Detonation

[tarzan]

My bike has bad spark knock from 3k on up almost to 5k.I read a service bulletine in an old post that the 80 and 81's had a problem like this.Does anyone else have it?And has anyone figured out how to get rid of it?

[randy]

My '81 Special had it when I got it in '88. It was only noticeable for me during very hot weather. I raised the needles by placing small washers under the clip. Ping went away.

[psycoreefer]

I read in another post that the "factory" timing needle can sometimes be off by 5%, possibly more if a PO has messed with stuff under the cover. Has your bike been modified in any way that could lean the mixture out... intake, exhaust ect?
I'd:
1. Check the plugs to make sure your not running really lean, causing the ping your hearing. If the plugs look good move on...

2. Check the timing with a timing light to see if it is on the factory marks. If it is move on....

3. Check to make sure the factory pointer actually lines up at TDC. I'm not sure how to do this as I've never done it but I hear it involves a degree wheel and some other tools I don't own. If you can't do that....

4. Possibly just retard the timing a few degrees and see if it goes away, if it does I wouldn't worry about it, I'd ride until I had the time to check it properly. Timing to late will only cost you some power and mileage. Timing to early can cost you an engine.

[olebiker]

Originally posted by psycoreefer


3. Check to make sure the factory pointer actually lines up at TDC. I'm not sure how to do this as I've never done it but I hear it involves a degree wheel and some other tools I don't own. If you can't do that....

I have used a pencil before. Bring the engine to indicated TDC put pencil in Sparkplug hole eraser first. Turn crank very small amount in both directions while holding the pencil straight. Gives you a pretty close idea if it is off.

[tarzan]

The bike is stock with a k&n air filter.I did drill out the tamper proof bolts holding the timing plate last week.I then put a timing light to it and adjusted it.It may have been a degree off according to the pointer.I will have to check the pointer to see if it is accurate.I dont have a degree wheel ,but I think a friend might.I should also check the plugs.Maybe it is running lean.I took it on a poker run through Idaho last weekend and put about 250 miles on it.It got about 44 miles per gallon.I thought that was odd since I have never seen anyone claim that kind of mileage from an XS.

[psycoreefer]

Were you having the same problem before you adjusted the timing?

[tarzan]

Yes psycoreefer,I adjusted the timing because of the detonation.
I probably should have tried retarding it though it sounds like.I would think I could get rid of the advance.Alot of bikes dont use this system.It has a mecanical and a vacuum advance.Why?

[mro]

44 miles per gallon

I've got that kind of mileage, but only cruising 45 to 55 MPH.

On an 80G I tried playing with the float levels to alleviate the knock. Was able to diminish it some but performance suffered.
Was going to play with the timing but never got around to it.


mro

[81xsproject]

Try going to mid-grade fuel. Would it be possible to have enough carbon build-up to raise compression enough to cause the knock...?

[TopCatGr58]

Originally posted by tarzan
I probably should have tried retarding it though it sounds like.I would think I could get rid of the advance.Alot of bikes dont use this system.It has a mecanical and a vacuum advance.Why?

Hey Tarzan,

Take a look at This Article in the Articles section, Paragraph 13 specifically for your explanation!

You could get rid of the vacuum advance, but NOT the Cent. Adv.!
T.C.

[tarzan]

Well I have been running premium gas in it.It could I suppose have some carbon buildup.But I do get on it some.So it should burn it out.

[olebiker]

I have just come to learn that high octane fuel in these bikes will build up carbon. I had always thought the reverse was true.

[tarzan]

Wow really,
I have never heard that before,but I have heard people say cars they have run for long periods of time on premium wont run well on regular.I guess I have had that happen with a car I had even.So that does sound reasonable.Maybe I should go to regular and run it hard for a while.That would be a gratifiing solution if it works.

[olebiker]

I dunno I just scraped out a pretty thick layer that was almost as hard as the piston. I have no idea if that would burn off.

[psycoreefer]

I don't think I'd want to run it "hard" while it was detonating like you say. Thats real hard on the motor, unless you really don't care if you have to rebuild it. You could try unhooking the vacuum advance and see if that helps at all, your mileage would suffer some but it might give you the chance to run it with regular for a while without having to worry about damaging things. You could also try Seafoam into the vacuum ports and see if that cleans things up.