2 stroke 80cc bike not running well

[Quintin]

about a year ago, i built a 2 stroke 49cc kit. ran excellent, no problems. at least until the engine seized. bought a new engine last week (80cc), installed it, got it running, but it doesn't run well at all. the only thing that has changed is the engine. when i rotate the idle needle on the side of the carburetor, literally nothing happens. I've thoroughly cleaned the carb, no change. when i pull the clutch in, most of the time it will stall. the rest of the time when i pull the clutch in it'll idle at low rpms (and moving the idle needle does nothing). also, when i open the throttle, the engine will pick up to about 15 mph (very slowly). my old engine never had this issue, and even when it was brand new it responded excellent and picked up speed very well. I'm not too sure what the issue is. any suggestions?

 

[crassius]

could be clogged muffler - try running it with muffler about 1/8 inch loose

also, once had a customer with slide in carb upside down that ran like that

 

[Quintin]

could be clogged muffler - try running it with muffler about 1/8 inch loose

also, once had a customer with slide in carb upside down that ran like that

i know for a fact that the muffler isnt clogged... i checked that earlier today. but the slide in the carb is also correct side up. cleaned that today aswell. thanks for the suggestions however. any other ideas?

 

[Quintin]

ive debunked the problem! loss of compression. i could see daylight right through the top gasket, even after it was tightened. now i have to figure out how to solve that. thanks!

 

[Chaz]

Check to see if you're not just seeing the normal gap between the cylinder and the head which is created by the head gasket.

If that is not the case then you probably have the stock acorn capped head nuts which can allow the bolts to top out. You can cut the caps off the nuts to make them regular "pass through" nuts and re-torque.

 

[Quintin]

Check to see if you're not just seeing the normal gap between the cylinder and the head which is created by the head gasket.

If that is not the case then you probably have the stock acorn capped head nuts which can allow the bolts to top out. You can cut the caps off the nuts to make them regular "pass through" nuts and re-torque.

hmm.... fascinating thought. how do i check to see if it is the normal gap? and yes, i do indeed have stock acorn nuts.

 

[Chaz]

The head gasket does not run to the full area of the mating surfaces so there will be a gap exactly the thickness of the gasket.

Nip or saw off the caps of the nuts so the bolts don't bind up amd give you a false sense of tightening. You also can't assume that the bolt length is identical to eachother.

 

[Quintin]

The head gasket does not run to the full area of the mating surfaces so there will be a gap exactly the thickness of the gasket.

Nip or saw off the caps of the nuts so the bolts don't bind up amd give you a false sense of tightening. You also can't assume that the bolt length is identical to eachother.

i tightened the bolts to specification, and there is now no gap in the gasket. also, im confident that the bolts arent topping off based off of how they felt when tightening, and the fact that there are enough washers on there to prevent this. i just took it for a spin, and i took the muffler off and noticed a slight increase of power, but im still convinced the muffler isnt the problem because it was fine on the old engine.

 

[crassius]

the next step is to do a compression test

 

[Tyler6357]

Start it up and see if you can feel any air shooting out around the head gasket. If it's leaking you can usually feel the air shooting out with your hand. If everything is sealed and the problem continues you probably need to jet down to a #65.

 

[Quintin]

Start it up and see if you can feel any air shooting out around the head gasket. If it's leaking you can usually feel the air shooting out with your hand. If everything is sealed and the problem continues you probably need to jet down to a #65.

what do you mean down to a #65

 

[Tyler6357]

what do you mean down to a #65

I mean that the main jet in the carb is too big causing your engine to run rich. Most carbs come with a # 70 jet. If you have made sure all the leaks are sealed than you probably need to lower your jet size.

https://www.ebay.com/itm/5-Jets-Hig...e=STRK:MEBIDX:IT&_trksid=p2055119.m1438.l2649

https://www.ebay.com/itm/5-JETS-Low...e=STRK:MEBIDX:IT&_trksid=p2055119.m1438.l2649

 

[Quintin]

I mean that the main jet in the carb is too big causing your engine to run rich. Most carbs come with a # 70 jet. If you have made sure all the leaks are sealed than you probably need to lower your jet size.

https://www.ebay.com/itm/5-Jets-High-60s-5mm-Jet-Set-for-Motorized-Bicycle-Fine-Tuning-Jet-Set/231458900645?ssPageName=STRK:MEBIDX:IT&_trksid=p2055119.m1438.l2649

https://www.ebay.com/itm/5-JETS-Low-60s-5mm-Jets-for-Motorized-Bicycle-Fine-Tuning-Jet-Set/271748133936?ssPageName=STRK:MEBIDX:IT&_trksid=p2055119.m1438.l2649

wow. i adjusted the needle and its running better. no compression leaks either. thanks for the suggestion... im going to look into this

 

[Tyler6357]

wow. i adjusted the needle and its running better. no compression leaks either. thanks for the suggestion... im going to look into this

Did you adjust it to the leanest setting?

 

[Quintin]

Did you adjust it to the leanest setting?

yes i did.

 

[Quintin]

alrighty... ordered the new jets, #61-65. i tried #64 first, not that great. messed around with some needle settings, got a little better, but not nearly the way it should be. tried #65, ran worse. then i tried #61, same issue. must've tried all kinds of different needle combinations, most were noticeably better at leaner settings, even with the #61. still however, didnt run good. any chance #62 or 63 might make a dramatic difference?

 

[Tyler6357]

If you put a 62 in and it didn't make any difference I shouldn't expect you will see much difference with the 63. Usually jetting down and leaning it out helps wake it up. I still think it's carb related. When you adjust the idle screw does the idling change? Are you certain the throttle cable is hooked up correctly? When you hold the carb in your hand can you see the slide going up and down when you turn the throttle? The idle screw should fine tune the amount it moves. It would be interesting to see how it runs with a different carb.

 

[Quintin]

I believe it's carb related aswell. Yes, the idle changes now. I'm 110% positive the throttle is hooked up correctly. And yes, indeed everthing moves as it should. I would be curious to see how it would work with a new one too, but I don't really want to buy a new carb, as this one worked perfectly fine with the old engine, and I can't imagine it wouldn't work with the new one. I'm surprised the jets haven't changed much...

 

[Quintin]

IT LIVES! IT LIIIIVES! i got it working like it should! i was stubborn in saying it wasn't the muffler due to the fact that when you blew through it, it had decent air flow. but after unscrewing it slightly AND with the new jets, suddenly all is well. i took the cap off only to realize that more air could've been able to run through, and with the bigger engine than the last one even more airflow was all it needed. Thanks to everyone for the help, i sincerely appreciate it! Stay speedy everyone.

 

[Tyler6357]

Hey, good news, I was actually thinking about what else you could try without spending any more money. I'm glad you figured it out. Be careful not to put too much oil in your mix, don't use 16x1 like they recommend, it will clog up in the pipe. Also, now that it is running correctly you should make sure you have the right jet in. You want to use the richest jet you can without it bogging or 4 stroking. If it's too lean the engine won't get enough oil and will run too hot, if it's too rich than the engine will bog and 4 stroke and you won't get good performance. Start high and move down until you find the right one for your engine and location. Also, it's best to get the engine hot before you make your analysis, sometimes it will 4 stroke a little bit until it gets warmed up.
Ride safe!