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head shave

2110 Views 3 Replies 2 Participants Last post by  CB750F
'76 CJ360T. I did the old compression check and with the new top end and shaved head im getting readings of 190 psi for each cylinder. Im thinking about 4 degrees timing retart well be a starting point for a serious retune. Id like to hear other thoughts like spark plug choice and carb issues or any other things I should think about
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Still that's not all that God-awful high for a high compression cranking pressure test. How much did you mill off? You'll probably want to go with slightly colder plugs. I would try the stock settings and keep an ear out for rattles and keep an eye on the plugs and header pipes. Run a minimum of 91 octane and I think you might be ok. Might want to richen up the carbs somewhat especially if you are running a more open pipe.

It's not so much the base timing advance it's more the mechanical advance that's going to give you problems IMO. You see behind the points the little weights and springs? That be the mechanical advance. The faster and higher the RPM goes up the more timing advance it dials in. You can control how fast it dials it in by switching to heavier springs to slow it down or lighter springs to speed it up. It be tough to find different springs for it but if you have any automotive speed shops local go in there and ask if they have a bunch of distributor springs and see if they fit your bike. Some automotive speed shops carry different weight distributor advance springs, they will be the same lenght but heavier or lighter wire, used for recurving the mechanical advance on car distributors. You may get lucky and a certain type may fit your bike. Even with different springs you'll still get the same amount of mechanical advance but it will occur at a higher (or lower) RPM point depending on what springs you have in it.

You can also limit how much advance can be dialed in mechanically by either making up bushings for the advance slots or welding them shorter.

Sounds like it's going to be one hell of a fun bike!
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Reactor Racing did my head, he went and took 30 thousandths off, I believe it was all taken apart and checked and measured, he sounded surprized when the old head passed all of his tests. Not sure if polishing the cumbustion chamber in the head would help anything but my manual said to do so with a wire brush. polishing with a dremill tool worked great to.

My exhuast was cut and welded by me from the collector back with the thought of making some power, while screaming far into redline. Im comfortable tuning the carbs and watching and finding the right heat range of plug for how ive got it setup.

Hunting down new advance springs to match the bike should be a good time. So generally with more fuel and air, Would the idea be to run stiffer advance springs to slow down the advance?
I would think so to keep it out of detonation. Really here's what I would do,try it with the stockers first and keep an eye on the plugs and listen for pinging. I know thats tough to listen for it on a bike but you know what you're doing.

The polishing will help eliminate "hot spots" so that should help.
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