165fmm and nibbi PE28FL tuning questions.

[NathanTC]

Ok so my XMOTOS 250 has the 165fmm engine and i just installed a Nibbi PE28FL carb. The factory air box is not a possibility with this carb so im running this pod filter found here.
https://www.amazon.com/dp/B0897KHTZB...roduct_details

So far the ONLY settings that make the bike run ALMOST perfect are as follows
Throttle needle at its absolute highs position
air screw two turns out
42 pilot jet
115 main jet

any deviation at all with jetting nets negative results, same for air screw and needle position

Bike runs way stronger than it ever did stock. Just there is a slight hesitation/bog right off idle. Sucks when you need to pop up the front tire. My dad suggested putting a sock over the air filter which helped but didn't eliminate it.

Spark plug is an E3 and shows that the bike is running about perfect. Both valves are set at 0.10MM or basicly 0.004

My question is this. Is it the filter making it impossible to tune perfectly? I can make it either to rich or to lean so i would think there is something im missing here.

[Megadan]

Slight apples to oranges considering our engines are vastly different for several reasons, but a 42 pilot is pretty darn big. I run a 38 with a cam and port work with a full exhaust. But, that is again apples to oranges given our different engines and mods, so neither here nor there.

Also, as aggressive as the needle taper is on the PE carbs out of the box, being all the way in the bottom notch is impressive. I am in the second notch lol. Admittedly, my main jet is quite big compared to what you are running, so that plays some factor, but even most near stock engines I know of don't go below the middle notch.

My method for getting a carb figured out is to first start with the pilot jet and have the needle in a neutral position (3rd notch). The main reason why is to try and eliminate variables. I actually adjust the needle last.

I figure out the pilot jet by first sorting out the idle mixture. This can give a clue into the pilot jet being potentially big or small. The 2 turns out is promising, but it never hurts to verify. With the engine up to temp - having run at least 5+ minutes - I will check and set the mixture based on the idle drop method. Simply turning the air screw in 1/4 turn at a time to richen the mixture until the idle drops off. I then back off 1/4 turn to the last adjustment where a steady higher idle was present. If you want to go a bit leaner you could back off another 1/4 turn, and as long as you don't get any lean symptoms like a hanging idle, this is a good range to be in. Generally, I aim to have this adjustment more than 1 turn out and less than or equal to 2 turns out.

Once that is achieved you can actually test to see if the pilot jet itself is lean or rich by simply riding the bike in a steady off idle to 1/8th throttle position (trying to keep the needle out of it). I find puttering around my neighborhood in second or third gear on flat ground at 10-20mph is a good way to achieve this. If it hesitates and bucks a bit, it's like too lean. You can always hold this state for a fair amount of time and do a plug chop to read the plug if desired.

Once you have that sorted, we gloss over the needle and do the main jet. We do this, because like the pilot on the bottom end of the throttle position scale, the main jet is the single biggest factor on fueling past 3/4 throttle. WOT plug chops are the poor mans best way of reading main jet fueling state.

Once you know for sure the main is a good size, then you can determine and make needle adjustments. The reason why? The needles fueling curve is greatly influenced by the main jet. I run a 135 main jet on my bike, which is a good bit bigger than a bike without all of my engine work, so the part throttle fueling on my bike is a lot richer than another persons might be. That is why I have mine in the 2nd notch.

[NathanTC]

Originally Posted by Megadan

Slight apples to oranges considering our engines are vastly different for several reasons, but a 42 pilot is pretty darn big. I run a 38 with a cam and port work with a full exhaust. But, that is again apples to oranges given our different engines and mods, so neither here nor there.

Also, as aggressive as the needle taper is on the PE carbs out of the box, being all the way in the bottom notch is impressive. I am in the second notch lol. Admittedly, my main jet is quite big compared to what you are running, so that plays some factor, but even most near stock engines I know of don't go below the middle notch.

My method for getting a carb figured out is to first start with the pilot jet and have the needle in a neutral position (3rd notch). The main reason why is to try and eliminate variables. I actually adjust the needle last.

I figure out the pilot jet by first sorting out the idle mixture. This can give a clue into the pilot jet being potentially big or small. The 2 turns out is promising, but it never hurts to verify. With the engine up to temp - having run at least 5+ minutes - I will check and set the mixture based on the idle drop method. Simply turning the air screw in 1/4 turn at a time to richen the mixture until the idle drops off. I then back off 1/4 turn to the last adjustment where a steady higher idle was present. If you want to go a bit leaner you could back off another 1/4 turn, and as long as you don't get any lean symptoms like a hanging idle, this is a good range to be in. Generally, I aim to have this adjustment more than 1 turn out and less than or equal to 2 turns out.

Once that is achieved you can actually test to see if the pilot jet itself is lean or rich by simply riding the bike in a steady off idle to 1/8th throttle position (trying to keep the needle out of it). I find puttering around my neighborhood in second or third gear on flat ground at 10-20mph is a good way to achieve this. If it hesitates and bucks a bit, it's like too lean. You can always hold this state for a fair amount of time and do a plug chop to read the plug if desired.

Once you have that sorted, we gloss over the needle and do the main jet. We do this, because like the pilot on the bottom end of the throttle position scale, the main jet is the single biggest factor on fueling past 3/4 throttle. WOT plug chops are the poor mans best way of reading main jet fueling state.

Once you know for sure the main is a good size, then you can determine and make needle adjustments. The reason why? The needles fueling curve is greatly influenced by the main jet. I run a 135 main jet on my bike, which is a good bit bigger than a bike without all of my engine work, so the part throttle fueling on my bike is a lot richer than another persons might be. That is why I have mine in the 2nd notch.

I did pretty much exactly what you described here. Starting with the needle in the middle position, and riding the bike at different throttle positions to see if it's surging popping or giving any other weirdness. Then after riding I pull the plug and see what the plug looks like. If I go smaller on the pilot I get terrible bog off idle. Same if I go up one size on the pilot. Regardless of needle position. I get about half of a turn on the air screw toward the Richer side before the idle changes. And about half a turn leaner before the idle changes. Right where it's set the idle is perfectly smooth with no rich or lean idle. And that was letting it idle for about 20 minutes and then pulling the plug to verify. The needle position has me baffled. It should be in the number five slot as per what the book calls the number five that came with the carburetor. Meaning the needle is at its highest position toward the throttle cable. In this position I have just the slightest hesitation when idling and then cracking the throttle well riding the bike. If I drop it down even one position, that slight hesitation turns into a bog. The farther down I go the worse it gets. When I go up on the main jet I get choppy wide open throttle. When I go down on the main jet, I get the same feeling you get when the bike is running out of gas. It basically cuts out and won't go at all.

The part that has me incredibly stumped is the air filter. When my dad suggested putting a sock, just a normal sock that you wear on your feet, over the air filter I decided to give it a try since I have been trying to get this bike tuned for 12 straight hours. Putting the sock over the air filter did make an improvement. It greatly reduced that hesitation but not completely. And it caused an almost unnoticeable difference in my top end, reducing power. So for the time being I ordered two completely different air filters that have a slightly smaller filter media to see if I can cause just enough restriction to solve the problem without causing too much restriction hurting my top end. I've heard other people have had issues tuning a carburetor with a pod filter, but I didn't expect it to be this much of a nightmare.

The reason it takes so long on this bike is you have to remove the seat and gas tank to get to the carburetor. Both seat and gas tank have to be removed just to change the needle valve setting. The entire carburetor and intake manifold have to be removed to change the Jets.. absolutely no room to work on anything while it's attached to the bike. That being said I have taken this bike apart so many times yesterday, I can guarantee you that I could do it blindfolded.

[China Rider 27]

You have hit upon what I think is the biggest issue with the PE style smoothbores that being "hesitation just off idle." I have this problem with a PE 30. The issue as I see it is how the air flow is created around that big slide when it starts to open. Your numbers sure indicate you are putting a lot of fuel on it as Megadan said "that bottom notch is impressive" and a 42 pilot. The answer hopefully is a combination of pilot and needle jet adjustment. Given your numbers I would think you could get a 40 pilot to work with a middle notch and less air. I am just thinking out loud. You might also try to go one size up on the pilot to see if it will smooth it out. One change at a time, one needle position at time.

[NathanTC]

Originally Posted by China Rider 27

You have hit upon what I think is the biggest issue with the PE style smoothbores that being "hesitation just off idle." I have this problem with a PE 30. The issue as I see it is how the air flow is created around that big slide when it starts to open. Your numbers sure indicate you are putting a lot of fuel on it as Megadan said "that bottom notch is impressive" and a 42 pilot. The answer hopefully is a combination of pilot and needle jet adjustment. Given your numbers I would think you could get a 40 pilot to work with a middle notch and less air. I am just thinking out loud. You might also try to go one size up on the pilot to see if it will smooth it out. One change at a time, one needle position at time.

The 40 pilot causes way worse bog with any needle position. The next size up that I have is a 45, and that causes a rich condition making the spark plug turn black. And hesitates just as bad as it goes with the 42 that does not make the bike run rich. I'm thinking that the pod filter I used is a little too free-flowing. The bike runs identically the same with the filter on or no filter at all. My dad's idea of putting a sock over the filter did help so that tells me maybe I have to have some kind of restriction at the filter for this carburetor to work properly.

[China Rider 27]

Yaa posted that and then saw your response. A sock over the air filter to me says you are restricting air flow, which you should be able to do with the pilot air screw. Sometimes when tuning at least myself I try to go to fast to get there and so I don't evaluate each position maybe as thoroughly as I should, needle position, air screw. The answer to me, if there is a good one, has to be some combination of pilot and needle position that will work. But maybe a foam pod filter will give you the air flow restricted so to get a pilot and needle position to work.

[NathanTC]

Originally Posted by China Rider 27

Yaa posted that and then saw your response. A sock over the air filter to me says you are restricting air flow, which you should be able to do with the pilot air screw. Sometimes when tuning at least myself I try to go to fast to get there and so I don't evaluate each position maybe as thoroughly as I should, needle position, air screw. The answer to me, if there is a good one, has to be some combination of pilot and needle position that will work. But maybe a foam pod filter will give you the air flow restricted so to get a pilot and needle position to work.

I ordered two different style filters when they get in I'm going to give them a try and see if it matters at all. I've run out of things to try other than that. Everything I modify from where it's sitting right now only makes things worse. If I get rid of the bog down low then I have it cutting out popping back firing and not running worth a damn up high. If I get it running extremely well up high then I have really bad bog down low and the bike running overly lean. And I've literally pulled the carburetor off which requires disassembling the whole bike to do so, to change the pilot jet trying every single jet that came with the jet kit. With the needle in the middle position and just set up is the only one that has the bike running almost perfect. When I say I spent 12 straight hours I mean exactly that. I got up in the morning and I never even stopped to get a bite to eat or anything. Worked myself sick from the gas fumes trying to get this thing right.

[Megadan]

Originally Posted by NathanTC

I did pretty much exactly what you described here. Starting with the needle in the middle position, and riding the bike at different throttle positions to see if it's surging popping or giving any other weirdness. Then after riding I pull the plug and see what the plug looks like.

That is not quite what I suggested to do.

I am specifically stating to check the plug only under certain conditions by performing a plug chop in those exact conditions. Checking it after just riding around checking every throttle position will not net the desired result. Running the bike in that specific condition for a period of time and then cutting the ignition off then immediately checking the plug will give you a much more accurate picture of what is going on with that specific jet.

My suggestion = narrow focus on specific throttle position to identify individual jet tuning.


Something else worth looking into is the float height. I noticed on my PE28FL that the float was set rather high. It would actually spit fuel out of the mouth if I cycled the throttle. Keihin's spec for the float height is 14mm on a PE carburetor, but I did my own digging and found that many other bikes that run PE's they set the floats to 18mm due to similar issues. This might be worth looking into, and could also explain that off idle bog.

Generic guide on how to set the float height.
https://www.jetsrus.com/FAQs/FAQ_flo...djustment.html

I will agree with you on one thing about the flanged carbs in general, this or even the stock PZ carburetor. Removing the carb to change jets is really annoying. I 100% get your frustration.

[China Rider 27]

I too feel your pain! I also have been through it. Spent 8 to 10 hour days trying to tune a carburetor on the likes of a Magician which also requires the fuel tank to be removed. Then did the same thing with a couple more carburetors.

I second working on one part of the throttle tuning spectrum at a time. That main in my opinion has a smaller affect on that off idle pilot hesitation.

[NathanTC]

Originally Posted by Megadan

That is not quite what I suggested to do.

I am specifically stating to check the plug only under certain conditions by performing a plug chop in those exact conditions. Checking it after just riding around checking every throttle position will not net the desired result. Running the bike in that specific condition for a period of time and then cutting the ignition off then immediately checking the plug will give you a much more accurate picture of what is going on with that specific jet.

My suggestion = narrow focus on specific throttle position to identify individual jet tuning.


Something else worth looking into is the float height. I noticed on my PE28FL that the float was set rather high. It would actually spit fuel out of the mouth if I cycled the throttle. Keihin's spec for the float height is 14mm on a PE carburetor, but I did my own digging and found that many other bikes that run PE's they set the floats to 18mm due to similar issues. This might be worth looking into, and could also explain that off idle bog.

Generic guide on how to set the float height.
https://www.jetsrus.com/FAQs/FAQ_flo...djustment.html

I will agree with you on one thing about the flanged carbs in general, this or even the stock PZ carburetor. Removing the carb to change jets is really annoying. I 100% get your frustration.

I did notice it spitting gas out of the intake of the carburetor while sitting in neutral and revving before I ever started messing with the Jets. Nothing comes out of the overflow, are you suggesting to lower the float level reducing the amount of gas in the bowl?

[Megadan]

Originally Posted by NathanTC

I did notice it spitting gas out of the intake of the carburetor while sitting in neutral and revving before I ever started messing with the Jets. Nothing comes out of the overflow, are you suggesting to lower the float level reducing the amount of gas in the bowl?

In simple terms, yeah, check it and set it to 18mm if it isn't there already. Mine wasn't at 18. It was at 13. By setting it to 18mm it lowers the fuel level in the bowl. In my case by 5mm. Still plenty high enough to supply the pilot jet.

A fun little cheat/life hack - you can hook up a clear fuel hose to the drain and tie it so it faces up and the end sits above the bowl a bit more, open the drain screw, and observe the fuel level. That is how I discovered my fuel level was damn near all the way up.

[NathanTC]

Originally Posted by Megadan

In simple terms, yeah, check it and set it to 18mm if it isn't there already. Mine wasn't at 18. It was at 13. By setting it to 18mm it lowers the fuel level in the bowl. In my case by 5mm. Still plenty high enough to supply the pilot jet.

A fun little cheat/life hack - you can hook up a clear fuel hose to the drain and tie it so it faces up and the end sits above the bowl a bit more, open the drain screw, and observe the fuel level. That is how I discovered my fuel level was damn near all the way up.

thanks, ill give that a try. Was just riding the bike and noticed if im going up a steep hill there is no bog at all. Also the bog seems worse if im putting around for a bit before trying to take off but lessens if im thrashing the bike. So you might have hit the nail on the head here. Going to tear it down when my new filters come in and try again to find the happy combo.

Side note. Do any of yall know of a good chain sprocket calculator. All the ones i find on google cannot possibly be correct. My bike right at 60 mph (gps speed) is at 7500-8000 rpm, but according to every sprocket calculator i find i would be doing over 180 mph at that rpm.

[Megadan]

If you have all of the gear ratios and primary drive ratio of the bike along with the basic specifications, you can select "add bike" to this page. This is the one us Hawk guys use. It is accurate as long as you put the right info in.

https://www.gearingcommander.com/

[NathanTC]

Originally Posted by Megadan

If you have all of the gear ratios and primary drive ratio of the bike along with the basic specifications, you can select "add bike" to this page. This is the one us Hawk guys use. It is accurate as long as you put the right info in.

https://www.gearingcommander.com/

engine is a 165fmm

According to a gear ratio calculator my 5th (final) gear is a 2.60 ratio not the .98 as advertised.

when i put the proper info in for the 13-48 sprockets (3.69 ratio) with a 27" tire and a .98 final gear i get 166.59mph at 7500 rpm and that is far from correct.
when i use 2.6 as the final gear i get the real 62 mph at 7500 rpm

from what i under stand is 48 divided by 13 is how i get my final drive ratio.
I use this gear ratio calculator for everything else and its always been spot on. Not sure where im messing up here.
https://www.tremec.com/calculadora.php

[Megadan]

Originally Posted by NathanTC

engine is a 165fmm

According to a gear ratio calculator my 5th (final) gear is a 2.60 ratio not the .98 as advertised.

when i put the proper info in for the 13-48 sprockets (3.69 ratio) with a 27" tire and a .98 final gear i get 166.59mph at 7500 rpm and that is far from correct.
when i use 2.6 as the final gear i get the real 62 mph at 7500 rpm

from what i under stand is 48 divided by 13 is how i get my final drive ratio.
I use this gear ratio calculator for everything else and its always been spot on. Not sure where im messing up here.
https://www.tremec.com/calculadora.php

I think you are forgetting the Primary Drive ratio. That is the ratio between the crank gear and clutch basket gear.