|08-12-2014 10:31 AM|
Another common event is the natural looseness in the timing chain and how it reacts to high RPM excitement. Then there are a number of clearances such as the clearance between the cam's distributor gear and the distributor's gear. The thrust clearance of the cam, typically the flat tappet chevy is unrestrained in forward thrust except by the distributor gear and there is a clearance of the distributor shaft and the housing allowing it some freedom of movement. Even roller cams have a little thrust freedom that works on the distributor gear mesh. All cams have a lot of rotational shake from the slowing when valves are opened against their springs to accelerations on the closing side. Nothing in this system is truly stable. All these clearances can and do gang up to change the timing. This is the reason why competition engines use the crankshaft as the timing reference to trigger the spark rather than a distributor at the end of a group of gears, chains and shafts. There are also other gadgets sold that apply tension to the timing chain for a more consistent holding of set position by not allowing any slack in the chain. Not as good as a crank trigger for ignition timing but very helpful in keeping the camshaft to crankshaft alignment steady. A thrust button on a flat tappet cam is also helpful to limit its fore and aft travel so that doesn't disturb the distributor gear mesh with the cam. Gear drives can be useful in eliminating the gear chain issues but you buy something of the quality and design of the Shaver-Wesmar to really get the needed effects the popular inexpensive drives don't solve this problem and make a lot of noise not doing it.
|08-11-2014 07:47 PM|
. Sounds to me like the carburetor was too small and high RPMs vacuum was pulling up the vacuum advance a bit...
|07-09-2013 10:41 PM|
not to be a smart ***** or anything... but did he have the light on the right ignition wire??
I ask this because I went down this road once... when my ignition wires were a rats nest, I missed the #1 wire and got the #2 wire instead.. couldn't figure out why my timing was WAYYY off, and when it got it where i thought it should be, the engine wouldn't run....
spent better part of an hour banging my head against the wall on that one..
|10-28-2012 10:32 PM|
Today, I use the MSD E-Curve. Worth every penny.
|10-28-2012 05:30 PM|
|Old Fool||back in the day a distributor was put on a distributor machine and had its advance curve set. no guessing, no oh ****s, you knew what you had. That was before they became dizzy's.|
|04-09-2012 04:14 PM|
Check out these bowls out.of course you have look at them upside down
|04-08-2012 10:25 AM|
|04-07-2012 12:02 PM|
If you want to prove if it is mechanical or electronic it is simple and has been stated in this thread several time.
Disconnect the vacuum advance
Lock the timing at something reasonable and see if it still jumps.
If you lock it and it still moves then it is not the advance weights or related parts. I would then look at the module or pickup. Either could cause the issue. A bad pickup may not be up to the rpm task and could be collapsing the field a little to soon at higher rpm or not building the field long enough. I would also check the air gap on the pickup it becomes more critical at higher rpm. The module could also be an issue but the pickup would be my first guess if it is electronic.
I think if the timing chain was stretching you would be loosing timing not adding and it would be inconsistant.
|04-07-2012 11:43 AM|
Is it possible that voltage rise time is the real issue.What I mean is hei pick up voltage will rise with r.p.m. and alter timing.
|04-07-2012 07:43 AM|
|cool rockin daddy||Can somebody please translate the above gibberish?|
|04-07-2012 01:18 AM|
no its just if the cam was dialed you have advanced the engine.so the ignition is just tuning the spark timing.i was allways taught the distributor is the moved bout an inch to a inch and a half prior to the spark of the .so if the #1 spark connection on the distributor is moved to spark bout an inch or so of travel before it lines with its own cable headed out to the #1 spark plug.so the spark is fired then by the time it shoots out to the spark plug ,it has occured ,if it went off at the same time of the distributor #1 being lined up to spark plug #1 it would be to late and the engine wouldnt run.i didnt understand the timing in that engine changing as the rpm raised.i allways buy the best i can afford in equipment and follow the instructions and research on you tube and internet and never once ran into that problem.i helped build a 65 mustang we built and now did a chevy 350 vortec 4 bolt main ,1 piece rear seal.just a 350 sbc but never bore to 60 for heat problems,i try to stay at .030 maybe .040 but even that i dont know cause you alter one part to much and then it starts throwing you Off in other areas.so i just port and polish the heads maybe 1mm off the exhaust chambers to wake it up,dont want my headers overlapping a 1.5 or 1 5/8 primariey.i really get technical bout the bowls ill just buy a pair of junk valves and port like any of us would go ahead and do.want he bowls like mirrors,mirrors i tell ya.lol .hope that cleared it up a little.sorry for confusing the masses.lol.if it makes sense and you wanns motor chat with ,hit me back anytime.bye dude
|04-05-2012 11:14 PM|
|4 Jaw Chuck||
Another explanation is called spark scatter, high cylinder pressure causes a higher load on the ignition system which can cause the coil to delay or jump across the plug gap only after the mixture self detonates.
High compression ratios combined with weak stock style ignition systems are a deadly combination, my bet is this phenomena you are seeing only occurs under load above a certain rpm...if so you are getting a clue of your ignition system weakness.
A good test is to physically lock the timing at a high initial like 18 degrees and repeat the test on the dyno, if your still seeing it assume your ignition system isn't up to the task and needs upgrading.
|04-05-2012 08:49 PM|
|cobalt327||Huh????? I read that twice and still have no idea on 90% of it.|
|04-05-2012 08:09 PM|
not the answer but feedback
i usually advance the cam then fine it with the distributor ,minor.it would make you lower the timing to hit 34 at 6000 rpm.that would hurt you low.but maybe youll have to retrace youre steps to the point of failure and adjust from there.i run a HEI with sbc vortec 350.dot to dot on the cam on comp cam mother thumper is fine 2-4 degree advance there or go alittle more.so i dont really want to have to do much more than an 1" to 1.5"advance manually on the dis.like the part bout mech advance only affecting low drive to an idle imma keep that in mind for when i next tune.love carbed out monsters and its true fine tuning and constant adjust comes with the territory.good luck with it brother.out
|04-05-2012 07:19 PM|
So no load means they set the timing running around the trackk? No seriously, this can be done on a dyno and I'm thinking it probably is although I haven't been near a dyno in just about as many years. I stick by the vacuum method and have used it on many hot rodded engines. Then I have used a light with an advance dial on it to "fine tune" timing. I do not trust all balancers, some yes, but not all. Many times I have found the timing ring on a balancer slipped or plain just improperly installed. Use the vacuum gauge first, check the timing mark to see where the balancer is and go from there.
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