I have a 1958 Corvette that is an original 290HP car. The engine it was rebuilt in 1985 with the correct Duntov cam and solid lifters but the rebuilt 4900R fuel injection unit was not installed. The then owner was told not to install the F.I. until the rebuilt engine had at least 3000 miles so a WCFB carburetor and distributor that looks much like a 110946 was installed. There are no identifying numbers on the distributor and no external oil tube or GITS oil port is present. When I bought the car five years ago it had been stored for 26 years and never run except for occasional startups.

Initially, I set the dwell at 30 degrees with my 50 year old analog meter and 10 degrees advance with my equally old timing light. I have since driven over 3000 miles. The engine started and ran with a bit of lugging at low rpm and some high RPM miss but no apparent knock. After reading the Forum’s Lars Grimsrud article Timing for Peak Performance I knew that I had to get a fix on what this temporary set up was doing. After buying a good digital, combination, tach, dwell, and timing light, the first thing I found was that my dwell was actually 34 degrees while my old analog meter read 30. After setting for 30 degrees, I followed Lars recommendation for removing one spring from the advance weights and disconnected and plugged the vacuum advance tube. The result was an all in advance of 41 degrees at 3000 RPM. Is it normal to have 31 degrees advance form the weights alone? I was surprised that it took 3000 RPM to max out the mechanical advance with only one spring. I can’t believe that I got anywhere near full mechanical advance even under hard acceleration. I then used the setback timing light to arrive at Lar’s recommended 36 degrees and locked down the distributor. The resulting initial timing is now 5-6 degrees. I then reconnected the vacuum line with a T connector connecting my vacuum gauge and started the engine , set idle speed a 600 RPM where no mechanical advance occurred and observed 29 degrees form my dial back light indicating 23 degrees from the VAC alone with 11 “ Hg vacuum. Again I was surprised to find this much advance since all that I read on the Forum indicated that I should expect about 15-16 degrees from a typical VAC. The VAC has B33 stamp which is not illustrated in any of the distributor information I can find for early Corvettes. From Lar’s article the total of 36 initial and 23 degree would give me an excessive 59 degree possible advance. I would like to follow Lar’s suggestion for installing lighter mechanical advance springs to get “All IN” advance at 2500 but I believe this could result in dangerously high advance with the current VAC. I plan on driving the car for at least another season before attempting to install the original 914 distributor and 4900R F.I.

What recommendations might Forum members make for the best fix for this temporary setup?

 

I would limit the total advance to 16 or so and crank in 20* of initial. You can use limiter bushings or weld up the slot etc.

JIM

 

Since you are only going to use that distributor one more year, I would get a distributor advance kit to bring the timing in sooner.

Check your distributor for a missing bushing in the advance slot, as it may be the reason you are getting 31* advance out of it. You could also fill in the slot a bit by welding and filing it so that you only get 16-18 degrees advance. (JB weld would even work for a season)

They make vacuum advance limiter plates that you can install to limit the vacuum advance to 10-12*. Lars advises limiting vacuum advance to 10-12 with today's gas.

I would take the distributor you plan on using and send it out to Lars for a rebuild. Then you will be all set when you go to use it next year.


Gerry

 

All of the above....for my '61 I just installed an Accel adjustable vac advance can #31034 and dialed it in where I wanted it... You have to watch those distributor spring kits - the springs aren't hardened and go out of spec many times... I would try to get your 'all in' number down some, IIRC I had it at 2500-2700 RPM on my '61.

 

What Mossy and Frank said. You are on the right path using Lars's materials for reference. None better. I would personally refrain from setting the initial timing at 20 degrees. Very hard on starters and flywheels, as you're trying to crank the engine against the too-early spark.

 

Some engines are happy with initial at 20, and some hate it. I guess it depends on the cam. It's easy enough to twist the distributor to 20* to see how it starts. In fact, it might be worth testing the initial from 12-20 to see how it fights the starter, and to see where you get the best idle. That way, when you send the distributor to get rebuilt, you can also have the curve tailored to your engine.

Gerry

 

Is it normal to have 31 degrees advance from the weights alone?

Yes, on most stock single point distributors the slot was large and sometimes GM used a rubber bushing, that over time deteriorated, which would cause excess mechanical advance. See the photo below.

I would limit the distributor centrifugal advance to 24 degrees, all in between 2,700 & 3,000 RPM, set the base timing at 12 degrees for a total advance of 36 degrees.
You can use a B-20 vacuum advance, NAPA VC1785 or Standard VC181 which is 16-18 degrees crank at 11 hg.
That gives you a total advance of 52-54 degrees at cruise.
Joe

 

plaidside has laid out a very nice advance map for these 283ci motors....here is what my 61 is set up at using medium weight advance springs, I set things up to be all in quite quickly as I like the snappier response; with the vac advance unplugged cruise vacuum was high (near 55*) but there was no pinging so I left it alone (48*-52* is fairly normal):

Distributor curve with vacuum advance hose disconnected and plugged (two medium weight advance springs, both silver, no advance limiter tabs installed):
12.0 deg @ 850 RPM (idle)
35.0 deg @ 2500 RPM (timing all in)
DO NOT EXCEED 38 DEGREES !!!

 

Use a dial back light to set total WOT advance at 38-40, as high in that range as possible without detonation. Prior to doing this you have to use the dial back light while revving the engine to determine where it stops advancing. It could be up to 30-32 degrees at up to 5100 revs.

Get a Mr. Gasket 928G spring kit and use the silver or gold springs to bring it in as quickly as possible.

The only VAC that passes the Two-Inch Rule with the Duntov cam is a B28 - Airtex 4V1053, and it must be connected to a full time vacuum source.

Duke

 

Thank you Plaidside and all other that have responded. As usual you have a wealth of useful information. I am inclined to start with Plaidside’s recommendation. I have already acquired the advance curve kit recommended by Lars and it includes bushings to replace the one likely missing under the advance plate. I will replace the B33 VAC with the B20 and go through the whole setup again with an initial 12 degrees advance. I did have the 914 distributor rebuilt last winter by Don Baker in Sandwich Illinois. His setup gave 10-12 degrees maximum advance at 6000RPM. This does not sound like much with no vacuum advance and the recommended 14 degree initial advance. I guess I will have more questions for you all when I get to the 4900R install. Most people tell me to keep the F.I. on the bench and just run with the carb if I really want to drive it but I would sure like to see it restored to original.

 

thank you Duke, a B28 it will be

 

Could Don be giving you distributor degrees? If so you have to double that to get crank degrees, giving you 20-24 degrees, plus the 14 will give you the 34-38 total.

When I got my fuelie, that sat for over 25 years, I put a manifold and carburetor on it and used it for over a year to get all the bugs out.
Then over the winter I rebuilt the FI unit and installed it that summer and have not looked back.
Joe