Least compression

Ok got out my 68 Norton Atlas and at the end of last season it would run on only the left cylinder. The right pipe had white ,unburned fuel smoke. I figured it was due for points or at least adjustment. Just put in new points, condenser and sparking plugs. Still runs on Left cylinder only. Removed the carb bowl everything clean, check valves ,clearances ok.. Sooo check compression on left side first, yes I did open throttle completely and got 110 psi. The right not so good 75psi.. I hate to pull it apart for what I might see. So I guess I'll pull the head and touch up the valves.. At what point ,psi, will the fuel mixture not ignite.? I know alot of the real old stuff had 6.5- 7.5 ratio and ran

Both should be around 110lbs sq in.
Check tappets on r. h. one first before stripping top off.

My oldest Norton has 4.9 compression ratio.
You say you changed condenser. I thought that a -68 have two. My -68 650SS have two.
I think you should check if you have ignition when running on both sides first. Could be a bad coil. Easily checked by switching them.

Yes it has two and both were changed Didn't think to switch plug wires Thanks

I preach the KISS, keep it simple stupid, repair method that I did not follow. When I switched plug wires it wouldn't start. The right plug wire was burnt off. I still might have a weak cylinder on the right, but if I get firing it might just be carbon holding a valve open a bit>

An excessively rich fuel mixture will not ignite.

Is the right plug wet with fuel?

Slick

Edit:
I presume you have concentric carbs. I once diagnosed a similar issue in another Forum member's bike to be in the pilot circuit flooding one cylinder. A loose or missing pilot jet can do this.

mean gene said:

Thanks I'm about to pull the carb off. I cannot tell if it's missing because of lack of spark or fuel. New plug wire made a difference, I used jumper wires to get around the wiring harness, no difference. I reset the float that was a little high no change. View attachment 80150If carb looks ok I guess I'll change the coil in case it's breaking down. This bike has been a storage shed since 1971. Don't plan on doing a thing in the way of cleaning but want to get running properly. Everyone needs a rat bike. LOL

Click to expand...

It would not let me view the attachment.
If you have a single carb, ignore my previous post.
Nice looking Atlas, BTW.

Slick

Might be you discovered why you put it in the shed in 1971. I can't remember a lot about 1971.

You did verify spark the old fashioned way, right? Plug out laying on head with plug wire attached look for spark when kicking it over.

If the plug on the dead side is bone dry, it's not getting enough fuel yet. My experience is an old cold Norton likes a bunch of fuel to start.

Does the Atlas have a little distributor on it? Cracked distributor cap, poor rotor contact maybe.

I would do everything I could think of before taking the head off. Once the head comes off, you never know what you might want to change once that can of worms is opened. Modern ignition, premiers,... the list goes on.

Been a while but totally rebuilt Atlas engine mostly to clean out (non-detergent oil) and to stop oil leaks. When attempting to get ignition timed my test light would turn off half way around the point cam. The reference book said point gap was to be .012 - .015. If I increased it to .018 the light stayed on until the proper position when it was supposed to turn off. There was plenty of room for more adjustment. The bearings and shaft were a close fit. So was the reference book wrong or bad point cam? This gives you one more thing tock hope it helps someone! Knock on wood I'm up and running!

What was the casue of the low compression that you initially found ?