005" stock is okay.
Theoretically the '63 PCV system does not need a flame arrestor at the inlet on the air cleaner housing base because the flow should only be one way, and the valve anti-backflow valve design acts as a flame arrestor for that part of the system that has combustible gas. Unless the blowby is so bad that it is forces blowby gas "backwards" into the air cleaner this inlet segment of the system should not have combustible gas.
I can't even guess how many times my engine backfired at high revs due to the flaky distributor for the first 60K miles. If that design didn't protect well against a backfire propagating through the PCV system to the crankcase, mine certainly should have blown up!
When you park in the garage it would be a good idea to open the hood and leave the garage door open - at least for five minutes of so. The open hood and garage door will allow fresh air to circulate and cool of the engine via free convection.
You can probably always find something to do in the garage for five minutes.
DukeComment
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