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

you may need more carb. i have seen a single dominator on the new billet blowers from blower shop which made 750 or so i believe.

Can you use vacuum reading to jet after installing blowers? Does that technic still work?

Al

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.

The fuel liens are 1/2 inch and using 110 GPH 12 psi carbs with Aeromotive regulators.

The vacuum reading method comes off the Holley carb site. We have been reading the plugs all along. While they are nolonger white white, they are whiter than they are tan/brown again indicating that we are still pretty lean. I'm beginning to believe the 1050 carbs are going to be what I need. I'll hope Cp Performance reads these threads; their the ones that recommended the 850's. Anyone need a set of Barry Grant Demon 850's with about 6 hours on them?

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..... :eek:

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.

I have seen a 250 blower set up for sale and the guy was using 1150 dom carbs

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

why don't you bring it to precision marine right there in kenner..worth paying the pro's $$$ instead of melting a ton of $$$$$$$$$...

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