Hi, I have a couple questions about my 1977 Corvette L48, 4 Speed, 3.70 rear. I recently pulled the motor to improve power over the stock 180 HP. I had the original block bored 30 over and had flat top pistons installed. The heads are Vortec 2.02 cast iron, with mild porting. I used the metal/shim head gaskets.

The engine is topped with a performer intake and a Jet Performance, 800 CFM, stage 2 Carb. Now, originally I used a Crane cam with a 530 lift and 1.6 roller rockers. Because I felt the car was to radical I pulled that cam and went with a Blue Racer Cam with a 490 Lift. Question is do you guys think I have to much carb for this set up? Vettes runs great at high RPMS but does not seem to like normal driving. If the carb is to much what would you guys reccomend?

Exhaust was upgraded to 2 1/4 true duals with flowmaster 40 series mufflers. I did not put headers on it I stayed with the stock manifolds. Do any of you guys have a problem with the shifter rattling at high RPMS? Vette did this even when it was all stock. Any help would be appreciated. If exact specs are needed on the cam I can post them. Thanks, Bill

 

If the Jet carb you are referring to is the reworked quadrajet, it won't be too much carb, since the airvalve will only allow as much air in as the engine can handle. The problem is more likely a jetting problem as far as your part throttle is concerned on your primary side. you could try running a smaller jet on your primary side and see if your problem improves.


Pat Kunz

 

To answer your question on the carb being too big. No it isn't too big. q-jets are never too big. You could technically run a q-jet on a 4 banger and it would perform just as well as putting a 2 barrel on it. Reason being that those secondaries only open as the engine requires, almost like fuel injection.

You probably just needa change the primary metering rods/jets.

 

Could I just use the rods/jets from the original carb? I've been told I should have gone with the Edelbrock or the Holly Street Avenger for better performance.

 

Whoever told you that is rather uneducated. A q-jet, from what I've seen, can pretty consistently perform better than the Edelbrock 1400 or 1600 series. Those are just Carter copies and Carters are NOT performance carburetors.

You need to see if you're rich or lean before you go changing the rods and jets, though.

 

Kalway,

You are dead on. A well set up Q-Jet is the way to go.

HT1663WB,

See the paper on Q-Jets that Lars wrote. In it, it gives the proper rod/jet combinations for optimum performance. But before doing this, take Kalways advice on checking the lean/rich condition on your carb.

 

IMO a 800 is a little big for a modifed stock engine, i heared the speed demon 650 or a 750 carb might work better at lower rpm but it's just my opinion.
as far as the rattling at high rpm....I have the same problem!!!

 

Thanks guys for the info. I always liked the Q-Jet. As for checking if it's running Rich or lean not to sure on that. I took the carb right out of the box and bolted it on the motor. Car idles fine, responds well under full throttle, electric choke seems to take forever to kick down. Under light throttle seem's to be sluggish. Any good carb tuners in the southern New Jersey Area?

 

With the engine idling and up to temperature, cup your hands over the primary side of the carburetor to restrict air flow. If idle speed increases and smooths out the idle mixture is lean. Open up on the idle mixture screws a little and see if it helps the light throttle weakness.

To check for a rich idle all you need to do is remove a vacuum hose. You may need to throttle it. If the idle speed increases when you create the vacuum leak the idle is overly rich. Readjust the idle mixture screws for the highest idle speed minus 50 RPM. This is a good clean idle. This may make the light throttle weakness worse so you will then need to turn to the main metering rods to cure that.

You'll need to open up the carburetor to get at the metering rods. See what size you have in there, then get one or two sizes smaller and see which ones work best. Make sure you get the right rods. As far as I know the Jet Performance Quadrajets are made on 170 series cores.

BigBlockk

Later.....

 

Well if it's an 800cfm q-jet it has to be a 170 series q-jet, because the 70 series q-jets are only rated for 750cfm. So your assumption on that would be correct bigblockk.

Guessing you don't have access to an air/fuel meter so your only method for figuring out what rod/jet combination will be trial and error. The primary metering jets adjust the mixture all around, but mainly you only change them to adjust the WOT mixture. The metering rods are what adjust your cruising mixture. The tips of primary metering rods are all the same size (like .026" I believe lars or bigblockk can correct me on that if I'm wrong) so if you're lean or rich at WOT changing your metering rods will pretty much do nothing to change that. If you're cruising and get the lean "surge" then you go to skinnier primary metering rods.

One thing that hasn't been suggested, that I'd try, is call up JET and see if their techs may be able to help you. Tell them what your engine specs are now and see what they suggest for a rod/jet combination. I would imagine that they'd give at least a good baseline to try or an idea what direction you need to go to. My knowledge on engine dynamics isn't good enough to really know which direction you'd need to go so that's my suggestion.

Just remember, a q-jet is NEVER too big for any engine up to like 550hp.

 

I decided to just leave the carb alone. It runs strong with no hesitation just doesn't feel responsive under light acceleration around town.

It pulls hard through all four gears under heavy acceleration nice crisp chirp through 2nd and 3rd. With the original L48 the Vette would hardley chirp the tires when you dumped the clutch from a dead stop.


I drove the Vette to work the other morning and the nose of the water pump shattered. Anybody else ever have this problem? No damage to the radiator. That pump didn't even have 1000 miles on it. It was replaced when the engine was done. It now has a brand new pump no more remans for me. Hey MDJICE77 since the pump was replaced, I no longer have the shifter rattle at high RPM'S.

Thanks again guys, Bill

 

Ithink alot of HEMI guys would disagree about the perfomance atributs of the carter AFB carbs . just my opinon tho

 

that sounds more like a timing issue to me, did yu disassemble the distributor and shim it and lube the shaft?

 

Bob, Funny you say that because when I first purchased the Vette it had been sitting for 7 years. The first thing I did was a tune up. When I pulled the cap to replace the rotor I noticed when you turn the rotor by hand it was very slow to return to its starting position. I did at that time lube it and it seemed ok. I'll have to recheck it.