Topic: Single cam timing and compromise...

[paulages]

Okay, so we all love our SOHC bikes and all, but as any tuner knows, having a single cam controlling both the intake and exhaust valves means that there is a limited amount that can be done optimizing timing. Cams are made with this in mind of course, and a compromise is struck. This is not the compromise in question though...

My megacycle 12620/12621 cam is giving me funny numbers. The intake centers are all averaging 106.5 (with a quarter degree or so variance) and the exhaust lobes are averaging 103.5. The cam card calls for 104.5 centers.

So... do I optimize the intake or exhaust, or go dead in between the two? That's where I am now. Megacycle reground my cam while fixing the broken flange... did they really grind it this far off?
 

[NitroHunter]

Intake CL numbers are far more important than the exhaust. You should still be able to hit 104-105 split. I like to close the intake early, more low and midrange torque.

[bwaller]

Were they trying to make it look new, or was there some scuffing?

I don't have a 650 cover to know if it can be done, but I cut the top of the camchain tunnel out of a 550 cover to time cams. It makes the job easier and then just install a good cover when done.

The 12620 I have settled @105LC intake 104.75 exhaust. Agreed you should focus on the intake.

What did you have when you installed it the first time?

[Bill/BentON Racing]

Intake closing most crucial one,can't build compression until it does.Bill

[754]

bellcow, I sent you a pM a few days back , did you get it?  Thanks 754

[paulages]

Were they trying to make it look new, or was there some scuffing?

I don't have a 650 cover to know if it can be done, but I cut the top of the camchain tunnel out of a 550 cover to time cams. It makes the job easier and then just install a good cover when done.

The 12620 I have settled @105LC intake 104.75 exhaust. Agreed you should focus on the intake.

What did you have when you installed it the first time?

i can't tell you how many times i've wished i had an extra 650 cam cover to cut open like this. oh well. anyway, it was at 105 degrees before. it wasn't scuffed as far as i could see, they just did it anyway. i'll call to find out exactly what they did. the receipt says that they checked and reground the lobes.. i can't imagine they did more than a light polish. still, it's driving me crazy that it won't time correctly, and it was just running in the engine a few months ago. if i retard it fully against the slot in one position, it's too far advanced, and if i skip a tooth and advance it fully against the slot, it's too far retarded.  slotting it more will be tricky, as there is a recess in the sprocket for the flange of the bolt head, which will also have to be milled.

i figured intake was best to focus on, but i don't want to end up with a situation where the exhaust valves are opening way too early and also causing loss of power.

[paulages]

Okay, so we ALL failed the test.. those numbers aren't contradictory. If the intake is advanced to be at 104.5, the exhaust number will go up, not down, as the degrees BTDC are decreasing as the piston rises.  Brain fart when I asked the question, I guess.

I did talk to the head machinist at megacycle though, and suggested that I leave it where it is and see how I like it. He says they usually suggest 104.5 degrees with stock compression, but that my 10.6/1 CR ought to allow for that much cam retard. We are talking about 1 degree or so at the crank, after all. Anyway, problem solved for now.