Timing question

[Bernie]

I put Howell fuel injection on my truck- Mallory Unilite, Edelbrock Performer manifold and awoke the truck from its Winter hibernation last weekend. I had always set the timing by advancing it until the engine just pings under load and then retarding it until it just didn't. My wife bought me a timing light with the advance function and I tried it out on the truck. It seems that at idle I have 600 rpm and 25 degrees of advance. The idle seems a bit fast, but the advance is a lot more than the manual says it should be. Is this ok ?

[msgdsrf]

Depends on whether you had your vacuum advance connected to distributor at the time and also what your timing is at say 2500-3000 RPMs. Base timing of 25 is not all that much (initial and vac together). If that's just initial then add say another 15'ish vac and say another 18 mechanical you're approching 60 total and that's not best VE (efficiency).

[70blackfish]

16 at idle and say 34 max...... above 16-18 at idle would be hard to start.

and pinging is the worst thing you can do to an engine...

[davepl]

Does the fuel injection control the advance? Because then you usually have to phase the rotor to be aligned perfectly at max cylinder pressure (peak torque). You enter that value in the software so it knows how far back 0 advance is, and it takes it from there.

If you're doing it right, you need to drill a hole in the cap so you can see the rotor-cap interaction. Then a timing light, run the engine (or just statically in the computer set) to max advance and then physically align the rotor tab with the appropriate cap tab when the timing light flashes. You can get away with bad alignment on a low compression engine, but if you've got a lot of air molecules to cross (supercharged or high compression) then it's got to be right.

So what's triggering the unilite distributor to fire?

[davepl]

Oh, and FWIW, most old cars require a lot more advance than the original tuneup sticker says. This is partly due to fuel and partly due to mods and partly due to people removing drivability aids like heat stoves and thermacs and so on.

[Bernie]

It starts up fine. The truck is a K20 with the engine, a 350, in the rear position so there is not much room to get at the distributor. A Unilite was about the only thing that would fit. I have the Howell fuel injection set up, but the distributor still uses the vacuum advance that I had with carb. I installed the Howell over the Winter and apparently the computer takes some time to adjust itself. The 25 degrees of advance is at idle with the vacuum tube connected to the distributor. The timing gun gives an idle of around 600 rpm.

[msgdsrf]

Originally Posted by Bernie

It starts up fine. The truck is a K20 with the engine, a 350, in the rear position so there is not much room to get at the distributor. A Unilite was about the only thing that would fit. I have the Howell fuel injection set up, but the distributor still uses the vacuum advance that I had with carb. I installed the Howell over the Winter and apparently the computer takes some time to adjust itself. The 25 degrees of advance is at idle with the vacuum tube connected to the distributor. The timing gun gives an idle of around 600 rpm.

25 is good then.

[davepl]

Originally Posted by msgdsrf

25 is good then.

Whoa, let's back up a bit. It probably is fine just as you say but you can't measure idle advance with the vacuum line connected. You can, of course, it just doesn't tell you much.

Cap it and set it like the factory tells you. I'm going to guess you need 12 degrees of initial advance. Regardless, figure out where your best idle is, set it there, and then get the distributor recurved limit total advance to 35 or so (or whatever is reportedly best for your motor, presuming no dyno is handy).

A lot of people crank the initial timing ahead and forget that the mechanical is still going to add what it thought it needed from the OLD base timing. So if you had 6 base and 29 centrifugal for a total of 35, now you'd have 12+29 for a total of 41 and it'd be just too much. Probably ping at part throttle with vacuum advance too.

All of this depends on the fact that everything else is right. You can compensate for a vacuum leak by cranking the timing ahead, but it doesn't fix it. So everything else has to be right too.

[msgdsrf]

Originally Posted by davepl

Whoa, let's back up a bit. It probably is fine just as you say but you can't measure idle advance with the vacuum line connected. You can, of course, it just doesn't tell you much.

Cap it and set it like the factory tells you. I'm going to guess you need 12 degrees of initial advance. Regardless, figure out where your best idle is, set it there, and then get the distributor recurved limit total advance to 35 or so (or whatever is reportedly best for your motor, presuming no dyno is handy).

A lot of people crank the initial timing ahead and forget that the mechanical is still going to add what it thought it needed from the OLD base timing. So if you had 6 base and 29 centrifugal for a total of 35, now you'd have 12+29 for a total of 41 and it'd be just too much. Probably ping at part throttle with vacuum advance too.

All of this depends on the fact that everything else is right. You can compensate for a vacuum leak by cranking the timing ahead, but it doesn't fix it. So everything else has to be right too.

He was just asking if 25 was within range (with vac.) It is.

[davepl]

Originally Posted by msgdsrf

He was just asking if 25 was within range (with vac.) It is.

I know, but so are 5 and 40. Depends on the relationship of the throttle blade to the transfer port (hence, ported vacuum).

My point, as you'll read in my post, isn't that I disagree with your conclusion that its in range, just that I don't think he should be relying on it as proof of anything.

[msgdsrf]

You have a good point, everything is in play. Just seems he had the motor running and assumed tuned including timing curve before the winter. I assumed he was getting the vac advance to acheive the 25 degrees because I doubt the starter would handle 25 initial. Now 25 degrees BTDC is typical with full vac advance, however you get there, with initial and vac. (my small block runs 16 initial w/ 10 vac and 18 mechanical.). So to the OP, it is less likely the high idle is the timing is more likely elsewhere (carb, leak, etc).

[Bernie]

Thank you for the replies. I have food for thought and need to experiment .