Sun Sep 18, 2022 2:52 pm

Hooked
'81 Vespa P200e, '85 Vespa PX125e, '70 Vespa 90 basketcase
Joined: 11 Jun 2020
Posts: 201
Location: St. Catharines, Ontario
 
Hooked
'81 Vespa P200e, '85 Vespa PX125e, '70 Vespa 90 basketcase
Joined: 11 Jun 2020
Posts: 201
Location: St. Catharines, Ontario
Sun Sep 18, 2022 2:52 pm linkquote
Hey all, I've noticed a discrepancy with my scooters here.

I have a p200e. Bone stock with a sip road 2 exhaust. The spark plug I have in the engine is a b8es (long threads) the bike runs like a top.

I have a second scooter with a p200 engine in it. Freshly built. The bike wasn't running right. I had a b8es in it. But the plug looked chocolately when I checked.

I then checked the Haynes manual and it calls for a b7hs (short threaded) plug.

I put that in the bike, and it ran so much better.

When I got home and checked the plug, it was very lean.


So why the discrepancy in performance? Was it just running leaner? Or was the plug and the thread length make that much of a difference?

Which plug is the right one? Is the Haynes manual incorrect?


Reading about spark plugs on the forum and it is littered with answers of p200e requires a b7-8es plug.

Can anyone clarify?

Thank you!
N
Sun Sep 18, 2022 11:12 pm

Hooked
Parmakit Primavera -74, Polini Primavera -68, Stock 150 Touring -59, VMC 177 Bajaj Chetak 125 -95
Joined: 22 Jun 2022
Posts: 388
Location: Finland
 
Hooked
Parmakit Primavera -74, Polini Primavera -68, Stock 150 Touring -59, VMC 177 Bajaj Chetak 125 -95
Joined: 22 Jun 2022
Posts: 388
Location: Finland
Sun Sep 18, 2022 11:12 pm linkquote
You should always run a plug thread lenght that matches your head.

The number on NGK plugs is a heat value. Lower number is slower to get rid of heat and runs hotter while higher numbers transfer heat faster to the head and thus run cooler.

Running too cold plug ends up coking itself up and too hot will melt in the extreme situations. Between these two is a zone where plug is hot enough to burn the crap off but still not hot enough to melt.

Coffee brown is good color for plug. Post images of your plugs so we can see what they look like.
Mon Sep 19, 2022 12:03 am

Hooked
125 GTR 1977, 50S 1975, GTS 125 Supertech 2021
Joined: 04 Aug 2014
Posts: 223
Location: Madeira Island
 
Hooked
125 GTR 1977, 50S 1975, GTS 125 Supertech 2021
Joined: 04 Aug 2014
Posts: 223
Location: Madeira Island
Mon Sep 19, 2022 12:03 am linkquote
One thing often underrated is the fact that when you use a wrong length spark it affects compression - A longer spark plug can reduce the chamber in 1cc which my rough calculations is 5 to 10% of the combustion chamber size on a 150-200cc engine.
Mon Sep 19, 2022 10:22 am

Molto Verboso
1958 Allstate 177VMC, 2005 70cc Yamaha Vino
Joined: 23 Aug 2020
Posts: 1098
Location: Philadelphia
 
Molto Verboso
1958 Allstate 177VMC, 2005 70cc Yamaha Vino
Joined: 23 Aug 2020
Posts: 1098
Location: Philadelphia
Mon Sep 19, 2022 10:22 am linkquote
PSMA wrote:
One thing often underrated is the fact that when you use a wrong length spark it affects compression - A longer spark plug can reduce the chamber in 1cc which my rough calculations is 5 to 10% of the combustion chamber size on a 150-200cc engine.
1% of 150cc is 1.5cc mate. 10% would be 15cc.
Mon Sep 19, 2022 11:30 am

Hooked
'81 Vespa P200e, '85 Vespa PX125e, '70 Vespa 90 basketcase
Joined: 11 Jun 2020
Posts: 201
Location: St. Catharines, Ontario
 
Hooked
'81 Vespa P200e, '85 Vespa PX125e, '70 Vespa 90 basketcase
Joined: 11 Jun 2020
Posts: 201
Location: St. Catharines, Ontario
Mon Sep 19, 2022 11:30 am linkquote
FINYoshi wrote:
You should always run a plug thread lenght that matches your head.
Where should the bottom of the plug be situated if I matched the threads of the plug to the head?

How deep should the plug be?

Thank you.
Mon Sep 19, 2022 11:40 am

Jet Eye Master
PX221 MHR, O tuned PX200, PX181 Quattrini and some motorbikes
Joined: 14 Jun 2017
Posts: 3434
Location: London UK
 
Jet Eye Master
PX221 MHR, O tuned PX200, PX181 Quattrini and some motorbikes
Joined: 14 Jun 2017
Posts: 3434
Location: London UK
Mon Sep 19, 2022 11:40 am linkquote
Nelluch125 wrote:
Where should the bottom of the plug be situated if I matched the threads of the plug to the head?

How deep should the plug be?

Thank you.
P200 has an ES plug. If bone stock it should be a B7ES.
If using a higher number plug (like 9) when not needed, it can give the illusion of running ok (tan) and actually on a 7 plug it would be running white.
Mon Sep 19, 2022 8:27 pm

Hooked
Parmakit Primavera -74, Polini Primavera -68, Stock 150 Touring -59, VMC 177 Bajaj Chetak 125 -95
Joined: 22 Jun 2022
Posts: 388
Location: Finland
 
Hooked
Parmakit Primavera -74, Polini Primavera -68, Stock 150 Touring -59, VMC 177 Bajaj Chetak 125 -95
Joined: 22 Jun 2022
Posts: 388
Location: Finland
Mon Sep 19, 2022 8:27 pm linkquote
Nelluch125 wrote:
Where should the bottom of the plug be situated if I matched the threads of the plug to the head?

How deep should the plug be?

Thank you.
The threads should not be visible when the plug is all the way in. With long plug on a short threaded head the threads of the plug are visible on the combustion chamber. The other way around the threads on the head are left exposed.

On many engines using too long plug will cause the plug to contact the piston top. This is not the case though with original Vespa heads with huge combustion chambers.
Tue Sep 20, 2022 5:53 am

Hooked
125 GTR 1977, 50S 1975, GTS 125 Supertech 2021
Joined: 04 Aug 2014
Posts: 223
Location: Madeira Island
 
Hooked
125 GTR 1977, 50S 1975, GTS 125 Supertech 2021
Joined: 04 Aug 2014
Posts: 223
Location: Madeira Island
Tue Sep 20, 2022 5:53 am linkquote
FridayMatinee wrote:
1% of 150cc is 1.5cc mate. 10% would be 15cc.
Wrong, mate

Combustion chamber is not 150cc, that's the cylinder + combustion chamber
On a 1/10 compression ratio engine, 150cc is compressed to 15cc, thats the combustion chamber volume
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