L-82kid

I have a 350 with sportsman II heads, magnum 280 cam, roller tip rockers, rpm intake, headers, msd 6al box, and a Speed Demon 750cfm carb with mech. secondaries. I used to run 373 gears with 4spd and the car ripped. the cam starts to pull at 2000-2500rpm so there wasn't alot of low end torque. But If I dumped the clutch at 4,000rpm it would burn my 275/60 tires up and leave long burnout marks. I went to track and it ran 13s and I had trouble getting traction.

I blew the 373s up last summer and now I have 336 gears just because i had an extra rearend already. Since I put the 336 gears in, the car hasn't run good at all. Today I tried tuning the carb, I swapped down one jet and played around with 4 idle mixture screws. I got the car to run good going through the gears. But now I can't launch the car and it wouldn't even spin the tires when I launch at 2000-2500rpm it falls on its face. The car bogs so hard it almost stalls. I tried to just drop the clutch at 3500-4000rpm to see if it would burn out but it chipped the tires and then bogged so hard I flew forward and then it almost stalled out completely. But it runs good through the gears if I floor it rolling in 1st after 2500rpm.

What is the deal? what could cause this? is it carb or engine trouble? it doesn't use oil and doesn't make noise. Im planning on 411 gears this summer but i don't think that the 336s are the problem.

Thanks, sorry for the long post

Jayson77

How large are your intake runners. That has alot to do with low end torque.

CheezMoe

You may have too high rpm of a cam to get enough low end torque to turn the 275/60's with the 336's. You could also try down-jetting the carb a little. If it's bogging down it could be getting too much fuel since it can't break free and wind up as freely. Just some thoughts.

blckslvr79

My guess is your cam needs more gear.
I once had a Camaro with a 355 motor built to around 350hp and a 280 Comp cam and it would not spin the tires from a stop with a 273 gear, but would chirp the tires when you hit passing gear at 55 mph.
Your gear is not far off....

bojangies46

If you go with the 411 you will see a big difference, your cam does not start to come on until about 3000 rpm, but not real good on highway, or gas with the 411----

Guru_4_hire

The L82 cam was 280 degrees in duration or so......

bashcraft

I would try a smaller vacuum secondary carb. Maybe a 650.

With the mechanical secondaries, it may be going lean at lower rpm's.

AutoX-Z06

Check you timing and look at the advance. With the new gear range you will be able to sit your carb up better because issues become more apparent.

L-82kid

[QUOTE][How large are your intake runners. That has alot to do with low end torque./QUOTE]
The intake runners at 200cc.


I was thinking along those lines too. I re-jetted the carb yesterday down a few jets. it runs better down low from a rolling start and through the gears but still can't launch it. I know the 275/60 are alot bigger then stock size but didn't think going for 373s to 336s would make that huge of a diff.

I never been in a car with 411s so I don't really know what to expect with them. I usually just take the car out a few times a week and cruise around town. I don't care about the highway too much anymore. I have a few buddies with flatbeds and thats how I'll be getting to the race track. I was even thinking or maybe going with 4.33 gears.

the car ran good with the 373s, the cam doesn't really start to pull until around ~2500rpm but I used to launch at that rpm or a little lower and it would spin the tires but still take off and put u in the seat. If I dumped it at 3500rpm this would happen:






I'm just trying to get the car to launch good now because I was gonna take it to track again this summer with the 336s. I won't have enough money for 411s until the end of summer. I know this motor needs at least 373s or steeper gear but I didn't think going from 373 to 336 would make that huge of a difference. If I went to the track the ways its running now I wouldn't be able to get off the line, it would run 2-3 seconds slowing then it should. I might have to just cruise around and forget about racing until I can get some steeper gears!!



I hope nothing is weird or broken in the motor. Could the clutch have something to do with this problem?

A8skyhawk

I have the exact same setup except with AFR 195's... and with a stock 336... I wouldn't say I have excess torque at low RPM, but I can definitely spin the stock tires launching above 3000 RPM. I've also run low to mid 13's repeatedly (On a G-tech, for what thats worth) and it doesn't falls on its face if I launch above 2000-2500 RPM. I suppose it could be your bigger tires bogging you down... I would definitely be interested to see how you like the 411's as I am considering a similar swap soon myself.

Steve20

to determine if it is the gears that are causing this problem,
how about tricking the car into thinking it's got lower gears?

Will a pair of 14 inch GM steel wheels clear the brakes and suspension?
Try driving one these and see if the problem goes away

L-82kid

When I had 373s the car would spin the stock tires like crazy, whenever I tried to practice my launch I couldn't hook. The 225/70s were pretty bald when I made the swap to 275/60. I did notice a little difference with the bigger tire, the rpms dropped a little when cruising on the highway and it launched different. I had the 275/60 on the last time I raced at the track. I had no issues with it bogging off the line, also the burnout pictures I posted were with the bigger tires. So the only difference in the way it runs is from the gear swap.

I'm going check the timing, and read the plugs and try playing with the carb again. I'm running 75/82 jets right now, it doesnt smell like its rich and runs strong through the gears. If I could race from a 15-20mph roll then I'll be fine. lol

wiseman79

That close of a gear swap isn't going to effect a strong motor that much.I ran 2.56 gears in a similarly built 350 for a while in my cutlass until I got the new rear built, it would anialate the tires, even did better burnouts because you could get so much wheel speed in first gear. Invetigate this better before you just throw money at it.

blckslvr79

I'm telling ya....gears.

JustinD

I got 4.10's and it is a whole other animal. Swapped from 3.08's was a huge difference from barely any tire spin to gratuitous amounts of it.

bojangies46

For what you want to do with the car 411 should do it with the cam you got like blckslvr79 said--

Jim_Harrison

I have 4:11's makes a huge difference even from the 373 I used to run. Only issue is you run out of RPM's at about 115 to 120 mph depending on tire diameter.

enkeivette

The gears aren't helping, but it sounds like you don't know how to tune the carb. When you got a cam recommendation they probably matched your cam up with a higher rear end ratio.

Set the idle mixture screws with a vacuum guage, they're cheap go buy a hand held one. Set them till you get the highest vacuum. How did you jet the carb? Squirter nozzles? What kind of carb is it? How are your plugs burning?

L-82kid

Its a Speed demon 750cfm with mech. secondaries. When the car was running good with the old gears it had 76/83 jets. the plugs were a light brown color but a few were a little darker. The car smelled a little rich so I swapped to 75/82 jets. I started with the 4 idle mixture screws 1 turn out and went from there. The squirter nozzle is whatever came with the carb out of the box.

Shirley1978

i got a 336 with a gm goodwrench repl crate, 260 hp or so.. got headers and a k&n, i burn out all day..