Detonation... very very annoying detonation

[1981XS]

Ok, first I must fall on my face and beg forgiveness for my extended hiatus... that being said, I have gotten the bike out, painted it to cover up the mismatched parts, installed my new led accent lighting (pimp, I know) and gotten everything running and riding nice again. Then it happened, the reminder of why I didnt get much riding done last year, at 3500 rpm there is a definite ping, so loud that while riding you will swear someone with a chain drive is passing you. Downshift or upshift and it is gone, I am thinking vacuum advance but not completely sure, anyone else encountered this? It is on my 1981 XS11 venturer and is only at 3500-3600 RPM

[Radioguylogs]

Possible causes for detonation:
(1) Spark too far advanced
(2) Engine too hot

How much paint did you put on the engine?

Could 3500 be a magic RPM that triggers a resonance of the cam chain or HiVo drive chain- and the chain is slapping against something?

[mack]

Info

we need more information. Try putting it on the center stand in neutral and taking it up to that RPM. Does it still do it? If so you can rule out the transmission and shifter. Try to get a buddy to work the throttle and you use a long wooden dowl or screw driver to your ear and try to locate the source (area) of the "ping". Pay attention around the CCT and the valve cover center rib.If the cam chain is stretched it can slap the valve cover and the original cct's are prone to issues from over tightening the shaft bolt.
Did it do it in all gears at that RPM? Is it only at that RPM?

[crazy steve]

The '81 bikes had 'semi-fully' electronic ignition, i.e. 'mechanical' ignition advance is done inside the TCI. Now, they still had two advance curves like the earlier bikes. You have 'mechanical' advance which is strictly controlled by engine RPM and is done electronically, and 'vacuum' advance, which is controlled by engine load (throttle opening/engine vacuum). Try disconnecting the vacuum line off the number two carb (plugging the hole at the carb) and see if the problem goes away. If it does, you have a problem in the vacuum canister or the advance linkage. The service manual shows the 'curve' at different vacuum signals, you'll need to check to see if the canister is giving the right advance. IIRC, Yamaha had issues with these on some '81s and had a 'service replacement' canister to address this problem.

Note that the vacuum line has to be connected to the carb, not the intake boot.

[skids]

Possible causes for detonation:
(1) Spark too far advanced
(2) Engine too hot

How much paint did you put on the engine?

Could 3500 be a magic RPM that triggers a resonance of the cam chain or HiVo drive chain- and the chain is slapping against something?

Wasn't there a thread where this was cured by inhibiting the pick-up coil movement with a small orifice inline with the hose? Or was that just to quiet down the clacking?

[IanDMacDonald]

Lol, I believe that orifice was a pilot jet. It solved the ping definitely, cuz the engine blew shortly after.

[TopCatGr58]

Wasn't there a thread where this was cured by inhibiting the pick-up coil movement with a small orifice inline with the hose? Or was that just to quiet down the clacking?

I think it was in regards to NON-XS11 carbs that someone was trying to use that didn't have a metered vacuum port...OR it was on some XS11 carbs that someone had drilled out the metered vac. adv. port opening so that it caused to much action.

Like Steve said, there is an official Yamaha bulletin regarding this issue, however the replacement is no longer available anyways.

The 81 static timing and plate is normally not adjustable....however the filled in timing slots are still visible, and one could use a DRILL or MILL BIT and ream out the filled in groove, so that you could adjust the static timing a bit. IIRC the static timing isn't as advanced as the earlier years, and folks have reported adjusting the timing a few degrees more advanced and get a little better throttle response in the lower rpm ranges.

I realize that you're talking about experiencing detonation/pre-ignition which is and indicator of too much timing advance to begin with....but aside from the "faulty" vac. adv., other factors may be contributing as well....the poor lower octane and ethanol fuels we have to use now for one. JAT.

T.C.

[ViperRon]

Not trying to lead you astray but are you sure it is an internal engine noise. These engines have a tendency to find a speed where it makes noise but there are several other things that do also. For me the horns would begin to vibrate at about 65 and hit the frame which made a terrible noise that sounded like engine I could not get the noise while running the bike on center stand and drove me nuts. I have known loose pipes or after market pipes to develop a vibration at certain speeds that cause a loud ringing that comes and goes and at times it can sound like a roar making you feel your crank bearings are gone. I only mention these because I do not want you working on the engine when it may possibly be something else vibrating or loose. One thing I did see a guy do to a bike one time was replace the springs on the mechanical advance with some different springs that did create a lot if issues. He had put weak ones on and had full advance just touching the throttle. You may try and take the hose off and plug it and run it to see if there is a change in the RPM range where it makes the noise and if so this my help ensure you are on the right track. Yes I know the bike will not run well but may help isolate the issue.

[IanDMacDonald]

. . . and had full advance just touching the throttle.

It's fun, everyone should try it