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Old 06-23-2008, 06:09 PM
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Carburetor mods with Thumper cam

I have a 70s Chevy 350 in my 65 Impala and have installed a Comp Cams Thumper cam. My problem is that I have lost control if the idle screw adjustment. I'm reading about 5" of vacuum at idle. I also have installed an Edelbrock Performer carburetor with the factory Quad manifold using the proper adapter. I'm wondering if the idle vacuum is so low that the metering pistons are reading it a part or full throttle and allowing the metering rods to come all the way out to WOT position. Would this cause me to loose the idle screw adjust ability? Should I get the Edelbrock kit and install the lighter metering springs? It's burning so rich at idle that it burns your eyes and nose.

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Old 06-23-2008, 06:24 PM
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There was another post in here about putting in a thumper cam and they had carb trouble . I cant seem to find it right know but I beleve he had to open the secondaries a little at idle and run the ignition 32* at idle and stop the mechanical advance in the distributor. If I happen to run across it I'll post a link.
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Old 06-23-2008, 06:56 PM
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Copied from Edelbrock Owner's Manual......
Long Duration Camshaft
If the engine has a fairly radical camshaft it may require an excessive amount of throttle opening for idle and/or have low idle vacuum levels. Either condition can lead to poor levels of
adjustability and erratic idles.
Another fix for the above condition is to run as much spark advance as possible at idle. If the distributor is fitted with a vacuum advance unit, connect it directly to manifold
vacuum. If you are not able to employ vacuum advance for some reason, then the mechanical curve should have a low limit, which will allow you to use plenty of initial spark advance.
Measure the manifold vacuum at idle. If it is below 7" Hg, there is a good chance that the Metering Rods are in the up (rich) position. When combined with a high idle air rate this can
cause the Nozzles to discharge fuel at idle. Use a weaker Step-Up Spring (see section on Step-Up calibration) to keep the Rods down at idle. With some cams, a stiffer spring (pink or
silver) is necessary. Experimentation is the best way to determine which is best for your application.

Part #1464 will get you a complete set of springs.

Last edited by techinspector1; 06-23-2008 at 07:01 PM.
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Old 06-23-2008, 07:52 PM
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Do you have the flat tappit thumpr or roller thumpr? I have the big mutha thumpr hydraullic roller wich is 243/257 @50 with 537/519 lift. I just paid over $600.00 plus for a new carb a q series quickfuel 650 mechanical secondary to go with it and I have a problem with it staying running after its in gear but it gets plenty of gas when you pump it. I had a 650 speed demon and it was not enough carb. I hope you can figure out how to get it running right. I did have an edelbrock and you will have to use different metering rods and spring to go with that cam with so much overlap. That is what I had to do with a 292h comp cam I use to have in one of my old motors. After i changed the rods and springs out and it idled at 1200 rpm it did fine other then the off idle stumble. Hope you get the kinks out. It seems to me a lot of people have problems with getting carbs to tune with the thumpr cams.
Eric
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Old 06-23-2008, 08:17 PM
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The biggest mutha thumper cam requires a lot lot lot of initial spark timing at idle.
Lock out the distributor mechanical advance and set the idle timing at 34-36deg BTDC.
Install the lightest/lowest tension edelbrock metering rod/power piston step up springs.
Be sure the PCV system is hooked up to the base of the carb and is functional.
the opposite valve cover needs a breather.
Edelbrock carbs do not idle off the secondaries like a holley does. They will be closed at idle.

Once you've modified the distributor and replaced the power piston step up springs and reset the carb idle mixture and idle speed adjustment the idle will be much cleaner. The throttle response will be much better.
A 3500stall torque converter is required. (10" th350 converter) (9" th700r4)

There is no proper carb adapter for the square flanged performer carb on a factory Qjet manifold.
Change the intake manifold. A edelbrock performer RPM is what you need.
Way to much fuel distribution upset with any adapter on a Qjet manifold.
Stop fooling yourself.

If when the motor is hot, the motor is hard to start with the timing locked out, install a SPST switch on the distributor (HEI) or external spark amplifier box power switch on wire to kill the spark during cranking.
Kick it over with the spark switch off, then throw the switch and it will fire right up. Any 10A or more rated automotive grade SPST switch will work.
Mount it on/under the dash where you won't kick or knock it while driving.

Which cam do you have and what distributor? What model # carb?
Get the proper intake manifold.
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Old 06-23-2008, 11:52 PM
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Square Bore to Q Jet Adapter

These adapters work great for square bore carb to Quadrajet intake. They are tapered from 4-hole square to fit the Quad pattern. These line up perfectly with between the 2 different bores. The adapter in the picture needs to be flipped over.

http://cgi.ebay.com/ebaymotors/EDELB...mZ230264904939
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Old 06-24-2008, 02:05 AM
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Quote:
Originally Posted by SSedan64
These adapters work great for square bore carb to Quadrajet intake. They are tapered from 4-hole square to fit the Quad pattern. These line up perfectly with between the 2 different bores. The adapter in the picture needs to be flipped over.

http://cgi.ebay.com/ebaymotors/EDELB...mZ230264904939

For around $50 you can get a nice 2nd hand performer rpm intake manifold
and really pick up the power. The fuel distribution is much better.
Performer RPM manifold (typical Ebay used) Well worth the bolt on.

I find these for sale locally all the time. $100 or less.
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Old 02-19-2010, 10:46 PM
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carb help w/ mutha thumper cam

i have a 351w bored .030 over and10:0:1 comp. broke cam in and i had to turn carb idol screw in alot to idol. edelbrock 600 carb. i read something about springs and vacume. the motor spits and sputters and is runni9ng rich too. can anyone walk me through this to set it up right? im new to this but not ignorent and learn fast w/a lil help. im running a mad w/b;aster 2 coil and msd street fire ignition box. also im using 45 plugs @ factory gap specs. shld i use a dif plug and @ what gap with this set up. or regap the ones i have. also i know 600 is a lil small but @ 3600 feet elev. what jet and needel shld i use too? in time i will go to a holley and is there issues with that carb i need to know about? can someone recomend which holley carb to go with. oh im running a c4 2500 stall. thx...SHAWN
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Old 02-19-2010, 10:59 PM
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Quote:
Originally Posted by 68stang73
i have a 351w bored .030 over and10:0:1 comp. broke cam in and i had to turn carb idol screw in alot to idol. edelbrock 600 carb. i read something about springs and vacume. the motor spits and sputters and is runni9ng rich too. can anyone walk me through this to set it up right? im new to this but not ignorent and learn fast w/a lil help. im running a mad w/b;aster 2 coil and msd street fire ignition box. also im using 45 plugs @ factory gap specs. shld i use a dif plug and @ what gap with this set up. or regap the ones i have. also i know 600 is a lil small but @ 3600 feet elev. what jet and needel shld i use too? in time i will go to a holley and is there issues with that carb i need to know about? can someone recomend which holley carb to go with. oh im running a c4 2500 stall. thx...SHAWN

What cam shaft is in this motor?. If its a biggy size long duration cam you will need to recurve the distributor for much more initial spark timing at idle yet limit the advance travel limit so the thing is not over advanced at high rpm (that involves more than swapping advance springs) and its very likely you will need a torque converter with more than 2500 stall
What cam is in the motor?
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Old 02-19-2010, 11:06 PM
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Carb Help

Quote:
Originally Posted by F-BIRD'88
What cam shaft is in this motor?. If its a biggy size long duration cam you will need to recurve the distributor for much more initial spark timing at idle yet limit the advance travel limit so the thing is not over advanced at high rpm (that involves more than swapping advance springs) and its very likely you will need a torque converter with more than 2500 stall
What cam is in the motor?
its the mutha thumper cam for a 351w. and im running the stall it recomends. now im running a vac advance and i dont know alot about what you mentioned about the dist. i have read about the spring change in the carb but im lost after that..ill look for my cam card too.
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Old 02-19-2010, 11:33 PM
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Quote:
Originally Posted by 68stang73
its the mutha thumper cam for a 351w. and im running the stall it recomends. now im running a vac advance and i dont know alot about what you mentioned about the dist. i have read about the spring change in the carb but im lost after that..ill look for my cam card too.
ok dur @ .050 is 235 int and 249 exh. lobe seperation is 107.0
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Old 02-20-2010, 12:49 AM
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I recomend you recurve the distributor to allow 24-26deg base initial timing at idle and 34-36deg at max mechanical advance. That means you have to physically limit the mech advance travel to around 10deg. 18 to 24deg is typical stock. (too much advance travel)

How you achieve this depends on what specific distributor you have.

From that point a combo of 1 med and 1 light tension advance spring (from a advance curve kit) will create the right rate of mech advance. peaking around 2800-3000rpm.

Vacuum advance also needs to be dialed in 10-15deg max at highest vacuum hiway cruise. The rate has to be custom tailored to your car from there.
Many factory vac advance cans have way too much advance travel and need to be restricted to 10-15deg.

ported /full manifold vac source,,,your choice. use what works for you.

Install a power valve/holley or power step spring ( qjet AFB) that will stay closed at idle. 3.5 to 4.5" rated.

To get you up and running until you can recurve the distributor properly just lock out the mecha adv curve and set the fixed timing at 34deg at idle.

You'll find you can now close the throttle setting back down and get it to idle cleanly at 800-900rpm


You will need a higher stall converter for correct in gear idling.
Get a 9"- 10" 3500 stall converter.

What is your favorite torque converter company? give 'em a call.

When you get all this sorted out it will idle like a taxi in gear and will not load up or burn your eyes out anymore.

Last edited by F-BIRD'88; 02-20-2010 at 12:59 AM.
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Old 02-20-2010, 06:07 AM
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Quote:
Originally Posted by F-BIRD'88
The biggest mutha thumper cam requires a lot lot lot of initial spark timing at idle.
Lock out the distributor mechanical advance and set the idle timing at 34-36deg BTDC.
Install the lightest/lowest tension edelbrock metering rod/power piston step up springs.
Be sure the PCV system is hooked up to the base of the carb and is functional.
the opposite valve cover needs a breather.
Edelbrock carbs do not idle off the secondaries like a holley does. They will be closed at idle.

Once you've modified the distributor and replaced the power piston step up springs and reset the carb idle mixture and idle speed adjustment the idle will be much cleaner. The throttle response will be much better.
A 3500stall torque converter is required. (10" th350 converter) (9" th700r4)

There is no proper carb adapter for the square flanged performer carb on a factory Qjet manifold.
Change the intake manifold. A edelbrock performer RPM is what you need.
Way to much fuel distribution upset with any adapter on a Qjet manifold.
Stop fooling yourself.

If when the motor is hot, the motor is hard to start with the timing locked out, install a SPST switch on the distributor (HEI) or external spark amplifier box power switch on wire to kill the spark during cranking.
Kick it over with the spark switch off, then throw the switch and it will fire right up. Any 10A or more rated automotive grade SPST switch will work.
Mount it on/under the dash where you won't kick or knock it while driving........ Get the proper intake manifold.
This is DEAD ON info, trust it, it will get you where you need to go,
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Old 02-20-2010, 07:14 AM
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Quote:
Originally Posted by 68stang73
i have a 351w bored .030 over and10:0:1 comp. broke cam in and i had to turn carb idol screw in alot to idol. edelbrock 600 carb. i read something about springs and vacume. the motor spits and sputters and is runni9ng rich too. can anyone walk me through this to set it up right? im new to this but not ignorent and learn fast w/a lil help. im running a mad w/b;aster 2 coil and msd street fire ignition box. also im using 45 plugs @ factory gap specs. shld i use a dif plug and @ what gap with this set up. or regap the ones i have. also i know 600 is a lil small but @ 3600 feet elev. what jet and needel shld i use too? in time i will go to a holley and is there issues with that carb i need to know about? can someone recomend which holley carb to go with. oh im running a c4 2500 stall. thx...SHAWN
[B]If you go to the Edelbrock site you can download the instrution manual. Also I know Summit carries the kit that includes the springs and metering rods you need. I never got to set mine up because i ended up having other issues with that engine and swaped it out. Good luck. That 351 sound sweet.[/B]
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Old 02-20-2010, 07:32 AM
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thx

Quote:
Originally Posted by Rickracer
This is DEAD ON info, trust it, it will get you where you need to go,
first off thank you for the help. the cam is the mutha thumper NOT bg mutha. and the mutha recomends 2500 i checked and cross checked. i will check into the mods you have mentioned as well and see if it works. im sure it will. im a lil unsure how to do some of tht but i dont mind asking questions and learn fast. i am runnibg th msd pro billit...NOT the ready to run style. this one requires a ignition box. any other help wld be great. thx again guys.
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