Need some help Carbed 4.8..
05-06-2012, 06:31 PM
#1
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Need some help Carbed 4.8.. Having problems with my setup. I picked up this motor used. Installed new Valve springs, pushrods and the biggest Comp Thumper cam 291HR. Running a Vic Jr. intake, and a 650 double pumper with a proform main body. It starts and idles good after some carb ajustment. I have tried all the msd pills with the same result with the 6010 box and with and with out the map sensor plugged in. If you try to give it any gas it wont rev and backfires out the carb and exhaust but not all the time. Its like an old sbc that has retarded timing or its not running on all cylinders. I have checked all primary tubes with a infared heat gun and all cylinders are fireing. I have verified proper cam timing also. And proper pushrod length. Im at a loss as to the problem. I think it may be to much overlap in the cam and not enough compression? any help would be great. Thanks.
Last edited by knuts82; 05-06-2012 at 06:50 PM.
05-06-2012, 07:27 PM
#2
Ya, tha is ALOT of cam for a small low compression engine!
I would custom program the timing curve.
try this:
0 - 500rpm 15*
500 rpm - 3500 38*
3500 -5000 ramp from 38* - 34*
5000 to top rpm 34*
that should work decent for timing
you are also going to need to work the carb, I bet it goes DEAD LEAN when you hit the gas. your probably going to need a BIG shooter, like a 40, and maybe a more aggressive pump cam. the blue cam on hole 2 give the most fuel with the least throttle movement.
I would custom program the timing curve.
try this:
0 - 500rpm 15*
500 rpm - 3500 38*
3500 -5000 ramp from 38* - 34*
5000 to top rpm 34*
that should work decent for timing
you are also going to need to work the carb, I bet it goes DEAD LEAN when you hit the gas. your probably going to need a BIG shooter, like a 40, and maybe a more aggressive pump cam. the blue cam on hole 2 give the most fuel with the least throttle movement.
05-06-2012, 07:35 PM
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Ill give that try. I already maxed out my pink cam and it did seem to help. I should have never bought this cam. The motor is in my bagged 68 c10 and i wanted it to sound like it had power.
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05-06-2012, 08:58 PM
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What plugs are you guys running in these motors. I tried NGK TRA 55's and the stock ones that came in it. I also noticed when I pulled them to change them they were pretty dry and black.
05-06-2012, 09:21 PM
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I was just thinking I could rob a 50cc pump and cam from one of my methonal carbs and give that a try but it may be to much?
Here is a pic from before i plumbed it.
Here is a pic from before i plumbed it.
Last edited by knuts82; 05-06-2012 at 09:30 PM.
05-07-2012, 06:54 PM
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Tried that timing and changed acc. pump cams and it still wont rev. Tried multiple cams and settings, I can give it so much fuel that it stalls out and every where between and I can only get about 1500 rpm out of it, its like the ignition turns on and off and on and off. Im going to try less timing and see if that helps. Could I have a bad sensor or? It just seems like its not hitting on all cylinders but I confirmed temps on all to show its firing. Any help would be great thanks.
05-07-2012, 07:18 PM
#12
have you run it with a laptop connected? you can display the map sensor , and the rpm/timing table. it will also tell you if there is a cam sensor failure, and it will tell you what voltage the msd is seeing.
The voltage is VERY important, if it gets low, the msd will act up if not shut off.
Grounds are another area that can effect the msd, run lots of GOOD grounds.
I would seriously consider pulling the front cover off again and re-verifying that the timing chain is installed dot to dot. I made this mistake ONCE
COOL truck BTW
The voltage is VERY important, if it gets low, the msd will act up if not shut off.
Grounds are another area that can effect the msd, run lots of GOOD grounds.
I would seriously consider pulling the front cover off again and re-verifying that the timing chain is installed dot to dot. I made this mistake ONCE
COOL truck BTW
05-07-2012, 08:41 PM
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I backed the timeing off to 28* till 2000 and now it will pull some rpm but it still cuts out but its not backfiring. I double checked the cam before I put the cover on but I may have to pull it and check. I have the computer hooked up also and see no sensor failure. Thanks
05-07-2012, 10:43 PM
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Ive got 6 psi at my fuel rail, floats are set. I can give it so much it floods out. Im thinking its just too much cam for the little thing. Im going to pull it apart tommorrow to double check the cam again but im 99.9% positive its dead on. It idles fine but as soon as you give it any its like the ignition cuts out and comes back and cuts out. I have the MSD grounded to the battery and when the computers hooked up it says it has 12.3 volts, I double checked all connections and I have a solid 12 volts to the pink ignition wire.
05-08-2012, 12:40 PM
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Yea sounds like its running lean. Check your float bowls to make sure you getting enough fuel in them, maybe you have a blown power valve in the carb or you need to jet it up.
Whats your fuel pressure?
Whats your fuel pressure?
