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Discussion Starter #1
Hello Folks,
What size Jets (slow and main) should I use for a 250 with only exhaust and intake modifications? Intake is a foam type and exhaust is a DG Performance RCM Steel exhaust.( Very loud) Any guidance would help! My guess is 40 pilot and 128 main but I have a feeling that is still too small.
 

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The std. #38 pilot jet should be fine unless fuel screw needs to be turned out more than 4 full turns from lightly seated - the pilot jet is the one it idles on - run engine about 20 minutes and then set fuel screw - connect a tachometer, adjust screw in until rpms drop by 50 - then turn screw out 1 full turn - if the screw is more than 4 full turns from lightly seated, increase jet size from #38 to #40.

Std main jet is #128 - a #145 should be fine

The decibel level of the exhaust has no effect on speed other than in most cases to hamper the acceleration and reduce the top speed. The RCM pipe is not 'very loud' unless the baffle is removed (not recommended) - the engine will perform better with some exhaust back pressure.
 

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Discussion Starter #3
The std. #38 pilot jet should be fine unless fuel screw needs to be turned out more than 4 full turns from lightly seated - the pilot jet is the one it idles on - run engine about 20 minutes and then set fuel screw - connect a tachometer, adjust screw in until rpms drop by 50 - then turn screw out 1 full turn - if the screw is more than 4 full turns from lightly seated, increase jet size from #38 to #40.

Std main jet is #128 - a #145 should be fine

The decibel level of the exhaust has no effect on speed other than in most cases to hamper the acceleration and reduce the top speed. The RCM pipe is not 'very loud' unless the baffle is removed (not recommended) - the engine will perform better with some exhaust back pressure.
Thanks for the reply! The oem main jet is a 125 if I remember correctly. I dont have a tachometer so I'll go off of sound. So what I take from this is that an upgraded #40 pilot should be fine. 145 for main sounds really big! So 128 would be still too small then? Do you have any recommendation for the mixture screw setting? 2.5 turns? How many screws beside isle screw do I need to adjust? Thanks for the answered questions. It's my first time messing with a carb. I am going to rebuild the carb and insert the new jets while I do it. The squad's spark plug is black and wet so a rebuild sounds about right.
 

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If the plug is wet and black, it's already getting too much fuel, rebuilding might help if the problem is the float needle or the float level.

There are only two adjustment screws on the carb - one increases the throttle opening to adjust idle speed - the other adjusts the amount of fuel mixed with the air bleeding past the throttle to help the engine idle smoothly without loading up or starving during periods of engine idling. Do not idle over 20 minutes with fresh oil or over 15 minutes with oil over 30 days old to avoid overheating and oil breakdown.

There is no initial pilot screw setting specified in the service manual - yeah, 2 1/2 turns will be fine to warm up prior to final setting.
 

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Discussion Starter #5
If the plug is wet and black, it's already getting too much fuel, rebuilding might help if the problem is the float needle or the float level.

There are only two adjustment screws on the carb - one increases the throttle opening to adjust idle speed - the other adjusts the amount of fuel mixed with the air bleeding past the throttle to help the engine idle smoothly without
loading up or starving during periods of engine idling. Do not idle over 20 minutes with fresh oil or over 15 minutes with oil over 30 days old to avoid overheating and oil breakdown.

There is no initial pilot screw setting specified in the service manual - yeah, 2 1/2 turns will be fine to warm up prior to final setting.

Sounds good to me! I was told it was running rich because carb needs rebuilding. I might also add that the idle is always all over the place. The previous owner never touched the carb. My plan is this...
1. Rebuild the carb
2. Install a #38 pilot jet and a #142 main jet
3. Upgrade the spark plug to a NGK DR7ES
4. Tune the carb by turning fuel needle out to 2.5 and idle screw 1.5 turns and fine tune from there.
Does this sound like it is going to work out? The float should be fine but I'll double check it. Should I play with the main needle at all? ( the one in the center of the carb with the c clip adjustment)? If so, should I lower or raise it?
THANK YOU!!
 

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Discussion Starter #6
If the plug is wet and black, it's already getting too much fuel, rebuilding might help if the problem is the float needle or the float level.

There are only two adjustment screws on the carb - one increases the throttle opening to adjust idle speed - the other adjusts the amount of fuel mixed with the air bleeding past the throttle to help the engine idle smoothly without
loading up or starving during periods of engine idling. Do not idle over 20 minutes with fresh oil or over 15 minutes with oil over 30 days old to avoid overheating and oil breakdown.

There is no initial pilot screw setting specified in the service manual - yeah, 2 1/2 turns will be fine to warm up prior to final setting.

Sounds good to me! I was told it was running rich because carb needs rebuilding. I might also add that the idle is always all over the place. The previous owner never touched the carb. My plan is this...
1. Rebuild the carb
2. Install a #38 pilot jet and a #142 main jet
3. Upgrade the spark plug to a NGK DR7ES
4. Tune the carb by turning fuel needle out to 2.5 and idle screw 1.5 turns and fine tune from there.
Does this sound like it is going to work out? The float should be fine but I'll double check it. Should I play with the main needle at all? ( the one in the center of the carb with the c clip adjustment)? If so, should I lower or raise it?
THANK YOU!!
I actually just read that for a lightly molded engine with exhaust and intake, you can keep similar to stock pilot size and only increase main jet by two sizes from #125 to #128
 

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First, how do you figure a DR7ES spark is an upgrade when it is one of the recommended stock plugs listed for the 87 TRX250X?
List of stock plugs:
98069-38719 NGK DR8ES-L
98069-57719 NGK DR7ES
98069-58719 NGK DR8ES
98069-57726 Nippon Denso X22ESR-U
98069-58726 NIPPON DENSO X24ESR-U
98069-59726 NIPPON DENSO X27ESR-U

Stock main jets:
99101-357-1250 JET, MAIN (#125) sea level to 3000 feet
99101-357-1180 JET, MAIN (#118) above 3000 feet

A 128 is technically a 1/2 size - the jets are only available in sizes of 5 (115, 120, 125. 130. 135. 140 etc) except through the manufacturer and the manufacturer does not offer consecutive sizes - 118 and 120 are technically the same calibration - the odd sizes were created to satisfy the EPA - if you order jets through Honda, they come in 118, 120, 122, 125, 128, 130, 132, 135, 138, 140...........

It's what makes race tuning so fun
 

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Discussion Starter #8
First, how do you figure a DR7ES spark is an upgrade when it is one of the recommended stock plugs listed for the 87 TRX250X?
List of stock plugs:
98069-38719 NGK DR8ES-L
98069-57719 NGK DR7ES
98069-58719 NGK DR8ES
98069-57726 Nippon Denso X22ESR-U
98069-58726 NIPPON DENSO X24ESR-U
98069-59726 NIPPON DENSO X27ESR-U

Stock main jets:
99101-357-1250 JET, MAIN (#125) sea level to 3000 feet
99101-357-1180 JET, MAIN (#118) above 3000 feet

A 128 is technically a 1/2 size - the jets are only available in sizes of 5 (115, 120, 125. 130. 135. 140 etc) except through the manufacturer and the manufacturer does not offer consecutive sizes - 118 and 120 are technically the same calibration - the odd sizes were created to satisfy the EPA - if you order jets through Honda, they come in 118, 120, 122, 125, 128, 130, 132, 135, 138, 140...........

It's what makes race tuning so fun

I'm not sure if fun is the right word for my situation! HaHa I picked up a 140 and a 38 pilot today. What sizes should I finally settle on? I'm just trying to figure this out so I dont have to keep pulling the carb off. I'm thinking I will just do a main on 130 and a pilot on 38. It sounds like a 140 will lead me too rich in fuel mixture. The quad is completely stock accept for the intake and exhaust. That all that is done to it.
 

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Discussion Starter #9
First, how do you figure a DR7ES spark is an upgrade when it is one of the recommended stock plugs listed for the 87 TRX250X?
List of stock plugs:
98069-38719 NGK DR8ES-L
98069-57719 NGK DR7ES
98069-58719 NGK DR8ES
98069-57726 Nippon Denso X22ESR-U
98069-58726 NIPPON DENSO X24ESR-U
98069-59726 NIPPON DENSO X27ESR-U

Stock main jets:
99101-357-1250 JET, MAIN (#125) sea level to 3000 feet
99101-357-1180 JET, MAIN (#118) above 3000 feet

A 128 is technically a 1/2 size - the jets are only available in sizes of 5 (115, 120, 125. 130. 135. 140 etc) except through the manufacturer and the manufacturer does not offer consecutive sizes - 118 and 120 are technically the same calibration - the odd sizes were created to satisfy the EPA - if you order jets through Honda, they come in 118, 120, 122, 125, 128, 130, 132, 135, 138, 140...........

It's what makes race tuning so fun

I'm not sure if fun is the right word for my situation! HaHa I picked up a 140 and a 38 pilot today. What sizes should I finally settle on? I'm just trying to figure this out so I dont have to keep pulling the carb off. I'm thinking I will just do a main on 130 and a pilot on 38. It sounds like a 140 will lead me too rich in fuel mixture. The quad is completely stock accept for the intake and exhaust. That all that is done to it
Also, my quad dealer ( milwake cycle) has 1/2 sizes in stock. They do indeed have a 128. The spark plug is not a big concern of mine. I have what OEM specifies in it already.
I really appreciate all the time and attention you are giving me. This helps me out immensely!! I want to do it right the first time and be done and enjoy it!
 

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Discussion Starter #10
First, how do you figure a DR7ES spark is an upgrade when it is one of the recommended stock plugs listed for the 87 TRX250X?
List of stock plugs:
98069-38719 NGK DR8ES-L
98069-57719 NGK DR7ES
98069-58719 NGK DR8ES
98069-57726 Nippon Denso X22ESR-U
98069-58726 NIPPON DENSO X24ESR-U
98069-59726 NIPPON DENSO X27ESR-U

Stock main jets:
99101-357-1250 JET, MAIN (#125) sea level to 3000 feet
99101-357-1180 JET, MAIN (#118) above 3000 feet

A 128 is technically a 1/2 size - the jets are only available in sizes of 5 (115, 120, 125. 130. 135. 140 etc) except through the manufacturer and the manufacturer does not offer consecutive sizes - 118 and 120 are technically the same calibration - the odd sizes were created to satisfy the EPA - if you order jets through Honda, they come in 118, 120, 122, 125, 128, 130, 132, 135, 138, 140...........

It's what makes race tuning so fun

I'm not sure if fun is the right word for my situation! HaHa I picked up a 140 and a 38 pilot today. What sizes should I finally settle on? I'm just trying to figure this out so I dont have to keep pulling the carb off. I'm thinking I will just do a main on 130 and a pilot on 38. It sounds like a 140 will lead me too rich in fuel mixture. The quad is completely stock accept for the intake and exhaust. That all that is done to it
Also, my quad dealer ( milwake cycle) has 1/2 sizes in stock. They do indeed have a 128. The spark plug is not a big concern of mine. I have what OEM specifies in it already.
I really appreciate all the time and attention you are giving me. This helps me out immensely!! I want to do it right the first time and be done and enjoy it!
Shall I change the needle position?
 

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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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Discussion Starter #12
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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Discussion Starter #13
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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