SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: Ty64 on August 27, 2017, 09:31:23 AM
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I need some help 468 chevy, Comp cam Duration 278/292, Lift .564/.566, RPM air gap intake, Holley 4150 marine 850, Pertronic flame thrower II 32 deg all in at 2800 and Hardin log exh. Here is where I am at with the carb started out with stock jetting 88 pir. 96 sec. 6.5 PV plugs looked real lean. everything is brand new. No vacuum leaks. Vacuum gauge reads 11" at Idle 15" at cruise 2" at WOT. replaced jets with 92 front 98 out back no change in plug color white as snow. not sure if i should go up more on the jets. when inspecting the carb there was 2 gaskets on the power valve is this common on a marine carb or did Holley just made a mistake ? It acts like the PV are not opening and I have put some 4.5 in there and 7.5 with no change.
Thanks for any and all help
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All gasoline power valves flow exactly the same. The number (4.5, 6.5, 7.5, etc.) is the point, in inches of vacuum, where the power valve opens. This is to help get the fuel flowing in the primary circuit, when the throttle is suddenly opened.
You will not notice any difference in the air/fuel ratio when changing the power valve number.
Where you will see a difference is, sorting out the power valve will help you overcome a momentary hesitation when you suddenly mash on the throttle. We can address this once you get the air/fuel ratio sorted out.
I'm assuming that you running pump gas. The additives in pump will make it a challenge. Do you have an otoscope or a 10x magnifying loupe?
Do you have an open spacer or has the divider in your Airgap manifold been modified? An unmodified Airgap will have a small window between the two plenums to help even out the vacuum signal on the carb venturi.
A true dual-plane manifold, with isolated plenums, will pull harder on each carb venturi than an open plenum because each cylinder is exposed to only 1/2 of the carb. An open plenum draws from all carb venturi. Spacers and plenum divider modifications can reduce the vacuum signal that each venturi experiences.
With all of that being said, there is a way to determine over/under carburation using a vacuum gauge, but I don't remember off hand how to do it and I can't put my hands on that information right this moment. My gut feeling is that 2" at WOT is not producing a very strong vacuum signal at the venturi.
In order for air flow to occur, there has to be a pressure differential. That pressure differential between the atmospher and the cylinder pushes air from atmospheric pressure through the carb venturi and ultimately into the cylinder. It is this airflow through the carb venturi that produces a low pressure area that the fuel in the float bowls are exposed to through the primary circuit (jets, power valve, et al). Atmospheric pressure present in the float bowls is greater than the pressure present in the venturi and fuel is pushed from the bowl in the venturi where it is atomized and homogenized with the air flowing into the intake manifold.
Just how much fuel flows out of the venturi is relative to the velocity of airflow through the venturi. When an engine is over-carbureted, the velocity is reduced along with a reduction in booster signal. To compensate, overly large jets need to be used. Don't be shocked if you have to substantially step up in jet size.
Just thinking now... could the spark arrestor be restricting the bowl vents? If you have any kind of open spacer under the carb, try it without the open spacer.
Good luck,
Joe
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All gasoline power valves flow exactly the same. The number (4.5, 6.5, 7.5, etc.) is the point, in inches of vacuum, where the power valve opens. This is to help get the fuel flowing in the primary circuit, when the throttle is suddenly opened.
You will not notice any difference in the air/fuel ratio when changing the power valve number.
Where you will see a difference is, sorting out the power valve will help you overcome a momentary hesitation when you suddenly mash on the throttle. We can address this once you get the air/fuel ratio sorted out.
I'm assuming that you running pump gas. The additives in pump will make it a challenge. Do you have an otoscope or a 10x magnifying loupe?
Do you have an open spacer or has the divider in your Airgap manifold been modified? An unmodified Airgap will have a small window between the two plenums to help even out the vacuum signal on the carb venturi.
A true dual-plane manifold, with isolated plenums, will pull harder on each carb venturi than an open plenum because each cylinder is exposed to only 1/2 of the carb. An open plenum draws from all carb venturi. Spacers and plenum divider modifications can reduce the vacuum signal that each venturi experiences.
With all of that being said, there is a way to determine over/under carburation using a vacuum gauge, but I don't remember off hand how to do it and I can't put my hands on that information right this moment. My gut feeling is that 2" at WOT is not producing a very strong vacuum signal at the venturi.
In order for air flow to occur, there has to be a pressure differential. That pressure differential between the atmospher and the cylinder pushes air from atmospheric pressure through the carb venturi and ultimately into the cylinder. It is this airflow through the carb venturi that produces a low pressure area that the fuel in the float bowls are exposed to through the primary circuit (jets, power valve, et al). Atmospheric pressure present in the float bowls is greater than the pressure present in the venturi and fuel is pushed from the bowl in the venturi where it is atomized and homogenized with the air flowing into the intake manifold.
Just how much fuel flows out of the venturi is relative to the velocity of airflow through the venturi. When an engine is over-carbureted, the velocity is reduced along with a reduction in booster signal. To compensate, overly large jets need to be used. Don't be shocked if you have to substantially step up in jet size.
Just thinking now... could the spark arrestor be restricting the bowl vents? If you have any kind of open spacer under the carb, try it without the open spacer.
Good luck,
Joe
Joe thanks for getting back to me.
Running 91 pump gas. and no otoscope or a 10x magnifying loupe but I can buy one.
The air gap has not been modified. I am also running an HVH Super Sucker Carburetor Spacers SS4150-1ALW but I have run with out it on also with no change in plug color.
spark arrestor is brand new 3" X 8" no scoop open to out side air.
So all that being said I should move down in carb size or keep jetting up.
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I just went back and reread your original post, where I had missed that you had only changed four jet sizes. I wouldn't go and dump a small fortune on another new carburetor just yet.
I would go back reevaluate your plug reading technique. You might find that you need to jump six or eight jet sizes. Remember, you have to read the plugs after a pass. You can't idle back to the marina and then take a proper reading.
I went through all of this with a 750 on a 340 Duster. Hindsight being 20/20, I wish I had just bought the complete jet kit instead of every single individual jet trying to sort it out, only to find out that I was reading the plugs all wrong. Then I had to change the cam and intake and I got to do it all over. At least I got a lot of practice. It makes me appreciate the proverb, "A smooth sea never made a skilled mariner." all that much more.
Here are some videos that you might find helpful/enjoyable:
https://youtu.be/IvWl-dUNUYg
https://youtu.be/z4cdCTAnwFE
https://youtu.be/uMYAEtxiYfc
https://youtu.be/lS5XofKnros
https://youtu.be/ld1rOW5v9AI
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I had a 72' Duster with the slant 6. First car, it got totaled before I could drop in the hemi.
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Thanks Flusher.
All plug reading has been done after wot run. my heat range might to hot also. I will go to a colder plug and up the jets to 94 and 102 and see how it does.