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Old 12-22-2007, 10:04 AM   #1 (permalink)
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BACKFIRE! Need Advice!

Hey guys. If anyone is able to help me out here, I'd greatly appreciate it, so thanks in advance and sorry this is so long.

I've been snowmobiling off and on since I was 8 years old....so I've been dong this for 20 years now. I build and race stockcars so I have a sufficient mechanical knowledge, but obviously 2-cycles are a different breed. I've always done all my own work to my sleds.

Here's the story...

I recently purchased a 1997 Yamaha VMax SX 700 Triple for $1100 (I know, what a deal! Guy didn't know what he had, I almost felt guilty!) Sled has 2300 miles on it, stored indoors, new track, etc... It's hardly been ridden the last couple years and has only had 380 miles put on it since 2001 according to the title. It has REALLY old gas in it, and it was never prepped for summer storage the last couple years. Sled starts up nice but occasionally has a dead spot in the throttle when you ride it.

Here's the deal. I know I need to go through the carbs and do the normal maintenance stuff to make sure it's all up to snuff. However, this thing has done some things I've never experienced a snowmobile do before. AKA, BACKFIRE occassionally when it's cold and you give it gas. It also won't idle consistently...it revs up, slows down, revs up, slows down...all on it's own!

I've never owned a triple before, but this thing sounds like it has a grumbly kinda knock coming from the engine when it's idling. Strange thing is, the sound doesn't really speed up when you rev it up and I can't hear anything necessarily coming from inside the engine when you put a screwdriver on it and listen. Could it be exhaust noise?

Compression tests out OK. I haven't put any new gas in the tank yet...still running it on 3 year old fuel. Old owner said that this sled had a tendency to foul spark plugs if he didn't run it hard. The sled came from upper Wisconsin and he thought maybe here in Iowa, with the warmer temperatures that it needed to be jetted down. It smells rich to me compared to my other sleds, FWIW.

My question is....do you guys think this sled is suffering from a bad case of skanky carbs? I'm hoping that carburetion is the only problem, but should I be looking for anything else?


Again, thanks in advance. I appreciate any and all opinions.


Tyler, IA
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Old 12-22-2007, 10:08 AM   #2 (permalink)
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You're on the right track!! First thing to do is get that old gas out of there!! Carbs will need a thorough cleaning....sounds like some plugged/gummed up passages.
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Old 12-22-2007, 10:15 AM   #3 (permalink)
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I agree with irs you need to get into the carbs and give them a good cleaning.
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Old 01-09-2008, 06:40 PM   #4 (permalink)
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just a little advice, when you go to pull the carbs do yourself a favor and first remove the top of the airbox then remove the bottom, the oil injection cable runs over the top of the air box and makes it a real pain in the butt unless you take the top off first(still a little tight but a LOT easier). Just trying to help somone else learn from my mistake. Same thing when you put it back together as you can now reach into the airbox to push the rubber boots back onto the carb flanges.
Good luck!
Oh yea make sure you pull the choke plungers too, mine had just a tiny bit of corrosion on the rubber seals in end of plungers and in choke bores, Sled backfired when cold just like yours, and used rediculius amonts of fuel .60-70 miles on a tank
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Old 01-14-2008, 09:57 AM   #5 (permalink)
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Thanks to everyone for their advice.

I went through the carbs and got them all cleaned out. Went through the entire sled actually. Since then, I've made some jetting adjustments because the old girl kept wanting to foul a plug or two every 100 miles. I also put a hotter spark plug in it which made an incredible difference. Apparently, this sled was jetted for running in much colder climates, according to the previous owner.

It runs like a raped ape now. I even raced my buddy who has a 2005 Arctic Cat, 700 twin EFI. Blew his doors off.

It's nice having an extra carb and cylinder!
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Old 12-15-2008, 08:45 PM   #6 (permalink)
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what did you use for a hotter plug??? because i am having the same problem with my 99 700 triple....right now i am running NGK. br9es plugs in it...i might have to try and run hotter plugs..to see if it helps my problem as much as it helped yours....

thanks
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Old 12-15-2008, 08:47 PM   #7 (permalink)
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typically results from various malfunctions related to the air to fuel ratio. Usually, backfiring occurs in carbureted engines that are running lean where the air fuel mixture has insufficient fuel. ("Running lean" is typically a sign of mal-adjusted carburetors or fuel injection where there is not enough fuel for the amount of air). Afterfire occurs in engines that have an emission system malfunction (air injection system diverter valve), exhaust leak or unburnt fuel in an exhaust system. When a driver shifts up and lets off the accelerator, the engine has a moment of running rich or with insufficient oxygen. This causes an incomplete burn which causes the fumes to explode in the exhaust system. The leak itself is the most dangerous aspect. Without it, the mixture would cool enough not to explode. A fuel injected engine may backfire if an intake leak is present (causing the engine to run lean), or a fuel injection component such as an air-flow sensor is defective.
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Old 12-15-2008, 08:54 PM   #8 (permalink)
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Quote:
Originally Posted by TheIntimidator View Post
Thanks to everyone for their advice.

I went through the carbs and got them all cleaned out. Went through the entire sled actually. Since then, I've made some jetting adjustments because the old girl kept wanting to foul a plug or two every 100 miles.
you dont let the sled just sit and run for periods of time do you??
thats a major cause in fouling plugs out.
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