Bike stutters at high revs

[frankO]

Hi,
My daughters bike is having problems when it reaches top revs. It feels like when a rev limiter cuts in. The bike is a 47cc cag standard. I have changed the spark plug and that made no difference.
Before I start to pull the carby to pieces I would like to know if anyone else might have an idea where to start.
I have 3 areas to look at
1) Timing - I haven't taken the side of yet. Does timing go off on these bikes. The bike still pulls out of a corner like normal. It is only when it reaches top revs.
2) Reeds. I thought maybe reeds but that would also affect mid range. Mid range is fine.
3) Carby - ?? Bike runs fine all through the rev range until it hits max.

Any ideas

Sorry wrong forum. I thought I was in the tuning forum

[CarterRaceEngines]

Hi There,

Ok a few things to consider.

1. The Vibration may be due to the bike reaching top speed and wanting to go further but is restricted due to gearing, suggest lowering rear sprocket size for more top end.
2. Vibration on those bikes is not uncommon , the cranks are not true and flywheels are not balanced.
3. New Fibreglass reeds will make the bike more responsive and give about 3-5mph more top end.
4.Carby seems limited until u change gearing then you need to re-assess the carby issue.

Cheers

[Steve]

Hi Franko,

My Cag style used to do the very same thing and it was a really simple fix.

I'll bet London to a brick that the bike is running out of fuel top end as the fuel flow through the tap on these things is very restricted to say the least. The piddling amount of fuel comming through the tap doesn't replenish the fuel level in the float bowl fast enough at peek revs and you end up with a surging affect at peek revs on the straight.

After bending the float arms to the max to raise the float level I still had the same trouble. Darren from Pocket Racing Australia put me on the right track when he suggested that I remove the tap assembly from the carb to drill the inlet and outlet of the fuel tap out to 3mm as well as the two holes behind the tap. I then also used the 3mm drill as a router the make the C shaped groove on the inside face of the plastic tap larger.

The bike screams now with no probs, however with the future addition of a full circle crank and higher rpm I may need to remove the inner part of the 4 holed rubber disc behind the tap to increase the flow even more and run a remote tap elsewhere.

Give the 3mm drill a go and let us know how it turns out. It worked a charm for me

[mynx]

cheers dude, I [and apparently my mates] have the same problem with our bikes. Future fix

[Ado]

putting in f/g reeds and changing gearing helped mine when it did that.

it happened again later but found out it was caused from the spark plug not screwed in properly :oops: