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Old 07-11-2007, 12:21 PM
oldbogie oldbogie is online now
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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.


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