Greetings all. So I just finished the restoration of my 1973 Norton commando including an engine overhaul, which was mostly replacing seals and checking components and tolerances . I replaced cam because of wear on the original made it just out of spec. Today I started it for the first time to do a cam break in. Right side piston was firing on every cycle where left side was firing every third or fourth cycle. My Norton has a single Mikuni carb, so this rules out fuel delivery issue. Compression shows 120 pounds in both cylinders. I swapped spark plugs, ignition wires, and coils. Problem persists on the left side. I’m stumped.
Puzzling misfiring from one piston
- Thread starter Semitone
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Intake manifold leak.
Idle mixture too lean.
Tight tappet clearances.
Sticky valve stem/guide.
Bad points.
Bad condenser.
Ignition timing wrong.
120psi is marginal for Norton compression. Throttle wide open during test? New rings not yet bedded in? Or see points 3 and 4.
Take your pick.
Idle mixture too lean.
Tight tappet clearances.
Sticky valve stem/guide.
Bad points.
Bad condenser.
Ignition timing wrong.
120psi is marginal for Norton compression. Throttle wide open during test? New rings not yet bedded in? Or see points 3 and 4.
Take your pick.
Tornado
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Air leak at dual manifold to head one side? 120psi seems low, buut at least even each side. Were the rings, bores or valves worked on? Throttle wide open, cold engine when testkng compression? What headgasket? Composites need quite a few rojnds of retorque before they settle down.
I get 145-155 psi after 6k miles since a rebore.
I get 145-155 psi after 6k miles since a rebore.
Electronic.
145-155 cold?
At idle, the right piston runs fine, so I’m hesitant to change anything on the single carb. If there’s a lean condition in the left I think it would be due to an air leak in manifold, but wouldn’t that effect both pistons since it’s a 2 to 1?
I haven’t ridden the bike because I need to break in the cam which instructions state should be done on center stand, 2000rpm for 20 min. -Paul
concours
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Yes.
But, it is NOT required to do it with an unloaded engine.
I rode mine, kept it revving at stop signs. I understand people live in a more urban setting, not so practical.
1) wait until the cam is done breaking in (you are using a window fan on HIGH, at the very least, correct?) then ride it, have realistic info on misfire.
2) go ride it, keep the revs up. Have realistic info on misfire.
P.S. the cam doesn't know or care if the engine is unloaded on the center stand, or loaded as in normal usage.
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Ok, so live with the misfire for now and get the cam broken in. I can see the merits of breaking in the cam loaded because I’ll get more realistic info regarding this misfire. On center stand I was planning to use a box fan and it still is about 50 deg. Fahrenheit here on the New England coast. I still was concerned about overheating, so another argument for a riding break-in? This all makes sense. Thank you.
Tornado
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No my numbers were on a hot engine.
marshg246
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No way would I start a rebuild and hold it at 2000 rpm for 20 minutes even with a box fan. The only thing I can think of that would be worse is letting it idle.
I'm not sure where this came from - on car engines where the cam is spash lubed and the engine is water cooled, this is often recommended - our engines are not like that.
The cam is spash and drip lubricated. There is a need to keep it well lubed especially when new and doubly especially if the tappets are worn. However, Norton, Triumph, BSA, et. Al, certainly never said to break-in by starting and holding the throttle at 2000 rpm. I carefully static time, quickly check with a strobe and then ride keeping the RPMs above 2000 as much as possible and not laboring the engine.
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concours
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I had my helmet & jacket handed to me after I started the new engine. Kept it at 2K
Once I had watched oil pressure build, no leaks, I rode it a half hour.
1) to break in the cam
2) to load the rings, make sure they seated.
Worked out great.
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o0norton0o
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The usual issue with the above kind of scenario which you describe is this,... If you actually are pumping fuel into a cylinder and the ignition misfires for 3 or 4 cycles and then finally ignites, then you are going to know it because the charge that ignites in the cylinder is going to ignite those other fuel charges that passed through the cylinder into the exhaust pipe and it's going to go, "POP". I'm not sure how loud of a pop you will get, but it would be noticeable. If you are not getting that observable condition, then the bike may not be firing on that side at all....
If you think it's occassionally firing because the pipe on that side is "warm" to the touch, that is a mistake. The pipe will be warm just from pumping air through the cylinder because compressing air heats the air up. For that cylinder to be firing, you should be able to spit on the exhaust pipe and it should boil the spit right off in a puff of smoke.... Being warm to the touch, does not mean the cylinder is igniting...
Hello again o0norton0o. Thanks for chiming in. No, that sporadic combustion sound sounded totally normal. I probably had it running for a total of 30 seconds or the course of 15 minutes. The right header was a bit warm to touch, the left cool. So I’ve got more into. After work I went over to the bike and rechecked tappet clearances. I was disturbed to see that at TDC on compression stroke, there was no lash between rod and rocker. In fact I had to back off the adjuster screw about 4 turns before I heard a “ping”. I think the rod was not seated properly. I’ve taken the head off many times over the years, and I’m usually very careful about making sure rods are seated properly before tightening head. Anyway, I set gap and checked others, which were fine. So I hope I didn’t damage the cam, follower, etc. during the short time I had it running. I checked compression and it was still at 120psi as it was yesterday. I’m not sure how I got this much compression yesterday with the intake rocker in the state it was in. Nevertheless I hope this was the cause of the misfiring issue. Unfortunately i didn’t have time to start the engine! Hopefully tomorrow.
In haven’t done this. Good suggestion!
Part of the reason I’m kind of paranoid about the break-in is that when I installed the cam I did not use any assembly lubricant. Since after putting the motor back together last year, I’ve become an addict to the Motor trend channel on TV. All these engine rebuilders stressing the importance of assembly lube. In the months since I put the motor back in the frame and filled the oil tank with oil, most of it has dripped into the sump. I removed about half of it back ti the oil tank hoping the excess oil in the sump would give the cam a good splashing during break-in.