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Discussion Starter · #1 ·
motor:
360, 10.5:1, hydraulic roller cam (no exact specs but mildly lopy), turbo 400 w/3000 stall, 3.55 gears.

I called Holley and spoke to a few very reputable engine builders here. All agreed that for what I have and want, the 650 double pumper was the way to go...so, my shiny new carb is bolted on and I've started some initial tuning on it.

It's idling very nicely. It has a fairly substantial stumble from dead stop to WOT and the plugs are reading lean. When I yank the throttle lever, I hear a GASPING of air when it stumbles. Is this the carb starving for fuel, hence the stumble?

I've got about 20" of vac at idle in gear. The stock powervalve is 6.5. Based on reading I've done, I should be using a PV that is roughly half of the vaccuum reading, right? Should I bump it up to a 8.5 or higher? Would this help the stumble?

To help with the stumble, should I work the jetting (lean plugs)? The stock jets are 68's in the front and 73's in the back. The previous carb had 68's in front and 74H (what is the H?) in the rear (but that carb was never quite right..think it was leaking fuel past the throttle plate slot...i could lean the mixture screw out completely and it would still idle). I canged the stock 28 squirters to the 31's I had in the old carb. If I am bumping the jets up, to what sizes? What is the appropriate difference in jet sizing from primaries to secondaries?

I haven't messed with the accelerator pump at all. I move the throttle a bit and I get a little fuel, but maybe not enough..that would cause the stumble right? Would larger jets help this (and the lean plugs) or do I need to mess with the accelerator pump spring and/or go to the larger accelerator pump and diaphragm?

Thanks for any suggestions.
 

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Before you do anything, make sure there is no "slop" in the accelerator pump linkage. Just adjusting the idle screw can create this slop, so make sure to check it after makes idle speed adjustments. I like to wiggle the accelerator pump arm and look for very little play.

I recommend that you do not mess w/squiters and or jets just yet. Just screwing around I ran 20 different carbs from 600DP's to 870 vac secs on my 454 and none, I mean none, ever needed any kind of accelereator pump work to get rid of a stumble...they all ran great w/factory specs. If you have insufficent timimg, I guess you could give it enough pump shot to mask it, but make sure you have PLENTY of timing. Anytime you get a big hesitation or backfore through the carb that mean timing in my book.

Start w/stock specs, make sure the transfers slots are relatively square, and go from there.
 

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first set the floats, then if your plugs are still reading lean go up 2 jet sizes at a time unitil the plugs look good, you dont want to start messing wit the acc pump until your jetting is correct because all you will be doing is masking a lean or rich condition. after you get it running smooth on the primaries adjust the secondary jetting for fastest acceleration
 

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Discussion Starter · #6 ·
cool...great info guys.

FLoats are perfect and mixture is too.

The accelerator pump has no slop. I made sure of that. I read the holley how-to on the accel pump adjustment:

WOT, push the pump lever down, check for .015 clearance. When I open to WOT the pump lever is all the way down. I can stuff a .015 gauge in there but fairly forcefully. I won't mess with it for now.

I've got the MSD pro billet dizzie. It's not hooked to the manifold vaccuum or the timed vac port. Should I hook it to one or the other? I think when last set and tested, I had like 38deg total timing advance (can't remember the initial timing). I'll check it today as well.
 

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try connecting it to a manifold vaccum source, you may have to reajust your idle speed as it will speed up a little, vaccum advance helps the engine run more efficent under part throttle. the engine runs cooler and you get better gas milage
 

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Couple of additional comments from what's posted so far -

#1 - you're timing has to be the first thing to get nailed down, it's one of the biggest causes of stumble.

#2 - Since it sounds like you're running lean all the time, up your jet sizes 2 or 4 as mentioned above.

#3 - the jets won't affect your "off-idle" stumble, that's the shooter size and the pump cam position. I never mess with the pump cam because I've never had to - others here do. Go up shooter sizes by 3 at a time until the off-idle bog goes away. My guess is you've got a shooter sized about 25 or 27 in there, and it should probably be 10 above that.
 

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Discussion Starter · #9 ·
I upped the fronts to 70's and in a quick spin and check, the plugs are a little less lean. I went up to 31 squirters from the 28s that were stock. I'll get something even bigger and see what I can do with the stumble. I had some great luck customizing a pump cam in an old Ford Truck I had built, but I had very good directions (had to even custom grind the cam a bit, but man what a great improvement it made).

Just so I'm 100% positive before I start messing with it, turning the dizzie counterclockwise advances the timing, right? Should I just be advancing it until I have pinging then backing off or is there a better way to adjust? Pinging can also be caused by lean mixture (too small jets) right? If I advance the timing, I may have to increase jet sizes as well, right?

again, thanks tons to all!
 

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is that your new carb you're working with now? dblpmpr is not normally a good choice for a heavy small block car with an automatic.
 

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Discussion Starter · #11 ·
Yes, the new carb is the 650 dblpmpr...per the recommendations of Holley and a few very known engine builders here. They said that with the 3000 stall converter and the performance of the motor, and that I am not overly concerned with mileage vs giddy-up, the doublepumper would be what I want. I took it for a longer driver...wow. I can tell you for sure, that the other carb wasn't right. The car had a ton of snap off the line (and I still don't have the hesitation worked out fully) and cruising at like 40mph, downshifting into 2nd and stomping it, she gets SIDEWAYS.

The plugs looked good after my longer drive with the 70's in the front and the stock 73's in the back...but shouldn't the secondary jets be larger with 70's up front? Whats the best way to tune the secondary jet sizes?

I'm going to pick up the larger squirters tomorrow and see if I can't get the stumble worked out.
 

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Discussion Starter · #12 ·
I upped the squirters to 37's (was going to get like 35's but the shop didnt have them). made a noticeable difference. There is still a bit of a stumble so is it fair to say that I should maybe step up again since the first jump helped a lot?

Should the secondary squirters be the same size or do I up them a few steps over the primaries for best performance? Whats the best way to set the secondary jet size for max performance?

I'm going to install an air/fuel meter and an O2 sensor. I had one in my last hotrod and it helped tremendously in tuning. that probably will be the best way to tune the secondaries...romp on it and watch the meter.
 

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Should I just be advancing it until I have pinging then backing off or is there a better way to adjust?
Best way to adjust your timing is with a timing light, I wouldn't trust any other method. If your like most of us you have way too much money in your engine/car to be guessing. Beg, borrow or steal one if you have to :D . I agree with everyone else, sounds like your timing could be off. Long duration cams usually want more timing. Keep us posted on your progress.
 

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If you had to go that far with the squirters and still have a stumble, then you probably have another problem and are just covering it up.
I agree - likely your timing is still not where it should be.
 

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Discussion Starter · #16 ·
i have a timing light and use it. It's not a fancy dial-back version, but it works. I'll recheck it again tonight. What would be a good initial advance number and where should I target for total? other than pinging, how do i know if I have too much total and how do I know if I have too much or too little initial?

again...much thanks
 

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i have a timing light and use it. It's not a fancy dial-back version, but it works. I'll recheck it again tonight. What would be a good initial advance number and where should I target for total? other than pinging, how do i know if I have too much total and how do I know if I have too much or too little initial?

again...much thanks
With a performance cam I would try somewhere between 16-20 initial and no more than 38 total. As far as knowing if there's too much timing...read your spark plugs and if you see tiny metal particles stuck to the porcelin (sp?) then you have a detenation problem which could be caused by too much timing. Basically you will know you have it right when the car doesn't stumble, no pinging and your spark plugs look good. Right now I have my timing locked out at 38* because the converter is not a good match with my cam (will be fixed soon). Question, if you drive the car for 20 to 30 minutes does your stumble go away? You can PM me if you don't feel like keeping this thread going.
 

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Discussion Starter · #18 ·
thanks again. I havent seen any metal on the plug insulators. I'll recheck the timing tonight and check on the "after driving it a while" question.
 
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