Misfiring T90

[T90 Arrow]

Hi all, just an update on my recently acquired misfiring 1967 T90. Sorry if you’ve read this on other Triumph sites but I’m trying to get it to as large an audience as possible. It starts ok and runs well up to about 4500 revs then starts to misfire, really noticeable if you drop a gear to overtake.
I’ve changed the plugs, points, condensers.
The carb is a 376/272, should be a 376/300, don’t know if that will make a lot of difference but have cleaned it out and replaced the 160 main jet with a 180 as per spec.
The coils are Japanese with sealed plug leads so they should be OK
Left cylinder is 100psi, right is 110psi BUT left plug is clean, right plug is sooty.
Have taken it for a quick spin after each attempt but no different.
HOWEVER today I tried something I should have done earlier, Just before I tried to wind it up I put the choke half way on and it accelerated without a problem. I’m no mechanic but I think that indicates a weak mixture.
1 Any probable causes and
2 Why would one plug be sooty (have changed them round).
Thanks, Barry

 

[gumpt1oo]

Hi Barry, another Cumbrian!
I had a similar symptom on my T140, It turned out to be the coil breaking down under load. It was OK up to about 3500, then the misfire started.
If the right coil is going bad that could be the reason for a right sooty plug.
Might be worth swapping the coils to see if the left plug then goes sooty.
Not sure if yours is the same problem though if you say it improved with the choke on, that does sound more like a fuel problem.
If you've cleaned the carb out and put a bigger jet in it, I wonder if you have an air leak around the carb/manifold?
Cheers
Steve

 

[gumpt1oo]

Just another thought, when you said you put the choke half on, I wonder if you already had the choke on, and turned it half off.
The choke works backwards, when the cable is pulled tight,the choke is off, when the cable is fully released the choke is on, if that makes any sense!

 

[grandpaul]

90% of carburetor problems are electrical. 90% of electrical problems are carb-related. hee hee

gump is on a good suggestion, swap coils and see if the problem follows the coil.

Also, you may have a blockage in the carb. A one-gallon can of Berryman's Chem-Dip will be a great fix. Point a fan (on high) to blow the fumes away from you. Tear the carb down and put all metallic parts to soak for 1/2 hour. Wash thoroughly and blast clear with compressed air.