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Old January 21st, 2016, 12:51 PM   #1
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Default vmax 600 hesitation backfire help

could use some outside input on something I have been beating my head against.

I have a 96 vmax xt 600. I got this in a partial trade. will probably sell it, but for now am using it and trying to get it running perfect. (make selling easier and feel better selling it in known working condition)

It had clutch issues when I got it. I put a lower mileage motor and clutch in it.

as part of the deal I have a complete harness/carbs/cdi box/coil etc.

OK, sled starts and runs and idles great. no problem. take off and ride it normal with a passenger (seat jack), put-sing along 30-40 mph no issues. everything is great road 80 miles like this. not one issue.

Now, we get back to the house. drop off the passenger. pull the seat jack off, and the guys go ride hard. we get to the trails and open them up. at about 50% throttle you lose power. I dont know that its a bog, but its like you lose one cylinder maybe. then it will back fire pretty descent. Now, if you ease out and back in to it you can avoid that bog or so it seems and then go WOT and it screams like a banshee and is surprisingly quick for such a heavy sled. if you gun it without easing in to it you can duplicate the issue at less throttle/lower speeds (the lack of power and backfiring).

What I have done... So I pulled the plugs one (let it stall holding throttle) and they were both wet/oily (not fouled). I put a different set of plugs in and duplicated the issue. I did a comp test on both cylinders 123 ish on both. Then I ran it and checked the intake seals/crank seals with carb cleaner. no idle changes etc. doesnt appear to be those. Carbs while I had the engine out were tore apart and soaked in carb cleaner and sprayed out then blown out with 130psi of air (air compressor).

one thing that hit me early was I have not checked the carb syncing (making sure throttle timing was the same) and ensuring AF screws set the same. Would that really cause this issue though?

I made sure belt alignment was good (wasnt when I first had the motor set, so had to reset it as it was trying to kill my belt). I cant imagine this would be a clutch or belt issue.

what do you guys think?
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Old January 21st, 2016, 03:14 PM   #2
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Sounds like you have hit all the basics. Carb sync usually shows up just off idle or at low speeds when you hammer it. It always helps to have them perfect but I don't think that's causing your problem.
It's an old sled...usually the clutch side crank seal gets replaced because of belt debris. I would pull the ignition side and replace that seal. You can do it with out splitting cases if you trim the seal a little to get it in and use yamabond to hold it.
I would verify timing while I was in there and if that doesn't fix it I would start swapping cdi, coils, etc..
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Old January 21st, 2016, 04:54 PM   #3
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air screws should be 1 and 1/8 turns out from lightly seated.
needles in number 3 position.
floats set 21 to 23 mm.

at least those were the stock settings.

turn the air screws all the way in, and then turn them back out to 1 and 1/8 out.
then take it for a ride. hammer it. see what happens.

its a place to start.
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Old January 21st, 2016, 05:02 PM   #4
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Bad power valve if it has them
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Old January 21st, 2016, 05:53 PM   #5
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No power valves.


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Old January 21st, 2016, 05:55 PM   #6
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Quote:
Originally Posted by joe_jeep View Post
air screws should be 1 and 1/8 turns out from lightly seated.

needles in number 3 position.

floats set 21 to 23 mm.



at least those were the stock settings.



turn the air screws all the way in, and then turn them back out to 1 and 1/8 out.

then take it for a ride. hammer it. see what happens.



its a place to start.

I will check the air screws first see where they are at. I have a second set of carbs. Everything is stock



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Old January 21st, 2016, 06:07 PM   #7
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if I remember correctly it is one throttle cable down to the carbs, then they are actuated by an arm the cable pulls on. should not need much adjustment!

try the screws.
if that dont do it, try the other carbs maybe?
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Old January 21st, 2016, 06:14 PM   #8
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I just remembered....its a Yamaha. Start by unplugging the throttle safety switch at the carbs and plugging the wires together. You can follow the wires from the throttle housing to the carbs..
They cause all kinds of problems. Just make sure the throttle works freely before each start up.
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Old January 21st, 2016, 07:06 PM   #9
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Quote:
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I just remembered....its a Yamaha. Start by unplugging the throttle safety switch at the carbs and plugging the wires together. You can follow the wires from the throttle housing to the carbs..
They cause all kinds of problems. Just make sure the throttle works freely before each start up.


lol. forgot to mention that I already did that. all to familiar with yamaha...

I will have another skidoo or two for next winter. super happy with skidoo's.
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Old January 21st, 2016, 07:07 PM   #10
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Quote:
Originally Posted by joe_jeep View Post
if I remember correctly it is one throttle cable down to the carbs, then they are actuated by an arm the cable pulls on. should not need much adjustment!

try the screws.
if that dont do it, try the other carbs maybe?
that is accurate. so I breifly peaked at the carbs (on the trailer ready to head to houghton lake tomorrow), and there is no air fuel screw on either side. of the carbs. gonna have to google that one.
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Old January 21st, 2016, 07:28 PM   #11
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not sure what jackass designed this carb, but the fuel adjustment is on the bottom of the carb facing down on the engine side of the bowl!?!?

WTF!?
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Old January 22nd, 2016, 09:40 AM   #12
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ya know.
I looked up specs for a 96 vmax 600.
but the listing said it had 38 mm flat slide mikunis.
now that I think of it, my yammi had kehin carbs?
so my specs are probably wrong.
is there a name or number on the carbs?
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Old January 22nd, 2016, 09:42 AM   #13
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Quote:
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Bad power valve if it has them
I love how you always have bad input on every topic.
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Old January 22nd, 2016, 09:45 AM   #14
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pretty sure they are mikuni.

I looked at my extra set last night and they are similar but the air/fuel set screw is in a sensible location that is adjustable when on the sled, and they were adjusted about 1.25 turns out. I set them back to that. after this trip I will probably tear those apart and clean them and install them and see how it runs. those carbs are the original carbs for the motor thats in it. I left the carbs that were on the sled on there to save some time. of disconnecting throttle cables and chokes etc...

that may have bit me in the ass.

It will just get babied this trip. if I want to ride hard I will have my wife swap me for the formula (its more fun anyway).
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Old January 23rd, 2016, 11:29 AM   #15
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Update. So playing around at the hill going up it the oil light came on. 200 miles to a tank of oil!? Could it be loading up with oil? Causing issues? The skidoo still has a full tank almost. Both started full. Klotz.


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Old January 23rd, 2016, 04:59 PM   #16
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that yamaha 600 will eat gas and oil compared to other sleds!
I never complain about using oil, thats why they dont blow up.
it will always be the first sled out of gas too!
when the gauge says empty, you are walking!
95 to 100 miles max on a tank of gas.
100 miles per quart is not abnormal.
iirc, it holds 2.5 qts, light comes on with bout a half quart left.
I know ive done 40 miles with the light on.

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Old January 24th, 2016, 09:14 AM   #17
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Quote:
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Update. So playing around at the hill going up it the oil light came on. 200 miles to a tank of oil!? Could it be loading up with oil? Causing issues? The skidoo still has a full tank almost. Both started full. Klotz.


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sounds like your using too much oil which will cause lean condition. check your pump and cable, may need simple adjustment. excessive oil consumption will also show up on your plugs.
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Old January 24th, 2016, 09:45 AM   #18
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sounds like your using too much oil which will cause lean condition. check your pump and cable, may need simple adjustment. excessive oil consumption will also show up on your plugs.
the oil cable starts pulling a fraction before the carb cable starts pulling. can be easily adjusted by tightening up the carbs cable. (has a tiny bit of slack).

I am not sure that is going to solve it though. I am going to try to disconnect the oil pump cable and unhook the oil supply and premix (the sled is on E right now).

That should be the easiest quickest way to know if thats the issue. I will mix a gallon or two of 50:1 and see how it runs. I have a pump on the old motor. I dont know the condition of that one though either.
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