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Another Carb Rejetting Question

  • Thread starter Thread starter ttfraz
  • Start date Start date
T

ttfraz

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Hey guys. So I tore apart my carbs today and was thinking about what I'd need to get if I wanted to rejet for pods and a 4-1. I've looked a the popular link here and read up on it. The wiki page it references says to go up 6 sizes in the example ex: 112.5 to 118.5. Is this correct seeing how 118.5 isn't a size since they go up by 2.5


also I have a 77 gs 750 and the carb reference here says that stock it runs a 100 main jet but when I tore my carbs up I had a 97.5. Should I base my calculations on the stock # or what I actually have for a jet?


Heres the the page I was looking at that was cited http://motorcycles.wikia.com/wiki/Rejetting
 
According to Mikuni, six sizes up from 112.5 would be 127.5, the 2.5 increments are half sizes.
 
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For what it's worth, my bike came with K&N pods, stock exhaust, and 110 mains. It rode decently when I bought it, however I have not been able to get it running well after cleaning the carbs. I'm hoping that the jetting isn't my problem.
 
Jordan that's a great picture. I thought I was going crazy with 97.5 mains and 27.5. Everywhere I looked it said 100 & 15

lifevsaver and a good place to start
 
I've been doing some research and a lot of reading about how to really tune in your carbs but no where can i find what turning in and out the pilot fuel screw does. al they ever talk about is adjusting the pilot air screw

Here is my stock carb setting (the 77 GS750 with 97.5 mains)
 
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So, the sizing is actually in millimeters, not the 2.5 mm jet sizes when calculating proper sizing? As in two sizes up from my 92.5's would be 94.5 not 97.5's but I'd buy 95's because they are no 94.5's ?
 
The pilot jet is not adjustable. The fuel screw is under the carb and regulates fuel, start off around 2 full turns out and 1 1/2 on the air screw.
 
allojohn said:
The pilot jet is not adjustable. The fuel screw is under the carb and regulates fuel, start off around 2 full turns out and 1 1/2 on the air screw.
Click to expand...

Way too far out for the fuel screw on VM carbs. Something like 3/4 turn out to start with.
 
tkent02 said:
Way too far out for the fuel screw on VM carbs. Something like 3/4 turn out to start with.
Click to expand...

Even with a 4-1 and pods? I set my 750 up the same way 5 years ago but have since gone back to stock ~ memory's a bit fuzzy.....
 
4-1 and pods add no airflow at idle and very low throttle, which is the only place the fuel screws are doing anything. I start with 5/8 of a turn out, but I'm up at higher elevation, you may need 3/4 or 7/8. And don't seat the screw to start counting, the tip is tiny and soft and will break off and stick in the port, sometimes ruining the carburetor. Very light bottoming is enough to break it off.
 
where is it mentioned that 5mm is a full size and 2.5mm is a half size? I missed that completely!:mad:
 
smilinbrad said:
where is it mentioned that 5mm is a full size and 2.5mm is a half size? I missed that completely!:mad:
Click to expand...

Not sure about the "size vs half-size" thing, but get rid of the "5mm" and "2.5mm" terminology. The sizes are NUMBERED, not necessarily MEASURED.

For example, the chart above says that the stock main jet for your carbs is 97.5. That is NOT 97.5mm.

The next available jet size (regardless whether it's a half- or full-size) will be 100. The next one after that is 102.5, etc.

The suggested jetting for pods and a pipe will depend on the pods and the pipe, but generally, add 2-4 sizes for the pipe and 3-5 for the pods.
So, for both pods and pipe, you might need to increase 5-9 sizes. Starting with your 97.5 jets, you would be looking for 110-120 mains.
Which ones you need will depend on how restrictive your pods and/or pipe are. More restrictive will keep the jet sizes lower.
 
Mikuni's jets are rated in flow rates, as in a certain amount of fluid will pass through the jet per minute at a certain pressure. Other manufacturers do it differently, some do use hole diameter, there are other methods as well. A lot time ago Mikuni jets only came in every five, such as 95, 100, 105, 110 but when they decided to go every 2.5 to enable more precise tuning those were now called half sizes so all of their old literature about tuning wouldn't be wrong. Going up a size is still going up a size, but now you can go up a half size too.

So if you read something that says go up two sizes from 100 it means 110. A lot of people don't know this, so they may say thats four sizes up, and others don't know how they are numbered at all would call it ten sizes.
 
Just to elaborate on what Tom said, a Mikuni jet measurement refers to the volume (in ccs of gasoline) that will flow through the jet, per minute, when released from a height of 50cms at sea level.

Hitachi jets differ in that the number relates to the actual size of the opening eg a #108 jet is 1.08mm diameter.
 
Hey tom thanks for the info. and all others input. either way ill grab a few different sizes and play around .

Anyone know of a place i can try different sizes and return the new i don't want? i may just have to call my local shop for that.

Also i was reading up on this website about tuning carbs. some one else can give it a look over if they want but seems like the way to go to get your cars truly tuned for these changes

http://www.smellofdeath.com/lloydy/jetting.htm
 
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