Check your plug and piston first.
The plug may have a broken or worn insulator if detonation was severe enough.
The piston dome can also tell you what the engine is doing...but you need to have the pipe removed to view the top of the dome with a small mirror (it may be a different story with the 85-86 engines, I've never had a chance to look).
Sometimes you can shine a light into the exhaust port and look through the spark plug hole to see the dome, or you can use a boroscope with the plug hole too.
This is a decent site with pictures to help you read your piston:
http://www.theultralightplace.com/pistons.htmOther than that, it's very difficult to tell if it's detonating just by listening to the engine.
Mild detonation may occur and the only symptoms will be the eventual elongation of the case where the bearings are held, requiring case inserts.
You might go through several top ends before the detonation wears away at the case if it's not serious.
Increasing levels of detonation cause increasing damage....piston destroyed, head studs pulled out of cylinder, broken cylinder ears, crank/bearing damage, etc.
So, here's the deal.
If your engine is suffering from mild detonation, it will eventually require case inserts.
If you can't tell if it's actually detonating or not (no telltale signs on plug/piston), then your engine is slowly destroying itself without any indication.
Lots of guys think that these engines just eventually wear out cases because it's a weak link, but they could be having detonation issues that aren't being detected.
Seems like a losing battle, but the easiest way to fix the issue is to simply run fuel with a safe octane rating, that way you KNOW that the engine won't detonate unless there's something physically wrong with the engine (or jetting is way too lean).
It may cost more for fuel, but balance that out with the cost of case inserts and other damage that could possibly occur with detonation.
What's a safe octane?
It depends on a lot of things.
An all-out 500 needs a lot of octane, which is why they're often running alky.
Dune ported 500's (mid level) need race fuel, preferably 110 or better.
Stock/mild builds can run 100 octane and still be safe if the head geometry is corrected.
250's can't produce a whole lot of power without tearing up the transmission/clutch, but 105-110 is a pretty safe bet for running up to that limit.
Mid-level 250's (nothing wild, but somewhere between), 100 octane minimum.
A mild or stock 87+ 250 can probably get away with premium pump fuel, and if throttle is rarely used and jetting is fat then the prospect is even greater.
An 85-86 250 is the least likely to detonate, as the power output is even less than the 87+
Keep in mind that you can still have really high compression in a mild build, or lower compression in a higher HP build, but generally speaking compression (and effective compression levels) increase when you're trying to get more power out of an engine.
The stuff above is just a generalization to help you figure out how much octane you'll want to run.
So, you've got an 85-86 engine that could theoretically run pump gas without detonation, but that doesn't mean you should.
As stated before, the problem with pump fuels is their irregularity.
The ethanol they add to the gasoline changes the stoichiometric ratio of the end product.
So if you jet with pure 93 octane gasoline right to the ragged edge of overheating and detonation, then when you run 93 octane fuel with 10% ethanol you WILL overheat and/or detonate.
That's not something I'd like to take into consideration when I ride my quads...is this fuel the same as the last batch, or am I going to melt a piston today?
We take a lot of stuff for granted these days with regard to octane, since we hardly ever see a problem when we cram the cheapest stuff we can into our EFI cars and trucks.
EFI systems monitor engine detonation and adjust parameters to reduce or eliminate the detonation...the engine loses some peak HP, but it's normally not enough for us to detect.
Since the car ran fine the last time 87 was pumped in, it should run good this time too, right?
And if my car can run 87 octane and make 200HP, then why can't my 30HP quad run it too?
Simple, non-EFI engines cannot adjust engine parameters to alleviate detonation problems, so they have detonation problems.
Our quads are even worse off, since the head geometry is a rather poor effort that is often the CAUSE of detonation.
It can be fixed, but detonation can still occur.
I'm mostly going off what I know about the 87+ engines, but a lot of what holds true for them also holds true for the 85-86.
If anyone else can think of something to add or amend the above info, speak up!