Preigniting when switching off bike???

[street bob]

My wifes bike has a couple times now given her a preignition "detonation" when switching off the bike.
It has the High flow air cleaner and K&N air cleaner. Also, cycle shack exhaust.
Some of you might remember that you all gave me pointers on
getting the carb jetted correctly and I did that. That was last fall.
The bike seems to run fine.
When I rode it, it seemed to me that it just didn't have that "snap" that a 1200 sporty should have when twisting the throddle, especially having a carb.
The timing has never been messed with.
Its a 2002 1200 custom.
Like I say, the bike seems to run fine, a little groggy on the throddle, and now a little pre-ingition.
Its only done it twice... but hey!! The temps were only in the upper 70's, so heat wasn't the issue.
I have tuned my older sporty many times.... a couple decades ago. (1000 cc)
I'm thinking its a bit of a timing issue.... any thoughts????

I just pulled the plugs... 6R12.. white, definitely running lean. I'm sure I'm running a 42 slow speed jet.... thoughts?????????

[shovelmike]

This sounds like a dieseling issue, otherwise known as run-on. It is caused by carbon deposits in the combustion chamber getting hot enough to ignite the mixture. I'm not a fan of additives, however in this case a little Marvel Mystery Oil in the gas might help. Others might have their own favorite additives to recommend.

[milindh]

Bottom line if your plugs are white you are still running lean and probably hot because of it. The outside temp doesn't have to be high to get dieseling although you'll get the dieseling more pronounced if it's hot outside because your engine will be running even hotter. I'll bet if you get the lean condition fixed the dieseling will stop. It would be good if you did plug checks at the various throttle settings. You might be lean in just one range. Not real likely but possible.

Sometimes you'll get dieseling when your engine is running hot and maybe even half a spark plug thread is exposed to combustion heat. That one little part of a thread will get heated up to the point that it acts like a glow plug. I guess the same could be true with a carbon build up like Mike said. Any sharp edge can become a source of ignition. Like I said, if you fix the lean conditon all this will likely become moot. I'd check the timing too. Might need more advance. Too much advance and you get pre-ignition but too far in the other direction will cause heat build up although it has to be pretty far retarded.

Good Luck

[shovelmike]

Milindh's advice is very good, a richer mixture could well be the cure, thanks milindh.I get the dieseling occasionally with the Shovelhead even with correct fuel/ air mixture the swill they sell us as gasoline is much to blame, those hemi chambers don't like it much.

[aceharleywrench]

If it happens when turning off the switch wouldn't that be considered "post ignition" ?

[street bob]

Ace........ I guess it would!
I'm going to richen up the mixture before I go messing with everything else.
Thanks gents... I'll keep ya'll posted as time wears on.

[frisco-rigid]

Hi Street

When you checked your plugs and found them to be white/lean, I bet it was after idling in to the garage huh..
That means that, at the very least, your idle Mix is prob tooo lean and that will contribute to pre-ignition/dieseling alot..!

As the temp rises little probs that don't show in cool weather will start to show-- as Mil touched on..

If you think it's a wee groggy on the throttle I would do a high-spd plug test and a low-spd test on the way home and compare with the plug colors you got after you jetted it last Fall..
I would also spray the connections looking for an intake leak cuz I'm guessing this is a kinda new symptom..

An example,,, I run a wee rich (tan or medium gray to medium brown) in All RPM ranges and alway have with whatever carbs I am using at the moment..
Each year here in the Phoenix Valley,, right about now,, my motors will sometimes Diesel at shut-off.. It is from the temp change..

Because I know for certain there are NO probs with timing, leaks, A/F, etc. I simply reach down and dial down the idle SPEED Immediately before shut-down, that just does the trick..
Soooo,, After you eliminate any Poss probs, that method is as simple as it gets as the temp rises..

[street bob]

Thanks frisco....

[wrongpaw]

How high do you have your Idle speed set at?
Too high a setting can cause this.

[milindh]

Frisco's last little statement brought out something about Mikuni's that non-users might not be aware of. They have a little knob/rubber tube extension for the idle mixture that just hangs from the carb. On my bike that is about 3" long and I can easily just reach down there anytime and hand turn that adjustment just before I hit the kill switch. That lowers the idle and the dieseling doesn't happen. No screwdriver needed.

This is such a cool feature of the Mikunis! First time I saw it I thought, "Huh...?" but when I had a little dieseling going on with my bike during a hot spell last summer I did just as Frisco said and lowered the idle speed. Voila! Problem went away and I could easily turn the idle mix back to its previous setting.

When you see a feature like this on a product you have to really appreciate the forethought of the designers. Other carbs can be pretty good street carbs for their app if tuned right in my opinion but I know of no other carbs other than the Miks that have this cool little feature. That and the slick tuning manuals and ease of adjustment make the Mikunis my faves hands down. I love 'em.