engine knock low end

[Bonz]

Hey motoman,

Good ideas, all!

Is the vacuum signal constant and even, coming off the carb? With a cylinder drawing fuel/air through the carb and the piston rising/falling, wouldn't that naturally lead to pulsing? i.e. pulsing in the lower rpm ranges would be more pervalent and par for the course.

Wouldn't a pinhole in the diaphragm pulse at all rpm? It is dead smooth in rotation for the final amount of travel to full rotation.

I need to look at the SG while running and the timing cover off. I never paid attention to pulsing or not on that bike, so it will give me a good comparison of the amount of pulsing and go from there.

The 80G runs as well as the 80SG in all areas of throttle response, roll-on, WOT, et al. It even averages 2-4 more mpg than the SG on the whole.

Ok, I'll keep you all posted over the weekend.

[BA80]

The vacuum is never steady when it's run off one cylinder as these are but much smoother at higher RPM. With the vacuum advance pod hooked to the carb there is no vacuum at idle though and is somewhat restricted by the orifice in the carb throat.

[Bonz]

Besides the random "knock" I get from the advance unit, the harmonic "tinging" that is being generated by the timing rotor, seemingly just from spinning, is annoying as heck too.

So, say I swap over the vacuum can from the SG and the "knock" goes away... Where do I find another vacuum can?

[motoman]

Besides the random "knock" I get from the advance unit, the harmonic "tinging" that is being generated by the timing rotor, seemingly just from spinning, is annoying as heck too.

So, say I swap over the vacuum can from the SG and the "knock" goes away... Where do I find another vacuum can?

Those vacuum cans WERE year specific based on timing and advance specs. Vacuum Hg's required varied advance and when, in what rpm range. For all GP though, if yours isn't functioning properly, any year vacuum can will work. Since the SG just sits to keep mileage low, looks like you've found one........

[Bonz]

Thanks moto!

[3Phase]

Howard,

My '80G does it too, sometimes, sort of... the ringing sound is probably the mechanical advance weights and/or the reluctor itself vibrating on its shaft.

Take off the Timing Plate and shoot some heavy oil (or take the timing assembly apart and use a few light dabs of high-temp grease) on the mechanical advance weights, springs, and on the reluctor shaft and its bushing. Put it all back together and fire it up to see if that cut down or eliminated the ringing.

If that takes care of it then it'll only be a temporary fix until the oil or grease goes away but it will at least let you know if that's what's causing the noise.


.

[Bonz]

Thanks Scoot.

[motoman]

The vacuum is never steady when it's run off one cylinder as these are but much smoother at higher RPM. With the vacuum advance pod hooked to the carb there is no vacuum at idle though and is somewhat restricted by the orifice in the carb throat.

Supposed to be restricted to eliminate piston pulses. Is no vacuum at idle cause it's a port vacuum not a manifold vacuum used for vacuum advance.......same in the cage world.

[Bonz]

I'd hate to see how much it pulsed if it didn't have the internally restricted carb bung to plug onto! It'd proabably clack around the same as hooking it to the intake boot.

Would a small jet in the vacuum line work to restrict the pulses even more, and still function like it needs? Read about that in some other threads.

[motoman]

I'd hate to see how much it pulsed if it didn't have the internally restricted carb bung to plug onto! It'd proabably clack around the same as hooking it to the intake boot.

Would a small jet in the vacuum line work to restrict the pulses even more, and still function like it needs? Read about that in some other threads.

Won't hurt. Vacuum won't change. Air volume is constant(or lack of). It might break up the pulses more having a different location than the restricted #2 carb orfice. A small Mikuni or Kiehn jet oughta fit I.D. of vacuum hose down at the can.

[skids]

I used to measure air quantities in main mine fans via air pressures. An old guy show me a trick where an empty jug acted as a dampener, otherwise the vacuum gage (magnahelic) vibrated a lot. It got me thinking that any kind of an air chamber with a smaller oriface could do the trick for dampening a jumping xs11 advance unit. The vacuum hose with a small jet in-line could do the trick or some sort of small filter canister inline with a hole-in-plate installed. Just thinking out loud. My bikes seem to be OK without any contraptions!

[Bonz]

Gents, I feel like I might just have it... Finally had some time tonight to get into the vac adv unit and it now rotates smoothly with engine vacuum, no pulsing. Too late to go for a ride so have to wait and ride it to work in the morning.

Took off the vac adv canister itself and it is sealing fine, no "pinhole" leak. Cleaned it up, put some silicone spray lube on the shaft and it seemed to move a bit easier now.

With the vac canister off, I removed the advancer assembly itself and cleaned it with contact cleaner and sprayed it with the same silicon lube, and that rotates much better now, even though I thought it was rotating easily before cleaning/lubing. Without the vac canister hooked up, the pickup coil rotates with little or no help from me. Yes, very little resistance, if any, to rotation.

Put it back together, started it up and rotation is smooth and steady from beginning to end as I apply light throttle, comes back smoothly as vacuum lessens when I give it gas or roll back to idle. No pulsing at all.

Man, I sure feel hopeful but only a ride will tell.

Can't say enough how those systems are old and not as smooth as they could be despite thinking they are.

[Bonz]

Ride report says... Didn't get rid of the thocking/knocking from the timing cover area, however the smoothness of throttle response and driveability is better than before, and it was good before.

There was a slight surge when I'd roll out from roll on at higher speed, that is gone from initial impressions.

FWIW, idle speed picked up about 200 rpm, have to adjust that back down.

I've been getting 37-41 mph on the whole, interested to see if it varies much from that over these first few tanks.

[WMarshy]

Gents, I feel like I might just have it... Finally had some time tonight to get into the vac adv unit and it now rotates smoothly with engine vacuum, no pulsing. Too late to go for a ride so have to wait and ride it to work in the morning.

Took off the vac adv canister itself and it is sealing fine, no "pinhole" leak. Cleaned it up, put some silicone spray lube on the shaft and it seemed to move a bit easier now.

With the vac canister off, I removed the advancer assembly itself and cleaned it with contact cleaner and sprayed it with the same silicon lube, and that rotates much better now, even though I thought it was rotating easily before cleaning/lubing. Without the vac canister hooked up, the pickup coil rotates with little or no help from me. Yes, very little resistance, if any, to rotation.

Put it back together, started it up and rotation is smooth and steady from beginning to end as I apply light throttle, comes back smoothly as vacuum lessens when I give it gas or roll back to idle. No pulsing at all.

Man, I sure feel hopeful but only a ride will tell.

Can't say enough how those systems are old and not as smooth as they could be despite thinking they are.

Sounds like this would be a great maintenance tip. I'm sure a lot of people could benefit from it. I will have to put it on the to-do list for this winter, take some pictures as I go if I can manage to find time.

[Bonz]

I am going to do the same thing to my 80SG. I cleaned it and lubed it when I got the bike 5 years ago, but I am sure a freshening up will not hurt.