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Old 09-20-2010, 05:42 PM
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What size carburetor should I use??

Hey guys, I am having a difficult time trying to decide what size cfm carburetor I should install on my 1990 chevy small block 350. The carb I have on there right now is to small(backfiring through the carb, all that good stuff), so I'm looking to buy a larger one, but I dont know what size to go with. First off the cam is a Edelbrock 7102, ( Duration (in degrees): 308/318, lift: 0.325/0.340 (int/exh)). It also has a Edelbrock performer intake and headers to a 2.5 in dual exhaust. Last it has a 4 speed so I usually shift at higher rpm's than a normal automatic. This car is for street use. I was thinking a 750 cfm's but was reading and most people were saying 650 is better. Let me know what you guys think. Thanks, Dustin.

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Old 09-20-2010, 05:46 PM
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Just my opinion but its hard to beat a Q-jet.
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Old 09-20-2010, 05:46 PM
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650cfm.

How long has the other carb been on it? Is this a new issue where it was fine before. Did you just add the cam? Backfiring throguh the carb is more likely timing than overly lean.
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Old 09-20-2010, 05:53 PM
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What is on it now? Like LS said, backfiring through the carb isn't always anything to do w/the carb- and certainly not because of the size- there are scads of 2-bbl 350's and bigger engines running fine.

Most likely, you need to tune the timing curve and perhaps adjust the carb, depending on what it is and where it was originally.

What is the current initial and total timing, and what is the vacuum at idle. Need that to go further on tuning it.

In the meantime, check for vacuum leaks, at the intake gaskets, the carb base, any/all vacuum hoses, PCV, etc.

Double check the plug wires- 1-8-4-3-6-5-7-2. Cylinder #1 is driver side, front. Alternates driver side to passenger side for #2, etc.
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Old 09-20-2010, 06:34 PM
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If you are looking for a new carb i would sugest the edelbrock 600cfm. I have a 350 with a medium sized cam and i had a 750 q-jet and it was horrible. It always ran rich and died when idling. I just put a reman edelbrock 600 and i love it. IMO a 600 or 650 is the perfect size for a 350; i wouldn't sugest the 750 because they just seem to be too big for a 350, unless it is real hot setup.
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Old 09-20-2010, 06:54 PM
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Quote:
Originally Posted by usa3990
Hey guys, I am having a difficult time trying to decide what size cfm carburetor I should install on my 1990 chevy small block 350. The carb I have on there right now is to small(backfiring through the carb, all that good stuff), so I'm looking to buy a larger one, but I dont know what size to go with. First off the cam is a Edelbrock 7102, ( Duration (in degrees): 308/318, lift: 0.325/0.340 (int/exh)). It also has a Edelbrock performer intake and headers to a 2.5 in dual exhaust. Last it has a 4 speed so I usually shift at higher rpm's than a normal automatic. This car is for street use. I was thinking a 750 cfm's but was reading and most people were saying 650 is better. Let me know what you guys think. Thanks, Dustin.
Dustin, I guess things just pop out at me because of the years of doing this stuff. I might be reading you wrong, but it's just the way your post is worded that tells me you have not built this motor around the cam, but have just stabbed the biggest cam you could into an otherwise stock 350 Chevy. Additionally, you have used the wrong intake manifold.

I don't think Edelbrock is doing anyone any favors by describing the camshaft you're using as making power from 1500-6500. Matter of fact, I personally think it is downright criminal. There is no camshaft on the planet that will make power through a 5000 rpm range and they know it. Seems to me they are just trying to sell more camshafts by fudging on the figures and I think that is just wrong. The best that the 7102 is going to do in my opinion is to make power from 3000 to 6500. They say in their description that it needs a Performer RPM intake manifold to work best. Maybe you didn't read that part.

The other part of this debacle and one that you can't be faulted for if you haven't that much experience or didn't counsel with someone with more experience before purchasing the cam is that you need between 10.5:1 and 12.0:1 static compression ratio to make this cam work. Since you didn't say anything about changing pistons or heads on your motor, I have to assume that the long block is otherwise stock and that you have added the cam, intake manifold, carb and headers and left everything else as it came from the factory. Now, I don't know for sure what the static compression ratio of your motor is, but I'd guess somewhere around 8.5:1, which would leave you about 2 full points shy of having enough squeeze to make the cam work. If you were all set on buying an Edelbrock cam, you would have been much better of using a 2102.

Now, for the rest of you guys who may not agree with me at first look, here are the specs on the cam. Decide for yourself.
308/318 degrees duration advertised
234/244 degrees duration @0.050" tappet lift
0.488"/0.510" valve lift
Timing events 10/44/59/5
IC 107, EC 117, LSA 112

Like I said, I could be wrong, but it looks like to me to be another case of choosing the wrong cam in the first place and nothing you do will make it right. You need less cam or more static compression ratio.
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Old 09-20-2010, 07:01 PM
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I agree Richard, It does read like a basicaly stock engine with to much cam.
So have you left out some info on this build or is it like we think simply to much cam for your engine ? I have to admit i have learned severl years ago that you can't just slap a big cam in any engine and expect it to perform.
In most cases something going to give up, Like maybe a valve through the top of a piston and through the side of the cylinder wall. Please don't ask me how i know this.


Cole
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Old 09-20-2010, 07:35 PM
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Quote:
Originally Posted by bigdog7373
If you are looking for a new carb i would sugest the edelbrock 600cfm. I have a 350 with a medium sized cam and i had a 750 q-jet and it was horrible. It always ran rich and died when idling. I just put a reman edelbrock 600 and i love it. IMO a 600 or 650 is the perfect size for a 350; i wouldn't sugest the 750 because they just seem to be too big for a 350, unless it is real hot setup.
Sounds like the power piston was stuck on the q-jet . I have run four different cars with q-jets and got over 20 mpg on everyone of them and one was a '67 toronado with the 385 hp 425 . My '75 Nova SS got 24 mpg with a non-tweaked q-jet. Getting ready to replace my eddy 1405 with a q-jet also .
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Old 09-20-2010, 07:45 PM
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The info from Edelbrock states, in part:

Quote:
These cams are for high performance street use only and are not for heavy vehicles. Use with Edelbrock or other aftermarket cylinder heads, adjustable valvetrain components, and at last 9.5:1 compression ratio. Headers, high energy ignition systems, lower than stock rear end gearing, and a high performance torque converter in automatic transmission equipped vehicles are necessary to gain all of the performance in these camshaft kits.
If this wasn't followed, then you cannot expect the cam to work as it was designed to.

That said, there's no reason- even if the engine is bone stock- for it to be backfiring through anything, be it carb or tailpipe.
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Old 09-20-2010, 07:59 PM
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Quote:
Originally Posted by cobalt327
The info from Edelbrock states, in part:

If this wasn't followed, then you cannot expect the cam to work as it was designed to.

That said, there's no reason- even if the engine is bone stock- for it to be backfiring through anything, be it carb or tailpipe.
You are correct Cobalt, Seems i got away from what the OP was saying . And he has either wiped a lobe on the cam already or cross wired,
Or out of time. JMO You think maybe he has the valves out of adjustment?


Cole
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Old 09-20-2010, 08:10 PM
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Quote:
Originally Posted by eloc431962
You are correct Cobalt, Seems i got away from what the OP was saying . And he has either wiped a lobe on the cam already or cross wired,
Or out of time. JMO You think maybe he has the valves out of adjustment?


Cole
Cole, I hadn't even thought about the cam lobes- I hope you aren't right- but fear that you may be exactly right, now that I think about it from that prospective.

Good catch, and a vacuum reading will confirm that- or a mis-adjusted valve(s), if the needle has a quick oscillation. If the reading is low and steady, there's hope..
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Old 09-20-2010, 08:26 PM
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Yeah, I'll admit to veering off on a tangent and not addressing the original problem, but it was just so evident that I had to say what I did. We may not hear from Dustin anymore after that broadside he got.
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Old 09-20-2010, 09:13 PM
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I hope that didn't sour him on us, too- but the truth can be a harsh taskmaster.
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Old 09-20-2010, 10:09 PM
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haha dont worry I try to take the most advice I can without taking anything personally. I am 16 and this is the very first engine I have rebuilt, so I am learning as I go. I really do appreciate all the advice you guys give me. What would you say my next step is. Besides, buying new everything, remember, I'm 16 and funds are very limited.
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Old 09-20-2010, 10:30 PM
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OK, you hang in there young man and ask questions about anything and everything. I think everyone on this board gets a kick out of helping a beginner get started in the right direction.

Cobalt told you that the carb size is not the problem and he is correct. You could run a 350 Chevy on a carb off a 5 horse Briggs & Stratton lawnmower motor without the motor snorting if push came to shove. You might not get it to a speed much above an idle without the airspeed through the carb exceeding its capability to mix fuel and air properly, but you get the idea. It's not the size of your carb that is giving you fits.

What delivery system is being used? What pump? What is the pressure of the fuel as it is delivered to the carb bowl? Have you set the floats according to trusted information? And adjusted the float drop also? What distributor are you using? When is the last time the cap and rotor were changed for new parts? Have you had the distributor apart to service the centrifugal weight pivots so the weight don't hang up? Have you disassembled the distributor to set the end gap on the shaft? The cam you're using will need a ton of initial timing at the crank. How much are you using at the crank? How much centrifugal are you using in the distributor? How about plug wires? How old? Have you run the motor at night in the dark with no light to look for spark scatter around the motor? Spark plugs? Have you researched and know positively that they are the correct heat range, gapped correctly and are the proper thread length for the cylinder heads you are using? There will be many more questions, but this is a start to get your cranial juices flowing.
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