98 VMAX XT Idle and Backfiring

mdkuni

mdkuni

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Apr 7, 2004
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Houghton Lake, MI
98 VMAX XT Idle and Backfiring **Update**

We have a 98 Vmax XT 600 and it was rebuilt last year. Ever since it was rebuilt the machine will rev on its own when it is could. The RPM's will fluctuate. As soon as she gets warm though this stops and she idles fine.
The second thing is on occasion when you are on the gas and then grab the brake pretty good she will backfire once.
Anyone have any suggestions on how to correct this? Thank you.
 
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I`m not saying it is, but it could be that the engine is sucking false air from somewhere. Was it a top end rebuild or did they remove the crank?.

False air can either come from the PTO side crank seal, the mag side crank seal, carbs not properly isntalled on the inlets, bad inlets. Try spraying around the mentioned places with start gas. If the engine idle changes while spraying, then you know where the problem is.

The wierd thing is that is goes away when the engine gets warm. Most materials expand when beeing heated, could it be and air leak that goes away when the materials expands or something?.

Have you checked your plugs?. Take them out and look at their color. Plugs can tell you alot.
 
kimoaj said:
I`m not saying it is, but it could be that the engine is sucking false air from somewhere. Was it a top end rebuild or did they remove the crank?.

False air can either come from the PTO side crank seal, the mag side crank seal, carbs not properly isntalled on the inlets, bad inlets. Try spraying around the mentioned places with start gas. If the engine idle changes while spraying, then you know where the problem is.

The wierd thing is that is goes away when the engine gets warm. Most materials expand when beeing heated, could it be and air leak that goes away when the materials expands or something?.

Have you checked your plugs?. Take them out and look at their color. Plugs can tell you alot.
Click to expand...

Honestly I do not remember for sure about the rebuild. I beleive it was just the top end though. I checked the plugs this weekend and attached a couple pics. I don't know if you can tell anything from the pics....
 

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Did the previous owner rebuild it or something?. How many miles does it have?.
It was kind of hard to read those plugs, but on the pic to the left it looks like there is some kind of "debris" that shouldnt be there, it also looks somewhat wet.

If your camera has a button named "Macro" or "Super macro" you should use it when taking close up pics, it makes the camera focus better on subjects that are near the lens.

Try spraying start gas on the places mentioned in the post above and take it from there.
 
Looks like a lean idle condition, but it could easily be idle adjustments, although an air leak can do it to. If it was sucking air, it would do it all the time though. Pull the carbs, and double check all adjustments. Good luck!
 
kimoaj said:
The wierd thing is that is goes away when the engine gets warm. Most materials expand when beeing heated, could it be and air leak that goes away when the materials expands or something?.
Click to expand...

Actually that isn't wierd at all.... "most" things expand when heated, some exceptions are water and clearances.... When something has a crack in it and then is heated, the material expands and closes up the crack, the crack does not get bigger, just a little trouble shooting advice, but yes check for air leaks at the carb boots and crank seal
 
1. Check wiring to your tors and to the throttle area.(loose/frayed connections)
2. let the motor warm up good 5-6 minutes and blip choke on/off to keep the motor running in the begining..no throttle....mine will backfire too if not warmed up proper.
3. when your on the gas, getting into it, excess fuel or the unburned portion will ignite in your muffler and pop/backfire....i see this from time to time.
4. your plugs look rich by those pictures..is it stock jetting??do you know the jetting?? going by those fuzzy photos.
5. i dropped my needles a half shim and it cleared up the mid-range...plugs should be cardboard brown.
....hope this may help....CP
 
Well here is the update. A buddy and I had to pull the pipes out to get to the starter as the wire from the battery came off of it. We removed the pipe at the exhaust ports on the motor and found that it was sealed with silicone sealant that was deteriorated! No gaskets. So to me this makes sense as to why she was idling rough, what do you guys think? Could this also of been the cause for the backfiring? Also, we found both of the engine mounting bolts were missing underneath the pipe! Good thing we pulled that pipe off.

The jetting has never been touched and it was a top end rebuild.
 
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Hmm mdkuni I don`t think the silicone seal is the one causing your problems. If there was an exhaust leak you should hear it, if it "pings" in your ears ALOT after riding, then you might have a leak:).

I`m not sure but I don`t think it`s the reason for your problems.

Good to hear that you discovered the missing engine mouting bolts!.
 
That seems to be the consensus that the seal was not causing the problem...

I did forget to add one more thing that I found. I am not sure what it was, maybe part of the baffle in the pipe, but it was some sort of hard metal screen that was floating around in my pipe. I was able to shake a small piece loose, but I can still hear that there is more in there when I shake it. Could this cause some restriction if it ended up in the wrong place causing the problems I am having??

I have yet to spray starter fluid on the places mentioned. I will try and do that this weekend.
 
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