Hi all i have a few questions regarding the 390 i built and put in my 1967 f-100. first of i will tell you a bit about the engine itself. the 390 block was bored over .060". bolted on are edelbrock performer RPM aluminum heads, edelbrock performer RPM intake manifold and 750 cfm edelbrock performer carb. ignition system is a full MSD system. MSD ignition unit, blaster coil, pro billet distributor and MSD super conductor 8.5mm plug wires. Now onto the questions. First, with vacuum hooked up what should the timing be at idle? with the vacuum unhooked and plugged? This is my first time fully tuning an engine and im not sure about vacuum advance and total timing etc. I am finding the engine runs great no hesitation under load and no pinging with the timing at around 45* BTDC (checked at idle, no load, vacuum hooked up) next question is, is the carb too much for the engine? i just pulled the plugs and all of them were black and carboned up like the engine was running too rich. and i am getting puffs of black smoke out the exhaust when quickly reving the engine up. any ideas? Any comments, thoughts etc are appreciated, thanks.

 

If you dist. vacuum can is hooked up to the ported vacuum port as it is suppose to be you will have "Zero" vacuum at idle and the timing will be the same whether connected or not. If you are hooked to manifold vacuum then it will change. How much depends on the vacuum unit and it's calibration.

For a street motor, never use manifold vacuum unless you screwed up and are using to much cam and need the addition idle timing to make the engine idle.

 

That sounds way off to me. Timing at idle, assuming semi stock cam should be 8-14º imo. Bear is correct as far as I'm concerned. The timing sounds like it's being pulled by vac/centrifugal weights due to high rpm. Or that is a typo. ??

 

You skipped the most important number in your specs, the compression ratio. A high compression ratio will use less timing. In the long run though any timing advice given even with all the info is a guess and the reactions of your engine will tell you exactly what it needs more then even the most experianced tuner.

There are 3 parts to tuning timing and should be done in this order before exact adjustment of the carb. What is best is really just determend by what works best with your engine while avoiding a ping, so pay attention and your engine will tell you what it likes. Ohh and just in case, ping is a knock, sounds like a deisel engine.

-Initial timing - this is the lowest your timing will ever be, is adjusted by turning the distributor, and is the timing at idle with the vacuum unplugged. Most engines idle better and have faster throttle responce with this rather high at 14-19deg. But how high you can go is often limited by having to start the engine. Too high of timing will advance the spark to the point where the engine will kick back or stop when sparked while attepting to start, more so when hot. You need to find the happy medium.

- Total timing - this is the high rpm timing WITHOUT the vacuum advance, representing a full throttle romp. This needs to be set for both the timing and at what rpm it advances to it's total, and is adjusted within the distributor, your MSD dizzy should have come with instructions and parts for setting this, generally between 34-38 in by about 2800 is best.

- Vacuum advance - This is the most complicated, Bear45/70 brought up the difference in tuning with the vacuum hooked up to manifold vacuum or ported. How the vacuum advance is tuned depends on which way you set it up. He and I STRONGLY disagree on what is best and have had many arguements. I believe manifold is best, however for a first time tuner ported is easier to start with.

Ported is using the vacuum port on the passenger side front of your edelbrock carb. It uses the throttle butterflies as a valve to turn on/off a vacuum signal that is more responsive to throttle position then actual engine load, load being better, but this easier. It's easier as it creates a timing advance with throttle movement with less risk of it creating too much timing at part throttle causeing a ping.

Manifold uses the port on the drivers side front of your edelbrock and gives the dizzy a way to actually adjust timing to the load on the engine because as load decreases more timing is neeed, vacuum increases increasing the timing. This is adjusted for tune with an adjustable vacuum can which your MSD dizzy should have come with. Should be in the instructions or you can find out by putting a 5/32 allen key in the hose nipple, if adjustable there will be an adjuster that key fits in. Either way vacuum advance should be adjusted last AFTER driving with it disconnected to ensure there is no ping before hooking it up so if there is after hooked up you know what to adjust.

If set ported the timing should quickly jump up when the throttle is touched. If set vacuum the timing will already be up there idleing with it hooked up, like it is now at 45, though that is a little high.

The black plugs and smoke are indicators that it's too rich, though likely not in the same way. Start with the idle mixture screws, likely the cause of the balck plugs, with a vacuum gauge or tack turn both out until you find the highest reading, then in until it starts to drop, and out half a turn. This should get you right about where you need to be until you can really drive it and make minor adjustments. The smoke revving is likely too much accelerator pump, this is adjusted on the lever on the top front drivers side of the carb by moving the rod. Moving it out a hole will lessen the pump shot.

No the carb isn't too big, but it's on the large side, just means you need to pay more attention to your tune to get the best performance.

Good luck.

 

Before we all get twisted up into a knot, lets start at the beginning.

45 degrees BTDC is just wrong. It can't be. It's either a typo, or your dampener ring slipped.

VERIFY the timing marks by getting cylinder #1 at TDC and make sure the dampener marks for TDC are dead on.

Or, post again and fix that typo

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The black soot coming out the tail pipe when you hit the gas is most likely that the accelerator pump is pushing too much fuel. Adjust that back until the soot goes away. Then recheck the plugs after driving it around for a while and see what they look like.

BUT - don't do anything until you verify that initial timing, and that the vacuum advance is not connected to straight manifold vacuum. Pull the hose off the distributor with it running and see if there's vacuum on it.

 

The 45 isnt a typo, and thats why i am asking, cause i thought it sounded way off. Thanks for all the input. I am going to check that the harmonic balancer hasnt slipped and check that my vacuum is hooked up right and start from the beginning. Thanks again for the input and please keep the ideas coming. I will post again after i have played around with it again.

 

45deg at idle isn't outside the relm of posibility, and is even likely if just turning the dizzy for a smooth fast idle with the vacuum can hooked to manifold vacuum. It will run great up there but likely ping while driving, many vacuum advance setups will add over 30deg. My truck out of gear has an idle advance of 38deg (16 initial + 22 vacuum) becasue it's hooked up to manifold, and I'd have it higher if it didn't cause a ping during certain part throttle driving conditions.

Not saying it's not a good idea to check timing marks, always a good idea, but the engine will tell you if something is wrong.

 

A typical vacuum advance unit won't pull the advance that far, usually. But 45 degrees, unless the cam is HUGE, there's no "it runs and idles great" with that.

And it would ping like a MOTHER under light throttle.

I suspect the dampener ring slipped.

 

Alright, just got back from playing around with the engine again. Checked the timing again with the engine idling in park and the vacuum line disconnected and plugged i am reading about 35* btdc. I checked the harmonic balancer TDC mark to the #1 piston and the ring hasnt slipped. #1 is at TDC when the balancer TDC mark is at the pointer. I Retarded the timing to 20* btdc and the idle gets rough and the timing jumps around and when you rev it up it backfires. Pulled the distributor back and advanced the timing and the idle smooths out and RPM increases. I am using a Mac tools inductive timing light with digital advance on it. I crank up the advance until the TDC mark is at the pointer and read what the advance says (which is between 35-40*) have you ever had issues with a timing light digital advance not reading correctly? Could it just be, that with all the Performer RPM aftermarket parts and MSD ignition system, this is just where the engine wants to run? To me it just seems insane. But when we retared the timing and took it for a drive the engine was powerless. as soon as we advanced the timing back it ran great, you could tromp on it and it would sit you back in your seat. even up at 5,000 RPM there are no signs of pinging. This is driving me crazy i cant believe the engine likes to run with the timing so far advanced. Thanks again for your comments, especially opossum your insight has been great. One more thing, i did look at the vacuum line and it is hooked to manifold vacuum, not timed vacuum. Any more insight would be greatly appreciated. Thanks

 

One question - you have #1 being the passenger-side front cylinder, right?

 

haha yup. thats the one.

 

Sure it will, many will go that far, like I said it's on the high side and likely would ping that high, but it's possible.

Try it out on your truck, put a vacuum pump on your vacuum advance and play around, you'll see what I'm getting at.

But back to the OP
OK 35 initial without the vacuum hooked up is WAY high and if it truely is 35 initial it would NOT start, it would kick back when the first cylinder sparks. This leaves 2 likely options, either your measurement is off, or 35 isn't actually the initial timing, which I think more likely.

First how are you finding TDC? of course the finger in the plug hole or feeling the piston with a screwdriver aren't exact methods, pretty easy to be 20 deg off with guestimation like that. The only real acurate way is with a piston stop, basicly a bolt in the plug hole that the piston will hit stopping it's upward travel. To find TDC doing this remove all plugs to make it easy to turn by hand and instal the stop. Turn the engine forward until the piston hits and stops then mark that spot on the balencer. Turn backwords until it hits and stops then mark that spot. TDC is exactly between the 2 marks.

Second check the advance on your light, though I've never even heard of a good Mac light being off. If you have other engines around find the one with the best timing marks with the widest range, so one that goes to 20btdc over only 10btdc and read the timing using both methods, with the markings, and with the light set to the advance and the marking reading 0. Should of course read the same using both methods, or better yet try a second light and make sure they read the same.

OK is the 35 reading correct but not really initial. This would happen if the centrifical advance is advancing at too low of an RPM or your idle is too fast. The timing with the vacuum advance unplugged should be the same during cranking as it is during idle and the centrifical advance should start as soon as RPM increases off idle, but not all idle speeds are the same so this may need to be tuned to your engine and be off. It may be advaning at idle. As it's rather hard to check timing while cranking you can't do that but it is still easy enough to tell if it's advaning at idle.

Your post wording indicates that the timing at idle isn't stable, this is a huge sign that it's doing this. Timing at idle, vacuum unplugged should be steady as a rock. To make sure just watch your timing marks while slowing the engine a little. Watch your timing while using your other hand to block the carb, close the choke, turn down the idle speed or put it in gear. Your timing should stay the same right down until it dies. If it drops, then your not actually reading initial timing but initial + centrifical advanace and need to put heavier srpings in your dizzy to increase the rpm it starts to advance or slow your idle speed.

This is all more important with your truck being an auto. RPM will be lower in gear, and if safe to do so ALL idle adjustments should be done in gear. With an ideal tune your engine will have higher timing in park then in gear becasue that slight increase in idle speed should increase the timing becasue that will give you the best responce while in gear, but will have a side effect of happening in park.

I'd start out first thing by just checking the timing in gear instead of in park, please be safe doing this put someone in the drivers seat with there foot planted on the brake if you can. I'm betting this is what's going on, and of course it's still just a number, how your engine runs is more important, but let us know, happy hunting.

 

Thanks Krewat. I am just looking for help with a problem, i dont want to know everyones stance on other topics. And thanks again to opposum. I got a second trouble light and am going to use a piston stop to find out true TDC on the harmonic balancer.

 

and by touble loight i mean timing light.

 

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