What is the compression PSI on an 850 commando Rebuild???

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Mar 1, 2007
I was wondering if my bike may not be starting due to low compression. I honed and re ringed it and it wont run??? I am getting about 77 PSI in both cylinders. Very consistent after several kicks. The rings are not seated yet because I can not start it----Thanks for any help
From my experiences anything under 100 psi means you tear the thing down . You first need to determine where the leak is . Check valves on compression stroke to ensure that they are in fact fully closed. Listen for air coming out through the exhaust or the carbs as the piston comes to the top. For rings , listen through the crankcase vent. Another quick check is to add a few ounces of oil into the cylinders , kick it through a few times and then do another compression check. If compression rises you can pretty well assure yourself of bad rings.

On the last rebuild of my MKIII , compression was within 2 psi of 150 psi on both sides .

Good luck .. been there .. done that . The reason for the last rebuild was because the new rings previously installed failed to seat in. Don't ask why because i don't have the answer . Another Norton mystery i suppose.
Mark, Have you tried push starting the beast? You will need one or two friends to help, but this is often easier with a recalcitrant bike after a rebuild, than kicking it until you are comatose. If it starts and runs for a while, it may bed in and then settle down,
Compression on Norton 850

Well, I borrowed my neighbors very good compresion tester. The type that screws in the plug hole. after one kick I get about 105 PSI. After 3 or 4 kicks I get about 128 PSI. Is this sufficient to start the bastard?? I guess I will have to bribe the neighbors kids to help with a push start. I am 53 and not that long winded anymore---Thanks for the info---Mark Ciginero
Are you aware there is a right and wrong way to do that test?
Panic gave this good advice on another site.
1. full throttle, choke off
2. engine warm
3. non-test cylinder plug is out
4. kick through as rapidly as possible, minimum of 10 times or until the gauge has not moved up for 2 kicks
5. results are typically between 15 and 20 × static compression ratio (9:1 = 135 to 180 psi)
6. 5% difference between cylinders is OK

100 lbs on a fist kick and a cold motor This is not your problem.
Thanks for the compression re-assurance.

At this point I am grasping for straws??? I am a very good shade tree mechanic and I am fairly used to my rebuilds starting first or second crank??? I did the same work on my buddies Commando last spring and it started second kick>> It did have a single Mukini though and an old Boyer that eventually failed during the rebuild and kicked the ever loving shit out of my foot. With your advise I did finally sort that problem out and replace the Boyer. It is still runing great. Bikes--Car, engines etc. Thisis one has me stumped and severly frustrated. Guess I should walk away from it for a bit till I figure it out and cool off?????At this moment torching it almost sounds theraputic but I know better. I will wait and re-vat the carbs and get some friends to help push start it---Thanks again---Mark Cigainero
Mark, I've been there!

When I rebuilt my Commando in 2006, having only ridden it for about 6 miles before doing a complete rebuild of everything, I couldn't get it to start. I would kick it and kick it but it would not start. There was a spark on both plugs, the plugs were getting wet indicating fuel getting into the pots, I'd re-check the timing, the compression etc etc, it would pop and kick back a bit. It had Amal MK1.5s on. I was sure that these were the problem as the choke on these are a plunger system similar to Amal MK2s, and so I bought two new carbs.......result? No change.

So in increasing desperation I bought a new Boyer, and it turned out to be a fault on the boyer ignition backplate. I dont know where as I had checked the wires and even resoldered them, but with the new back plate fitted (before fitting the new box) one kick and vvvvrrrrrrooooom the engine fired up immediately :shock: . I was fairly suprised although much relieved.

I know that this isn't much help to you, but if your compression is OK and you are confident that the valve timing is OK, see if you can substitute (preferably from a friend so that you don't have to purchase them) other bits of ignition, and or carbs.

My Commando now has new carbs and ignition, so now I've got over the expense, I'm quite pleased! 8)
FWIW, on my non-rebuilt 73 850 with a suspect valve, I got 155/170 warm with throttle fully open.
Howdy! I have a question, how much kicking compression should an 850 with a 2S (Combat) cam have? I'm only getting 130, same in both holes. I haven't checked the valve timing thoroughly but the intake does close with the piston about halfway up the bore and the spec is 90 degrees ABDC. I checked another 850 with stock cam and shaved head, it had 190 with same gauge.

If it's the same in both cylinders, odds are things are just fine and it's your gauge, if anything, that is faulty.

How's the power? How do the exhaust pulses feel if you hold your hand behind the muffler outlets? Have some percussive quality - some "punch" - to them? If the power feels okay and the exhaust is "punchy," I'd say you're okay.

Very scientific, I know!
The 2S cam on a combat makes it easier to kick over than the 10:1 compression would suggest, this is due to the greater overlap allowing more of the incoming charge to escape at low kicking over speeds. So as as suggested already as long as the 2 sides readings are close 130 is ok.
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