speedtigger

iTrader: (16)

I have been chasing a lean tip in problem with my Pro-Systems 780 since I put it on the car. I have tried just about everything. I even sent it back to Pro-Systems and had them completely gone through it and send it back. Still, it would go dead lean just over idle.

Well, today I really went after it. I started swapping everything from my Holley 750HP. I tried the metering plate, high speed and low speed bleeds and nothing worked. Then, I tried the one thing that should not have made a difference. I swapped on an old 750 base plate off the shelf and whamo! Lean tip in problem gone. I have no idea what is wrong with the Pro-Systems base plate. It looks fine to me. Obviously I am missing something. So, I will call Brad on Monday and tell him what I found. I am relieved to have found the culprit, but puzzled as to what the problem is with this base plate.

2efnfast

iTrader: (2)

that's pretty weird

Paul57

Interesting...Good to hear you found it. Any chance the bad plate is warped or the gasket didn't seal well?? I am just throwing things out for ideas.

Pop N Wood

iTrader: (1)

I changed out my Holley baseplate because of a sticking throttle condition and saw a similar change in the carb performance.

I moved up to a quick fuel baseplate and I know they use much better throttle shaft seals. Or maybe the machining was just more precise.

ZONES89RS

iTrader: (25)

Hmmmm, my 750 vac sec on my 4.8 does this as well.

GC99TA

iTrader: (30)

Congrats on getting it figured out! I admire your patience and diagnosis technique. I'll be real curious to hear what Prosystems has to say about this since they use their own baseplates. Was your lean tip in condition only apparent on the wideband or could you notice it otherwise?

TJ

iTrader: (10)

what baseplate gasket was used? did you compare the baseplate gaskets betweeen the 2 carbs?
there are several different base gaskets.

speedtigger

iTrader: (16)

I know which is which. I know the <750 from the 850> from the others. I have been at this for 25 years. In fact. I used the same throttle body gasket that came with the Pro-Systems carb.

speedtigger

iTrader: (16)

Oh yeah. It would actually stall. And, at a cruise it would lean misfire and get rough.

rojs234

iTrader: (5)

Hey, "tigger". Good job finding the fix. A couple of the tuners on the Innovate web site were having the same problem a few months ago and solved it the same way that you did, by swapping to a Holley base plate. If I remember right, they finally decided it was the thickness of the aftermarket plate and the length of the transfer slots that caused the problem.
Ron

speedtigger

iTrader: (16)

This is really an interesting situation Ron. I also tried my Pro-Form base plate off of my home-built 750 HP and it worked with that one too. We shall see what Brad or Patrick thinks on Monday. It can't be common for them or they would have know right away when I complained.

ZONES89RS

iTrader: (25)

Well, I am thinking that my 4.8 just doesn't get a great signal down low possibly for the existing air bleeds on the 750 vac sec. They are not adjustable though so I think finding a 650 would get me set. Only does it at slight roll I to throttle down low when taking off.

Pop N Wood

iTrader: (1)

Drill them out and tap them for a 6/32 brass set screw. That is what I had to do. You can buy everything you need from McMaster Carr for probably $20

rojs234

iTrader: (5)

Did you mean 10/32 instead of 6/32 for the air bleeds? A 6/32 brass set screw won't accept the drill sizes normally used for air bleeds. I have a 3310 Holley that I drilled and tapped for 10/32 set screws in the air bleeds, 6/32 set screws in the idle fuel restrictors and 8/32 set screws in the power valve restrictions. As you suggested, I used brass set screws in those sizes that were 3/16" in length that I got from McMaster Carr. Excellent place to buy all kinds of hard to find stuff.
Ron

ZONES89RS

iTrader: (25)

Well, if i get a really good deal on a 650, i think that would be the smarter choice.

allblowdup

I have seen the throttle cable adapters that bolt under the carb create a bad stumble off idle. The plate acts like a shear plate and pulls the fuel out of suspension then it just runs down the side of the intake.

ten5nova

I have a 5.3 with a carburetor, and my 750 Quick Fuel did the same thing. I tried everything, squirters, air bleeds, transfer slots, etc. Finally gave up on it, and put my buddy's AED 850 on the thing. It didn't work at first either, but was better. It finally started backfiring through the carb on the hit, then I knew what was wrong. I put on bigger squirters, and the biggest pump cams I could find, and it worked great. Took it to the track and had a 1.78 60 ft. in my Nova. Carbed LS engines are tricky to tune, but once you get them right, the work awesome.

Good luck finding your problem Tigger!!

speedtigger

iTrader: (16)

I wanted to update this thread because I have learned more since facing this issue. Putting the old baseplate on the newer carburetor was not really a true fix for what was wrong with my carburetor. The reason it helped is because the old baseplate was a two corner idle baseplate. They have idle transfer channels and different secondary idle ports. So, while it made the problem go away, it was not really a real fix per se.

The real fix was to drill the idle feed fuel restriction orifices in the metering blocks out .002". In this carb that meant going from a .035" idle fuel restrictor to a .037" idle fuel restrictor.

My particular combination just needed more fuel at low speed cruise and tip in. I hope what I learned helps somebody in the future.