87 trx250xPerformance exhaust and intake carb Jetting Question. Please Help! - Honda TRX Forum
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post #1 of 12 Old 11-14-2019, 08:11 PM Thread Starter
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87 trx250xPerformance exhaust and intake carb Jetting Question. Please Help!

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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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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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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Originally Posted by Here 2 help View Post
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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Quote:
Originally Posted by Here 2 help View Post
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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Quote:
Originally Posted by Flamin 250 View Post
Quote:
Originally Posted by Here 2 help View Post
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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Quote:
Originally Posted by Flamin 250 View Post
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Originally Posted by Flamin 250 View Post
Quote:
Originally Posted by Here 2 help View Post
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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