Modern Vespa

Hello Y'all !
Well I'm restoring my 2001 ET4 Vespa and I've finished the mechanical and body work so I figured I'd change out the original spark plug at 10,00 miles. The forum article showing dropping the engine was very helpful and I was very careful with installing the new plug. The bikes back together and feels strong but when I took it out for its test run something is not right. Giving gas to it it it hesitates at about 20 mph. I think I have a compression problem. I was careful not to cross thread the new plug, the gap is correct, maybe I did not secure the spark plug cap securely? Maybe the spark plug wire was damaged, maybe I did not tighten the plug correctly? So today I'll drop the engine again and maybe put the old plug back in to see what happens.
Any advise before I start over? Thank you,
Joe in Carefree, AZ.

An update on my ET4 sparkplug change. I am feeling air/compression coming from the plastic box that holds the sparkplug wire. I'll buy a new spark plug and a tube of anti-seize and hope for the best?
Joe in Carefree.

T I think the airflow you are feeling is from the fan which blows air over the cylinder head. You can remove the spark plug without dropping the engine, it just harder to line it up when installing. The hesitation is probably carb related.

I also think that it is not spark plug or compression related.

Before buying another spark plug first test the old one and if that does not make any difference you can be sure it is not spark plug related.

A spark plug you best screw in manually, that will reduce the risk on damaging the thread to zero. Then at the end you secure it with a proper tool.

It could be carb related, indeed.

Or maybe it is related to the transmission. 20mph is somewhere the speed at which the clutch goes from slipping to sticking. It could be that transition from slip to stick does not go smoothly but rather brutal. You will notice a shock and a sudden drop in rpm. Does that describe the hesitation you feel?

I agree with this assessment. Check the function of the the accelerator pump and make sure that the the ports are clear on the main jet atomizer tube.

I recently had issues with my LX150 (same engine as ET4).

Here are 3 things to check that I encountered:

1. clear both jets
2. Make sure the carb diaphragm is seated properly. Grease it when seating to hold it in while putting the cover on.
3. Check that your plug wire has not come loose from the plug cap. Mine exactly well out of it.

Good luck.

Make sure you feel the top of the spark plug cap click onto the terminal. You don't need to drop the engine to remove the spark plug, and if you have the original spark plug wrench that came with the bike, it's about the easiest tool to take out and reinstall the plug. I'm not sure what you think you need anti seize for. Are you planning to do something besides remove and reinstall the plug? The spark plug is not something that you would use anti seize on.
Do check the main and pilot jets and make sure they are clean, and run premium gas from a station that sells a lot of gas. You don't need ethanol free or a bunch of mechanic in a can treatments, just fresh pump gas.

Hello Y'all. I have been successful with my spark plug change after dropping the engine three times to get at the little bugger. As I discussed above the ET4 ran poorly after my new plug install. So on the second engine drop to retriev and scrutinize the new plug it was discovered that the 2'' hose that goes into the gear box had become disconnected and was not secured. Both hoses that go into the air filter box and gear box come loose when dropping an engine. I noticed the air box hose my first attempt but did not see the gear box hose had come off. I returned the old plug to verify I had no spark plug issue and the test drive was successful! My buddy and I flipped a coin to see should we drop the engine a third time to install the new plug and "heads", change the plug. With no more guessing on sequence, that change took about 20 minutes.
Thanks everyone who took the time to message me. I'm glad I have a Modern Vespa Forum to go to. Next, changing out the gear transmission oil.
Joe in Carefree AZ.