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Hi Guys,

Looking at a T120, points, two 12v coils, mikuni carbs.

Bike runs only on left cylinder. Right cylinder has spark (verified with spark tester and pulled lead with bike just running on the left side) has plenty of fuel (bowl fills quickly, hand choke shows gas is being sucked up). Verified pilot and pilot passage way are clear. Cycled engine thru by hand with plugs out and can see valves opening and closing... So this brings me to my title... Can the bike run on one cylinder if the plug leads are crossed? Just figured I would ask those who know before doing any experimentation, or digging any deeper.

Thanks for any and all help,
-Rob
No, each spark is ~360 deg crank rotation apart. So if its working on one cylinder, there must be a fault with the other cylinder, its ignition or carb.
As you've got a spark on the right, the carb on that side would be the first thing to look at (particularly the cold start circuit).
After that, a compression test (actually its not a bad idea to do that asap).
Hi Rob, What does right side to if you shoot a little carb cleaner into carb throat while motor running on left side?

If lean that will bring right side to life. Not all cleaners will react, but most will.

If you are way too rich on right side plug will be super sooty or wet with gas.
Don
If you have a basic Boyer or Pazon electronic ignition it fires both sides every 360 degrees. The way they can get away with this is only one cylinder is on compression, the valves are open on the other so harmless, ie a "wasted spark". If you have one of these ignitions, crossing the plug wire should fire the other cylinder.
As noted above, it depends on the ignition system and how it is wired. There is insufficient information in the original post to provide a precise answer.
First post:
"!Looking at a T120, points, two 12v coils, mikuni carbs."
Originally Posted by htown70
If you have a basic Boyer or Pazon electronic ignition it fires both sides every 360 degrees.

Originally Posted by mondtster
As noted above, it depends on the ignition system and how it is wired. There is insufficient information in the original post to provide a precise answer.

Original post: "points".
Thanks Guys,

Koan58 you were spot on. No compression. Intake valve is not sealing as I can here the pressure come back thru the carb when I seal it with a finger and turn the motor over.

Many thanks for all the replies,
-Rob
Originally Posted by koan58
First post:
"!Looking at a T120, points, two 12v coils, mikuni carbs."

I see that now. Somehow I looked past it the first time.
There’s always some pressure back through the carburettor at cranking rpm.
Posted By: koan58 Re: Will a T120 run with plug leads crossed?? - 03/12/21 11:00 pm
TT +1.
One of the benefits of having 2 cylinders is being able to compare them. Any differences between them is informative.
Thanks again,

I should have mentioned that I did an actual compression test as Koan58 suggested in addition to the very scientific "finger in the hole approach" and the offending cylinder came back at zero PSI. I can also do a leak down test, but seeing zero on the compression test, and hearing the pressure come back thru the carb (while blocking the spark plug hole with a finger) makes me pretty confident the head is coming off.

Thanks again you guys are the best!
-Rob
Originally Posted by Robert William
Thanks again,

I should have mentioned that I did an actual compression test as Koan58 suggested in addition to the very scientific "finger in the hole approach" and the offending cylinder came back at zero PSI. I can also do a leak down test, but seeing zero on the compression test, and hearing the pressure come back thru the carb (while blocking the spark plug hole with a finger) makes me pretty confident the head is coming off.

Thanks again you guys are the best!
-Rob

Robert, you didn't mention if you had checked valve clearance...no clearance and you have the symptoms you describe. .002" clearance isn't much and can go away pretty quick if you tighten head or cylinder bolts etc. without resetting clearances. Mark R.
Mark,

You point is well taken, and so astute that I am forced to admit my own "duh" moment. Checking valve lash obviously makes a whole lot more sense then hastily removing the head in anticipation of the worst. I really appreciate you pointing out my lapse in logic, and will get those gaps checked straight away.

Thank you,
-Rob
Originally Posted by Robert William
Mark,

You point is well taken, and so astute that I am forced to admit my own "duh" moment. Checking valve lash obviously makes a whole lot more sense then hastily removing the head in anticipation of the worst. I really appreciate you pointing out my lapse in logic, and will get those gaps checked straight away.

Thank you,
-Rob

Debris can also get between valve and seat and do the same...by putting air pressure in the cylinder with valves closed and putting your ear to either the intake or the end of exhaust you can determine which valve may be leaking, (I know you said intake) one can even rap lightly on the top of rocker arm of leaking valve and bounce the valve off the seat to try and clean off carbon or whatever...the air pressure will help here. Common procedure on aircraft engines before pulling cylinders...staking the valves. Mark R.
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