sluggish down low! help!

Hey guys. So today I drove my car around my neighborhood today for the first time ever! I was super pumped! Brakes work great. Steering is tight! Little backround on it.

It's a 75 trans am with a 4 bolt main Chevy 350. It's punched to a 355. 10.5:1 compression. Running on Sunoco 93 pump gas. It has cast iron 2.02 camel back heads. Ported and polished. Has a edelbrock victor Jr. Highrise intake with a carb spacer. It's feed by a Holley 750 double plumper mechanical choke. The choke is wired open. HEI distributor. The cam is unknown spec. But it only pulls 6-10 inchs of vacuum.

Now the bad. It's just extremely sluggish down low. Below 3k I'd say. It breaks up a little also. The car starts fine! No hesitation. Also the idle jumps around a little. Between 900-1100. I'm not exactly sure where the timing is set. It's been a few months since I set it. I wanna say it's at 16 degrees. It did back fire once thru the carb. Should the power valve be replaced? I was wondering if this was a carb/fueling issue or if it's a timing issue or a combination of them. Any insite or things to try is greatly appreciated!

Thanks in advanced guys! And when in doubt burn out!

I'm not a tech guru, just an old shadetree wrencher, but from what you have told us, I'd say too much cam, carb not tuned right, and timing way off. The real eggspurts may tune in here pretty soon. Need way more info to address this properly.

Thanks Rrumbler. I'm thinking its carb tuning seeing it was my first carb I ever did. Plus the powder valve might be blown. Also timing.

For any other replies if you need more info just ask I'll give you as much info as I Gah! Thanks for the help!

Any backfire/pop back through the carb points to too much initial timing. I'd back it down to about ten to twelve degrees of base timing and see what it does. And yes, if your Holley runs a power valve, and if you backfired it any then the power valve is likely toast, so get your timing set first, see if it feels any better and then replace the power valve and check plug color to see where your mixture's tracking across the range.

Holly has a kit to fix the blowing of the power valve. Depending on how old the carb is. The later ones won't need it. Not knowing what the cam is could cause you problems with timing.

my guess is your power curve does not start until 3k + . high comp - big valves- big carb - daily driving it will be a dog most of the time and likely load up a lot .

It definitely sounds like some basic tuning is on order to optimize the engine and the advice above is all good.

After that is taken care of I suspect the car will still be a bit of a dog for the very reasons that shine wrote

"......my guess is your power curve does not start until 3k + . high comp - big valves- big carb - daily driving it will be a dog most of the time and likely load up a lot ......."

Add to that that that it is a fairly heavy car and the standard rear gear for the car (with automatic) was/is a 2.56, it will be no ball of fire from a dead stop.

Personally I'd start saving for a deeper set of gears, possibly a looser convertor (depending on the cam) and an overdrive tranny if you plan of doing any highway driving.


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I'm with the others, timing is probably off and check the power valve. A single plane intake like your Victor won't even start working till 3500 RPM... A dual plane intake and a 600 or650 vacuum secondary carb would move your power band down in the RPM range (probably some big lumpy sounding cam, too?) would make the car a whole lot more street friendly and better power. Bigger is not always better on cams and carbs!!!!!!

I would lean towards a dual plane manifold over a single plane for the street, like something from the Edelbrock Performer line that's effective from idle up to 5500rpm or so - Intake Manifolds - Introduction - Edelbrock, LLC.

a 750 on a sbc is way too much for street use. you cant tune away cfm. drop to a 600 first then work from there.

If I can be blunt, it's not a carb/fueling/timing issue. It's a package issue. The carb, cam, timing, manifold, compression ratio, displacement, trans type, differential ratio have to complement each other. However, it's pretty hard to make any comment without knowing the rest of the package. What trans and rear axle ratio do you have? The cam specs are also an absolute must.

I will venture this, though. The carb size and type are probably wrong for the engine, and the manifold is made for high rpm. Unless you have a manual trans, and a low rear gear, the double pumper is going to dump too much air/fuel mixture for the engine to run until it winds up. It appears you have a "package" that's always going to be soft on the low end unless you change some major components.

Welcome to CHR!

I have to concur with most of the above. If this vehicle is intended to see street use, you have too much carburetor, probably too much compression and in all likelihood too much cam. More information would be helpful for a diagnosis and as Jack mentioned – the entire package needs to function together.

Do you have the casting numbers for the heads? Not trying to be negative, but not all camel humps were created equal. Did you assemble? How did you come up with 10.5:1? Are you certain they're 2.02 valves? Did you have the heads surfaced? What was the final volume of the combustion chambers? Was the block decked? If so, what was your final measurement (looking for a number somewhere between 9.025 and 9.000).

If it were mine, I’d pull the heads and measure the deck clearance to determine if I could get the C/R down below 10:1 with a thicker head gasket. I’d then e-bay the Victor Jr and go with a Performer RPM and a 600CFM Edelbrock 1406 with the electric choke. You may need to lean down one size on the initial out-of-the-box jets and metering rods.

Assuming the valves are correctly adjusted, the backfiring is probably caused by 16 degrees of initial timing – much too high as is attested to by the fire happening while your intake valve is still slightly open, but with the cam you have it may not idle much less than 12 degrees. Ideally, the initial timing should be 8-10 degrees and 34 degrees all in (around 2000-2200 RPM). If you cannot get to this point, the cam may be too big.

6-10 inches of vacuum is unacceptable (in my opinion) for a street driven car. 20 inches of vacuum is a better number, 18 is about as low as I’d go on a street car with power brakes and below 18 is just scary for overall braking a 3800 pound car in a hurry.

My two cents - keep us in the loop and once again, welcome to CHR!
Glenn

Thanks for the input all. Like I said I'm only 23 and learning this whole carb and timing thing when it isn't done by a computer. But I work graveyard and only get time to play on the weekends ill check some other stuff out this weekend. And get back to you guys! thanks a bunch!

Hey guys stupid question but could it be total advanced timing? Or would that not matter since its vacuum advanced?

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Originally Posted by t.thompson259

Hey guys stupid question but could it be total advanced timing? Or would that not matter since its vacuum advanced?

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Like Glenn said immediately above, your total advance should be in the 34 degree range and will be all in by 2000 to 2200rpm. It builds on your base timing, so if you set the base timing (no vacuum at idle) to 8 degrees your total timing might be 30, but if you set your base to 12 it would then be 34. If your base is now at 16 then your total may be up in the 40 range which is pretty tall, but might still be OK depending on your piston choice and cam.

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Originally Posted by rspears

Like Glenn said immediately above, your total advance should be in the 34 degree range and will be all in by 2000 to 2200rpm. It builds on your base timing, so if you set the base timing (no vacuum at idle) to 8 degrees your total timing might be 30, but if you set your base to 12 it would then be 34. If your base is now at 16 then your total may be up in the 40 range which is pretty tall, but might still be OK depending on your piston choice and cam.

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Okay thanks. I set the timing with the vacuum advanced hooked up (rookie move) so imma redo the timing this weekend and take it around the block and see what it does then. Hope for no cops cause she ain't got plates on her yet!

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Originally Posted by t.thompson259

Okay thanks. I set the timing with the vacuum advanced hooked up (rookie move) so imma redo the timing this weekend and take it around the block and see what it does then. Hope for no cops cause she ain't got plates on her yet!

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You want to unhook the vacuum line from the advance can, but plug the line. A sharpened pencil or piece of dowel rod works great.

You might also check the vacuum source the line comes off. Best bet is to use a source directly off the intake. If you are using a source off the carb base, it may or may not have vacuum at idle. The carb nipple which has vacuum at idle is the one you want, and is referred to as 'ported'.

At some point in this process, someone is going to point out that at wide open throttle (wot) you loose vacuum, and you centrifugal advance is in control, and you can swap springs for controlling the centrifugal curve, but first let's nail down your base setting.

And lastly, don't forget to test that the vacuum advance chamber isn't leaking. The simple way to test is to apply vacuum and see that it holds that setting while you have the light hooked up. I have a little vacuum pump I use sometimes, but my cheapy way is use a new long piece of hose and "pull" a vacuum with my mouth and place my tongue over the hose while the other end is connected to the vacuum can. If it bleeds off vacuum, you can see it and feel it! Change the can and retest.

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Originally Posted by glennsexton

If it were mine, I’d pull the heads and measure the deck clearance to determine if I could get the C/R down below 10:1 with a thicker head gasket.

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Glenn, before learning differently, that may have been my suggestion as well. But reading through the Keith Black Piston catalog, I noted that John Erb, Chief Engineer for KB has stated that using a thicker head gasket to lower static compression ratio will increase the squish measurement and may allow the motor to detonate worse than it would have with the tighter squish, thinner gasket and higher static compression ratio.

Piston crown configuration and combustion chamber volume are ways to change static compression ratio. Piston deck height and gasket thickness are ways to change squish.

The fact is, this combination is not going to be street friendly because the OP has chosen the wrong parts.
1. Static compression ratio is too high for iron heads. 9.5:1 max for today's fuels. 10.5:1 max for aluminum heads.
2. Heads are steaming piles of dog poo. Much better heads are available for less money than re-furbishing these old relics.
3. Wrong intake manifold. Use a single-plane if you need to make power over 6500. At low rpm's, there is too much volume in a single-plane intake to generate good vacuum to fill the cylinders. Use a dual-plane, high-rise such as the Edelbrock 7101 Performer RPM, Holley 300-36 or Weiand 8016 Stealth if you want to make good street power from idle to 6500. Holley and Weiand part numbers are out of production, but these manifolds can be found in good used condition on ebay, craigslist, racing junk, etc.
4. Combination needs a 10", 3500 stall converter and a minimum 3.73 rear gear. More gear than that would be OK too, but 3.73 minimum. Use a vacuum-operated secondaries carburetor with a stock converter. Use a double-pumper with a looser aftermarket converter or manual trans.
5. Not enough ignition timing. Disable centrifugal weights and lock timing out at 36 degrees at the crank. Install momentary push-button (normally closed) with 12-15 amp capacity. Cut the power wire to the coil and install the switch on the dash or some other easy-to-get-to place in the driver's compartment. When starting the motor, push the momentary-off switch with your left hand to interrupt power to the coil so that the motor will turn over easily with the starter switch that you are operating with your other hand. Once the motor has spun up, release both switches and she will be idling, no muss, no fuss.

Use this tutorial to find true top dead center so that you can intelligently set the ignition timing with a light.......
http://www.crankshaftcoalition.com/w...op_dead_center


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Originally Posted by firebird77clone

You might also check the vacuum source the line comes off. Best bet is to use a source directly off the intake. If you are using a source off the carb base, it may or may not have vacuum at idle. The carb nipple which has vacuum at idle is the one you want, and is referred to as 'ported'.

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firebird, your theory is correct, but you have the incorrect term. Ported or "timed" vacuum ports are above the throttle plates and have no vacuum at idle.

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Originally Posted by Henry Rifle

firebird, your theory is correct, but you have the incorrect term. Ported or "timed" vacuum ports are above the throttle plates and have no vacuum at idle.

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Not always the case Jack, some carbs have internal passages that allow manifold vacuum to be seen with a port that is above the throttle blades. Idle the motor. Place a small piece of paper on the port. If it gets sucked down against the port, it's origin is manifold vacuum. If it will not stick to the port, it is a ported vacuum source.

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Thx Henry, I dunno how I got it backwards.

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Originally Posted by t.thompson259

Thanks for the input all. Like I said I'm only 23 and learning this whole carb and timing thing when it isn't done by a computer. But I work graveyard and only get time to play on the weekends ill check some other stuff out this weekend. And get back to you guys! thanks a bunch!

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That's the attitude we really like to see here. We're all still learning - some are just a bit further down the road.

As to your timing question, you could certainly try and set your initial timing at 8-10 degrees (vacuum advance disconnected and plugged) and see what difference it makes.

Another handy thing to keep in you toolbox is a couple of golf tees; use them to plug vacuum hoses or other things that need temporary plugging.

Well did some research and found out that the carb isn't a 750. It is a 650 double pumper. A Holley 4150. I ran the list number on it and its a 80777. So that's plus in my favor as to not over fueling. But we are inching closer to the weekend and time to get my hands dirty!

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Originally Posted by Rrumbler

Another handy thing to keep in you toolbox is a couple of golf tees; use them to plug vacuum hoses or other things that need temporary plugging.

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Also good to keep extras around if you should decide to slide out for the afternoon and shoot a round!!!!!:LOL::LOL::LOL:

Ok guys. So I set my timing light at 36 degrees. Cranked her up to 3k. And check the timing. (all the vac lines off this time. :P) and well she was at 10 advanced. So I'm guessing that means it was at 46 all in? Yes I know I suck. Lmao. Sooo anyway. I set it at 36. Tightened the distributor. And double, cripple and quadrupled checked it. And it didn't move! So out of curiosity. I put my t light back to zero and checked it at idle. And its at 8 degrees. I haven't got a chance to road test it yet. I had some other projects to button up. But it ideas better. Down around the range of 950rpm. I haven't played with the carb yet. But it's coming. Anything else you guys think I should check? I'll also keep you guys in the loop when I road test it.

Also do you guys know a efficient way to check to see if your v belts are lined up properly? The machinist side came out in me and I had to modify my alt bracket to work with my heads since there is no holes in them. Every once in a while when you goose it. You get a quick belt squeal.