Troubled problem

G'day boys, long-time user, second-time poster.

My issue has the possibility of being an expensive one, or one that has eluded me. I have a MKI 73' 850.

The bike a month ago she started backfiring and spitting out of the ride side exhaust, running on one cylinder, and running lukewarm down the right side exhaust.

I was convinced it was the original Amal's (as i was putting off buying premiers until I ultimately needed to). They were shot. So I Installed the new premiers.

Now no backfiring and spitting. However still running on one cylinder, right exhaust lukewarm, could touch it. Left still running hot & firing.

I purchased new CNW exhaust roses and some header pipes a while ago and thought I'll put them on now so I can take a peep through the ports. The left (which is firing) is the proper dark carbon colour. I look into the right. Which is BRIGHT WHITE!! This is after the tri-spark testing, which showed the tri-spark was working perfectly, swapping coils and leads- did not show error. The petrol isn't even igniting?

AND installing the brand new Amal premiers with which the needles, slides and mixture are working perfectly.

I am about to install a fullauto head with brand new kit from Jim Comstock. The RH4 on mine is 40 years old, and to be honest I don't want to put it on to have the attitude that replacing the entire top end will automatically fix the problem - Just to have something much more sinister wrong with the bike . I.e.: BOTTOM END.

The original wiring harness needs replacing, but I cancelled out electronics, i.e. Testing trispark and swapping leads, coils, proved that the plugs were sparking perfectly, the new Amal's letting plantttttty of fuel in, and the fuel still not igniting?

Any help here would be hot..

Usually about now, most mechanics would be reaching for the compression gauge - to give it a compression test, on both cylinders.
AFTER trying a new set of plugs in it first.
And checking all the valve clearances are still good.

Seeing that it still has good compression on both cylinders would be a good first step.
If the non-firing cylinder is low on compression, you may be looking at a blown head gasket, or burnt valve.
Or, less possibly, a worn cam lobe maybe.

You are right, fitting all brand new components to fix a fundamental problem somewhere would not be good.
With a new cylinderhead up top, most owners would be looking at a rebuilt bottom end, and new or good pistons in a known good bore.

Hopethishelps, havefun !

I am sorry for being a little vague, tested for compression - fine. & with brand new NGK resistor plugs...

I should have included that rebuilding the bottom-end is something that will be tiring for me, considering I've spent a lot of time on this bike already.

First I would check that there is a good spark. That can be done by kicking the engine with the removed spark plug inserted in the cap lying on the head fins. If you see a good spark and the plug is dry it could be a problem of an air leak or wrong valve clearance (too much ?). When the spark plug is dry and bright white there is not enough fuel or too much air.

I did this with the tri-spark ignition tests. Plugs lying on the head. Both plugs(new) were sparking fine, and after swapping coils and leads.. Both plugs are dark-brown to black. It is looking inside the port and inner head that is White.

If the fuel would not ignite the plug would be wet and you could smell the petrol. So I suppose it is still a matter of fuel starvation.

The fuel taps/filters are brand new as are the stainless braided fuel lines, both sides flowing fine. Sorry to troubleshoot so late.. I can't smell petrol, the spark plugs never dampen.

Is there no exhaust emission at the not running cylinder ? I would still check the valve clearance.

seanthomasking said:

The fuel taps/filters are brand new as are the stainless braided fuel lines, both sides flowing fine. Sorry to troubleshoot so late.. I can't smell petrol, the spark plugs never dampen.

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Have you tried swapping the carbs to see if the problem moves?

Funny I just had a similar problem on my Trident. One pot had white ex valve and white interior on header pipe.
It ran well enough. Discovered extremely loose main jet on the problem cylinder.
Lucky I didnt hole a piston but no damage seen upon removing head with cylinder piston or valve and seat.
Just a thought....

There is emission . No black smoke. Just " pantering". But the exhaust pipe is touchable and not hot AT ALL.

Brand new premier carbs, working perfectly. changing them over would be obsolete.

Hands behind both exhausts- the one in question feels like it's gasping for air. The other, put put put put - like it should.

Sounds like an ignition issue. Possible problem at the pickup. Try switching those two wires and see if the problem switches sides. If compression is good, fueling good and ignition good, both cylinders should run the same. Somehow, you're missing something.

Danno said:

Sounds like an ignition issue. Possible problem at the pickup. Try switching those two wires and see if the problem switches sides.

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I seriously suggest he does NOT try to swap the Tri-Spark module wires.

Even swapping the wires on a conventional Boyer, Pazon, etc., where the pickup is a separate component would not transfer the problem to the other cylinder, only alter the timing considerably and 'fix' the advance.

seanthomasking said:

Brand new premier carbs, working perfectly. changing them over would be obsolete.

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How do you know that?

Are you certain the right hand carb slide is moving upwards when you twist the throttle? Maybe you have a broken cable end inside the splitter. What happens if you give it a little choke when running? Does gas overflow from the RH carb when you press the tickler? Are you still using the stock coils?

Paddy_SP said:

Have you tried swapping the carbs to see if the problem moves?

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Difficult (although not impossible) to temporarily swap Commando carbs over, (if that's what you meant, and not swap with a substitute) because the carb bodies will clash, if left and right handed carbs are swapped over.

Often it is advisable to check your last work steps. Perhaps the needle clip isn't located correctly or something like that.

seanthomasking said:

The fuel taps/filters are brand new as are the stainless braided fuel lines, both sides flowing fine. Sorry to troubleshoot so late.. I can't smell petrol, the spark plugs never dampen.

Click to expand...

Seems like it must be fuel related if your plug is dry and you can't smell gas
Craig

I know when I have complicated problems, my mind races with all the complex theories of which combinations of issues are causing my troubles...

I have learned to test for the easy stuff first, even though the symptoms don't always point to the easily fixable or easily identifiable problems...

So...
-Install 2 new plugs and run the bike (I've had an intermittently bad plug that was torture to detect)
-take a can of carb cleaner and spray around the intake manifold and carb while the bike is running to see if it has any effect. I had a cracked manifold that gave me a misfire that I found with this method.
-visually inspect that both of the carb bodies are going up and down when you twist the throttle so you know the cables are working
-check the battery voltage
-check the resistance of each coil
-swap the coil leads (which you did already)
-get the bike running and pump the bleeder on the carb for the cylinder that is misfiring to see if it causes a change to how the bike runs
-add another ground directly from the battery to the engine and coils
-check the static ignition timing (just so you know that the magnetic rotor didn't slip and move)

Moving on to more invasive proceedures
-compression check
-check the valve gaps and valve lift as you turn the engine over by hand

My guess is cracked or leaking intake manifold, if you have good compression in the cylinder that isn't firing. OR a valve issue if you don't have good compression.