GAV settings with hi comphead & B cam grind

Steve_Mack_CT

Steve_Mack_CT

Senior Member
Curious if anyone else with a similar set up finds that a slightly richer mix to be optimal. I have a fresh engine (maybe 5 hours on it so far). Snyder's 5.5 head, B cam grind and 0.100 overbore. Runs great with exception of stalling twice on deceleration. GAV essentially closed as I would ordinarily run once up to temp. Before looking at the float I am wondering if the issue could be as simple as running too lean. I am unsure if these minor mods would call for a richer mix but have also read issue could be as simple as a lean mixture.

I don't seem to have an issue on corners which I know could be flooding - tough to tell for sure as it happened quick but felt more like starvation vs flooding. I know some variables at play here but am curious what others experience has been woth a similarset up.

Thanks, Steve.
 
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My car has a high compression head and cam from Burtz. I had to richen up the mixture on my Weber carburetor. I know the Zenith carburetors have a tendency to stall on deceleration.

You may want to experiment with the idle mixture. On the Zenith, try screwing the adjustment screw in 1/4 turn in at a time. As far as the GAV is concerned, that depends on many factors and I suggest you just experiment with it's setting to see what works best.
 
Yes, it's entirely possible that it now needs a bit richer GAV (and possibly idle mix) setting. Give it a try...the engine will tell you what it wants.
 
I don't see how your set up would change the air to fuel requirements?
Just because you have a slightly higher compression ratio that shouldn't be causing a rich condition. If anything, the hotter mixture would cause a lean condition due to hotter or faster burning of the fuel. Often timing must be adjusted more than anything.

Once you set the fuel level at 5/8" below the gasket line with an external sight tube, no condition in the carburetor good or bad would ever require this to be changed. The jets and the adjusting needles control how it runs not the level in that float bowl. That is just a reserve so that you can go up and down hills without starving the jets. The Jets pull fuel from the very bottom of the carburetor, the fuel level never ever gets close to this low, even if the car was on a sharp angle.
 
RENNERS CORNER said:
I don't see how your set up would change the air to fuel requirements?
Just because you have a slightly higher compression ratio that shouldn't be causing a rich condition. If anything, the hotter mixture would cause a lean condition due to hotter or faster burning of the fuel. Often timing must be adjusted more than anything.
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To be fair, he actually said he's running a different or rebuilt engine with larger pistons and a more aggressive cam.

On the other forum, you recently said "Even though the compensator jet under gravity might keep up with supplying the idle jet while only running on the idle circuit, once you involve the cap jet, pulling away that idle jet source somewhat, the return to idle from high speed, the idle circuit might struggle due to a undersized compensator jet."

I wonder if that's happening here, but it wasn't noticed before because the volume of air at a given RPM was lower in his previous engine.
 
Appreciate all the input here gents. Going to try the simple adjustments and will keep the group posted.

Appreciate any and all thoughts!!

Oh - been a while since I have been here. Car in question is our new to us 31 Deluxe Roadster. Just updated my avatar.. ��

Steve
 
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alexiskai said:
To be fair, he actually said he's running a different or rebuilt engine with larger pistons and a more aggressive cam.

On the other forum, you recently said "Even though the compensator jet under gravity might keep up with supplying the idle jet while only running on the idle circuit, once you involve the cap jet, pulling away that idle jet source somewhat, the return to idle from high speed, the idle circuit might struggle due to a undersized compensator jet."

I wonder if that's happening here, but it wasn't noticed before because the volume of air at a given RPM was lower in his previous engine.
Click to expand...


Very possible, that's why knowing your jet flow rates is so important.

You can use the flow data to make educated changes in jet sizes to fix problems.

Though often I think these engine hop-ups like heads and cams don't make as much difference as we might think, not that you don't notice them, just that they might not require as drastic of changes to the other components as we might think....
The Zenith carburetor is easy to make rich, a half turn of the GAV can add typically up to 75 more cc of fuel than the engine typically can use.
This is another reason why all of my Cap Jets flow 300cc so that the GAV can actually add a good amount of fuel if needed, it basically becomes a pathway for metered GAV and metered Compensator fuel not a controlling orifice.

Well hopefully we are kept updated....
 
I installed the Snyder's 6.1:1 head and had a similar experience. I toyed with the idle screw a bit and it went away. Even on the coldest starts, I never need to open the GAV....it runs too rich even at a 1/4 turn..causing fouled plugs. Otherwise it runs much better with the higher comp head. I live in a very hilly area and the added power allows better hill climbing without down-shifts. The ONLY down-side of the new head is that it increased blow-by and caused a fine mist of oil accumulating under the oil pan which had been sealed with silicon. I dropped the and installed the correct gasket...and the problem went away.
 
Thanks again guys - RENNERS CORNER will keep you updated but won't get anywhere with it till the weekend. That said, 75 degrees here in CT. Want to be anywhere but here in my office working....

This is our third A - I was pretty regular here 2004 - 2010 ish & rejoined in 2015.
 
I run a 6:1 head on a Model B engine with a stock Model B camshaft and a Zenith carburetor. The slide has my GAV setup & driving habit. I time the ignition with the spark lever down 5 notches because the 6:1 head needs to have more spark retard ability than the stock compression head. The gasoline I use is 87 Octane with 10% Ethanol.
 

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Well a couple folks were interested in an update. Did 50 miles over the weekend and stalled once pulling into a parking lot coming off a relatively hi speed road (40 - 45 mph). I had adjusted mix a little richer but GAV was closed or close to closed. So I think float adjustment is needed as this time no hill was involved.

Otherwise a great run - the extra power, whatever it is in HP really makes a difference as we have a lot of hills in NW CT and the car really didn't struggle at all.
 
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