Just an FYI, the Ds650 does require Premium fuel in stock form (93 or 94 Octane). Detonation from a lower quality/lower octane fuel could indeed bend the spark plug electrodes - especially when the engine gets hot from running in the dunes. If the prior owner increased compression - via a high compression piston, then the need for high octane fuel is even more critical! Check your owner's manual for the plug specs and buy a new set (Don't buy the same plugs that you removed - without checking to see if they are the correct model and temperature range).

Also, be sure to check the dome of your piston for a hole - which can and does happen with bad cases of detonation.

 

I do, and always have, run 91 octane in all of my bikes (the DS manual calls for 91 ((R + M)/2) or higher octane). I do not think the installed piston is especially high-compression although planing the head can increase compression due to decreased cylinder displacement at TDC.

I can try mixing with 100LL Av gas - I run a 100-91 octane mix in my buggy and always have some on hand, however, wouldn't low-octane fuel cause pre-detonation in high compression situations which would manifest itself as pinging? I have not heard any pinging or any other signs of pre-detonation.

I will be checking the piston to make sure all is well in there, so thanks for the tip. At this point, I am ready to try just about anything.

I am still a little worried about some dents in the ground electrode on the inner plug, these are irregular and look like the plug is being hit, or was hit before it was installed.

 

Despite what the manual says a stock DS will run better on plain ol 87 octane.

 

I installed new plugs today and the bike started right up, sounds good. I used plugs that were the same type and style but went with ones that had a shorter thread section so they could not get hit again.

I will be taking it out this weekend to test it, I hope everything is OK.

 

I would think very hard about that decision - by installing a shorter plug, you are leaving a certain amount of threads in the cylinder head exposed to the combustion process - these sharp areas are detonation prone... it may not cause issue at 1st - but over time, carbon will collect on these edges & will eventually begin to glow.

Unlike in a car - trying to hear engine ping in an atv is quite difficult.

I am still firm on the notion of fuel level being your original problem.

Old - low octane fuel, coupled with running lean due to decreasing fuel levels will quite easily bend the electrodes.

Even a steep hill with proper float settings can cause a lean condition as the fuel is being consumed at a very quick rate while the majority of the fuel is at the back of the bowl - potentially uncovering the emulsion tube.

Back in the day - I could ride a very slow (1mph) 1st gear wheelie - just balancing the front end in the air, and after a few seconds the bike would cough & die from fuel starvation. While your hill probably won't have the bike in the same atitude - keep in mind my example was while only using the pilot jet. Lower the angle of the bike, but increase fuel usage via the main jet & the same issue will apply.

Lastly, while I agree the clear fuel line/float height test is OK ... it needs to by done more critically - a couple of millimeters can/will make a difference - I suggest removing the bowl again & performing the manufacturers suggested float height test/measurement.

 

Well said Forcefed.

 

So on this problem I actually ended up consulting a Can-Am mechanic who used to be a dealer. The sharp thread issue is one that many DS's deal with when they have the OEM specified plug installed - it is just that the sharp edges are on the protruding plug and not in the head recess. This is especially true after the head has been machined. My head has been machined.

I am pretty sure the 1/2 inch plugs in my case are not exposing any head-side threads, and even if they are, what is the alternative? With the way my 3/4 inch reach plugs were installed, they did not protrude into the cylinder at their full length anyway (not tightened to spec) and even at this minimal insertion the 3/4 inch plugs were definitely getting hit by the piston.

So you are saying that I should stay with the same length plugs when they are being whacked by the piston, just to avoid carbon build up? A spark plug thread chaser once a year can solve a carbon problem in an instant. I did think about indexing washers, but then I calculated that a 3/4 inch reach plug just put in finger tight (no crush on the washers) compared with a 1/2 reach plug tightened to spec only produces .15 of an inch difference in overall plug depth. I can live with .15 inch difference as long as it keeps my plugs from getting whacked by the piston.

I am 99.999% positive this is not a fuel problem. Earlier in this thread I wrote about swapping the carbs between my two DS's. Both carbs worked admirably on both machines. I test-rode them pretty hard. The swap was done after thoroughly cleaning the tank, petcock, and carburector AND replacing the fuel line and making certain the breather tube is totally unobstructed. Fuel flows out of the fuel line like crazy.

I never run old or low-octane fuel, because I buy gas on the way to the dunes, and I always run 91 octane in my machines. Except my buggy which gets a 96 octane AV-gas mix.

I did think about the carburetor angle when climbing issue but what is the alternative there? The float level is already over spec as far as I can tell. So if the angle is causing the main to suck air - I guess I need a better carb. I find it hard to believe that the OEM DS carb can't handle hills.

Anyway - I am testing both bikes out tomorrow, I will post the results. I will also pull the new (shorter) plugs and let you know how they look and how the plug threads look in the head.

 

dude this whole situation has gone so far I don't even know where to begin..

First off, back to beginning talking about float level, there is only ONE way to measure float level - you remove the bowl and measure from the float to surface where bowl mates with a ruler (carb upside down in hand, angled such that the needle valve is closed BUT the tiny spring between it and the float tab is NOT compressed) and compare the result to the spec. Bend tab to correct. A couple mm off is enough to cause problems, either starvation if its off one way or flooding if its off the other. Clear tubes and such are not gonna provide corretc info. Swapping carbs between bikes may not provide an answer due to one motor being built different than the other.

Plugs being smashed? I have a 14:1 piston with a dome so high it forms a sharp point on top and it does not hit the plugs! There is so much clearance with a stock piston there that I cannot possibly imagine how your plugs could get smashed, providing they are the correct DR8EA plugs and not something longer, whether the head was shaved a few thousandths or not. IF the piston truly is hitting the plugs, then you must have a 14:1 because it is the only one with a tall enough dome to do it if the plug is incorrectly installed by bottoming out the cush washer. And IF you have anything over 11.5:1 then it is being destroyed by pump gas. 93 minimum octane for 11.5, I MUST run 110 with my 14:1. WTF man, if somebody mods a motor, shouldn't they tell the guy buying it??!!?! Somebody needs their *** beat.

 

Couldnt agree more Hightower - there is no way the piston is hitting the plug.

"So on this problem I actually ended up consulting a Can-Am mechanic who used to be a dealer. The sharp thread issue is one that many DS's deal with when they have the OEM specified plug installed - it is just that the sharp edges are on the protruding plug and not in the head recess. This is especially true after the head has been machined. My head has been machined."

You got some bad info from your can-am tech ...
I have held countless heads, in my hand, with the plugs installed, looking at this very scenario. It DOES NOT exsist on factory plugs, they stop flush. & machining the head WILL NOT make this more prone as you are removing material from the mating surface of the head, not the thread area ??

 

How did you confirm the head was milled ?

"I am pretty sure the 1/2 inch plugs in my case are not exposing any head-side threads, and even if they are, what is the alternative? With the way my 3/4 inch reach plugs were installed, they did not protrude into the cylinder at their full length anyway (not tightened to spec) and even at this minimal insertion the 3/4 inch plugs were definitely getting hit by the piston."

If that is true, & they dont protrude past the threads in the head - but is still getting hit, then the piston would be smacking the head too - huh ?