OP
UTC

Molto Verboso
'07 GTS250, '07 LX150, '81 P200E, '78 P200E, '64 V90 and 3 Ciaos
Joined: UTC
Posts: 1182
Location: Tucson, AZ
 
Molto Verboso
'07 GTS250, '07 LX150, '81 P200E, '78 P200E, '64 V90 and 3 Ciaos
Joined: UTC
Posts: 1182
Location: Tucson, AZ
UTC quote
The patient isn't a Vespa, but it is a 2T and I'm hoping the 2T gurus can provide some insights to help me properly solution this issue.

So, the tooted-up Buddy 50 crapped out last night. Lost power coming up to a traffic signal. Would sort of pop when cranking it, but wouldn't run. Earlier in the week, it briefly acted like it was soft-seizing after a two mile WOT pull when it encountered a stiff headwind. Speed was quickly walked down and it seemed to recover. A borescope inspection looked safe, but I still wasn't satisfied with the wash buildup on the piston. Prior to the failure, I swapped the Mikuni 95 main (105 Keihin) for a Mikuni 100 main (110 Keihin) to see if it would splutter up top or if it would at least show more wash build up on the piston.

I tore down the top end last night and saw the least carnage I've encountered when one of these little guys goes pop. The piston was showing more wash, but not nearly as much as the cylinder head. The piston has gotten hot, judging by the buildup on the back of the crown and the erosion by the piston ring locating pin. It seems like it was still lean on the Mikuni 95 and I upjetted too late. However, I think there was another issue at play: insufficient squish.

When I built this thing, I used the good calipers and the good torque wrench. I even read the included manual and followed the recommendations for setup. One detail in particular should have been a warning. The manual indicated that the squish should be set to 0.6mm, +/- 0.10mm. when I originally stacked and torqued everything, the squish came to .77mm, so I swapped out a spacer for a thinner one and settled on a .72mm squish. I figured I was playing it safe and leaving myself a small buffer.

Upon disassembly, I found that the O-ring was getting squeezed out of it's channel at the 3:00 and 9:00 positions, despite the head still being torqued to spec. It had melted a bit too, gluing the head to the bore. Impressive, when considering that the combustion chamber is recessed 2.6mm below the O-ring sealing surface.

In doing some reading, I came across a Stage6 official blog posting on this very kit, which now recommends a squish between 0.75mm and 0.85mm for setup. I also read some 2T squish discussions on other sites which suggested that having less than 0.8mm squish can cause weirdness in the fuel/air mixture. Finally, having small squish causes pistons to run hotter.

Since the cylinder and head look new, the exhaust gasket and crank seals show no signs of leaking and I feel like I'm overjetting to build a healthy wash, and the piston looks surprisingly good in consideration of excess heating and localized erosion near the piston pin, I feel like too much squish was my killer.

Would those with considerably more 2T kung-fu be willing to render judgement on the evidence. I'd like to fix it STAT and not blow it up again quite so quickly.

Pictures follow.
Buildup on the bottom of the piston crown.
Buildup on the bottom of the piston crown.
Piston, driver's side.
Piston, driver's side.
Piston, passenger side.
Piston, passenger side.
Piston crown.
Piston crown.
Piston pin.
Piston pin.
Cylinder bore. Note the O-ring distortion/residue. O-ring came out in one piece.
Cylinder bore. Note the O-ring distortion/residue. O-ring came out in one piece.
Borescope view of wash prior to kablooie. Sorry for the camera-on-camera view.
Borescope view of wash prior to kablooie. Sorry for the camera-on-camera view.
@chandlerman avatar
UTC

Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 79 P200E, 66 Lammy S3
Joined: UTC
Posts: 9984
Location: Nashville

78 Days Since Last Explosion
 
Lucky
@chandlerman avatar
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate, 79 P200E, 66 Lammy S3
Joined: UTC
Posts: 9984
Location: Nashville

78 Days Since Last Explosion
UTC quote
I don't think it's squish. I think it's a combination of too much timing advance and maybe a head leak where the gasket failed causing the overheating.

The detonation on the piston crown is the red flag to me.

I run my 187's and the smallstate at .8mm squish and I'm guessing they live much harder lives with a much longer stroke than that little thing.
@charlieman22 avatar
UTC

Nedminder
62 VBB1T Round Tail W/ leaner sidecar
Joined: UTC
Posts: 4110
Location: california
 
Nedminder
@charlieman22 avatar
62 VBB1T Round Tail W/ leaner sidecar
Joined: UTC
Posts: 4110
Location: california
UTC quote
I'm with CM.
Mostly.
- woulda been interesting to pressure test - though perhaps a leak only occurring when hot.
- ignition advance. Take a couple degrees out.


Couple comments/ questions.

Top of piston looks lean lean lean.
Should be mostly black.
May be significantly under jetted main?
Or air leak for a while.

Your piston bottom doesn't look bad to me. Barely a dark spot.

Edge of piston has pre detonation at circumference. How does that align with gasket leak areas and exhaust port(s)?

Normally that erosion due to timing would be at exhaust - but u have pinging all around and central.

My move would be:
New ring
New o ring and pressure test.
Re jet staring much richer - and walk down.
OP
UTC

Molto Verboso
'07 GTS250, '07 LX150, '81 P200E, '78 P200E, '64 V90 and 3 Ciaos
Joined: UTC
Posts: 1182
Location: Tucson, AZ
 
Molto Verboso
'07 GTS250, '07 LX150, '81 P200E, '78 P200E, '64 V90 and 3 Ciaos
Joined: UTC
Posts: 1182
Location: Tucson, AZ
UTC quote
chandlerman wrote:
The detonation on the piston crown is the red flag to me.
Agreed. The annoying bit is, there hadn't been any detonation on the crown prior to last Tuesday's minor soft seize. I saw that Friday night when I borescoped it (hadn't been run since Tuesday). Immediately upjetted and went out for a retest.
chandlerman wrote:
I run my 187's and the smallstate at .8mm squish and I'm guessing they live much harder lives with a much longer stroke than that little thing.
I'm sure things can run fine at 0.8mm if set up properly. It's that sub-0.8mm area that concerns me. Considering that the vendor changed their setup guidance from "0.6mm +/- 0.10mm" to "between 0.75mm and 0.85mm", I suspect that they had their reasons for the updated guidance.

This one had run well for 200 miles, until it decided not to.

My apologies for the bush-league blowup Master, I will strive to better scatter parts on the next iteration.
OP
UTC

Molto Verboso
'07 GTS250, '07 LX150, '81 P200E, '78 P200E, '64 V90 and 3 Ciaos
Joined: UTC
Posts: 1182
Location: Tucson, AZ
 
Molto Verboso
'07 GTS250, '07 LX150, '81 P200E, '78 P200E, '64 V90 and 3 Ciaos
Joined: UTC
Posts: 1182
Location: Tucson, AZ
UTC quote
charlieman22 wrote:
I'm with CM.
Mostly.
- woulda been interesting to pressure test - though perhaps a leak only occurring when hot.
- ignition advance. Take a couple degrees out.


Couple comments/ questions.

Top of piston looks lean lean lean.
Should be mostly black.
May be significantly under jetted main?
Or air leak for a while.

Your piston bottom doesn't look bad to me. Barely a dark spot.

Edge of piston has pre detonation at circumference. How does that align with gasket leak areas and exhaust port(s)?

Normally that erosion due to timing would be at exhaust - but u have pinging all around and central.

My move would be:
New ring
New o ring and pressure test.
Re jet staring much richer - and walk down.
If I've learned anything from you gents, it's that when I build a new setup, I procure spare parts. Had a fresh gasket kit and piston kit on the shelf. Already reassembled the little bugger, albeit with 0.1mm more squish in the stack. Heat cycles completed and ready to begin road testing anew. Leaving the "overkill" jet in and put in a fresh NGK BPR8HS plug.
New parts already installed. These are the backup backups.
New parts already installed. These are the backup backups.
Another look at the back of the crown.
Another look at the back of the crown.
Decided not to re-used this due to the exposure of the ring land.
Decided not to re-used this due to the exposure of the ring land.
@popgunandy avatar
UTC

Hooked
61 VBB 76 Rally 200 79 P200 80 Sport 100
Joined: UTC
Posts: 288
Location: Folsom CA
 
Hooked
@popgunandy avatar
61 VBB 76 Rally 200 79 P200 80 Sport 100
Joined: UTC
Posts: 288
Location: Folsom CA
UTC quote
It's entirely possible the o-ring was installed improperly at the get-go or was a bad one, slowly leading to your issue (think Challenger accident). Hopefully no issues after you redo things.

+1 on always buying duplicates of cheap stuff like rings, gaskets, and hardware...I've had to redo a couple rebuilds and din't have to wait on the UPS guy as a result.
@charlieman22 avatar
UTC

Nedminder
62 VBB1T Round Tail W/ leaner sidecar
Joined: UTC
Posts: 4110
Location: california
 
Nedminder
@charlieman22 avatar
62 VBB1T Round Tail W/ leaner sidecar
Joined: UTC
Posts: 4110
Location: california
UTC quote
Hahaha.
Not only rebuilt but heat cycled already.
Great!

I thought about it after posting.
My bet:

U sprung head leak.
Leak = heat.
Heat = melted Oring
Melted O ring = more heat…

Option B:
Maybe it just got too hot on its own - and above cycle occurred where melted O ring was start of cycle.

If that's the case - perhaps I were only a little lean (until the O ring died).

By a little lean - I mean: fine for daily but too lean for that WOT run u made.

¯\_(ツ)_/¯

Let us know how u go.
OP
UTC

Molto Verboso
'07 GTS250, '07 LX150, '81 P200E, '78 P200E, '64 V90 and 3 Ciaos
Joined: UTC
Posts: 1182
Location: Tucson, AZ
 
Molto Verboso
'07 GTS250, '07 LX150, '81 P200E, '78 P200E, '64 V90 and 3 Ciaos
Joined: UTC
Posts: 1182
Location: Tucson, AZ
UTC quote
Took the Angry Buddy out for a 45 mile cruise around town tonight. Rode with my buddy who has a slightly less tuned-up Zuma 50. We generally cruised at 40-45mph, but after about 30 miles of good behavior, I picked a few traffic light fights and loosened the Buddy's leash a bit.

Power is excellent for a tooted-up weed eater and it transitions from pilot to main jet much more cleanly than before. At the top of the rev range at WOT, the engine falls a bit flat. Closing throttle a touch perks it up, so I think I'm one size large on the main jet now. No rev hang when stopping from a hard pull, just settles nicely back to idle.

I'll poke it with a borescope tomorrow after work and may try a few plug chops, though those are a bit harder to get from a CVT scoot. I would have fitted a Runleader, but the plug fits into a recessed well that a plug probe can't fit into. When I build my own scoot with this kit, I'll be sure to drill/tap the head for a temperature probe.

Timing on this scoot is identical with a Honda AF18 and is fixed. There's a pocket that the Hall sensor drops into, with little room to adjust it. Fortuitously, I learned of a Taiwanese firm that just came out with an auto-retarding system for a Honda AF18; I'll be looking into that to ensure longer engine life. That Zuma has a Top Performances 3-degree advance key on the crank, in addition to a curved CDI that retards timing by 9 degrees between idle and 10K RPM. Wish there was a similar easy solution for the Buddy!

We load up for LA on Thursday and will be participating in the SuperSunday ride on the 30th. Hoping the Buddy continues to run well for it's new owner.

Need the lift clear for a bit so I can fix my P or the V90.
Dumb stuff on yard equipment. Testing hill climbing on Sentinel Peak.
Dumb stuff on yard equipment. Testing hill climbing on Sentinel Peak.
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