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Bike is a 78 T-140E with twin AMAL Mk2s. So over the summer the bike was running fine, but my rocker box gaskets started leaking quite a bit, so I replaced them, set the valve clearances and re-assembled everything. Now the bike isn't running right. It pulls strong from a stop but as soon as you get up to speed at higher RPMs it misfires. I' tried re-synching the carbs which i've done before after owning the bike 10 years now, still getting the same issue. Plugs were new N4C and after I took my test ride they look lean. In neutral with the bike running if I rev it up it will miss and pop out the left side muffler. I re-checked my intake valve clearances and they were dead on. I haven't had a chance to check the exhaust valve clearances yet. Any ideas? Thanks in advance!

I wonder why the rocker box gaskets developed significant leakage in the first place.
The tightness of the bolts and nuts do need checking periodically. Both the headbolts and rocker box.

When the boxes have been removed to replace the gaskets, that would have been the best time to retorque the head (if its been running fine for years it likely doesn’t need it, but as you’re having issues I’d check it anyway).

Any new gaskets will compress a little over time, especially in the first 1000 miles after installation.
I’d suggest that you check all top end torques first, then re-set valve clearances, then see how it goes.

If the bike was off the road for some months, you have drained and replaced with new fuel?

You sound confident in carb synchronising, so I won’t waste time on that.
It would be unlikely to exhibit problems at higher throttle anyway, more suggestive of tight valve clearances, plug/coil possible but less likely.

Is it contact breaker ignition, or is this the first of the Lucas Rita?

Best of.

Do I understand correctly, that it was running fine before the service?

What all did you disturb when you were working on the engine? It sounds like you've double checked some of your work but maybe not all of it. The first thing I think I'd do at this point was put the old spark plugs back in that you removed during the service and see if it changes anything.

Sounds like an issue I had that turned out to be loose wire connectors on the ignition coils. Others on the forum have had that happen also. Check all your ignition wire connections.
It ran fine until I got up to cruising speed (40-50) at which point engine vibration would cause the connector to "dance".

My thoughts exactly. Something moved. As long as you are checking connections, clean them up and squirt a little dielectric grease into them.

Carb mount nuts coming loose letting air sneak in?

A '78 ('X' model year) T140E would originally have had contact breakers. RITA (and negative earth/ground) from '79 ('A') model year.


AMAL Mk2s (T140E) are rubber mounted.

Yes, and any air leaks would create misfires at idle and low revs anyway. As I understand it, his misfires happens at higher rpms.

SR

Looked over my connections at the ignition coils and what do you know? Connector to the left ignition coil wasn't connected. I assume it got pulled loose when I removed the battery trickle charger leads from the battery or it rattled loose at some point. Thanks for the replies from you all, I appreciate it!

Untitled by Joe, on Flickr

They have a saying over on the Honda forum ... ninety percent of carburetor issues are electrical

Lol!

glad you told us what was solution👍