NEED HELP no top end!

[Rjb13]

So i tried posting a thread and it disappeared because I didn't have an account, well I'm back and I will use copy and paste until it works.
Its a lot to read but if u can help me It'd be great!


So I'm having trouble with my 66/80cc China 2 stroke. I recently built a new bike because my frame broke, It was running great before putting it on new frame. Easy 35mph+
New frame is BBR Tuning gas tank frame, (which eliminates the need for chain tensioner) basic HP Carb carb and HP magneto

The problem is not noticeable when going slow ( 1- 20mph ) but as soon as 25 hits it will not go faster. It just sounds like s**t. Like the engine is maxing out, but it's not over revving, it's just gets louder with no speed change. Slow down a bit and it runs good again. Speed back up and-- you know.

I RAN OUT of gas. Right before it ran out, it had full power, it got up to 31mph and would have kept going but,, it ran out of fuel... hmm??

I have tried adjdusting idle screw/cable stays, cleaned the carb ( float is not level with carb? also, found green liquid on air filter? Why is this here?? ) , tighten head bolts, check spark, ride without gas cap, checked fuel flow from tank... ALL made no difference to performance.

I heard about jet size? I guess I have 77 but they sell 60, 66, 70, 77, and so fourth... What what are these numbers and what do I need?

My stock exhaust broke in half but when it did, the bike got a lot faster so I don't see how this would slow it down. New exhaust coming tomorrow!

I may have missed mentioning some things. Please ask question if you have them but more importantly help me solve this!

Thanks

 

[Rjb13]

Oh, maybe vacuum leak? It revs down slow when idling

 

[Rjb13]

Oh, maybe vacuum leak? It revs down slow when idling

But leaking from where??

 

[mark20]

So i tried posting a thread and it disappeared because I didn't have an account, well I'm back and I will use copy and paste until it works.
Its a lot to read but if u can help me It'd be great!


So I'm having trouble with my 66/80cc China 2 stroke. I recently built a new bike because my frame broke, It was running great before putting it on new frame. Easy 35mph+
New frame is BBR Tuning gas tank frame, (which eliminates the need for chain tensioner) basic HP Carb carb and HP magneto

The problem is not noticeable when going slow ( 1- 20mph ) but as soon as 25 hits it will not go faster. It just sounds like sh*t. Like the engine is maxing out, but it's not over revving, it's just gets louder with no speed change. Slow down a bit and it runs good again. Speed back up and-- you know.

I RAN OUT of gas. Right before it ran out, it had full power, it got up to 31mph and would have kept going but,, it ran out of fuel... hmm??

I have tried adjdusting idle screw/cable stays, cleaned the carb ( float is not level with carb? also, found green liquid on air filter? Why is this here?? ) , tighten head bolts, check spark, ride without gas cap, checked fuel flow from tank... ALL made no difference to performance.

I heard about jet size? I guess I have 77 but they sell 60, 66, 70, 77, and so fourth... What what are these numbers and what do I need?

My stock exhaust broke in half but when it did, the bike got a lot faster so I don't see how this would slow it down. New exhaust coming tomorrow!

I may have missed mentioning some things. Please ask question if you have them but more importantly help me solve this!

Thanks

could it be 4 stroking?
those hp carbs need to be tuned (jetted) for them to work right (im a fan of the nt carbs myself, easy to adjust and nearly maintenance free)

basically those numbers are sizes of the jet drilled out (ie a #70 jet is .70mm)
you want to down jet a bit (i would start out with 66 and go from there, jetting depends on many variables, i cant tell you what you need for certain)
the green liquid is the 2 stoke oil and fuel (when a 2 stroke engine runs, it blows a bit of oil back though the carb, this is normal)

heres some jets to mess around with (9$ for 5 jets is not bad)

5 Jets "high 60's" 5mm Jet Set for Motorized Bicycle Fine Tuning Jet-set for sale online | eBay

Find many great new & used options and get the best deals for 5 Jets "high 60's" 5mm Jet Set for Motorized Bicycle Fine Tuning Jet-set at the best online prices at eBay! Free shipping for many products!

www.ebay.com

hope this helps!

 

[gary55]

Ok lets start with the facts.
1. It was running fine before changing frames. This suggests that it is most likely something that changed in that time frame . So if it is speeding up to a certain RPM then slowing down, then speeding up, then slowing down in a cycling fashion without no change in throttle position you have a lose motor mount.
The reason this happens with a lose mount is that it allows the motor to have excessive vibration at a certain RPM and the fuel foams in the float bowl. When it foams it is not supplied properly to the engine. Once the RPM drops below the speed of the vibration the fuel in the bowl stops foaming and feeds normally. Thus the speed up slow down cycling without changing throttle position.
While looking at your other symptoms this may vary well not be your problem, however it is something you should check before continuing on to the next steps just to eliminate that possibility.
2. The next symptom you have described leads to think that it most likely is not the engine change because you say it ran fine until you ran out of gas.
So if it was on the new frame when it ran out of gas and was running fine till it ran out let's look at that.
A. When it ran out possibly some crud that was in the new tank got into the carb. bowl and blocked the jet. I know you said you cleaned the carb, but did you make sure nothing was stuck in the jet?. This would cause a lean condition as you described in the engine taking a long time to return to idle , and hamper fuel flow restricting top end. Another thing that could cause the slow return to idle is some fresh air being taken in through the broken exhaust depending on where it was broken.
B. This is a unlikely case but still possible. When you ran out of fuel you were running at wide open throttle. This would cause a extremely lean condition for a brief period, and though unlikely could have caused internal damage. If it immediately ran poorly after replacing the fuel, and it ran fine before doing so this is most likely the area causing the problem. Still occasionally there is a coincidence that something else occured at the time such as a loosened mount or a blown crank seal etc. etc..
When checking for leaks I use a unlit propane torch. You can use this while the engine is running. Blowing the propane at possible leaking areas if their is a leak the engine will usually slow down. You can check for the sound you are looking for by blowing the propane into the air filter, then look for a similar reaction at other suspected places. Generally a crank seal problem will cause a high idle that can not be adjusted down by the idle screw, and poor top end due to the diminished pressure it causes in the crank case.
With what you have described it is unlikely that it is the angle of the carb. or the jet since it ran fine before running out of fuel.
3. The broken exhaust could be a problem depending on where it broke at. If it is above the muffler this could allow some fresh air into the mix. This lean condition may cause it to run good, a little to good and cause over heating and engine damage could occur. The engine oil on the filter is normal, but if excessive it could also be a indicator of cylinder damage.
So go through the easy stuff first then if all fails pop the jug and have a look at the cylinder. This is kind of long winded, but I tried to touch all that your described conditions. Good luck.

 

[Rjb13]

could it be 4 stroking?
those hp carbs need to be tuned (jetted) for them to work right (im a fan of the nt carbs myself, easy to adjust and nearly maintenance free)

basically those numbers are sizes of the jet drilled out (ie a #70 jet is .70mm)
you want to down jet a bit (i would start out with 66 and go from there, jetting depends on many variables, i cant tell you what you need for certain)
the green liquid is the 2 stoke oil and fuel (when a 2 stroke engine runs, it blows a bit of oil back though the carb, this is normal)

heres some jets to mess around with (9$ for 5 jets is not bad)

5 Jets "high 60's" 5mm Jet Set for Motorized Bicycle Fine Tuning Jet-set for sale online | eBay

Find many great new & used options and get the best deals for 5 Jets "high 60's" 5mm Jet Set for Motorized Bicycle Fine Tuning Jet-set at the best online prices at eBay! Free shipping for many products!

www.ebay.com

hope this helps!

That! It is definitely 4 stroking. I believe I am running too rich? And when I ran out of fuel it ran perfect which leads me to believe there was too much fuel ? Maybe the expansion chamber I got can help with this because it pushes air back into the cylinder?? Will see how that works out before re-jetting.

The reason i got the HP card is because my NT carb wouldn't idle, I would have to give it a lil throttle to keep it alive

 

[Rjb13]

Ok lets start with the facts.
1. It was running fine before changing frames. This suggests that it is most likely something that changed in that time frame . So if it is speeding up to a certain RPM then slowing down, then speeding up, then slowing down in a cycling fashion without no change in throttle position you have a lose motor mount.
The reason this happens with a lose mount is that it allows the motor to have excessive vibration at a certain RPM and the fuel foams in the float bowl. When it foams it is not supplied properly to the engine. Once the RPM drops below the speed of the vibration the fuel in the bowl stops foaming and feeds normally. Thus the speed up slow down cycling without changing throttle position.
While looking at your other symptoms this may vary well not be your problem, however it is something you should check before continuing on to the next steps just to eliminate that possibility.
2. The next symptom you have described leads to think that it most likely is not the engine change because you say it ran fine until you ran out of gas.
So if it was on the new frame when it ran out of gas and was running fine till it ran out let's look at that.
A. When it ran out possibly some crud that was in the new tank got into the carb. bowl and blocked the jet. I know you said you cleaned the carb, but did you make sure nothing was stuck in the jet?. This would cause a lean condition as you described in the engine taking a long time to return to idle , and hamper fuel flow restricting top end. Another thing that could cause the slow return to idle is some fresh air being taken in through the broken exhaust depending on where it was broken.
B. This is a unlikely case but still possible. When you ran out of fuel you were running at wide open throttle. This would cause a extremely lean condition for a brief period, and though unlikely could have caused internal damage. If it immediately ran poorly after replacing the fuel, and it ran fine before doing so this is most likely the area causing the problem. Still occasionally there is a coincidence that something else occured at the time such as a loosened mount or a blown crank seal etc. etc..
When checking for leaks I use a unlit propane torch. You can use this while the engine is running. Blowing the propane at possible leaking areas if their is a leak the engine will usually slow down. You can check for the sound you are looking for by blowing the propane into the air filter, then look for a similar reaction at other suspected places. Generally a crank seal problem will cause a high idle that can not be adjusted down by the idle screw, and poor top end due to the diminished pressure it causes in the crank case.
With what you have described it is unlikely that it is the angle of the carb. or the jet since it ran fine before running out of fuel.
3. The broken exhaust could be a problem depending on where it broke at. If it is above the muffler this could allow some fresh air into the mix. This lean condition may cause it to run good, a little to good and cause over heating and engine damage could occur. The engine oil on the filter is normal, but if excessive it could also be a indicator of cylinder damage.
So go through the easy stuff first then if all fails pop the jug and have a look at the cylinder. This is kind of long winded, but I tried to touch all that your described conditions. Good luck.

You are right. The only thing I changed with the rebuild was the carb. This is most likely the culprit? After extensive video searching I figured out that I am 4 stroking. Too rich maybe? Because when I ran out of fuel it was running great for about 5 seconds before it ran out. This leads me to believe there was too much fuel and not enough air, but just before it ran out it was perfect.
I ordered an expansion chamber (which forces air back into cylinder) I'm assuming this might help cure the mix problem? Will also try re- jetting

But if it's lean at idle, with a slow rev return, why is it rich at WOT? I cleaned with carb cleaner and compressed air, everything was clear. Could my gas/oil mix be off and causing this? I was thinking of adding some more oil to the tank to see how it changes (too much oil isn't bad, right?)

I can still adjust idle screw and the idle will go up or down, so maybe not a crank seal issue?

I'm assuming the propane trick won't damage anything?? Will try it later on today.

Thanks for your help

 

[gary55]

You are right. The only thing I changed with the rebuild was the carb. This is most likely the culprit? After extensive video searching I figured out that I am 4 stroking. Too rich maybe? Because when I ran out of fuel it was running great for about 5 seconds before it ran out. This leads me to believe there was too much fuel and not enough air, but just before it ran out it was perfect.
I ordered an expansion chamber (which forces air back into cylinder) I'm assuming this might help cure the mix problem? Will also try re- jetting

But if it's lean at idle, with a slow rev return, why is it rich at WOT? I cleaned with carb cleaner and compressed air, everything was clear. Could my gas/oil mix be off and causing this? I was thinking of adding some more oil to the tank to see how it changes (too much oil isn't bad, right?)

I can still adjust idle screw and the idle will go up or down, so maybe not a crank seal issue?

I'm assuming the propane trick won't damage anything?? Will try it later on today.

Thanks for your help

I didn't realize you had changed the carb. at the same time. I missed that part. The dellorto clone (rundong) carbs. do tend to run rich, and is probably the issue. Jetting smaller will cure the 4 stroking. Don't sound like a leak issue at all. The speeding up thing when running out of gas always happens. Even when carb. is tuned properly. The expansion chamber returns unspent fuel as well so you may or may not need to rejet again after it's on. Sounds like Mark20 hit the nail on the head. If you like some good reading on the workings of these engines check this out. Long winded, but it helped my out a lot.Jennings: Two-Stroke Tuner's Handbook - tuners.pdf

 

[Rjb13]

I didn't realize you had changed the carb. at the same time. I missed that part. The dellorto clone (rundong) carbs. do tend to run rich, and is probably the issue. Jetting smaller will cure the 4 stroking. Don't sound like a leak issue at all. The speeding up thing when running out of gas always happens. Even when carb. is tuned properly. The expansion chamber returns unspent fuel as well so you may or may not need to rejet again after it's on. Sounds like Mark20 hit the nail on the head. If you like some good reading on the workings of these engines check this out. Long winded, but it helped my out a lot.Jennings: Two-Stroke Tuner's Handbook - tuners.pdf

I may have forgot to include the carb switch in my question, however your information is valuable and I will retain it for future diagnosis

 

[mark20]

You are right. The only thing I changed with the rebuild was the carb. This is most likely the culprit? After extensive video searching I figured out that I am 4 stroking. Too rich maybe? Because when I ran out of fuel it was running great for about 5 seconds before it ran out. This leads me to believe there was too much fuel and not enough air, but just before it ran out it was perfect.
I ordered an expansion chamber (which forces air back into cylinder) I'm assuming this might help cure the mix problem? Will also try re- jetting

But if it's lean at idle, with a slow rev return, why is it rich at WOT? I cleaned with carb cleaner and compressed air, everything was clear. Could my gas/oil mix be off and causing this? I was thinking of adding some more oil to the tank to see how it changes (too much oil isn't bad, right?)

I can still adjust idle screw and the idle will go up or down, so maybe not a crank seal issue?

I'm assuming the propane trick won't damage anything?? Will try it later on today.

Thanks for your help

basically a slide carb works on two principles, the idle jet (affects idle to 3/4 throttle mixture), and the main jet (affects 3/4 to WOT mixture)
your a bit lean on the idle jet but i think it should be fine. your main jet is the problem. as its 4 stroking badly at higher rpm (ie its runs too rich so the engine is hitting its limit) if you down jet (making it run "leaner") your introducing less fuel so it revs higher and you get back those speeds (but dont go too lean, or else you will be needing a new engine)
its a test and tune game.
good luck!
(fyi, your idle pin is probably clogged on your nt. thats why you cant get it to idle well)