Stutter at top speed

[Lamaddict]

I'm running the engine in but opening it up a bit today and it stutter at top speed. What could cause that it's been very smooth through the gears?

[coaster]

It depends where your throttle is at when the stuttering starts, you haven't given any details of ypuyr engin/carb but it sounds like it might be a bit rich on the main jet. The main doesn't really have much effect below 3/4 throttle. Also, have you tried a new plug, could be a bit gunked up through running in?

[Lamaddict]

It depends where your throttle is at when the stuttering starts, you haven't given any details of ypuyr engin/carb but it sounds like it might be a bit rich on the main jet. The main doesn't really have much effect below 3/4 throttle. Also, have you tried a new plug, could be a bit gunked up through running in?

It's a PHBH 28, MB SIL 210 with Clubman. 123 main and I'm thinking it's a bit weak, the plug is light brown but not white showing leaness

[coaster]

It's a PHBH 28, MB SIL 210 with Clubman. 123 main and I'm thinking it's a bit weak, the plug is light brown but not white showing leaness

When you say stuttering do you mean 4 stroking like when you back the throttle off? or do you mean jerkiness/hesitation?
If you haven't already done so, you will need to do plug chops at various throttle settings. Try holding at the point where it is stuttering and put the choke on to see if it gets better or worse. It might be an idea to get some larger and smaller main jets, try going richer first though.

[Special X]

123 is a big main jet for a 28PHBH in this application try a 118 for starters - even that may be too rich. Can you tell us the rest of your jetting. I reckon 50 idle, AV266 atomiser, X7 needle should be a good safe starting point. 45 slide would be good as well but you'll probably have the std 40 in. Get a grown up to modify the slide to save buying a new one.

[Lamaddict]

123 is a big main jet for a 28PHBH in this application try a 118 for starters - even that may be too rich. Can you tell us the rest of your jetting. I reckon 50 idle, AV266 atomiser, X7 needle should be a good safe starting point. 45 slide would be good as well but you'll probably have the std 40 in. Get a grown up to modify the slide to save buying a new one.

Nice one Special X, so if I carefully file the throttle cut away so the distance is to 45mm?

[Special X]

Thats right just takes away that over rich spot as you ease the throttle off tickover. It'll still be a little rich there at 45 but thats a good thing as it prevents damage when shutting down after a bit of a thrash.

[diesel]

so if I carefully file the throttle cut away so the distance is to 45mm?

Could you explain that to me please? Where will he be measuring from on the slide?

[Lamaddict]

so if I carefully file the throttle cut away so the distance is to 45mm?

Could you explain that to me please? Where will he be measuring from on the slide?

Pls correct me if I'm wrong Special X.

Diesel, if you look at the slide and measure the angle/slope at the bottom of the slide it will be 40mm from the front to the back so I'm going to file that angle to make it stepper to 45mm. When you look into the carb after the angle will be slightly bigger, more open.

[diesel]

Thank you