Carburation
Thread Starter
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Fleet Mechanic
Joined: Jan 2014
Posts: 1,529
Likes: 271
From: Billings Montana
Carburation
I have a 292 that is bored 60 over (300 cu in). Mummert Y 270 S cam, Tim McMaster built a set of iron heads with bigger valves on it. Mummert High Comp pistons and Blue Thunder intake with Mummert ramshorn exhaust manifolds and Holley manual choke vac secondary 600 cfm 4bbl. I have never been happy with the way it has run! I want a good driver, it will never be raced and rarely will it see 5000 rpm. I have the tach redline at 4500. I have come to the conclusion after reading and reading and reading that it is way over carbureted. After doing the carb calculation that people have said to use (cu in x rpm x v e divided by 3456) at 4500, 5000 and 5500 rpm I came up with needing a 332, 367 and 405 cfm carb respectively. So with that said I am planning on installing a 390 cfm Holley manual choke vac secondary carb and see how it works. Have any of you guys tried the 390 carb on a 292? How did it work for you? And if you started with a 600 and went to a 390 did it make an improvement over idle quality and more responsive at street cruising?
Mountain Pass
Joined: Sep 2021
Posts: 209
Likes: 52
From: Rincon GA
Since the no one else has responded, I thought I would. So here is my two cents:
The cam will cause a slight lope per Mummert’s website which will affect the idle a little. The engine timing should be somewhere near 20 degrees (my guess). I assume it has not been on a dynamometer or you would have mentioned that. Can you verify that there are no vacuum leaks?
I had 66 mustang with the 289 and a 650 cfm Fomoco carb from the factory, I believe. Did Ford put a carb on that engine that was twice what it should have been? Also, you only get 650 cfm when you open the secondaries, otherwise it is a two barrel. The problem may be in the carb idle circuit. That motor should burn all the rubber off the tires very fast.
The cam will cause a slight lope per Mummert’s website which will affect the idle a little. The engine timing should be somewhere near 20 degrees (my guess). I assume it has not been on a dynamometer or you would have mentioned that. Can you verify that there are no vacuum leaks?
I had 66 mustang with the 289 and a 650 cfm Fomoco carb from the factory, I believe. Did Ford put a carb on that engine that was twice what it should have been? Also, you only get 650 cfm when you open the secondaries, otherwise it is a two barrel. The problem may be in the carb idle circuit. That motor should burn all the rubber off the tires very fast.
Thread Starter
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Fleet Mechanic
Joined: Jan 2014
Posts: 1,529
Likes: 271
From: Billings Montana
Never been on the dyno. I forgot to add that last fall we put a Mummert gear and chain on it so we could degree the cam. We advanced it 4 degrees on Johns advice. Before doing that we had the timing set at 24 btdc to get a fairly decent idle. We now have it a 16 btdc and the idle is better. The problem is it seems pig rich at idle and can't seem to get it to lean out, Transfer slots are not even visible on primaries or secondaries.Fuel screws are 2 turns out for best idle. screw them in 1/2 turn and it will die. I thought Ford used a 480 CFM Autolite carb on a 66 A code 289?
5th Wheeling
Joined: May 2025
Posts: 27
Likes: 14
From: Atlanta,GA
Since the no one else has responded, I thought I would. So here is my two cents:
The cam will cause a slight lope per Mummert’s website which will affect the idle a little. The engine timing should be somewhere near 20 degrees (my guess). I assume it has not been on a dynamometer or you would have mentioned that. Can you verify that there are no vacuum leaks?
I had 66 mustang with the 289 and a 650 cfm Fomoco carb from the factory, I believe. Did Ford put a carb on that engine that was twice what it should have been? Also, you only get 650 cfm when you open the secondaries, otherwise it is a two barrel. The problem may be in the carb idle circuit. That motor should burn all the rubber off the tires very fast.
The cam will cause a slight lope per Mummert’s website which will affect the idle a little. The engine timing should be somewhere near 20 degrees (my guess). I assume it has not been on a dynamometer or you would have mentioned that. Can you verify that there are no vacuum leaks?
I had 66 mustang with the 289 and a 650 cfm Fomoco carb from the factory, I believe. Did Ford put a carb on that engine that was twice what it should have been? Also, you only get 650 cfm when you open the secondaries, otherwise it is a two barrel. The problem may be in the carb idle circuit. That motor should burn all the rubber off the tires very fast.
Mountain Pass
Joined: Sep 2021
Posts: 209
Likes: 52
From: Rincon GA
Could be but not in my memory banks. (You know a person can get it wrong with a 50 year old memory) However, even 480 cfm is bit too much per Summits cfm calculator.
Myy 292 was rebuilt with a 57 plus cam and larger intake valves, also with a 4 barrel intake and electronic ignition. It was put on a dyno with a 600 and then a 650 cfm carb. The 650 performed better. Now if I could just install it in my truck… I am stuck in a low motivational state.
Myy 292 was rebuilt with a 57 plus cam and larger intake valves, also with a 4 barrel intake and electronic ignition. It was put on a dyno with a 600 and then a 650 cfm carb. The 650 performed better. Now if I could just install it in my truck… I am stuck in a low motivational state.
Mountain Pass
Joined: Sep 2021
Posts: 209
Likes: 52
From: Rincon GA
Never been on the dyno. I forgot to add that last fall we put a Mummert gear and chain on it so we could degree the cam. We advanced it 4 degrees on Johns advice. Before doing that we had the timing set at 24 btdc to get a fairly decent idle. We now have it a 16 btdc and the idle is better. The problem is it seems pig rich at idle and can't seem to get it to lean out, Transfer slots are not even visible on primaries or secondaries.Fuel screws are 2 turns out for best idle. screw them in 1/2 turn and it will die. I thought Ford used a 480 CFM Autolite carb on a 66 A code 289?
How does it run when accelerating or at cruising speed?
Have you tried increasing the fast idle and then turning the fuel screws in?
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Fleet Mechanic
Joined: Jan 2014
Posts: 1,529
Likes: 271
From: Billings Montana
It has no vacuum leaks anywhere on the engine. I have tried the fuel screws going it to 1 3/4 or 1 2/ turns out almost kills the engine and if you turn the idle screw in so it will idle then you have opened up the idle transfer ports and you are just adding more fuel at idle. If you use the secondary stop screw to raise the idle then you are exposing those idle transfer ports to much and adding fuel. As far as drivability is concerned it works but not like IMO it should. Stick your finger in the tailpipes and they come out black. Plugs look to be a little rich. I have changed the jets all the way down from 66 to 63 one at a time with no noticeable change. I have a 390 coming. I will kit it and install it in the next week or two and I will report back here. Everything I read is smaller is better unless you drive around with your foot on the floor all the time. The original carb that this pickup came with from the factory was a 290 CFM 2 bbl and if you do the carb size calculation for a 292 it comes out to be 287 CFM.
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Cargo Master
Joined: Mar 2002
Posts: 3,132
Likes: 217
From: W (BY GOD) V
Again, the CARB has to be dialed into the application,
There is no bolt-on and run with a HOLLEY.
You had all of that engine work done for what?
The FORD F4100 4V 1.12 was rated @ 480 CFM approx.
CARB flow is also dictated by the engine modifications. Rated flow is performed on a flow-bench.
There is no bolt-on and run with a HOLLEY.
You had all of that engine work done for what?
The FORD F4100 4V 1.12 was rated @ 480 CFM approx.
CARB flow is also dictated by the engine modifications. Rated flow is performed on a flow-bench.
Thread Starter
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Fleet Mechanic
Joined: Jan 2014
Posts: 1,529
Likes: 271
From: Billings Montana
Again, the CARB has to be dialed into the application,
There is no bolt-on and run with a HOLLEY.
You had all of that engine work done for what?
The FORD F4100 4V 1.12 was rated @ 480 CFM approx.
CARB flow is also dictated by the engine modifications. Rated flow is performed on a flow-bench.
There is no bolt-on and run with a HOLLEY.
You had all of that engine work done for what?
The FORD F4100 4V 1.12 was rated @ 480 CFM approx.
CARB flow is also dictated by the engine modifications. Rated flow is performed on a flow-bench.
Cargo Master
Joined: Mar 2002
Posts: 3,132
Likes: 217
From: W (BY GOD) V
Well, first of all those so-called formulas are not 100% correct ...
They are a sellers way of not having someone buy too big of a CARB for their application and then having to listen to the whining.
Just because a CARB is rated @ 600CFM, if the engine cannot take that amount of flow, it will not flow 600CFM. Your problem(s) are in the CALIBRATION(s).
You have a highly modified engine (why?) and to operate properly needs correct fuel mixture flow. I see you have it governed low. This is another factor.
The HOLLEY is infinitesimally (? - college word) adjustable -
CLICK HERE - https://cartipsdaily.com/how-to-adju...ley-carburetor
There is plenty more TECH INFO out there to get it to perform correctly.
!!! DO NOT SHOOT THE MESSENGER !!!
Now if you feel the CARB is too big, try a SUMMIT OR EDEL.
.
They are a sellers way of not having someone buy too big of a CARB for their application and then having to listen to the whining.
Just because a CARB is rated @ 600CFM, if the engine cannot take that amount of flow, it will not flow 600CFM. Your problem(s) are in the CALIBRATION(s).
You have a highly modified engine (why?) and to operate properly needs correct fuel mixture flow. I see you have it governed low. This is another factor.
The HOLLEY is infinitesimally (? - college word) adjustable -
CLICK HERE - https://cartipsdaily.com/how-to-adju...ley-carburetor
There is plenty more TECH INFO out there to get it to perform correctly.
!!! DO NOT SHOOT THE MESSENGER !!!
Now if you feel the CARB is too big, try a SUMMIT OR EDEL.
.
Last edited by KULTULZ; Apr 4, 2026 at 07:52 PM.
Thread Starter
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Fleet Mechanic
Joined: Jan 2014
Posts: 1,529
Likes: 271
From: Billings Montana
Have a tach and a vacuum gauge fuel pressure is 6 psi. I just read that link you posted and have done everything that article said to do 100 different times a hundred different ways. You keep telling me I don't have it dialed in but you do not give me any ideas as to what you would try. I realize those formulas are not 100% but I also know that anyway I worked the formula changing rpm or volumetric efficiency on a 300 cubic engine does it come anywhere close to a 600 cfm carb needed. Even at 85% VE and 7000 rpm it says 516 cfm is needed and at 75% VE 7000 rpm calls for a 458 CFM carb. For this engine stock Ford put a 290 CFM 2 bbl carb on it from the factory. If you do the carb calculation for what Ford says the stock HP rating is at 4000 rpm it comes in at needing 287 CFM carb. So there as got to be something to be said for the accuracy of the calculator. As far as a Summit or an Edelbrock the smallest they make is a 500 CFM which is still at least 100 cfm bigger than the calculated numbers. If the 390 works I at least will have my answers and in the future might be able to help someone else down the road.
Last edited by skidoorulz; Apr 4, 2026 at 08:09 PM.