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Post by tvnacman on Nov 18, 2014 3:22:19 GMT -5
well I have a stock 150cc with a pod air filter and free flow exhaust , I use it in the cold weather so to aid in cold start ups I installed a 40 pilot jet I get some engine break up in the upper rpm about 7k I have a 115 main jet I was thinking of dropping in the 120 or 125 main jet and see what happens , I'm using a Keihin 24mm CVK . What are your thoughts ? John I would like to see the results with a 26mm carb and the 115 main . the 26mm will pull more fuel through the 115 then the 24mm carb. I would stick with whatever pilot that the 26mm carb comes with . a 125 main with the 24 mm carb will prob be to rich for the stock bore and stock head . but the extra air flow the 26mm carb will allow should match the extra fuel it will pull with the 115. I have read this several times , it seems just not right . I'm going to step outside the box here for a moment . I'm going to apply some hydronics theory here . They say that the max flow of water through a 3/4" pipe is 8.5 GPM (gallon Per Minute) . The point I going for here is if you take two carbs one with a 26mm and the other with a 24mm throte what did we change if they both have a 115 jet ? Remember we changed the throte size not the jet size . I think we just increased the amount of air while the fuel flow rate stayed the same . I would think you can pass more air through a larger opening . The flip side here might be the larger carb will have a larger pilot jet , do we know this for certain ? Doesn't the pilot jet drop out when the butterfly is opened ? What do you guys think ? Have I gone off the deep end here ? John |
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Post by geh3333 on Nov 18, 2014 4:51:31 GMT -5
I hear ya , is this one of those myths ? Is It really that a 32mm carb will not actually pull more fuel at top end out of a 125 main then a 24mm would be able to but will allow for more fuel to be used ? Meaning a 24mm carb will not be able to match the airflow of a 32mm thus the 24mm will need to be tuned in a way its not burning or using as much fuel? The 32 mm will be able to burn a higher quantity of fuel then the 24mm thus its said that the bigger carb pulls more fuel? Or does the bigger carb as it reaches the top end actually gain more suction then the 24mm at top end ?,possibly due to the engine running at a higher HP and much stronger ? I've never done any tests nor do I have the equipment to do the tests . when it comes to most experts they say the bigger carb pulls more fuel from the same jet used in a smaller carb . very good question
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Post by scooter on Nov 18, 2014 17:17:47 GMT -5
I hear ya , is this one of those myths ? Is It really that a 32mm carb will not actually pull more fuel at top end out of a 125 main then a 24mm would be able to but will allow for more fuel to be used ? Meaning a 24mm carb will not be able to match the airflow of a 32mm thus the 24mm will need to be tuned in a way its not burning or using as much fuel? The 32 mm will be able to burn a higher quantity of fuel then the 24mm thus its said that the bigger carb pulls more fuel? Or does the bigger carb as it reaches the top end actually gain more suction then the 24mm at top end ?,possibly due to the engine running at a higher HP and much stronger ? I've never done any tests nor do I have the equipment to do the tests . when it comes to most experts they say the bigger carb pulls more fuel from the same jet used in a smaller carb . very good question I'm lost here but I'm going to guess that a smaller carb is going to have higher vacuum and will thus pull more fuel out of the same size jet. Obviously it cannot supply more air than the larger carb though. I believe we're trying to achieve a 14.7:1 stoichiometric mix. If you use a bigger carb, maybe you'll need a bigger jet? |
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Post by scooter on Nov 18, 2014 17:28:14 GMT -5
When I was testing jets on my 150cc, I just kept putting in bigger jets and trying them out at top speed. At 115, I felt the throttle had the most power, and I could easily hit 60 on the Chinometer with no trouble. When I went up one more step to a 120, it took longer to get to 60mph. I believe that means I should use the 115 for maximum power.
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Post by tvnacman on Nov 18, 2014 17:55:37 GMT -5
When I was testing jets on my 150cc, I just kept putting in bigger jets and trying them out at top speed. At 115, I felt the throttle had the most power, and I could easily hit 60 on the Chinometer with no trouble. When I went up one more step to a 120, it took longer to get to 60mph. I believe that means I should use the 115 for maximum power. What kind of air filter and exhaust system ? John |
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Post by scooter on Nov 18, 2014 18:03:55 GMT -5
When I was testing jets on my 150cc, I just kept putting in bigger jets and trying them out at top speed. At 115, I felt the throttle had the most power, and I could easily hit 60 on the Chinometer with no trouble. When I went up one more step to a 120, it took longer to get to 60mph. I believe that means I should use the 115 for maximum power. What kind of air filter and exhaust system ? John Stock filter and exhaust. At sea level. Started with 105 jet in the carb and went up from there. Engine ran pretty hot with the 105. 115 gave me max hp as far as I can tell, and no more "hot" smell and "clinking" sounds as the the engine cools down. I think the 105 jet was seriously bad for this engine.    |
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Post by xyshannen on Nov 19, 2014 15:28:40 GMT -5
Scoot, a lean main jet will give you a little better acceleration. Wile your 115 might have been optimum the only real way to tell is to do a plug chop.
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Post by geh3333 on Nov 19, 2014 20:35:37 GMT -5
I hear ya , is this one of those myths ? Is It really that a 32mm carb will not actually pull more fuel at top end out of a 125 main then a 24mm would be able to but will allow for more fuel to be used ? Meaning a 24mm carb will not be able to match the airflow of a 32mm thus the 24mm will need to be tuned in a way its not burning or using as much fuel? The 32 mm will be able to burn a higher quantity of fuel then the 24mm thus its said that the bigger carb pulls more fuel? Or does the bigger carb as it reaches the top end actually gain more suction then the 24mm at top end ?,possibly due to the engine running at a higher HP and much stronger ? I've never done any tests nor do I have the equipment to do the tests . when it comes to most experts they say the bigger carb pulls more fuel from the same jet used in a smaller carb . very good question I'm lost here but I'm going to guess that a smaller carb is going to have higher vacuum and will thus pull more fuel out of the same size jet. Obviously it cannot supply more air than the larger carb though. I believe we're trying to achieve a 14.7:1 stoichiometric mix. If you use a bigger carb, maybe you'll need a bigger jet? I believe a 30mm will pull more fuel through a 115 main " at top end " then a 24mm will because when it comes to the bigger carb you will be able to tune the carb to use more fuel " because of a higher air flow" and when you reach top end your engine is running harder and with more HP. Let's say you have a straw with both ends open. Towards the end of the straw you have a very thin hose attached to the straw " the hose piercing through the side of the straw " with the other end in a bowl of fuel. If you lightly suck on the straw no gas will enter through the hose due to the air being much lighter and less dense . but the harder you suck the more gas will enter the straw . so as more fuel and air is allowed to enter the head and cylinder the bigger the combustion " causing more hp " and the harder " faster" the cylinder will drop and suck the fuel from the jet. Thats my theory , hope it makes sense . |
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Post by geh3333 on Nov 19, 2014 20:52:36 GMT -5
When I was testing jets on my 150cc, I just kept putting in bigger jets and trying them out at top speed. At 115, I felt the throttle had the most power, and I could easily hit 60 on the Chinometer with no trouble. When I went up one more step to a 120, it took longer to get to 60mph. I believe that means I should use the 115 for maximum power. From my experience a 115 main is about tops on a stock setup . however if u would get a high flow air filter and exhaust You should be able to go with a 26 or 28 mm carb and draw in more fuel through the 115 or even up to a 120,main . these stock setups are really restricting the amount of air and fuel the engine can take . |
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Post by geh3333 on Nov 19, 2014 20:54:47 GMT -5
Scoot, a lean main jet will give you a little better acceleration. Wile your 115 might have been optimum the only real way to tell is to do a plug chop. Yes it it best to go by the plug , a little rich is good but a little lean is not . |
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Post by JerryScript on Nov 20, 2014 1:31:03 GMT -5
Here's a little demo you can do at home to get an idea of what is happening. Take two soda cans of differing dimensions, one larger in diameter than the other, but both the same length. Cut the top and bottom out of each can, and punch a hole just large enough for a coffee stirrer straw in the side. Put the straw in the hole you punched, and place the other end in a cup of water. Now put your mouth to one end of the can while holding a paper towel in front of the other end, and blow as hard as you can. See which makes the paper towel more wet, the larger can or the smaller one.
Air flowing through a smaller orifice will create a larger vacuum over the same size straw (jet). It sounds counter-intuitive, until you think of it in reverse. Vacuum is pulling from all sides, and if there is more open space, the vacuum effect is spread over a greater area, making it's effect on a small point such as a jet opening less drastic. Condense that vacuum to a smaller area, and it's effect on a small area (the jet opening) is greater.
There are other considerations to take, a carb is a balanced system, but hopefully this helps explain the vacuum issue a bit.
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Post by tvnacman on Nov 20, 2014 5:38:01 GMT -5
Here's a little demo you can do at home to get an idea of what is happening. Take two soda cans of differing dimensions, one larger in diameter than the other, but both the same length. Cut the top and bottom out of each can, and punch a hole just large enough for a coffee stirrer straw in the side. Put the straw in the hole you punched, and place the other end in a cup of water. Now put your mouth to one end of the can while holding a paper towel in front of the other end, and blow as hard as you can. See which makes the paper towel more wet, the larger can or the smaller one. Air flowing through a smaller orifice will create a larger vacuum over the same size straw (jet). It sounds counter-intuitive, until you think of it in reverse. Vacuum is pulling from all sides, and if there is more open space, the vacuum effect is spread over a greater area, making it's effect on a small point such as a jet opening less drastic. Condense that vacuum to a smaller area, and it's effect on a small area (the jet opening) is greater. There are other considerations to take, a carb is a balanced system, but hopefully this helps explain the vacuum issue a bit. very well said !!!!!!!!!!!!!! How are you doing Jerry ? John |
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Post by JerryScript on Nov 20, 2014 12:15:59 GMT -5
Here's a little demo you can do at home to get an idea of what is happening. Take two soda cans of differing dimensions, one larger in diameter than the other, but both the same length. Cut the top and bottom out of each can, and punch a hole just large enough for a coffee stirrer straw in the side. Put the straw in the hole you punched, and place the other end in a cup of water. Now put your mouth to one end of the can while holding a paper towel in front of the other end, and blow as hard as you can. See which makes the paper towel more wet, the larger can or the smaller one. Air flowing through a smaller orifice will create a larger vacuum over the same size straw (jet). It sounds counter-intuitive, until you think of it in reverse. Vacuum is pulling from all sides, and if there is more open space, the vacuum effect is spread over a greater area, making it's effect on a small point such as a jet opening less drastic. Condense that vacuum to a smaller area, and it's effect on a small area (the jet opening) is greater. There are other considerations to take, a carb is a balanced system, but hopefully this helps explain the vacuum issue a bit. very well said !!!!!!!!!!!!!! How are you doing Jerry ? John Thanks John! I've been swamped all holiday season, it starts in Oct and I don't stop working 14 hr days till the new year. Good news is, I'm looking forward to semi-retiring next year!  |
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Post by geh3333 on Nov 20, 2014 16:56:09 GMT -5
very well said !!!!!!!!!!!!!! How are you doing Jerry ? John Thanks John! I've been swamped all holiday season, it starts in Oct and I don't stop working 14 hr days till the new year. Good news is, I'm looking forward to semi-retiring next year! That is a good way to test , I do believe it has more to do with the ability to increase the amount of air to fuel with a bigger carb that comes into play . I believe if we were able to spin the engine without ignition the vacuum will cause greater fuel flow through a smaller carb . however we must take the fact that we need ignition to occur and the more fuel and air that can enter the chamber the greater the combustion and the faster the piston drops causing greater suction. I do believe thats the reason why its only pulling more fuel through a same size jet at top end , because that's were u hit your top HP . it sounds as if this theory has certain Guidelines to follow such as how much fuel can be pulled through the jet and still achieve a proper burn . this is the only way I can see how a bigger carb will pull more fuel , because a smaller carb will be tuned in a way so the fuel matches the lower air flow and the result is as follows . 24mm with 115 main 38pilot vs 26mm with 115main 38pilot 26mm will be tuned to match greater airflow This means more fuel flow from pilot at low and mid The greater fuel and air flow = more HP At top end the engine is performing greater with the extra HP and the vacuum caused by running the 2mm bigger carb is enough so that it not only equals but surpasses the vacuum of the 24mm thus pulling more fuel through the 115 main. This is the only way I see this theory being possible. |
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Post by scooter on Nov 20, 2014 17:04:47 GMT -5
Scoot, a lean main jet will give you a little better acceleration. Wile your 115 might have been optimum the only real way to tell is to do a plug chop. I've never done a plug chop and I'm not sure how well I'll be able to read the plug. I was looking at a page about jets and it said you get max hp when running a little rich. I have no idea. I would have thought lean gave more power as well. |
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