rage
n00b Ruckster
Posts: 8
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Post by rage on Apr 16, 2021 18:38:27 GMT -5
Just deleted the airbox and hooked up a uni filter. 38 slow jet, 85 main jet, shimmed needle. Engine is running rich at 1/4-1/2 throttle but seems badly lean at wot. Bright white sparkplug. Is it that i simply need a larger main jet or could there be some other problem?
Only thing i was confused about duting installation was the 2 hoses for the PAIR valve. Plugged the one going to the cylinder cover and was told to leave smaller one on the manifold in front of the carb open to air.
Hard to find any info on the older models with extra hose from pair valve to intake. Any advice appreciated!
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rage
n00b Ruckster
Posts: 8
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Post by rage on Apr 16, 2021 20:26:40 GMT -5
Doing a little research does it make sense that my diaphram in the carb may be pinched or ripped not allowing it to expand? Lost about 8 mph at full throttle and lean
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Post by Baizy on Apr 18, 2021 14:54:52 GMT -5
Hey rage you've just opened up the same can of worms most of us rucksters have. Trial by error tuning. A torn diaphragm would lead to it not being able to rev up at all so its not a diaphragm issue. Going straight to an essentially open system you're going to be running lean in the middle and rich up top believe it or not. You should shim the needle up as high as you can and slap an 80 in there. Most spark plug checks are innacurate as it'll fire a few more times on the mid and idle circuits giving you false readings. Try what I've just suggested and check back in with us. I run an 02 sensor in my exhaust to watch the afr, so you can trust the science behind what I've suggested. The air flows much better in the low range than it did before but you wont have gained too much up top. Happy Tuning! |
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rage
n00b Ruckster
Posts: 8
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Post by rage on Apr 20, 2021 19:37:32 GMT -5
Hey rage you've just opened up the same can of worms most of us rucksters have. Trial by error tuning. A torn diaphragm would lead to it not being able to rev up at all so its not a diaphragm issue. Going straight to an essentially open system you're going to be running lean in the middle and rich up top believe it or not. You should shim the needle up as high as you can and slap an 80 in there. Most spark plug checks are innacurate as it'll fire a few more times on the mid and idle circuits giving you false readings. Try what I've just suggested and check back in with us. I run an 02 sensor in my exhaust to watch the afr, so you can trust the science behind what I've suggested. The air flows much better in the low range than it did before but you wont have gained too much up top. Happy Tuning! Im a bit confused! Youre saying it will run lean run richer at wide open throttle than day half throttle? Spark plug check may not be a good test but it seems to be saying opposite. Is shimming the needle further and putting smaller jet going to increase fuel? Today i changed fuel filter, pump, and cleaned fuel check valve. Wasnt getting good fuel pressure before. Didnt help anything though still looks lean and bogs at WOT. |
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Post by Baizy on Apr 20, 2021 21:10:03 GMT -5
I'll try to break it down so I can try to clearly communicate a few ideas without me getting them jumbled up.
-Different airboxes will flow air differently through the rpms
-the stock airbox is mean to provide consistent airflow across the whole rev range and the carb spring and needle is tuned to work with it
-pod filters flow significantly better than stock in the 2000-6500rpm range
-pod filters flow a little better than stock in the 6500+ revs
-The issue is that as your revs are building up the carb flows more air into the cylinder (more air volume) but is only pulling a bit more fuel as the calibrated spring is sensing the same velocity of air. To fix that issue you need to raise the needle which adds more fuel in the midrange, or cut the spring so it will raise the needle earlier in the revs. However the stock carb has a VERY LIMITED amount of space for needle adjustment compared to most other carbs so you cant tune to get enough fuel in the low revs and inevitably have to jet up.
-Jetting up fixes the midrange being too lean and now you can make it into the high rpms. In the higher rpms you're flowing a bit more air than before (bit more volume), but way more fuel! So you end up running rich up top and you can't fix that as you're already cramming as much air into your carb as you can.
- a different needle shape would be the only way to fix the fuel delivery. Thinner at the top than stock and thicker at the tip. More fuel in the middle and less up top.
I hope that makes sense to anybody who reads it in the future.
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rage
n00b Ruckster
Posts: 8
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Post by rage on May 3, 2021 12:20:13 GMT -5
Thanks for the in depth explination Baizy!
Got the jetting down and still had the speed decrease. I put lighter variator weights in at the same time as the intake mod which was the problem. Learned my lesson not to change too many things at the same time!
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Post by Baizy on May 3, 2021 21:29:42 GMT -5
Happy to hear I made sense that time around. Even happier to hear you're back out on the streets hitting the speeds you were after. Enjoy the ride rage |
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