Compression Test

[ddragon63]

So I'm preparing myself for tomorrow nights Carb Sync adventure on "Old Man"
The old dog hasn't ran since my little trip 2 weekends ago. So in prep I decide to pull the plugs and check em all out in my reader, and Yes I am THAT guy the guy that owns a plug scope... It's one of the few tools that did not leave with the race car, trailer, and tools. anyhow... I wanted to take a look @ them so I can get a better understanding of where I'm to start and what problems He may have. (that and I was looking for a good excuse to hide in the garage) 1 and 2 are pretty lean since the valve adjust, no detonation freckles tho, probably cuz I run super in it, 3 is kinda dirty, 4 looks like I would expect it to look with the little idle time it had putting it back in the garage.

So @ this point I haven't even finished my first beer so I thought.... hmmm haven't checked the compression in about well EVER Lets get the Boy warmed up while I finish my beer.

Warmed up, plug wires off. comp gauge out. Now some may run different procedures @ this point, this is how I do it. pull plug, Roll it over with throttle closed and gauge off, to clear out cylinder. attached gauge to cylinder... open throttle to WOT roll motor over till needle stops, without letting go of the throttle clear gauge reading, repeat 2 more times taking mental note of the last 2. The first one don't count as you just compressed some fuel. average the last 2 and record.

This is what I came up with. #1 167psi #2 171psi #3 169psi #4 170psi

So I know the book says 142psi @ sea level.... I'm just gonna assume that's a base line for @ sea level under perfect atmospheric and barometric conditions. I'm pretty certain this thing doesn't have a big bore kit in it. I would have remembered Dad doing something like that. anyways am I right in assuming that 142# is a base line? the high pressure conditions I've got outside right now could cause the extra? I know when we use to put cylinder heads on the flow bench the flow thru a port could change with the rise and fall of the weather, that's why we recorded such things @ time of testing....

Ok that was long winded.... as usual

[bikerphil]

A little carbon in there from using premium fuel can cause higher readings.

[ddragon63]

A little carbon in there from using premium fuel can cause higher readings.

hmmm yea I never though of that.. carbon build up with statically change the combustion chamber volume which would raise the compression a bit. good call Phil

[crazy steve]

I think Yamaha posted a low number in the manual to avoid any complaints from owners....

[ddragon63]

I think Yamaha posted a low number in the manual to avoid any complaints from owners....

maybe.... then people think they have some super bike with extra power! LMAO

[ddragon63]

oh and by the way I will be checking the timing for the first time tomorrow before sync and tune. after reading that section in my Clymer I'm gonna assume I can't loosen the timing plate and adjust it on the fly like a car? Looks like I have to shut off, make a small adjustment, and recheck? this sounds time consuming to get it spot on.... More time to drink beer I guess

[DGXSER]

I have seen it done with the engine running on an XS11. All of mine so fr have been 81's non adjustable.

But, beer time can never be overlooked.

[ddragon63]

I have seen it done with the engine running on an XS11. All of mine so fr have been 81's non adjustable.

But, beer time can never be overlooked.

Ya it just looks like a finger grabbing, new word creating, cuss factory if you try it running. I'd rather not messing up the plate or pickups.. or break something. I'll just move it a bit @ a time and lock it down. That is if I need to do it at all.

and beer time is never over looked in my garage

[TADracer]

My compression numbers have always been in the 160-170 range and I might know why....length of the threads on your gauge hose. I have both long and short threads on different hoses and when I use the longer threads (matching the length of the spark plug) I get readings like yours and the shorter length threads I get back around the 145-155 range. That little bit of area in the spark plug hole makes a difference in the compression reading.

As far as the timing plate thingy while running, I do it all the time. Loosen up one of the screws completely and use the other to just snug the timing plate until you got it set.

[ddragon63]

My compression numbers have always been in the 160-170 range and I might know why....length of the threads on your gauge hose. I have both long and short threads on different hoses and when I use the longer threads (matching the length of the spark plug) I get readings like yours and the shorter length threads I get back around the 145-155 range. That little bit of area in the spark plug hole makes a difference in the compression reading.

As far as the timing plate thingy while running, I do it all the time. Loosen up one of the screws completely and use the other to just snug the timing plate until you got it set.

that would make sense... I checked it with the long reach adapter so it matched the plug. I'll give the timing adjustment running a shot I guess..

[BA80]

Before you go adjusting timing you better make sure the pointer is correct at TDC. It could have been bent or moved over the years.

If the tamper proof bolts are still in the adjusting plate you probably don't need to adjust it. There's no reason it should be off.

[ddragon63]

Greg to the best of my knowledge the 79 special didn't have tamper proof bolts. But then again when I've had the cover off to roll it to the c mark to play with the cct I have never looked. Still it would be good to put the light on it and see where its @ one of those things that down the road you know it is correct.

[BA80]

I'm pretty sure it does have them on the slots in the adjustment holes on the breaker plate mount.

[ddragon63]

Well I will have to investigate. If there were tamper proof screws I'm sure they are still there. The old man was more into riding than tinkering. He would have seen those and decided yamaha didn't want him to mess with it.