This is for those of you running a carb blow thru system.
Well, heres the deal. I first read and listened to what turbomustangs post says on how to modify a Holley DP for Blow Thru setup and did everything it says except drill the pvcr's and it ran horribly rich. I have a 4150 Holley 750 double pumper with mechanical secondaries and it ran fine when the motor was N.A. now when I would put the carb hat on it would just flood. It used to run #71 primary and #78 secondary jets with 3.5 power valves on front and back due to my cam with little vaccum. After having it just flood out the second it saw a little air comming from the turbos at idle it would just flood and die.
I fixed this by putting in 1.5 hg holley power valves and using a #60 jet for primaries and a #82 in the secondaries. Now it runs great. Idles incredibly good and responds magnifisant. Runs a little rich at idle but not noticibly rich.
Make sure you use plastic nitrophyle floats ( i tried using my stock brass ones for a while and first time the engine saw 4 lbs of boost, they got crushed and gushed fuel into my engine) and make sure you JB Weld the choke linkage hole if you have one. Also put some JB weld on the ribits on top of the metering blocks.
My engine is now running very nice.Dont listen to the part that says you have to jet bigger if you go blow thru. Thats bull crap. The carb see's pressure and it forces more fuel thru the jets and your speeding up the air going thru the venturies, therefor you get more fuel and have to go about 4 jet sizes smaller on the primaries.
Any questions just ask. Its not as hard to do.
I dont know why all the web sites say jet up 3-4 sizes, when in reality you have to jet down. Also try to put a low number power valve that way it kicks the extra fuel at 0 hg which is when you start to get boost. Hope this helps the people asking me how I did it. Dont waste your money on a $599.00 Ebay blow thru carb when you could build your own.

 

That is odd. I am on my second blow thru set-up and have had to add jets at the minimum. Current set-up even drilled pvcr and installed annular boosters.

Do you have any data logging of a full pull for a/f, etc? What hat are you using? I would just be worried that at the top end you will lean out. It may seem fine at low rpm/boost where the added air is just cleaning up your rich a/f mixture, but I would be careful

I do agree you can successfully build your own carb.

I would recommend that people start conservatively (too much fuel) before they just pull jets out

Matt
blow-thru 383

 

Hey that is odd but its happened to me and a friend of mine running about the same setup. Could it be the smaller cubes?
Or could it be the cam I am running or the carb. I am using my friends wide band (PBX model) and it shows me around 13.2 at idle, but its constanly changing from 13's to 14's at idle due to the large cam. I do plan on drilling the pvcr's in order to have it tuned better for top end. On the upper rpms like 5800 it will start to lean out slightly. That is the only dangerous part leaning out on higher boost. I will drill them bigger and jet the secondaries slightly bigger this weekend. What do you recommend me to drill the pvcr's to?
Thanks for your input.

 

I believe that carb may be too big for your set up. I think you would get a better signal from a 650. Are you running a secondary pv? Do you have adjustable air bleeds?
Try just increasing jets first. Drilling is permanent so do it in small steps.
*Just as a reference- my low boost (6psi) set up just needed jets; current set-up (12+) drilled to.078

Matt

 

Thanks I will check in to it. I am running the pv in the secondary also. It gets it to run better on top end. I will do more testing. Thanks

 

Hey, I am considering boosting my 305. I have a Holly 650 and a Torker intake. I wanted to know what happens to the timing when all the extra air is introduced from the turbo?

Tim

 

Nothing happens when all the extra air goes in. You are the one that has to retard the timing via a boost timing master, or by locking out your initial timing backed off a little. If you have high or higher than stock compression, you engine will ping like hell when the boost comes in if you dont adjust your timing and fuel settings. I am using the BTM which does the retarding for me and low boost and it still reads a couple of volts on the knock sensor,but its not as bad. I still need to play wth my timing a little more though. Hope that helps.