A question for the Holley/Demon carb experts
11-01-2017, 08:22 AM
#21
Melting Slicks
Thread Starter
You may want to try the .050 IAB to see if it improves.
Is this a vacuum or double pump carburetor?
Closing the secondary blades completely gives no improvement with the .037/.055
What is the IFR and IAB in the secondary metering block
Last edited by 65air_coupe; 11-01-2017 at 02:12 PM.
11-08-2017, 06:55 PM
#22
Melting Slicks
Thread Starter
So I've gone up two sizes in primaries (70 to 74) and one size in secondaries (78 to 80) and re-tested with no changes to IFR or IAB and find that I'm no longer lean from idle up to 3,000 RPM but I still have the stumbling at light load, low throttle. But more troubling is the inconsistency of the readings. How steady should they be at any load/throttle position - I would expect less than 1? If they're changing as much as 2-3 is that a carb issue or something else like a vacuum leak?
11-09-2017, 04:55 PM
#23
Okay, no more guessing, I installed a wide band sensor with gauge and while it does indicate I'm lean at low speed, light throttle, it also indicates I'm lean all the way to WOT.
So I've gone up two sizes in primaries (70 to 74) and one size in secondaries (78 to 80) and re-tested with no changes to IFR or IAB and find that I'm no longer lean from idle up to 3,000 RPM but I still have the stumbling at light load, low throttle. But more troubling is the inconsistency of the readings. How steady should they be at any load/throttle position - I would expect less than 1? If they're changing as much as 2-3 is that a carb issue or something else like a vacuum leak?
So I've gone up two sizes in primaries (70 to 74) and one size in secondaries (78 to 80) and re-tested with no changes to IFR or IAB and find that I'm no longer lean from idle up to 3,000 RPM but I still have the stumbling at light load, low throttle. But more troubling is the inconsistency of the readings. How steady should they be at any load/throttle position - I would expect less than 1? If they're changing as much as 2-3 is that a carb issue or something else like a vacuum leak?
If you are no longer lean from idle up to 3000rpm then could the stumble be the accelerator pump or pump check valve adjustment. FWIW, the goal of primary main jet tuning is to get the engine to run as lean as possible during cruise short of surge. When you find the lean limit go back up one or two on the main jet then look at the power valve opening point and power valve channel restriction hole size.
11-10-2017, 08:04 AM
#24
Melting Slicks
Thread Starter
I can't answer your question about the variation in wide band readings, maybe someone else here can help.
If you are no longer lean from idle up to 3000rpm then could the stumble be the accelerator pump or pump check valve adjustment. FWIW, the goal of primary main jet tuning is to get the engine to run as lean as possible during cruise short of surge. When you find the lean limit go back up one or two on the main jet then look at the power valve opening point and power valve channel restriction hole size.
If you are no longer lean from idle up to 3000rpm then could the stumble be the accelerator pump or pump check valve adjustment. FWIW, the goal of primary main jet tuning is to get the engine to run as lean as possible during cruise short of surge. When you find the lean limit go back up one or two on the main jet then look at the power valve opening point and power valve channel restriction hole size.
What is interesting to me was I did have a couple of experienced mechanics tell me that my transition mixture would richen with a larger main jet which was in direct contradiction to just about everything I've read but I've now experienced it for myself.
If I was confident my problem is solely the carb, I'd spring for a new one in an instant but this carb ran fine when it was on my mostly stock L79 and only started running poorly when I put it on this 406. I'd suspect I've got some other problem going on and the next steps are new coil then new points/condensor. I've already ruled out any vacuum component by disconnecting them which also should rule out too much ignition advance.
11-10-2017, 11:02 AM
#25
Thanks for responding. Let me be clear, there is no hesitation on acceleration at any speed/rpm. The stumble/trailer hitching or whatever one wants to call it is at steady throttle at rpms less than 2500. As soon as I add throttle, it disappears until steady rpm is reached at the new throttle setting unless I'm fully into the main jet.
What is interesting to me was I did have a couple of experienced mechanics tell me that my transition mixture would richen with a larger main jet which was in direct contradiction to just about everything I've read but I've now experienced it for myself.
If I was confident my problem is solely the carb, I'd spring for a new one in an instant but this carb ran fine when it was on my mostly stock L79 and only started running poorly when I put it on this 406. I'd suspect I've got some other problem going on and the next steps are new coil then new points/condensor. I've already ruled out any vacuum component by disconnecting them which also should rule out too much ignition advance.
What is interesting to me was I did have a couple of experienced mechanics tell me that my transition mixture would richen with a larger main jet which was in direct contradiction to just about everything I've read but I've now experienced it for myself.
If I was confident my problem is solely the carb, I'd spring for a new one in an instant but this carb ran fine when it was on my mostly stock L79 and only started running poorly when I put it on this 406. I'd suspect I've got some other problem going on and the next steps are new coil then new points/condensor. I've already ruled out any vacuum component by disconnecting them which also should rule out too much ignition advance.
IMO, the trailer hitching is caused by the overlap in this camshaft when both valves are open and light throttle vacuum pulls exhaust back into the combustion chamber which dilutes the mixture causing poor combustion.
Take a big swing at it and put a .010 smaller IAB in the system and see if that helps as it may give a indication if you are on the right path because at the surge rpm's you are on the idle/transition circuit.
11-10-2017, 05:55 PM
#26
Melting Slicks
Thread Starter
I think the larger main jet allows more pressure to the idle jet so it does make a difference.
IMO, the trailer hitching is caused by the overlap in this camshaft when both valves are open and light throttle vacuum pulls exhaust back into the combustion chamber which dilutes the mixture causing poor combustion.
Take a big swing at it and put a .010 smaller IAB in the system and see if that helps as it may give a indication if you are on the right path because at the surge rpm's you are on the idle/transition circuit.
IMO, the trailer hitching is caused by the overlap in this camshaft when both valves are open and light throttle vacuum pulls exhaust back into the combustion chamber which dilutes the mixture causing poor combustion.
Take a big swing at it and put a .010 smaller IAB in the system and see if that helps as it may give a indication if you are on the right path because at the surge rpm's you are on the idle/transition circuit.
I am beginning to feel like the problem has gotten worse since I started working on it which wouldn't be out of the question since I've actually done modifications to the carb. Drilled and tapped both the IAB in the main body and the IFR in the metering plate and while I was careful in all steps of doing so and blew everything out then and each time I change either, something may not be right. The fact that the mixture ratio bounces around so much is certainly a clue something's not right. I'm going to see if I can locate a carb to borrow to try and isolate the problem.
11-10-2017, 06:50 PM
#27
Interesting thoughts about the cam, could be part of the problem but I've about decided I've got something else going on. I've swapped out the coil and ignition components with no change, plugs and wires are new but problem was there before changing them.
I am beginning to feel like the problem has gotten worse since I started working on it which wouldn't be out of the question since I've actually done modifications to the carb. Drilled and tapped both the IAB in the main body and the IFR in the metering plate and while I was careful in all steps of doing so and blew everything out then and each time I change either, something may not be right. The fact that the mixture ratio bounces around so much is certainly a clue something's not right. I'm going to see if I can locate a carb to borrow to try and isolate the problem.
I am beginning to feel like the problem has gotten worse since I started working on it which wouldn't be out of the question since I've actually done modifications to the carb. Drilled and tapped both the IAB in the main body and the IFR in the metering plate and while I was careful in all steps of doing so and blew everything out then and each time I change either, something may not be right. The fact that the mixture ratio bounces around so much is certainly a clue something's not right. I'm going to see if I can locate a carb to borrow to try and isolate the problem.
11-10-2017, 06:55 PM
#28
Interesting thoughts about the cam, could be part of the problem but I've about decided I've got something else going on. I've swapped out the coil and ignition components with no change, plugs and wires are new but problem was there before changing them.
I am beginning to feel like the problem has gotten worse since I started working on it which wouldn't be out of the question since I've actually done modifications to the carb. Drilled and tapped both the IAB in the main body and the IFR in the metering plate and while I was careful in all steps of doing so and blew everything out then and each time I change either, something may not be right. The fact that the mixture ratio bounces around so much is certainly a clue something's not right. I'm going to see if I can locate a carb to borrow to try and isolate the problem.
I am beginning to feel like the problem has gotten worse since I started working on it which wouldn't be out of the question since I've actually done modifications to the carb. Drilled and tapped both the IAB in the main body and the IFR in the metering plate and while I was careful in all steps of doing so and blew everything out then and each time I change either, something may not be right. The fact that the mixture ratio bounces around so much is certainly a clue something's not right. I'm going to see if I can locate a carb to borrow to try and isolate the problem.