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I'm not a savior!!!
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148 Posts
Discussion Starter #1
I have an '89 750 with a header, Hindle muffler,36MM carbs. i was told it has a jet kit.I was only able to read the numbers on 2 of the main jets,they are 112.5.These 2 had bigger heads than the 2 other ones. The other 2 also looked like they had different numbers. It;s really hard to tell what they are. Is 112.5 stock jets? What should the correct size be? I'm near L.A. elevation is around 400' or so. Thanks
 

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the important thing is how does your bike run? are you having issues? the intake is the big factor in having a "jet kit"

to answer your question; 112.5 Mikuni main jets are stock size.
 

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Likes to race old junk.
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1,875 Posts
There are too many variables to accurately predict what the perfect jet size is...weather, elevation, temperature, relative humidity on that particular day, each bike's individual quirks etc.
You're already close with the factory jetting...assuming it runs, idles, carbs are in good condition etc.
You just have some bolt-on mods, the fuel demands haven't changed that much...we're not trying to feed a 1340cc beast all of a sudden.
You still haven't responded to why you are changing the jets...what changed? what are you trying to do?
It's all trial and error...check the plugs, how did it run, change the jets again...getting better, change the jets again...oops now you made it worse...2 steps forwards, 1 step back.
Even on a dyno, it's still back and forth, but you can accurately measure your results.
Welcome to carbs.
 

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I'm not a savior!!!
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148 Posts
Discussion Starter #7
Sorry for the delay. I figured the stock jets were perhaps on the leaner side, since i have a header and an aftermarket muffler,giving more airflow. I do plan on dyno'ing one of these days, but for now,i just wanted to go 1 size up and see how it runs. It's been sitting for about 2 years now and I'm anxious to just get it on the road already. I plan to also hook up a wide-band meter to see what the ratio is ,too. Thanks for responding!!!
 

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Accurate Butt Dyno
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Nothing wrong with going up a size and evaluating your plugs, but unless your plugs are white white with the 112.5 you are probably safe? Are you getting lean backfires on deceleration? If not, then you are probably safe as is. Just my $0.02
 

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I'm not a savior!!!
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148 Posts
Discussion Starter #9
I don't remember if it backfires. Wouldn't backfiring be from a lean pilot jet,since the throttle is closed and not on the main circuit? Also, if I go 115,should i do it on the outers only and leave 112 in the inners? Thanks.
 

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I have similar mods to a 90 1100 with 36mm flat sides. It took me days to get the carbs dialed in. Those days were spent changing, testing and reading plugs on a good stretch of unpopulated road. Mikuni has a great method for dialing in carbs that is based on throttle position and acceleration.

In this order: You essentially find the main jet that has the best 3/4 to full throttle acceleration. Then you set the needle to find the best acceleration at 1/4 to 1/2 throttle. The Pilot screw sets idle to 1/4 throttle response. All can be done in 2nd gear roll-ons on your unpopulated road. It takes time and patience plus a sense of "feel" for how the engine accelerates.
Based on my results, the stock settings were extremely close. I have header, canister, single pod filters and a better ignition. I am at sea level and found I only needed to richen the main by one size. I would guess that the stock settings will definitely be in the "window" of being correct for your bike, assuming everything is in good mechanical condition within your engine.
 

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I forgot to mention........as a basic rule, you do not vary jets between cylinders on a street bike. The intake runners are equidistant so leaning out or enriching cylinders causes an unbalanced power stroke. Also, the needle jet hardly ever needs to be changed to fit a stock to "hot" street bike setup. The needle in a jet kit has enough of an adjustable range to tune the mid range correctly.
 

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I'm not a savior!!!
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Discussion Starter #12
I've heard middle jets are 1 size smaller because those inner cylinders run hotter than the outer,which have a little more cooling.
 
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