With or without vacuum advance hooked up?
Erratic Timing
- Thread starter ScamperTom
- Start date
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ScamperTom
Supposed to be working
With vac hooked up to manifold. And by “load” I just mean “driving “. Big difference I guess
Got it. Thanks.
If the timing looks anything like this, you can see how much timing could be with vacuum advance added in your normal driving range.
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Definately leaning toward welding the inside of the slots.
Using the '72 318 timing curve as the example.
Taking 4 distributor degrees out of the slots and setting the timing at 8* would get exact same curve but with more initial.
If you want the advance to start moving before 950 rpm, we know how to get that, right?
Take a little tension off the primary spring by turning the perch.
Worst case, bend the spring hooks open just a bit.
Using the '72 318 timing curve as the example.
Taking 4 distributor degrees out of the slots and setting the timing at 8* would get exact same curve but with more initial.
If you want the advance to start moving before 950 rpm, we know how to get that, right?
Take a little tension off the primary spring by turning the perch.
Worst case, bend the spring hooks open just a bit.
ScamperTom
Supposed to be working
Well the sight plugs finally came in and I got the chance to run a few tests, which now confuse me more than ever. I must be doing something wrong.
First, my balancer had 3 marks on it, and I thought I should double check TDC to make sure I was using the right one. Sure enough, my previous measurements were based off the wrong one, which appears to be maybe 20* advanced. The factory groove is correct at 0* TDC. I reset the initial at 12* and got it to idle down to 600 rpm in gear.
Bonehead mistake. I should've caught my TDC mixup right at the beginning so I didn't waste your time, but I really appreciate all your help with the timing diagrams, Mattax!
That verified, I had someone run the motor up to 4000 and noted the timing, which I marked in red.
I must be doing something very wrong to be getting a curve like this. The Distributor slots allow for 30*, which should put me at 42* at some point. What is with that spike at 2500 rpm? The engine really seems to want more initial timing, like 15*+. I'm really holding it back at 8*-10* I'll try hooking the vac advance can up to manifold vacuum and seeing how that changes things. I'm gonna go back out and check these numbers again, and I still have to check more precisely how the advance moves from 600-1000 rpm.
First, my balancer had 3 marks on it, and I thought I should double check TDC to make sure I was using the right one. Sure enough, my previous measurements were based off the wrong one, which appears to be maybe 20* advanced. The factory groove is correct at 0* TDC. I reset the initial at 12* and got it to idle down to 600 rpm in gear.
Bonehead mistake. I should've caught my TDC mixup right at the beginning so I didn't waste your time, but I really appreciate all your help with the timing diagrams, Mattax!
That verified, I had someone run the motor up to 4000 and noted the timing, which I marked in red.
I must be doing something very wrong to be getting a curve like this. The Distributor slots allow for 30*, which should put me at 42* at some point. What is with that spike at 2500 rpm? The engine really seems to want more initial timing, like 15*+. I'm really holding it back at 8*-10* I'll try hooking the vac advance can up to manifold vacuum and seeing how that changes things. I'm gonna go back out and check these numbers again, and I still have to check more precisely how the advance moves from 600-1000 rpm.
This is when you start thinking how nice it would be to have a distributor machine!
If it seems to like more intial, that's fine. We don't know the details of the cam or compression so we are just guessing. I was a little surprised it had kickback at 10*, now we know why. Glad you cleared it up!
I don't have an answer to the curve shape a 2000 - 2500.
Can you get the second spring to engage earlier by turning the perch?
If it seems to like more intial, that's fine. We don't know the details of the cam or compression so we are just guessing. I was a little surprised it had kickback at 10*, now we know why. Glad you cleared it up!
I don't have an answer to the curve shape a 2000 - 2500.
Can you get the second spring to engage earlier by turning the perch?
ScamperTom
Supposed to be working
Yeah no kidding right about now I'm thinking it would be nice to have a computer handling this! I haven't messed with the spring perches yet; I want to be sure of what I have first.
OK I checked the timing curve again with a more reliable person watching the tach and I got the line in orange – much more reasonable. Some of the points could be off by a degree or so, but I'm confident I'm very close this time.
I still can't get the idle to drop less than 100/150 rpm going into gear. And if I quickly jab the throttle, after revving up the engine drops another 100 rpm before returning to stable idle. If I brake when doing that it stalls the motor. It just lacks power at low idle. I've tried all kinds of timing: 8*-18* initial with and without vacuum advance, but it doesn't seem to help. I cracked the secondaries because the cam needed more air (not enough to get into the transfer slots); maybe this is causing a lean condition?
OK I checked the timing curve again with a more reliable person watching the tach and I got the line in orange – much more reasonable. Some of the points could be off by a degree or so, but I'm confident I'm very close this time.
I still can't get the idle to drop less than 100/150 rpm going into gear. And if I quickly jab the throttle, after revving up the engine drops another 100 rpm before returning to stable idle. If I brake when doing that it stalls the motor. It just lacks power at low idle. I've tried all kinds of timing: 8*-18* initial with and without vacuum advance, but it doesn't seem to help. I cracked the secondaries because the cam needed more air (not enough to get into the transfer slots); maybe this is causing a lean condition?
Its hard to be sure how accurate the tach and timing readings are. Plus/minus a degree is certainly to be expected.
Since this setup allows plenty of intial timing, use ported vac advance. Using manifold vac advance can be useful when its difficult to achieve enough initial mechanically and when small changes in vacuum caused by rpm or load change aren't going to effect the vacuum advance. I leave the vac supply hose plugged until the mechanical and initial are pretty well sorted out.
Based on the rpm drop and the need to open the secondaries a crack, I think you should go for more initial timing than the 12*. 16*? and then retune the fuel mix. Its going to be iterative testing. 16 seems like the next step since you've tried it at 12 and 20 and you know 22 is too much and 8 is too little.
Computer wouldn't be a lot of help. LOL. Vacuum or MAP is probably our best indicator of power and efficiency. Wideband O2 interpretations get fooled in situations like this, and they can't tell you what the power is. CO is more reliable indicator of fuel/air mixture at idle, but again doesn't tell us what the power is. We know from industry and military testing that idle AFR mixtures somewhere in the 13.5:1 range are the most efficient, but every engine is different and needs to be worked out emperically. Often more information is not all that helpful. If we turn a mixture screw 1/8 and observe the effect on the key objective, better vac or rpm in gear, what the WBO2 and CO meter indicates is not all the helpful.
ScamperTom
Supposed to be working
That's what I'm thinking. Don't really need the extra timing and vacuum is all over the place at idle anyway. I'm gonna try bumping up the timing and closing the secondaries a little... Right now 16* and 18* idle well in park, but drop a lot in gear. In any case it's time to double check my T-ports.
Computer wouldn't be a lot of help. LOL. Vacuum or MAP is probably our best indicator of power and efficiency. Wideband O2 interpretations get fooled in situations like this, and they can't tell you what the power is. CO is more reliable indicator of fuel/air mixture at idle, but again doesn't tell us what the power is. We know from industry and military testing that idle AFR mixtures somewhere in the 13.5:1 range are the most efficient, but every engine is different and needs to be worked out emperically. Often more information is not all that helpful. If we turn a mixture screw 1/8 and observe the effect on the key objective, better vac or rpm in gear, what the WBO2 and CO meter indicates is not all the helpful.
TBH I wouldn't know how to work a diagnostics computer anyway. But you're totally right: I have to experiment on this engine, at the very least to figure out how engines respond in practice to changing parameters. Thanks again for the help! I think I'm getting there.
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