SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: jamiemwedmore1234 on August 20, 2015, 05:14:52 PM
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Need some help fellas I have a 9:1 496 stroker will say hp ball park 550-600 I have a hp 950 Holley carb runs real hard at 5800 rpm wot the problem I'm having is getting on the gas from a dig right of idle has a bad stumble from idle circuit up I changed the power vaulve from a 6.5 to a 4.5 per a discussion with Holley Tec I've watched YouTube on tuning and sounds like I need to put a bigger squirter on it ?? When do you decide to go to a bigger accelerator pump motor runs good all through the rpm range and one more question this carb has 4 corner idle adjustment I closed them to so what they where at the primary are 2 turns out and the 2nd are 1/2 turn out I thought all four needed to remain the same am I wrong
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Hey Jamie, you know who I am…I really don’t think you need to upgrade to 50cc accelerator pumps, but check the size on your present pump shot nozzles, they should be at least .031. Have you ever hooked up a vacuum gauge up to see what your engine actually has at idle and cruising? This is also a good tuning tool for adjusting your idle circuit. Ballpark adjustment for your four corner idle screws (all four) would be 1 ½ turns out to get it close. The vacuum gauge also is very important to see what a good power valve size should be; I believe a 6.5” is still fine. Another good tip to prevent fuel starvation in the secondaries on the hit, are jet extenders. Is your ignition advance coming in quick enough? Mine reaches total advance at 1,800 RPM and that is 36*, jet boats like a lot of lead quick to get up and running. Other than that according to CFM requirements for that CID, RPM and volumetric efficiency going off 85-90% you may be a little on the big side. If you don’t have a strong enough signal at the boosters just off idle there isn’t enough velocity to pull fuel into the air stream therefore the engine will encounter a lean stumble. Just for shit’s and giggles if you know someone that has a Holley 850 CFM DP, bolt it on and see how it responds, that will tell you a lot. It’s just going to be something simple and stupid, it always is in the end.
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Hey Jamie, you know who I am…I really don’t think you need to upgrade to 50cc accelerator pumps, but check the size on your present pump shot nozzles, they should be at least .031. Have you ever hooked up a vacuum gauge up to see what your engine actually has at idle and cruising? This is also a good tuning tool for adjusting your idle circuit. Ballpark adjustment for your four corner idle screws (all four) would be 1 ½ turns out to get it close. The vacuum gauge also is very important to see what a good power valve size should be; I believe a 6.5” is still fine. Another good tip to prevent fuel starvation in the secondaries on the hit, are jet extenders. Is your ignition advance coming in quick enough? Mine reaches total advance at 1,800 RPM and that is 36*, jet boats like a lot of lead quick to get up and running. Other than that according to CFM requirements for that CID, RPM and volumetric efficiency going off 85-90% you may be a little on the big side. If you don’t have a strong enough signal at the boosters just off idle there isn’t enough velocity to pull fuel into the air stream therefore the engine will encounter a lean stumble. Just for shit’s and giggles if you know someone that has a Holley 850 CFM DP, bolt it on and see how it responds, that will tell you a lot. It’s just going to be something simple and stupid, it always is in the end.
i pick up a vacuum gauge yesterday hooked it up on the trailer briefly and pulled 12 on vacuum I'm heading out to avi today going put the boat on the water warm it up and see what it reads under load and as far as the 4 corner adjustment I'm going to make them all the same see how it affects my vacuum ##
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It would be good to have enough feet of vacuum line so that you (or passenger) can monitor the gauge while under way. Adjust your idle circuit while the boat is in the water (pump loaded). Can you get an accelerator pump cam kit?
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Maybe he will get some answers over the weekend, I really don’t think changing or trying to tune the rate of the accelerator pumps is going to help matters any. I think the carb is just too big. It’s about 100 CFM too much for that engine. That 496 even at 6,000 RPM and at 100% volumetric efficiency would only require 861 CFM. He says it runs good mid range to top end okay, that’s because there is enough piston speed to create enough velocity at the booster venturi’s to pull fuel through them. Also lowering the power valve opening rate can cause a rich stumble because again velocity isn’t high enough to handle the additional fuel along with poor atomization. Maybe in time one can tune around this problem to mask it, but it’s still not going to be right. Old school hot rodders rule of thumb to follow with carburetion is…run as little carburetion that you can, without sacrificing performance. In other words, run an engine with a CFM size that may be a tad on the smaller side because the signal at the boosters will be very high to pull fuel, the response will be much better and you will have better atomization due to higher velocities and turbulence and that alone will make more power because you are handling and managing the fuel better. On the other hand much too small of a CFM rating will just choke the engine down at mid range and top end, drastically affecting HP potential. The better the atomization, the more you will burn during the combustion process, remember any fuel not burnt is wasted potential energy, and raw fuel doesn’t burn.
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Maybe he will get some answers over the weekend, I really don’t think changing or trying to tune the rate of the accelerator pumps is going to help matters any. I think the carb is just too big. It’s about 100 CFM too much for that engine. That 496 even at 6,000 RPM and at 100% volumetric efficiency would only require 861 CFM. He says it runs good mid range to top end okay, that’s because there is enough piston speed to create enough velocity at the booster venturi’s to pull fuel through them. Also lowering the power valve opening rate can cause a rich stumble because again velocity isn’t high enough to handle the additional fuel along with poor atomization. Maybe in time one can tune around this problem to mask it, but it’s still not going to be right. Old school hot rodders rule of thumb to follow with carburetion is…run as little carburetion that you can, without sacrificing performance. In other words, run an engine with a CFM size that may be a tad on the smaller side because the signal at the boosters will be very high to pull fuel, the response will be much better and you will have better atomization due to higher velocities and turbulence and that alone will make more power because you are handling and managing the fuel better. On the other hand much too small of a CFM rating will just choke the engine down at mid range and top end, drastically affecting HP potential. The better the atomization, the more you will burn during the combustion process, remember any fuel not burnt is wasted potential energy, and raw fuel doesn’t burn.
Totally agree.
Just trying to help a brother out, working with what he has.
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A little update on my carb I end up putting 50 cc accelerator pumps on it and just on the trailer it got rid of the flat spot I had I'll know more when it's in the water
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A little update on my carb I end up putting 50 cc accelerator pumps on it and just on the trailer it got rid of the flat spot I had I'll know more when it's in the water
Sent from my iPhone using SoCal Jet Boats
Hey Jamie,
Hows it goin bud?
I had a nasty flat spot in my carb too. (850 Demon on my 5.3)
I fixed it by keeping the 30cc cups. but drilling my nozzles out. If memory serves im at a 33 front and a 35 rear.
If you need to borrow my jet drills and pin vice to drill yours you are more than welcome!
I started with a 28, and drilled one size up at a time til the problem was gone. My engine is very responsive all the way through the powerband once its warmed up. (and that doesnt take long)
Otherwise, buying the nozzles they are about 12 bucks each if i remember correctly