ET: Engine (Twin) Checking the Cam timing on a twin

greg brillus

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Mr Freebird.......looking at your picture above......it shows you have equal lift at TDC.......therefore your cam is retarded........rotating the cam clockwise one tooth is advancing it 7.5 degrees........this is at the cam, so at the crank it is half that......3.75 degrees which is very close to what you need.......so rotate the cam clockwise one tooth........however, you might find the front cam out as well........I know it takes time and a bit of head scratching looking at the degree wheel, as from TDC you need to put a mark 50 degrees anti clockwise to show TDC on the front cylinder.......if the front cam is retarded, then you can alter that one at the same time.......you may not know that the crank gear has 5 keyway slots in its center so the overall timing can be adjusted........normally a small puller is used to extract this gear off the shaft once the thin E80 nut is removed.......this keyway adjustment allows overall changes of 3 degrees per slot at the crank........if your front cam is out, it may be a simple case of altering each cam.....or removing the crank gear and relocating the gear via a different key slot to alter the timing........keep asking away........if you intend on riding this bike for some miles, I might add that changing that alloy large idler gear to a new steel item is the norm.........the alloy gears work ok, but tend to readily shed fragments of alloy which spreads nastily throughout the engine and fills the oil tank........not good and oil tanks are difficult to clean especially whilst still attached to the top of the engine........Good luck with it........Cheers........G.
 

Pete Appleton

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Errr. Doesn't the crank travel twice as far as the cam? 7.5 degrees of cam will give 15 degrees of crank.
 

greg brillus

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Hi Peter, yes sorry too many things going through my head at present........he'll need to pull the crank gear off and use a different keyway......I'd still check the front cam to see if it's out the same.......Cheers.
 

Freebird

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Thanks Greg.

I checked the front cam and it’s pretty good at 5 degrees BTDC.
 

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greg brillus

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I did some checks on engines I have here and you will need to alter the rear cam and pinion by pressing the two apart, then advance the cam a small amount........distance wise it is around the width of the top of one of the teeth on the pinion gear..........each tooth on the cam is 7.5 degrees and you need to alter it by 2 degrees, thus 4 at the crank........Cheers.
 

Freebird

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The reason I am looking into this is that the rear cylinder has always run a lot cooler than the front one.
After a short run of a few miles, the temperature of the rear downpipe is around 55 degrees C whereas the front one is around 145!
I have had the carbs checked and all is well, a colourtune test also looks normal, so I don’t think it’s mixture related.
A leak down test indicated a small leak on the rear inlet valve but was well into the green on the leak down gauge at 30%.
The ignition timing is a tad advanced (I think) at 38 degrees when fully advanced and both pots are the same.
I now find the valve timing is out by 4 degrees on the rear, but would that be enough to cause the difference in temperature? Seems unlikely to me.
 

davidd

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30% is too high. Broken compression rings ran at 60%. Half a percent new is not unusual, and 1% to 7% seems reasonable. 7% was what the racer was running after four seasons.

David
 
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