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Discussion Starter · #1 ·
i got my ultra shes not running good. its got alot of miles so im thinkin compression. i know from workin on chainsaws 100 is pretty much as low as you can go. what should the range of my compression be? thanks
 

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anything under 110 in my book would be just cause for a new top end
 

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Discussion Starter · #4 ·
whats the normal amount??

i looked in the sevice manuel and it didnt say anything.
 

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120psi is normal for the ultra,all motors run different psi on pump gas. Some of the cats are 140+ stock.
 

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Discussion Starter · #7 ·
ya i relize that how come different motors are different psi. because my 81 air cooled ski do has like 150
 

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polarisripper28 said:
ya i relize that how come different motors are different psi. because my 81 air cooled ski do has like 150
It has to do with port location which effectivly alters timing. I am not talking about ignition timing but rather event timing (intake charge verses exhaust cycle. If the port is higher/lower, the cylinder will bleed off or retain more or less of the cylinder pressure at cranking speeds. That is assuming that the engines being compared are relatively tight and are not suffering from any other mechanical issues. That is about as simply as I can explain the difference in compression readings in different engines.
 

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tc66nova said:
polarisripper28 said:
ya i relize that how come different motors are different psi. because my 81 air cooled ski do has like 150
It has to do with port location which effectivly alters timing. I am not talking about ignition timing but rather event timing (intake charge verses exhaust cycle. If the port is higher/lower, the cylinder will bleed off or retain more or less of the cylinder pressure at cranking speeds. That is assuming that the engines being compared are relatively tight and are not suffering from any other mechanical issues. That is about as simply as I can explain the difference in compression readings in different engines.
Back in the day we used to mill the piston skirts on 750cc Kawi triple's to change the port timing,along with porting,those thing's used to haul the mail.
 

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Id say less than 110
 

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Check it warmed up, after you get it warmed up and shut down, pull the plugs, screw in the gauge, hold the throttle wide open and pull a few times.
 

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my ultra had 90 psi on all three cyl the day i bought it and ran fine for 7 years before i decided it was time to freshen her up, still at 90 psi on all three after 7 years, fresh top end it was 125
 

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Mine is at 125 across all three. I have seen engine's run at 80 but had to start and it run like crap.
If your within 10psi difference between any of the reading's then rebuild. At 100 I would rebuild it!
 

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tc66nova said:
polarisripper28 said:
ya i relize that how come different motors are different psi. because my 81 air cooled ski do has like 150
It has to do with port location which effectivly alters timing. I am not talking about ignition timing but rather event timing (intake charge verses exhaust cycle. If the port is higher/lower, the cylinder will bleed off or retain more or less of the cylinder pressure at cranking speeds. That is assuming that the engines being compared are relatively tight and are not suffering from any other mechanical issues. That is about as simply as I can explain the difference in compression readings in different engines.
Port location doesn't affect compression all that much, and is not a main factor as to the differences in compression between different engines. Compression ratio is the biggest factor, and port location doesn't affect compression ratio. The compression ratio is the total volume of the cylinder with the piston at BDC divided by the volume of the combustion chamber with the piston at TDC.

The shape of the top of the piston and the shape of the cylinder head directly affect the compression ratio, and therefore significantly affect the actual compression.

Having intake ports cut lower in the cylinder and having reed valves both help increase compression to some extent, but they increase running compression more than they affect static compression (the number you see on the gauge).

A 2 stroke motor with reed valves (like an ultra) can typically still run with compression that is too low for a piston port 2 stroke (like an xlt).
 

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My 1997 680 Ultra used to be 135 across all three cylinders. Two years ago I was sitting at 130, 125 and 130 and now I'm at 120, 120 and 120.

Cheers
 

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ok on this one i gotta go with old school. i dont see how port timing can affect compression(to a degree thatd be noticabl) lowering the ports allows more fuel air mixture in, but compression is based on what the cylinder can hold, mostly affected by the tdc measurements...head and piston design. with lower ports you may see a quicker highest compression reading????? but i dont think itd change the overall reading.
 
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