How do you set initial timing on a carb/distributor engine? Do you use a vacuum gauge and adjust it until you get the best vacuum reading?
I just reassembled a mild 351C (9.0:1 CR) with a healthier than stock cam (0.529/0.540", 220/226 @ 0.050"). I am running a low-rise Weiand single plane intake with a 600 CFM Edelbrock Performer carb (might be a bit too small now).
After playing with a vacuum gauge, it would appear that the car wants almost 20º initial timing to idle with decent vacuum (16-17"). If I set timing around 10-12º, vacuum is 7-10" and actually increases once I crack the throttle and increase engine speed. I fast idled the engine during cam break-in and got around 17" vacuum at 3,000rpm.
I've checked for vacuum leaks and as a result disconnected the power brake booster until I can replace it. Other than that, I found no leaks. The cam called for a 108º intake centerline. With my timing set, the closest I could get was 110.5º. Cylinder/ring seal is solid.
Re: Need basic tuning help (setting initial timing)
a vacuum gauge reads vacuum, a timing light reads timing. yes, more advance makes more vacuum but we can't have it all
using a gauge for best vacuum to set timing will result in a very over advanced setting, i doubt the engine would crank & restart when hot? take care not to push the total timing too far either, when the distributor dials in its' advance, the total timing wil be way too much and you'll be pinging & detonating at top end(bad)
does your balancer read degrees out past 40* BTDC? on a budget you can stick on a timing tape to read you total timing. with the timing light set up, slowly rev until the advance stops and max's out and read your total timing. a dial back timing light can also be used if you trust the gauge on the dial
what timing set did you use, how many key positions? what position is it set in?
Re: Need basic tuning help (setting initial timing)
Quote:
Originally Posted by tinman351
a vacuum gauge reads vacuum, a timing light reads timing. yes, more advance makes more vacuum but we can't have it all
using a gauge for best vacuum to set timing will result in a very over advanced setting, i doubt the engine would crank & restart when hot? take care not to push the total timing too far either, when the distributor dials in its' advance, the total timing wil be way too much and you'll be pinging & detonating at top end(bad)
does your balancer read degrees out past 40* BTDC? on a budget you can stick on a timing tape to read you total timing. with the timing light set up, slowly rev until the advance stops and max's out and read your total timing. a dial back timing light can also be used if you trust the gauge on the dial
what timing set did you use, how many key positions? what position is it set in?
That’s good information to know, regarding more base timing typically increasing vacuum. I’ve worked on my fair share of cars over the last 18 years, but sadly this was my first pushrod V8 cam swap. I wasn’t sure if what I was seeing on the vacuum gauge made sense. I think I’ll set it at 14-16º for now so long as I don’t hear detonation under load. That should give me 38-40º total (mechanical advance adds 24º per manufacturer). At some point I’ll dial it in on a chassis dyno.
I didn’t pay attention when I had it off the crank, but there are timing markings on the damper past 20º so I think it goes up to 30º at least. In hindsight, I should have marked the 20 and 30º with a light colored paint because it’s pretty hard to read.
My timing set is an Erson True Roller (9-3121a) which I think is made by or was bought out by Cloyes. It allows straight up or advance/retard by half a crankshaft tooth, which would be 7.5º on a 24 tooth crank sprocket. The cam called for an intake centerline of 108º. The closest I could get was a straight up setting of 110.5º, so the cam timing is a bit retarded. That is what the engine had with the old cam as well. It worked well for the 5 years I drove it regularly.
Re: Need basic tuning help (setting initial timing)
i think the factory balancer is scaled out to 30*, it's not too bad to paint a couple marks from underneath with a jack on one side. most show 6*, but this Q code emission sticker shows 16* for the initial timing setting and is considered to be about the max for dependable hot starts, just keep an eye on the total even if you just guesstimate it from the edge of the 30* mark. i've heard of Clevelands still making power out to 45* with open chambers & big dome pistons but more efficient chambers only need as little as 24* to make max power
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