Cold seize problem
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Hooked
1974 200 Rally, 1970 180 Rally, 1980 P150X and P200E
Joined: 22 May 2017
Posts: 145
Location: George, South Africa
Tue Nov 20, 2018 11:06 am quote
I have recently rebuilt a P150X motor. While testdriving the Vespa, I had a cold seize. After it had cooled down, it started without any problems and I drove back to my shop. I turned the air mixture screw in by half a turn and again took it for a ride. And again I had a cold seize. (First time this has happened to me).

Now my questions are: is it the wrong air adjusting screw, or must I change a jet somewhere?
Addicted
Old douglas ..smallie with polini 115..super with nasco 177
Joined: 07 Nov 2013
Posts: 521
Location: New Zealand
Tue Nov 20, 2018 6:35 pm quote
cold sieze?show us the munted parts
twould be rather hard to munt an engine with just the air fuel mix out of whack.......and whats a cold sieze any way...it heated up..expanded and stopped....prob not that screw

More likely an air leak from a poorly installed seal would cook and engine in a few minutes.of riding

a leaking head may take 15minutes before the [piston gets too hot

Diagnostic can only take place with a pic of your barrel and piston

Poor timing will leave a hole in the piston...that takes about half an hour to show

Post some pics of muntings (ps "munted" is a local term for broken )
Molto Verboso
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 1086
Location: London UK
Tue Nov 20, 2018 7:15 pm quote
As your experiment has proved perfectly, the mixture screw does absolutely nothing once the throttle is more than a few mm open.

Being just rebuilt, then as said, more likely an air leak. To actually heat seize from jetting it does need to be quite a way under.
Hooked
1974 200 Rally, 1970 180 Rally, 1980 P150X and P200E
Joined: 22 May 2017
Posts: 145
Location: George, South Africa
Tue Nov 20, 2018 8:44 pm quote
Ok, we call it a cold seize, although it is actually caused by the piston being overheated. So far I have rebuilt/serviced over 50 engines, thus I am pretty sure it is not a leaking seal. But I will have a look anyway, everybody makes mistakes! Will keep you posted with what I have found.

Jack221, what do you mean with a pilot jet (in another post)?
Ossessionato
LXV 150 Midnight Blue (SOLD)
Joined: 06 Dec 2010
Posts: 2463
Location: Bangkok
Tue Nov 20, 2018 8:51 pm quote
Was it properly warmed up? Some two-strokes are notorious for "cold seizing" when the inlet side of the piston is a different temp to the exhaust side when the engine is cold.
Hooked
68sprint,65super,64Allstate,55 Allstate
Joined: 20 Jul 2017
Posts: 245
Location: Central california
Tue Nov 20, 2018 8:58 pm quote
Cold seize
Well Iím on my first rebuild. I did make a pressure tester...to pressure test the engine before I took it apart to verify the problem. You have a lot more rebuilds under your belt than I have. Iím surprised you dont have one. To me it is an invaluable tool especially if your searching for a problem. Scott
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2:6
Joined: 11 Jan 2007
Posts: 7375
Location: San Francisco
Tue Nov 20, 2018 9:58 pm quote
I had a VNB that didn't have enough clearance piston to bore. it would go 3 blocks, power would drop and then seize. pulled the top end to measure and found the issue. had it honed for more clearance and it was perfect from then on.

if the distance is always the same-ish then cold seize is likely and a bit more clearance should fix it. If it's all over the map, the I'd look for air leaks or jetting.
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 6711
Location: Victoria, Australia
Wed Nov 21, 2018 6:04 pm quote
I had a CDI go bad and at first would randomly vary the timing and caused several seizes. Chased all sorts of other things before finding the real culprit.
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69 VLB, 79 P200, P125/Stella and a couple frames
Joined: 06 Jul 2012
Posts: 908
Location: Alabama
Thu Nov 22, 2018 1:01 am quote
I assume this is a stock rebuild. I've never ran into this with a Vespa rebuild but I had an RD350 set up with tight piston to cylinder Clarence. If this is truly what some of us refer to as s cold seize, it may be worth measuring the size of the piston compared to the cylinder. I don't recall the proper measurement but it shouldn't be hard to look up. With out knowing the whole set up of your motor it's all speculation, but the scuff marks on the piston will give lots of information.
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 6711
Location: Victoria, Australia
Thu Nov 22, 2018 2:15 am quote
Very useful photos and descriptions here to help identify the issue.

http://polariswatercraft.com.au/pistondiagnosticguide.htm
Molto Verboso
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 1086
Location: London UK
Thu Nov 22, 2018 8:00 pm quote
Seen this before sometime. All good info. If it also said never, ever run a 2 stroke after a rebuild, without first doing a pressure test, it would be perfect.

I think the term 'cold seize' comes from steam engines when they used to hydro lock with condensation in a cold cylinder. Hot cold or otherwise any seize with our scooters is a bad day.
Hooked
1974 200 Rally, 1970 180 Rally, 1980 P150X and P200E
Joined: 22 May 2017
Posts: 145
Location: George, South Africa
Thu Jan 17, 2019 8:04 pm quote
I took the piston out and checked the ring clearance: was about 0.02mm. I then increased it to about 0.35mm. Took the Vespa for a ride yesterday and after about 2km it happened again. So what now?
Molto Verboso
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 1086
Location: London UK
Thu Jan 17, 2019 9:49 pm quote
That clearance was small but 0.35 should be ok. If you are sure there is no air leak and the timing is set correctly, then it can only be jetting.

What are all the jets in the carb? Stock PX150 is usually something like 48/160 AC160 BE3 MJ102
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Mongrel 187
Joined: 21 Oct 2018
Posts: 9
Location: Belfast
Sat Jan 19, 2019 2:29 am quote
danie wrote:
I took the piston out and checked the ring clearance: was about 0.02mm. I then increased it to about 0.35mm. Took the Vespa for a ride yesterday and after about 2km it happened again. So what now?
Is it a new piston and rebore? What's the piston/cylinder clearance? Might be too tight?
Hooked
1974 200 Rally, 1970 180 Rally, 1980 P150X and P200E
Joined: 22 May 2017
Posts: 145
Location: George, South Africa
Wed Jan 23, 2019 3:54 am quote
It looks as if the piston clearance is too tight. Normally the size that the barrel must be machined to is stamped on the piston. However, this (new) piston does not have a size stamped on it. The piston measures 58.4mm (4th oversize), so what should the barrel be machined to?
Addicted
69 VLB, 79 P200, P125/Stella and a couple frames
Joined: 06 Jul 2012
Posts: 908
Location: Alabama
Sun Jan 27, 2019 12:31 pm quote
Something else to note is that the proper clearance for a forged piston may differ from a cast one.
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'64 Motovespa 150S (177) , '76 V90 ( 133)
Joined: 27 Mar 2013
Posts: 521
Location: S.Salem, NY
Mon Jan 28, 2019 4:12 am quote
When you fitted the 4thOS piston you must have had the cylinder bored for it, no ? What spec did you use for clearance ?
Hooked
1974 200 Rally, 1970 180 Rally, 1980 P150X and P200E
Joined: 22 May 2017
Posts: 145
Location: George, South Africa
Mon Jan 28, 2019 8:04 am quote
Since this (new) piston did not have the bore size engraved on it, I had the cylinder bored to the size written on the box the piston came in. I never measured the piston, since you assume what is written on the box should be correct. The piston did fit a bit tight, but I thought it was because of the new rings.

Alas, it looks as if the wrong piston was put in the box! I took the cylinder and piston in today to have the cylinder rebored.
Hooked
1974 200 Rally, 1970 180 Rally, 1980 P150X and P200E
Joined: 22 May 2017
Posts: 145
Location: George, South Africa
Tue Feb 05, 2019 10:18 am quote
Problem solved. Engine runs like a dream now that the cylinder has been rebored to the correct size.

Thanks for all the advise.
Addicted
2006 LX150 (carbed) | 2007 GT200
Joined: 29 Jun 2016
Posts: 575
Location: Toronto
Thu Feb 07, 2019 5:20 pm quote
danie wrote:
Since this (new) piston did not have the bore size engraved on it, I had the cylinder bored to the size written on the box the piston came in. I never measured the piston, since you assume what is written on the box should be correct. The piston did fit a bit tight, but I thought it was because of the new rings.

Alas, it looks as if the wrong piston was put in the box! I took the cylinder and piston in today to have the cylinder rebored.
Regardless of a bore size stamp, I thought machine shops usually (always?) want to have the new piston in hand precisely so that they can bore-to-measure? Since you had the piston available, I dont understand why the shop didn't measure it?

I'm glad it's running well now That's the important part!
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