this is on a Demon 750DP. I've been playing with pump cams all day and it seems to be getting me nowhere.

When i hit the throttle quickly, but only enough to open the primaries, it's sluggish, but it doens't really lean out...

however when i hit the throttle wide open from idle i get a huge backfire, and the af goes waayyy lean.

i can't seem to get the throttle response crisper, or keep it from backfiring when i open the secondaries. I almost think the rear power valve might not be functioning or something. In which case i'm thinking about maybe just jetting up the rear bowl and blocking the power valve. Does this seem like a decent idea?

I still have the stock accel. pump, and the stock squirters. Would larger versions of either help these problems? The sluggish throttle response on the primaries is really buggin me, and the backfire...well thats got to go...


Just to get more specific..when i gradually build up rpm to like 3500 or so and then floor it, it doesn't bog, and it doesn't go lean.

Any thoughts on what the problem is, and how to fix it?

Eric

It does sound like you need a bigger shotter size.

Try two or three size up, and see what happens.

Are you running an automatic? It's not uncommon for people to run 50cc pump or 40+ shotters with autos.

Ron

But have you checked your fuel pressure lately? My pump diaphragm was shot, and it would idle ok, and sort of accelerate ok but would hit a wall with the secondaries open.

I'm in the process of testing out a stock 1978 Chev. Pick-up truck pump instead of the stock 'bird one, but I have to wait for a radiator to show up at the door.

And don't power valves usually go bye-bye on a single backfire? It might be the other issue.

fuel pressure is good, 7.5 psi. I'm still not having any luck with the lazy throttle response.. ive tried adjusting the stop nut on the linkage that hits the accel. pump arm, and tried bending it slightly as per the demon video and instructions. Still nothing. i think maybe ill try stepping up the squirter size.

any thoughts on a good starting point for a squirter?

Eric

IIRC our combos are pretty similar...so I will give you my carb combo that may help ya out.I am currently at 73/80 jets...40/37 squirters...and a orange pump cam/stock pink...primary/secondary respectively.This is also on a Demon 750DP.I still have a small flat spot exactly as you describe but way better than in stock form from BG..I can stab it from a 5mph roll in second and ignite the tires whereas it used to backfire.I am thinking it is time to get a 50cc kit because of the 40 squirter on the primaries...I remember reading you should run a 50cc on any squirter over a 37.HTH


Oh yeah..another thing I did that helped out and cleaned up the over rich idle was install all 4 air bleeds that came with the carb.

oh man thats perfect...thanks for the help man. I'm gonna match your combo exactly and go from there.

you've got an xe274 cam too don't you?

Eric

I guess were all in the same boat I'm still trying to get rid of mine.
I run XE284 with a Victor Jr. intake & 830CFM carb @ 6300feet elevation, so you can imagine my problems.
However I did drop in a 1" four hole spacer to pick up a little more signal.
Tomorrow I was going to make a day of carb tuning.
'zupmanZ28', you need to have a full line of squirters, and cams to test & tune.
I started buying a **** load over the winter, and pretty much have every PV, jet, squirter, & cam Holley makes.
Your just going to have to narrow it down. Like today I had a .028 on there that was to lean, then I threw on a .045 which was to rich due to black smoke when a blip the throttle. So tomorrow I'm going to start off with a .037, and go from there.

I'm also using a AF gauge to help out a little bit. I can tell right now it's damn right accerate on the lean side, however the rich side sucks. That's when I pop out a plug & start reading.

I let you know how it goes, and btw I run at 6 PSI...

Ron

Yeah I love that 274 cam..midrange is wicked.But like ronterry said...get yourself an assortment of parts and swap till it gets better...I have been tinkering with this Demon for two years now...but have learned how each and every part of the carb works and what effects what.Thats the fun part though...beating on the car.

i've got some parts. I've got the jet kit, and a few power valves, and the pump cam kit. Thats about it.

Is there some kind of official term for a squirter? I was on Holleys site and i couldn't find anything.

Eric

Accelerator Pump Discharge Nozzle

Straight Type - $8.00 a pair
http://www.holley.com/HiOctn/ProdLin.../StrtNzzl.html

Tube Type - $16.00 a pair
http://www.holley.com/HiOctn/ProdLin.../TubeNzzl.html

Ron

ahh there we are..last question, whats the difference between the tube type and the straight type?

As much as I have learned about carbs..I have no idea!Anyone else?Is there a performance difference to one or the other?I stayed with the straight type as that is what came on the Demon..but...now I wonder.The tube type is also more $$$....that has got to mean more performance right???JK.Anyone that can help us understand the differences??

The tube type is suppose to direct the shot closer to the boosters for better atomization.

Ron

well i just changed the squirters, the throttle response is now live-with-able, but i still get the backfire when i try to go wide open from idle.

Any thoughts?

i have 40/37 squirters, 72/84 jets, green cam/pink cam. What else could be the problem? I'm going to step the back squirter up to 40 and see what that does.

btw, this is a lean backfire. Whenever it happens the AF goes waayyyy lean.

Eric

Did you try the orange pump cam?What made you choose green?Just curious..I found the orange to work the best for me so far but most others get great results with the green..kinda like 50/50 split.You may have to get a 50cc accelerator pump because of the large squirter..I think that is where my bog is coming from...the pump shot is aggresive enough now..just doesn't have enough volume to get it through the "hole".

i have a 700r4 and a 750 dp carb and i am running dual 50 cc accelerator pumps, and a .42 and .47 shooter front and back respectively and 16 degrees base 36 total timing and my car runs liek a rape date.
joe