Cold weather screws up engine - carburetor air/fuel or jetting?

[RB55]

Hey All,
My bike's been running great since I re-jetted the carburetor to 0.61mm a few weeks ago. I ride it almost every day, with the temperature usually around 45 F in Kentucky at this time of year. Last Sunday I rode it at 60 degrees and the engine performed as well as ever, no 4-stroking, bogging, power loss, etc. Smooth all throughout the power band. Today, I decided to go for a ride at 30 degrees and the bike ran horribly! I didn't touch anything since the last run, not even adding more gas. It was extremely hard to start, would barely idle, sputtered throughout the entire power band, jolted backwards after a few seconds of WOT, and the engine died whenever I stopped. So I replaced the jet with the stock 0.70mm which helped a little but it was still 4-stroking pretty badly. I don't understand this because I thought cold air leans out the air/fuel mix since it is denser than normal air (concept behind a car cold-air intake). I want to be able to run my bike in all temperatures and conditions without messing around with the jet every time since I ride almost every day regardless of the weather. Any suggestions and insight would be greatly appreciated. Tomorrow it's supposed to be around 42 degrees and sunny so I'm going to re-jet (solder+drill) to 0.61mm like it was before.

 

[Purple Haze]

the 61 jet may be slightly lean, but using a little choke should compensate for that. Cold air is more dense, but once the engine is warmed up it won't make that much of a difference. I would stick with the jetting that ran good before, and try a hotter spark plug, along with judicious use of the choke.

 

[Street Ryderz]

Hey All,
My bike's been running great since I re-jetted the carburetor to 0.61mm a few weeks ago. I ride it almost every day, with the temperature usually around 45 F in Kentucky at this time of year. Last Sunday I rode it at 60 degrees and the engine performed as well as ever, no 4-stroking, bogging, power loss, etc. Smooth all throughout the power band. Today, I decided to go for a ride at 30 degrees and the bike ran horribly! I didn't touch anything since the last run, not even adding more gas. It was extremely hard to start, would barely idle, sputtered throughout the entire power band, jolted backwards after a few seconds of WOT, and the engine died whenever I stopped. So I replaced the jet with the stock 0.70mm which helped a little but it was still 4-stroking pretty badly. I don't understand this because I thought cold air leans out the air/fuel mix since it is denser than normal air (concept behind a car cold-air intake). I want to be able to run my bike in all temperatures and conditions without messing around with the jet every time since I ride almost every day regardless of the weather. Any suggestions and insight would be greatly appreciated. Tomorrow it's supposed to be around 42 degrees and sunny so I'm going to re-jet (solder+drill) to 0.61mm like it was before.

OK when you say 0.61mm is that what you are drilling your jet to,or are you meaning the jet size #61 because they are not the same.And with this being so a #70 jet is no where near 0.61mm,numbered drill set from 61-80 also don't collate etheir.

 

[Greg58]

Maybe this will help.

 

[RB55]

OK when you say 0.61mm is that what you are drilling your jet to,or are you meaning the jet size #61 because they are not the same.And with this being so a #70 jet is no where near 0.61mm,numbered drill set from 61-80 also don't collate etheir.

I mean that I drilled the jet hole to 0.61mm, as measured by my digital caliper. I understand the mix-up though, that almost made me order a 1.00mm jet

 

[RB55]

Update to all - I followed PurpleHaze's advice and re-drilled the carburetor jet to 0.61mm like before. It 4-stroked a lot at first but after a few minutes it warmed up nicely and gives the same amazing power like it used to. This engine is really strange, sometimes it just wants to work well and sometimes it doesn't. Also, the power distribution throughout the RPM range is like a racecar; it gives good power at 0 to 5/6 throttle but once I reach WOT there's a huge surge of power at around 7,500 RPM (digital tach). Awesome feel!! . It probably helped that it was 40 degrees F today instead of the 29 degrees it was yesterday; this is the temperature I usually ride at. Next up - installing analog tach, repacking exhaust, and flamethrower!

 

[Greg58]

Here's a better chart, I only included the sizes that apply to most of our applications.

 

[Greg58]

RB is your engine a 66cc or 48cc? The reason I ask is a number #73 drill is the one that is correct for one of my 48cc bikes.

 

[RB55]

RB is your engine a 66cc or 48cc? The reason I ask is a number #73 drill is the one that is correct for one of my 48cc bikes.

66cc Chinagirl

 

[Frogslayer]

Have you checked if your float works recently or your tab may possibly be bent slightly for your needle valve