Modern Vespa

Hi all. I was searching for some specific information about the spark plug for a 1964 Sears Allstate. I recently restored the scooter and am running it on the road. It has run really well for the first 160 kms, but now am finding it sluggish and not able to get up to speed well. It can be hard to start and cuts out under idle. I have been using the NGK B7HS plug but am wondering if the B6HS would be a better plug.

Anyone has any experience or suggestions?

Thanks.

Moved to NSM.

The 7 plug should be fine for general riding. The number only indicates the operating temperature of the center electrode and very likely has nothing to do with your problem.
I'd look at the condition and settings of the carburetor, ignition timing, or compression.
Of course, plugs have been known to go bad.

And of course............test for and resolve air leaks.

Thanks Moto64 and garncarz. My friend and I went over the scooter today checking timing and points gap as well as checking mixture and idle. Put a new B7HS plug in and the scooter seems to be running as before. Have to take it on a ride to really test what we did, but it seems like a good start. We did talk about compression, but I don't have the tools to test that. Apparently the previous owner rebuilt the engine (he is a motorcycle and car mechanic) but we forgot to ask him if he tested the compression. If the problem still persists, I will get back to him.

Appreciate the suggestions.

I know this is a dead horse that I am beating but....If the engine has been apart there is nearly 100% probability that you will have an air leak.

If you can, ask the guy who rebuilt the engine if he performed a leak test (with a gauge and how long it held stable pressure) after he reassembled the engine. Go from there.

Having just pressure tested a newly build engine, I basically agree with garncarz. Test for leaks.

But my two cents … a non-running engine holding some amount of stable pressure over an extended period of time isn't necessary. With 5 lbs of pressure in there, any leak is going to be immediately obvious.

True