Idle and low throttle issues. Where should I start?

[basilfawlty]

I put a malossi 70cc kit into my buddy today and have not been able to get it to idle after that. The issues may be unrelated to the new cylinder but I'd love some guidance on what to look at first.

Symptoms: After putting in the new cylinder the only way I could get it to fire was to give the fuel petcock vacuum hose a little manual suction. After that it seems to start ok, but revs very high for about 10 seconds and then slowly reduces it's rpm to the point of stalling. I took the scoot for a spin and it does very well at 3/4 to full throttle, it's just the low end that's the issue.

I tried adjusting the idle screw, but that seems to have zero effect, which seems odd on it's own.

What is the next thing I should look at?

PS: Added a video. I rev the throttle at the end a bit and you can hear it stall shortly after.

<iframe width="560" height="315" src="http://www.youtube-nocookie.com/embed/BY0LXaHcvMI?rel=0" frameborder="0" allowfullscreen></iframe>

[az_slynch]

DId you change the jetting in the carburetor after fitting the cylinder kit? You will probably need a bigger idle jet, the original one may just be too lean to run. The main will need to be upsized as well.

[basilfawlty]

Thanks for the suggestions.

I did upsize the main jet (86). The new cylinder was a replacement for another 70cc (Stage 6) kit. The idle jet worked just fine with the previous setup.

I took the carb apart and checked the idle jet. I could blow air through it (barely) but without a frame of reference it's hard to tell if it's ok.

I'm not convinced that the idle jet is the culprit because the idle actually runs really high for the first while.

Could the choke be doing this? If I unplug the electric choke, will it stay off or does that make it stay on?

[az_slynch]

Unplugging the choke will leave it engaged. The choke unit moves the needle as it heats up and closes off the enrichment circuit once it is warm.

If the motor is running fast no matter how you position the idle screw, you might have an air leak. Check the intake manifold to make sure it's clamped tight to the carb and that the seals around the reed block are good. That would have been swapped during install, so it's a good place to check for a leak.

The fact that you have to manually generate suction to pull fuel makes me think there's a low vaccum condition in the manifold between the carb and the cylinder. If you are keeping the throttle closed, there should be enough suction to pull fuel. Again, could be a sign of an air leak.

Did you examine the reed block when you swapped the kit? The reeds could be cracked or chipped, or just not seating right.

[basilfawlty]

That's good thinking.

I did take the reed block off in an attempt to switch out the reeds with the ones that came with the cylinder. I couldn't get the intake and block off all the way though because the big tube that holds the whole rear end onto the scooter was in the way.

I will check that for sure. It's quite possible that something didn't get sealed right when I bolted it back on.

PS: Is there an easy way to get that tube out of the bearings? It might be a good idea for me to replace the reeds if I'm taking it all apart again.

[basilfawlty]

Today I took everything apart.

I re-read the project frenchy thread and realized that the reed block isn't attached to the intake manifold. So, I took the block out and replaced the reeds with the malossi ones that came with the cylinder kit. I added a bit of gasket maker to the reed block before I re-installed it to make sure there was no leak at that point.

I also took the carb apart and cleaned it all.

The situation is unchanged. Argh!

My next step may be to pick up a performance intake and carb, just to see if it works. It was something I intended to do anyway, just not quite yet. I'm still open to any other suggestions before I take the plunge.