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Topic Review (Newest First)
01-25-2013 12:06 PM
CHEVILLAC Pulled the valve covers today and found out these are no mystery heads, they are 3947041's which should be 64cc chambers if the weren't resurfaced but I'd lay odds they were so maybe 63cc's? I'm guessing the CR is somewhere around 10.5:1 because from what I've read you have to run 91 octane or higher fuel at that ratio to avoid detination and I asked the previous owner about what fuel he ran and he said premium because the engine would sputter and ping on anything less. I bought a 3500 stall converter and I'm looking for a used 3 series posi carrier with 3.73's in descent condition for my 10 bolt. In the meantime I'll have to run this motor with my open 3.08 12 bolt because I don't think the motor that's in my truck now is going to last much longer. I'll get the new motor in and going next weekend with my 600 edelbrock and post an update. Hopefully I won't have any more carb questions. Thanks again for all your help and suggestions.
01-20-2013 06:05 AM
cobalt327
Quote:
Originally Posted by CHEVILLAC View Post
Ok, I know ported vacuum for the vacuum advance is hooked to the carb so I'm guessing it is operated in a similar way as the vacuum secondaries, otherwise a motor that has very little manifold vacuum wouldn't be able to operate the advance. If the motor runs well as it is, what would be the advantage of adding the vacuum advance back into the equation? Will it be better for for driving conditions between idle and WOT?
Ported vacuum (passenger side vacuum port on the Edelbrock carb) is only there when the throttle blades are opened. When closed, no vacuum. What that means when applied to a vacuum advance is there will be no added advance at idle. You already have a high enough amount of initial advance (or you should have, like 20 degrees-plus) for the engine to idle well w/that cam, so no additional advance is needed, or wanted.

The vacuum advance will only add advance to your engine at light throttle/light load conditions. This helps the engine to run smoother, maybe a tick cooler, and will use less fuel. There are no downsides to using it- as long as it's done as I've outlined.

Once you get it running, check the initial and total advance to be sure it's what it should be.
01-19-2013 10:35 PM
F-BIRD'88 I would change the camshaft to a new summit cam that will run fine the the stock torque converter and gears
you got. A new summit k1104 camshaft and lifter set is $99 and solves all your expensive problems.

Plenty of performance and a good match to everything else and the purpose. The price is right too.

The camshaft mismatch is the problem that needs a fix. Any carb will work with this cam.

You will like this cam in your truck.

Another good cam choice for you is Summit cam #SUM-1788. Either one of these cams fixes your (I got the wrong cam in my motor for my truck) problem, correctly, for a small cost.
01-19-2013 09:17 PM
CHEVILLAC
Quote:
Originally Posted by cobalt327 View Post
That's good that the distributor is dialed in already. Just give it a check w/a dial back timing light (or whip up a temporary timing tape) to double check it. If the damper is a stock part, it's a good idea to double check TDC to be sure the outer ring hasn't moved. You can mark it w/a line to keep an eye on it.



One thing more- you can use a vacuum advance, and I recommend you do so. The caveat on this is twofold:

Use ported vacuum
Use a Crane p/n 99619-1 limiter plate to keep the added advance to about 10-12 degrees (or make one)
Ok, I know ported vacuum for the vacuum advance is hooked to the carb so I'm guessing it is operated in a similar way as the vacuum secondaries, otherwise a motor that has very little manifold vacuum wouldn't be able to operate the advance. If the motor runs well as it is, what would be the advantage of adding the vacuum advance back into the equation? Will it be better for for driving conditions between idle and WOT?
01-19-2013 03:53 PM
cobalt327 That's good that the distributor is dialed in already. Just give it a check w/a dial back timing light (or whip up a temporary timing tape) to double check it. If the damper is a stock part, it's a good idea to double check TDC to be sure the outer ring hasn't moved. You can mark it w/a line to keep an eye on it.



One thing more- you can use a vacuum advance, and I recommend you do so. The caveat on this is twofold:

• Use ported vacuum
• Use a Crane p/n 99619-1 limiter plate to keep the added advance to about 10-12 degrees (or make one)
01-19-2013 12:32 PM
CHEVILLAC
Quote:
Originally Posted by cobalt327 View Post
CHEVILLAC, in case I wasn't clear on my recommending you use the small Edelbrock carb, this is to get the engine up and running. If the carb is in good shape, you can basically bolt it on and w/a possible/probable change to the step up springs, you'll be able to at least drive it to see what else is needed.

I always advise guys to first set the distributor up w/a performance timing curve, before tuning the carb beyond adjusting the idle mixture screws and the curb idle speed.

A performance timing curve for a stock GM HEI consists of lighter mechanical advance weight springs (the weights are usually OK as-is) and limiting the mechanical advance to about 10-15 degrees. The rest of the timing (about 20-25 degrees) will come from the initial timing. Details are at the link I added several posts ago.

I seldom recommend locking the mechanical advance out so it doesn't work, but in this case- w/the cam as big as it is- you can do this to get it up and running. Then later on you can go back and set it up w/a curve if you want to. This is easy to do but it may require an ignition interrupter switch be installed so the engine doesn't kick back against the starter when you start the engine (locking the mechanical advance and the interrupter switch are also covered in the link). I'd use about 32 degrees timing to begin with- not optimal, but enough to see what you have. This isn't what you want w/a high rear gear, but as long as you don't mat the throttle from a too-low rpm you should be OK. If you hear detonation though, you'll need to back off the timing, back off the throttle/load on the engine, or add the curve I talked about above.
The previous owner says it's setup to be all in (initial + centrifugal) with 36 or 38 at 3500rpm and no vac adv. The motor was setup and tuned on the dyno and the only thing he did since was pull the motor from the car and remove the fuel injection. I know it ran like a scalded dog at that point so I just want to leave it as is, get it carb'd with the proper stall and go from there. I read the link on performance dizzy setup and bookmarked it for later in case I decide to go that route. I'm just reluctant to try to fix something that isn't broke, I just want to adapt it to carb.
01-19-2013 12:18 PM
CHEVILLAC
Quote:
Originally Posted by T-bucket23 View Post
I would get the gears changed as soon as possible. They 3500 stall isnt going to help much with the 308 gears.
I agree with Cobalt on the carb. Use the one you know is good at least to start.
I would love to improve the gears immediately but unfortunately that's the most expensive part. The 3500 stall I can get for about $150. I think I want to use a vac sec 750 holley for ease of driving, I don't want to have to drive the truck like it's a manual all the time, a good used one can be had for $100 or so, but I'll get the motor going with the eddie for now. The gears gears and everything that go's with changing them will run me almost as much as I paid for the motor. I'm on a fairly limited budget right now but I need to get this thing going.
01-19-2013 11:32 AM
cobalt327 CHEVILLAC, in case I wasn't clear on my recommending you use the small Edelbrock carb, this is to get the engine up and running. If the carb is in good shape, you can basically bolt it on and w/a possible/probable change to the step up springs, you'll be able to at least drive it to see what else is needed.

I always advise guys to first set the distributor up w/a performance timing curve, before tuning the carb beyond adjusting the idle mixture screws and the curb idle speed.

A performance timing curve for a stock GM HEI consists of lighter mechanical advance weight springs (the weights are usually OK as-is) and limiting the mechanical advance to about 10-15 degrees. The rest of the timing (about 20-25 degrees) will come from the initial timing. Details are at the link I added several posts ago.

I seldom recommend locking the mechanical advance out so it doesn't work, but in this case- w/the cam as big as it is- you can do this to get it up and running. Then later on you can go back and set it up w/a curve if you want to. This is easy to do but it may require an ignition interrupter switch be installed so the engine doesn't kick back against the starter when you start the engine (locking the mechanical advance and the interrupter switch are also covered in the link). I'd use about 32 degrees timing to begin with- not optimal, but enough to see what you have. This isn't what you want w/a high rear gear, but as long as you don't mat the throttle from a too-low rpm you should be OK. If you hear detonation though, you'll need to back off the timing, back off the throttle/load on the engine, or add the curve I talked about above.
01-19-2013 11:12 AM
T-bucket23 I would get the gears changed as soon as possible. They 3500 stall isnt going to help much with the 308 gears.
I agree with Cobalt on the carb. Use the one you know is good at least to start.
01-19-2013 09:29 AM
cdminter59
Confused about carbs, need help!

Here is a summits description of the sum-1108 cam. Basic Operating RPM Range: 3,400-7,000. Rough idle, excellent high-end horsepower. Needs 10.5:1 and higher compression, 3,500+ stall, and gearing. 350+ c.i.d., bracket race cam. With that cam you'll want a 3500 or 4000 stall converter. Holley 750 DP and use the single plane intake since you already have it.
01-18-2013 08:35 PM
CHEVILLAC
Quote:
Originally Posted by cobalt327 View Post
The idle vacuum doesn't matter as to whether to use a vacuum secondary or DP carb. When you floor it, either way the vacuum drops off to nearly nothing, regardless if it has a small or big cam.

Heavier vehicles and/or higher (lower number) rear gear ratio equipped vehicles are generally easier to drive w/a vacuum secondary carb. Light vehicles w/low rear gears take to a DP like a duck to water and aren't as likely to bog on you.

Either will work (the DP requires a learning curve- think of it as the difference between an AT and a stick shift). But for the street most guys tend to like a vacuum secondary-type carb better than a DP, and the mileage will tend to be better as well.

There's a lot of trickery that goes into a race engine. Glad you know the cam in this engine. I guess it's the heads that nothing is known about and I confused the two. My mistake.

Good luck.
Ah yes, I think I got it!
01-18-2013 06:57 PM
cobalt327
Quote:
Originally Posted by CHEVILLAC View Post
Cam specs are in my original post but it is a summit cam, k1108. This is setup as a race motor and has very little vacuum at idle but the previous owner was able to drive it on the street. This motor was in an all steel 41 willy's 4 door and clocked 10 flat on the strip with a 150 shot of NOS, several times. I know vacuum secondaries are generally better for vehicles less than 3000lbs or so with automatic transmissions and mechanical secondaries are generally better suited for heavier ace vehicles. I want to take this truck to the track but be able to drive it to a cruise-in or car show too, hopefully without changing anything except MAYBE the distributor and the stall converter for now and add my posi later. Which brings me here asking which carb to run. It looks like a 750cfm is the consensus and I'm guessing mech secondaries since I have very little vacuum. Unless a vacuum secondary carb can be tuned to operate correctly with little vacuum, So because I'm also reading that vac sec carbs are more street friendly and pretty good at the track when a person is not so into racing that they're trying to shave tenths off their et. So I'll put the 600 eddy on there so I can get a vacuum number because the previous owner doesn't remember how much the motor had because he didn't care since he ran with fuel injection. From there maybe I'll have a better idea which carb to use. I do the fab and body work part of building custom cars and truck. This is my personal ride and I want it to be fricken fast and track worthy. I'll upload some pics and maybe that will help some of you understand my perspective a little better, if you don't already. I've never had more than 300hp and never been the driver at the strip but I drive my truck like I stole it.
The idle vacuum doesn't matter as to whether to use a vacuum secondary or DP carb. When you floor it, either way the vacuum drops off to nearly nothing, regardless if it has a small or big cam.

Heavier vehicles and/or higher (lower number) rear gear ratio equipped vehicles are generally easier to drive w/a vacuum secondary carb. Light vehicles w/low rear gears take to a DP like a duck to water and aren't as likely to bog on you.

Either will work (the DP requires a learning curve- think of it as the difference between an AT and a stick shift). But for the street most guys tend to like a vacuum secondary-type carb better than a DP, and the mileage will tend to be better as well.

There's a lot of trickery that goes into a race engine. Glad you know the cam in this engine. I guess it's the heads that nothing is known about and I confused the two. My mistake.

Good luck.
01-18-2013 06:35 PM
CHEVILLAC
Quote:
Originally Posted by cobalt327 View Post
If you're going to use a Q-jet, the amount of vacuum the engine makes has a direct effect on what spring to use for the power piston. Too much spring/too little vacuum will have it running pig rich (will be always on the power circuit) anywhere except WOT. Same concept as the power valve on a Holley or the step up springs on an Edelbrock/Carter.

Depending on what the Q-jet carb was originally from, it may need other changes (APT adjustment, jets/rods, bypass air orifice size, more idle fuel, to name a few). I don't say this to dissuade you from using it, just want you to be aware of the possible pitfalls.

Again, for the purpose of getting the engine up and running: Use the Eddy and a dual plane. Use a properly set up distributor.

Once you know what the idle vacuum is and how the cam is, then you can go about tuning it for real.

Also, the cam is the big unknown. It might be any sort of cam. Depending on what class it was built for, it could have a stock lift cam (often they'll have a ton of duration), it could be a cam ground for a vacuum rule, it could be set up to intentionally loft the valves off the nose of the cam lobe to circumvent the rules, could have hydraulic lifters on a solid cam (or vice-versa), it may be excessively advanced/retarded, et cetera.

If you cannot get a straight answer on cam specs/grind number from the guy who owned the engine, you really should take the time to measure it or see if there's some ID on the cam- whatever it takes. Because the way it stands now, you're (and we're) totally in the dark. This makes giving meaningful recommendations a total crap shoot.

3500 stall on the street is not as bad as you may think. But again- w/o knowing what the cam specs are, picking a stall speed is a total guess.
Cam specs are in my original post but it is a summit cam, k1108. This is setup as a race motor and has very little vacuum at idle but the previous owner was able to drive it on the street. This motor was in an all steel 41 willy's 4 door and clocked 10 flat on the strip with a 150 shot of NOS, several times. I know vacuum secondaries are generally better for vehicles less than 3000lbs or so with automatic transmissions and mechanical secondaries are generally better suited for heavier ace vehicles. I want to take this truck to the track but be able to drive it to a cruise-in or car show too, hopefully without changing anything except MAYBE the distributor and the stall converter for now and add my posi later. Which brings me here asking which carb to run. It looks like a 750cfm is the consensus and I'm guessing mech secondaries since I have very little vacuum. Unless a vacuum secondary carb can be tuned to operate correctly with little vacuum, So because I'm also reading that vac sec carbs are more street friendly and pretty good at the track when a person is not so into racing that they're trying to shave tenths off their et. So I'll put the 600 eddy on there so I can get a vacuum number because the previous owner doesn't remember how much the motor had because he didn't care since he ran with fuel injection. From there maybe I'll have a better idea which carb to use. I do the fab and body work part of building custom cars and truck. This is my personal ride and I want it to be fricken fast and track worthy. I'll upload some pics and maybe that will help some of you understand my perspective a little better, if you don't already. I've never had more than 300hp and never been the driver at the strip but I drive my truck like I stole it.
01-18-2013 05:32 PM
cobalt327
Quote:
Originally Posted by CHEVILLAC View Post
I don't want to blow it up but I really don't have a lot of money to invest in it either. I think I'll rebuild my q-jet, put it on the motor, put the motor in with a 3000 stall and see what it feels like. Then when I can save some money I'll put the 3.73 posi setup in the rear and my 29"x10" pie shell cheater slicks and head for the track. Does that sound reasonable?
If you're going to use a Q-jet, the amount of vacuum the engine makes has a direct effect on what spring to use for the power piston. Too much spring/too little vacuum will have it running pig rich (will be always on the power circuit) anywhere except WOT. Same concept as the power valve on a Holley or the step up springs on an Edelbrock/Carter.

Depending on what the Q-jet carb was originally from, it may need other changes (APT adjustment, jets/rods, bypass air orifice size, more idle fuel, to name a few). I don't say this to dissuade you from using it, just want you to be aware of the possible pitfalls.

Again, for the purpose of getting the engine up and running: Use the Eddy and a dual plane. Use a properly set up distributor.

Once you know what the idle vacuum is and how the cam is, then you can go about tuning it for real.

Also, the cam is the big unknown. It might be any sort of cam. Depending on what class it was built for, it could have a stock lift cam (often they'll have a ton of duration), it could be a cam ground for a vacuum rule, it could be set up to intentionally loft the valves off the nose of the cam lobe to circumvent the rules, could have hydraulic lifters on a solid cam (or vice-versa), it may be excessively advanced/retarded, et cetera.

If you cannot get a straight answer on cam specs/grind number from the guy who owned the engine, you really should take the time to measure it or see if there's some ID on the cam- whatever it takes. Because the way it stands now, you're (and we're) totally in the dark. This makes giving meaningful recommendations a total crap shoot.

3500 stall on the street is not as bad as you may think. But again- w/o knowing what the cam specs are, picking a stall speed is a total guess.
01-17-2013 06:33 PM
CHEVILLAC I'll give it a shot. 3500 stall just doesn't sound like it would be any fun on the street at all but I can't say I've driven anything with that high of a stall on the street so I really don't know if it's as bad as it sounds to me.
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