Engine Trouble BT80 running rich and nothing fixes it

drcool52

New Member
Joined
Nov 15, 2019
Messages
2
I bought a bt80 and out of the box it ran super rich. To the point that it wont get over 15 MPH even with a 36 tooth on the back. I had to jet down to a 60 and full leaned out needle just for it to get 23 MPH with the cap off it and it still 4 stroked at the top. I tried a new plug gapped at .35. I'm running a 35:1 oil ratio. Everything is stock besides the intake, its an offset intake. I'm just at a loss here and bikeberry is absolutely no help at all. I've been working on regular china girls for almost 4 years now and I've never had this kind of problem.
 


LewieBike

Active Member
Joined
May 21, 2014
Messages
685
The BT engine is similar to the newer all-in-one head and cylinder HT engines and a previous poster with a stock, unitized head engine was having the same problem with 4 stroking. Needing to really jet down further than #60 jet to get it running cleanly.

A numbered drill set and careful measuring of your stock #60 jet, fill in the jet with solder and redrill to the next size smaller numbered jet.

Remember that numbered drills and numbered jets are backwards to each other, smaller diameters are bigger # drill sizes.
 

Karl Snarl

Active Member
Joined
Sep 20, 2019
Messages
138
if you are running a #60 jet, your problems are right there. to small a jet, you're gonna burn it up. try a #67 and run at 28:1 to 32:1. let it break in too. you want it a bit rich on break in. running rich on break in keeps it cooler while thermal stress is induced to set the engine in. break in is more than just setting rings. it's coating all the bearings in oil so you dont have dry starts. a lot of people fail to remember the engine doesn't have an oil reservoir. so they break in the engine wrong and ruin the bearings way before they would have actually lasted. yeah it might run great, but the problem shows up in loss of longevity. when the engine could have ran 10k miles or more, it conks out at 3k instead. then they blame it on shoddy manufacturing. its the biggest mistake i hear on these sites. so many people think they understand these engines, and they don't. they go off all half cocked when you tell em what there mistake is too. it's ok with me, i make mad cash off people with hard heads. they are always coming to me to fix this or that because they failed to listen to instructions.
 

Street Ryderz

Well-Known Member
Joined
Oct 14, 2016
Messages
3,195
Do you have a compression tester? I ask this because lower comp will want a leaner burn and these being one piece cyl/head if it's too tall comp drops as does power.I would still try a new or different cdi,also curious as to when/where and how you define 4 stroking?
 

ESmalik

Member
Joined
Oct 22, 2019
Messages
54
if you are running a #60 jet, your problems are right there. to small a jet, you're gonna burn it up. try a #67 and run at 28:1 to 32:1. let it break in too. you want it a bit rich on break in. running rich on break in keeps it cooler while thermal stress is induced to set the engine in. break in is more than just setting rings. it's coating all the bearings in oil so you dont have dry starts. a lot of people fail to remember the engine doesn't have an oil reservoir. so they break in the engine wrong and ruin the bearings way before they would have actually lasted. yeah it might run great, but the problem shows up in loss of longevity. when the engine could have ran 10k miles or more, it conks out at 3k instead. then they blame it on shoddy manufacturing. its the biggest mistake i hear on these sites. so many people think they understand these engines, and they don't. they go off all half cocked when you tell em what there mistake is too. it's ok with me, i make mad cash off people with hard heads. they are always coming to me to fix this or that because they failed to listen to instructions.
mmmm not really. If its still 4 stroking its definitely not lean. 28:1 is dumb rich, youll never seat your rings with that much oil. Bearings are plenty fine running dry at 1000 rpm for hours on end. As soon as bearings are initially coated they're good to go to full rpm. Anyone that builds real motors will go balls out for a few heat cycles and then you're good to go.

As to OP, its still 4 stroking because the offset intake, those intakes are so restrictive and they dont allow the fuel to stay atomized. When the fuel travels though the intake, it adheres to the walls and turns it into little drops of fuel instead of the mist. Thats what caused the 4stroking. A china offset will never run correctly.
 

Street Ryderz

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Joined
Oct 14, 2016
Messages
3,195
mmmm not really. If its still 4 stroking its definitely not lean. 28:1 is dumb rich, youll never seat your rings with that much oil. Bearings are plenty fine running dry at 1000 rpm for hours on end. As soon as bearings are initially coated they're good to go to full rpm. Anyone that builds real motors will go balls out for a few heat cycles and then you're good to go.

As to OP, its still 4 stroking because the offset intake, those intakes are so restrictive and they dont allow the fuel to stay atomized. When the fuel travels though the intake, it adheres to the walls and turns it into little drops of fuel instead of the mist. Thats what caused the 4stroking. A china offset will never run correctly.
The op is running 35:1 witch is fine as is 28:1 witch is oil rich not fuel rich, it's leaner on the fuel count with more oil! Anyone whom builds REAL engines knows that a motor is electric!Then break in consist's of wot pulls with the load removed at the peak by clutching and not allowing engine breaking,this seats the rings under load properly and still allows for proper lubrication of the bearings.The offset intake works just fine! Many people use it with no issues as many bikes don't allow the carb to sit inside the framed area,anyone using a large reed setup is running an offset intake essentially.Also you do realize that the am6 and all the scooter guy's run 16-20:1 oil mix NOT the 40:1 or higher that you guy's use! No wonder your engines don't last!
 

ESmalik

Member
Joined
Oct 22, 2019
Messages
54
The op is running 35:1 witch is fine as is 28:1 witch is oil rich not fuel rich, it's leaner on the fuel count with more oil! Anyone whom builds REAL engines knows that a motor is electric!Then break in consist's of wot pulls with the load removed at the peak by clutching and not allowing engine breaking,this seats the rings under load properly and still allows for proper lubrication of the bearings.The offset intake works just fine! Many people use it with no issues as many bikes don't allow the carb to sit inside the framed area,anyone using a large reed setup is running an offset intake essentially.Also you do realize that the am6 and all the scooter guy's run 16-20:1 oil mix NOT the 40:1 or higher that you guy's use! No wonder your engines don't last!
35:1 is fine, id say 28:1 is on the rich side. I stick with 50:1 Opti-2 from the start so that's my opinion. And for clarification yes oil rich. As far as the motor vs engine debate this isn't ye old english. In modern times the two terms are nearly interchangeable when talking about IC, hence why we call this "Motored Bikes" not engined bikes. Either way youre really looking to nitpick anything at this point. I 100% agree with your break in, thats what i mean by balls out heat cycles. When I hear offset intake I think of the one linked. The bends are terrible and the ID is all of 11mm or so, theyre just plain bad. I have used many and none of them can be tuned well and always run like butt. Large reed offsets are a different story, lots more volume to keep atomization and no big hard corners. AM6 and scooters commonly run around 40:1 for street driving and 20-32:1 for track, mainly because iron bores, which is a whole different story. Iron bore will always need ran richer, a china girl chrome plated cylinder doesn't do well with that much oil 40:1 is a sweet spot for these. Our engines last just fine, not sure what you're referring to exactly?
 
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