1968 Corvette 427 390HP Rochester Q-Jet Issue

[Joseph M.]

[Wayne W.]

Re: 1968 Corvette 427 390HP Rochester Q-Jet Issue

Check the operation of the choke pull off. It should crack open the choke as soon as it starts. The dripping is certainly flooding. Q-jet carbs are known for problems with soaked floats. You should look at that.

[Jeff G.]

Re: 1968 Corvette 427 390HP Rochester Q-Jet Issue

I think Wayne might be on to it. Sounds like a stuck float...

[Michael M.]

Re: 1968 Corvette 427 390HP Rochester Q-Jet Issue

Not sure if related to your problem, but when my original 427/390 was rebuilt the exhaust cross-over passages in the intake manifold under the carb were plugged.

[Stuart F.]

Re: 1968 Corvette 427 390HP Rochester Q-Jet Issue

Joe;

Does your 68 use a thick (phenolic) base gasket/spacer and an SS plate? Some years did, others didn't. That combination is necessary to control/limit heat soak or percolation of the gas. Even then, it is important that the SS plate be intact. If, as you say, it is dripping while at idle, that may be an entirely different matter. The most common fault is the float. However, being as it is only on one side it may open up a few other possibilities. First, when was the last time the carb was serviced? Was it thoroughly cleaned at that time? Reason I ask is that it may be possible, but rarely, that the venturi cluster has a plugged or restricted air bleed/vent, causing a siphoning effect on that one side.

It is a common problem today on many old carbureted Vettes to experiece hard starting when hot. That's nothing new, but the problem has been made worse by the pump fuel available today (w/ethanol). I have had a very bad problem on my 63 L-76 w/AFB. I had to add the phenolic spacer and double gaskets to limit the problem. By so doing, I noticed about a 35 to 40 degree drop in carburetor temperature vs. manifold temp. Without the spacer, there was only about 5 degrees difference.

Stu Fox

[Joseph M.]

Re: 1968 Corvette 427 390HP Rochester Q-Jet Issue

Hello,
[FONT='Verdana','sans-serif']In addition, to my original matching # carb, I do have a 2nd matching # overhauled carb and a clone one with the correct casting numbers so I probably should try putting one of these other carbs on my engine to see if the problem goes away. If the problem remains, do you think that blocking the cross-over exhaust ports under the carb may help?

[/FONT]

[Stuart F.]

Re: 1968 Corvette 427 390HP Rochester Q-Jet Issue

Joe;

You didn't mention where you live. That can be important due to the ambient conditions you may deal with through the year.

Certainly, the first thing you should do is try your spare carb and see how it acts in similar circumstances.

There are a number of ways to plug the exhaust heat ports in the manifold, some easy and temporary, others more difficult and more permanent. If you use plugs, the best way is to remove the manifold and tap the holes for threaded plugs. If you wanted to plug the entire cross over passages in the head(s), you would need to pull the manifold and replace the gaskets with ones that have block off plates. I have no idea if they are available for the big block (I have made plates using simple tin cans, but other more substantial materials are available). A simple method to block off just the holes under the carb would be a base gasket the seals them off. Again, if you can find one for a big block. You might also try using a SS plate below the base gasket or spacer. Just make sure it seals well w/o any vacuum leaks.

Good Luck;

Stu Fox

[Michael M.]

Re: 1968 Corvette 427 390HP Rochester Q-Jet Issue

I do not profess to be an expert but my understanding is that the exhaust crossover can create gas percolation issues for the obvious reason that hot exhaust gases are heating up the carburetor. Blocking off the cross over stops the exhaust heat from heating up the carburetor.

I've had no gas percolation issues or drivability issues with my car since it was done.

Having said that, I would first address the fuel drip issue. Could be debris in the seat or a float problem as others have indicated. You may have to do more research to see whether excessive heat being transferred to the carburetor could cause the dripping. Sorry I couldn't help more.

[Paul H.]

Re: 1968 Corvette 427 390HP Rochester Q-Jet Issue

I have an original motor 68 427/390. I got so tired of farting around with the Q-Jet that I bought a replacement spreadbore Holley for it. What a difference. Faster starts, better idle and great throttle response. I have not blocked off the crossover but have the correct gaskets under the carb. No percolation problems.

There aren't many people out there that really know how to properly rebuild the Q-Jets. Most of the rebuilds are problematic.

[Jack H.]

Re: 1968 Corvette 427 390HP Rochester Q-Jet Issue

"There aren't many people out there that really know how to properly rebuild the Q-Jets. Most of the rebuilds are problematic."

My experience is few rebuilders adequately check for wrapped body parts then compound the problem by using an incorrect carb to intake gasket that lacks the proper thermal insulation characteristics. That accelerates the issue of carb body component wrappage...

The QJet on my '71 was badly wrapped when I did a frame off, so I replaced it with a clone for a while. I sent the original out to Jerry Luck for a complete restoration and he had to mill/true a number of body component surfaces.

Plus, when he shipped the finished, bench tested, carb back he included the correct THICK insulating gasket. It was VERY different from what was under the carb prior to frame off!

Since, I've logged 65K miles on the odometer without a hiccup in terms of carb performance. What I learned was: (1) there's a big difference between a simple carb rebuild and a competent/professional carb restoration, (2) using the correct carb mounting gasket is important, and (3) when done right, these QJets perform and last VERY well...

[William V.]

Re: 1968 Corvette 427 390HP Rochester Q-Jet Issue

Joseph

There could be an improperly adjusted float or a fuel soaked float. When q-jet floats get old they soak up fuel and become heavy allowing fuel to leak past the needle & seat. I suggest you replace the float. A second problem with q-jets is the led plugs at the bottom of the fuel bowl drying up and shrinking allowing fuel to leak out of the bowl into the intake. This can cause hard starting and or stumble at idol. The fix is to seal the plugs with epoxy. Use to type of epoxy recommended to seal leaky fuel tanks. A marine supply store should have this type of epoxy.