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Post by manisobsolete on Nov 10, 2011 11:58:57 GMT -5
Summary: 85 jet causes soft seizes on bbk, 90 jet seems too rich.
I have a 99 Yamaha Jog, horizontal minarelli. Installed the 70cc kit from Scrappydogscooters, upjetted from the stock 76 to 90, and put in a 9 heat range NGK spark plug. I left everything else stock.
The acceleration and throttle response were ok, but I couldn't get over 35 mph. WOT bogged badly. The spark plug was black and oily. I put in an 85 jet and it ran a lot better: 40 MPH, no bogging, spark plug was dark brownish. Top speed was still lower than stock (45 MPH). However, on my first ride with the 85 jet, the piston soft seized at med-high rpm. It did it again the next morning.
I put the 90 jet back in and lowered the needle clip 2 notches (to the lowest notch). The scooter doesn't soft seize anymore, but it's way too rich above 3/4 throttle.
I'm nervous to go lower on the main jet, due to the traumatizing soft seizes with the 85. Once I figure out the jetting, I'll address the issue of my disappointing top speed, which should be at least 50 MPH given that it was ~45 MPH stock. I've tried 5g, 6g and 7g rollers. The belt travel on the variator is the same as stock. I might get a LV sp3 pipe in the future, but I want to see what the bbk can do alone.
Anyway, advice on the jetting? Advice in general?
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Post by 90GTVert on Nov 10, 2011 13:35:56 GMT -5
I would try checking for leaks. If it's not sealing somewhere, perhaps around the base since the cylinder was just installed, it could cause some kinda erratic behavior. Having one be a bit picky isn't all that unusual, but having a soft seize to rich bog within 5 numbers on the jet size is. You could try an 87-88 of course, but I think there's more going on.
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Post by manisobsolete on Nov 10, 2011 14:34:07 GMT -5
I checked for leaks the best I know how. All the gaskets are in great shape, the cylinder nuts are torqued to 15 ft lbs, and done again when warm. Spraying carb cleaner around all seams while idling doesn't reveal anything. Exhaust is on tight, carb and manifold are snug, spark plug is plenty tight. The cylinder was pretty hard to install, though: I had to use a rubber mallet to get it in there. It seems to be in pretty snug, but I could try retorqueing again when its hot.
I've been very careful to get everything right, and I've eliminated every possible factor that I can. Maybe I'll put an 87-88 in there and see what happens. If that doesn't work out, I'll be tempted to go back to stock bore and just get a pipe. I once borrowed my friend's YMS V8 exhust for a day and really liked the performance with the stock bore.
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Post by Enviromoto on Nov 10, 2011 14:47:25 GMT -5
I would put the clip back into the center slot, and move up to a 8 range.
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Post by manisobsolete on Nov 10, 2011 15:06:16 GMT -5
Ok, I could try an 8 range plug. My understanding is that an 8 would be hotter than a 9 and less prone to fouling, but I don't understand how that would affect my situation.
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Post by Enviromoto on Nov 10, 2011 15:54:09 GMT -5
If the 90 is rich it could be because its not burning off the plug since its too cool.
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Post by manisobsolete on Nov 10, 2011 17:00:52 GMT -5
OK, that sounds reasonable. I will try that tonight.
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Post by jmkjr72 on Nov 10, 2011 17:40:22 GMT -5
i belive the jog still used teh 12mm carb and that has been a bad point of the prebugs with the vertical engine the small carb
buguy over on zf and scooterinvasion may be able to help you with your tuning he had a wicked fast jog
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Post by manisobsolete on Nov 11, 2011 11:59:24 GMT -5
I double checked and tightened everything. It's possible that I had an air leak at the time of the soft seizes, and that I've since fixed the problem. Last night I left in the 90 jet, moved the needle clip to the center slot, and put in an 8 range plug (NGK BR8HS). This was still too rich, with a blackish, dry plug. This morning I put in the 85 jet, gently rode a couple of miles (no top speed runs), then did a plug reading after some WOT. Here are the results:   I'm happy with the performance, though I haven't ridden it hard to see if it soft seizes. It's possible that I've fixed whatever was causing that problem, but only time will tell. Does the plug look good, or shall I move up to an 87.5 jet? |
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Post by 90GTVert on Nov 11, 2011 13:42:02 GMT -5
That looks OK since it's only been a couple of miles. You can see plenty of coloring down deep in the plug. Adjust your idle again and try riding a little more before you trust it to really ride. Make your WOT runs close to home just in case. I'd like to see a little more color on the ring around the end there, darkening, if it's allowed to run at low speed for any amount of time. If that's wet at all when you remove it, it could be washing it clean at low engine speeds.
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Post by skuttadawg on Apr 17, 2012 1:11:59 GMT -5
Have you adjusted the fuel/air screw ? My dealer told me a customer had too hot of a plug and it made the piston swell up so much it would seize until cooled . Stock exhaust is restricted with a catalytic convertor which limits flow so a HP exhaust that is catless will flow better . Do you have the stock air box ? Check for vacuum leaks ?
I have a 2011 Jonway Magnum 2T 49cc with a 75 jet , 5 gram rollers as 4 was too light , NGK iridium and unlimited CDI . I weigh 200lbs and can hit 50 mph according to my speedo , but radar test says I register 4 more mph than the radar displayed . It run perfect at any speed and I have a 19mm PWK . Did you install a larger idle jet too ? Do you run premium gas and premium synthetic 2T oil ?
I want to put on an Air Sal 72cc BBK after it starts to fade as its a sin to take apart an engine that runs so darn good
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