Cam Timing

[plattey]

Alright, I made a 17 thou thick copper head gasket and used yamabond for a sealant between the jugs and engine cases to increase the engine's cr. I calculated how much the change in distance between the cam and crank would affect the cam timing and I didn't have the correct tools to degree in the cam at the time. When I pressed the cam sprocket off to re-clock it, I moved it a smidge too far forward of my calculated mark and didn't think much of it. Well it has come back to bite me in the ass (specifically the pocket that has my wallet in it) because the bike is now getting pretty bad gas mileage (33 average) and the plugs are definitely not showing a rich condition.
Today I used a degree wheel and dial indicator to find out when the intake valve is opening and closing. After taking measurements, the cam is 5 degrees advanced. The engine runs great and has great torque down low. When I did research about cam timing, all I found is that advancing the cam creates great torque in the low rpms while sacrificing top end power. Retarding the cam creates great top end power while having a doggy low end. I never ran into any material that mentioned gas mileage. I talked with some of my friends that build small block chevy's and they said that even 4 degrees is a big change for a smallblock. So I guess my question is;
Is the 5 degrees of advance the culprit of the poor gas mileage? Thanks!

 

[nj1639]

The copper gasket that you made, is that thicker or thinner than stock? I'm no expert here, just a tinkerer. I've been thinking of doing a cam for the 8-1 xsjohn compression mod for my new acquisition- as my runner is already set up from a modded cam by xsjohn, more of a top end cruiser as opposed to a low end screamer.
Here's a link to the lower compression, might be able to glean some info for the higher.......
http://www.xs650.com/forum/showthread.php?t=2426

 

[plattey]

The copper head gasket (0.017" thick) is thinner than the stock head gasket by 0.030" and eliminating the base gasket free's up 0.020". Doing this decreases the combustion chamber volume by a little bit, but enough to increase the compression ratio from 8.4:1 to 9.4:1.
I think I'll just have to tear the motor out again and set the cam right.....this will be the 3rd time this semester that I've pulled the engine . I should be able to do it blindfolded at this point haha

 

[nj1639]

Third time's a charm! Seems to get easier and faster for some reason.........

 

[nj1639]

Wonder what would happen using the new copper gasket and no change in cam?

 

[plattey]

the timing would get retarded around 3 degrees (off the top of my head)...i'd have to my check calcs. retarding timing shifts power into the upper rpm range