Ive got a 1998 MXZ with a highly modified engine. At wide open throttle engine sputters and looses a little power, when you let off intermidently backfires from exhaust? At part to half throttle it pulls real hard like it should. At first I thought it was a lean sputter but then I checked my max egts on my digitron and they were ok. Im wondering if someone can give me an idea of what is going on with my sled? I bought from a guy that snow dragged it. I think the engine work done was done by specialty sleds and mots machining, previous owner has had no problems with it. It has 15:1 comp thanks to a machined head, milled out carbs with bobs, k+N pods, custom built pipes, stubby rotary cover, Im running pure sunoco race fuel, main jets are 480 in both, needles in middle slot, my max egts are 1124,1127, which are perfect if you ask me. Although my cylinder head temp is 127 f which seems a little low, I checked ignition timing-OK, I checked secondary clutch setting for possible over revving-OK, Took carbs apart and inspected-OK, verified float hight-OK, Has anyone seen a drivability issue due to an engine running too cold?, Could a thermostat cause this concern? What should my cylinder head temp be at operating temp?, Any suggestions would be appreciated.
Modified skidoo 670 sputter at wide open throttle
- Thread starter modz
- Start date
- Staff
- #2
- Joined
- Oct 21, 2006
- Messages
- 48,007
- Reaction score
- 32,148
- Location
- Edmonton/Sherwood Park
- Website
- www.bumpertobumper.ca
you max egt's are actually a little low..... I'd be running 1250 max, after really hard long pulls. This almost sounds like the rotary valve timing might be out........has this always happened, since you've owned it? Did the previous owner say it was happening too? How fresh is the race fuel you're using? You would probably be okay, 50/50 race and supreme fuel........... Too many unanswered questions? Have you talked to mots about it?
Id be looking at the electrical, what plugs are you running in it? Are the plug tops loose? Boots loose on the ends of the wires? Ric is right is right 1127 is low there is safely another 100 degrees left to go. That being said though how far from the exh side of the piston are your probes mounted??
It may seem simple but it should also be noted that a 670 will run like $h#t if you don't gap your plugs correctly.
Last edited:
And what do your plugs look like? If plugs look OK, EGT readings are fine then, the pipes probably like it hotter though. 480's could be rich depending on alt, what size are the carbs milled to?
- Thread starter
- #6
Hi, quys
Thanks for the feedback. I am running brand new NGK race wires and ends, brand new (gapped) B9EG NGK race plugs (nice and tan), the egt probes are mounted 2" from exhaust ports, thanx deano ill give you a call and ill update the thread.
Thanks for the feedback. I am running brand new NGK race wires and ends, brand new (gapped) B9EG NGK race plugs (nice and tan), the egt probes are mounted 2" from exhaust ports, thanx deano ill give you a call and ill update the thread.
- Thread starter
- #7
Oh ya i forgot to say i talked to the previous owner and the last time he raced it in alberta beach he said it ran real good, when I test drove it before I bought it in april on the sturgeon it was sputtering, too lean I thought. I could not check egts becaause the digitron was broken. He told me just to fatten up the needle which I did after I bought it, He had it set to the leanest slot. I installed a new digitron so i could verify. My first ride this season it was sputtering again in top end.
rsaint
Active VIP Member
I had the same symtoms on my 98 670 summit NOT a X and chased this same problem finally went and changed the coil and found the wire feeding the coil the wire was broke inside the rubber coating right where it made the 90 degree and clipped on the coil. I would check that both wires are good.
- Thread starter
- #9
I had just relocated my coil after i bought it, it was mounted on a makeshift bracket off of an engine mount, i built a new bracket and mounted it on the bulk head, i inspected wiring when that was done? Im still leaning towards a thermostat, 127 degrees is too low maybe? im going to put one in and give it a shot, if its not at operating temp you think you would get a sputter, incomplete burn?
team dirt
Active VIP Member
- Joined
- Oct 17, 2008
- Messages
- 2,078
- Reaction score
- 5,443
- Location
- brooks ab/seymour arm
- Website
- www.sledseymourarm.ca
127 is deffinetly low. u would want to be more in the 180F zone or even up to 200. It sounds like an ignition issue though to me, seen it lots on 670 engines. it does not have the jam at hi revs and load for complete burn and that is where the backfire is happening on reduced throttle. also low pyro temps are probobly due to some unburned fuel leaving the cylinder.
127 is just about perfect for a 2 stroke. The stock XP t-stats open at 108F and most others are around 100F. If you run it at 200, you won't have a motor left.
2 strokes and 4 strokes run at way different temps.
I would suspect electrical or that your fuel pump might be giving up the ghost and its not keeping up at WOT.
2 strokes and 4 strokes run at way different temps.
I would suspect electrical or that your fuel pump might be giving up the ghost and its not keeping up at WOT.
- Staff
- #12
- Joined
- Oct 21, 2006
- Messages
- 48,007
- Reaction score
- 32,148
- Location
- Edmonton/Sherwood Park
- Website
- www.bumpertobumper.ca
ya, i had that happen, when the fuel pump gave up.... Just went with a high volume fuel pump and problem solved....... I think nac.00 still has one in my old chassis, that he's parting out.
- Thread starter
- #13
So i got in touch with tom at mots machining and he said operating temp is 120-140. Im right in the middle. He wants me to put 520 or 540 main jets in it. He thinks it still too lean. He said not to trust the digatron. Ill give it a shot and see how it goes.
Pappillion
Active VIP Member
Thats good advice from a pro. Tom knows his chit!
you should be watching egt's, plugs ,and piston wash ,with the wash being the most important for tuneing.have seen things to lean and still good egt's.egt's are neccessary to keep watch ,but your set up should be determined by piston wash..and with straight race gas is hard to read,mix in a appropriate amount of 91 pump gas if applicable ,ask the engine builder about ratio,if you dont watch the wash it can be really easy to be off tune and think you are right..then as mentioned I would rebuid or replace fuel pump for precautionary measure if nothing ealse. sounds like your set up is right on the edge of needing a high flow pump as well.if its restricted or blocked at all your engine will starve at WOT....
- Thread starter
- #16
Hey Dave
With the mssg you left me on profile page. If the previous owner had 480 mains in and the sled ran and lived on 50/50 fuel and now you are at 620 mains granted the sputter is gone but seems like its running rich, I be would looking for air leaks. Carb boot cracked, base gasket blown out, crank seal gone?? Typically these failures will make one of the two cylinders hot not but not both, that being said rotary valve o ring pinched,case halves leaking. It seems like you have taken your jetting to the point at which it wants to compensate for an intake leak but now runs like a bucket. Does it have a stock rv cover or aftermarket??
Deano
MAD MOTORSPORTS
With the mssg you left me on profile page. If the previous owner had 480 mains in and the sled ran and lived on 50/50 fuel and now you are at 620 mains granted the sputter is gone but seems like its running rich, I be would looking for air leaks. Carb boot cracked, base gasket blown out, crank seal gone?? Typically these failures will make one of the two cylinders hot not but not both, that being said rotary valve o ring pinched,case halves leaking. It seems like you have taken your jetting to the point at which it wants to compensate for an intake leak but now runs like a bucket. Does it have a stock rv cover or aftermarket??
Deano
MAD MOTORSPORTS
- Thread starter
- #18
I know its been a long time since iv e posted and left feedback. i figured it out last year.. thanks for all your guys input though.. the problem was tooo much compression. i spoke with a few well known engine builders and we concluded that there was around 16:1 compression.. lol.. i put in thicker base gaskets which raised port timing and vwala kicked ass at the races. problem solved
Similar threads
