Topic: Valve clearance with modified cam

[Ottawa_550F]

My bike has a KH cam with DS stamped on it.  Somebody else had posted that megacycle bought out the Kenny Harmon stock years ago so I went to the Megacycle site and it states that the valve clearance for their cams should be set at .004/.005 instead of .002/.003 as stated in the manual.  With this cam and the 592 kit I can only get around 5cm-hg of vaccuum when doing a synch.  I do have my valves at the stock clearances and am wondering if anyone has run into this issue with an aftermarket cam on?  Would loosening up the valve clearances potentially reduce the valve overlap enough to get better vaccuum?   Thanks everyone,

Adam

[dusterdude]

yea,your tighter clearances may be holding the valves open a tad bit.

[MRieck]

It's also easier on the rockers and allows better valve cooling being that the valves spend extra time on the seats.

[Ottawa_550F]

Thanks DusterDude.  I will re-adjust the valve clearances and try to synch again. I do worry that going to .004/.005" may be a lot and end up being noisy.  I am definately learning as I go here.  Is it normal to have to increase valve clearances when replacing the cam on any of these sohc's?

[Ottawa_550F]

Thanks MRieck...  your post kinda answers my question.  That totally makes sense that I should free up the valve train a bit with the cam I have.  I am hoping to get more vacuum from my synch than the 5cm it likes to balance at now.

[hevykevy420]

Ottawa-
 
It is normal with a higher than stock lift cam to have to run a bigger valve gap.  I too am running a higer than stock lift cam in my 836 rebuild and the manaufacturer (Web) asks for a valve clearance of .004 for both intake and exhaust.

When I synched my carbs using a vacuum tool purchased from JCWhitney, my vacuum readings were low like yours.  I don't remember the exact vacuum numbers, but on the synch. tool, the vacuum was in the red zone on the tool, which I took to mean "bad, your reading is low".  It idles pretty good, i was perplexed about this as well.  I double checked the valve gaps, and it was .004, perfect.  Bike runs good so i'm not worrying about it.  Just make sure you get even readings for all the carbs...

Check and make sure you don't have any leaks in your manifold rubbers and that the clamps are holding securely.

[dusterdude]

if these cams have more overlap between the opening and closing of each valve,there will be less engine vacuum available.

[Ottawa_550F]

Thanks Duster and Hevy.  TwoTired said something similar responding to my question that  5cm-hg was way to low:

"Certainly would be on a stock bike.
Camshaft grinds that work well at high RPM, usually don't work well at low speed, and have poor vacuum charateristics at low speed.  Do you know your cam grind specifications?   How far after BDC does the intake valve close relative to the stock grind?"

And I did adjust the valve clearances to .004/.005, the bike is now running pretty well with the main jets drilled to 118 so I am not going to worry about it either.  It makes sense that an aftermarket cam is going to change the vacuum characteristics of the machine.  And I do not know the actual specs of the cam anyway.   It is likely similar to the megacycle mild cam which is open 15 Btdc , 45 ATDC close on the intake and 45BBC/15ATC on the exhaust side.

[TwoTired]

Stock cam:
 Exh open 35 bbdc
 Exh Close 5 atdc
 Intake open 5 btdc
 Int close 35 abdc

Intake/exh overlap 10 degrees  (piston pushes air into both exhaust and intake ports for 5 degrees then piston draws air from intake and exhaust ports, for 5 degrees.
Intake valve open to intake port during compression stroke for 35 degrees of crank rotation.

Mega Cycle 126 cam:
 Exh open 45 bbdc
 Exh Close 15 atdc
 Int open 15 btdc
 Int close 45 abdc

Intake/exhaust overlap 30 degrees  (piston pushes air into both exhaust and intake ports for 15 degrees then piston draws air from intake and exhaust ports, for 15 degrees.
Intake valve open to intake port during compression stroke for 45 degrees of crank rotation.

Manifold vacuum will most certainly be lower at idle because actual intake suction time is lower and more time is spent pushing air into the intake port.

Such a cam will require higher air velocities to negate the low speed vacuum loss.  However it should "work" better at higher RPMs where the air flow aids the engine's breathing.  The power and torque at low speed will be significantly less due to inefficient low speed vacuum and carburetion.  Lower gearing is called for, as well as operation in the lower trans gears.  With the megacycle 126 cam, I imagine the redline should be about 1000 - 1500 RPM higher than with the stock cam.  Yes, the cylinders will wear out faster.  Part of the trade off.  More power/performance; shorter engine life.
Should be fun while it lasts though. 

FWIW

Cheers,

[MRieck]

.....that cam also requires a slotted cam sprocket and cam timing for those numbers to be actually happening. Overlooking cam timing with aftermarket cams is hit or miss at best.

[Ottawa_550F]

Wow thanks TT and MRieck.  That is great info.  Is it reasonable to just file out the holes on a spare cam sprocket to make it slotted or is this something you can purchase?  I have seen the sprockets on the megacycle site but I think they are for the 750's.  I have often thought about the cam timing and whether it would make a difference.   Something new to tinker with...and I can even buy some new tools...woo hoo!