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Old 01-15-2003, 09:48 AM
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Post Timing SBC by vacuum

I have a 350 SBC showing a number of symptoms of having timing too much retarted. It has a non OEM cam but I do not know it's specifications. I wanted to check optimum timing by checking vacuum. What is exact proceedure? Should you have vacuum line going to distributor disconnected and plugged? Anything else I should know before trying this method. Last time I put vacuum gauge on intake manifold, needle just fluctuated all over gauge making it tough getting exact reading?? Any suggestions appreciated THANKS DAVE

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Old 01-15-2003, 10:58 AM
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-Go here- for diagnosing vacuum gauge readings. This may give you some insight on what is wrong with the engine.

To set initial timing with a gauge, you merely unplug the ported vacuum advance, plug the vacuum source and find correct initial timing wihen you come to the highest vacuum reading. The engine has to be in fairly good condition to go about it this way.
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Old 01-15-2003, 02:26 PM
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This is a direct quote from an old post by Jim Weeks on vacuum timing.
[quote]The reason this came up in conversation to begin with is that one hot rodder had an engine that could not be identified thus you could not even look up the timing specs. Another hot rodder had an engine with a slipped vibration damper, and slack in the timing chain so you know before you start your marks are going to be off. A timing light is almost useless in both of these situations.

Here is the procedure as explained to me.

l. Adjust your carburatur settings and then bring the car to an idle.

2. Advance the distributor until you get the highest vacuum.

3. Back up the distributor until you get one less unit. (Example is your highest reading is 14 you back it up to 13 and lock your distributor down.

OK I took this advise and applied it to a 1965 Plymouth 273 that I completely rebuilt a few years ago, so the engine was in good shape. The factory spec with a timing light is 10 degrees before top dead center, and that is what I had it set at. I hooked up the vacuum gauge, rotated the distributor and the best vacuum I got was 20. I then backed it up to read 19 and locked the distributor down. I ran the car thru a series of tests and the car started faster, idled smoother, excelerated faster, and ran a little cooler than it ever had before. I re-cheched the timing with a light and it read 14 degrees before top dead center.

I wish some of the rest of you would try this technique and report your results good or bad. I was sceptacle about this myself in the begining.

This is a copy of the results from my car as I tested it after Jim's post.
[quote]OK so I finally got a few minutes together to try the vacuum test. I found that I reached max vacuum at approx. 23 degrees advance. I backed off one degree and was at 18 deg. advance. Dropped another deg. vacuum and was around 11 deg. advance. So with my engine it took me about 2 degrees vacuum drop to get to an acceptable advance. I still feel safer with my light but it is an interesting test. I suppose in a pinch I could use this information to get an engine running and set it with a two degree vac. drop to be safe until a light was available. It's better than nothing. Just wonder how a radical long duration cam or a burnt valve would affect the test. I saw BowTie's results, hope others try also. Maybe we can get info on a stock engine and a real radical one too. Thanks for the test. <hr></blockquote>
Jim originaly quoted a one inch vacuum drop for setting timing but I actually found that almost a two inch drop in vacuum put me where I run her. I would also check out the vacuum link Kultulz gave to ensure you don't have some other problem going on.
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