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Main jet question

  • Thread starter Thread starter helmutholderbenz
  • Start date Start date
H

helmutholderbenz

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Hi, I have a 83 750e I just got finished putting together. I put a Wiseco 816 kit in, 4-1 exhaust, supertrapp r series canister w/6 disks, pod filters, 85" 700 cams. My bike started right up and with a little tuning is running strong. My carbs are the bs32ss, they have been drilled and the main jet has been changed to 140. I think this is too big. With what I've stated, what is a good main jet size I should start from? I got better response with my air screw turned out 3t. Thanks for any bodys help!
 
The general "rule of thumb" for jetting increases will ask you to go up 3 to 5 sizes for an exhaust system and 4 to 6 sizes for pods. Since you have both, something in the range from 7 to 11 sizes larger than stock. Stock jets are 117.5. 7 sizes larger would be 135, 11 sizes would be 145, so you could be close. I have also heard that a bore increase will require a slightly smaller main jet, but I have no experience with that, so I don't know how much. Just a wild guess here, but I am thinking you might need something in the 130 to 140 range.

And, ... you don't have "air screws", those are your "pilot mixture screws". Three turns out is a good starting point and might be where your bike needs to be, with those modifications, but a little fine-tuning won't hurt anything. If you find that it runs even better when they are turned more than about 4 1/2 turns, consider upping the size of the pilot fuel jet one size, from 37.5 to 40.

What are your spark plugs telling you?

.
 
Steve said:
The general "rule of thumb" for jetting increases will ask you to go up 3 to 5 sizes for an exhaust system and 4 to 6 sizes for pods. Since you have both, something in the range from 7 to 11 sizes larger than stock. Stock jets are 117.5. 7 sizes larger would be 135, 11 sizes would be 145, so you could be close. I have also heard that a bore increase will require a slightly smaller main jet, but I have no experience with that, so I don't know how much. Just a wild guess here, but I am thinking you might need something in the 130 to 140 range.

And, ... you don't have "air screws", those are your "pilot mixture screws". Three turns out is a good starting point and might be where your bike needs to be, with those modifications, but a little fine-tuning won't hurt anything. If you find that it runs even better when they are turned more than about 4 1/2 turns, consider upping the size of the pilot fuel jet one size, from 37.5 to 40.

What are your spark plugs telling you?

.
Click to expand...
My plugs look good, the electrodes are grayish and the ceramic is a light tan. Yesterday I changed main jets to 127.5s but the motor seemed to like the 140s better so I put them back in. While I was in there I checked all my float heights and they are all at factory specs. I did all this with my pilot air screws set at 2 1/2 turns out. the bike ran good but it wouldn't idle very well and would change when I got back from a test ride. The last thing I did last night was change my mixture screws to 3 turns out and the bikes idle is now rock solid at where ever I set it(1100rpm) It was sluggish at under 1/4 throttle but not too bad. Today I changed the mixture screws to 3 1/2 out and the bike runs better but if I am in 2nd gear around 5 grand under 1/4 throttle I feel a little surging but if I shower down on it it really jumps to life.
 
I've got my bike running real well now but I believe I can get it even better. No more surging, rock steady idle, crisp throttle response. I have 140 main jets and want to try maybe 135s. my question is when I change main jet, do I have to change other settings on the carbs?
 
helmutholderbenz said:
I've got my bike running real well now but I believe I can get it even better. No more surging, rock steady idle, crisp throttle response. I have 140 main jets and want to try maybe 135s. my question is when I change main jet, do I have to change other settings on the carbs?
Click to expand...

I think what posplayr is saying is "yes". Let me expand on this based on my own experience. Everything setting in a carb overlaps another setting. Main jet size is supposed to be the WOT tuning, but I've noticed that changing it will affect the mix curve as it were. For larger, say, not as much effect on idle, maybe, but as the needle starts to rise, there is more fuel than there used to be the higher it gets. But then you might still need to fiddle with your A/F screws to keep your idle/off idle in the sweet spot. Before you know it, you're starting over.
 
GSCafe said:
I think what posplayr is saying is "yes". Let me expand on this based on my own experience. Everything setting in a carb overlaps another setting. Main jet size is supposed to be the WOT tuning, but I've noticed that changing it will affect the mix curve as it were. For larger, say, not as much effect on idle, maybe, but as the needle starts to rise, there is more fuel than there used to be the higher it gets. But then you might still need to fiddle with your A/F screws to keep your idle/off idle in the sweet spot. Before you know it, you're starting over.
Click to expand...
Thanks GSCafe! I'm not sure if I should even do anything. the bike is really running good. Any bike if you put the coals to it in a tall gear is going to have to wind up a bit right? I mean its not a bog really, kinda sounds like a quadrajet carb if you know what I mean. If I'm in the rpms the bike really gets it. the only real reason I wanted to try the 135s is because the table guidelines say with my mods I should go up in jet size 9 to 12 sizes larger than stock. stock for my bike is 117.5 + 12=130s. I'm using 140s
 
John you seam to have done a fine job so far ; just follow what the experts say about jetting, it is not wrong.
 
helmutholderbenz said:
Thanks GSCafe! I'm not sure if I should even do anything. the bike is really running good. Any bike if you put the coals to it in a tall gear is going to have to wind up a bit right? I mean its not a bog really, kinda sounds like a quadrajet carb if you know what I mean. If I'm in the rpms the bike really gets it. the only real reason I wanted to try the 135s is because the table guidelines say with my mods I should go up in jet size 9 to 12 sizes larger than stock. stock for my bike is 117.5 + 12=130s. I'm using 140s
Click to expand...

I sure don't bill myself as an expert but it seems you have already reached the benchmark for your tuning. I would say that trying different main jets would be fine as it is a fixed adjustment; I mean you can just switch back. However, the temptation to turn a mix screw will be strong if you change the mains. I'd say I've reached the same point as you. I'm weighing having the bike put on the dyno with an o2 sniffer as I can't get any closer. And yes, inline 4, the power band is peaky especially if you go pods and pipe. You want torque down low you need a V-twin.
 
Some people will ask a question they already know the answer to to get a conversation started. I want to talk about my bike and what I'm doing to it and how I can make it better. I come to this forum to get help and answers from people I believe have similar interest but most people in this forum get mad at the new guy for asking questions. I thought thats what a live forum was for, to ask questions, give answers and just shar the love we have for these bikes. You guys got me scared to even ask a question. Think about that! Helmut
 
helmutholderbenz said:
Some people will ask a question they already know the answer to to get a conversation started. I want to talk about my bike and what I'm doing to it and how I can make it better. I come to this forum to get help and answers from people I believe have similar interest but most people in this forum get mad at the new guy for asking questions. I thought thats what a live forum was for, to ask questions, give answers and just shar the love we have for these bikes. You guys got me scared to even ask a question. Think about that! Helmut
Click to expand...

I'm not sure what you are referring to. I do know that people in the know can sometimes forget what it was like to be the new guy and lose patience. I'm not like that. Let me know if I've taken a wrong turn. Ask away without fear. You only need to read the stuff that helps you. :)
 
I appreciate you GSCafe, and everyone who has taken time to answer my questions even though you've heard them asked countless times before. Thanks, Helmut
 
helmutholderbenz said:
I appreciate you GSCafe, and everyone who has taken time to answer my questions even though you've heard them asked countless times before. Thanks, Helmut
Click to expand...

Thank you. Honestly, I'm pretty good with the electrical questions but I have some carb input because I struggle with mechanical stuff and have learned it the hard way sometimes. Hard earned knowledge sticks with you though.
 
helmutholderbenz said:
Some people will ask a question they already know the answer to to get a conversation started. I want to talk about my bike and what I'm doing to it and how I can make it better. I come to this forum to get help and answers from people I believe have similar interest but most people in this forum get mad at the new guy for asking questions. I thought thats what a live forum was for, to ask questions, give answers and just shar the love we have for these bikes. You guys got me scared to even ask a question. Think about that! Helmut
Click to expand...
Thank you ; that is one of the most enlightening posts I have read in a very long time.
 
The first post I read I was thinking I'd start with a 130-135 main.
It is always good to start large and work your way down. Keep an eye out for speckled spark plugs which is the beginning of a bad ending.
 
chef1366 said:
The first post I read I was thinking I'd start with a 130-135 main.
It is always good to start large and work your way down. Keep an eye out for speckled spark plugs which is the beginning of a bad ending.
Click to expand...
Thanks chef1366, I will be sure to keep an eye on my spark plugs. (going to go check them now) Thanks everyone for your replies and advice, its much appreciated. I think today I'm not going to work on my bike, I'm going to ride it! (after I check my plugs) Its been a long process and I'm going to enjoy the fruits of my labor! Keep it on two wheels everyone! Helmut
 
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