Jetting after installing 250 blowers with 850 Barry Grants
04-28-2007, 10:53 AM
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Jetting after installing 250 blowers with 850 Barry Grants
It's been a while , but here I am again needing more help form the keepers of all the world knowledge regarding these go fast boat.
Long story made short, we had popping (through carbs) on top end rpm's and after several trys (and a dropped intake valve) we got the engines jetted correctly (taking vacuum readings as we changed jets) with no more popping running 88 primary jets and 90 secondary jets in Holley 750 dual feed carbs.
Then we need more power so, I installed 250 blowers from the Blower Shop and 850 Demon carbs from Barry Grant. They came with #88 primaries and 92 secondaries and we set total timing at 30 degrees; popping on top rpm range is back. We went back to taking vacuum readings and re-jetting. After jumping up to #96 primaries and #98 secondaries. Still popping over 3500 RPM's; I know we are still lean: plugs pretty white.
My question centers on jeeting by the vacuum reading techinic after blowers are installed. Before blowers, you watched vacuum increase (at the same powers setting) while changing to large jets; when you went past the righ jet (to big) vaccuum reading decrease (moved towards zero at the same power setting). With blowers, these readings appear to be staying the same at the same power setting yet I've increase jets from 10 sizes. I'm beginning to believe you can not use the vaccumum reading technic to jet once you install blowers or we need a hell of alot more gas than we are feeeding them even after dramitc increases in jet sizes. Any thought or suggestions? I certainly don't want to break another engine or blow these super chargers apart. Based on that top end popping through the carbs, I think it's still way lean. Would appreciate any help.
Al
Long story made short, we had popping (through carbs) on top end rpm's and after several trys (and a dropped intake valve) we got the engines jetted correctly (taking vacuum readings as we changed jets) with no more popping running 88 primary jets and 90 secondary jets in Holley 750 dual feed carbs.
Then we need more power so, I installed 250 blowers from the Blower Shop and 850 Demon carbs from Barry Grant. They came with #88 primaries and 92 secondaries and we set total timing at 30 degrees; popping on top rpm range is back. We went back to taking vacuum readings and re-jetting. After jumping up to #96 primaries and #98 secondaries. Still popping over 3500 RPM's; I know we are still lean: plugs pretty white.
My question centers on jeeting by the vacuum reading techinic after blowers are installed. Before blowers, you watched vacuum increase (at the same powers setting) while changing to large jets; when you went past the righ jet (to big) vaccuum reading decrease (moved towards zero at the same power setting). With blowers, these readings appear to be staying the same at the same power setting yet I've increase jets from 10 sizes. I'm beginning to believe you can not use the vaccumum reading technic to jet once you install blowers or we need a hell of alot more gas than we are feeeding them even after dramitc increases in jet sizes. Any thought or suggestions? I certainly don't want to break another engine or blow these super chargers apart. Based on that top end popping through the carbs, I think it's still way lean. Would appreciate any help.
Al
04-28-2007, 01:07 PM
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It's been a while , but here I am again needing more help form the keepers of all the world knowledge regarding these go fast boat.
Long story made short, we had popping (through carbs) on top end rpm's and after several trys (and a dropped intake valve) we got the engines jetted correctly (taking vacuum readings as we changed jets) with no more popping running 88 primary jets and 90 secondary jets in Holley 750 dual feed carbs.
Then we need more power so, I installed 250 blowers from the Blower Shop and 850 Demon carbs from Barry Grant. They came with #88 primaries and 92 secondaries and we set total timing at 30 degrees; popping on top rpm range is back. We went back to taking vacuum readings and re-jetting. After jumping up to #96 primaries and #98 secondaries. Still popping over 3500 RPM's; I know we are still lean: plugs pretty white.
My question centers on jeeting by the vacuum reading techinic after blowers are installed. Before blowers, you watched vacuum increase (at the same powers setting) while changing to large jets; when you went past the righ jet (to big) vaccuum reading decrease (moved towards zero at the same power setting). With blowers, these readings appear to be staying the same at the same power setting yet I've increase jets from 10 sizes. I'm beginning to believe you can not use the vaccumum reading technic to jet once you install blowers or we need a hell of alot more gas than we are feeeding them even after dramitc increases in jet sizes. Any thought or suggestions? I certainly don't want to break another engine or blow these super chargers apart. Based on that top end popping through the carbs, I think it's still way lean. Would appreciate any help.
Al
Long story made short, we had popping (through carbs) on top end rpm's and after several trys (and a dropped intake valve) we got the engines jetted correctly (taking vacuum readings as we changed jets) with no more popping running 88 primary jets and 90 secondary jets in Holley 750 dual feed carbs.
Then we need more power so, I installed 250 blowers from the Blower Shop and 850 Demon carbs from Barry Grant. They came with #88 primaries and 92 secondaries and we set total timing at 30 degrees; popping on top rpm range is back. We went back to taking vacuum readings and re-jetting. After jumping up to #96 primaries and #98 secondaries. Still popping over 3500 RPM's; I know we are still lean: plugs pretty white.
My question centers on jeeting by the vacuum reading techinic after blowers are installed. Before blowers, you watched vacuum increase (at the same powers setting) while changing to large jets; when you went past the righ jet (to big) vaccuum reading decrease (moved towards zero at the same power setting). With blowers, these readings appear to be staying the same at the same power setting yet I've increase jets from 10 sizes. I'm beginning to believe you can not use the vaccumum reading technic to jet once you install blowers or we need a hell of alot more gas than we are feeeding them even after dramitc increases in jet sizes. Any thought or suggestions? I certainly don't want to break another engine or blow these super chargers apart. Based on that top end popping through the carbs, I think it's still way lean. Would appreciate any help.
Al
04-28-2007, 01:36 PM
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fuel lines are 1/2" all the way back to the tanks. Pumps are 110 gph, 12 psi with aeromotivce regulator reducing pressure back down to 8 psi at the carbs.
04-28-2007, 03:52 PM
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04-28-2007, 05:06 PM
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Are both motors backfiring through the carbs?
04-28-2007, 05:59 PM
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bigger carbs
nickerson set me up with 1250 dominators,96 pri. 102 sec. mine are on 509ci. 6.5lbs. boost on dyno they pulled just over 900cfm. nickerson reworked them before he sent them.. careful don't go lean..
04-28-2007, 06:20 PM
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Sounds like new carbs. They have much bigger jets for the 850. Are we basically saying there is no way to get these bradn new 850's to work regardles of the jets size?
Al
Al
04-28-2007, 06:59 PM
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Don't know but...
Nickerson has done many B3M 250's with reworked 1050's. Give him the cam spec's boat weight and a few other details. Take it out of the box, set the idle and close the hatch.
Nickerson has done many B3M 250's with reworked 1050's. Give him the cam spec's boat weight and a few other details. Take it out of the box, set the idle and close the hatch.
04-29-2007, 07:25 AM
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First let’s discuss how a carburetor works:
Carburetors work based upon pressure differential. As your engine draws air through the carburetor it creates a low pressure in the center of the carburetor. The difference of this pressure, and the higher pressure (from the atmosphere) pressing down on the fuel in the bowls is what causes the fuel to flow through the carburetor into the engine.
Now with that out of the way, if you have to jet up excessively from a carburetors baseline it usually indicates one of two things. It could be the carburetor being too large for the combination, or your fuel system not keeping up with the demands of your engine.
With that being said, going to a larger carburetor is not going to help you at this point. The first thing to look at would be your fuel delivery system. A 110 GPH fuel pump would be on the small side for a supercharged engine.
As far as your original question, jetting by vacuum most likely isn't going to work on your current combination.
Carburetors work based upon pressure differential. As your engine draws air through the carburetor it creates a low pressure in the center of the carburetor. The difference of this pressure, and the higher pressure (from the atmosphere) pressing down on the fuel in the bowls is what causes the fuel to flow through the carburetor into the engine.
Now with that out of the way, if you have to jet up excessively from a carburetors baseline it usually indicates one of two things. It could be the carburetor being too large for the combination, or your fuel system not keeping up with the demands of your engine.
With that being said, going to a larger carburetor is not going to help you at this point. The first thing to look at would be your fuel delivery system. A 110 GPH fuel pump would be on the small side for a supercharged engine.
As far as your original question, jetting by vacuum most likely isn't going to work on your current combination.
