Carbureted LSX forum - 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.

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.

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.

lol
what converter you running?

Muncie 4 speed..

thats good, because you'd need alot of converter with that cam!!

Made the timing map and its a little better, going to get a bigger squirter and cam tommorrow and try that. I only have a 31 in it now.

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.

Im sure the oem plugs from the 4.8 truck application are fine. 55's might be a bit cold

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.
http://i704.photobucket.com/albums/ww43/knuts82/picstruck028.jpg

http://i704.photobucket.com/albums/ww43/knuts82/truck25.jpg

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.

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

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

I am surprised less timing helped
Unplug the map sensor and try it.

The map sensor is disabled and unhooked right now.

Hmm, hard to help much through the interweb
I assume you have checked that it's getting fuel??

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.

Sounds like ur running out of fuel. Check the secondaries make sure their opening.

Oh, and sweet truck man

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?

Pulled the timing cover to find the cam set correctly but when I turn it over to line the dots I noticed one of the cylinders is leaking down into the block and the oil is full of gas. Looks like it needs to come back out. Motor must have been bad before I bought it.

shitty deal,
look for a 6.0, and your cam will be happy. dont forget to upgrade the valvesprings

The place I bought the motor from just shipped me another 4.8 that is newer with less miles so looks like im in luck. Thinking about a Texas speed 228r to replace the thumper.

Oh man, well least you found the problem and a quick replacement.

That cam will still want that little 4.8 to spin, let us know how she does, I am determined to spin one to 7700 or so all stock with a cam, but that is with a 24x, I have a spare 54x 4.8 short block that will have a good signal past that:)

The place I bought the motor from just shipped me another 4.8 that is newer with less miles so looks like im in luck. Thinking about a Texas speed 228r to replace the thumper.

Is your current engine a Gen III 24x or IV 54x. Make sure you get the same or you will have to get another MSD box.
And cam for that matter.
FYI you might want to look into a vaccum pump or canister for the brakes.

Is your current engine a Gen III 24x or IV 54x. Make sure you get the same or you will have to get another MSD box.
And cam for that matter.
FYI you might want to look into a vaccum pump or canister for the brakes.

Im rockin manual disc brakes all around so no need for vaccum. Its still a 24 tooth reluctor also so that works. Im going to rebuild the one thats in it and put in another project of mine and run a single turbo efi setup since I now have two complete truck efi setups.

Got the new motor ready to go in so i tore the other one apart. Cylinders look good rings look good head gaskets look good. I found no cracks in the heads or block. This one has me stumped. Cylinders still had crosshatch marks. Any ideas on what it could be.

You would have to magnaflux the cylinder. Some cracks do not easily present themselves.

Very true, we a friend pro charged a ls1, nearing 1000 rwhp we ended up leaking coolant. You cannot visually see the fracture in the block.

Well after ordering a ASA cam and installing new heads on the second motor I finally figured out the problem. I would like to slap myself. I totally forgot about the resistor in my ignition wire since it used to be points and when I had my lap top hooked up I only looked at the voltage at idle. As you gain rpm the msd box was dropping out since it was only getting 8 volts. Damn thing runs great now.

Haha, well at least it's solved now! I would love to hear how that 4.8 runs with such a radical cam

Second that, it will want 7800 RPM no problem.

Ill have to make a video for you guys on how it runs. The sm465 4 speed really doesn't like 6500rpm. Yeah the 4.8 sounds nasty with the ASA cam but it needs an auto or T56.

I bet it does, I wanna know how much it will want to spin, that short stroke will let the rod bolts and pistons to live allot higher than the 5.3, 5.7,6.0 and 6.2 with a much less piston speed. Seen many spin over 7500, 7800 is the usual.

The truck is running frickin great after some timing and carb adjustments. I have a 347 injected mustang and the old chevy will smoke it. The power on this little thing starts coming on at about 5800 and pulls super hard to 7200 where I backed off since the tranny was yelling at me. I would bet its still making power to about 7700-7900 if it would stay together with the stock bottom end. Video to come.

:headbang:
Keep us informed how the 4.8 does !!

Here's a little vid of before any tuning. ASA cam ported 706 heads, lightweight retainers, locks and Custom single springs from another setup I had.
http://i704.photobucket.com/albums/ww43/knuts82/th_picsphone2008.jpg (http://s704.photobucket.com/albums/ww43/knuts82/?action=view&current=picsphone2008.mp4)

After some tuning but still not perfect.
http://i704.photobucket.com/albums/ww43/knuts82/th_picsphone2013.jpg (http://s704.photobucket.com/albums/ww43/knuts82/?action=view&current=picsphone2013.mp4)

It sounds much better with the Dynomax Ultra Flows on it than the open headers. Ill post up some vid of it tuned in action in the next day or so.

It will hold together at 7800 with a 4.8, my 6.0 and 5.3 hold 7200 all day, the shorter stroke reduices piston speed and allows higher revs with the stock bottom end. I will be doing a 4.8 in the next camaro, not sure what cam yet, but will hit 78-7900 and be set up for auto cross.

sounds killer cant wait to see some nore of this.