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Discussion Starter · #1 ·
Hi im new to this site, i have a 1979 chevy big ten silverado with a factory manual 4speed ive been trying to fix up, I have a question on the 350, its not original i was told it was outta mid 80s halfton but its a 4 bolt main, eldebrock intake with new 600cfm carb, but the question is with vacuum advanced unhooked i run 18 degrees. this truck runs great, sometimes takes a few cranks in the cold to get it going but i think thats just a old carb motor, i know that a stock 350 runs around 10ish i believe, so does this mean i have a mild cam?? i dont know a whole lot about internals of motors but i have a pretty good idea as i rebuild small engines and do alot of maintance and minor mods to my 3 trucks, any help will be nice thanks!!
 

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I would say someone changed cams inthat engine. Most aftermarket cams require more initial timing somewhere 16-20 degrees. If you have a timing light with an advance knob on the back have someone hold the rpms at 3000-3500. Point the light at the scale and turn the knob until the line on the balancer aligns to zero. How many degrees is the scale on the back of the timing light reading? Should be 36 degrees. When your done run a vacuum hose from the right port on the carburetor to the distributor. Now check the timing with the vacuum hooked up. Set the advance knob to zero then point at the scale and turn the advance knob until the balancer line aligns to zero. The scale on the timing light should be on 50 degrees.
 

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Discussion Starter · #3 ·
cool ill try this tommarrow, i ask someone that knows there fair share about small blocks and they said it probably has...ugh i cant quite remember what he said but its a factory height on the nobs just alot longer durantion is what he said...this truck has some crazy torque let me tell you, so im going with what this guy told me. thanks for you info really appreciate it!!!
 

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I am running a mild cam in my '75 GMC K25 (350 engine, Summit 1101 cam, Edelbrock performer manifold and quadrajet) and use about 16 degrees of initial timing and 18 degrees mechanical (vacuum advance also adds about another 18). It starts and runs very smoothly with that combo, which is good for hauling with a work truck. I have about 18-19 degrees of vacuum at idle.

I have not tried going for max initial timing, but most articles I've read over the last few years don't recommend going over about 18 degrees initial with a stock engine if your mechanical advance still works correctly (and adds 18-20). The stock GM setting for base timing is usually about 6-8 degrees, and that's much too low.

I also have 4.10 gears and stock height tires, so low end pull is excellent.

Bruce
 

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.. Most 1970's engines and some 1980's engines had horrible very low compression ratio with dished pistons and large chambers in the heads... this big combustion space requires early cam timing to get the fire burning fast enough to fill all that space before the pstons go down in order to get what little power was available out of them... BTW, the Summit 1101 cam is almost identical to a stock 327/350/400 cam... However, I wouldn't go any bigger cam without increasing the compression ratio from the stock ' 8.5:1 ' (actually about 7.6:1).... to at least 9.0 - 9.5:1... otherwise, a bigger cam acts like a vacuum leak...

.. Newer engines with higher compression ratio and especially true Vortec engines with fast burn chambers as well may only need 32-34 degrees total timing at WOT above 2500 to make max power... even with a mild performance cam...

.. If you think your engine has power now, drive one with a properly built 350 with good quench...!
 

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20 at idle and 38 total works well on old school 350's.

However, you need to use an adjustable vacuum advance canister and set it to add only 10 degree of timing instead of the factory 20. As 48 to 50 degrees should be the max timing (mechanical + vacuum).

I would also use manifold vacuum with the vacuum advance. so timing would be 30 degrees at idle (20 mechanical and 10 vacuum).
 

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Discussion Starter · #8 ·
i beleive it has 3:07 gears, i did do a compression test on it and i was surprised on how low it was! idle at 600rpms it was about 75lbs, although it was about 5 degrees outside and i just started it, but when i revved it up to about 1500-2000 it only built up to maybe 125ish....is that bad? i know it has the normal small block valve seat problem when u first start it smokes blue for a few seconds but clears out quickly, do you guys know how rare a silverado 4speed is? it is factory! but i was told it was pretty rare and havnt seen any other trucks with the 4 speed and if they do there the cheyenne or scottdales not silverado
 

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.. Yes, compression test is done with engine off and throttle wide open (or removed)... check all 8 cylinders, you're especially interested in any cylinder way lower than the others... checking it at idle or with throttle closed lets little or no air in and gives a low reading... vacuum can be checked with engine running...
 

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.. Yes, compression test is done with engine off and throttle wide open (or removed)... check all 8 cylinders, you're especially interested in any cylinder way lower than the others... checking it at idle or with throttle closed lets little or no air in and gives a low reading... vacuum can be checked with engine running...
Warm the motor up. Turn the motor off. Wire the throttle blades wide open so the motor can breathe. Remove the 12v feed wire to the coil so the spark plug wires will not fire and light the mixture that will be blown out of the spark plug holes when you turn the motor over with the starter. Mark the spark plug wires for the cylinder they belong to. Remove all the spark plugs. Insert the gauge tip into the first spark plug hole and turn the motor over with the key. Let the motor turn over until the gauge needle will rise no farther (usually 4 or 5 "chuffs" from the cylinder). Write down the results and go to the next cylinder.
 

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Hi im new to this site, i have a 1979 chevy big ten silverado with a factory manual 4speed ive been trying to fix up, I have a question on the 350, its not original i was told it was outta mid 80s halfton but its a 4 bolt main, eldebrock intake with new 600cfm carb, but the question is with vacuum advanced unhooked i run 18 degrees. this truck runs great, sometimes takes a few cranks in the cold to get it going but i think thats just a old carb motor, i know that a stock 350 runs around 10ish i believe, so does this mean i have a mild cam?? i dont know a whole lot about internals of motors but i have a pretty good idea as i rebuild small engines and do alot of maintance and minor mods to my 3 trucks, any help will be nice thanks!!
On a deal like this you need to know that TDC is being accurately shown by the damper line and timing tab. There were several different damper/tab combos used through the years and if they're mixed up you can get a false reading. If the outer ring of the damper has slipped you will get a false reading as well.

Setting up a performance advance curve is always needed w/a stock distributor. This alone can make a big difference in how well the engine runs.
 

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.. X 2... I forgot to mention that. A piston stop tool (~$5) can be used to verify that your timing tab is the correct one and/or the outer ring of your engine damper hasn't slipped around on the rubber putting its mark in the wrong spot...
 
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