Thread: 70 350 chevy engine misses/bogs

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I have a 1970 chevy vette, 350ci 300hp pretty much stock. The engine has run great for years. However, recently I have been experiencing terrible bogging/missing across the mid rpm range.

Engine starts easy and idles fine. Also runs fine through the entire rpm range when it is warming up. However, once it reaches normal operating temp (still ok at idle and at low rpm's) it begins to bog/miss/buck in the middle rpm range. If I work it above ~3,500 rpm, it grabs hold again with smooth good power.

I am not an experience mechanic. Pretty much learning as I go. Any ideas or simple things to check? Any help is much appreciated.

If your Corvette is stock, it should have L46 350, 11:1 compression 350hp Engine. Stock carburetor is a Q-Jet.

The optional LT1 was a solid lifter engine, 11:1 compression, a high-performance camshaft, and used a Holley four-barrel carburetor on an aluminum intake to produce 370hp.

In either case, I’d recommend running a tank of the best fuel you can find and a couple cans of performance cleaner (additive) and see if that helps. If not, a good rebuild on the carburetor may be in order. It sounds like something is hanging in the transition to wide open throttle. Rebuild kits are readily available, but make sure you get all the numbers (especially from the Q-Jet). If you’ve never done a rebuild, you may want to solicit some help.

Pull your air cleaner and take some pictures and there will most certainly be some folks with ideas and suggestions for you.

Regards,
Glenn

Originally Posted by glennsexton

In either case, I’d recommend running a tank of the best fuel you can find and a couple cans of performance cleaner (additive) and see if that helps. Regards, Glenn

Glenn,
How do you feel about SeaFoam, sucked slowly into the system using the vaccum hose normally hooked to the power brake booster as a cleaner with another can added to the gas tank to clean the carb? I have done that, letting it suck the last 1/4 can fast to drench internals, let it soak overnight and then start it in the morning. Smokes to high heaven, but loosens up deposits.

I’ve never used it personally, but have heard others say it has helped them with rough idle. I believe there are good instructions, i.e., use a third in the gas tank, a third in the crank case and a third should be sucked in through a vacuum port. I would make sure it goes directly into the manifold as it could be not-so-good on a check valve. That said, this may not provide a strong enough concentration in the gas to address any carburetor internal issues, and I’d be tempted to use the entire can in the gas tank.

It’s made up of Pale Oil (petroleum) , Naphtha and Isopropyl Alcohol so it will burn like a banshee but make sure you run it through fairly quick and don’t let it sit in your fuel system. Maybe add it and go for a nice long

Years ago we used a similar product made by Rislone and it worked pretty well – but it does smoke like there’s no tomorrow when you suck it into the manifold. If you decide to ingest some into the intake, you may want someone to hold your RPM at about 2,000 to make sure the engine doesn’t stall. Do it at night or you may have the DEQ or the cops at your door - and for heavens sake, don't do this in a garage, rather out in the open.

Let us know,
Glenn

Describe your entire ignition system.