Jetting after installing 250 bloowers - Help!
04-28-2007, 10:59 AM
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Jetting after installing 250 bloowers - Help!
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, 11:38 AM
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What is your fuel pressure? What pumps and what size fuel lines? Monstaa may be right on too small of carbs. If I remember reading here 1050s are common on those blowers.
04-28-2007, 01:21 PM
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04-28-2007, 01:27 PM
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you need a minimum of a 1050 dominator even with those blowers on a 454.A good mechanical fuel pump with an electric back up on full throttle switch.At least #8 fuel line from pump to carb.Jetting with a vacuum guage,WHAT?Break the vacuum guage and read the plugs or you will have a meltdown.
04-28-2007, 04:17 PM
#7
Geronimo36
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reading plugs on motors with the new fuels is very hard compared to the old days. You'll only see a slight tan ring around the tip ceramic on the electrode. And thank god you're only popping out the carb and not burning pistons...
A long time ago I was trying to jet a 502' with a big cam and couldn't get it right... We started with 83-93's with powervalves and went all the way up to 90-100's and it still looked like it was lean on the plug reading.....
I learned the hard way that the plugs couldn't be read the same way anymore... turns out we were WAY to rich....
I just say send it to the dyno...unless you have a kean eye for plug readings. That has been my expereince.
A long time ago I was trying to jet a 502' with a big cam and couldn't get it right... We started with 83-93's with powervalves and went all the way up to 90-100's and it still looked like it was lean on the plug reading.....
I learned the hard way that the plugs couldn't be read the same way anymore... turns out we were WAY to rich....
I just say send it to the dyno...unless you have a kean eye for plug readings. That has been my expereince.
04-29-2007, 03:01 AM
#9
Charter Member # 55
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Never even heard of using a vacuum to set jetting on carb. use a vacuum guage to set the idle mixture and then read the plugs. i also think your carbs are way small. You are using a single carb.....right?????
Plug reading 101 link
http://www.empirenet.com/pkelley2/sparkplugreading.html
Plug reading 101 link
http://www.empirenet.com/pkelley2/sparkplugreading.html
