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Discussion Starter · #1 ·
Once warmed up, if I'm putzing along at 4500-5500 rpm and pin it, the sled usually backfires and then takes off. Above 6 grand, it seems fine. From a standing start, pinning it doesn't make it backfire, although it seems to me lacking "get up and go" If I blip the throttle a few times then pin it, either from a standing start or when cruising along, it seems to respond much better. It does seem pretty boggy around 5 grand even if it doesn't backfire..

Sled has 40 pilots, and has the "updated" 5.5 slides, waste of time I'm told, but I didn't have to pay for it. Above 6 grand, sled is fine. A friend of mine who's a good mechanic went through the carbs thoroughly, so they are 100% clean, and float height has been set exactly according the the Poo service bulletins, and the floats and pins are in good shape according to him.. He drained all the gas, fresh gas and isopropyl in the tank. The stator specs out, there was a bulletin for this as well. Compression and leakdown test is good, as are the reeds. Replaced Champions with New NGK plugs, and I sniped off 1/4" from each end of both plug wires and rethreaded them. Adjusting the air screws from 1/2 to 1.5 turns, not much difference that I can notice.

So far, all of this has done just about 0 to alleviate the backfiring.

Everyone says "adjust the pilots", but since I see the worst drivability problems around 5 grand when cruising, I'm thinking maybe leaner needles? The off-idle response seems to be pretty decent. Right now it has 1368G on the middle clip. I've heard lots swap the needles for 1371G, should I try this? Picked up the sled cheap, low miles, and a reverse kit on it. If I could get this one issue sorted out I would be a happy camper.
 

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Once warmed up, if I'm putzing along at 4500-5500 rpm and pin it, the sled usually backfires and then takes off. Above 6 grand, it seems fine. From a standing start, pinning it doesn't make it backfire, although it seems to me lacking "get up and go" If I blip the throttle a few times then pin it, either from a standing start or when cruising along, it seems to respond much better. It does seem pretty boggy around 5 grand even if it doesn't backfire..

Sled has 40 pilots, and has the "updated" 5.5 slides, waste of time I'm told, but I didn't have to pay for it. Above 6 grand, sled is fine. A friend of mine who's a good mechanic went through the carbs thoroughly, so they are 100% clean, and float height has been set exactly according the the Poo service bulletins, and the floats and pins are in good shape according to him.. He drained all the gas, fresh gas and isopropyl in the tank. The stator specs out, there was a bulletin for this as well. Compression and leakdown test is good, as are the reeds. Replaced Champions with New NGK plugs, and I sniped off 1/4" from each end of both plug wires and rethreaded them. Adjusting the air screws from 1/2 to 1.5 turns, not much difference that I can notice.

So far, all of this has done just about 0 to alleviate the backfiring.

Everyone says "adjust the pilots", but since I see the worst drivability problems around 5 grand when cruising, I'm thinking maybe leaner needles? The off-idle response seems to be pretty decent. Right now it has 1368G on the middle clip. I've heard lots swap the needles for 1371G, should I try this? Picked up the sled cheap, low miles, and a reverse kit on it. If I could get this one issue sorted out I would be a happy camper.[/b]
Swap out those needles. But, the 98 took FOREVER to warm up. If its really cold, that sled will almost always backfire, fart and pop. No thermastat. I always blamed it on that. When a sled does that, it is either a lean bog/pop or rich one. I usually just half choke it while riding for a second and punch it and it goes away. I'm guessing it has to do with the cold temps up there. If its around 20+ degrees F, i would bet it doesn't pop at all. What is your jetting too? Stock mains were 185.

Keihn carbs blow dogs.
 

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Discussion Starter · #3 ·
Swap out those needles. But, the 98 took FOREVER to warm up. If its really cold, that sled will almost always backfire, fart and pop. No thermastat. I always blamed it on that. When a sled does that, it is either a lean bog/pop or rich one. I usually just half choke it while riding for a second and punch it and it goes away. I'm guessing it has to do with the cold temps up there. If its around 20+ degrees F, i would bet it doesn't pop at all. What is your jetting too? Stock mains were 185.

Keihn carbs blow dogs.[/b]
It was around -10C today, that's around 13 or 14 F I think. I don't remember what the mains are, except they are the stock ones, so they must be 185. I have ridden it when it's warmer, and it still popped. My mechanic friend suggested trying to richen the needles and see what happens since at around 5 grand I should be into the needle circuit, he thinks the pilot settings probably have little to do with what I'm experiencing. If it gets worse with richer needles, then I'll try leaner. I think it is actually too rich since it doesn't seem to do this UNTIL it gets good and warmed up, but better to go richer first to be on the safe side.

On another idea, does a boost bottle help this engine much? It seems it could use some real help below 6k regardless of whether it is backfiring or not.
 

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It was around -10C today, that's around 13 or 14 F I think. I don't remember what the mains are, except they are the stock ones, so they must be 185. I have ridden it when it's warmer, and it still popped. My mechanic friend suggested trying to richen the needles and see what happens since at around 5 grand I should be into the needle circuit, he thinks the pilot settings probably have little to do with what I'm experiencing. If it gets worse with richer needles, then I'll try leaner. I think it is actually too rich since it doesn't seem to do this UNTIL it gets good and warmed up, but better to go richer first to be on the safe side.

On another idea, does a boost bottle help this engine much? It seems it could use some real help below 6k regardless of whether it is backfiring or not.[/b]
i have ridden my 98 700 a few hundred miles this season, same sled basically. Never popped or farted once when it was warm, but as soon as it got down into the teens, it did it. No need to go richer than stock mains at those temps, especially if you have no mods...air box...reeds....etc. The first thing I would do to that sled if i were you, boost bottle and a good set of reeds. The was like the only sled that actually needed those two things. I laugh when guys throw reeds on newer sleds, does basically nothing.
 

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Discussion Starter · #6 ·
i have ridden my 98 700 a few hundred miles this season, same sled basically. Never popped or farted once when it was warm, but as soon as it got down into the teens, it did it. No need to go richer than stock mains at those temps, especially if you have no mods...air box...reeds....etc. The first thing I would do to that sled if i were you, boost bottle and a good set of reeds. The was like the only sled that actually needed those two things. I laugh when guys throw reeds on newer sleds, does basically nothing.[/b]
I have no intentions of richening the mains, but I might richen the needles to see what happens. I've heard boyesen reeds are a good addition to this sled. I realize Keihins are crap, but my brother has a 99 700 SKS that runs way better than this thing, and the carb settings are almost identical. Anyone know an easy Mikuni setup to drop on a big-block 600? ;) The small block 600 VES in my other sled is a dream compared to this thing. :)
 

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Discussion Starter · #7 ·
oh yeah, set your idle to about 1800rmps when warm. Whats your clutching like?[/b]
The warm idle is around there, maybe a bit lower. Now idea on clutching, other than the primary spring looks like a dark blue one. Pretty sure it should be stock, as the rest of the sled is.
 

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Usually a backfire then take off is lean. If you cannot tune it out with the air screw at 1/4 turn, you likely need richer pilots. As mentioned, those sleds are very cold blooded, some more so than others. I had a couple customers who had to ride for 10 minutes or more with the choke on half, until the engine was good and warm not to pop. If we richened the air screw setting until it worked well when we thought it was warm (short ride), once it really warmed up (engine), it would load up. Have you checked the reeds for the rubber overmolding delaminating?
 

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Discussion Starter · #9 ·
Okay, took the sled out today, and found out my bog/backfire is definitely a lean condition, but not idle circuit's fault, because the sled even loads up a bit at idle, and doesn't bog from a dead stop, only when you pin it after putting along at 30mph... It was only -4C today, that's 24F, and I completely solved it by using the richest needle setting? I shouldn't have an air leak, since the leakdown test was fine, and their doesn't seem to be fuel delivery problems, since starting and idles fine, even loading up at bit at idle if you let it sit idling for a few minutes. As far as I can tell, for some reason it is just lean at about 1/4 throttle with stock carb settings, but no where else. If I run 80mph for a couple of miles, and shut it down, the rad hoses were barely warm, so I doubt it's starving for fuel! I'm going to replace the reeds when I put a boost bottle on it, since I might as well do both at the same time, got Boyesen Power Reeds on the way and an SPI boost bottle. I'll just leave the needles on the lowest clip for now, but am a little worried what this thing will do if it's -25C.
 

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Discussion Starter · #10 ·
Usually a backfire then take off is lean. If you cannot tune it out with the air screw at 1/4 turn, you likely need richer pilots. As mentioned, those sleds are very cold blooded, some more so than others. I had a couple customers who had to ride for 10 minutes or more with the choke on half, until the engine was good and warm not to pop. If we richened the air screw setting until it worked well when we thought it was warm (short ride), once it really warmed up (engine), it would load up. Have you checked the reeds for the rubber overmolding delaminating?[/b]
Mine never seems to get that warm, but I tried running with the choke on half, and basically it just loads up like crazy. If I richen the air screw, it just loads up at idle but still has the bogging if you pin it from 20-30mph. Weird that many people recommend leaner needles, but I have to richen mine way up to get more responsiveness at 4500 rpm?

edit: it just occurred to me that I don't know what KIND of leak-down test my buddy did.. I'm thinking probably just cylinder leak-down test? Although if I had an air leak, you think I'd see problems like the idle racing or similar? There is also quite a bit of spit-back in the airbox, is that normal?
 

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Discussion Starter · #12 ·
I decided to have a look at the pistons, and the wash looks like it is definately running pretty lean, the wash on both pistons is tiny, like half the size of the my pinky's fingernail. Both are the same, though, so I don't know what to think. With 185 jets, stock needles on the richest settings, and a temp of -4C, I would expect the wash to look pretty rich, especially since the last thing I was doing was putzing around and loading the sled, not blasting down the lake... For lack of something else to try, I'm going to put a new kit in the fuel pump and change the fuel filter.
 
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