Vacuum Advance on a 1980 Special

[jacked_72]

I've got my 1980 XS1100SG running and took it for a test drive. A couple of things have come up. First, the tach intermittently jumps around. Second, when the bike is warmed up, the idle RPM goes from about 1000 to 2000 plus.

I'm going to take a close look at the vacuum advance device tonight and check for broken wires. I've been into the timing cover before, and the wires looked good externally, but I can see that to really get to them, I'm going to have to remove the timing plate. I've read a couple of things on line and if I need to remove the centrifugal mechanism it looked pretty straight forward. Then I read the Clymer manual which says the 1980 has special torque bolts holding it in that have the heads sheared off when they're installed. None of the on-line things I've read mention these special bolts. Will it be necessary to remove these bolts to get to the wires? Can the wires be disconnected from any point for easier acceses?

This centrifugal device rotates forward with hand pressure, but I was planning on lubing it up as well because it doesn't just flip flop like some posts described.

[Bonz]

Welcome, jacked_72

Neither of what you have is an issue with the vac advance. Lubing the vac advance is a great thing to do though as most have not been done since they left the factory!

Tachs on these bikes get a "wobble", you can take it apart (pain) and lube in there with a shot of silicon spray. Search for "tach wobble" and you will find lots. My 80SG has the wobble too. Got it to go away with lubing but it came back.

The idle going up is a lean condition usually caused by an air leak somewhere, not much else, if anything, causes that.

Double check intake boots from airbox to carbs and carbs to engine first and foremost. Check your exhaust headers where they mount to the engine, make sure carb diaphragms don't have pinholes in them, that your fuel screws are correctly adjusted and sealing up with the little o-ring washer, and that your fuel screw tips are not broken. Probably missed something, but you get the idea!

[BA80]

The tach issue we pretty much ALL have to deal with unless you spend a small fortune to have it rebuilt.

The Idle issue sounds like a sync issue. Have the carbs been PROPERLY synced?

[Bonz]

Ah, BA80, wise thought on the sync. Now that you mention that, I had a hanging idle when installing carbs on another bike recently. Adapted ZZR 1200 carbs onto my ZRX 1200 (holy cow what a perormance difference, by the way...). Chased the dang lean idle thing around with all kinds of adjustments. Went up to CZ's place to use his sync sticks and the issue went away.

YES, check the sync as one of the first things you do.

[TopCatGr58]

Hey there,

Good info from the others. The tach is electronic, and gets it's signal from the ALT. Sometimes jumping can be just a poor connection, so cleaning the harness connectors can help, especially the ones behind the main fuse panel, the large white plugs, clean, dielectric grease....but if you find it partially melted, that will be a sign of extended corrosion and heat build up from the charging circuit/ALT.

There is an idle adjustment screw that you can turn out a bit once the bike is warmed up to lower the idle speed....if it will lower when turning that. IF it won't...then what was commented on about the carbs being out of sync or a vacuum leak may be applicable. But if the idle will come down with adjusting the screw, then it was just set too high when it was cold.

Just be aware that when warming up the bike while sitting, using the "CHOKE"/fuel enrichener lever will allow the bike to idle when cold, and then as the RPM rises as it warms up, folks then push the lever in 1 stop, and then when it rises again, then push it all the way in. And then it should be idling in the 1000 to 1200 range. However, most folks don't sit/wait for the bike to fully warm up, they start it on FULL CHOKE, finish getting gear on, and then just start riding, and push the choke lever in after a minute or so of riding.

Just some extra thoughts.

T.C.