My motorcycle loses power at high RPMs and full throttle.

[Ucvmn]

My motorcycle loses power at high RPMs and full throttle, and it does this: I accelerate fully reaches 7k rpm starts to lose acceleration > accelerate > lose acceleration > and it doesn’t go past 9,500 rpm, no matter what gear it’s in.

Sometimes, when I’ve been maintaining speed for a while and then accelerate fully, the bike takes a while to respond.

It has a new air filter, new fuel lines, and a new fuel filter. I also replaced the spark plugs, cleaned the carburetor a few months ago, and always use 95 octane gasoline (E5). Additionally, I changed the intake manifold o-rings.

Whenever I check the fuel filter, it’s almost empty, I think it’s around 3%.

I’ve also checked the vacuum lines, the fuel pump, and the fuel tank cap with a vacuum pump + gauge, and they don’t have any leaks.

I’m going to leave two Paint images showing how I have the fuel lines and vacuum lines connected.

PS: I installed the fuel filter as seen in the Paint drawing, and the arrow labeled ‘TO CARBS’ is pointing towards the fuel pump.

 

[Ucvmn]

I forgot to mention that the bike is a Hyosung GT125R with a 250 engine. Because the 125 died and i had a 250 EFI near for a replace.

[Simon]

I assume as it’s a 250efi engine & yours was a carb bikes your using the carbs from the 125?

[Ucvmn]

Simon wrote:

I assume as it’s a 250efi engine & yours was a carb bikes your using the carbs from the 125?

Quote

Yes, but I’ve changed the main jets on the carburetors to FRONT = 92.5, REAR = 95, and I’ve put the pilot jets at 20.

[♠️ MARCEL]

Check your float height ,  after every ride on my carb bike = the big filter is usually full.   Or your float needles are jamming causing some internal engine slow leak of fuel in to cylinders = Your manifolds will be “wet” inside as a hint.

Also open “OIL FILLER CAP”  ( on the clutch cover) and sniff inside hard. ==> If you smell fuel  => change oil before riding again.

Can you also clean your spark plugs , go for a short ride => wait to cool down => Take pictures of spark plugs so we can see the mixture  ?

Simon also has the EFi 250 motor, he knows if the fuel system is too rich or too lean,.  Pictures of spark plugs will help

OR

It could be electrical!  (Coils? Stator unit?) (Since stator is also part of ignition system  = high revs , more pulses to cdi  , cdi talks to coils to spark the plugs. , dashboard tells you how much RPM engine is doing

// Meditation doesn't mean you have to sit still....

[Ucvmn]

♠️ MARCEL wrote:

Check your float height , after every ride on my carb bike = the big filter is usually full. Or your float needles are jamming causing some internal engine slow leak of fuel in to cylinders = Your manifolds will be “wet” inside as a hint. Also open “OIL FILLER CAP” ( on the clutch cover) and sniff inside hard. ==> If you smell fuel => change oil before riding again. Can you also clean your spark plugs , go for a short ride => wait to cool down => Take pictures of spark plugs so we can see the mixture ? Simon also has the EFi 250 motor, he knows if the fuel system is too rich or too lean,. Pictures of spark plugs will help OR It could be electrical! (Coils? Stator unit?) (Since stator is also part of ignition system = high revs , more pulses to cdi , cdi talks to coils to spark the plugs. , dashboard tells you how much RPM engine is doing

Quote

Okay, I’ve never checked the float height. One question, should I change the needle jets on the carburetors? Because in the manual, I see that for the 250 and 125 they are different. The spark plugs now are new CR8E, I’ve ridden 150 km today, I’ll take them out and upload a photo. This might take a bit because I’m a bit busy this week.

If the float needles aren’t working, the fuel filter should lower the level when the bike is off, right? It doesn’t. Last week, I removed the airbox and the carburetors to change the spark plugs, and the intake manifolds weren’t wet with gasoline. The fuel filter only empties if I accelerate to the maximum.

Do you then see it’s alright how I’ve connected the vacuum lines?

Thank you very much to you and Simon for the help.

[Ucvmn]

Should I also change the main air jets to the 250 ones?

[♠️ MARCEL]

Ucvmn wrote:

should I change the needle jets on the carburetors? Because in the manual, I see that for the 250 and 125 they are different.

See this link   (click here for tutorial)    , and if you want new  needles   (click here for shop link)
Also you should use 250cc slides   (click here to see 250cc slides) , because 250cc slides were different from 125cc

Ucvmn wrote:

The spark plugs now are new CR8E, I’ve ridden 150 km today, I’ll take them out and upload a photo. This might take a bit because I’m a bit busy this week.

No rush! Just take it easy with her 😉

Ucvmn wrote:

If the float needles aren’t working, the fuel filter should lower the level when the bike is off, right? It doesn’t. Last week, I removed the airbox and the carburetors to change the spark plugs, and the intake manifolds weren’t wet with gasoline. The fuel filter only empties if I accelerate to the maximum. Do you then see it’s alright how I’ve connected the vacuum lines?

Okay, if you ride next time, go 5-10 miles at lower revs (steady cruising) = Look at your filter again and let us know if its half-full  (50% or more IS OK)
We are justing making sure there is no internal carb leaks in the float seats  (because float seats have O-Rings too)  = slow leak in to engine sometimes.

Should I also change the main air jets to the 250 ones?

Not really, it should be fine but keep the carb cleaned as there is lots of holes inside.

This video is 100% explain how our carbs work, because it is a CV carb.

https://www.youtube.com/watch?v=wyspAHrMbb8

// Meditation doesn't mean you have to sit still....

[Ucvmn]

Simon wrote:

I assume as it’s a 250efi engine & yours was a carb bikes your using the carbs from the 125?

Quote

♠️ MARCEL wrote:

Check your float height , after every ride on my carb bike = the big filter is usually full. Or your float needles are jamming causing some internal engine slow leak of fuel in to cylinders = Your manifolds will be “wet” inside as a hint. Also open “OIL FILLER CAP” ( on the clutch cover) and sniff inside hard. ==> If you smell fuel => change oil before riding again. Can you also clean your spark plugs , go for a short ride => wait to cool down => Take pictures of spark plugs so we can see the mixture ? Simon also has the EFi 250 motor, he knows if the fuel system is too rich or too lean,. Pictures of spark plugs will help OR It could be electrical! (Coils? Stator unit?) (Since stator is also part of ignition system = high revs , more pulses to cdi , cdi talks to coils to spark the plugs. , dashboard tells you how much RPM engine is doing

Quote

Today I came back from the garage feeling defeated. The bike won’t start anymore.

Firstly, I removed the spark plugs, and they were black, indicating that the engine might have been running rich.

• I checked the float needle, and they seemed fine.

• The float height was very off. They were at 8 mm and should be at 17 mm according to the manual. I adjusted them as the manual instructed, but I didn’t check the fuel height. Is it important?

After all that, I tried to start the bike, but it wouldn’t start. When I engaged the starter, it made an attempt to start but didn’t go any further. During the starting attempt, the fuel filter only filled to about 5%.

Could this be due to the starter jet, main air jet, and the 125 slides? Because according to the manual, they are different between the 125cc and 250cc models.

I’m thinking of buying second-hand carburetors from a 250cc. I don’t know what to do.

 

P.S.: I opened the oil filler cap and smelled it, but it didn’t smell like gasoline.

[♠️ MARCEL]

The manual is incorrect , our floats in the shop are around 7mm (petrol is full inside carb) and they lift up to 17mm (empty fuel)

So the height from that picture should be 7mm. when the carb floats is facing the sky.

Clean the carbs again,   125cc carbs work on 250, just requires “Size 20 Pilot Jets” and “Size 90 and 925 or 92.5 and 95” main jets.

The starter jet gets very clogged easily, you can’t remove it but you can use air compressor all over the carb where you see any holes.  it is very crucial.

We do liquid cleaning , and after carb is dry, we use air compressor to blow everything clean inside the skeleton body

Check for leak or damaged “O-Rings” on Intake Rubber pipes attached to the cylinder heads because they affect everything including vacuum for fuel pump + tap , difficult starting , poor running

 

This picture ,  remove the gold seat , and check O-Rings are OK and clean the mesh filter attached on the other side.

// Meditation doesn't mean you have to sit still....

[Ucvmn]

Alright, tomorrow I’ll set the float height to 7 mm and give the carburetors a thorough cleaning.

I’ve installed size 20 pilot jets and 92.5 + 95 main jets on the carburetors, yes. I’ve also replaced the intake manifold o-rings (I bought them from you along with other things, and I’m happy with the purchase, thank you).

The intake manifolds have cracks on the outside, but I don’t see any cracks on the inside. I’ve also conducted a test by spraying brake cleaner on the intake manifolds while the bike is idling, and the idle didn’t change. Is this a reliable test?

Thank you very much for your help.

[♠️ MARCEL]

If the revs did not change. That is good.

 

Make sure the airbox is not leaking on top of the carbs.

// Meditation doesn't mean you have to sit still....

[Ucvmn]

♠️ MARCEL wrote:

If the revs did not change. That is good. Make sure the airbox is not leaking on top of the carbs.

Quote

I come with good news and bad news.

The bike has started, you were right, 17 mm isn’t correct. When I opened the carburetors, there was no gasoline at all, so it’s no wonder the bike wouldn’t start. I set the floats to 7 mm and checked all the vacuum lines again, and they’re not leaking anything, absolutely nothing. I also cleaned the carburetors as you suggested.

I cleaned the spark plugs as much as I could and checked the ohms with a tester for the black plug cap, HT wire, and the terminal pin.

Here are the values:

(BLACK PLUG CAP) R – 8.76K Ohm; F – 9.58K Ohm.

(HT WIRE – GROUND) R – 6.05K Ohm; F – 5.54K Ohm.

(TERMINAL PIN – GROUND) R – 0.2 Ohm; F – 0.3 Ohm.

I started the bike and it felt weird, the tachometer was acting strangely and stayed at 5000 rpm even though the bike was idling. I rode it for a while, then stopped to check what was happening: I touched the rear exhaust manifold, and it was hot, but the front one was cold. At that moment, I remembered that I hadn’t connected the terminal pin of the front cylinder coil, so I connected it on the street by reaching between the radiator and the cylinder, and the bike started running smoothly again. However, the tachometer still didn’t work properly. I think I might have damaged the tachometer because it still does the initial check, but when I start it, it doesn’t show the rpm, it stays at 0.

The idle is bad (like before), and I still don’t have power at high RPMs. It doesn’t go over 110 km/h at full throttle in fifth gear (it still has the transmission from the 125cc). The fuel filter is still almost empty.

I don’t know what else I can do. Should I give up?

PS: I’m going to attach a video that I recorded before this adventure so you can see what the idle of the bike looks like.

https://www.youtube.com/watch?v=

[Ucvmn]

The video:

https://www.youtube.com/watch?v=

[Ucvmn]

Today I went out to test the bike, and I don’t know, the fuel filter seems to fill up, and it empties regardless of the circumstances. The idle is also very bad. Sometimes when I come to a stop, the bike stalls. And to top it off, the tachometer only shows 0, I’ve lost the tachometer.

[♠️ MARCEL]

Okay,

Try this…  Take out the spark plugs and show me ?   Are you also using standard CR8E ? (I hope so)

Next , remove dashboard ==> Remove fairings ==> Remove tank ==> Follow the “BLACK & YELLOW” wire from dash board to front coil = check for any ohms

It should go from 0 omhs to infinity 9999999999 ohms = If it stays zero,  harness is broken somewhere and dash board is not able to read signal from front coil.

Also check black yellow wire from front coil to  rear CDI unit = check for circuit is complete with your meter ?

When your bike is running =-==> Check under bike for “sparks” escaping the ht wires on the coil  (coil damage)

Soon = we may need to replace your stator and regulator again ,   but i cannot remember if i send you Stator & Regulator for you 125cc a long time ago and you need new parts for a 250cc now ?

// Meditation doesn't mean you have to sit still....

[Ucvmn]

♠️ MARCEL wrote:

Okay, Try this… Take out the spark plugs and show me ? Are you also using standard CR8E ? (I hope so) Next , remove dashboard ==> Remove fairings ==> Remove tank ==> Follow the “BLACK & YELLOW” wire from dash board to front coil = check for any ohms It should go from 0 omhs to infinity 9999999999 ohms = If it stays zero, harness is broken somewhere and dash board is not able to read signal from front coil. Also check black yellow wire from front coil to rear CDI unit = check for circuit is complete with your meter ? When your bike is running =-==> Check under bike for “sparks” escaping the ht wires on the coil (coil damage) Soon = we may need to replace your stator and regulator again , but i cannot remember if i send you Stator & Regulator for you 125cc a long time ago and you need new parts for a 250cc now ?

Quote

Yes, I’m using CR8E spark plugs.

A few weeks ago, I bought a regulator from you along with other things, but not a stator.

[♠️ MARCEL]

We will see if stator is a problem but try to test what i said in the previous replies and spark plug pictures

// Meditation doesn't mean you have to sit still....

[Ucvmn]

Hi, the bike was stored for 2 months. I was very busy due to work and study.

This week, I had a little time to change the chain and sprockets to the 250cc ones and tested the wiring from the CDI to the front coil and from the front coil to the dashboard (up to the blue connector). The tester gave me a reading of 0.1 ohms. However, the tachometer still doesn’t work.

I tested the bike and it still jerks at full throttle and top speed (only reaching 123 km/h). If I release the throttle slightly, say from 100% to 95%, the bike gains a little more power and reaches 127 km/h. The idle remains unstable.

I forgot to remove the spark plugs and take a photo.

Also, I tested the stator:

Yellow wires in all combinations: 0.6 ohms.

Blue and green wire: 133.3 ohms.

[♠️ MARCEL]

Sounds like we have to see your spark plug colours   (side view and  center view too)

 

123 kmh / 77mph = I would say  90mph is the factory speed because over 100mph all the time will be big issues later.    So   76 is not too far away from 90mph

Hopefully also you can take a video of the “unstable” idling   (but alas , the rpm rev counter isn’t working) – Is there also a wrecked bike in your country to try another matching display dashboard ?

 

Regards

// Meditation doesn't mean you have to sit still....

[Ucvmn]

77 mph was the top speed of the 125cc engine (under the same road conditions, wind, etc…).

 

I will try to remove the spark plugs to upload a photo. Also, I will make a video of the idle. If I give it full throttle and release in neutral, the RPM hangs above idle and then “stabilizes”. If I give it a little throttle and release, the RPM drops below idle and then “stabilizes”.

What kind of signal does the dashboard receive? Can I replicate that with an Arduino or something similar to test the dashboard?

[Ucvmn]

PS: here is a video showing the idle, was posted in this thread:

[♠️ MARCEL]

Ucvmn wrote:

What kind of signal does the dashboard receive? Can I replicate that with an Arduino or something similar to test the dashboard?

I forgot (as i drank today lol) ,  but i can tell that the RPM signal comes from the front ignition coil  + CDI + stator  to count flywheel speed  (faster flywheel means more sparking)

// Meditation doesn't mean you have to sit still....

[Ucvmn]

Hi, this doesn’t seem to end, hahaha. Here are the pictures. I think the bike is running rich. I don’t know why.

The bike has a large fuel hose and fuel filter, spark plugs changed 3 months ago, coils tested, stator tested?, plug caps tested, new air filter, new oil and oil filter, carbs with new jets and cleaned, and no vacuum leaks…

At low to mid rpm, it has plenty of power, but when it gets to high rpm, the bike becomes jerky.

[Ucvmn]

Hi, this doesn’t seem to end, hahaha. Here are the pictures. I think the bike is running rich. I don’t know why.

The bike has a large fuel hose and fuel filter, spark plugs changed 3 months ago, coils tested, stator tested?, plug caps tested, new air filter, new oil and oil filter, carbs with new jets and cleaned, and no vacuum leaks…

At low to mid rpm, it has plenty of power, but when it gets to high rpm, the bike becomes jerky.

[♠️ MARCEL]

That is very rich!!

How about this ?

• Look at carb slide needles on diaphragm, and (click here)
  Put your needles to “MIDDLE” setting  (this is factory)
• Go back to Factory 92.5 and 95 jets mains, to see if this improves mixture
• Get new plugs again as those ones will be fouled, even if cleaned up, they won’t spark as good anymore

Or check you have no blockage in the air system (check air filter ,  or “test” with a STOCK NEW air filter first to eliminate possible air issues / chocking up at high revs with not enough air flowing)

Pilot Jets = I hope they are size 20 and  make sure mixture screws 2.5 FULL turns maximum from closed. Leave it there forever.

// Meditation doesn't mean you have to sit still....

[Ucvmn]

I have news. Today, I tried testing the motorcycle without the air filter, and without it, there’s plenty of power at high RPM. I reached 140 km/h smoothly.

Does this have something to do with the airbox or something? Do 250cc motorcycles have a different airbox than the 125cc ones?

P.S.: The air filter isn’t old, and I’ve only used it for a few kilometers.

[Ucvmn]

The needles are in the middle position and the main jets are 92.5 – 95, pilot jets 20 2.5 turns. Air filter is almost new, i bought it from here. Is the 125 – 250 air filter different?

[♠️ MARCEL]

Do you have K&N upgrade air filter ? – It is not good to ride without airbox but…. you can use 125cc airbox and 125cc intake pipes , they are just longer pipes and GT250 pipes are just shorter and wider but less with 125cc intake pipes you lose maybe 1-5% power , it is not a big issue.   And 140 km is usually the “safe” max speed a factory model 250cc will do,  otherwise bigger revs will make valves start “floating” , and premature wear/damage happens soon.

I think maybe you need more air ?  So if you have stock air filter, it is a good idea to use K&N

No pod filters , Hyosungs really hate them.  you will be in more hell lol.

// Meditation doesn't mean you have to sit still....

[Ucvmn]

I’m happy with that top speed, especially since the engine runs smoothly at high RPM.

Now, since I shouldn’t use the bike without the air filter, I will try a K&N to see if it does not reduce airflow too much. So, the engine running rich is due to the 125cc intake, isn’t it? Because everything else is “stock” for a 250cc engine (main jets and pilot jets). Well, the exhaust headers and muffler are the 125cc ones.

Another problem that I have to solve is the tachometer. I will see what I can do about that.

I think we are close to making the bike run perfectly. Thank you very much for your help.

[♠️ MARCEL]

The intake pipes won’t be a problem  5% is loss of power is like 1-2bhp but it is not a big deal, i would focus in getting more air via an improved intake like K&N Catridge box or  try  BMC air filter  (Since you are close to italy , find a local distributor that imports BMC filters as they are a little better than K&N, just a little – Just ask them for Carb Model GT250/GT650)

Exhaust headers = it is possible it will be restricting exhaust flow a little bit!  = I would try to get  OEM GT250 headers and use a full size muffler/silence with DB Killer

We don’t know the muffler you have, so i am guessing at this point, but yes 125cc headers will need changing so it breathes more easier at higher rpm.

// Meditation doesn't mean you have to sit still....

[LaserBeam]

Hi, Uvcmn. my GT250R has similar problems to yours – not as bad.

I haven’t totally fixed it but it runs better than when i bought it. My plugs were a bit sooty and the exhaust is sooty.

My Front Cyclinder does not get as warm as the rear, ever. I can smell unburt fuel out the pipe.

I get FUEL on the AIR FILTER coming from Cylinder-1 Carby, i think.

And i get this horrible CUT-OFF of power when i have the throttle near fully open, (this cut-off used to be at 5000-6000 revs).

I still get the cut-off of power but it’s now at 9500-10000rpmw, which is a lot better.

Power completely cuts out while i’m on the gas and head down – it’s a  bit scary & even more annoying.

I’ve not fixed the issue(s) but i guess my issue is with the CARBYS.

To IMPROVE the situation i changed all fuel lines, new fuel vacuum pump, bigger fuel filter, new spark plugs, new ignition coils & a new CDI unit.

I also bought a Voltge Regulator but it’s not installed yet.

I also added a strong FUEL SYSTEM CLEANER to the PETROL TANK, several times & gave the bike long runs at high revs.

Then i sprayed a bit of CRC Carby Cleaner into the INTAKE of the carbys. If you are taking the CARBS apart, you can spray this CRC all over the metal components.

This stuff eats paint but it’s amazing for cleaning the carbs – stonger than a brake cleaner.

All of that made the bike run much better but DIDN’T fix the cold front cyclinder or eliminate the power cut-out.

But it idles much better, runs much better & only cuts-out at the top of the rev range.

I will have to do my Carbs. I would love new carbys but i can buy half a Hyosung motorcycle for the same price.

If anyone finds out why the front cylinder is colder, i’d like to know. It doesn’t sit right with me – it’s not right.

Engines get hot, don’t they? 😆

-----LaserBeam---->

[Ucvmn]

@laserbeam, did you properly tighten the wire to the HT caps? You need to twist it clockwise to tighten it.

Also, you can test the HT caps with a multimeter, as I did in this thread.

[LaserBeam]

The power cut-off issue, my bike has, is not as bad as it was. The coils, leads & caps are new; they problem was there before.

I read that the CDI unit has some sort of cut-off that causes this problem. It’s the result of some emission laws, they said.

I can’t verify if they are right. Apparently you cut one of the wires to the CDI unit & it MAY make the problem go away.

My green wire to my CDI was already cut & they didn’t leave any wire for me to reconnect it for test purposes.

When i put a new CDI unit onthe bike i still have no green wire connected to it. They say cutting it was meant to fix this cut-out sceanrio but it hasn’t.

I got the GT250R to 10,000RPM today in a few gears but i should really fix her before revving her too much. Big service is due.

-----LaserBeam---->

[Ucvmn]

According to my wiring diagram, the green wire connects the CDI with the pickup coil. I don’t think the bike could run at all without that connection.

Do the front and rear spark plugs have the same color? If yes, and they are sooty, you can remove the air filter to conduct a quick test run to see if the problem is a rich running condition (which is the case for me).

[Ucvmn]

Ladies and gentlemen, I am pleased to inform you that the problem is officially resolved.

I cut the snorkel from the air filter, and now the bike runs perfectly. It just needed a bit more air. Thanks to everyone for your help. Now, I just need to fix the dashboard.

[Ucvmn]

Hi, this doesn’t seem to end, hahaha. Here are the pictures. I think the bike is running rich. I don’t know why.

The bike has a large fuel hose and fuel filter, spark plugs changed 3 months ago, coils tested, stator tested?, plug caps tested, new air filter, new oil and oil filter, carbs with new jets and cleaned, and no vacuum leaks…

At low to mid rpm, it has plenty of power, but when it gets to high rpm, the bike becomes jerky.

 

[Author]

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