When is it the carb?
03-23-2015, 03:50 PM
#22
Racer
Just disconnect the vacuum advance line, put a short piece of clean hose on the can and siphon all the air out of it. It should hold it's position until you let the air out. It's a simple way to make sure the diaphragm doesn't leak air. If it leaks, then you're probably not getting enough vacuum advance in the timing curve.
03-29-2015, 03:02 PM
#24
Racer
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Silvertone, you were absolutely correct. I bought a vacuum pump and tested the vacuum advance can. It was leaking badly. So I purchased a new one (BWD V384) and installed it. Still no vacuum advance coming in at idle. Turns out the manifold vacuum port on the carb that the vacuum can was hooked to wasn't pulling any vacuum. Hooked the can up to a different manifold vacuum port and voila, I now have vacuum advance. Had to reset the timing, idle and mixture screws again, here's where I'm at now.
Base timing 12 degrees at idle
With vacuum advance 31 degrees at idle.
Idle in park 800 RPM
Idle in drive 600 RPM
Vacuum at idle 20 inches.
Car idles much better now, I'll have to wait for a test drive to see if it performs better now at part throttle.
Base timing 12 degrees at idle
With vacuum advance 31 degrees at idle.
Idle in park 800 RPM
Idle in drive 600 RPM
Vacuum at idle 20 inches.
Car idles much better now, I'll have to wait for a test drive to see if it performs better now at part throttle.
03-30-2015, 12:59 PM
#25
Racer
Silvertone, you were absolutely correct. I bought a vacuum pump and tested the vacuum advance can. It was leaking badly. So I purchased a new one (BWD V384) and installed it.
Base timing 12 degrees at idle
With vacuum advance 31 degrees at idle.
Idle in park 800 RPM
Idle in drive 600 RPM
Vacuum at idle 20 inches.
Car idles much better now, I'll have to wait for a test drive to see if it performs better now at part throttle.
Base timing 12 degrees at idle
With vacuum advance 31 degrees at idle.
Idle in park 800 RPM
Idle in drive 600 RPM
Vacuum at idle 20 inches.
Car idles much better now, I'll have to wait for a test drive to see if it performs better now at part throttle.
OK that's great! That can is the equivalent of Ecklin 1836, which apparently is correct for our '77s stock but intuitively in my mind seems like it comes on too early for your high vacuum signal. Great deal if it works for you, but this can starts pulling at just 3-5hg and is all in at 11-13hg. If you get pinging off idle just might need a stiffer model (just get an adjustable Moroso for $28). Factor into the equation this one adds up to 18 crank degrees. Lars says you don't want more than 52* combined vacuum+centrifugal, but it's easy enough to find or make a limiter.
03-31-2015, 06:03 PM
#26
Racer
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Took the Vette out last night for a test drive and it runs and idles so much better now! Thanks again to everyone for all the help and suggestions.
03-31-2015, 06:54 PM
#27
Racer
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Location: Wellington, New Zealand
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I checked my timing yesterday and I am running at 12 degrees also (I think I stated I was at 11 degrees), unless we limit the vacuum advance canister this seems to be the maximum we can be with a stock L48. It may not be ideal but as you have found out, too much advance can make your car run pretty rough.