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/cgi-bin/YaBB.pl General Category >> Rubber Side Down! >> Simple jetting test question - full throttle/decel /cgi-bin/YaBB.pl?num=1250728861 Message started by voldigicam on 08/19/09 at 17:41:01 |
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Title: Simple jetting test question - full throttle/decel Post by voldigicam on 08/19/09 at 17:41:01 2007 S40 stock carb, 55 / 152.5 1/2 spacer, dyna muffler. Full throttle up a hill in 4th, then back off a little. With the 150, got a little bump of power. With the 152.5, rpm drop, then hesitate briefly, then decline to steady state. Is this perfect, slightly lean, slightly rich? I really don't know, but I do know this is the best this bike has run so far. I have a 155 jet, but I don't want to pop it in if I'm fine. Thanks! |
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Title: Re: Simple jetting test question - full throttle/d Post by Serowbot on 08/19/09 at 18:15:15 If you're no longer getting a little bump up,... you're good...beyond that you'll just waste gas.... ;) |
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Title: Re: Simple jetting test question - full throttle/d Post by DrunkenDwarf on 08/19/09 at 18:36:01 voldigicam, I've been trying to do that test and I'm doing something wrong because I can't feel that "little bump of power". Could you explain exactly how you tested? Speeds, durations, etc? Thanks. -D. Dwarf |
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Title: Re: Simple jetting test question - full throttle/d Post by Serowbot on 08/19/09 at 19:28:59 To test the main, accelerate from 30 or 40 mph, in 4th or 5th gear at full throttle for several seconds, then reduce throttle by about 1/8,...if power increases for a second, you are lean on the main jet. Go up one jet size and test again. |
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Title: Re: Simple jetting test question - full throttle/d Post by dasch on 08/19/09 at 21:41:03 Dwarf, in that case your main jet seems to be fine. You only get it if it's slightly (or a lot) small. |
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Title: Re: Simple jetting test question - full throttle/d Post by voldigicam on 08/20/09 at 03:55:54 I put a throttle gauge on made of high-tech masking tape, so I can dial in 1/4 or 1/8 or whatever throttle. Turns out I rarely use more than 1/3 throttle unless I'm popping into traffic on the 4 lane US highway. The problem here is that I really don't have much room on most roads to run up from 30 in 4th gear for very long! The hill is useful for that. With the original jet & stock muffler I could run up for about 3 seconds full throttle then just roll off a tiny bit. The bike would give a nice little short vroom and pull. That stopped going to the 150, with nothing but a steady drop no matter what. I suppose I'm just right on the edge of lean at this point. So the bike is running just about perfect I suppose. I can't feel the pilot/main transition at all. Comes off idle well. Still get the rare exhaust pop on closing the throttle. Real pop, rather than the standard burble. The dyna muffler sounds better, I suppose. Women swoon and men cower, as is appropriate. The squirrels still won't get out of the road, but at least they look up!!! |
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Title: Re: Simple jetting test question - full throttle/d Post by voldigicam on 08/20/09 at 07:47:38 Addendum: Presuming I am the very tiniest bit lean at 1/2 throttle and more . . . I presume I would pull the needle up a little. That is, less spacer. Just a bit. I think I'm at 1/2 spacer. So perhaps it would be worth trying 1/3 spacer????? Opinions? Just the next time I have the seat off. I can pull the carb top and do that easily. |
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Title: Re: Simple jetting test question - full throttle/d Post by srinath on 08/20/09 at 08:06:42 3A2C3B263E2B263D490 wrote:
Oh yes this is a great test. The key though, you need to get that throttle to WFO and stay there for a few seconds till you see it really start pulling ... And speed is almost irrelevant. You need load, so the slowest it can run in 4th or 5th ... And throttle position determines what jet you are using, not rpm or speed. In a way, I find it is easier and less confusing to tune a bike wihtout a tach. You mark on the grip 0 (where the free play ends and actual throttle action begins) 1/8, 1/4. 3/8th, 1/2, as it goes on till full in increments of an 8th. Then go by that when you make your observations. Also I wont say use a good average day for jetting ... I would jet it to run right and be just rich enough on the coldest day in the region you live in. Then check it on the hottest day and you will probably be just under being too rich for that day, which is the best way to be on a hot day. Usually in most of the eastern US summers are humid. Winters are less humid and that makes for near perfect jetting for all types of weathers. Cool. Srinath. |
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Title: Re: Simple jetting test question - full throttle/d Post by voldigicam on 08/20/09 at 08:13:24 Makes sense to me. We're humid today, but when the roads dry I get to run into town, so I'll be on a real highway with some hills. Easier to test without worrying about driveways and stray cats. Or suicide squirrels (common here). I do not want squirrel bits in my cooling fins!!!!! |
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Title: Re: Simple jetting test question - full throttle/d Post by DrunkenDwarf on 08/20/09 at 11:13:29 3B3A2126293C20480 wrote:
Someone please correct me. The piston valve (needle) isn't directly connected to the throttle. It's raised up by the vacuum in the venturi. To really test the main jet, we need to get that valve as far open as possible. Otherwise the needle will obstruct some of the flow. That means we need the biggest vacuum in the venturi. Since the vacuum is caused by the movement of air, we need the maximum amount of air moving through the venturi. Opening the throttle completely provides the opportunity for the maximum amount of airflow. Wouldn't a high RPM also be required to actually pull the maximum amount air through? The alternative is that the piston valve is so sensitive to the pressure drop in the venturi that even with low air velocity, the high volume will open it completely. -D. Dwarf |
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Title: Re: Simple jetting test question - full throttle/d Post by verslagen1 on 08/20/09 at 11:40:19 474E4B42584F4B2A0 wrote:
You talking about the slide? there you've been corrected, happy? |
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Title: Re: Simple jetting test question - full throttle/d Post by DrunkenDwarf on 08/20/09 at 12:07:14 716275746B6660626936070 wrote:
You talking about the slide? there you've been corrected, happy?[/quote] Yeah, slide. I've seen it called different things in different documentation. -D. Dwarf |
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