Installed correct unbroken distributor... let's see how she runs

[KTPhil]

I have had overheating and pinging issues since my rebuild, and it's been a strange trail to work to resolve them. After switching to a 20W-50 oil from 30 weight I had been using for the last year, the oil pressure is where it should be and it's running a bit cooler as a result. And I relocated my oil temp sensor to get it out of the hot cooling air stream, so it's more correct now. Other threads detailed those past adventures.

But I've still had wonky ignition advance, and so I investigated a bit. After rebuilding (and no doubt before as well, no reflection on my meticulous engine guy--I did all assembly past the long block stage), we checked the advance, and found way too much.

From TDC at idle, it was hitting 33 degrees just on the mechanical, and the vacuum added another 10 or more. At high loads and speeds, these would work in tandem, and the whopping 45 degrees made it ping under load, and no doubt also contributed to the high oil temps. So we removed and plugged the vacuum advance. With FI, you must keep timing at TDC statically or at idle, so the practice of just playing with the distributor position was not an option.

So I went to Bentley to see what advance I should have. Turns out I was way over spec on the mechanical side. At 2500 rpm, I should be looking at about 20, and then add 10-12 for vacuum, for a reasonable 32 total. So why the heck did I have 33 mechanical?!

I figured the weights or springs must have broken inside, which would also explain the changing idle timing and also my tach being very jumpy. Thinking back to last year's Type 3 Invasion and my variable timing, this seemed like the answer.

From an idle, it ran better if I gave it a bit more static advance, but that made the pinging worse, so I backed it off and put up with a little hesitation and the elusive Type 3 FI bucking if I jammed the pedal at 2000 rpm or less.

I had a spare distributor, but it was for a '71 automatic. Looking at Bentley, that actually seemed like a more reasonable curve for my 1776, higher compression engine:
- quicker mechanical advance off the line
- less mechanical at higher rpm where I had pinging
- WAY less vacuum advance
Frustratingly, the Bentley graphs are not to the same scale, so I scanned them and overlaid them to compare:

The Bentley graphs are expressed in distributor shaft, not engine, RPM and degrees, so double these figures for your tach readings.

The orange dotted line was my readings after the rebuild. With sky-high mechanical, something was wrong.

So I put the '71 AT distributor on, including the vacuum advance, and it drives much better. The bog is gone, so throttle response is smoother and a bit quicker. I took a brief run at freeway speeds and it accelerated great, with no pinging; however, it's cool out today so I can't really push her to see if the pinging is gone. Next weekend should be 90 or above, so that will make a better high speed test.

The weirdest thing I found, is that I had the wrong distributor in it! The unit I pulled is a 1970 MT "AB" unit, not the correct "AC" version. I know the history of this car and engine; my dad bought it new and gave it to me. Nothing in the docs I have shows a distributor change! This was an early '71 so maybe they used the older unit, but I doubt it-- '71 federal smog rules made that unlikely.

But look at the blue AB curve-- high mechanical advance at high rpm. I was even higher than that (orange dotted line) due to some malfunction. But the stock '70 curve would probably have been too much even if it was working correctly.

No wonder it was pinging!

Fingers crossed for next weekend's testing in hotter weather, but I may be onto the root cause!

[Nate M.]

The joys of 45+ year old EFI and distributors. . .
_________________
Regards,

Nate M.

Squarsche build
Heavy Metal Affliction feature

For heaven's sake, put a type4 and a Porsche 5-speed in there. . . It's the right thing to do!!

[KTPhil]

Update:

Still a higher reading on the temp gauge than I'd like, but it's not calibrated, so it's more the behavior not the absolute reading, so...

It's 90+ out today and I ran it for long stretches on the freeway at 70-75mph, and oil temp slowly rose to 260F. A bit high but I could not get it any higher no matter how hard I pushed it further.

Before the swap, I could get the oil temp that high faster, at lower ambient temps, at 65mph, and it was still rising so I would cut it down to 55 or 60 max and worry a lot.

So this is real progress. Much smoother, not a hint of pinging, very much better low end smoothness (no bucking even if I boot it at 1500 rpm).

I'll work on checking the gauge with an IR gun. Last time I did it was reading about 20F too high. If I convince myself that differential is real, I'll put a resistor inline to calibrate it.

So far so good, thanks for listening.

[danielheff]

Hey just wondering your outcome of this new distributor? I'm thinking my distributor could be contributing to my engine running hot at high RPMS as well.

I'm using the VW 311-905-205AB, Bosch 0231 163 008
Which has this advance range:
Advance/Retard Range: Vacuum: 8-12deg Adv; Centrifugal: 10-14deg @ 1500rpm, 20-23deg @ 2300rpm, 27-30 @2800rpm

So with full vacuum and mechanical advance it can be pulling 42º of advance. I can't hear any pinging, but seems hard to imagine that so much advance would be necessary.

I'm also running dual carbs instead of FI.

[Donnie strickland]

[KTPhil]

[danielheff]

Glad its running better.

I had Tasb test and inspect my distributor and it's very close to factory spec. Vacuum can pulls exactly 12º and this is what he found for the mechanical advance:

Advance Specification RPM My Distributor
10-14 1500 15
20-23 2300 26
27-30 2800 31

I too have a larger engine and wonder if the vacuum can is making things worse at higher rpm. I have an 1835cc.

Is it possible for the engine to be pinging lightly? Or in a way that I can't hear it?

I had my carbs tuned professionally from a shop that hooked it up to the exhaust analyzer so I'm pretty sure my fuel/air mix is correct. Thats why I'm questioning the ignition now. It really only runs hotter on the fwy, especially climbing any kind of hill.

[KTPhil]

[danielheff]

I'm going to re-install the distributor soon after I get the cap & rotor back from Tasb. He installed a shim in the distributor to take up some slack. But the engine is pretty new. Around 2k miles on it.

Maybe I'll try plugging the vacuum can, but I really like the way it gives a little life to the lower RPM torque. Around town its awesome.

Right they didn't check the tuning under load.

Should I modify this mechanical advance to maybe only pull 28º and let the vacuum do the rest? Or is there a different distributor that will have a more agreeable advance curve? Maybe replacing the vacuum can to something meant for a bigger engine?

[Slow 1200]

I'm assuming you are running stock PDSITs? which of them? if they are setup for vacuum only distributors they will pull vacuum at full throttle for sure

[danielheff]

Yes they're PDSIT's, re mfg by Bert. But since the engine is larger than stock I made the Venturi's 28mm and re-jetted to match.

Sorry for hi-jacking your thread Phil.