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Weird Chevy 350 timing problem - HELP!

This is a discussion on Weird Chevy 350 timing problem - HELP! within the Automotive Support forums, part of the Tech Support Forum category. My son bought a 1974 corvette with non matching numbers. Turns out the engine was from a 1975 full size

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Old 10-17-2011, 09:21 PM   #1
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My son bought a 1974 corvette with non matching numbers. Turns out the engine was from a 1975 full size - the engine code is "CMJ". It ran fine until the other day when it apparently jumped time. I say that because I was able to get it running again using the timing light and rotating the dist but it had zero power. (the mark had moved significantly)

After it jumped time, I got the edelbrock timing set that allows for three installs - retarded, straight, and advanced. The jegs guys said install it straight up for a stock cam. I now don't think there is a stock cam in there.

We knew the setup was weird because when we first timed it months ago, the timing mark on the balancer did not line up with the marks on the cover, rather it pointed straight up. At first maybe I thought the outer ring on the balancer had slipped. But when I started to tear down the front of the engine the other day, I set the engine to tdc (as I knew it) with the timing mark straight up. After pulling the balancer, the crank key was straight up too.

When installing the new timing set, the key ended up pointing around 130 as viewed from the front of the engine.

Now the engine won't run at all. It tries but won't stay running. I have found TDC with a compression gauge in #1 and the timing mark now lines up with the plate on the timing cover and the rotor is pointing to number 1. We took great care to make sure the marks on the cam and crank gear were perfectly aligned. The only thing I can think of is the cam is not stock and the gear set should have been installed differently. But which way and why???

Any insight or ideas would be helpful. One thing I was thinking was to just get a cam from edelbrock so at least I knew precisely what was in there. However, I want to minimize the money we put into this engine because, well, it is a CMJm the heads suck, and the casting is most definitely a 2 bolt. The long term plan is to run this engine for another year or two and then get a decent stroker crate motor and drop it in.

If you have experience with 350sb cams and whatnot and you can help I would greatly appreciate it!
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Old 10-18-2011, 07:04 AM   #2
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Hi active123

If you plan on stripping the engine to replace the camshaft you might as well do a complete job by replacing the timing chain to bring it to original specs. You are correct in your assessment that the key on the balancer shouldn't be straight up but on a 130 angle, on the sprocket the timing marks on the cam gear should be pointing downward to align with the crankshaft. The timing chain has jumped significantly for it not to start, by turning the distributor 90 either way is not a good sign so far all signs are pointing that the timing gears are stripped. You have to be aware of two things; 1) When replacing the chain on a 75 corvette the cam gear has three 1/2 inch or 9/16 bolts to remove and it comes out fairly easy, but the crank gear is different, you will need a puller to remove it because it has two key-ways on the shaft these things are on super tight. 2) When replacing the timing sprockets with new parts, do not buy the nylon type sprocket set if you plan on constantly booting the engine get the steel version it will last allot longer.
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Old 10-20-2011, 07:22 AM   #3
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ignore the compression gauge and marks, just pull the number one plug and rotate engine with a straw sitting in it and resting on the top of the piston. The straw will rise, pause, then start to drop, when it starts to drop, stop and go back. TDC is midway between the rise and fall points.

When installing the chain and gears, use the marks on the cam gears ignore all others, and when you're done, make sure the rotor is pointing at the number one tower in the distributor.

If engine has been reworked, it isn't uncommon for a different timing cover to be used, often timing covers have the marks in a different location to enable viewing with different accessories installed, so if the cover's changed but balancer wasn't the marks won't always agree.
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Old 10-20-2011, 08:53 AM   #4
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Originally Posted by kendallt View Post
ignore the compression gauge and marks, just pull the number one plug and rotate engine with a straw sitting in it and resting on the top of the piston. The straw will rise, pause, then start to drop, when it starts to drop, stop and go back. TDC is midway between the rise and fall points.
This is correct......but.......it could be TDC of the exhaust stroke. The best way to tell is to remove the valve cover on the bank where #1 is located and observe if the valves are even (both closed) when the piston is at TDC....if the valves are not even, the engine is on the exhaust stroke.

This pertains to older engines not controlled by computer!!
As for placing the distributor on #1.......I can make most mechanics scratch their heads on that one. You can place the rotor any place you want as long as you match #1 spark plug wire with the rotor position and follow the correct firing order and rotation.

Another thing to be aware of is that the dist must drop completely for correct installation.....especially the SB Chevy engines. Locating #1 and dropping the dist in place does not mean the oil pump drive has engaged the oil pump. (My son did that once....and only once. I got a frantic call one night and he said he had no oil pressure after reinstalling the dist on his 350. I asked him if the dist was fully seated......after a "what?", I told him I would be home soon and would show him what was wrong. He was just young and starting his mechanical "career"......)
It's a good thing I never had any sanity......I would have lost it by now.
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Old 10-20-2011, 10:17 PM   #5
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The jegs guys ... were meaning if you have a high lope cam you might want to set it at a couple deg adv on the timing chain gears ... they also sell off set dole pins for the cam gear to advance it ... this offsets the valves not the ing timing
So even if you went a couple degrees on the cam timing gear it really isnt going to have much effect on the reg timing marks .... offsetting the cam gear is not a big thing when its a every day driver dont worry about that and set it at standard valve timing NO off set

your probably going to have to pull the timing cover again BUT first i would check my timing ... remove #1 plug and plug the hole with your thumb then bump the engine till you feel and hear the compression then do like kendallt said and use a straw and turn the engine by hand forward or backwards till the piston is at the top of it stroke ... that is TDC compression and #1 plug will be firing ... now pull the dist cap and see if the rotor is pointing at #1 ... if its pointing 180 away from #1 remove the dist and reinstall it so its pointing at #1
Remember the crank turns 2 times to the camshafts once so having the timing mark up doesnt mean diddly cause the cam can still be off 180 deg

If the dist is lined up when you do the TDC pull the timing cover you got it wrong ... (off a tooth or so)...
before you reinstall the timing cover start it for a couple seconds to make sure all is right
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Old 10-21-2011, 03:24 PM   #6
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Also remember the firing order is 1 8 4 3 6 5 7 2. So whichever cylinder rotor is pointing to at TDC , so make sure its pointing to cylinder #8 the wires t go on in a clockwise direction. And #1 cylinder is the front one on the drivers side.

You probably already know this, but its a VERY common mistake to make.
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