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Old 07-09-2007, 04:48 PM
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Ignition timing woes

Hey guys,
So I've been trying to get my ignition timing figured out on my sbc 350 lately and im having a lot of trouble with it. The problem im having (as it seems) is that im not getting enough timing in the lower rpms (it sort of stumbles on hard throttle low rpm runs). So what I had done was I cranked the timing way up, which resulted in great low-mid rpm power but ended up dogging on the top end.


Just to get a solid start, I threw a piston stop on #1 today and hand cranked it both directions, made marks, and took the halfway point to verify TDC. It matched fine with my balancer mark.

So I pulled the vacuum advance off of the dist. and set total timing to 35* (a bit conserative), initial ended up at about 20* (still without vaccum advance hooked). I threw the vaccum on from direct manifold port (ive tried direct and ported with same result) and it brought timing up to around 30* (10hg vacuum pull at idle).

So with everything set as described, it just doesnt run right. It idles high with the direct vacuum (even with idle screw all the way out), mid range power is bad and it doesnt even feel right on the top end until its turning around 4500+rpms. The only way my motor feels right is when it's pulling a buttload of static timing.

So what gives?

I figure it may need the timing curve to be changed, but should it be fairly decent with a range of 20* initial to 35* total? Total timing comes in at about 2200rpms

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Old 07-09-2007, 05:33 PM
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Quote:
Originally Posted by KNanthrup
Hey guys,
So I've been trying to get my ignition timing figured out on my sbc 350 lately and im having a lot of trouble with it. The problem im having (as it seems) is that im not getting enough timing in the lower rpms (it sort of stumbles on hard throttle low rpm runs). So what I had done was I cranked the timing way up, which resulted in great low-mid rpm power but ended up dogging on the top end.


Just to get a solid start, I threw a piston stop on #1 today and hand cranked it both directions, made marks, and took the halfway point to verify TDC. It matched fine with my balancer mark.

So I pulled the vacuum advance off of the dist. and set total timing to 35* (a bit conserative), initial ended up at about 20* (still without vaccum advance hooked). I threw the vaccum on from direct manifold port (ive tried direct and ported with same result) and it brought timing up to around 30* (10hg vacuum pull at idle).

So with everything set as described, it just doesnt run right. It idles high with the direct vacuum (even with idle screw all the way out), mid range power is bad and it doesnt even feel right on the top end until its turning around 4500+rpms. The only way my motor feels right is when it's pulling a buttload of static timing.

So what gives?

I figure it may need the timing curve to be changed, but should it be fairly decent with a range of 20* initial to 35* total? Total timing comes in at about 2200rpms
More data, what engine (Ok somebody's 350), how big (figure it's 350), how old/miles on it, what cam, how much compression, type of piston, what carb, what distributor, what spark plug and wires, what intake, what kind of exhaust, what car/weight, what type of transmission, what are the rear gears? All this plays heavily with ignition tuning.

Bogie
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Old 07-09-2007, 07:16 PM
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Quote:
Originally Posted by oldbogie
More data, what engine (Ok somebody's 350), how big (figure it's 350), how old/miles on it, what cam, how much compression, type of piston, what carb, what distributor, what spark plug and wires, what intake, what kind of exhaust, what car/weight, what type of transmission, what are the rear gears? All this plays heavily with ignition tuning.

Bogie
Fresh crate 350, maybe 2000 miles, cam is Comp XE274H 230/236@.050 .520/.520 lift, 10:1cr, stock cast pistons, Holley 4150 DP 750cfm carb, std HEI dist., bosche platinum plugs, 8.5mm MSD wires, Edelbrock performer RPM intake, dual 2.5 exhaust thru 1 5/8 shorty headers, all in an s-10 pickup weighing ~3000lbs, trans is NV3500 5 speed manual, rear gears are 3.73
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Old 07-09-2007, 08:40 PM
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What heads are on it? What fuel are you running? Have you done a compression test on it?

If these are 76cc smogger type heads with dished pistons you might need closer to 40 degrees advance total.

If the idle is high even with the idle adjustment screw completely out it sounds like there probably is a vacume leak somewhere or the secondary blades are open to far.
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Old 07-09-2007, 10:13 PM
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They're #450 60cc w/ 2.02/1.6 valves and hand porting. The idle thing I guess I shouldn't have mentioned because the carb needs a little tweaking here and there. It was a race built carb for a circle track car and I haven't jetted it down enough, etc. so the idling problem is most likely from the carb itself
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Old 07-10-2007, 07:47 PM
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Anyone got any ideas? Maybe it's just the way the motor combo works is that it likes lots of timing? My understanding has always been that lower compression engines like more timing (like 8.5 and below) and higher compression engines use less. My motor being around 10:1 and with 60cc chambers I would think would not want to run a whole lot... I don't know.

Should I look in to "locking out" the distributor? Someone had mentioned that to me. But would locking it out be a good idea for vehicle driven on the street? It seems like it would have too much timing for low rpm throttle stabs.

Anyone?
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Old 07-10-2007, 08:09 PM
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Quote:
Originally Posted by KNanthrup
Fresh crate 350, maybe 2000 miles, cam is Comp XE274H 230/236@.050 .520/.520 lift, 10:1cr, stock cast pistons, Holley 4150 DP 750cfm carb, std HEI dist., bosche platinum plugs, 8.5mm MSD wires, Edelbrock performer RPM intake, dual 2.5 exhaust thru 1 5/8 shorty headers, all in an s-10 pickup weighing ~3000lbs, trans is NV3500 5 speed manual, rear gears are 3.73


Have you checked to see if the Balancer has spun?



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Old 07-10-2007, 10:01 PM
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Quote:
Originally Posted by Rob Keller
Have you checked to see if the Balancer has spun?



R
Yeah I verified TDC with a piston stop, so the mark is still correct. I almost thought that was the case though.
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Old 07-11-2007, 11:21 AM
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Quote:
Originally Posted by KNanthrup
Yeah I verified TDC with a piston stop, so the mark is still correct. I almost thought that was the case though.
In addition to this is the need to find out how the cam is timed to the crank. You need a degree wheel attached to the crank. The first part of the drill looks a lot like setting up to find TDC which you've done. Then with number 1 set on compression firing for the known TDC position. You put a dial indicator with magnetic base on the exhaust so it's on the spring keeper and is absolutely parallel with the valve in the direction of its travel. You zero the indicator up from its maximum extension by a tenth of an inch, this is so you just don't over-run its length at full extension. With a friend pulling a wrench on the crank, the engine is rolled around in the normal direction of rotation, plugs out makes this a lot easier. You mark the degrees of rotation where the indicator starts to move and in separate columns also mark the degrees at .006 lift this is SAE standard, again at the lash if the cam is a solid tappet, and again at .050 where a lot of manufacturers measure for advertising purposes. You do the same as the valve closes.

Stop at full closure, if you're happy with the quality of your measures take the dial indicator and move it to the intake side and repeat all the measurements. Now compare these measurements to the cam card to see if it's installed correctly and certainly machined so the events are what they should be. What I'm looking for here is whether the cam is timed correctly to the crankshaft and piston movements. Errors in setting up the timing gears and chain or errors with the installation of timing bushings will be apparent from this test. A couple common errors are not having the cam and crank gears in proper alignment or putting a 4 degree advance bushing on a cam that's already ground with 4 degrees of advance, now you have 8 and that causes problems.

Timing the distributor, the fact that the engine wants more advance indicates the cam may be miss-timed. However, the problem of a flat bottom end can be as much a carburetion problem as a timing problem. I don't know what you mean by a stock cast piston! Are these flat tops, domes, dished if so what shape dish? Also not known is deck height and whether the heads have been milled. All this plays into compression ratio. If you have a flat top piston with a .025 nominal deck clearance and a.021 shim gasket, I compute your compression to be 11.2:1, which is too high for pump premium fuel.

Engines only like about 35 to 45 degrees of advance. The specific amount is primarily dictated by the shape of the combustion chamber, compression ratio, and cam timing. Modern fast burn chambers like timing around the 35-40 degrees, older SMOG heads like timing above 40 degrees reflective of their slower burn rate.

Timing is divided into static or initial and dynamic which is a function of vacuum, centrifugal, or is done by computer with digital systems. Also important is the rate at which the variable comes in to play. Another important aspect is that the vacuum needs to come out as the centrifugal comes in so that the two together do not over advance the engine.

The division between static and variable must be watched carefully. Unless the variable degrees are modified with bushings or another method to reduce total variable timing travel, simply twisting the distributor to increase the static timing will result in an over advanced ignition. In other words if the distributor is designed to work with a 10 degree static/initial setting and has 30 degrees of variable in it for a total of 40 degrees of lead. If you twist the distributor to 20 degrees of static timing, with no other changes, the 30 degrees of variable now is putting 50 degrees of timing advance into the engine. An over advanced engine will tend to surge at WOT if detonation doesn't take out a piston first.

Regardless of compression ratio as cam duration, intake closing point and overlap are increased dynamic pressure within the cylinder from idle to mid RPMs goes down. This effect is compensated for by increasing the static compression ratio so what is there works harder, it's a thermodynamics thing with lot of really ugly math. The other is to increase the ignition advance because less dense mixtures burn more slowly. But there is a balancing relationship between compression ratio and timing advance, so as compression comes up the advance doesn't need to as much as it would with less compression ratio. Again somewhere in there is a happy medium for your engine.

Most competition engines do away with the vacuum advance since with a really long duration cam there isn't enough manifold vacuum to run the it with sufficient sensitivity to conditions and it's just another failure point that's eliminated by it not being there. These engines are not concerned with running clean or efficient at low speeds. The usual solution is to run the static lead up to 20 degrees or so and cut the variable (centrifugal) total back to keep the full advance around 35-40 degrees all put in by 1500, 2000 rpm. This makes the engine a bit hard to start with so much static advance, but other wise has no drawbacks so long as you don't go slow. Drag engines running on "Fuel" (ethanol, methanol, nitro-methane, benzol) need a lot of advance on the bottom end as these fuels burn slowly compared to gasoline. So these distributors usually are locked with full advance all the time and the cars are push started rather than electric motor cranked as they are hard to start with so much advance being willing to push the piston back where it came from instead of around in the "forward" direction.

Street engines live in a much more complex environment and need variable timing schemes to cover the broad range of operating conditions satisfactorily.

So what you need to do is become convinced the cam and crank are timed properly to each other. Once that's done the distributor needs to be set up so that you know how much static and variable timing is in there and the rate at which the variable comes in. The XE274 is getting to the edge where there isn't enough manifold vacuum to operate vac advance properly. If you can't use vac advance, then the static lead will need to be increased and a like amount removed from the variable. The variable may have to come in sooner in the RPM band as well. These type changes don't work well in a heavy vehicle say 3000 pounds up with a high ratio rear end say 3.23 or less with a stick and 3.08 with an automatic.

The carb can be causing the problem you're seeing as well, a mixture ratio measuring device can be most useful in triming the carb. One of the big problems as cam timing grows is getting too much primary throttle opening to sustain idle, this exposes the mid range circuit which then makes it hard to get the right mixture at idle and creates a lean hole before main metering gets going. I don't remember the Holley you said your running, but those with 4 corner idle circuits are the simplest to set up since you can get to everything. But regular Holley's have an upside down screw on the secondaries that you can use to crack them open for more air then you can dial the primary opening down till you get the throttle blades out of the mid range slot. I wouldn't be surprised if this wasn't the root of your problem.

You also need to map engine vacuum against RPMs then take the distributor to somebody with a Sun machine that can take that data and coordinate the vacuum and centrifugal advance events to insure they work together. But 10 inches of idle vacuum isn't very much, unless you can get that up with carb adjustments, you might find you're better off with a bit more static and less, but sooner, centrifugal with no vacuum advance.

Bogie

Last edited by oldbogie; 07-11-2007 at 11:37 AM. Reason: I punch in lots of letters, some form words others do not
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Old 07-14-2007, 12:06 PM
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Oldbogie,

Thanks for the detailed information. Cam timing may be my problem. I did degree the cam when I installed it, however, at the time I did not know that Comp grinds them with 4* advance already. I put 3-4* advance in it when I installed it. So it must theoretically be at about 7-8* total. How much cam timing is excessive, where do you draw the line? It sounds like I may have a real issue if the cam wants timing farther in but the piston timing doesn't. I don't know if it would be safe to run it hard like that... which means I need to figure out the quickest and easiest way to get that cam back in place.
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Old 07-14-2007, 11:47 PM
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I don't think 4 more degrees is going to throw your intake closing point off enough to cause problems unless you were on the very edge. Do yourself a favor before tearing the front end down and retarding the cam-do a compression check to see where it is. 150-170 psi and you're Ok for pump gas, 180+ and it'll take some tuning and probably octane booster to make it work. Let us know what she's pumping, Bob
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Old 07-15-2007, 05:57 PM
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Quote:
Originally Posted by baddbob
I don't think 4 more degrees is going to throw your intake closing point off enough to cause problems unless you were on the very edge. Do yourself a favor before tearing the front end down and retarding the cam-do a compression check to see where it is. 150-170 psi and you're Ok for pump gas, 180+ and it'll take some tuning and probably octane booster to make it work. Let us know what she's pumping, Bob
Thanks a lot bob, i will be sure to do that and report back. I just wish I knew if it was pinging... I don't mind running 45* total timing if it's not detonating... but i hesitate to think that piston timing would be safe.
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Old 07-15-2007, 07:16 PM
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What about the mechanical advance? Say you get about 18 at idle with the vac disconnected. Rev the engine up, vac still unhooked, what have ya got now. Just for an example. 20 degress initial, rev it up 20 more for a total of 40 with the mech, now hook up the vac and see if it goes to about 50-60 degrees total advance. Sounds like the mechanical weights in the dizzy may be stuck.
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Old 07-16-2007, 11:25 AM
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Quote:
Originally Posted by KNanthrup
Oldbogie,

Thanks for the detailed information. Cam timing may be my problem. I did degree the cam when I installed it, however, at the time I did not know that Comp grinds them with 4* advance already. I put 3-4* advance in it when I installed it. So it must theoretically be at about 7-8* total. How much cam timing is excessive, where do you draw the line? It sounds like I may have a real issue if the cam wants timing farther in but the piston timing doesn't. I don't know if it would be safe to run it hard like that... which means I need to figure out the quickest and easiest way to get that cam back in place.
Advancing the cam is a way to recover bottom end and mid range torque. This is an effective trick where too much cam is used in an engine that can't be run toward its power peak RPMs. Manufacturers recognize that most buyers over cam their engine so many grind advance into hot street sticks to reduce complaints about a weak bottom to mid power band.

In the real world of race engines it may be necessary to advance or retard the cam to tune the engine to specific track and weather conditions. This is usually not more than 4 degrees either side of a straight up setting. Competition engines usually include an adjustable cam gear rather than bushings. There is a two piece timing case cover where the cam gear and its adjustment can be quickly accessed for between round tweaking.

In your case, putting a 4 degree bushing in addition to a cam with 4 degrees advance built in is 8 degrees advance, by any standard this is too much advance and will make the engine ping prone regardless of distributor settings.

I'd go back to a straight up configuration using only Comp's built in 4 degrees. If the engine still pings than take it back with a 4 degree retard bushing to straight-up timing.

Bogie
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Old 07-16-2007, 03:44 PM
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was that carb used on a circle track?

most times they remove the power valve and jet up 8 or 10 jet sizes. Makes for a bad running street car.
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