78 F150 400 Distributor Timing??
 

Ok, some of you chimed in on my last post and I want to provide some new information and get your opinions. This is on my 351m-converted to a fairly stock 400.

Been trying to get rid of all the spark knock I get (sometimes) on light throttle-steady cruising speeds. I get no knocking on hard acceleration. I've had my timing set at 9 degrees base and it has eliminated 98 % of the ping. But, I know I'm underpowered with this low of a mechanical setting. I run a 180 degrees Cleveland correct thermostat, from Tim Meyer. I actually run about 175 in the winter which is almost too low for my comfort. But, I change the oil twice a year with Brad Penn. My cooling system is very efficient. Possibly because there's no fan clutch? So I'm definitely not running hot, even in the summer Atlanta temps. I think my compression ratio is about 9.0-1. I run an open 6" air cleaner element.

I got use of a set-back timing light today and here's what I found, to my delight. Base with vacuum advance disconnected was at 9 degrees, 980 rpm in park right where I set it last year. It hit 13 at 1200, 20 at 1500, 31 at 2000, 34 at 2500. It hit the max dead at 2500. The vacuum advance added exactly 12 degrees at idle. I use manifold vacuum. It raised the max from 34 to 46 absolute total.

Based on my curve, I think it's pretty close based on what I have learned, and to get the max in by 2500. I'm at about 2300 rpm at cruising speed of 55-60 mph.

I think the only thing I need to do is get the mechanical back up about 3 degrees for a base of 12 and total of 37ish. But, the vacuum advance then needs to be lowered -maybe from 12 down to 6 degrees to prevent that spark knock when it's fully deployed at light throttle.

My vacuum can is marked a number 6, meaning it should pull 12 degrees, and it's dead on the money. I had a hard time finding this particular can. It actually fits a 1978 Ranchero with a 400. Different from an F150 in the catalogs.

I would like to modify the vacuum arm and think I can drill a small hole in it, install a roll pin and have it limit the pull length. The roll pin would hit the inside base of the distributor. Thoughts? Then, I can bump the timing back up to 12 or so. Don't tell me to just adjust the vacuum can, the adjustment on these type cans just changes the "when" , not the total.

What do y'all think of my curve readings? Am I maxing out too soon?

The funny thing is, as far as I know, when the engine builder put this new reman distributor in, it was a cheapy and except for it having had a high vacuum advance which originally was a number 12 -translating to 24 degrees which was made for EGR engines, it is curved near perfect, I think. Since I don't have the EGR , the vacuum advance doesn't need to be as strong. But would 6 degrees be too little?

I had thought about having a distributor built and curved, but now I don't think I need to. I need to just get the vacuum advance down a little more than what it is. I still haven't found a chart anywhere that lists a vacuum can with a smaller degree than the one I have.

Comments please.

This sounds more like an air/fuel ratio problem. I think you're Lean. Have you noticed a stumble, cough, or backfire through the carburetor when lightly accelerating from cruise, especially up hill? If you're running lean, light throttle at cruise is where knock / pre-detonation is gonna be most prevalent. Butterflies have opened a little to allow more air in, but you're vacuum signal has not yet dropped enough for the carburetor to meter enough fuel to match the air. I'm assuming you have a carburetor, not EFI.

A quick bandaid would be to use a good octane boost to reduce pre-detonation. Not the parts store or Walmart stuff. Klotz is good. I would install an AFR gauge, and tune the carburetor accordingly. That gauge in conjunction with a vacuum gauge make it very easy for me to tune my carburetors at installation, and for weather condition changes.

I don't think my carb is running lean. It runs too well and has no performance issues really at all. If anything, being a Holley, it may be a little on the rich side. I just can't help feel that due to the lack of EGR, it just doesn't need that much vacuum advance. I don't have access to the equipment needed to check AFR while cruising. If you stab it once moving, it goes and pulls like a horse with no spark knock! If you stab it from a dead stop, it might falter a little, and I think that's due to the mechanical timing being a little low.

So have you tried ported vacuum to see if there is the same problem? also drilling and putting a pin in is what I had to do also,works great! I do think ranger might be on to something I fought the same thing with a 460 that I had for a long time and a buddy of mine told me fattin it up a step and see what happens,it all went away!Yep adjusting the diaphragm only changes the amount of vacuum it takes to pull it in not the total it pulls

Without a gauge, you have no way of knowing if your carburetor is lean or rich. Holley, Edelbrock, Carter, or whatever, they all need tuning. The symptoms you described in your first post, and this one, are classic lean conditions.

What equipment are you talking about? The gauges are readily available from any automotive website or parts store. All you need is to install the gauges. It's no more involved than installing an oil pressure gauge. A decade ago, Ignorance was Bliss. We tuned until we could smell fuel, and had no stumble on acceleration. No longer. Air fuel ratio gauges are too easy to install now. That in combination with a vacuum gauge, which is even easier to install, will tell you more information than any repair shop diagnostic machines.

I'm listening and thankful for the suggestions. I have tried ported vacuum and get the same thing. I like the manifold vacuum to get the most complete combustion at idle and low throttle conditions. I can disconnect the vacuum advance and it totally disappears.

This is not bad spark knock. in fact it's so slight that most would ignore it. It still just seems like it's a few degrees over advanced with timing for when it occurs.

What do yall think of the curve readings I first listed? Are they in line with where I want to be?

You should be using Ported/Timed Vacuum. Manifold vac will be advancing the curve at idle, and you could be reaching total advance at 1500 or less rpm.

The spark knock isn't bad? Do you know what spark knock is? It's pre-detonation. Combustion is reaching peak pressure while the piston is still moving upward. Every time it knocks, it's eating the top of the piston away. Ideal combustion peak should happen when piston is on it's way down, about 7 degrees After TDC. All spark knock is bad.

A good performance curve for a 400 would be 20-21 degrees from the vacuum advance, with total timing at 32-36 degrees, at 2700-3300 rpm.

Ported vacuum was a crutch for emmissions. I wouldn't use it. Your numbers sound fairly close. I have modified vac advance modules to limit at 10 degrees. That lets you ad more initial. It would help to limit the rate/max rpm for the mechanical advance so it doesn't top out untill 3500.

If all else fails Snow Performance water/meth injection worked wonders for me.

Beartracks..how did you modify your advance?

It was on an MSD unit. I drilled and tapped a hole for a screw in the arm and used a small piece of metal (oblong) that I could turn a little this way and that to adjust as a stop.