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"Sons of Arthritus"
Jan 14, 2004
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I recently acquired a 73 750 that had been gone through. The PO had run Mobil 1 since the rebuild and represented the bike ran great. The gas tank was even full which proved he knew how to store a part time running bike with a steel tank.
Tickled the carbs, lit right off on the first kick. (This was after at least 6 weeks of delivery time) Bike ran great, pulled hard to 7000, sounded a little noisey, I attributed to the 15/50 Mobil 1. Had a few drips, some on the pipes for that nice aroma.
A little rough around the edges, but a decent bike. I was pleased.

Fired it up the next day to experience that smooth, strong pull, somethings different, it's flattening out in midrange and not pulling. Won't hit 7000. Idles fine, don't tell me the Amals are gummed up already.
Limp back to the garage, look it over, still idles fine, seems noisey, sure does smell, what gives. OH KNOW, I forgot to turn on the oil tap. YEP, the PO had install a manuel shut off on the oil line. I quickly turned it on, no change in the noise. Oh well, damage done.

But what damage? I started it up again, idles fine, sounds a little noisey but I can't tell if it is noisier than before. It just runs flat, a lot like when there is no advance. It has a Boyer, so I will assume for now it is OK.

Obviously, a tear down is in order. But how much?? How good is this Mobil 1?

Before I do it tear it down, I will double check the Amals, Boyer etc.
If these are OK, I am thinking the reason for the flat running is a flattened camshaft or followers. Would scored pistons and cylinders cause this?

Any thoughts?
The most likely first call will be the Big end shell bearings, these are soft metal linings and need oil pressure for them to float on. Next one will be the crank which these bearings run on. The rest works on oil splash and may be in better condition but as you will need to strip the engine you will be able to inspect them. On a fully bedded in engine you may get away with a crank regrind as the clearances will be loose any way, put new rings in even if it does not look like it needs it.

Fit a good Norton specific non return valve immediately (but you knew that anyway)

Good Luck
My first question would be How long and how far did you run it like this?

The first thing to give out should be the rings. It will idle ok on low compression but will certainly faulter at higher rpm. Before you start tearing it a part do a compression check. This in itself may tell you the story. Once the head is off you will be able to see if the cylinder wall is damaged. Go from there. You may have had enough oil in the sump to have prevented lower end damage. The oil tap was there to prevent "Wet Sumping " which is no real big issue if the engine was half decent to start with. Typical cause for this is a oil pump that is wearing. Make sure you take it off and lap it back in. As far as the inlet valve goes my thoughts are to get rid of it altogether. I have a friend that was concerned about wet sumping so he installed a GOOD one way valve. Immediately you could hear the difference . The engine had more top end noise. Remove the valve and it went away. Reinstalled and it came back.Needless to say that the valve has a certain restriction to it. Are you ready to take another chance
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