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Post by scootdoggydog on Jul 18, 2018 0:28:23 GMT -5
Recently I lost some power in the mid to full throttle range. At first I had no idea what the problem could be but we just recently had a heat wave here with high humidity so I was thinking it might be running a little rich. My scooter is a 155cc bbk with a K&N filter and free flow exhaust. I have been running it like this for almost 10 years with a 118 main jet. the bbk is about 4 years old. Ever since I installed the bbk it has had extremely high compression and is hard to start sometimes especially when its colder out or has been sitting for a while. Im thinking about getting a high torque starter but theyre quite expensive so I have been putting it off for a while. Anyway I went to start it recently and it wouldn't start at all. So I checked the valve gaps. The intake was almost completely closed and the exhaust was a little bit too big so I set them to specs .004 intake and .005 exhaust. It was still hard to start but I eventually got it started the loss of power issue was still there but not as bad. So I was thinking maybe it was still running rich because of the temp and humidity. I rejetted to a 115 main and the issue seemed about the same or maybe a little worse so I rejetted to a 120 main jet and that was absolutely terrible it ran fine at slow speeds but when I gave it like 1/3rd throttle or more it just seemed like it didn't want to accelerate at all. So I rejetted to a 112 main jet and It's running much better now but it's still stumbling at higher speeds it seems like it's the worst around mid to full throttle. Im guessing I'll probably have to lower the needle or go to a lower main jet but with my setup that doesn't really make much sense to me my next size smaller jet is the stock jet it came with no # on it. Any thoughts on what could be causing my issues here? And I dont think fuel delivery is an issue either it's gravity fed and the fuel filter always looks full. Maybe theres an air leak or something somewhere? But wouldn't that effect the idle too?
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Post by pistonguy on Jul 18, 2018 19:04:39 GMT -5
CVK carb?
May want to check the Diaphragm for a tear,
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Post by scootdoggydog on Jul 18, 2018 20:04:54 GMT -5
Today I checked the diaphragm it looks fine. I raised the clip to lower the needle and put in a stock main jet with no number on it and it runs a little better at slower speeds now and mid range speeds have improved for the most part but theres still a little hesitation every once in a while. Top speed has improved but it still hesitates or stumbles a little but not as bad as before. I figure it should be running very lean now but it never surges. It sounds like it is backfiring more often now especially at slower speeds. It has always backfired on deceleration though. But now it's more noticeable and a little louder I think. Anyway I ordered a 110 main jet and I should get it within a week. Im still a little bit confused as to why I would have to use such a small jet with my setup though. It has always run great with a 118 main jet so why would I be having problems now?
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Post by pistonguy on Jul 18, 2018 21:14:34 GMT -5
Well, your Jets are Fixed so that was a constant, Something Else has Changed to cause you to alter the tune to try and compensate.
Do you still have the egr?
I don't know what kind of Scoot so do you still have the OE Vacuum operated Fuel Pump?
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Post by scootdoggydog on Jul 19, 2018 3:22:59 GMT -5
No egr I replaced my motor a long time ago and the engine I have doesn't have that option. And no It is not vacuum operated I replaced the petcock with a shut off valve so it is now gravity fed and the fuel filter stays mostly full.
Recently I couldn't get it started. So I decided to check the compression and as soon as I looked at the plug I noticed it was most likely carbon fouled, I guess the high heat and humidity finally did it in. It's an iridium plug but it has been in there for a while without me checking it. I guess a 118 main jet is just way too rich for me at least in the summer with the higher heat and humidity anyway.
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Post by wheelbender6 on Jul 19, 2018 8:02:59 GMT -5
I had that mid-range problem a while back. I was fortunate that adding Seafoam to a few tanks of gas fixed it.
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Post by chewbaca on Jul 19, 2018 12:55:42 GMT -5
sometimes the cap in the CDI will start to go bad and give you a week spark at higher rpm.
checking your valves for pitting might be useful
also your starter motor can get filled with oil when the seals go bad
it makes the brushes fail to conduct full current and adds lots of friction
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Post by scootdoggydog on Jul 27, 2018 20:22:08 GMT -5
Well I'm stumped I got the 110 main jet put it in and it ran worse than the 112. So I put the 112 main back in and it runs pretty good except for top speed it still stumbles a bit sometimes. I also tried a spare cdi I had laying around and didn't notice much of a difference. This is weird and confusing what could cause me to have to go down 6 jet sizes and still have performance issues at WOT?
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Post by cyborg55 on Jul 28, 2018 7:55:10 GMT -5
Your running out of fuel at hi rpms,,,carb float set too low maybe so your only running half a bowl to begin with,,,partially obstructed fuel fitting,,maybe some casting flash inside the fittings somewhere ,,,,your getting flow but no volume,,,
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Post by scootdoggydog on Jul 28, 2018 9:28:05 GMT -5
I don't think fuel delivery is the issue. And I really don't think it is casting flash. If I was running out of fuel wouldn't switching to a larger main jet make more sense unless there is a problem with the float which it doesn't look like there is and why would it happen all of the sudden after years of running fine. The problem does seem to happen at high rpm's though. So I'm thinking the problem might be more spark related or inside the combustion chamber maybe in the head somewhere. Because when I checked the spark plug there was a lot of carbon build up on it but there was a tiny clean area on the central electrode and on the sides at the tip of the side electrode were clean good thing I was running an iridium plug. But I changed the plug to be safe and still having issues. Maybe theres too much carbon build up somewhere internally there? I'm not sure. I'm thinking of maybe getting a bigger head to match the 58.5mm 155cc bbk I installed a while back. Could the problem maybe be in the ignition coil or have something to do with the stator possibly the pick up though?
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