Modern Vespa

Hello I have recently built my T5 with a Malossi 172 Kit and a SIP Road 2. I have increased the main jet to 122. My air corrector is at 100.

It's currently running a little rich at the moment and I am ok with this while the engine is breaking in.

I am having issues where it stalls on the lower end, when riding it wants to bog down and die when I pull in the clutch.

We verified it isn't an air leak and feel it can be due to timing or improper jetting.

The Malossi website says I should set my timing from 14-16 degrees but I do not have information on how to setup my jetting.

Can someone tell me what success they had with the idle jet size, main jet size air corrector size and timing?

Thank you

My 172 with a 26mm carb and JL exhaust plus external air filter runs a 136 main jet and 15 degrees timing, has done for many trouble free miles

Accelerates v well and will do 70+ on the flat with most of the torque in the higher RPM


Not a good basis for you to compare against though as yours is different

How did you test for an air leak?

What cylinder head are you using?

What spark plug?

Do you mean a 120 air corrector or are you really using a 100?

Start with the 50/100 idle jet.

Hello. Air leak was tested by covering the exhaust.

It died immediately.

I have made some adjustments since my original post and I am now using the following setup:

150:be4:122 Main Jet

55/160 Idle Jet

12,5:1 Cylinder head

I will have to confirm the spark plug.

The issue is now different. She runs well with alot more power then before. Now she stalls after approximately 30 mins of riding when pulling the clutch in.

Hopefully there is some useful information here to give me some guidance.

Air leak can only be tested using vacuum.

If you're actually using a 24g.
Your jet stack should be 120/BE 5/probably 122ish. You're on the right track. You can use the be4 for now.

50/100 idle.

The fact you said 55/160 makes me think you are running a 24e because with a 24g you need to cut one down to fit under the air filter or drill the air filter so it can pole through.

NGK B8ES or cross referenced other. If it's a resistor plug it makes no difference.

Need the make and model of your head. 12.5 isn't enough to offer advise on the spark advance.

On my T5. Malossi 172, SIP XL, MRP head, stock timing, 24/24G, 50/100, 120/BE4/125. Careful with the mixer, BE4 is the only one suited for a T5, nothing else…

Thank you for your feedback I will get you the missing information today after work. I will also do a carb cleaner test around the air intake box to see if there are any leaks there. I do not have the tools for a vacuum test at the moment.

Very good catch about the 24g as the air filter is drilled out to accommodate the 55/160 idle.

Here is the link to the head I purchased:

https://www.sip-scootershop.com/en/product/cylinder-head-sip-t5-for-malossi-172cc-for-vespa-t5_22439310

The timing is currently set to 13 degrees from TDC.
I am also using a high flow fuel tap.

If there is any other information aside from the spark plug I can get for you please let me know.

The spark plug is B9ES. I attached a photo of how it looks. It's not a plug chop.

Oh dear, look at that. If the engine is not damaged, put the jetting as Safis said. Plug 8. Timing 16. And then it will start to run correctly.

Hello ..it's my understanding setting the timing at 16 makes the engine run hotter and 13 is the safer option. I will certainly review the jetting.

16 degrees is mild to start with. Bigger testicles and you'd be running 18. With variable timing it could be over 25.

16 degrees is very safe. I run my t5 at 18 degrees and that's with a 54mm crank and MMW head. I had a hunch you had the timing too low.

As stated by others, use a B8 plug and get rid of that idle jet for a stock one. That plug is so fouled I'm surprised it runs at all.

I don't concur about the be4 being the only one that will work as I've tried nearly every one. The be5 works really good for me and it wouldn't currently be in my bike if it didn't.