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Discussion Starter #1
Like everyone else, I'm trying to achieve that "perfect" timing setting that will allow my engine to run it's best. Since the whole thing is built out of aftermarket parts, any factory tuning specs are out the window. Without going into all of my engine's particulars, it there a systematic way to determine the total timing that is appropriate for any engine. I realize that almost any of the engines components (cam, heads etc.) can determine the total timing number, but how do you find it? I could just jack the distributor up until I get detonation, then back it down some, but it seems that this wouldn't really indicate the optimum advance. You guys are very helpful and I really enjoy reading your responses to my questions. Thanks!
 

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OK the best way to do it is to put a timing light on it, record the idle time stock is around 6.
Then take it to about 3500 rpms and record the time again should be not more than 35 degrees total advance.
I just had this issue do to a bigger cam I had to change the advance curve, mine would run up to 60 degrees at 300 rpm
I noted that mine ran best at idle around 17 degrees advance and if I took it to 6 degrees like the book it idled for crap.
You see the problem is that if you changed the cam your advance curve is wrong and your getting to much advance.

The best thing to do is get timing tape from the auto parts store, set the timing so the engine so it runs the fastest at an idle and starts without killing the starter.
Next check the timing at 3500rpm, it should be no more than 35 degrees total advnce, if it is you need to put stronger counter weight springs in the distributor to bring down the advance. any auto pro shop sells them, they are cheap.

Be for warned that id your engine is running with high advance is can burn the valves out and blow pistons.
Good luck

Chuck
 

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borrowing a timming light with a varible advance can do wonders--vacuum @ idle is a great place to start---my old fe injected motor liked 42 deg total--so who knows---if it doesnt ping and will still start w/o bogging,try some more advance
 

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Question; Is the vacuum advance @3500 RPM in neutral really meaningful? Seems like you'd get a higher vacuum in neutral than under load. Until I can afford something better, I'm running my 289 with mechanical advance only, 12 deg advanced at idle & about 40 deg at 3500.
 

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Discussion Starter #6
Right now, my initial timing is 15deg btdc. at my total advance is 38deg btdc. All in at 3000rpm. Not really sure how much advance the vacuum can is pulling. I've adjusted it to where I no longer get pinging under load. I don't time the engine with the vacuum advance hooked up.
 

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MAX vac at idle is good depending on how well it will do under load.....Once you think you have it - drive up a steep hill and see what it acts like. Shouldn't take much tweeking from there unless your advance needs changing. It took me a fair amount of time to get the correct spring combo's in my dizzy where I matched it good. The best performing spot is also the hardest starting spot for me so I backed it off a bit to make starting easier.
 

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Discussion Starter #8
MAX vac at idle is good depending on how well it will do under load.....Once you think you have it - drive up a steep hill and see what it acts like. Shouldn't take much tweeking from there unless your advance needs changing. It took me a fair amount of time to get the correct spring combo's in my dizzy where I matched it good. The best performing spot is also the hardest starting spot for me so I backed it off a bit to make starting easier.
I tried that once. Max vacuum with my engine is about 25btdc. Even if you back it down a pound or two, It'll knock so bad that you can't drive it.
 

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The best thing to do is get timing tape from the auto parts store, set the timing so the engine so it runs the fastest at an idle and starts without killing the starter.
Next check the timing at 3500rpm, it should be no more than 35 degrees total advnce, if it is you need to put stronger counter weight springs in the distributor to bring down the advance. any auto pro shop sells them, they are cheap.

Be for warned that id your engine is running with high advance is can burn the valves out and blow pistons.
Good luck

Chuck
Springs DO NOT determine the total advance - they determine the rate of mechanical advance. The slot in the advance plate controls the amount of mechanical advance. Here is a pic of a distributor with a 10* limiting slot. This number is doubled to determine the mechanical advance allowed.



The advance plate can be changed for another with a different total advance. 8* 10*, 12*, 14* and 16* plates are available.

DuraSpark distributors are slightly different in that they have two different slot 'limits' per plate so you can pick one or the other. Again, other plates are available with different limits.

Vacuum advance is controlled by the ported vacuum (venturi vacuum, not manifold vacuum!) This helps with part-throttle power. At idle (closed throttle blades), manifold vacuum is high and ported vacuum is low since there is very little air going through the carb. At wide-open throttle, manifold vacuum is near zero. There are aftermarket vacuum advance cans that are adjustable for both rate and total vacuum advance allowed.

Total advance needed for best power is NOT set in stone at 35*, especially when the engine varies from stock components! Dynamic compression ratio, rod angle (and the associated piston dwell time at TDC), valve timing, type/grade fuel used all take part in the process.
 

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This thread is very interesting to me because of how my 351C has "acted" from the first start. It is a true 10:1 motor with 302C heads (and .080" custom head gaskets to get the compression down to that) with a Comp XE262H cam, Air Gap manifold, 4777 Holley double pumper mechanical secondary carb, Duraspark with spring kit and Crane? adjustable vacuum can triggering MSD 6AL, full length headers and 2 1/4" stainless exhaust with stainless Magnaflow mufflers. Manual trans. The problem was/is a couple of things: 1. Setting the timing by ear to what "sounds right" ends up being a lot of (too much) advance (and pinging under load as a result), something like 15-17 BTDC initial (balancer checked and good) and as the thread starter has experienced, 6 BTDC runs totally horrible. 2. There is what I would call a stumble right off idle, wondering if the Air Gap is causing this (1500 up stated vs/idle up on Performer) but hoping tuning will solve the problem. I would hate to have to resign myself to going back to the (less sexy) Performer (was not called Performer back in the day, just Edelbrock 3750 - what I still have). I could run a carb spacer with the 3650 though, no room with the Air Gap.

For timing I eventually settled on no vacuum advance and 12 BTDC initial timing (about 35 degrees total advance), but am unhappy with the off idle stumble. The other day I put vacuum advance back and the stumble seems slightly improved but still there. Also I know have too much total advance in this configuration and it will ping under some conditions.

Perhaps take out some mechanical advance and add some vacuum keeping the total to around 35?

I need to get to the bottom of this, pretty sure there is a lot to be had in tuning, but am somewhat confused as to what to do (but trial and error and your guys help/suggestions should get me there!).

Hope this is not a hi-jack but a similar issue and so on topic.

Thanks,

Bob
 

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Bob, with that cam and compression, your timing seems about right. You could bump the initial timing up a couple of degrees, but no more than 16 initial with that setup. As far as the stumble is concerned, it might not be timing related. Have you tried changing the squirters or pump cams?

In my experience, setting the timing by ear or vacuum at idle will usually result in way too much initial timing. Always use a timing light.
 

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Discussion Starter #13
Bob, with that cam and compression, your timing seems about right. You could bump the initial timing up a couple of degrees, but no more than 16 initial with that setup. As far as the stumble is concerned, it might not be timing related. Have you tried changing the squirters or pump cams?

In my experience, setting the timing by ear or vacuum at idle will usually result in way too much initial timing. Always use a timing light.
OK Adam, in my 390 my cam is a Comp 275DEH. Compression is 9.5:1. Like I said, I'm starting my initial timing at 15. Can you tell me what you feel my initial timing range might be?
 

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Within about 4 degrees, I'd just guessing. 15 @ idle, & 38 total seems pretty good to me. I think it could take a couple more degrees if you wanted to experiment, but it sounds like you bumped it up until it started pinging which means you are right there. If your engine doesn't ping and you aren't straining the starter, you're close enough. Take it to the track on a test n' tune day and experiment.
 

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Bob, with that cam and compression, your timing seems about right. You could bump the initial timing up a couple of degrees, but no more than 16 initial with that setup. As far as the stumble is concerned, it might not be timing related. Have you tried changing the squirters or pump cams?

In my experience, setting the timing by ear or vacuum at idle will usually result in way too much initial timing. Always use a timing light.
I did adjust the screw/spring on the primary side accelerator pump on the off chance that the squirt was not happening immediately upon throttle movement, even it appeared to be doing so. So no change by the screw/spring adjustment anyway.

Otherwise it is a brand new carb out of the box, just with an electric choke fitted.
 

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I did adjust the screw/spring on the primary side accelerator pump on the off chance that the squirt was not happening immediately upon throttle movement, even it appeared to be doing so. So no change by the screw/spring adjustment anyway.

Otherwise it is a brand new carb out of the box, just with an electric choke fitted.
The accelerator cam pump profile might not be right for your application.



Here's a bigger diagram PDF

My carb came with a pink cam. I swapped in a blue one and the stumble went away. However, the blue one also gave a puff of black smoke when I took off so I dropped down to a green one. It is perfect now. I still have a pink on the secondaries. Since you have a 4150, you can also swap your squirters (aka accelerator pump discharge nozzles) for bigger or smaller ones. On my 4010, they are not replaceable. That's why I went with a more aggressive accelerator pump cam.

 

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How much vac do you get at idle >13 i would imagine? That cam has plenty of intake duration -- If you back it off, you get major knock/ping....hmmm I imagine youv'e beat the vac leak issue to death by now. Maybe your getting to much advance to soon then...? What happens if you leave it off and cap it?


Bob - your off idle thing sounds more like carb/jetting/secondary not dialed in....vac secondaries are easier to set up, but that carb has plenty -o- adjustability. Try figguring if it's starving or flooding under the gun first. Pull a plug quick after you romp it see if it's wet/black. If not, maybe you need a bigger squirter?....LOL (just gonna leave it alone right there!)
 

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Thinking starving, but how to decide on trying: cam, nozzle or (new third choice) 50cc pump? I seem to recall somewhere in my distant past that I may have run a 50cc pump, maybe even for the same sort of problem, but my CRS has made this memory quite fuzzy (but at least I still recall a little of it!).
 

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Start with cams. A cam kit is cheap (http://www.summitracing.com/parts/HLY-20-12). Since you will need to experiment with a few sizes, nozzles will be more expensive ($20 per pair for tube type and $12 per pair for straight type). Also, if the cam is too slow, putting in a bigger squirter won't help.
 

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