Hot Rod Forum banner

Confused about carbs, need help!

9K views 38 replies 7 participants last post by  CHEVILLAC 
#1 ·
I've searched this forum quite a bit and I think I have a basic understanding of carbs. My confusion comes from several sources of conflicting information so I'll give all the information I can about my truck and hopefully you guys can help me figure out what changes I need to make.

I have a '61 chevy short wide that is my daily driver right now but fairly soon will be driven on the street mostly and will see a few runs down the strip several times a year during the summer months. I live in Oregon so that should give me about 4 months a year to play around, so I want it to be streetable and good fun down the strip too.

It has a 12 bolt truck open rear end with 3.08 gears, a TH350 with stage 2 shift kit and 2300 stall. I just bought my boss' old race motor and this is where it gets fun. It's a 350 bored .030 over, flat top 4 relief pistons, forged crank, h-beam rods, 2.02/1.60 valves, 1.6:1 high tensil stamped rockers, Weiand Team G single plane air gap manifold, hei dizzy with MSD module, high output coil, no vacuum advance and the lightest springs available on the centrifigul adv. so it gets full adv. right off idle, I think he said total timing was 30deg. at 3500rpm? 10.5:1 CR and the cam is a summit k1108 (specs below)


RPM Range3,400-7,000
Duration at 050 inch Lift: 244 int./254 exh
Valve Lift with Factory Rocker Arm Ratio: 0.508 int./0.533 exh.
Lobe Separation (degrees): 114
Camshaft Manufacturers Description: 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.

He says this motor ran 450HP at 6200RPM on the Dyno with some kind of carb style fuel injection system which did not come with the motor so I need to figure out what carb will work on this motor for my application (described above) without changing anything except mayme my stall and rear end. Which brings me to another question (perhaps for another thread)?

I have a corporate 8.5" 10 bolt out of a 79 camaro with 2.41 posi that I was thinking of putting under the truck but I don't think it would hold up with that gear ratio and nobody makes a 3.08 or 3.73 gear set for the series 2 carrier so I don't know if I should change get a series 3 carrier and gear set for that rear end or for the truck 12 bolt, and will the 3.08 12 bolt open rear get me by until I save up the money for the propper rear end solution?

Thanks in advance for all help. You guys are a great wealth of knowlege and I'm somewhat new to the technical aspects of hot rodding though I do have a basic grasp on what will generally make things go faster or slower.
 
See less See more
#2 ·
Confused about carbs

Someone can correct me if I'm wrong,you only need about a 750cfm carb with vacuum secondaries ,my personal preference is a Holley because of the ease of making quick adjustments.You might run into trouble with running a single plane intake with street cruising.You might also consider going with a bigger stall.I'm not sure what your truck weighs,but I would get some shorter gears and bump the stall to al least 3500 and go with a dual plane intake and a 750 cfm carb.You also want to run a vacuum advance.Hope this helps and ill be glad to try and help with your set up as much as I can
 
#4 ·
The truck weighs about 3250lbs. Could I get away with 3000 stall and the 3.08 gears? I know I don't want to get the stall too low for the cam but I don't want to have to get too far into the pedal before the truck starts to move either. I'm used to my 2300 and even it seems a bit high for the factory spec cam that's in the 350 that's in the truck now.
 
#5 · (Edited)
First, what heads? You may not have enough compression ratio for that cam- or it may be too high for pump gas. You'd really like to know the exact chamber volume to take milling the heads (if it was done) into consideration.

All CR figured w/a zero deck/0.041" head gasket:

76cc heads, unmilled decks = about 9.1:1 compression ratio. Milled down to 70cc, will be about 9.7:1.

64cc heads will get you enough, just under 10.4:1, and all the CR you can use w/iron heads, if not too much.

IMO that's just too much cam unless you bring the rest of the engine in line w/it (correct CR, sufficient valve spring pressure, a higher stall converter and lower rear gears).

A 3310 could work OK, or a double pumper (650 to 750) once the stall/gears are dealt with.

The distributor needs to be set up w/a lot of initial timing and a limited amount of mechanical advance. You can use a vacuum advance limited to 10 degrees, hooked to a ported vacuum source.

The 8.5 corporate rear end is strong, but the bearings ride on the axles and are often worn and need replaced. If that's the case, upgrade the rearend to a 30 spline limited slip differential (Eaton is often recommended) and axles to match. It would be better if you posted this on the Trans/Rearend forum...

Edit- my usual calculator is misbehaving, had to edit the results.
 
#6 ·
I think the question relating to carbs does belong here.Cobalt,Im certainly not arguing with you as you know the respect I have for your knowledge.

Its just that my opinion is: The carb fits the engine not the gear ratio. The engine is built for a purpose and the driveline is built for a purpose,not because of the others parts short comings.
good example is myself; I use a 3.50 gear ratio because that is what the car needs to perform what I want(not drag racing) My engine has a big cam that a lot of people would say"you should have 4.56 gears for that cam"

4.56 gears would not suit my purpose at all.
so I say,imo,a 750 ish cfm carb is a good choice for that engine.

Will that truck make a fast pass at the drags?
different question
 
#11 ·
3.08 gear ratio is not a good gear for launching,true. From 25 MPH up,whats the difference from the 3.08 guy accellerating in low gear and the 3.73 guy accelerating in second gear?

At 60 MPH,in my car,using a 950 double pumper,I can yard the secondaries wfo with out bog or hesitation.In fifth gear my over all gear ratio is 2.87.
I have 5 gears so it makes more sense to go down a gear and use 1/2 throttle.
just saying,if the carb works,it should work at all RPMs
 
#12 ·
That's alot of cam for "mostly street" and if your not enjoying the small stall you have now your going to hate what that cam will actually require to idle and be right. 373 and 3000 won't cut it, will it work? Depends on your level of tolarence, but you will be much happier with a 355-373 and the stall you have now. If you want to keep it brutal, 3500 - 4000 stall, 750 dp, and 411 gears and hope it never hooks cause you will lunch that 10 bolt.
 
#13 ·
I honestly have no idea about the heads. The story I was given is " There's an old circle track guy in the Denver Co. area that built these heads for all the circle track guys within at least a couple hundred mile radius, about 15-20 years ago. He wouldn't divulge his source for the bare heads. They have no casting #'s and never did. they weren't ground off, they were never there." My boss had this motor in his 41 willy's, which I rode in, and it ran strong. I can't say it ran right because as I said I'm new to the technical stuff, but it ran strong. He did say that when he first built the motor he had an 850cfm Demon carb that was setup by a performance shop on the car and kept sucking the fuel bowls dry, under what conditions I don't know. He also said it has very little vacuum which creates a problem for my power brakes but I can get an electric vacuum pump for that. Seems there's alot of heresay but I do know it's a strong motor in great shape. If I put my 600cfm edelbrock on there, will it run? I have a Q-Jet on my other 350, but it could use a rebuild. Would it get me by?
 
#16 ·
Mystery heads, a race car cam that likely needs race car fuel, a single plane intake......with all due respect, your going to hate, or break it. And regret it. If you own it, and can't change that, be smart here, pull the race car parts, sell em, make them garage art, whatever, buy some decent heads, the right cam, swap the intake for someones dual plane, sell your 600 Carb, or trade for a 750, and enjoy your truck. I have done, hell I'm kinda doing now what your going to do, and its a terrible idea.....I'm a moron sometimes, don't be like me.
 
#14 ·
The 600 will work,not a perfect match.You can get a vacuum canister for your brakes. Try that before you buy a vacuum pump. Low vacuum on my car mostly bothers the head lights,brakes work ok as I keep the revs up if Im sport driving.When driving in traffic I notice a firmer pedal. I g
 
#15 ·
Confused about carbs, need help.

The best way to go would be to buy an Eaton 3series carrrier, Motive 373 ring and Pinion. The GM corporate (big 10) rear is just as heavy duty as that 12 bolt. Get the 30 spline carrier and a pair of moser axles. A Hughes GM40 stall converter will help that cam shaft you have. Try a Holley or Demon 750 DP carburetor with that Victor JR. intake.
 
#17 ·
Which would be better for short term (a few weeks max), the freshly rebuilt Eddie or the Q-Jet in need of a rebuild? $40 for a canister, $55 for a pump that provides 16hg and is smaller....? My other motor is getting tired so I'm trying to get this carb thing figured out asap so I can swap motors and be back up and running in a weekend because for at least a couple more months it is my daily driver. I already get 11mpg so how much worse can it get? Ultimately I'm thinking Holley 670vs or 750vs but for some reason everyone in the Portland area thinks those are worth $150-$200 all beat up. Seattle area has fresh rebuilt ones for that price but I'm leary of buying carbs sight unseen.
 
#19 · (Edited)
bygddy

He ran it on pump gas for almost 20yrs up until about 6 months ago when he built a 383. I paid $600 for this motor. You don't think it would be worth it to just put a dual plane intake and 750 carb on it? In part, my thinking is... having 170hp or so more than I had, even if it can't perform to it's "full potential" will feel better than what I have. Even if I don't change anything else, as long as it's not going to destroy the motor. If you go from a good running factory straight 6 to a small block v8 smogger it's still going to be more fun.
 
#23 ·
Ahhhh......OK, some of this I didn't know and I apologize, if he ran it on pump, and you only paid 600$, then absolutely, have at it, nothing to lose but a few bucks and some time. I would still try and swap your 600 for something bigger, and would up the gear and converter, but sounds like a fun, maybe grenade, but absolutely fun. :)
 
#20 · (Edited)
Maybe that's why I recommended the DP to him in the first place, w/reservations. You're preaching to the choir. FWIW I used a 4777/4779 DP on 8 of the 11 engines I had in my DD Camaro.


But what ET's better (WOT) doesn't necessarily make the best all around street carb, either.
The idea that the carb is only about the engine, and is apart from and not affected by the rest of the combo and vice-versa, is a flawed theory IMO. The carb is just as much of a part of the whole combo as is any other important part. The CFM is about the engine as far at max output goes, but when you're looking at the whole picture and not just WOT performance (power under the curve), you'd have to see that there are some carbs better suited to a combo than others. Else why is there different types of carbs in the first place?
 
#22 ·
I really appreciate everyone's help on this. I'm just trying to make my truck a mean summertime street machine that will be fun at the track, not trying to break any records. I got a really good deal on the motor and figured I'd put a carb on it and see how I like it. If it feels too much like a race car then I'll make a couple changes and have what I'm looking for at a reallydiscounted price. At the most I would change to this Voodoo Hyd Cam Kit - Chevrolet Small Block 276/284 - Lunati Power cam and a dual plane intake like the performer rpm with the same holley 750vac sec carb you guys are recomending and just add a posi carrier to my 3.08 rear end or maybe go to 3.50 or so gearing so I can keep it fairly street and highway friendlyish but still be fun on the strip. The motor came with a 150 shot NOS setup as well. Thanks again for all your help on this.
 
#31 ·
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.
 
#34 ·
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.
 
#33 ·
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.
 
#35 ·
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.
 
#36 ·
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)
 
#37 ·
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?
 
#39 ·
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.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top