Sacaps
XS650 Member
So let's start with a little history. I built this motor after building my first Tracker and was all hot to build a second hot rod and built this 750cc 277 rephased motor. A lot of other stuff then got in the way and this project has been in the corner for fourteen years. The lockdown/covid thing has now got me into the bottom of the project barrel and here we are.
Before I spend anymore time or money on it I thought I would get it running. Easy right? I hooked up an ignition, an IV bottle for fuel and can't get the right cylinder to fire.No amount of ignition checking, carburetor cleaning, valve checking has provided any success.
When it is running, it seems to be trying to blow out of the right carburetor rather than suck in. So I'm thinking valve timing. But if the left side runs then the right should follow since it is only one cam.
Remember the motor is rephased 277 degrees with the LEFT cylinder ahead of the right by 83 degrees.
So I think let's check the cam timing again. Anybody that has not done this, pay attention and anybody with some knowledge on the subject, show me were I am wrong.
First a degree wheel. and a positive stop for the piston on the side I want to check.
turn the crankshaft forward until it hits the stop, write down the number from the degree wheel. Turn the motor backwards until it hits the stop, Note the number. Top Dead Centre will be halfway between these two.
(be careful here as the valves are moving and the stop will damage the valves. On the XS motor if you back off completely the valve adjusters there will be clearance) remove the stop, turn the motor to the number halfway and you are at true Top Dead Centre.
Now since the opening of a valve is VERY slow, it is impossible to measure when it starts to move so a dial indicator is set onto the top of the valve and with NO valve lash (rocker to valve clearance) the opening of the valve is measured at .04" (1mm) or North American car guys use .050". I used .050"as this is what was specified in the cam timing chart.Note the number on the degree wheel, The is the valve opening value. It should be before top dead center.In my case I'm at 25 degrees before top dead center BTDC.Continue turning the engine forward until the dial indicator is an .050: and again note the number. Im at 45 degrees ABDC after bottom dead centre.
Now move the indicator to the exhaust valve and to the same. DO NOT MOVE THE DEGREE WHEEL.
I got Exhaust valve opening 50 degrees BBDC and closing at 20 degrees ATDC.
Now move to the left cytlinder and do the same.
Here are my readings
R/S Intake R/S exhaust L/S intake L/S exhaust
open 25btdc 50 bbdc 25btdc 47bbdc
close 45abdc 20 atdc 48 abdc 20 atdc
if you cant read it, the specs are 25 and 45 degrees for both the intake and exhaust. For this motor, that is as close as we are ever going to get.
Question 1.. is there anything wrong with the way I have installed the Cam?
Just for fun, while I had the degree wheel set up, I checked the opening of both the left and right side points opening.
with a spec of 15 degrees BTDC I would say I'm close enough.
Notes: if I swap coils and condensers left to right it makes no difference
New pistons, rings, and valves Compression in both sides is over 150psi
A timing light shows both cylinders firing.
So I have compression, spark at the correct time, cam timing correct and fuel I think
Question 2 ...Why wont it run?
I'm into one of those wrench throwing frustrating moments.
Anybody have a constructive idea?
Before I spend anymore time or money on it I thought I would get it running. Easy right? I hooked up an ignition, an IV bottle for fuel and can't get the right cylinder to fire.No amount of ignition checking, carburetor cleaning, valve checking has provided any success.
When it is running, it seems to be trying to blow out of the right carburetor rather than suck in. So I'm thinking valve timing. But if the left side runs then the right should follow since it is only one cam.
Remember the motor is rephased 277 degrees with the LEFT cylinder ahead of the right by 83 degrees.
So I think let's check the cam timing again. Anybody that has not done this, pay attention and anybody with some knowledge on the subject, show me were I am wrong.
First a degree wheel. and a positive stop for the piston on the side I want to check.
turn the crankshaft forward until it hits the stop, write down the number from the degree wheel. Turn the motor backwards until it hits the stop, Note the number. Top Dead Centre will be halfway between these two.
(be careful here as the valves are moving and the stop will damage the valves. On the XS motor if you back off completely the valve adjusters there will be clearance) remove the stop, turn the motor to the number halfway and you are at true Top Dead Centre.
Now since the opening of a valve is VERY slow, it is impossible to measure when it starts to move so a dial indicator is set onto the top of the valve and with NO valve lash (rocker to valve clearance) the opening of the valve is measured at .04" (1mm) or North American car guys use .050". I used .050"as this is what was specified in the cam timing chart.Note the number on the degree wheel, The is the valve opening value. It should be before top dead center.In my case I'm at 25 degrees before top dead center BTDC.Continue turning the engine forward until the dial indicator is an .050: and again note the number. Im at 45 degrees ABDC after bottom dead centre.
Now move the indicator to the exhaust valve and to the same. DO NOT MOVE THE DEGREE WHEEL.
I got Exhaust valve opening 50 degrees BBDC and closing at 20 degrees ATDC.
Now move to the left cytlinder and do the same.
Here are my readings
R/S Intake R/S exhaust L/S intake L/S exhaust
open 25btdc 50 bbdc 25btdc 47bbdc
close 45abdc 20 atdc 48 abdc 20 atdc
if you cant read it, the specs are 25 and 45 degrees for both the intake and exhaust. For this motor, that is as close as we are ever going to get.
Question 1.. is there anything wrong with the way I have installed the Cam?
Just for fun, while I had the degree wheel set up, I checked the opening of both the left and right side points opening.
with a spec of 15 degrees BTDC I would say I'm close enough.
Notes: if I swap coils and condensers left to right it makes no difference
New pistons, rings, and valves Compression in both sides is over 150psi
A timing light shows both cylinders firing.
So I have compression, spark at the correct time, cam timing correct and fuel I think
Question 2 ...Why wont it run?
I'm into one of those wrench throwing frustrating moments.
Anybody have a constructive idea?