Setting up a holley carb
04-03-2002, 10:42 AM
#1
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Setting up a holley carb
Last year I rebuilt my engines (409 SBC's). I put the old carbs back on. The carbs are Holley 4150 750 CFM 4 barrel double pumpers. The engines ran great but weren't developing the expected HP which according to desktop dyno will be around 425 HP. I only had breakin time on the engines but there are some things I noticed.
For starters, reading the plugs they are black and wet, so I figure things are running to rich. What jets and power valve should I start with for this size of engine both for primaries and secondaries? Someone here mentioned 75's and 10.5 PV. I don't know what jets I have now, the boat is still under cover.
Next, when I was running it back home for the last time just for grins I hammered it. I had marked on the throttle where the secondaries open they are mechanical. I was at 5000 when they started to open but as they opened there was no increase in RPM's with throttle movement even though I know the throttle plates are still opening. I figured that may have been due to the restrictive 350 flame arrestor on it. It just couldnt get more air. Sound accurate? I know I need to reprop and adjust the secondaries (mechanical secondaries).
Any suggestions would be great. I would like to get things set-up right before the first OSO Lake St. Clair fun run. I don't want to be embarassed too bad. Thanks guys.
For starters, reading the plugs they are black and wet, so I figure things are running to rich. What jets and power valve should I start with for this size of engine both for primaries and secondaries? Someone here mentioned 75's and 10.5 PV. I don't know what jets I have now, the boat is still under cover.
Next, when I was running it back home for the last time just for grins I hammered it. I had marked on the throttle where the secondaries open they are mechanical. I was at 5000 when they started to open but as they opened there was no increase in RPM's with throttle movement even though I know the throttle plates are still opening. I figured that may have been due to the restrictive 350 flame arrestor on it. It just couldnt get more air. Sound accurate? I know I need to reprop and adjust the secondaries (mechanical secondaries).
Any suggestions would be great. I would like to get things set-up right before the first OSO Lake St. Clair fun run. I don't want to be embarassed too bad. Thanks guys.
04-03-2002, 10:50 PM
#2
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Jeff,
Have the motors ever back-fired before? That would destroy the power valves in one shot and would lead to a very rich condition at idle and would cause wet plugs. Since you've made changes and such I'd hook up a vacuum gauge and find out what your idle readings are. From there choose a power valve roughly 2" HG lower and that will keep the valves closed at idle (somewhat the old rule when dealing with more radical setups). A milder engine that pulls a lot a vacuum at idle (let's say 19" HG) would use the rule of idle vacuum divided by two (9.5 PV by the above example). You really need to know what idle vacuum is first in order to make a decision.
As for jetting... before changing anything, I'd make a good WOT run and immediately kill the engines. Do not go to idle, etc. just kill it. Then pull the plugs and see if they're wet and/or sooty. That'll give you an idea about whether or not the carbs are rich or lean, etc. Use fresh plugs during your run.
As for the secondaries, those throttle blades should be beginning to open at about 50% throttle. I haven't had my hands on a mechanical Holley in quite a few years but I do seem to remember that there wasn't much to them as far as getting the throttle blades to open up. Screaming along at 5k RPM with the secondaries just coming on sounds odd to me (like there was no load at all... I would expect that with a mal-adjusted vac. secondary Holley). With your boat I would think that you'd be into the secondaries somewhere in the mid 3k RPM range at load. Sound right to you?
Outside of the tuning issues... how's she runnin'?
Tom
Have the motors ever back-fired before? That would destroy the power valves in one shot and would lead to a very rich condition at idle and would cause wet plugs. Since you've made changes and such I'd hook up a vacuum gauge and find out what your idle readings are. From there choose a power valve roughly 2" HG lower and that will keep the valves closed at idle (somewhat the old rule when dealing with more radical setups). A milder engine that pulls a lot a vacuum at idle (let's say 19" HG) would use the rule of idle vacuum divided by two (9.5 PV by the above example). You really need to know what idle vacuum is first in order to make a decision.
As for jetting... before changing anything, I'd make a good WOT run and immediately kill the engines. Do not go to idle, etc. just kill it. Then pull the plugs and see if they're wet and/or sooty. That'll give you an idea about whether or not the carbs are rich or lean, etc. Use fresh plugs during your run.
As for the secondaries, those throttle blades should be beginning to open at about 50% throttle. I haven't had my hands on a mechanical Holley in quite a few years but I do seem to remember that there wasn't much to them as far as getting the throttle blades to open up. Screaming along at 5k RPM with the secondaries just coming on sounds odd to me (like there was no load at all... I would expect that with a mal-adjusted vac. secondary Holley). With your boat I would think that you'd be into the secondaries somewhere in the mid 3k RPM range at load. Sound right to you?
Outside of the tuning issues... how's she runnin'?
Tom
04-03-2002, 10:53 PM
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Oh... and as long as the flame arrestors were clean they should have provided enough air flow to get the job done though the air flow may not have been as smooth it still should have been adequate I'd think... though I could be wrong.
04-03-2002, 11:00 PM
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Jr,
I will ask a friend of mine what jets he has. He is running close to the same set up you are. I think you guys have close to the same boat too.
On the desktop dyno his engines made 440hp so his jetting should get you in the ballpark.
I will try and get ahold of him tommorrow.
I will ask a friend of mine what jets he has. He is running close to the same set up you are. I think you guys have close to the same boat too.
On the desktop dyno his engines made 440hp so his jetting should get you in the ballpark.
I will try and get ahold of him tommorrow.
04-03-2002, 11:55 PM
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For a base line take the carb no.stamped by the air horn a holley book will list the original jets for that carb. or guess a small block at 75 front and 80 rear and go from there.Several books at the book stores that explain how they work if you dont run a rear power valve go up about 6 jet sizes buy a jet assortment and reusable gaskets.
04-04-2002, 06:12 AM
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Thanks guys, thats a great place to start. Mortec.com has listings for carbs and the original jetting. So I'll check there. I have little faith that the guy that first put the carbs on the doomed rebuilds the first time had any idea what he was doing. So he may have started with the wrong carb and jets.
Tom, as long as I have had the boat it has never back fired. But I have suspected that it was running rich at idle. Have adjusted the idle mixture almost closed and I feel it is still running rich. I will check the power valves.
Thanks guys.
Tom, as long as I have had the boat it has never back fired. But I have suspected that it was running rich at idle. Have adjusted the idle mixture almost closed and I feel it is still running rich. I will check the power valves.
Thanks guys.
