Mech Advance Only Distributor Timing

[olddog]

I don't know if you read the links I told you about, so I will explain one thing that may be important.

A lean fuel mixter burns slower. Richer mixtures burn faster. The slower the burn rate the more timing advance is needed. If your carbs are too lean, the engine may well act like it needs more timing.

You could get away with a lot of initial timing, as long as you never load up the engine until you are at or above 2500 rpm. Remember when you open up the carbs the power valve richens up the mixture and it is going to burn faster. If you have enough timing to run good at light load (lean mixture) and step on it at low rpms you could ruin the engine. Vacuum advance fixes all this. Can your distributor accept a vacuum advance canister?

[Excaliber]

One problem with vacuum advance is it goes away (retards) faster than it advances. When you go deep with the throttle the vacuum signal drops to zero or close to it in a heart beat. Depending on the return spring of the vac advance unit that means fairly instant retard, back to whatever mechanical advance was/is in play.

I use a hand held vacuum pump to check the vac advance myself. There fairly inexpensive and that way you can tell just how much advance at what vac signal your getting. How fast it responds, how fast it drops out, etc.

[Barry_R]

That graph is actually very similar to the one from the MSD billet distributor instructions. The combination of blue and silver springs is one of my personal favorites. Whats happening is that at cranking speed you have minimal advance so it cranks easily.

At idle speeds of +/- 1000RPM you have a modest amount of advance because of the light springs that you can set to the desired 20ish for light throttle off-idle driving.

After that 1300or so RPM it advances in a non-linear fashion because the real light silver spring gets stretched out sooner than the blue one. You limit the max amount of advance by using different stop bushings - the black one gives the least amount of distributor advance - another favorite...

[patrickt]

Originally Posted by Barry_R

That graph is actually very similar to the one from the MSD billet distributor instructions. The combination of blue and silver springs is one of my personal favorites.

So in your professional opinion I'm not running an antiquated cam and an antiquated advance curve?

[69boss429]

Originally Posted by scottj

My motors always ran best without any advance so I've always lock out my distributors, but my cams have always had 260+ degrees at .050". On my 11:1 motors I didn't need the MSD start retard box as they always started without issue, but it was required (-20deg) on my high compression motors or they would blow teeth off the starter ring gear. A dyno is needed to determine optimal timing.

Scott

Question,
When you set the timing with your locked dist where do you set the mark?
With a digital timing light do you set total at TDC (say 32 deg or whatever your max is) mark or BTDC

[Argess]

OldDog: Yes, I read all the links. Thank-you I appreciate them.

BobCat: Major help....thank-you....you'll see why below.

Everybody Else: Yes, "Thank-you".....I am getting somewhere:

I have been making some mistakes:

1/ Paying too much attention t the bog. I've re-adjsuted the mixture each time I try some timing now, and find I can get the bog to dissapear other than the slight hiccup which I can probably conitinue to dial out, or play with accelerator pump cams.

2/ Paying too much attention to the damper slippage correction. As I really have no idea when or how much at a time the damper was slipping, I have now ignored all previous settings and started over....with some success.

3/ Not remembering correctly what detonation "feels like" (as I can't hear it) and thinking my engine currently sounds like it has retarded timing vs advanced.

4/ Thinking the excessive engine heat at low advance settings indicated I needed a much higher advance.

5/ Thinking the best idle (max rpms and max vacuum) should be a starting point for intial advance.

OK, so starting over, I went with the max Barry Grant figure for my cam. I set the intial to 16 Deg BTDC and re-adjusted the idle. Mixture and idle stop screws. I did not re-adjust the dist plate yet for a max of 38, but I am interested in the lower rpms where the advance works for now. Went for a test drive. Engine felt light, revved easily, snappy reponse, bog gone 'cept for slight hiccup. Higher rev's seemed a little down on power, but since the dist plate was still limiting to 10 over idle (26) I wasn't worried or surprised.

Idle is not great.....slow hunting and sometimes it gets low after I take my foot off the gas and sort of stays there until I clear it with a shot of gas. But I am getting 17 in Hg from the max of almost 20 that's possible at best advance ( at best fuel effieciency).

So today I'll try it again at 18 Deg and then 20 Deg, and that will be enough. I am interested in a little higher mostly to help the idle and keep down the engine heat, but naturally I dont' want to get into the detonation range again. 16 may very well be it.

Once that's set and the dist locked down, I'll re-adjust the plate for max advance. I may not go straight to 38, but may try 34, and then 36 first. I don't want to take any chances. FE re-build is slow and although I do it myself, it's a tad pricey with gaskets, parts, machine shop work, etc. Last, I'll work on the carbs for that "hiccup".

So I have high hopes that I will find good timing (not perfect, but acceptable), and in the range that most of you are finding to be acceptable.

I still plan to try a vacuum dist. With no question, at a minimum, it would solve the idle issue. I thik it will give me better light throttle reponse too. Just trying to find one that will fit. At any rate, I will have the timing numbers from my mech distributor to set the vacuum one up (vacuum temp dissconnected) once I get one.

Thanks again everyone. Things are starting to come together and if it doesn't rain today (we're getting the June rain late this year), I hope to have it finalized.

[Argess]

Great success today. Intial at 18, Max at 37 and all in around 3000, or perhaps a bit less. (It was getting close to max @ 2800 and I don't like revving it too high when un-loaded).

Idle is still hot, but not terribly bad. Idles about 18 in Hg, Sometimes it comes down from WOT and idles a bit low till I clear it. Slight delay when I floor it, but so small, it's acceptable. Seems ot happen at any rpm I try to accelerate from, so I figure a little playing with the accel pump cams should fix it.

Higher rpms sound OK, but I'm not sure. I started at 37 intending to work down vs lower (say 34) and working up, so I may try to drop the max down a few degrees to see if it makes a difference.

So far, it works better than ever and "no smoking" for the first time. Interestingly, it was also the first time I used Sealed Power rings vs Hastings. Major different oil seperator design. I credit that ring for no more smoking.

[olddog]

Glad your having success.

All credit on the links goes to Tom Kirkham.

[scottj]

Originally Posted by 69boss429

Question,
When you set the timing with your locked dist where do you set the mark?
With a digital timing light do you set total at TDC (say 32 deg or whatever your max is) mark or BTDC

I use ATI dampers that we mark with white paint at the actual deg BTDC as determined on the dyno. We only use the cheapo $20 timing lights, no dial-backs or digital. If for some reason we had to time the motor at the track, at night, it is a fool-proof method. While this procedure is for our circle track motors, I didn't have any good reason to do it differently for my Cobra motors. With locked advance and a good custom carb, my Cobra motors had better street manners than most others having 20 degrees less cam. In fact my girlfriend could easily drive it until I put a 3-disc button clutch and the Jerico in it.

Scott

[Excaliber]

I really don't like those adjustable timing lights myself, wouldn't have one!

[Argess]

Mine is a dial-back (but digital). A Snap-On I bought from a mechanic friend who changed careers. Setting the dial-back to say 38, and then quickly revving the enigne to see if TDC lines up and then stop, seems easier on the "unloaded" engine than holding it at high rpms to read the damper lines (TDC additionlly marked with a bright paint blotch). Having a tach on it is a nice feature too.

I really like it, but it would be nice to be able to check the calibration somehow. So far, my static max advance seems to match the light with engine running. My old light went to another friend, so no double checking the basic function.