Detonation on 800 with twins

[Goinboardin]

I rebuilt an 04' 800 Vertical Edge this summer, bad rod bearing and PTO piston. It has the SLP race twins, and jugs from an 02' so no decompression holes, no porting, stock head. I've put 100 miles on it, no issues first 90.

Had it out on Sunday and when going down the trail the deto sensor would flash at me if between 6200-6600 RPM, but never below or above that. I heard it too, so I'm confident its not just a bad sensor. Never flashed in the deep. Plugs look about right: dark tan/brownish, even, and dry. Checking piston wash tomorrow to be sure.
I unload at 9k and only move up from there, been running 91 non oxygentated fuel. Timing is at 29.5* BTDC with TPS unplugged @ 3250 RPM (I'd like to retard it a little bit).

Could it be lean on the midrange (or maybe just crap gas)? It was on the cooler side that day.

Should I pick up a barrel of race gas?

Or just getting a little hot on the trail?

This makes me want to pick up a head meant for twins...

Any thoughts? Thanks for your input in advance.

 

[sled_guy]

The SLP twins put you close on DET with the stock head, but at 9k you should be ok.

I assume you are running the '04 needles and not the '02 needles is that right?

Sounds like just the normal lean spot in the midrange that has always been on those motors. You say you heard it so it isn't just exhaust spring vibration.

If you didn't see it for the first 90 miles then I would almost suspect a bad load of fuel, but that's the problem with being right on the edge, you can get bumped over it with a little poor quality fuel.

Try richening your needles one clip and see if it takes care of it without making it run like crap. If not, then you'll either have to go to the head change or start dumping a gallon or 2 of av gas/race fuel in.

sled_guy

 

[cascadesnowjunky]

I agree with sledguy probably just a little lean in that area, those sleds all ran lean there. Possibly a bit of bad gas put you over the edge. I ran my 03 edge 800 with twins for years with no deto. at 4000'-6000', I started to detonate after I had the engine fully ported by Jack. I then had to dump a couple gallons of sunoco 110 octane with it, I did run it on the edge for jetting though.

 

[Goinboardin]

Yeah I'm running the 04' needles. I went from #2 E-clip position to the #3 position and it cleared up the deto. Ran perfect on saturday that way, then on sunday it was a little warmer and it had a bit of a stumble mid range off trail but very minor. Besides that its rarely that warm until spring, so I'll likely be running this for the majority of the season.
Thanks for the tip guys!