Tip-In Stumble

[Joe C.]

Re: Tip-In Stumble

Joe,

Sounds like the idle circuit is tuned good. At low speed cruise, mixture is available from idle port, transfer slots, and main circuit. Transfer slot is fixed and idle circuit sounds good. Do the plugs look like main jets could be increased two sizes?

I still think accellerator pump shot not timed properly. You need heavy shot with light movement of throttle. The volume is the same but timing needs to be increased by changing cam location or squirter nozzle size.

One other thing you probably have considered is timing. How much inital timing are you running because the centrifugal comes in quick with 365 HP engine. Engine is going lean and pop back through intake valve, I would think 12-14 is good.

Timothy,

Accel pump cam is in the #1 hole. I've never tried it in the #2 hole, but I'll do it today. Somehow I don't think that that's going to change things, either. I'll keep you posted. If one squeezes the throttle slowly enough, then accelerator pump should not squirt, correct. That's the situation where the lean pop occurs. I swapped in the stock .025 nozzle and car runs exactly the same, so they'll stay in place. I do not think that the accel pump shot is the culprit.

You might be right about the jets. I have stock jets (65/76) all around. Plugs burn light tan insulators. Dyno pull from 2 years ago shows A/F ratio @ WOT between 14.5 and 15:1, until the secondaries open, where it drops to 12.0 and slowly climbs back to 13:1 @ about 5300 RPM (see attached). I'll try 67's and 69's today.

Only one more idea (besides the pump cam and pri mains) that I have left. The far flung possibility that there is a problem with the emulsion tubes in the pri metering block due to a defective casting. Idle and main air bleed orifices are stock, never drilled, free and clear with compressed air and/or Gumout.

I reworked my distributor's advance map, by limiting max centrifugal advance to 21 degrees by modifying the advance weights, all in @ 2200 RPM. I have initial set @ 16 degrees, for a total @ WOT of 37 degrees, all in @ 2200. Vac can is a 236, verified to be fully deployed @ 17 degrees at about 8 in-hg.

[Clem Z.]

Re: Tip-In Stumble

Timothy,

Accel pump cam is in the #1 hole. I've never tried it in the #2 hole, but I'll do it today. Somehow I don't think that that's going to change things, either. I'll keep you posted. If one squeezes the throttle slowly enough, then accelerator pump should not squirt, correct. That's the situation where the lean pop occurs. I swapped in the stock .025 nozzle and car runs exactly the same, so they'll stay in place. I do not think that the accel pump shot is the culprit.

You might be right about the jets. I have stock jets (65/76) all around. Plugs burn light tan insulators. Dyno pull from 2 years ago shows A/F ratio @ WOT between 14.5 and 15:1, until the secondaries open, where it drops to 12.0 and slowly climbs back to 13:1 @ about 5300 RPM (see attached). I'll try 67's and 69's today.

Only one more idea (besides the pump cam and pri mains) that I have left. The far flung possibility that there is a problem with the emulsion tubes in the pri metering block due to a defective casting. Idle and main air bleed orifices are stock, never drilled, free and clear with compressed air and/or Gumout.

I reworked my distributor's advance map, by limiting max centrifugal advance to 21 degrees by modifying the advance weights, all in @ 2200 RPM. I have initial set @ 16 degrees, for a total @ WOT of 37 degrees, all in @ 2200. Vac can is a 236, verified to be fully deployed @ 17 degrees at about 8 in-hg.

any movement of the throttle should send a squirt of fuel. if the check ball bearing is not set correctly in the pump well it could not be sealing when you move the pump just a small amount. the idle transfer slots should do the job first as they cover the time it takes for the fuel from the squirters to reach the bottom of the carb. make sure the main body and throttle plate are flat and sealing.

[Timothy B.]

Re: Tip-In Stumble

Joe,

Clem has a good point about the check ball in the pump I think even with slow movement of the lever fuel should be present.

I would change the primary jets to 67 but not secondary, I doubt if that's the problem. Set timing at 12 just to see if that helps because you are running 33* idle timing plus what ever centrifugal is in at low RPM.

Probably nothing wrong with emulsion in metering block but if the carburetor is apart take a straight edge and check the main body primary and secondary and if you have a spare gasket check the bottom and the top and bottom of the throttle plate. On my 3810 the four corners where the main and throttle plate attach were slightly pulled from someone using thick gasket and tightening screws and needed to be filed flat. A internal vacuum leak could cause vacuum to pull air or fuel from somewhere other than transfer slots and booster venturies.

I think with patience you will get it right.

[Joe C.]

Re: Tip-In Stumble

Joe,

Clem has a good point about the check ball in the pump I think even with slow movement of the lever fuel should be present.

I would change the primary jets to 67 but not secondary, I doubt if that's the problem. Set timing at 12 just to see if that helps because you are running 33* idle timing plus what ever centrifugal is in at low RPM.

Probably nothing wrong with emulsion in metering block but if the carburetor is apart take a straight edge and check the main body primary and secondary and if you have a spare gasket check the bottom and the top and bottom of the throttle plate. On my 3810 the four corners where the main and throttle plate attach were slightly pulled from someone using thick gasket and tightening screws and needed to be filed flat. A internal vacuum leak could cause vacuum to pull air or fuel from somewhere other than transfer slots and booster venturies.

I think with patience you will get it right.

Clem/Timothy,

Over the years, I have rebuilt this carburetor.......twice. I normally don't do this, but finally sent it to Jerry Luck. He rebuilt/restored it beautifully!
Not to take away from his work, which is BETTER THAN TOP NOTCH, the carburetor, although it came back looking beautiful and new, worked in exactly the same way that it did before I sent it to him.

All of that being said, I can vouch for all surfaces being flat, all passages open, gaskets installed correctly, floats set properly (although I did raise it about 1/16" above where it was when Jerry sent it back to me), pump check ball in place and free-and-clear. That's not to say that the casting might not have a small irregularity preventing a perfect seal for the check ball!

[Timothy B.]

Re: Tip-In Stumble

Joe,

I think you should try to open the secondary throttle blades about 1/2 turn more on the stop screw to allow secondary to feed idle air/gas which will allow primary blades to be closed even if the transfer slots are covered. This way when the throttle is moved vacuum is acting directly on the transfer slots thus pulling mixture to cover up transition (along with accellerator pump) to main circuit. You may need to play with this adjustment and it's OK to cover the transfer slots on the primary.

Again, make sure that accellerator pump is squirting with movement of arm. #1 hole should give full shot.

I think the carburetor needs to be tuned to the 365 because it is used on the L79 engine so it is a compromise with factory settings.

[Clem Z.]

Re: Tip-In Stumble

Joe,

I think you should try to open the secondary throttle blades about 1/2 turn more on the stop screw to allow secondary to feed idle air/gas which will allow primary blades to be closed even if the transfer slots are covered. This way when the throttle is moved vacuum is acting directly on the transfer slots thus pulling mixture to cover up transition (along with accellerator pump) to main circuit. You may need to play with this adjustment and it's OK to cover the transfer slots on the primary.

Again, make sure that accellerator pump is squirting with movement of arm. #1 hole should give full shot.

I think the carburetor needs to be tuned to the 365 because it is used on the L79 engine so it is a compromise with factory settings.

if you cover the transfer completely you will have a flat spot because they are not supplying any fuel till they are exposed. that is the reason there are the transfer slots to cover the time it takes for the fuel from the squirters to reach the bottom of the carb.

[Timothy B.]

Re: Tip-In Stumble

Clem,

If the primary transfer slots are covered the air/gas screws can be richened to compensate plus the secondary side has idle ports and transfer slots. On my 3810, the primary transfer slots are covered and I was told by a carburetor restorer this is OK. Big difference in camshaft overlap but I think this will help Joe's problem.

I understand what your saying and it may take a few adjustments to get it right but it's a step in the right direction. If accellerator pump is adjusted and working right that only leaves idle air bleed which is last resort.

[Clem Z.]

Re: Tip-In Stumble

Clem,

If the primary transfer slots are covered the air/gas screws can be richened to compensate plus the secondary side has idle ports and transfer slots. On my 3810, the primary transfer slots are covered and I was told by a carburetor restorer this is OK. Big difference in camshaft overlap but I think this will help Joe's problem.

I understand what your saying and it may take a few adjustments to get it right but it's a step in the right direction. If accellerator pump is adjusted and working right that only leaves idle air bleed which is last resort.

i am not that familiar with a 3810 but i know when i converted 2 corner idle carbs to 4 corner idle i had to drill the idle ports to .081 as the one in the base plate were very small just to allow the fuel in the secondary float bowl to be refreshed in case the secondary was never opened for a long period of time.

[Joe C.]

Resolution!!!

Thanks all, for your helpful replies, especially Clem, who pointed me in the right direction.
If you care to read about how the condition was cured, and probably learn a thing or three, you can read about it here:

http://www.innovatemotorsports.com/forums/showthread.php?t=9303

I am the thread starter, "1965tripleblack".

Joe