Full throttle all the time, 20 mph max speed

A

aMotorMan

Guest
Hello, motoredbikes gurus!

I have been a long time lurker, and built my first bike over the course of the last few weeks. Everything went really well, and I carefully followed the advice of many of you. I am currently in the break in period of my bike. I am using a 32:1 mix. I have been through just over one tank of gas. From day one, I have noticed that my bike pretty much only performs at full throttle. Less than full is around 5-10 mph, and sounds bogged down. At that point, I find myself just peddling to keep speed. This doesn't feel right, even for the break in period. I have checked that the throttle is lifting the slide in the carb all the way up. I performed a plug chop and found that I need to adjust the c clip for a less oil heavy mixture, but I have held off on this as I've seen many say that adjustments to the carb are unnecessary during the break in period. Overall, I built my bike with a lot of care, and I'm disappointed in its performance. I have also made sure that the tire pressure is where it should be. I don't feel any air shooting out around the engine.

What I'm asking is this: is this normal? Full throttling everywhere for a top speed of 20? What can I expect during this break in period, and what should I be paying attention to if I want better performance?
 


gary55

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Joined
Nov 27, 2012
Messages
3,957
Most find it needed to go down a few sizes in the jet to stop it from 4 stroking. Depending on your elevation you may need to decrease the jet size several sizes smaller to clean it up. First move the clip to the top notch on the needle and see if that helps at all. Decreasing jet sizes should be done 1 size at a time to find the sweet spot without going to lean and damaging the engine. If you have the NT type carb stock jet is #70 so these should work for you.Fine Tuning Jets - High 60s | Bicycle Motor Works
 
A

aMotorMan

Guest
Most find it needed to go down a few sizes in the jet to stop it from 4 stroking. Depending on your elevation you may need to decrease the jet size several sizes smaller to clean it up. First move the clip to the top notch on the needle and see if that helps at all. Decreasing jet sizes should be done 1 size at a time to find the sweet spot without going to lean and damaging the engine. If you have the NT type carb stock jet is #70 so these should work for you.Fine Tuning Jets - High 60s | Bicycle Motor Works
Thanks! I'll give that a try. Is it unwise to tune the carb like that while I'm still using an oil rich mixture? I did a half tank at 16:1 and made my next tank (current) 32:1 per the advice of many on here. When should I move to a leaner mix than that?
 
A

aMotorMan

Guest
Most find it needed to go down a few sizes in the jet to stop it from 4 stroking. Depending on your elevation you may need to decrease the jet size several sizes smaller to clean it up. First move the clip to the top notch on the needle and see if that helps at all. Decreasing jet sizes should be done 1 size at a time to find the sweet spot without going to lean and damaging the engine. If you have the NT type carb stock jet is #70 so these should work for you.Fine Tuning Jets - High 60s | Bicycle Motor Works
Also, I have the "high performance" carb, similar to this one. It has a silly star shaped screw head so I'm trying to find a screwdriver to get in there. What kind of jets would be appropriate for this?
 

gary55

Well-Known Member
Joined
Nov 27, 2012
Messages
3,957
Thanks! I'll give that a try. Is it unwise to tune the carb like that while I'm still using an oil rich mixture? I did a half tank at 16:1 and made my next tank (current) 32:1 per the advice of many on here. When should I move to a leaner mix than that?
32:1 is good. That is what I run with full synthetic. The jets I showed you are a good place to start. That carb tends to run rich so you may have to go lower than 66, but I can't suggest taking such a large step right away. I wouldn't want to tell you something that might cause damage to the engine.The packs are pretty cheap so you might consider getting both the high and low 60's selections. As soon as it stops 4 stroking at wide open throttle you're there. Ignore what I said about needle clip adjustment as that carb has no needle.

When talking lean rich conditions we are referring to air fuel mixture not oil fuel mixture so the 32:1 is good for oil, the jets will determine the air fuel mix which is what will effect the bogging at open throttle.
 
Last edited:

LewieBike

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May 21, 2014
Messages
812
No mention from the OP if this is a 66cc or 48cc engine.

Just for the record, my "Zimtown" 48 cc engine came with it's intake manifold to cylinder gasket restricted down to a 9 mm opening. And yeah, it was pretty miserable performance with this restriction, not unlike a VeloSolex engine except without the bottom end torque. Opening it up made a big difference.
 

LewieBike

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Joined
May 21, 2014
Messages
812
Thanks! I'll give that a try. Is it unwise to tune the carb like that while I'm still using an oil rich mixture? I did a half tank at 16:1 and made my next tank (current) 32:1 per the advice of many on here. When should I move to a leaner mix than that?
It's always best to tune for the mixture you intend to run the bike with, adding extra oil leans out your fuel mixture. Adjust accordingly.

I would stay with 32:1 as these engines aren't exactly built to Echo or Husky quality and the slight extra oil will help carry away some of the heat compared to 50:1 mix.
 

gary55

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Joined
Nov 27, 2012
Messages
3,957
I agree with Lewis on tuning carb to the mix it will be using. Especially when cutting the oil mix in 1/2. 32:1 will be fine full syn., and he also has a point on intake gasket.

The intake gaskets on newer engines with 40mm intakes are usually matched well, but the ones with 32mm mount spacing in my experience have always been restrictive and in need of trimming to match intake ID.

The posting you show for the carb says something about 6mm jet. Out of all the Dellorto SHA clones I have worked on I have only seen 2 that use a 6mm jet. The standard thread size is 5mm. Since I have seen 2 it is possible that this one is a 6mm thread, but I doubt it as it has been 5 years since I have seen one. The other thing that makes me doubt it is the sellers description shows he hasn't a clue what he's talking about.
 
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