Vintage Vette

[Bonnewagon]

Buddy of a buddy has a '74 Vette, 350, HEI, T-400, rebuilt the original motor. The motor is the problem. Someone stuck a BIG cam in, to an otherwise stock motor, and plopped a Summit 850 on top. Of course it's a dog, even after fixing all the vacuum and electrical screw ups. Cam is unknown, but she barely pulls only 15"-17" at idle with the vacuum advance hooked to full manifold vacuum at 1000 rpm. I'm trying to convince the guy to back down on the cam, and ditch the carb. I suggested the stock cam, or perhaps a smidge better, and of course a Q-jet. What would you Chevy guys recommend?

 

[CWPottenger]

Well Biggest issue is the 850... Way too big for a 350sbc

 

[Bonnewagon]

CW, do you think a smaller carb, like a Q-jet, would still work well with a too big cam on this Chevy? I have run a Q-jet on a Pontiac 400 with a 278 duration cam and had poor vacuum and a crummy idle, yet pulled mightily after 1500 rpm. I figured the smaller 350 motor exaggerates the affect of the cam.

 

[83partshauler]

Ditch the 850 carb - way too big. Look at the Quick Fuel Technology - slayer series 600cfm. If it's an automatic car go with vacuum secondaries, 4 spd car you can go with the dble pumper. They're inexpensive and construction quality are really nice over similure style carbs (ie. Holley) 15"-17" ain't too bad (not great but enough to run power brakes.) The nice feature of those Slayer carbs besides being affordable is they have removable air bleeds so you can tune them for a larger cam like what your buddy has as well as if you go with a vacuum secondary they have an adjustment screw for the secondary diaphragm spring to either soften or stiffen depending on how you adjust it. Holley and other brands usually requires you buying a spring kit.

 

[Bonnewagon]

That sounds like a good carb but I don't think this guy is capable of any serious investment at this time. He's keeping it at my buddy's shop and is starting to piss him off with his lack of funds. So I scrounged around and came up with an '80's Chevy Q-jet from a 350 Suburban. Jets are #74 and rods #52. That seems to me like a decent starting point. I'm no Chevy guy but I do remember pulling my hair out over this same Suburban years ago. Someone had put in an "LT-1" cam, whatever that means, since no one had the cam card. I do remember ending up with an early Camaro Q-jet with #76 jets and like #39 or so rods just to get that behemoth moving. It even had 3:42 rear gears. I had kept this original carb because the Suburban just ran terribly with it, and I couldn't understand why. Hopefully the Vette can use it.

 

[Bonnewagon]

OK, I found the original specs. #75 jets and #46 rods if it's the base 350. #75 jets and #43 rods if it's the L-82. CH secondaries on both. I can build that. I did also read that the L-82 cam was designed to work well with the "rams horn" stock manifolds, which he has. Hopefully I can talk some sense into him.

 

[axisg]

Buddy of a buddy has a '74 Vette, 350, HEI, T-400, rebuilt the original motor. The motor is the problem. Someone stuck a BIG cam in, to an otherwise stock motor, and plopped a Summit 850 on top. Of course it's a dog, even after fixing all the vacuum and electrical screw ups. Cam is unknown, but she barely pulls only 15"-17" at idle with the vacuum advance hooked to full manifold vacuum at 1000 rpm. I'm trying to convince the guy to back down on the cam, and ditch the carb. I suggested the stock cam, or perhaps a smidge better, and of course a Q-jet. What would you Chevy guys recommend?

Qjet you mentioned sounds great. I have swapped the same qjet from a 454 to 305 to a build 350. Only the secondary rods really needed to be swapped out.
I too have been guilty of too much cam in a stock block. No matter what you do its still a smogger block with low CR. Vortec heads are cheap and an xe268 cam will really wake things up without going overboard.

 

[Bonnewagon]

Funny how guys think just stuffing a huge cam in will create a screamer motor. They never get it that everything else has to compliment the cam. Anyway I used the 80's Q-jet with #77 jets and #47 rods and CH secondaries. Best I could do with what I had lying around. I dropped it off yesterday, his linkage is different, choke too, so he'll have to McGyver a few things. I'll check on him tomorrow and see what he accomplished, if anything.

 

[Bonnewagon]

DONE! Crank timing at 8º, vacuum advance hooked to full manifold. Q-jet w/#77 jets, #47 rods and the idle ports drilled out to .100". Idles at 850 rpm at 19" vacuum, pulls strong and revs nicely. Not my first Chevy and I always need to re-educate myself with these. Both buddies very happy!