Re-Attaching Vacuum Advance

[IanDMacDonald]

To be honest, I'm really at a loss to do right now. With the current timing, the bikes runs good, just very poor mileage. I'll look into getting an advance from a '78, and see if I can start over, utilizing the vacuum advance unit.

[IanDMacDonald]

Sorry for blowing this thread up, but I found if I retard the timing all the way clockwise as far as the plate will turn, the bike idles almost @ 10°, one notch to the right of the F mark. However, full advance shows somewhere around 25° @ 5200rpm. What should I do? Re-tune carbs to the 10° @ idle, or leave it where it is @ 19° where it runs well.

[crazy steve]

You can try running it with 19 degrees of initial advance, but if it knocks/rattles once it's on the road and under load, you'll have to retard the timing back closer to 10. I strongly suspect that's what you'll end up with.... until you replace the modified parts.

[IanDMacDonald]

Well, right now it's running @19° during idle, and runs smooth as butter. When I took that bolt out of the vacuum pod and allowed it to go back to the normal position, the engine died and would not idle well. I should have tried re-timing it after I allowed that vacuum to go back to stock position.

[GLoweVA]

gotta ask a stupid question

I haven't been keeping up on this thread too closely, but have you checked that the cam dots line up the the arrow when the pointer is at T ?
The PO may have advanced them by one link or something like that.

Just a thought and it may be a stupid question, but the thought popped in my head and I had to ask.

[IanDMacDonald]

George- Great thought, but I put the top-end back together myself, and made sure it was right due to messing it up before. Cam timing is spot on. The timing being off is a mod PO did. He locked-out the vacuum pod arm, and altered the mechanical advance, thus my initialtiming being 19°. I have the option to turn the plate all the way clockwise to the stops and have Iinitial timing @ 10°, but full advance would be @ 25° at 5200rpm.

[oldnortonrider]

Okey dokey. So, I just went out there to see where exactly it is @ idle, and again @ 5k. With her warmed-up, the plate shows 19° @ idle, and 35° @ 5k.

Note: With my ear next to the plate, I just noticed the metallic pinging noise while idling and @ 5k.

Suggestions?

I'm thinking this 'Scott' fellow might have been an out-of-the-box hot-rodder. If he knew to run cold-est (heat range) plugs ~~ like Denso 24s or NGK 8's, and had access to good fresh high octane fuel (above 100, preferably higher), or knew of the really effective high octane boosters, and added small amount of acetone just before running ~~ it wouldn't ping ~~ and I could see why he'd want high degree setting at the low rpm, and not go past 36 degrees at the high rpm. Really interesting; why he did this; what he was after...................

[3Phase]

Ian,

It looks like Scott did some good work there. He locked the vacuum advance to the max to keep it from moving and to get a metric shipload of advance for the Base timing without enlarging the holes on the main ignition timing assembly. Then he limited the mechanical advance slots to account for the extra initial Base timing.

Locking the vacuum advance to the max would keep the engine from going nuclear if the lock-out screw vibrated loose or, for example, someone reconnected the vacuum advance pot without resetting Base ignition timing before running the engine -- fail safe.


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[IanDMacDonald]

Yes, I've mentioned in other posts as well as Scott himself that he was no dummy converning this bike. He owned it 20 years, and did all the work himself including CNC machined dog gear fix, as well as ported/polished head, etc. I am the ignorant one when it came to his modifications to the bike. I messed-up big before and learned to ALWAYS ask questions before I undertook something mechanical on the bike. The reason behind my want to try the '78 advance and start over is due to poor gas mileage. I just saw another video on the thread as to how the advance is supposed to work. At-least now I know what to look for.

[IanDMacDonald]

I've located two different advances off a '78E for my bike. Before I go dropping money on this thing, I was wondering if I could get some guidance with this. What should I do if I bought this thing? Just put it on, hook the vacuum advance up, unscrew the locked-out pod and re-time?

[crazy steve]

That's pretty much it. Make sure you're getting the right amounts of advance at the right RPMs and you should be good to go....

[3Phase]

That vacuum advance looks pretty grim

Ian,

As Steve said, just set the timing. It should be set to 10 degrees BTDC or the 'F' mark if you got the '78 timing wheel along with new mechanical advance.

Double-check the movement of the advance plate and its bearing before you reconnect the vacuum advance. The advance plate (the one with the pickup coils attached) should turn freely like a Lazy Susan and you can't tell if the plate or the bearing is binding when the vacuum advance is connected to it.

The vacuum advance itself can be a little temperamental (or just plain mental! ). It starts to go to work at barely 2 inHg (50 mmHg) and it's 'all in' with 16 degrees of advance at only 6 inHg (150 mmHg). That's a pretty narrow 4 inHg (100 mmHg) range between 0 and 16 degrees of advance.

In a nutshell, the vacuum advance, "floats like a butterfly, but stings like a bee!"


If it's just coyote ugly but it still works then give that wallowed out hole and the sleeve a quick shot of your favorite cleaner to rinse out out any metal bits and any other creeping crud, then add a little bit of oil.

After you reconnect the vacuum advance and tighten the screw, be absolutely certain that the screw is not touching the bar again and that no part of the advance assembly is sticking or binding.

Go ride!


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[IanDMacDonald]

Great, thank you gents!

[IanDMacDonald]

I have a '78 advance and a few tidbits on the way from Scott. I'm excited to get that vacuum advance working and to start saving some gas.

[IanDMacDonald]

Scott sent the wrong part. I got the vacuum pod in the mail, but not the requested mechanical advance. Couple more days.