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Old 03-26-2005, 07:30 AM
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Proper Vacuum Reading for 350 w/ Cam

I have just installed a rebuilt 350 in my '76 El Camino. The previous owner had pulled it out of his '70 C10 and put in a ZZ4/700R4 combo. My buddy who goes to church with this guy picked it up and installed it in his Ď75 GMC one-ton crew cab pickup that he used for camping. My buddy is now putting a big block into his truck so he sold me the engine and tranny for $400 (saved me quit a bit of dough). The motor was recently rebuilt before it was yanked the first time and was not used much during the last two years (no papers came with the engine). It sure runs very strong and it has a cam in it. Iím trying to run down the builder, since he goes to church w/ my buddy, and find out what type of cam was installed. I initially set the timing at 12 degrees BTDC but was having problems w/ dieseling after a hot shutdown. Yesterday I reset the timing to 8 degree BTDC, added an insulated carburetor gasket and checked the vacuum. It runs a steady 15 inches at idle. My manuals say it should be running about 20 inches with a stock cam. What is a good reading for a motor w/ a mild cam?

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Old 03-26-2005, 08:13 AM
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If it "runs on" ad 12 BTDC either the idle is too high, there are vacuum leaks or the plugs are too hot.(or it's not really at 12*).

15" is ok if it's steady, probably around a 270(+/-5*) adv duration cam.
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Old 03-26-2005, 08:19 AM
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I had a Comp XE268H cam with Edel heads and could only pull 9 in of vacuum. Ran good though. Don't fret over vacuum. It's used to set the carb. Idle mixture is set by vacuum or rpm. Power valve on low vacuum cars have to be changed.

Last edited by bracketeer; 03-26-2005 at 08:50 AM.
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Old 03-26-2005, 08:58 AM
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i would have to diagree with you. in my expireience the less vacuum it makes, the worse it idles, and usually have to make it idle higher. 12 is good, 15 is fine, and ya 20 is like a stock engine.

hey ghetto didnt your xe268 make 15"?

Last edited by spinn; 03-26-2005 at 01:10 PM.
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Old 11-28-2009, 05:03 AM
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vacumwith a mid cam

I've got a sbc with a XE268h and pull 16 inches of vaccum @ idle rpm timing is set at 16 initial total 36 she runs like a charm.
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Old 11-28-2009, 05:44 AM
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ANOTHER THING THAT WILL CAUSE RUN-ON IS LOW OCTANE GAS IN A HIGH COMPRESSION ENGINE. I bought a 71 javelin new and had a problem with run-on and after burning a couple of valves, discovered it had a 70 engine in it.
10:1 compression doesn't like reg. gas. If that's what your useing, you need to switch now before any damage occures. It might take a couple tanks to straighten out the problem. Also, 1970 engines didn't come with hardened valve seats and if you have pre 70 heads, the UN-leaded gas ain't helping either. Might want to ask the previous owner what gas he used in it and any additives he might have put in it.
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Old 11-28-2009, 07:37 AM
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Bracketeer, what rpms are you getting 9 lbs at?? Gotta be careful here as any lower vaccum reading, and power brakes could be a problem.. Even with a vaccum can, you might only get one or 2 more pumps on the binders with power brakes..
I have a REAL stock old 2bbl Rochester 350ci 145 hp engine that I get 22lbs from.. Can't get no more stock than that, eh?? Try and set up your unit with a good reading vaccum guage getting the highest steady HGs at the lowest idle you can deliver without stalling. You'll know the "sweet spot" by ear and sometimes smell..
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