Switched intakes, now stumbling, 650 DP

Nuke427 · #1 (**permalink**) 01-21-2004, 12:36 PM

I've replaced my air gap intake with a victor on my 302. I did this since I've installed nitrous and wanted a single plane intake so there's nothing in the way of the nitrous/fuel bar on the plate. I also bought a 4" tall air cleaner and replaced the 3" one, not that it should matter. Just trying to point out what's different.

Anyway, I've just taken the car for a spin and at part throttle the car is stumbling badly. Once I give it full throttle, it takes off and actually runs great to 7k. I'm sure I need to rejet now but not sure which way to go. The victor has much larger ports and I bought the fel pro 1262 and gasket matched the head ports. I had the fel pro 1250 on the air gap and didn't gasket match and some of the head port was covered a touch. I'm sure I'm flowing more at higher rpm's with the victor, and it really doesn't matter what rpm's I'm at, it stumbles at part throttle, even at 4-5k.

My float bowls are to the bottom of the sights, I never get any kind of back-fire, even letting off the gas in gear. My carb is a Holley 650 double pumper, ANY IDEAS? Other noteworthy engine upgrades are under my ID.

By the way, I have not used any nitrous during my testing.

Mike Simard · #2 (**permalink**) 01-21-2004, 01:45 PM

When you put the old carb back on, did you have to change the idle speed?

mikiec · #3 (**permalink**) 01-21-2004, 01:49 PM

My guess would be that the Victor Jr. is sending a different velocity/vacumn signal to the carb. You might have to adjust the accelerator pump to match the new manifold.

Mike

Nuke427 · #4 (**permalink**) 01-21-2004, 03:08 PM

I'm thinking the car is running lean. Vacuum has probably been lowered with the victor and it's not getting enough fuel with part throttle. Adjusting the accelerator pump? How's that done?

I guess a new question to you carb guys would be, how do I get more fuel under part throttle without changing the jets? Or is that what I need to do?

Nuke427 · #5 (**permalink**) 01-21-2004, 03:19 PM

Quote:

Originally posted by Mike Simard

When you put the old carb back on, did you have to change the idle speed?

Yes, but I also was adjusting timing since I had the distributer out and not marked too well. So I did a little of each of those. I'm sure I ended up bumping idle up since there would be less vacuum with this direct port single plane intake.

Mr.Fixit · #6 (**permalink**) 01-21-2004, 03:35 PM

You have lost velocity of the intake air charge. That plus the single plane replacing a dual plane intake means you lost vacuum signal at the carb. You may get satisfactory results by adding more fuel to the accell pump shot, or maybe even running a smaller squirter to lengthen the pump shot duration.

I am assuming that when you say it stumbles at part throttle, you really mean it stumbles as you transition from part throttle to slightly more throttle, but not at WOT.

Mike Simard · #7 (**permalink**) 01-21-2004, 03:45 PM

The reason I asked about the idle was because I might be inclined to suspect an old fashioned vacuum leak.

Nuke427 · #8 (**permalink**) 01-21-2004, 03:53 PM

Quote:

Originally posted by Mr.Fixit

I am assuming that when you say it stumbles at part throttle, you really mean it stumbles as you transition from part throttle to slightly more throttle, but not at WOT.

That would be correct, it does not stumble at WOT.

Nuke427 · #9 (**permalink**) 01-21-2004, 03:56 PM

Quote:

Originally posted by Mike Simard

The reason I asked about the idle was because I might be inclined to suspect an old fashioned vacuum leak.

I've checked for that, and I can't find anything. A vacuum leak would create the same symptoms I am having due to I have less vacuum now than I had with my old intake. Just need the cure!

Mr.Fixit · #10 (**permalink**) 01-21-2004, 03:57 PM

it doesn't stumble if you cruise at a steady (say 2500 rpm) part throttle, does it?

did you use the end seals. or silicone when installing the new intake?

Nuke427 · #11 (**permalink**) 01-21-2004, 04:10 PM

Actually, it does have a slight "miss" sound at cruising speed, steady at any rpm. I do have the intake sealed well. I used the end seals and a bead of silicone on top and slightly up the head. I've replaced many O' intakes and gotten pretty good at it. Carb tuning on the other hand, not so good.

Nuke427 · #12 (**permalink**) 01-21-2004, 06:01 PM

Possible vacuum leak located! Hole on passenger side of intake plenum was sticky taped over instead of an alan bolt in it. What the crap! It was kinda hidden behind my fuel rail. Too late now to see if it was the culprit.

By the way, this was my stupid mistake! When I intitially installed everything I didn't have an alan bolt so I used some foam sticky tape to allow me to start the car and adjust the idle/timing etc. This was a little while back and I'd forgotten about the bolt even though I had bought one. Anyway, it's put in place now, I just need to go on a test run to see if it cures my problem. It may snow tonight! ERRGHH!

bullseyeprec · #13 (**permalink**) 01-22-2004, 09:15 PM

Nuke, I am running a vic jr on my sttroker and I have pretty good vacuum signal. As I recall at idle I have around 15 inches and cruising I have more than that so I went with a 10.5 power valve. If you have really low vacuum, you might indeed have a leak. Nitrous..Man you are a wild man. Dont ever buy anything used from this guy...."grandma only drove it to the store on sunday for groceries" HAHAHA. Hope you get it fixed. Let us know what you figure out. Hope you dont get any snow.

Nuke427 · #14 (**permalink**) 01-23-2004, 06:09 PM

Still haven't been able to get it out! Today was 55 degrees but I just got home from work. I hate long work days! Gotta work all day tomorrow too. I'm off on Monday, but it doesn't look good weather wise. I keep kicking myself, if only I would have located it earlier and was able to do another test run!

I said I looked for leaks! I just didn't look good enough. I should have went out and started looking hard right after Mike Simard brought up the "classic leak". I was in denial. If I would have, I'd driven it and known now if I need a carb adjustment. Am I rambling?