HM 341 Exhaust drain holes

Does anybody know if Honda ever drilled small drain holes in the lowest point of the HM 341 exhaust?

Thanks Scott

I've read that there are small 2mm drain holes in each pipe but never looked for them. Typical rust spots for the 341s are at the header to muffler seams and at the ends on all four pipes.
I've read that a bit of 2-stroke oil with every fill up or so will coat the insides of the pipes and help lubricate the fuel system.
Once my bike is back on the road this spring I plan to use Marvel Mystery Oil with every other fill up or so. I use it in the oil and fuel on other vintage/antique carbureted engines that sit from time to time. The few times I've taken things apart on those, it's all been impressively clean and I've never had any issues with pipes rotting other than a couple blown out Simthys that had premature paint loss and rust formation on the outside too.

MMO had been used by mechanics for decades. Great stuff

Scott750 said:

Does anybody know if Honda ever drilled small drain holes in the lowest point of the HM 341 exhaust?

Thanks Scott

Click to expand...

These are the original 341s on my '74 K. Two on the upper pipes and one on the lower inner pipes.

My Yamiya no-nos have one hole in about the same spot on each pipe.

Hey guys, sorry for the long delay. Here are the images of my HM341’s and their associated drain holes. Please feel free to comment on what you guys are seeing and how this relates to your bike.

My pipes.
SL, SR have two holes each.
IL, IR have one hole each.

I have arranged the pictures by the pipe position (SL, IL, IR, SR) and its associated drain.


My bike - 74’ CB7502311671