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@rb_prior avatar
UTC

Hooked
1963 Allstate, 2005 Vespa PX150, 2001 Harley 95 ci Dyna
Joined: UTC
Posts: 320
Location: Central Ohio
 
Hooked
@rb_prior avatar
1963 Allstate, 2005 Vespa PX150, 2001 Harley 95 ci Dyna
Joined: UTC
Posts: 320
Location: Central Ohio
UTC quote
I have a 1963 Allstate with a new 2 port 150 cc top end upgrade. This included the cylinder, piston, wrist pin, rings, and head. The engine was broken in as per the Allstate manual not exceeding 30 mph in 3rd gear for the first 1000 miles. The bike runs great. It starts on the first or second kick always, revs fine, and has good torque as judged by the seat-of-my-pants test. The bike responds well and is smooth and even throughout the throttle range. Overall, it is a pleasure to ride.

I have checked the compression with four different testers and all read about 70 psi. By all standards, this is too low. However, the engine runs great. Would it run better with more compression? Should I be concerned about the low compression tests or ignore them and ride on.
Compression test, warm engine, WOT, at least five hard kicks.
Compression test, warm engine, WOT, at least five hard kicks.
@chandlerman avatar
UTC

Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate
Joined: UTC
Posts: 9393
Location: Nashville

118 Days Since Last Explosion
 
Lucky
@chandlerman avatar
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate
Joined: UTC
Posts: 9393
Location: Nashville

118 Days Since Last Explosion
UTC quote
If it's running great, then I wouldn't be worried. That reading is so far out of bounds as to indicate a measurement issue.

How long is the hose on your compression tester? That can make a big difference on the readings off a small cylinder.
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@rb_prior avatar
UTC

Hooked
1963 Allstate, 2005 Vespa PX150, 2001 Harley 95 ci Dyna
Joined: UTC
Posts: 320
Location: Central Ohio
 
Hooked
@rb_prior avatar
1963 Allstate, 2005 Vespa PX150, 2001 Harley 95 ci Dyna
Joined: UTC
Posts: 320
Location: Central Ohio
UTC quote
Thanks for the informative comment Chandlerman. The total length of the hose from cylinder head to gauge is 19 inches. The rubber hose alone is 17 inches.
@dasscooter avatar
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'63 VNB, '66 Bluebadge, '88 Elite 250, '17 Grom, '08 Burgman 400
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@dasscooter avatar
'63 VNB, '66 Bluebadge, '88 Elite 250, '17 Grom, '08 Burgman 400
Joined: UTC
Posts: 246
UTC quote
Are you testing it with the throttle open?
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@rb_prior avatar
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Hooked
1963 Allstate, 2005 Vespa PX150, 2001 Harley 95 ci Dyna
Joined: UTC
Posts: 320
Location: Central Ohio
 
Hooked
@rb_prior avatar
1963 Allstate, 2005 Vespa PX150, 2001 Harley 95 ci Dyna
Joined: UTC
Posts: 320
Location: Central Ohio
UTC quote
Yes, the throttle was wide open during the compression testing.
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Addicted
Joined: UTC
Posts: 920
Location: The Bank
 
Addicted
Joined: UTC
Posts: 920
Location: The Bank
UTC quote
I have an engine that seized twice due to lack of lubrication and still runs. gets high 80's in compression. Hard as well to get started in the cold, and after a good warm up runs ok.
@sdjohn avatar
UTC

Veni, Vidi, Posti
'15 GTS300, '86 PX125EFL, '66 VBB, '01 ET4
Joined: UTC
Posts: 8149
Location: San Diego, CA
 
Veni, Vidi, Posti
@sdjohn avatar
'15 GTS300, '86 PX125EFL, '66 VBB, '01 ET4
Joined: UTC
Posts: 8149
Location: San Diego, CA
UTC quote
Have you measured your squish? If it is really big you can get more compression and performance by getting it more reasonable.
UTC

Veni, Vidi, Posti
Joined: UTC
Posts: 6486
Location: Tega Cay, SC
 
Veni, Vidi, Posti
Joined: UTC
Posts: 6486
Location: Tega Cay, SC
UTC quote
Are you testing it after a 3 - 5 mile ride(warmed up), or on a cold engine?
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@rb_prior avatar
UTC

Hooked
1963 Allstate, 2005 Vespa PX150, 2001 Harley 95 ci Dyna
Joined: UTC
Posts: 320
Location: Central Ohio
 
Hooked
@rb_prior avatar
1963 Allstate, 2005 Vespa PX150, 2001 Harley 95 ci Dyna
Joined: UTC
Posts: 320
Location: Central Ohio
UTC quote
My initial compression test was on a cold engine. I have noticed before that having the engine hot had little or no effect on my compression readings. However, I just came back from a 15 mile ride and re-tested the compression on the hot engine. I got the same results, 70 psi. Since the bike runs so nice I have to conclude the 70 psi reading I am getting is an aberration in the testing method as suggested by Chandlerman. The bottom line is the bike starts easy and runs great. Compression testing in my situation is not a good indicator of engine health.

On a side note, an old Vespa 150 service manual says the compression ratio is 6.8:1. Correcting for the 1000 feet elevation where I live, the expected compression I should get would be 95.9 psi. Perhaps my squish is too big and lowering it would improve the compression reading. The photo below shows where my piston is at TDC. There is a small distance from the piston edge to the top of the cylinder and perhaps this distance should be zero. But in view of the bike's splendid performance, I am going to leave it as it is for now.
Compression reading on a hot engine.
Compression reading on a hot engine.
Piston at TDC showing small distance from piston edge to top of cylinder.
Piston at TDC showing small distance from piston edge to top of cylinder.
@chandlerman avatar
UTC

Lucky
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate
Joined: UTC
Posts: 9393
Location: Nashville

118 Days Since Last Explosion
 
Lucky
@chandlerman avatar
76 Sprint V, 63 GL, 62 VBB, 05 Stella, 66 Smallstate
Joined: UTC
Posts: 9393
Location: Nashville

118 Days Since Last Explosion
UTC quote
RB Prior wrote:
But in view of the bike's splendid performance, I am going to leave it as it is for now.
You could measure the squish, then plane down the head to get a little more compression, but if you're happy with it, leaving well enough alone is definitely the smart play.

...even if the ethos for most of us around here is, "If it ain't broke, fix it 'til it is."
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Addicted
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Location: The Bank
 
Addicted
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Location: The Bank
UTC quote
RB Prior wrote:
There is a small distance from the piston edge to the top of the cylinder and perhaps this distance should be zero. But in view of the bike's splendid performance, I am going to leave it as it is for now.
Pretty sure the head has a lip on the fits down in the cylinder a bit to center itself which closes that gap a little.

I have a mid 70's super that runs too well. starts first kick, all lights are intact and work, plug chops well, gps'd at 58 mph, but something has to need fixing right?
UTC

Veni, Vidi, Posti
Joined: UTC
Posts: 6486
Location: Tega Cay, SC
 
Veni, Vidi, Posti
Joined: UTC
Posts: 6486
Location: Tega Cay, SC
UTC quote
If you remove the head and there is no indication of leaking, then it's fine to leave alone. However if there is leaking, it will be a good idea to go ahead and mill it down flat and you will pick up the compression a little. Meanwhile, I'm glad you are happy with it as is. Great scooter for putting around town.
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