Chris98Buffalo

Hey guys, I have a 70 455 in a 98 - seems to have a bigtime vac leak.

Idle is rough, brand new plugs have been installed, and the old ones looked fine. Compression showed 160-170 on all cyls. Timing is at 10, dwell angle at 30. Still lousy idle. Manually closing the choke blade some brings the idle up significantly and it smooths out some, so it's obviously lean. Idle vaccuum is at around 15, with a small (1-2 in of vac) rapid flutter in the gauge. (I am thinking the flutter is from it being a non dampened gauge and I had it on one of the intake ports, not base of the carb)

Where are the common places to find vac leaks on these? I do plan to a cap, rotor, wires and points just to eliminate any variables, but the choke experiment clearly shows it's lean. I have the idle mix screws out about a turn past where they have any effect; I am thinking unmetered air is getting in elsewhere.

Ideas??

Thanks everyone!

oldcutlass

Spray carb cleaner around the base of the carb and around the outer edges of the intake. Cap or plug all the vacuum ports/hoses to eliminate them also.

Olds64

Chris, have you tried to use ether (starting fluid) to id the vacuum leak? It's definitely a "redneck" trick, but supposedly it's worked for folks in the past. All you have to do is spray ether around the base of the carb and near the vacuum lines and the bottom of the intake. If there's a vacuum leak it will change the idle when you spray the ether.

You can also use cigar or vape smoke by blowing it into the engine though a vacuum line with the engine shutoff. If there's a vacuum leak you will see the smoke creeping out from under the carb or at the bottom of the intake, etc. This is all an attempt to emulate a tool professionals use to id vacuum leaks that makes smoke with a small burner.

If your idle stabilizes when you close the choke plate that definitely indicates a vacuum leak. You can also test it by holding onto a shop rag and slowly covering the carburetor with the engine idling (with the choke off of course).

Chris98Buffalo

Update for any of those following - I decided to have the carb and distributor rebuild to factory spec.... although I am confident this isn't the problem, I want to eliminate variables and go for reliability. Both carb and distributor are the correct numbers, and will be totally blueprinted to factory spec, excepting eliminating the points.

While this is being done, all of the vac lines will be replaced, and sorted to original configuration.

If anyone needs an amazing carb and distributor guy, I have one!

svnt442

I had a similar issue with my 70 442 a few years back and it was so bad that I could hear the hissing from the leak. It turned out to be my brake booster leaking, yet I still had power brakes...go figure.
I replaced the booster and the problem was fixed.

Chris98Buffalo

Once I have fuel and ignition rock solid, I am going to start it and run it with all external vac lines capped to rule out an internal leak. I did consider the brake booster but since the brakes work well I discounted it... so, one by one we shall see.

svnt442

It's an easy test. Just pull the check valve out and cap it with your thumb. If the idle smooths out you know you found your leak.

oldcutlass

I would have done this prior to sending them out.

Chris98Buffalo

The carb had several things wrong in it from an amateur rebuild prior - float hung wrong, wrong gasket between throttle plate and main body, bound up choke linkages, just to name a few. the distributor had gummed up weights, a frayed ground lead, cheap and poorly adjusted points, and will be converted to electronic as well as have the correct factory curve verified on a distributor machine. Like I said, I know this isn't the root problem, but if I am under the hood I might as well un-f**k anything else that will affect how it runs. Doing this also eliminates variables when finally nailing down the leak and tuning it properly.

oldcutlass

I see... Tell the distributor company to put a performance curve into your distributor with the proper vac advance canister. It will allow a bit more initial advance and bring your total in sooner.

Chris98Buffalo

He is a local artisan, wonderful at what he does. We discussed that, and since the car is bone stock, he feels keeping it exactly the way it was built is best, since I just want reliable driveability and I don't plan to race it. He has old the old books with the factory curves, etc. - since he's got 40 years in doing carbs and distributors, I am gonna let him make the call at this point. Everything I've ever had him build in the past (race or stock) has always been "set it and forget it" Carmen's work.