[biagionim]

I had originally posted this in another section but thought I'd do it here also.

Hey Guys i just wanted to start off saying this is my first post to the forum. Well I have quite the issue that i cant seem to figure out. So lets get the specs out of the way, I have a 67 lemans with a 350 .030 over 4c heads ported and polished and about .015 shaved off the deck. the cam is a crower 60916, thats what was in it when i bought the motor. Carter AFB 750cfm carb, I believe? The number stamped on it is 9755s, from what I could gather it's a 750. Edlebrock performer intake and HEI ignition, headers and thats about it. Anyways when I start it it runs fine and all but as soon asa its time to go anywhere the engine bogs down and almost dies. Ive tried just about every timing degree there is all the way up to as much as 16 advanced! When its that advanced it dosent stumble at all! and Im kinda left scratchin my head on this one. Cause isnt initial timing suppose to be somewhere in the 6-8 range? any ideas would be great thanks in advance.

[azbirds]

Automatic or manual trans? Have you checked the accelerator pump to make sure its working? Hard or light throttle? Stock timing settings go out the window when you start to modify the engine parameters.

[b-man]

Try setting the total timing around 32 to 36 degrees (vacuum advance disconnected, vacuum line plugged). Take note of what your initial timing is, but the total number with all of the mechanical advance in is what really matters.

If the distributor is still stock and has not been recurved for performance, get that done as soon as possible. That's a good-sized cam and it needs an optimized advance curve in the distributor to run properly.

Unfortunately the Edelbrock AFB 'clone' carbs are known for bogging under hard acceleration on our Pontiacs and sometimes it's impossible to tune out. These carbs are calibrated for the Chevy crowd, their biggest customer base. The weighted flaps on the secondary side are not adjustable for opening rate, unlike a stock Quadrajet carb that's a lot more tuneable with its adjustable air-valve secondary design.

[moontower69]

hmm, that's kind of a big cam (same as crower 60242) for a 350, 14-16 initial might be what it takes to get rid of that stumble, you need about 34-36 total timing all in by about 2800- search timing and you get all kinds of info on this (plus vaccum advance debate, etc.)

[biagionim]

Yeah I found that about 16 initial it tends to clear up! But I'm looking into a holley 750 and maybe it'll help! Good idea about the advance curve on the distributor I didn't think of that I'll look into these ideas! Thank a bunch guys for all the insight

[lust4speed]

Hey Bart, I believe that biagionim is still trying to get out of the driveway - so the use of his secondaries before hitting the street might be a bit premature.

The use of the 750 AFB isn't that terrible of choice on his 350. Of course, I would also argue that the pre-set air flap doesn't see anything more than the volume of air passing by it and doesn't know whether it is on a 327 SB, 400 SB, or the similar displacement Pontiac blocks. I know that two 650 4's on my 455 never had a bog. I also know that old and new AFB's come jetted so lean out of the box that there is always a bog until properly jetted.

Many of us run 14-16 degrees initial after insuring that total timing doesn't exceed 34-36 degrees. Many GM distributors don't have much mechanical advance built into them and it might be safe to run the 16° and not have too much timing on the top end, but you won't know until you get a timing tape or dial back timing light. As Bart said, there's a lot of potential in a properly curved distributor.

[b-man]

Originally Posted by lust4speed

Hey Bart, I believe that biagionim is still trying to get out of the driveway - so the use of his secondaries before hitting the street might be a bit premature.

Mick, I guess gettin' out of the driveway means different amounts of throttle for different people.

I'm always at full-throttle.

[biagionim]

Well it does run and I do drive it quite a bit! I'm just very disappointed with the performance and I am trying to find out what the issue is! It seems to like it when initial is set at or around 16. I just thought it was odd of it to be that high! But I am gonna try rejetting the it and see where that leads me! I will say it runs great after I get past the hiccup! And the heads or ported and polish quite a bit so maybe what I'm thinking is a 750 AFB is not and it's just starving! At this point it's gonna be a long Saturday of test and tune

[b-man]

No way is that 750 carb size too small for a street 350, more than plenty.

I raced a moderately-built 455 with a Holley 650 double-pumper on it for 10 years, ran 12-seconds flat in the 1/4 mile with it. Tried a 750 DP and it didn't run any quicker, I was running a cam that's much larger than the one you have (221/229 duration @.050 for the 60916, 244/252 for the HC-03 I ran).

Your carb is big enough, but I'd replace it with either a Holley or a Q-jet if it was my car.

[bluzurb]

I have the 800 edelbrock on mine and i had a little stumble. stock 400 with lunati cam .490 lift. We jetted down the primaries one size and initial timing is 18 and at 2000 rpm its at 36. Got rid of the stumble.

[Socrates]

Originally Posted by biagionim

Ive tried just about every timing degree there is all the way up to as much as 16 advanced! When its that advanced it dosent stumble at all! and Im kinda left scratchin my head on this one. Cause isnt initial timing suppose to be somewhere in the 6-8 range?

Back to the basics, are you setting your initial timing with the vacuum advance disconnected? If not, do so. Nothing wrong with 16 initial if the car starts easily and if it gives you the total advance you engine wants, which could be from 30-40deg, depending on other variables. IF you're running low compression the engine might like a ton of timing with your current cam. Don't fight what works best for you combo just because you heard or read some other amount of timing was best for someone else! You no longer have a factory engine, the factory timing specs went out the door a long time ago.

How much vacuum do you have at idle, and how much do your RPMs drop from park to drive (assuming an auto). Working with the idle circuits or just upping the idle speed might help with your driveway bog, too. Get the timing right first, then play with the carb. Experimenting with more pump shot on the carb is a good place to start, along with just making sure you're getting a shot at all...

[biagionim]

It is a auto trans, a th400. When we initially dialed in the carb I was getting about 12-14 vaccuum, I haven't checked since the heads have been done. And as for the timing, when it's set to 16 initial (with advance disconnected) it starts not no easily. I'm off the next couple of days so I am definately going to be playing with it!

[azbirds]

Those carbs also have a spring under the valve that moves the needles. If you have the wrong springs in there for the amount of vacuum you are making at idle that could cause a rich condition.

[biagionim]

So should I put a stiffer springs in it? I don't even know what springs are in it!

[biagionim]

So I just checked manifold vaccuum and all I have is 8

[biagionim]

Ok so here is another update, I just checked total timing and you guys are flip on this haha, it's 55!!!! And I haven't had a chance to mess with it after that. How would I go about reducing total timing?

[b-man]

Checked the total with the vacuum disconnected, revving the engine to around 3,000 RPM or until the timing mark stopped moving under the timing light?

[steve]

My Initial is 16 and the car would run 18 if I let it. Sounds to me like you need to rebuild or replace that dist before your drive her again. You may be quite surprised at the performance increase.

[biagionim]

Heres the thing the dist is brandy new! I know it could be jacked up off the git go! Well dang this sucks! Anyways thanks y'all

["QUICK-SILVER"]

Originally Posted by biagionim

Ok so here is another update, I just checked total timing and you guys are flip on this haha, it's 55!!!! And I haven't had a chance to mess with it after that. How would I go about reducing total timing?

With vacuum advance hooked up and working 50°+ is not unusual with the engine reved up. Especially true on 8.0:1 CR smog engines.

There's been several posts about changing spray pump springs on these carbs. They need a stronger spring on the pump, or a weaker spring in the well.

The cam you're using will kill a lot of idle/off idle power compared to OE. It's advertised to be used with 10.0:1 plus CR engines. With the 8.0:1 350 it'll feel extra weak till you get it reved up.

So without a good stall converter you may have a problem that can't be tuned out. This happens when you run mix matched parts.

At your CR you shouldn't be having hard start problems with base timing at 16°~18°. I've run 20° plus base timing in cammed up 8.0:1 CR 455's without hard start issues.

You can tune it where it will do the best it can. With the wrong combination of parts, what you've got may be close to the best it's going to do.

IMO A lose converter that would allow the engine to catch its breath needs to be thought about. That would be after you get carb and timing straightened out.

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