Over jetted on main jet ???

Bill Hilly · 04-20-2021, 05:48 PM

My pilot jet kit from AliExpress finally came today, and I installed the 38 pilot, and replaced the 115 main that I have been running with a 125 . I skipped the 120 main because I am pretty sure that the 115 was a hair lean . i have gutted the cat ,and removed the small baffle pipe from the stock muffler .It has about a 2.5" exhaust outlet now ,and is very loud ,and free flowing. I get a great idle, and it runs great, it ran great with the 115 as well, except it seemed to have just a little subtile flatness at high rpm,and wide open throttle lately .It actually pulled just a hair harder if I just backed off the throttle just a slight hair.. With the 125 it pulls just fine ,and runs great, and seems to thrive at high rpm, but when I just slightly reduce the throttle after running wide open at high rpm it just make a brief stumble and then is fine . It's nothing I couldn't easily live with, but would rather it not occur .. Does this sound like I am over jetted on the main?

Bill Hilly · 04-20-2021, 06:49 PM

I put the 120 in it, and it took care of the issue.

CheapThrills · 04-20-2021, 07:31 PM

Curious, I'm running a 110 main and now wondering if a 120 will make a bigger top end difference. Plus, looking at adding a foam filter and better exhaust. What make/model you moding, and what elevation are you at?

thx!!

Bill Hilly · 04-20-2021, 07:52 PM

I am at about 600 foot elevation. I first installed the 115 when I gutted the cat, and I could tell that it seemed more at home running high rpms. I later de restricted my muffler ,and ordered a pilot jet kit from Ali Express for the stock carb. With the intention of changing the pilot,and up sizing the main at the same time. The pilots took a long time to get here. It ran great with the gutted muffler,and cat, but it seemed like as the motor was completing break in it got a little leaner. I stalled it in town once, and had to choke it to re start it on a hot motor,and it had never need choked on a warm motor before. I can tell the 120 provides enough fuel for all the air the carb can flow. I think it has a little more willingness,or freeness to pull at high rpms in high gear. It's hard to explain,but it almost feels geared higher, even though it still buzzes down the road at 6,000 rpm. I forgot to mention .I have a 2020 Brozz250. Right now I am running a 42/15 sprocket combo.

China Rider 27 · 04-20-2021, 10:22 PM

Good to see the stock PZ30 running well! And with the new jets you should have no problem tuning it to what you need. Does the new "cold start" pilot jet run better than the 38 you modified?

Bill Hilly · 04-20-2021, 11:03 PM

Quote:

Originally Posted by China Rider 27

Good to see the stock PZ30 running well! And with the new jets you should have no problem tuning it to what you need. Does the new "cold start" pilot jet run better than the 38 you modified?

I never was able to tune the one I modded . It ran fine out on the road, but I never could get a smooth idle , and while it always started, it was either finiky , or inconsistent on wether it needed choked, or not. I cut down another , I think it was the 40, and realized that something was not right with them, so I had to put my original 34(I think) back in. It started, and idled fine with it, but the mix screw had to be backed way out. With the 38 from the kit ,I just turned it out 2 full rotation to be close, figuring that I would need to adjust it, but I must have gotten lucky, because it idles smoothly, so I haven't touched by it again.

TxTaoRider · 04-21-2021, 05:58 AM

With the use of mikuni and keihin main jets being used in these carbs (they both fit but they flow different amounts of fuel even though the numbers are similar), then throw in the Chinese copies of the jets with thier poor quality control and inconsistentsies, and you have the makings for jet sizing being all over board.

Personally, I've been buying genuine Mikuni main jets for constant results.

Something else to consider, is the tip diameter of the needles can play a role in main jet choice. If its large enough it can still create a blockage requiring a larger main jet choice (this is pretty common in Japanese dirt bikes / quads). And who knows the quality control or anything on the Chinese carbs?

Anyhoo, what I'm saying is you can have 2 exact same bikes, and have much different jetting due to so many variables, especially with Chinese mix and match carbs.

China Rider 27 · 04-21-2021, 09:08 AM

Quote:

Originally Posted by Bill Hilly

I never was able to tune the one I modded . It ran fine out on the road, but I never could get a smooth idle , and while it always started, it was either finiky , or inconsistent on wether it needed choked, or not. I cut down another , I think it was the 40, and realized that something was not right with them, so I had to put my original 34(I think) back in. It started, and idled fine with it, but the mix screw had to be backed way out. With the 38 from the kit ,I just turned it out 2 full rotation to be close, figuring that I would need to adjust it, but I must have gotten lucky, because it idles smoothly, so I haven't touched by it again.

Not surprising, given that standard type Kehein pilot jet has a much larger hole all the way through. The Cold Start has a pin hole at the one end. They are definitely a different jet and cupped to fit snuggly against a brass piece. Good to see they work. Curious if it worked.

tknj99 · 04-21-2021, 09:41 AM

Quote:

With the 125 it pulls just fine ,and runs great, and seems to thrive at high rpm, but when I just slightly reduce the throttle after running wide open at high rpm it just make a brief stumble and then is fin

This is exactly what i experience with my PE30.. its most noticeable on the highway where if im past 7500k it will hesitate and then fall flat, feels like i hit the rev limit but i know i havent.. Im running a 140 with my current setup.. not sure though if its just better to keep it as is and stay on the richer side than risk it running lean at full throttle

China Rider 27 · 04-21-2021, 10:00 AM

Quote:

Originally Posted by TxTaoRider

With the use of mikuni and keihin main jets being used in these carbs (they both fit but they flow different amounts of fuel even though the numbers are similar), then throw in the Chinese copies of the jets with thier poor quality control and inconsistentsies, and you have the makings for jet sizing being all over board.

Personally, I've been buying genuine Mikuni main jets for constant results.

You are talking about the large style Mikuni round jets that fit Keihin style carburetor mains correct? Good strategy to try and standardize but expensive if you do a lot of exotic jetting or upgrades.

Bill Hilly · 04-21-2021, 10:03 AM

Quote:

Originally Posted by China Rider 27

Not surprising, given that standard type Kehein pilot jet has a much larger hole all the way through. The Cold Start has a pin hole at the one end. They are definitely a different jet and cupped to fit snuggly against a brass piece. Good to see they work. Curious if it worked.

I think I was getting a jetted pin hole when I first cut them, but when I tried cupping them it removed what small jetted portion that was left on the jet, thus making them all over sized. Here is a picture of of the ones I experimented on ,along with a proper one.http://

China Rider 27 · 04-21-2021, 10:05 AM

That cold start is weird, on one end it has jet size then other is like smaller exiting passage to the carburetor? For those who don't know, the cold start is farthest jet on the left in the row.

TxTaoRider · 04-21-2021, 03:00 PM

Quote:

Originally Posted by China Rider 27

You are talking about the large style Mikuni round jets that fit Keihin style carburetor mains correct? Good strategy to try and standardize but expensive if you do a lot of exotic jetting or upgrades.

It's cheaper than the time waisted trying to out guess what the heck the is going on with the cheap jets... and i use a wideband tuner which saves time too.

China Rider 27 · 04-21-2021, 04:09 PM

A wideband tuner has got to make it an easy task and allows precision. For the average guy who is gonna do an exhaust mod, a couple of 2 or three name brand jets not all that expensive except for shipping which is like $12 so figure out exactly what you need first. Jets R Us good thing. I've swapped a few of the chinese cheap jets out to OEM style brand jets and so far I have not seen a difference I could tell but there is always that doubt.

04-20-2021, 05:48 PM	#1
Bill Hilly Join Date: Feb 2021 Posts: 1,003	Over jetted on main jet ??? My pilot jet kit from AliExpress finally came today, and I installed the 38 pilot, and replaced the 115 main that I have been running with a 125 . I skipped the 120 main because I am pretty sure that the 115 was a hair lean . i have gutted the cat ,and removed the small baffle pipe from the stock muffler .It has about a 2.5" exhaust outlet now ,and is very loud ,and free flowing. I get a great idle, and it runs great, it ran great with the 115 as well, except it seemed to have just a little subtile flatness at high rpm,and wide open throttle lately .It actually pulled just a hair harder if I just backed off the throttle just a slight hair.. With the 125 it pulls just fine ,and runs great, and seems to thrive at high rpm, but when I just slightly reduce the throttle after running wide open at high rpm it just make a brief stumble and then is fine . It's nothing I couldn't easily live with, but would rather it not occur .. Does this sound like I am over jetted on the main?
	0

04-20-2021, 07:31 PM	#3
CheapThrills Join Date: Jan 2021 Location: Lehigh Valley Area Posts: 166	Curious, I'm running a 110 main and now wondering if a 120 will make a bigger top end difference. Plus, looking at adding a foam filter and better exhaust. What make/model you moding, and what elevation are you at? thx!! __________________ - My 3 Bikes: 1. TaoTao TBR7 (My TBR7 Upgrades) 2. Boom Vader (FYI: My Boom Vader Upgrades) 3. 1978 Suzuki SG750 --------------------------------------------- Testing a YouTube Channel
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04-20-2021, 07:52 PM	#4
Bill Hilly Join Date: Feb 2021 Posts: 1,003	I am at about 600 foot elevation. I first installed the 115 when I gutted the cat, and I could tell that it seemed more at home running high rpms. I later de restricted my muffler ,and ordered a pilot jet kit from Ali Express for the stock carb. With the intention of changing the pilot,and up sizing the main at the same time. The pilots took a long time to get here. It ran great with the gutted muffler,and cat, but it seemed like as the motor was completing break in it got a little leaner. I stalled it in town once, and had to choke it to re start it on a hot motor,and it had never need choked on a warm motor before. I can tell the 120 provides enough fuel for all the air the carb can flow. I think it has a little more willingness,or freeness to pull at high rpms in high gear. It's hard to explain,but it almost feels geared higher, even though it still buzzes down the road at 6,000 rpm. I forgot to mention .I have a 2020 Brozz250. Right now I am running a 42/15 sprocket combo. Last edited by Bill Hilly; 04-20-2021 at 08:25 PM.
	2

04-21-2021, 05:58 AM	#7
TxTaoRider Join Date: Oct 2020 Location: Flower Mound Texas Posts: 904	With the use of mikuni and keihin main jets being used in these carbs (they both fit but they flow different amounts of fuel even though the numbers are similar), then throw in the Chinese copies of the jets with thier poor quality control and inconsistentsies, and you have the makings for jet sizing being all over board. Personally, I've been buying genuine Mikuni main jets for constant results. Something else to consider, is the tip diameter of the needles can play a role in main jet choice. If its large enough it can still create a blockage requiring a larger main jet choice (this is pretty common in Japanese dirt bikes / quads). And who knows the quality control or anything on the Chinese carbs? Anyhoo, what I'm saying is you can have 2 exact same bikes, and have much different jetting due to so many variables, especially with Chinese mix and match carbs. __________________ 2021 Tao Tbr7 - "Lucille" Mods so far- Brozz swingarm, 21" front rim (Bridgestone Tw302 rear/Dunlop D606 front tires), Digital gauge cluster, pz30b pumper carb, after market hand guards, aftermarket brake and clutch levers, round fold away mirrors, Fly handlebars shortened slightly, 13t front sprocket 2009 Q-link Legacy 250 1982 Suzuki GS1100ES - "Jolene"
	1

04-20-2021, 06:49 PM	#2
Bill Hilly Join Date: Feb 2021 Posts: 1,003	I put the 120 in it, and it took care of the issue.
	0

04-20-2021, 10:22 PM	#5
China Rider 27 Join Date: Dec 2018 Location: PNW Posts: 984	Good to see the stock PZ30 running well! And with the new jets you should have no problem tuning it to what you need. Does the new "cold start" pilot jet run better than the 38 you modified?
	1

04-21-2021, 10:05 AM	#12
China Rider 27 Join Date: Dec 2018 Location: PNW Posts: 984	That cold start is weird, on one end it has jet size then other is like smaller exiting passage to the carburetor? For those who don't know, the cold start is farthest jet on the left in the row.
	1

04-21-2021, 04:09 PM	#14
China Rider 27 Join Date: Dec 2018 Location: PNW Posts: 984	A wideband tuner has got to make it an easy task and allows precision. For the average guy who is gonna do an exhaust mod, a couple of 2 or three name brand jets not all that expensive except for shipping which is like $12 so figure out exactly what you need first. Jets R Us good thing. I've swapped a few of the chinese cheap jets out to OEM style brand jets and so far I have not seen a difference I could tell but there is always that doubt.
	0