Sled loses RPM / bogs after WOT runs

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sleeper_dave

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Oct 26, 2009
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I finally got my vmax running reasonably well, but i'm having an issue now that's got me kind of stumped.

Sled is a '96 vmax xt, with carbs and gears from a '94 ST, SLP pipes, slightly milled head, "cleaned up" exhaust ports, and the intake hole on the piston skirt opened up slightly. Clutch has been replaced with a clutch off ebay, from (i believe) a '98 vmax sx 600 or something like that. When I looked at the specs it was in the ballpark for what came with the '94-'96 vmaxes. Motor is fresh (a few hundred miles), passed a pressure test, and is jetted on the rich side still.

I got the jetting close to where I want it but still on the rich side a few weeks back during tip-up-town. Seemed to run pretty well. Took it out for a spin back home (lake st. clair) afterward and it seemed like it didn't backshift well, so I increased the spring tension on the secondary.

Finally got it up north for some trail riding last weekend and I'm having a wierd problem where it just doesn't want to rev. If I run it WOT for long enough, revs just slowly drop and it won't go past about 5k RPM. If I take a nice WOT blast then slow down for a corner, same thing. Doesn't want to go past 5k rpm.

When I start the sled cold, it runs fine. Rips to WOT, makes power up there, and does what I want it to do. Shifts out at just under 8k RPM and pulls hard. Once it's been run for a while, it won't hit the same rpm (mid 7's or so), and makes noticeably less power. After a bit longer, if you let off it just doesn't want to go past 5k. Almost feels like it's loading up. The longer you run it the worse it gets. If I stop for a break and let the sled cool down a bit, it runs great for a little while, then gets bad again.

I know it's rich because I killed it a few times while doing this and checked the plugs. Dark and wet, and piston wash visible at least as far as the plug hole.

I tried jetting down a little and it didn't fix the problem. Tried new plugs and it didn't fix the problem. Checked the fuel pump by bumping over the ignition (i put electric start on it) with the lines unhooked and it seems to be pumping plenty of fuel. I don't think it's a failing fuel pump because it still seems rich even when it's exhibiting the symptoms. And I'm still only getting about 6 mpg, so it's pumping fuel somewhere.

I'm thinking it could be the ignition. I suppose it could also be in the clutches, but I doubt it - it seems to be more a symptom of the way the engine is running than the way it's shifting. So much so that I initially thought it was just still way too rich.

Any thoughts? When I search for ignition problems on here, I see references to failed stators more than failed coils. Does this sound like a possible failure mode for either? Could it be a bad connection somewhere? I was thinking it could even be a bad ground between the coil and the engine that just increases in resistance with heat and eventually causes the spark to break up at high RPM.

I'm planning to poke around with a multimeter tonight and try to at least check the coil and stator, but I'm afraid it might check okay even if there's a problem because i'm not going to be able to get it warmed up enough.
 
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I had the same issue with 2001 srx. I believe my issue was the snow conditions. Really sloppy almost grainular snow acrossed the lake. It would shoot right up to max RPM and then drop down to 7000. almost like it was dropping a cyclinder. After we were done with the trail rides the next day took the sled back out on our local lake and she rips again. The only thing that was different was the type of snow I was on. Hope this helps.
 
Hey,

My sled was doing the same thing :

http://www.youtube.com/watch?v=J-eieZhvRF0

It would idle just fine, cruise at slower (30 mph) speeds for quite a while but I'd lose the right cylinder after a few hundred yards. If I'd stop and let it idle for 45 sec. or a minute, the right cylinder would start firing again. It ended up being a little piece of plastic between the valve needle seat and the body of the carb - obstructing fuel flow :

image4_zps3b261996.jpeg


Here's the thread I'd started, a bunch of guys posted with things to try and a list of stuff I did/tried :

http://www.totallyamaha.net/forums/showthread.php?t=90454
 
The snow conditions do affect how it runs, to some degree, but it has demonstrated this symptom on groomed trails and on 2 different lakes.

I had the carbs apart recently, and replaced the needle valves, set float height, and cleaned everything.

Also, it doesn't seem to be losing a cylinder. It just loses power. If I stop as soon as it happens, it'll idle fine. It just won't rev past about 5k RPM, and if it does, it won't make power like it normally does.

I did run it WOT until the problem came on, and as it was losing power and gradually slowing down, I killed it, stopped, and checked the plugs. Black and wet. Also did the same when I couldn't get it to rev past 5k rpms - I ran around holding the throttle and letting it blubber along, then killed it and checked the plugs. Black and wet again. Wet with fuel, not oil. Black / dark brown. I also tried the starter jet when it was having the problem. Made it slightly worse, so if anything, it's probably getting too much fuel.

This thread seems to have a similar problem. http://www.totallyamaha.net/forums/showthread.php?t=87870&highlight=bad+coil

I picked up a coil at lunch today, for the price I can throw the part at it and if it doesn't fix the problem at least I have a spare. Not looking like I'll have time to work on it tonight now, though. Hopefully tomorrow or wednesday.
 
If you had the carbs apart recently and it ran good before this maybe something happened. It sounds like floats to me too, getting way to much fuel. If it wasn't a yamaha it probably wouldn't even start.
 
It still ran fine after I had the carbs apart. Float levels are set to spec.

If I start the sled, let it warm up a little, and go run it to WOT it'll make plenty of power and rip right up to about 100 mph, maybe more if I have enough room. If I leave the throttle pinned for 2-3 minutes, THEN it starts losing power.

I did have a pin back out of the float on my phazer, letting the float and needle valve drop into the bowl. It bogged and ran like crap all the way home. Similar to the way this one is running, but it wasn't intermittant in that case.

It is rich, and I thought maybe it was just getting way too much fuel. So I reduced the size of the main jet. No change.
 
Had the exact same problem on my 98 sx 600 twin, wot through a long field with good amount of fresh snow, slowly starts losing power until tach is around the 5k mark. My 95 600 would scream through it no problem. Took plugs out, black, wet, ended up turning my fuel/air screw in the bottom of the carb from 1 3/4 turns out to 1 turn out and seems to run like a champ now, plugs look nice and brown. My 2 cents.
 
I tried adjusting the air screw from 1 1/4 to 2 1/4 in 1/4-1/2 turn increments, no change.

Last night I checked the ignition system with an ohm meter. Spark plug caps, ignition coil, pulser coil, and charge coil all were within spec. I replaced the ignition coil and caps anyway.

I also removed and checked the carbs last night as well. Everything looks good. I went back to the larger main jet so that I don't have to worry about shifting lean if it is in fact an ignition problem. Float level is set to spec and needle and seat are operating as they should.

Hopefully I'll get an opportunity to get it out on the lake and run it this weekend, to see if it's still having problems.
 
If your plugs are wet and black, putting a larger jet and turning the fuel/air screw from 1 1/4 to 2 1/4 is only going to make things worse. Your running way too rich by the sounds of it, so lean that thing out. Running it super rich can be just as bad as running it lean, keep leaning it out running it and checking your plugs until Its properly tuned and that should fix your problem.

Your stock main should be a 156.3 and your stock pilot should be a 52.5.
Also the clutch off a 98 is set up a lot different than the older vmax.
it uses 11kg more pre load, different free length, it uses a red spring not a pink, and the wrap is set differently.

Once the sled is tuned and parts are changed to this extent its hard to say exactly what the issue is. Id say just keep trying things until it works.
 
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Which is why I turned the air screw leaner (more turns out equals leaner) and put a smaller jet in it. And saw no change. Prior to this problem starting, the sled was running great in 10°-30° weather, running slightly on the rich side based on plug readings. Since I knew it was rich, the first thing I did was shift it leaner, and I didn't see an improvement. I returned it to the specs that it was at previously, because if spark is an issue it could make the plugs read richer than they really are, and I didn't want to burn the motor down after fixing a spark issue due to leaning it out trying to chase the problem.

When this sled is not exhibiting symptoms, it shifts out just under 8k RPM, holding the same RPM as it shifts out. The pipe mfg'r says 7700-7900 rpm is optimal, so whatever the differences are in the clutch, it's close enough. Back shifting left a little bit to be desired, but it was close. Given the amount of time I have to ride, a 100% solution on the clutch is just not an option. If I have an 80% solution, it's good enough for now. Once I have the sled in a condition where I can take a nice trail ride, maybe then I'll start taking the time to tune the clutch a little bit better.

The '94-'96 Vmaxes have 4-5 different clutch configurations. The '98 is a lot closer to the '94-'96 than you're making it out to be. Yes there are differences, but just about every tunable part in the '98 clutch is used in one or another configuration between '94 and '96.
 
sleeper_dave said:
Which is why I turned the air screw leaner (more turns out equals leaner) and put a smaller jet in it. And saw no change. Prior to this problem starting, the sled was running great in 10°-30° weather, running slightly on the rich side based on plug readings. Since I knew it was rich, the first thing I did was shift it leaner, and I didn't see an improvement. I returned it to the specs that it was at previously, because if spark is an issue it could make the plugs read richer than they really are, and I didn't want to burn the motor down after fixing a spark issue due to leaning it out trying to chase the problem.

When this sled is not exhibiting symptoms, it shifts out just under 8k RPM, holding the same RPM as it shifts out. The pipe mfg'r says 7700-7900 rpm is optimal, so whatever the differences are in the clutch, it's close enough. Back shifting left a little bit to be desired, but it was close. Given the amount of time I have to ride, a 100% solution on the clutch is just not an option. If I have an 80% solution, it's good enough for now. Once I have the sled in a condition where I can take a nice trail ride, maybe then I'll start taking the time to tune the clutch a little bit better.

The '94-'96 Vmaxes have 4-5 different clutch configurations. The '98 is a lot closer to the '94-'96 than you're making it out to be. Yes there are differences, but just about every tunable part in the '98 clutch is used in one or another configuration between '94 and '96.
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Those are fuel screws you are turning.. arn't they? dont think there are air screws with them type of carbs! which means turning them out is fatter not leaner...more turns out is richer . turns in is leaner.
 
x2 on the fuel screw, turning mine in leaned it out more, it is the only external screw adjustment on my carbs and its on the bottom side of it.
 
If not for the wet plugs I would say you were running out of fuel. I had a sled once that had a bunch of debris in the tank because it had sat for a long time and all the gas dried out and crystalized. It would run really good for about 3 minutes and then it would slowly start to slow down as the float bowls ran low cuz it couldn't keep enough fuel flowing. Does your sled have a fuel filter that you could check?
 
I don't think it has a fuel filter, it might have the one in the tank though. I might have to check that out.

And it is an air screw. Carbs from a '94.
 

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As the diagram says the air screw is for low speed tuning. It really should not affect your wot running much at all. It's more about how good you are at idle and off idle.
 
You MAY have a stator problem as the resistance increases when the motor gets warm and heat soaked.

It also sounds as if it is jetted to rich.... Just saying sounds...
Dave is everything pretty much stock??? Just trying to clarify. Check the spark when the motor is hot to see if the spark has gotten weak... Thats all I got.

Reminds me of when I had a 1987 vmax 540. The sled would start and run fine for the first 10 mins or so then would just quit and had no spark... let it cool and it would fire right up again...
 
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Well, I THINK it's fixed.

There were two distinct issues. One was that it was too rich. But I knew that, and wanted to make sure the other problem was taken care of first.

I replaced the coil, checked resistance on the stator, checked the fuel pump with a vacuum pump on the pulse line (making sure it's not sucking fuel through the diaphragm) and replaced the fuel filter in the tank. Also gapped the spark plugs and double checked float levels, and made sure fuel was flowing into the carbs freely.

I took it out on the ice on Sunday and ran it around a bit. It was weak up top, but weak because it was rich. It would still pull, still keep the RPM up, it would just load up a bit in the mid range and up top. I ran it around a bit and tried to get the problem from last weekend to come back. I didn't exactly have a nice long trail run, but when I rode it hard for maybe 5-10 miles on the ice, the problem didn't come back. So I swapped the main jets for the next size down, and ran it around some more. That had the sled running good pretty much all around, with no bogging at all (except off the line - but clutch tuning will be another adventure entirely). Reading the plugs, I was concerned that I was a little leaner than I wanted to be in the mid range (I checked the plugs after a WOT run, then after a 6k RPM cruise, then after a 7k RPM cruise). It looked about perfect, but weather was in the high 20's / low 30's and I want to make sure it's fat enough that it won't shift too lean if the temperature drops. So yesterday, i took off work a little early and adjusted the needle up half a notch (added a shim). The clip is now second notch from the top with a shim under it. Took it back out on the ice yesterday and ran it around some more. I think it's a little rich in the mid range now, but I don't want to remove that shim until I have the chance to ride it in some really cold weather and see how it does. Now, the plugs show it about where I want it but it loads up a bit after a mid range cruise, and takes a couple seconds at WOT to clear out. But again, I could not get my main problem to come back.

So, as I said, I THINK it's fixed. It might come back if I have the chance to put more miles on the sled in one sitting, but for now I'm thinking it's all set. I think I got enough time on it to put enough heat into it to cause any problem it was having to recur, but of course I won't know for sure until I can take it for another long trail ride. I think it was most likely the coil getting a little bit weak.

Thanks to all of you for your input, I did check into just about everything suggested.

sxr70001 said:
I get that, just saying it shouldn't really cause you to start slowing down at WOT.
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I tend to agree, but I think that either we underestimate the impact the air screw has at WOT, or SLP is crazy in their jetting recomendations. According to SLP, when you put their dual pipes on this sled, you go to a SMALLER main jet (156.3 stock to 155), and the ONLY thing you do to shift the mixture richer is adjust the air screw from 1 1/8 turns stock to 1/2 turn! That would lead one to believe that the air screw has a pretty significant impact on fuel delivery throughout the RPM range. Or, that SLP is nuts.

I thought that was kind of screwy so I went with a 160 main jet and actually tried a 165 main jet also (which was part of my "too rich" problem). I went with the 165 jet trying to get a "rich" reading on my plugs when I was tuning it in 5° - 10° weather, and it was too rich for more "normal" weather. I'm back to the 160 main jet now, with the air screws at 2 turns out, and it seems to be running well in weather in the 20's to 30's.

Sxr700Bandit said:
You MAY have a stator problem as the resistance increases when the motor gets warm and heat soaked.

It also sounds as if it is jetted to rich.... Just saying sounds...
Dave is everything pretty much stock??? Just trying to clarify. Check the spark when the motor is hot to see if the spark has gotten weak... Thats all I got.

Reminds me of when I had a 1987 vmax 540. The sled would start and run fine for the first 10 mins or so then would just quit and had no spark... let it cool and it would fire right up again...
Click to expand...

When I started thinking spark, I started thinking coil. When I came here and started reading, I started fearing it could be the stator. Coils are cheap, stators not so much so... so I hope you're wrong! I think it was the coil though. Old coil tested good, but I replaced it anyway because they're cheap. I wasn't going to throw a stator at it until I could verify the problem.
 
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