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Discussion Starter #1
Hi,

It’s been a bit of up and down recently with my 1996 SS SP. I got this bike earlier this summer, and I went through all the basic stuff. The belts, valves, carbs, etc. it runs well for the most part, except that it has a pretty distinctive sluggishness around 3500-4000 rpm, but only in high gear. I recently installed ExactFit coils (there was nothing wrong with the original coils so far as I knew, I just replaced it as a preventive thing to replace 23-year old parts). It made the throttle response crisper, but otherwise it was the same as before, that was two weeks ago. The day before yesterday, I thought that the vibration got a bit worse (I can’t say for sure, but it just might be only in my head), and I also heard a backfire through the air box when the bike was cold (but just one pop, with puff of smoke came out from the airbox, everything was normal after that), so I removed the plugs to take a look. The horizontal cylinder looks pretty good, but the vertical cylinder is dark, and slightly wet with fuel (not oil). I cleaned the plugs and put it back together, and I started it up, from dead cold with the tank up and no air filter so I could see. I saw it pop on the vertical cylinder since of the carb, I could see a flame, too, twice (at this point I turned it off, I had a bad past experience with a bike catching fire on me from spitting from the carbs). Backfiring, or coughing from the intake normally indicate that it’s too lean, right. Checked the Timing belt marks just to be sure, and all looks fine. I have good spark, could it be a carb issue...the carbs were rebuilt with kit that includes new needles and all. I think the bike is running generally on the rich side, so I was thinking about raising the needle clips by one notch (from the 4th from the top to the 3rd notch, is this a bad idea?). Anyway, I need to organize my thoughts and tackle it in a logical and organized way, and I was hoping that more experience folks here can give me a bit of a pointer. I’d really appreciate input.
 

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What needles did you fit??? I had this on my bike when I upgraded to Factory pro kit with std pipes. Had to run different between each cylinder until I went to a equal length 2-1 pipe.
 

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Swap your new coils across the cylinders to see if the problem moves, or put the OE coil back on the rear cylinder to see if it goes away completely. There are a couple of instances of the Exact Fit coils bench testing fine but under-performing in real world conditions.

Check your spark by pulling the plugs and grounding them to the frame while you crank the bike. They should be sizzling blue-white and strong. If they are yellow and/or weak, that is also an indicator of something amiss. Test both plugs against both coils to identify potential culprits.

Also, check your caps and connections, they may not be up to par after the swap. If you re-used your wires, you might snip 1/4" off the ends and re-attach.

This is what mine was doing. Going back to the OEM coils resolved it.

 

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Discussion Starter #4
What needles did you fit??? I had this on my bike when I upgraded to Factory pro kit with std pipes. Had to run different between each cylinder until I went to a equal length 2-1 pipe.
You know, I don’t honestly know, I need to look it up. It came with the rebuild kit, but I recall the needle was thicker than the one that was in the carb.
 

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Discussion Starter #5
Swap your new coils across the cylinders to see if the problem moves, or put the OE coil back on the rear cylinder to see if it goes away completely. There are a couple of instances of the Exact Fit coils bench testing fine but under-performing in real world conditions.

Check your spark by pulling the plugs and grounding them to the frame while you crank the bike. They should be sizzling blue-white and strong. If they are yellow and/or weak, that is also an indicator of something amiss. Test both plugs against both coils to identify potential culprits.

Also, check your caps and connections, they may not be up to par after the swap. If you re-used your wires, you might snip 1/4" off the ends and re-attach.

This is what mine was doing. Going back to the OEM coils resolved it.

Thanks for the video. Yeah, that’s very similar to what my bike is doing. I’m going to swap them out tonight and see what it does. I need to be methodical and change one thing at a time...
 

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use a micrometer to measure your needles the factory needles are different from the dynojet as well as the dynojet stage 2 needles. 3 different base diameters so different amounts of fuel when the needle is buried in the needle jet. If you used a knock off needle who knows what size it is so start by measuring to get an idea if it is the same or different than what you had before.
 

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Discussion Starter #7
use a micrometer to measure your needles the factory needles are different from the dynojet as well as the dynojet stage 2 needles. 3 different base diameters so different amounts of fuel when the needle is buried in the needle jet. If you used a knock off needle who knows what size it is so start by measuring to get an idea if it is the same or different than what you had before.
It is from this kit. Yeah, the needle looked slightly different. I kept the old ones, too, so I'll check when I get home.
 

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I’ve used that same kit. Keep in mind it comes with all the oem size jets and the fat, oem size needle. If you used all the parts in the kit then you’ve swapped out the dynojet (assuming this is the kit that you started with) mains and pilot jets but kept the dynojet needles.


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Discussion Starter #9
I’ve used that same kit. Keep in mind it comes with all the oem size jets and the fat, oem size needle. If you used all the parts in the kit then you’ve swapped out the dynojet (assuming this is the kit that you started with) mains and pilot jets but kept the dynojet needles.
Dynojet - huh, I didn't think of it, but is it possible that I had Dynojet to begin with. I recall that the needle looked different from the one that I took out. Hmm, I'll need to go take a look tonight. It's been running fine though, but that may be the reason I had what feels like a weird spot between 3500-4000rpm in high gear. The thing is, I didn't run the bike before I rebuilt the carbs with this kit, so I have no reference to what it was like before.
 

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These carbs are more sensitive than most in regards to float level. I would check actual fuel level with a clear line alongside the float bowl. I would lower the fuel level from whatever you have now by 1 mm or match the front carb if it’s lower. Examine your float needle carefully. If it’s old, replace it anyway. These carbs can leak internally ( past the gasket under the seat Assy) allowing in metered fuel to enter the float bowl or get sucked into the venturi. ( past gasket under the main jet holder) . If your rebuild didn’t include replacing them , do it now. You can’t tune the carb if you can’t control the fuel going through the float bowl.
 

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Discussion Starter #12
These carbs are more sensitive than most in regards to float level. I would check actual fuel level with a clear line alongside the float bowl. I would lower the fuel level from whatever you have now by 1 mm or match the front carb if it’s lower. Examine your float needle carefully. If it’s old, replace it anyway. These carbs can leak internally ( past the gasket under the seat Assy) allowing in metered fuel to enter the float bowl or get sucked into the venturi. ( past gasket under the main jet holder) . If your rebuild didn’t include replacing them , do it now. You can’t tune the carb if you can’t control the fuel going through the float bowl.
Float needle is new, along with the o-ring and all that. I'll double check the float level tonight, too, though I'm pretty certain that I set it correctly when I rebuilt it...
 

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Dynojet - huh, I didn't think of it, but is it possible that I had Dynojet to begin with. I recall that the needle looked different from the one that I took out. Hmm, I'll need to go take a look tonight. It's been running fine though, but that may be the reason I had what feels like a weird spot between 3500-4000rpm in high gear. The thing is, I didn't run the bike before I rebuilt the carbs with this kit, so I have no reference to what it was like before.
The different needle is the giveaway. That rebuild kit comes with an oem style needle which has very little taper. The tapered/pointy needle you describe is likely dynojet or factory pro. If you still have the old main and pilot jets that you removed it might help if you can determine what sizes they were vs what the new rebuild kit provided (likely 140 main and 40 or 42.5 pilot). You’ll find the numbers stamped on the old jets :)

Cheers


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Discussion Starter #14
The different needle is the giveaway. That rebuild kit comes with an oem style needle which has very little taper. The tapered/pointy needle you describe is likely dynojet or factory pro. If you still have the old main and pilot jets that you removed it might help if you can determine what sizes they were vs what the new rebuild kit provided (likely 140 main and 40 or 42.5 pilot). You’ll find the numbers stamped on the old jets :)

Cheers
Hmm, interesting. I vaguely recall that the jets that came with the rebuild kit was bigger. I of course kept all the old ones, so I'll take a look at it tonight.

This is great, I have pretty good idea of where to begin tonight. Thanks all!!
 

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Float needle is new, along with the o-ring and all that. I'll double check the float level tonight, too, though I'm pretty certain that I set it correctly when I rebuilt it...
What I’m saying is to lower the float level no matter what level you think it’s set at if you can verify you don’t have internal leaks like I explained. Set the fuel level , don’t just check the float level and think you have it right. Also, I’m saying if you find nothing else wrong then doing this may cure your problem. I’m saying checking fuel level will tell you if you may have an internal leak. You will not be able to correct that with float level adjustment.
 

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Discussion Starter #16
What I’m saying is to lower the float level no matter what level you think it’s set at if you can verify you don’t have internal leaks like I explained. Set the fuel level , don’t just check the float level and think you have it right. Also, I’m saying if you find nothing else wrong then doing this may cure your problem. I’m saying checking fuel level will tell you if you may have an internal leak. You will not be able to correct that with float level adjustment.
Eh, got it!
 

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Genuine mikuni jets are marked on each and every one, if you have no maker mark then they are aftermarket. Dynojet jets are marked only with a number of the jet size as are most mikuni knock off jets. None of them meter the same amount of fuel even if the numbers match unless they are made by mikuni. I see the kit is very complete and includes needle jets as well so know that if when the bike was originally jetted it may have been done with needle jets that were also flowing more fuel and you just fixed that.

If you just jetted back to stock know that the bike would probably need to be stock as well to make that work , no slip ons or air box mods. because different jets meter fuel different even with the same number if you need to change the jets you need to know that a larger number will give more fuel not less so stay with whatever brand you can get an assortment of or you can be chasing your tail.

Airbox backfiring is classic lean condition and if you just took out richer needles and needle jets and mains and installed original size parts (in theory) then you would expect it to be lean.
 

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Discussion Starter #18
Well...
I took the carb apart tonight. It all kid of came back to me naw I disassembled the carbs. I had to do a bit of research online, but Apparently, it came with Dynojet kit when I bought it. When I rebuilt the carburetors with the rebuild kit, I only used certain parts from it, but cleaned and reused the needle and main jet (that’s because I read somewhere that the stock jet is on the lean side, so I kept the larger one - rebuild kit is 140, the jet that came with my bike was 150, and as for the needle, simply the big fat needle looked like nothing I’ve ever seen before in the carburetors and didn’t like it). So here I am, Dynojet stage 2 from the specs, according to the Dynojet, because mine has airbox with top cut out, and slip on, 150 main and all that, just as it was before. This makes me wonder that maybe it’s not carb related issue...
 

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Did you change the pilot jets?
I just finished 3 different carby 900's and all were spot on with 37.5 mikuni pilots. fuel screw ended up between 3.5-4 turns out on all of them. The main being dynojet (as well as needle) simply gives a known starting point to jet from but from your description it did not sound main or needle related unless the new needle jet is leaner than what you had. I would double check the float heights as mentioned if you have it apart as I had 2 of the 3 off quite a bit. remember with carbs the smallest differences will be noticed.

Also keep in mind with a carby on a cold motor you should be lean otherwise you may be too rich when at normal riding temperatures. plus running with no air filter leans it out even more so make sure you are jetting for running conditions and not bench conditions. Figure out the rich plug first and you might be on track with dropping the needle I more often than not am 3-1/2 on the needle. at those settings it may be the stage 2 needle is just bleeding a bit more fuel by as it is thinner at the base than a stage 1 ,factory or original.
 

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Discussion Starter #20
Did you change the pilot jets?
I just finished 3 different carby 900's and all were spot on with 37.5 mikuni pilots. fuel screw ended up between 3.5-4 turns out on all of them. The main being dynojet (as well as needle) simply gives a known starting point to jet from but from your description it did not sound main or needle related unless the new needle jet is leaner than what you had. I would double check the float heights as mentioned if you have it apart as I had 2 of the 3 off quite a bit. remember with carbs the smallest differences will be noticed.

Also keep in mind with a carby on a cold motor you should be lean otherwise you may be too rich when at normal riding temperatures. plus running with no air filter leans it out even more so make sure you are jetting for running conditions and not bench conditions. Figure out the rich plug first and you might be on track with dropping the needle I more often than not am 3-1/2 on the needle. at those settings it may be the stage 2 needle is just bleeding a bit more fuel by as it is thinner at the base than a stage 1 ,factory or original.
Thanks for the advice - Yeah, the pilot was replaced with the ones that came with the kit, but that was 42.5, so that's a big difference there. The bike did originally come with the OEM 37.5. 37.5 will make it leaner, but I get what you are saying about tuning to the actual riding condition. 99% of my carburetor experience is on two-stroke motorcycles, and they are much more severe and vocal when settings are off. I don't have a good feel for four-stroke engine carburetor settings, so a lot of learning here. I'll double check the float height before putting the carbs back on.
The plugs look like this. This is only after two hundred miles since the ExactFit ignition coil kit (Ca Cycleworks, comes with new, cooler plugs). Horizontal cylinder is pretty clean, the vertical cylinder is, well...

 
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