john robert bould said:
Over a pint or two the great subject of cam timing cam into force...the debate was what happens when the cam is advanced or retarded.
the answers where quite differant....
So how would the engine respond to 50 degrees's advance..and say 50 Degree retard. what effect on compression air/fuel,mixture etc...i except only a few will have played with adverse timing...mayby some by mistake.
Postby john robert bould » Tue Apr 29, 2014 3:34 pm
OK it's time to come clean, i should'nt have posted this on the commando forum...and will except a big slap from the moderators!
Truth is i have been working on a 1936 Morgan ..Jap 1000cc dog eared water cooled V twin ...single amal carb T branch ...evey thing {so far] checked out...compression 100psi and 120 psi spark timing same both side 680 thou and 700 btdc ...But it runs crap! colour tune shows yellow oneside ...RICH as hell with cool header,,,other side pale blue WEAK as hell...red hot header....can incorrect cam timing cause this? the 120 psi is on the weak side..is the lower compression[100] due to bore wash? all joints made air tight with tape...
Postby pete.v » Tue Apr 29, 2014 10:31 am
For crying out loud, 50 degrees? Is this a trick question? After some serious clash I think the rest of the question would be void other than the compression part which would be nill.
How about something closer to reality like 10 or 15 Degrees.
Although my comments will be simplistic compared to other when they respond, the timing characteristics will depend on a particular cam I'm sure but generally speaking,
It was my finding that retarded cam raises the compression at kickover but kills rpm obove 3500 or 4000 whereas advanced cam lowers compression on kickover and bringing the duration of the stage 1 cam into play for higher RPM running. My JS cam is 10 degrees advanced.
By the light of day and being of clear mind this AM some of our beer talk from last night isn't quite setting right today. But that doesn't mean the beer wasn't fun does it?
pete.v,
If the cam is retarded, the IN closing event is likewise retarded (occurs later) which lowers, not raises, the pressure during compression. Similarly if the cam is advanced the IN closing event is advanced which serves to raise compression. Regarding your JS1 cam being advanced 10 degrees, I thought JSM posted earlier in some other thread here that, due to a manufacturers error, the cam indexing reference was misallocated and that in fact advancing the cam some number of degrees actually placed it in the normal straight-up installed position. This may not be the case for your specific cam, but I know JSM mentioned it wrt to at least one of his cams.
JRB,
In very general terms if a cam is an appropriate design for a given engine, installing it in the "straight-up" position, i.e., where IN and EX overlap is centered at TDC, provides the best starting point and a condition where the engine should run well. Advancing the cam shifts the useful rpm range lower and retarding the cam shifts the useful rpm range higher.
I'd also agree with Matt S and kerinorton regarding visualizing the cam timing on the engine. Find TDC, and observe the IN and EX valve motion as the system approaches and departs from TDC, where the IN and EX valve motion should mirror one another. Given the vintage of your vehicle I would expect the cam to be quite short duration and consequently the overlap at TDC between IN and EX should be short. If the IN and EX valve motion is not centered at TDC, the cam was likely installed incorrectly. Attached is a plot of an old Indian Chief cam profile to illustrate the valve motion you might see with a vintage short duration cam.
Does the engine idle well? I ask because this would indicate the IN manifold is well sealed. IN leaks on v-twins can cause some very peculiar running issues and I wonder if that might be the case in your situation. On vintage HD v-twins the std protocol is to pressure test the IN manifold by applying 15 psi air to it and verifying that no leak-down occurs.
Regarding the red hot header, in my experience this is most often caused by very late timing, but if the cams were way out of whack this could also be a contributor. As noted above it should be trivial to assess whether the cam is in the right ballpark or not.
Good luck and keep us posted.
And just in case anyone is intrested in NORTONs , 
