87 trx250xPerformance exhaust and intake carb Jetting Question. Please Help! - Page 2 - Honda TRX Forum
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post #11 of 13 Old 11-16-2019, 02:27 PM
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Don't change the needle position unless you need to. The needle controls fuel in the mid range (steady state throttle position).

Here's a crash course in carburation jetting:

A simple carburetor has three fuel control circuits, idle, mid-range and full throttle. The pilot circuit controls fuel at idle to about 1/8 throttle - above 1/8 throttle you could eliminate the pilot circuit with very little effect on operation. The mid-range is 1/8 to 3/4 throttle - this is controlled by the slide needle. Throttle opening above 3/4 is full throttle - the main jet is the only control of fuel for throttle openings above 3/4.

Changes to the exhaust have very little impact on jetting except at full throttle. Changes to the intake affect jetting at all throttle settings with the least affect on idle. While idle is the most difficult to set accurately and is the most critical at low speed, most changes to the intake and exhaust have zero to little effect on the original setting. Most changes to the exhaust and intake have little effect on the mid range. Putting on a low restriction exhaust and installing a low restriction air filter may or may not have a noticeable effect on the mid-range. If the mid-range is affected, it generally leans out due to the low restriction intake and the smoothness of the engine may be improved by raising the slide needle one notch (non-adjustable needles can be shimmed with small washers to raise). The most affected by changes is full throttle settings. If there is less to no intake restriction and no exhaust restriction (straight pipe of the same diameter as the exhaust port) then the air flow is increased and more fuel will be required to mix with the greater volume of air entering the engine. This is not a precise science as variables include type of air filter media, type of oil used on the air filter (if any), length and straightness on intake tract, distance between carb and intake port, size of valves in the engine, valve lift and duration of valve opening, overlap between intake and exhaust valves being open at the same time, diameter of the exhaust pipe, length of the exhaust pipe, number of and radius of the bends in the exhaust pipe and design of the muffler (if any).

The is no magic formula that determines what size jet is right. Two identical machines with the exact same modifications might be jetted differently due to manufacturing differences within the carburetor, camshaft and cam timing. And precise jetting will be affected by air density and humidity.

Unfortunately, the only way to get the jetting correct is to ride it and adjust the jetting as required for the ambient conditions. Fortunately, the carb manufacturer has installed an access cap on the float bowl so the main jet can be changed without removing the carb. They don't provide access to the pilot jet, because once the idle is right is doesn't change due to the atmospheric variations and while mid-range can be adjusted without removing the carb, it doesn't usually require adjustment due to changes in weather and atmosphere. Only the main jet requires changes due to the atmospheric changes, but only for those serious about maximum performance. I have never changed the jetting on my race bike since it was set up. I have raced on 100 degree summer days and in the winter when the temp was below freezing and there was close to a foot of snow on the ground, but I am racing off road natural terrain, if I was racing flat track where the throttle was wide open most of the time, then I might have to change the jetting as required by conditions. Off road is brief wide open throttle followed by slamming the throttle shut and snapping back to wide open while interspersed with occasional steady state partial throttle settings.

Hope you find this helpful
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post #12 of 13 Old 11-16-2019, 03:30 PM Thread Starter
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Quote:
Originally Posted by Here 2 help View Post
Don't change the needle position unless you need to. The needle controls fuel in the mid range (steady state throttle position).

Here's a crash course in carburation jetting:

A simple carburetor has three fuel control circuits, idle, mid-range and full throttle. The pilot circuit controls fuel at idle to about 1/8 throttle - above 1/8 throttle you could eliminate the pilot circuit with very little effect on operation. The mid-range is 1/8 to 3/4 throttle - this is controlled by the slide needle. Throttle opening above 3/4 is full throttle - the main jet is the only control of fuel for throttle openings above 3/4.

Changes to the exhaust have very little impact on jetting except at full throttle. Changes to the intake affect jetting at all throttle settings with the least affect on idle. While idle is the most difficult to set accurately and is the most critical at low speed, most changes to the intake and exhaust have zero to little effect on the original setting. Most changes to the exhaust and intake have little effect on the mid range. Putting on a low restriction exhaust and installing a low restriction air filter may or may not have a noticeable effect on the mid-range. If the mid-range is affected, it generally leans out due to the low restriction intake and the smoothness of the engine may be improved by raising the slide needle one notch (non-adjustable needles can be shimmed with small washers to raise). The most affected by changes is full throttle settings. If there is less to no intake restriction and no exhaust restriction (straight pipe of the same diameter as the exhaust port) then the air flow is increased and more fuel will be required to mix with the greater volume of air entering the engine. This is not a precise science as variables include type of air filter media, type of oil used on the air filter (if any), length and straightness on intake tract, distance between carb and intake port, size of valves in the engine, valve lift and duration of valve opening, overlap between intake and exhaust valves being open at the same time, diameter of the exhaust pipe, length of the exhaust pipe, number of and radius of the bends in the exhaust pipe and design of the muffler (if any).

The is no magic formula that determines what size jet is right. Two identical machines with the exact same modifications might be jetted differently due to manufacturing differences within the carburetor, camshaft and cam timing. And precise jetting will be affected by air density and humidity.

Unfortunately, the only way to get the jetting correct is to ride it and adjust the jetting as required for the ambient conditions. Fortunately, the carb manufacturer has installed an access cap on the float bowl so the main jet can be changed without removing the carb. They don't provide access to the pilot jet, because once the idle is right is doesn't change due to the atmospheric variations and while mid-range can be adjusted without removing the carb, it doesn't usually require adjustment due to changes in weather and atmosphere. Only the main jet requires changes due to the atmospheric changes, but only for those serious about maximum performance. I have never changed the jetting on my race bike since it was set up. I have raced on 100 degree summer days and in the winter when the temp was below freezing and there was close to a foot of snow on the ground, but I am racing off road natural terrain, if I was racing flat track where the throttle was wide open most of the time, then I might have to change the jetting as required by conditions. Off road is brief wide open throttle followed by slamming the throttle shut and snapping back to wide open while interspersed with occasional steady state partial throttle settings.

Hope you find this helpful
That was actually extremely helpful! I will leave the float and needle positions alone as it does run very well in mid range. The only real noticable running issue with it is the idle. I'm going to still rejet to 38 pilot and to 130 main because I know I can adjust the mixture if they are too big. This was extremely helpful!! I still dont know why it would be running rich as is but I suspect it is because of this carb hence why I'm rebuilding it. Thank you for all of the immediate and great help!! I owe you one!! In conclusion... I'll leave float position alone, rejet to these specs and change out main jet if it's too rich and bogging down. I'm glad to know that I can change the main jet without pulling the blessed thing off!
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post #13 of 13 Old 11-17-2019, 01:48 PM Thread Starter
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Quote:
Originally Posted by Flamin 250 View Post
Quote:
Originally Posted by Here 2 help View Post
Don't change the needle position unless you need to. The needle controls fuel in the mid range (steady state throttle position).

Here's a crash course in carburation jetting:

A simple carburetor has three fuel control circuits, idle, mid-range and full throttle. The pilot circuit controls fuel at idle to about 1/8 throttle - above 1/8 throttle you could eliminate the pilot circuit with very little effect on operation. The mid-range is 1/8 to 3/4 throttle - this is controlled by the slide needle. Throttle opening above 3/4 is full throttle - the main jet is the only control of fuel for throttle openings above 3/4.

Changes to the exhaust have very little impact on jetting except at full throttle. Changes to the intake affect jetting at all throttle settings with the least affect on idle. While idle is the most difficult to set accurately and is the most critical at low speed, most changes to the intake and exhaust have zero to little effect on the original setting. Most changes to the exhaust and intake have little effect on the mid range. Putting on a low restriction exhaust and installing a low restriction air filter may or may not have a noticeable effect on the mid-range. If the mid-range is affected, it generally leans out due to the low restriction intake and the smoothness of the engine may be improved by raising the slide needle one notch (non-adjustable needles can be shimmed with small washers to raise). The most affected by changes is full throttle settings. If there is less to no intake restriction and no exhaust restriction (straight pipe of the same diameter as the exhaust port) then the air flow is increased and more fuel will be required to mix with the greater volume of air entering the engine. This is not a precise science as variables include type of air filter media, type of oil used on the air filter (if any), length and straightness on intake tract, distance between carb and intake port, size of valves in the engine, valve lift and duration of valve opening, overlap between intake and exhaust valves being open at the same time, diameter of the exhaust pipe, length of the exhaust pipe, number of and radius of the bends in the exhaust pipe and design of the muffler (if any).

The is no magic formula that determines what size jet is right. Two identical machines with the exact same modifications might be jetted differently due to manufacturing differences within the carburetor, camshaft and cam timing. And precise jetting will be affected by air density and humidity.

Unfortunately, the only way to get the jetting correct is to ride it and adjust the jetting as required for the ambient conditions. Fortunately, the carb manufacturer has installed an access cap on the float bowl so the main jet can be changed without removing the carb. They don't provide access to the pilot jet, because once the idle is right is doesn't change due to the atmospheric variations and while mid-range can be adjusted without removing the carb, it doesn't usually require adjustment due to changes in weather and atmosphere. Only the main jet requires changes due to the atmospheric changes, but only for those serious about maximum performance. I have never changed the jetting on my race bike since it was set up. I have raced on 100 degree summer days and in the winter when the temp was below freezing and there was close to a foot of snow on the ground, but I am racing off road natural terrain, if I was racing flat track where the throttle was wide open most of the time, then I might have to change the jetting as required by conditions. Off road is brief wide open throttle followed by slamming the throttle shut and snapping back to wide open while interspersed with occasional steady state partial throttle settings.

Hope you find this helpful
That was actually extremely helpful! I will leave the float and needle positions alone as it does run very well in mid range. The only real noticable running issue with it is the idle. I'm going to still rejet to 38 pilot and to 130 main because I know I can adjust the mixture if they are too big. This was extremely helpful!! I still dont know why it would be running rich as is but I suspect it is because of this carb hence why I'm rebuilding it. Thank you for all of the immediate and great help!! I owe you one!! In conclusion... I'll leave float position alone, rejet to these specs and change out main jet if it's too rich and bogging down. I'm glad to know that I can change the main jet without pulling the blessed thing off!
I have great news. The carb rebuild and rejet was a success. I'm currently tuning by adjusting the mixture screw, ripping on it and the looking at the plug. The plug is a very light sandy color which seems about right! I'm going to continue tightening it up since its winter here and see if I can get the plug a little darker because I'm thinking it is still on the lean side. Any tips for tuning procedure?
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