Big Bore Pinging

[3Phase]

That's a good idea: limiting the final advance while leaving the base advance alone. I think I'll try that if lowering the compression a tad doesn't do the trick.

Before you tear apart the engine, if you haven't already done it then try disconnecting the vacuum advance and capping the #2 carb nipple. The '79SF adds up to 16 degrees of advance, depending on throttle position and load, between 2 inHg and 6 inHg.

You can't change the spring but the vacuum advance range/max advance can be adjusted by removing the blob of epoxy on the end of the dashpot to uncover the locknut and setscrew.


The mechanical advance for the '79 Special starts at 1,800 rpm it's all-in at 5,400 and 31 degrees. Trim a couple of small machine screws like a 10-32 and use RTV to temporarily glue them in the advance slots and limit the max advance.

It won't stay, of course, so it's totally temporary but you can get the upper timing dialed in, then figure out something more permanent if you have to.

[TopCatGr58]

Hey Scott,

I like the idea of unplugging and capping the vac.adv. for a start, but we are forgetting an aspect of the vac. adv. pot. He states pinging under hard accel, that requires lots of throttle input, and when you crack open the throttle plates, the vac. signal DROPS and the vac. advance actually RETARDS the timing back closer to the max Cent. Adv. levels of 35 degrees which are supposed to be the best adv. amount for the maximum amount of POWER....the higher adv. amounts under low throttle input were primarily for fuel economy.

SO...the timing should be dropping back to that ~35 degree level under hard accel and so I would think shouldn't produce pinging especially with premium. I have a 1179 big bore kit, but in an 81 with TCI programmed cent. adv. but still manual vac. controlled adv. and haven't experienced any pinging with the premium that I ran, and just in the last couple of years have run Mid grade and still haven't experienced pinging! But I realize that my engine may not have as much compression ratio as his with the earlier heads, and perhaps my timing curves are not as much advanced as the early model.

T.C.

[Bonz]

For timing advance, my FSM says 36 deg BTDC at 5200 rpm (which I assume is the rpm at which max advance would occur and stay there) with the vacuum adv disconnected and plugged. Correct me if I am wrong.

Have you run the bike like this, so the vacuum advance is out of the picture for the sake of trying it? That would either eliminate or bring into play an issue with the vacuum advance not releasing, IMO.

Hope that may help. Bonz.

I was saying that back on post three... above is the quote of the first couple paragraphs. I know when it gets to the end of the first page and onto second page of the thread the first posts get to be stale.

All of the good info still doesn't change that he has what looks like too much compression for a perfectly working advanced system. Doubling up a base gasket like he said he was going to do is the way to go, IMO.

[3Phase]

Bonz, I saw that you asked about disconnecting the advance but it wasn't answered.
(shrug) I'd try a lot of simple (to me) things before I pulled the jugs and doubled the gaskets.

T.C., there is vacuum advance during acceleration, I don't know why people keep saying it goes away.
It drops but then it comes back and that's when it'll ping if it's going to ping.

When I made an experimental video for it, I zeroed and taped the gauge for the advance to show the active range.
I should have used colored tape because it's hard to see it during the ride.


The active range for the advance pot for my '80G is from 1.5 inHG initial to 4.4 inHg max, measured with a vacuum hand pump.

The vacuum pot for my '79 Special is similar but initial is about 2 inHg and max is a little over 5 inHg, so it's sort of like the book.

In the video, the manifold gauge is heavily damped or the needle would go insane.
The vacuum advance gauge is only damped by the restrictor in the carburetor nipple.


XS1100G Ride Vacuum