compression

mattmincer · Post by **mattmincer** » Mon Jul 12, 2010 7:11 pm

Hey All,
I have a 66 CA77. My cylinders are only getting 115 psi on the right and 125 psi on the left. Other than a rebuild, is there anything I can do?

Peace
Matt

brewsky · Post by **brewsky** » Tue Jul 13, 2010 5:07 am

Welcome to the forum!
Before you can interpret the numbers, make sure they are correct.
Valve adjustment checked first?
Engine warmed up?
Throttle held wide open during test?
Choke open during test?
Air filter clean or removed?
Tester hose sealed tight?
Engine spun enough to get maximum reading?

Any one of these can affect the reading.

Has the bike been sitting for a while?
Does the engine run?

jensen · Post by **jensen** » Tue Jul 13, 2010 5:34 am

Hi,

Maybe it's time to introduce a "how to do" section, Ed's tuning recipe, of course, is the first one.

Jensen

versuspop · Post by **versuspop** » Tue Jul 13, 2010 9:04 am

I thought the throttle wide open only applied to CV carbs. Can you explain the principle behind doing it with silde carbs?

brewsky wrote:Welcome to the forum!
Before you can interpret the numbers, make sure they are correct.
Valve adjustment checked first?
Engine warmed up?
Throttle held wide open during test?
Choke open during test?
Air filter clean or removed?
Tester hose sealed tight?
Engine spun enough to get maximum reading?

Any one of these can affect the reading.

Has the bike been sitting for a while?
Does the engine run?

paperslammer · Post by **paperslammer** » Tue Jul 13, 2010 3:30 pm

jensen wrote:Hi,

Maybe it's time to introduce a "how to do" section, Ed's tuning recipe, of course, is the first one.

Jensen

That's a good idea! That was my idea behind creating the "walk-through" scenarios with pictures. I think it would answer a lot of questions.

teazer · Post by **teazer** » Tue Jul 13, 2010 5:13 pm

Versuspop,

The carbs have to be wide open to allow the motor to suck in as much air as possible. Any restriction through closed throttles, poor air filters etc reduces teh amount of air going in and that reduces the compression.

Think of Compression being the amount of air sucked in and the space it's squeezed into. More air in=more compression. Smaller space to squeeze it into=higher compression.

High compression pistons (tall dome) or machining metal off the head reduce the space to squeeze gas into therefore compression goes up.

Conversely, partially closing off the intake makes it harder for the motor to "suck" in gas and the less gas in there to compress the lower the compression.

That's one reason why everything is related in a motor and when we want more performance we start to chase different compromises to optimize output. For example: Long duration cams allow more time to fill at high revs but fill worse at low revs, so we have less gas to compress at low revs and more at higher revs, so we lose power at the bottom and hope to gain some at the top.

versuspop · Post by **versuspop** » Wed Jul 14, 2010 6:54 am

Thanks.