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  #1  
Old 12-01-2013, 11:17 AM
davidmij davidmij is offline
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Default trouble with timing?

Hey all, I have been tinkering with tuning my newly built motor and have a question or two. First off, for those who don't know me it's a 390 with a 428 crank and CJ cam, edelbrock RPM performer intake and edelbrock 750 manual choke carb. Standard transmission.

The builder said to set the initial timing to 10-12 degrees. I did this, adjusted the idle mixture screws, and then went to set the timing. (Dizzy vacuum disconnected and line plugged) If I turn the distributor clockwise the RPM goes up from 650 to 1000. However this causes the timing advance to go up to 30 plus degrees. I then back off the idle adjusting screw to put it back round 650, closer to where it should. But the timing is still pretty advanced I think around 20 degrees - I'll check again today to be sure.

If I'm looking at the motor from the drivers side the dizzy vacuum module is pointing to about 10 o'clock, I started at about 9 oclock.

I also tried setting it with a vacuum gauge. It more or less did the same thing. HOWEVER, as I pull the vacuum gauge out of the brake line the idle goes up. I thought it was supposed to drop or even die? When I plug it back into the brake booster the idle drops to the same as when it was plugged into the vacuum gauge. Does this make sense, am I missing something? I have no real experience setting timing other than my old motor so I'm not real sure what I should be seeing.

thx, Dave J
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  #2  
Old 12-01-2013, 12:14 PM
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jopizz jopizz is offline
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To correctly set the timing you need to have the idle below 550. Anything over that and you are introducing mechanical advance and your reading will be off. The ideal distributor setting is at 9:00 with the timing set to around 6 degrees. That should give you plenty of room to advance the timing without hitting anything. Yes, the idle will go up when you remove the vacuum hose because you are allowing more air to be sucked in. You are probably running too rich. It will upset the fuel/air mixture especially at low rpm.

John
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  #3  
Old 12-01-2013, 12:14 PM
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Let's start with your rich mixture. I didn't hear anything about black smoke coming out your exhaust but I would not be surprised if that is what you have.

With a normal idle mixture, idle drops and the engine runs like crap when the booster's vacuum line is pulled off because two things happened: The vacuum advance pancake lets go which makes your timing retard. More air is introduced into the mixture.

Plug all your vacuum leaks. Lean your mixture out more at idle. Yours is way too rich. Those adjustment screws are designed to work at idle speeds (when the butterflies are closed). If the passages are blocked or clogged, the idle adjustment screws will not work.

Ignition timing Total Advance comes from three adjustments:
  • The centrifugal throws and springs under your points plate (this is your distributor 'curve'),
  • The setting you leave the distributor at when turning it,
  • The vacuum advance pancake (that pulls on the points plate)

When timing the engine we normally pull the vacuum hose off and plug it (so the engine doesn't run too lean). This takes the pancake out of the equation.

Ford FE distributors should have a total advance of 36-38 and peak around 2,500-RPM. Total Advance includes the initial advance. For example, if we start with 6BTDC at idle with the vacuum hose off, and slowly increment the speed by 500-RPM, the timing marks should steadily advance to ~36 at 2,500-RPM and hold there as we increase RPMs.

Vacuum advance is totally separate as it performs a completely different function. At steady idle, we should expect to see some vacuum advance. As the throttle opens, vacuum goes down. When climbing hills there is very little vacuum. So vacuum advance is designed for steady throttle conditions and it adds to Total Advance.

Too much advance will cause pinging and knocking. Starting will labor because the starter motor is fighting against pistons that fired too soon before TDC.

Hope this helps. - Dave
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Old 12-01-2013, 02:05 PM
davidmij davidmij is offline
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Thx Gents, that does help a lot.

Another thing that will tell us something is that one time after I had adjusted it with the vacuum gauge, I accidentally took it for a spin without the dizzy vacuum connected. It actually ran no different even at 2800 RPM. As a matter of fact it seemed to be less "poppy". I have short little glass packs and as the rpms go up to around 2200 it gets a louder, poppy sound to it.

It's kind of hard to get it to idle as low as 550 because of the cam. I think I will try it as low as possible and see how it turns out.

More to come, thx, Dave J
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Old 12-01-2013, 02:40 PM
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Wow, did you see that? John and I posted in exactly the same minute.

Oh yeah, your cam (with lots of duration). I forgot about that. Hot cams do a good job of fouling plugs unless they are run hard. It's just not practical to run them on the streets, but they're great at the track.

Call your cam company and ask them for specific tuning instructions. The advice I have is more for 'street' cam setups. Trying to tune a 'loping' cam turns into a dance that usually ends up running high rpms at idle because your engine produces very little HP down low. - Dave
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Old 12-01-2013, 03:02 PM
davidmij davidmij is offline
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thx Dave, that's why I'm going with what the builder told me about idle timing being around 10-12 degrees, and it should idle fine at stop lights around 650-700 RPM. He knows FE's and he knows the cam specifically.

With that, should I still set the timing around 550-600? That's what I just tried, I started it, twisted the dizzy to the best, smoothest, highest RPM's. Then I adjusted the throttle screw back down to just over 550 RPM and checked the timing light. It was way up at 40 degrees! Does this mean I am most likely off by one tooth on the dizzy?

thx, Dave J
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Old 12-01-2013, 04:13 PM
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I don't think that 40 degrees of timing is even possible at 550 rpm. I'm wondering if your damper may have slipped and your timing marks are off.

John
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Old 12-01-2013, 04:21 PM
davidmij davidmij is offline
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I thought of tht too Joe, so I pulled the number one plug, stuffed a rag in there and bumped the starter till TDC. Then I looked at the timing and found it to be right on.

Can having the dizzy one tooth off cause that?
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Old 12-01-2013, 04:47 PM
cdhowell cdhowell is offline
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Your "initial timing" means without the vacuum hooked up to the distributor. Turn your idle back up a little and retard the timing to 10* at 650 rpm. Then hook up your "ported" vacuum to the distributor. It should not change much if at all at this point. Now tune your Carb with a vacuum guage. Now check total timing at 2800 rpm.
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Old 12-01-2013, 05:53 PM
davidmij davidmij is offline
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Sorry John, I typed Joe earlier.

So I started over and managed to set the timing at 580ish RPM with the dizzy vacuum unplugged and the line plugged with a golf tee as it should be. I finally got it to stay running (just barely) and set the timing right at 11ish degrees. Hooked the dizzy vacuum line back up and it went up to about 750 RPM. Thought I would check it at that point (just for reference) and the timing light at that point was around 25 degrees. I adjusted the throttle screw down to idle at about 710 RPM.

Charlie, I don't know how to tune the carb with a vacuum gauge. When I initially set up the carb I used the metering rods and jets that Edelbrock recommended for our altitude. Oh wait I just snapped, you mean for me to adjust the idler screws with the vacuum gauge, is that correct?

I checked it at 2800ish RPM's with the vacuum hooked up for normal use and it reads about 50 degrees with the timing light. Is that normal? I just don't know what to expect.

So it drives pretty well when I hit the gas - no misses etc.

anyway, thx, Dave J
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