timing mark way off the scale???

[orange-c10]

been having alot of hesitation problems when i take off. replaced plugs and points. nothing. checked timing. it was at 4 degrees btdc. so i decided to loosen the distributor and drive down the street and make adjustments until the problem went away. got home and pulled timing and now im way advanced past the indicator. it accelerates good now but its knoking like crazy when i get in the throttle. anyone know whats causing this?

also just read that some 350s have a timing mark at around 2 o'clock and some at around 12. my timing mark is lining up at about 12 right now with the indicator gauge at 2. but if i advance it more wont my knocking get worse?

[be_el]

Have you checked your timing advance unit..... Sounds similar to a problem I once had from not having hardened valve seats and running unleaded gas. Caused my valve seats to begin pitting and eroding...seems like I was always messing w/ my distributor and advance unit 'til I changed my heads.

[orange-c10]

not quite sure how to check it.

[BobRitts]

re: checking the vac advance can... One way is with one of those little handi-vac hand pumps (like for bleeding brakes... harbor freight probably has one for under $20 or you might could borrow one from the local parts place).

Attach the vacuum lead to the dist. vac advance can port, put the timing light on, and watch how far it advances and when. Should start advancing the timing at about 5 inHg and be all in by 15 or so, with a total of 15 - 20 degrees advance when you have the pressure at max.

If it knocks under throttle it's probably not the vac advance, b/c the vacuum goes way down when the throttle plates are open. I'd suspect the mechanical advance (initial + centrifugal) is too high for that load/mixture combo.

Mixture may be off, hence the stumble off idle. IIRC, too lean will ping, so richening it up could make it not ping so much. Real cause might be a vacuum leak messing up the mixture.

HTH

[jacobs]

I have personally known timing marks on harmonic balancers to be off by as much as 8 degrees new from the factory. Although this is not common, it can happen. Also, harmonic balancers can and do slip causing timing marks to be off. It sounds to me like you advanced your timing too much and what you are hearing is just pinging. Back your timing off a little at a time until the pinging goes away and you should still have the performance you are looking for.

[jacobs]

High performance Corvettes in the 1950's and 1960's didn't even have vacuum advance. A vacuum advance that isn't working WILL NOT cause driveability problems. They are only there to boost gasoline milage.

[70rs/ss]

Vacume advance most certainly will effect driveability, if the distributor doesn't advance how does your spark lead advance with the rpm?? They have mechanical or vacume, but not none!

[Billla]

Originally Posted by jacobs

High performance Corvettes in the 1950's and 1960's didn't even have vacuum advance.

Not the case to my knowledge for the 60's 'vettes. Even the top-of-the-line L-46/L-88/ZL-1 engines had vacuum advance. Can't speak for the 50's.

[alblogg]

Originally Posted by jacobs

I have personally known timing marks on harmonic balancers to be off by as much as 8 degrees new from the factory. Although this is not common, it can happen. Also, harmonic balancers can and do slip causing timing marks to be off. It sounds to me like you advanced your timing too much and what you are hearing is just pinging. Back your timing off a little at a time until the pinging goes away and you should still have the performance you are looking for.

Got a 307 now if you put a timing lite on it and set it to factory specs will not run. When I tune it up I idle it up I turn the distributor one way till it misses then the other way then come back about in the middle take a spin then I know if I need to advance or retard.

[jacobs]

High performance Corvettes only had centrifugal advance whereas everything else including trucks had BOTH centrifugal and vacuum advance. Nothing uses vacuum advance only - it just won't work.

[cdowns]

well to properly time an engine it's first necessary to determine where what marks on the balancer and timing indicator are in relation to TDC on the compression stroke
in 35 years the balancer could of been changed the timing indicator could have been changed the timinig cover changed // you can dance araound all you want with the distributor but if you dont know what you're lookin at it aint going to do much good //this is where a service manual comes in handy
personally i'd change the fuel and ar filters b-4 the timing

[Billla]

I would start by ensuring the TDC mark is right - this is pretty easy with one of the spark plug piston stops, let me know if I need to detail the process.

The rest of this comes from another post of mine at:

http://67-72chevytrucks.com/vboard/s...d.php?t=211331

I would disconnect the vac advance and check centrifugal advance first. Advance should be all-in before 3000 RPM and should not exceed 34-36 degrees. Set static timing ensure no more than 36 degrees total advance. It's not uncommon to need 12-14 degrees initial advance on a modified engine to see 34-36 degrees total, but a stocker should be closer to 8-10. Anything really wacky here in terms of the RPM that it's all-in at, total advance or required static advance means someone's been monkeying with the centrifugal weights or springs or they're sticking. They should rotate very loosely on the pins and have a bit of moly lube on the dimple underneath that they bear on. Replace the springs and weights and lubricate the points if you end up with something weird.

Next, ensure the vacuum advance is connected to a ported vac source on the carb and hasn't been moved somewhere else - too strong of a signal means too much advance under load. Ported vac is taken from directly above the throttle blades. Then take the engine to that all-in RPM slowly and see how the advance changes - vac advance should add another 12-14 degrees of advance at part throttle. If you're seeing WAY more than that then consider replacing the can - it's an easy swap, and an adjustable item is your best bet.

[orange-c10]

would i get a knock or ping if the timing wasnt advancing enouph

[orange-c10]

also if i back the timing off anymore ill get the flat spot back

[Billla]

Originally Posted by orange-c10

also if i back the timing off anymore ill get the flat spot back

You kinda need to decide if you're actually going to troubleshoot this and fix it, or if you're just going to change stuff and see what happens A piston stop is like $10 and will let you be SURE the timing is set correctly. A little work with an advance timing light - $60 for a basic one at Sears - will let you be SURE the centrifugal and vacuum advance is working correctly. This is at most a couple of hours work and you don't even have to take anything apart.

[orange-c10]

ill have more time to work on it this weekend. for now i just need to get to work. would napa have a piston stop. i agree i need to find out where the true tdc is. now when i do is that where i make a mark on the balancer or do i have to turn the crank back until it hits tdc again and place the mark in the middle?

[Billla]

Originally Posted by orange-c10

ill have more time to work on it this weekend. for now i just need to get to work. would napa have a piston stop. i agree i need to find out where the true tdc is. now when i do is that where i make a mark on the balancer or do i have to turn the crank back until it hits tdc again and place the mark in the middle?

Sorry, Dude - been there :/ I would retard the timing until the ping goes away and live with the flat spot for the next few days, IMHO.

NAPA might have one, but you *may* have better luck with one of the other chains...or someone in your area might offer to help you out! ANYONE?

The second is absolutely correct - turn one way to hit the stop, make a mark, then turn back and make a second mark and it's midway between. It's a heckuva lot easier if all the plugs are out, but if you take it slow and let it bleed down it's not too bad. If you find the balancer mark is off, IMHO the best bet is to buy an aftermarket pointer that you can adjust.

Let us know how it goes.

[GMC Jim]

per Chev truck chassis service manual, a 350 cu.in engine, auto trans.; specs are :- Initial timing 8 (degrees) @ 550 RPM (idle). Centrifugal advance (degrees) @ (RPM) are 0 @ 865; 2 @ 1335; 11 @ 2400 and 18 @ 4200. Vacuum advance is 0 @ 8" vac. and 15 @ 15.5" vac. Proper vacuum advance is somewhat critical for best engine operation. Test the vacuum unit for leak down by applying vacumm; closing of the vac source and observe that the unit holds the vacuum applied.

[Werewolf]

I don't know if your question was answered or not, as I didn't have time to read through all the replies.

But serious pinging while driving, especially under load, is often caused by the timing being too advanced.

Although serious carbon build-up can do it too. Other things, like a nonfunctional EGR, will also make a big ping. Of course, you shouldn't have an EGR setup on this truck.

[70clone]

just wanna make sure, do you have the vacuum line disconnected from the dist and blocked when your running the timing light, because if you dont that will cause it to do exactly what ur talking about.

[Werewolf]

A stretched timing chain might do this, too.

[orange-c10]

yes i plugged the vacum on the distributor as well as the carb. ive only been experiencing these symptoms since i installed an edelbrock performer manifold. i initially thought it was a vacum leak. sprayed around the carb and manifold with carb cleaner and no change in idle. im wondering if maybe its a leak between exhaust and intake runners. could possibly cause me to be running lean maybe? first im going to check to see whether my timing mark is right and if so im going to take the manifold off and reinstall.

[Billla]

Ah, that's a key piece of information - knowing of a change is always good

I'd definitely suspect a vacuum leak at the most common points - the front or rear of the manifold. Did you just use a good RTV there, or did you use the gaskets?

[orange-c10]

i used the gaskets. i noticed in the instructions it said to just apply 1/4 inch of rtv

[Billla]

Originally Posted by orange-c10

i used the gaskets. i noticed in the instructions it said to just apply 1/4 inch of rtv

Well, not a certainty but the gaskets are a definite no on the SBC. They're VERY easy to displace and don't hold despite the little rubber pins that fit into the block. I'd go ahead and check the TDC, but then IMHO I'd pull the intake, dump the end gaskets and use a good high-temp RTV. If that's not the problem now, it will be :/