63 carb replacement

[Duke W.]

Re: 63 carb replacement

More information is needed: transmission type and axle ratio. Idle vacuum at idle speed (in Drive if an auto)

If someone installed a high overlap cam, that could be an issue. Idle vacuum @ idle speed will reveal if it has an OE or hot rod cam. They are tough to start from a dead stop without stumble, especially with tall gears.

Also, what is the data stamped on the VAC mounting bracket.

When you say timing was adjusted you need to provide data - real measured numbers. Saying you checked vacuum and reset the timing is worthless in terms of trying to help you out.

Post all the numbers. The data should tell a story.

Duke

[Joe T.]

Re: 63 carb replacement

you have to realize I'm a real novice. Transmission is the original muncie 4 speed that came with the car(at least the case casting numbers and dates are right), I do not think it is close ratio, the axle is 3:08 by the stamp code, the idle vacuum is 17 PSI, the timing was advanced to 5 degrees from almost neutral. I have no idea about the cam. I did not have time today to remove shielding and distributor for vacuum mount data, I think the first digit I can see is 0 on it. In terms of the clunk in the rear I took out the drain plug and had to stick my finger way down to get any gear oil on it so tomorrow I will get gear oil(80w90 GL-5 and 4 ounces of friction modifier and see how that goes. If I can get the stumble under control and the clunk to stop I will be in heaven with this really cool car, Joe

[Duke W.]

Re: 63 carb replacement

Manifold vacuum is measured in inches of Mercury, not psi. Think of barometer measurements you see on weather reports.

If manifold vacuum is 17" Hg, then the cam is probably OE, but you need to state the idle speed that was associated with your manifold vacuum reading.

The initial timing should be at least 10 degrees assuming the centrifugal curve is the OE 24 degrees. Determining the spark advance map is important to diagnosing and solving your problem. The basic specs are in your '63 Corvette shop manual. Then use a dial back timing light and Mity Vac to determine if the centrifugal and vacuum advance meet spec.

Positraction clutch wear increases differential gear backlash, and this can be determined by side yoke end play, which can be measured with no load on the rear suspension and pushing in and out on the tire. The only way to repair it is overhaul the axle by replacing the clutches, or buying a new loaded differential case.

Duke

[Joe T.]

Re: 63 carb replacement

a new mechanic spent a great deal of time on my stumble problem and feels that he has it solved by advancing the timing again, but further. He said I had it a 0, it wasn't good at plus 4, he went to plus 6 and he said the car no longer stumbles. He feels the rear needs to rebuilt as you suggested. I heard 63's were a problem, is that true?

[Duke W.]

Re: 63 carb replacement

I guess you missed my last post.

Duke

[Joe T.]

Re: 63 carb replacement

NO I read the post, your posts have been very helpful. I don't have the equipment to make the measurements and changes you suggested on my own, I also work 60 hours a week in a non-related job. I got frustrated because i didn't get to drive the car much this Summer due to the stumble and the clunk. I found a garage that re-builds their own differentials, tranny's, engines and specializes in old cars so I sent the car there, in hopes of getting some driving in before snowfall. The mechanic there went thru all the steps that you and other posters suggested(he knew what to do already). So I was reporting his results . I did jack the car up with the right rear wheel off the ground and I was able to move the wheel back and forth quite a bit confirming I believe your suspicion that the rear axle needed a rebuild. My question then became "did a 63 rear axle require special rebuilding" it just seemed to me that some places advertise rebuilding 1965-79 rear axles and not 63. I figured there are one year only parts that are going to be difficult.

[Duke W.]

Re: 63 carb replacement

a new mechanic spent a great deal of time on my stumble problem and feels that he has it solved by advancing the timing again, but further. He said I had it a 0, it wasn't good at plus 4, he went to plus 6 and he said the car no longer stumbles. He feels the rear needs to rebuilt as you suggested. I heard 63's were a problem, is that true?

It's only takes a few minutes to set initial timing, so what the heck did the guy do in "a great deal of time"? Your owner's manual says to set it at 8. I suggested in post #33 that it should be at least 10, but your guy only set it to 6. I'm wondering if he knows what he's doing. Did he disconnect and plug the signal line to the VAC and set the initial timing at an engine speed below where the centrifugal advance starts? Who knows, but apparently he had no technical documentation on the engine so he could make proper adjustments. He was probably just guessing.

You can't expect local mechanics to have a proper shop manuals for vintage Corvettes, so the owner should provide a shop manual or write down the proper adjustment specs and hand them to the guy before he begins work, or better yet, write the information on the RO.

The ring/pinion and companion flange on '63-'64 axles are unique to those years, but later parts can be used if the current ring and pinion is bad, which is probably not the case. The axle likely needs a new loaded Positraction case, and an overhaul would also call for new bearings throughout, which will require gaging and checking pinion depth and possibly replacing the shim. The quality among local mechanics varies greatly and averages very low. It's best to have the axle overhaul done by someone with experience and an excellent reputation, so I suggest you contact Gary Ramadei through this board and send the axle to him for overhaul.

As far as minor maintenance and tuneups are concerned, because local mechanics are such a crap shoot, I advise owners to buy the appropriate shop manuals and tools/equipment so they can do their own work. You need to determine the spark advance map and correct as required, then go through the idle speed mixture adjustment procedure and the engine should idle butter-smooth at 500 in neutral and have excellent driveability and performance.

I can do the above in about five minutes because I've done it a zillion times. It probably takes more time to setup and tear down the equipment than to get the data and make any necessary adjustments as long as no parts need to be changed.

There have been many cases reported here where owners don't have the knowledge or don't feel confident enough to do the evaluation, analysis, and correct problems, so they rely on some "mechanic" to get the car running properly, and often they just stumble along, spent a lot of money, and their car never runs right. Sometimes the "mechanic" says you need a new carburetor and a new igntion system and a new... when most of the time it's a simple problem like an improper or dead VAC, sticky centrifugal advance mechanism, or the distributor gear installed backwards, which prevents the attainment of the proper initial timing range - problems that can be corrected with zero to 20 bucks worth of parts and an hour or two of labor.

Maybe there is someone in your local chapter who can evaluate the spark advance map and verify that the engine is set up correctly. I often bring my equipment to chapter meetings and end up evaluating at least one car, and I usually find evidence that bubba has been there.

The following pdf should help you get educated on how to set up a vintage Corvette engine, and everything else you need to know is in the owner's manual and 1963 Corvette Shop Manual.

http://stlouisncrs.org/news_files/DukeWilliamsEngineTuningPerformance&EconomySeminar.pdf

Duke

[Rob M.]

Re: 63 carb replacement

http://stlouisncrs.org/news_files/DukeWilliamsEngineTuningPerformance&EconomySeminar.pdf

Duke

Hi Duke,

That is a very interesting presentation! I wish I could attend that seminar, is it also available as webinar?

[Duke W.]

Re: 63 carb replacement

Nope, only the pdf of the PowerPoint charts, which are missing "California Dreamin'" by the Mamas and Papas, Jimi Hendrix "Crosstown Traffic", some "movies" that are imbedded in the PowerPoint file, and, of course, my excessive yaking.

Duke