Low compression while on the benceh
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Enthusiast
1965 vespa 150 sprint VLB 1T
Joined: 12 Aug 2017
Posts: 52
Location: Ojai, California
Thu Oct 31, 2019 10:09 am quote
After rebulding my Vlb 1965 sprint 150 I have been getting low compression while testing engine mounted on a bench vice. I have replaced the rings three times with same results. Also new cylinder and head. Compression is about 60psi I was wondering if I could get more by kicking it instead of using my hand to push kick starter. I do not have carb on so I should get full air flow. The new piston arrow is facing the exhaust port. Rings are gaped to .500mm. What am I missing? Any advice would be appreciated.

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Molto Verboso
Joined: 26 Oct 2015
Posts: 1825

Thu Oct 31, 2019 10:45 am quote
You'll want to kick it over with multiple hard swift kicks. Doing it by hand won't cut it.
Hooked
piaggio hexagon
Joined: 17 Aug 2013
Posts: 440
Location: lincolnshire uk
Thu Oct 31, 2019 10:50 am quote
A compression test should be carried out when the engine is at operating temperature. You won`t get a meaningful reading using your method.
Hooked
1955 Allstate 1958 lambretta 1965 Allstate 1968 sprint
Joined: 20 Jul 2017
Posts: 397
Location: Central california
Thu Oct 31, 2019 12:46 pm quote
low comp.
whodatschrome is 100% correct....needs to be kicked over by foot. If i tried to kick start my scooter over by hand....I would be afraid i would break my arm.
Hooked
P200E
Joined: 11 Apr 2010
Posts: 462
Location: The Town that dreaded Sundown.
Thu Oct 31, 2019 6:11 pm quote
Just rebuilt my VSE and 60 psi on the bench also. Once I got it back in the bike it runs good and will have good compression once I’m able to get it hot. Wouldn’t worry
Molto Verboso
Joined: 26 Oct 2015
Posts: 1825

Thu Oct 31, 2019 6:29 pm quote
I probably wouldn’t worry too much about doing a comp test on a hot engine. I’m going to guess that 90% of all engines out there (car, truck, moto, scooter, boat, tractor) get a comp test when they’re cold. A comp test on a hot engine will give you a more accurate reading on how well everything is sealed while the engine is at its standard operating temp though.
Hooked
1980 Honda Twinstar and 2004 Yamaha V Star
Joined: 16 Apr 2010
Posts: 477
Location: Akron, Ohio
Sat Nov 02, 2019 11:33 am quote
hexatony wrote:
A compression test should be carried out when the engine is at operating temperature. You won`t get a meaningful reading using your method.
That's right. I tested the engine on my Honda Twinstar at 50psi when it was cold. If that was the full compression on the bike it would be blowing smoke and running poorly if it ran at all. But, the bike runs good and strong with no smoke or other problems.
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