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Author Topic: Anatomy of a HM341  (Read 1311 times)

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Offline KayOne

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Anatomy of a HM341
« on: November 20, 2013, 10:16:51 pm »
Had a set of left side 341's that were pooched. So, for the betterment of science and SOHC.net I pulled out the  sawzall and sectioned the lower pipe.....don't torch me purists, I have the photo proof they were beyond repair.

Seems to me I have read on this forum that the 341's had five chambers. This pipe has four if you include the last chamber that the "baffle" is fastened to.
 
I had no idea that the header pipe carried so far into the muffler. You may also notice it is perforated for a portion of its length. The first separator plate (of three) carries a tube that connects to the third chamber. This tube reduces in diameter as it enters the 3rd chamber. The first separator plate has two small holes drilled in it that carries pressure to the second chamber. The second separator plate has larger holes than the first plate (still 2 holes).

The last tube is slightly less in diameter than the first one, but is constant in its diameter. The last separator plate has four larger (1/4") holes.

It looks like the second chamber is the most restrictive as virtually all the flow (except for two 1/8" holes) must pass through the flow tube that has a diameter reduction at its end.

I used this info and a extended length 3/8" drill bit from Home Depot to drill three additional holes in the first and second separator plates. I also fabricated an extension to get the bit into the recesses of the muffler. Use the four holes in the last separator plate as a guide to drill through plates 1 and 2. You have to be careful and set the drill with a hammer first so that it doesn't spin off to the sides and mar the outer casings.

As far as the rust pattern on the pipes, water seems to have accumulated at the header/muffler joint before the perforations, after the perforations you can plainly see where exhaust heat kept the metal dry. The separator plate flanges look like they trapped water behind them, causing corrosion by the spot welds. Maybe a combination of short rides and the restrictive baffles that didn't allow enough exhaust/heat through them was the downfall of these pipes.

My 341's won't flow as much as the HM300's but at least they will make lots of noise now :), besides, I don't have a lot of $ into the pipes and the set are really quite nice.


http://i1283.photobucket.com/albums/a555/kayone71/sept13066_zps7ade6244.jpg[/IMG][/URL]



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1984 VF45F Interceptor - all original, 12000 km
1968 S90 - all original, 2100 miles
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Offline dave500

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Re: Anatomy of a HM341
« Reply #1 on: November 20, 2013, 10:49:24 pm »
reverse flow chambers.

Offline nccb

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Re: Anatomy of a HM341
« Reply #2 on: November 21, 2013, 03:39:16 am »
Thats pretty cool.  I may not be thinking of this right, so you drilled holes in the first plate then extended the bit through the holes you just drilled to drill the second plate?

Offline HotCarl73

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Re: Anatomy of a HM341
« Reply #3 on: November 21, 2013, 05:22:32 am »
thanks for sharing. now i don't have to crash my bike so i can get a look inside my pipes. ;D
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Offline KayOne

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Re: Anatomy of a HM341
« Reply #4 on: November 21, 2013, 08:48:13 am »
nccb, I drilled from the outermost plate behind the baffle using the four existing holes as a guide through plate two and then on to plate three. If you open up the holes on the first two plates it makes it easier to gain access to the third plate.

I don't know if it will have any improvement on performance. If any one has any thoughts please chime in. Maybe I wasted a Sunday on this job :)
CB750 K1 restored
1979 CB750Fz - original except for exhaust, 14000km
1984 VF45F Interceptor - all original, 12000 km
1968 S90 - all original, 2100 miles
1973 H2a, Restored
1973 H1D, next project
CB750K1 (sold)
1976 KZ900 (sold)
1981 CB900F (under restoration)
2015 Yamaha FJ09, my appliance rider

Offline Irukandji

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Re: Anatomy of a HM341
« Reply #5 on: November 21, 2013, 04:10:28 pm »
I really like that the picture of the muffler joining at the weld to the header, shows I could probably drill another weep hole closer to the weld to eliminate water settling. One of the most rust-outs places other than the baffels.
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Offline Jerry Rxman Griffin aka MuthaF'er

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Re: Anatomy of a HM341
« Reply #6 on: November 21, 2013, 05:00:02 pm »
OK, now who has some 300's to open up?
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Offline fmctm1sw

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Re: Anatomy of a HM341
« Reply #7 on: November 21, 2013, 05:53:58 pm »
very nice.  I like things like this..
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Offline HondaMan

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Re: Anatomy of a HM341
« Reply #8 on: November 21, 2013, 07:12:10 pm »
There are/were actually 3 different versions of the HM341, at least up to 1975 when I quit keeping track of them. The first were like yours, we called them the "3-baffle" pipes. They had the best performance of the HM341, and that's what my very early K2 had when new.

Soon after that, they came out with the ones we called "7-baffle" pipes. These had a shorter pipe into the muffler and it did not have the diffuser holes in it like yours does. The pipe ended about 2" in, and there were 7 visible sets of spot-welded baffles on the muffler. These rusted out so fast that Honda stopped making them in about 4 months, and many were warranteed-away so Honda would not "lose face" over it. They rotted right at the upsweep where the pipe joined the muffler, and at the next 2 baffle spot welds on the insides (i.e., bike side) of the pipes. They were VERY quiet, while they lasted!

Then came the ones that seem to have remained the longest, and are being replicated in the ones we've seen recently from a cloner somewhere, maybe in Thailand? These were called the "5-baffle" pipes, and they seem to be in 2 variations: the earlier ones have no inner diffuser on the end of the pipe, but the pie sticks in about 4" inside the muffler. These have a throaty note that comes from the drain hole that is punched from the inside-out at the upsweep bend on the mufflers. These have 5 visible sets of spot welds along the pipes to hold the baffles, but I have not seen one apart, yet. the other variation has a diffuser end on the pipe, much like the one you show here, and 4 baffles that can be seen as spot welds along the mufflers. These first appeared on the later K3 bikes and were most of them until I quit tracking in 1975. They last a long time, are not too restrictive (due to the reverse-flow design) and only the #2 pipe tends to plug up its too-small drain hole and rot at the upsweep site. The chain lube usually clogs these holes shut, on the left pipes (a good place to keep an eye on!). These made the 55 MPH touring more practical during that era, because they both boosted the midrange torque (3800-6500 RPM range) and let Honda install a #105 mainjet to 'help' everyone save gas when we were suffering the Arab Oil Embargo of the later 1970s.

I had the 4-baffle, 5-chamber pipes on my 750 until 3.5 years ago when I bought a set of the clones. These are lighter metal, lesser chrome, and a little louder overall than the Honda version, and need a slightly bigger mainjet (like 2.5 size bigger than whatever you had before) to cool off the engine a little at hiway speeds. This hints that they don't have a good reverse-flow pattern inside, so there is more header effect 'sucking' on the exhaust valve at speed.
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Offline KayOne

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Re: Anatomy of a HM341
« Reply #9 on: November 21, 2013, 07:55:25 pm »
Thanks HM.

We need a volunteer to section the later variations of the HM341......if someone gets bored this winter maybe they can document this. My replacement left side 341's are externally identical in every way to the right side pipes so I  have a complete set of early three chamber mufflers. :)

If the reproduction pipes are of lighter gauge metal and poorer finish (and who knows about internal construction), maybe the original 341's should be considered more desirable than they are now. I was thinking of buying a set of reproductions in the future, but if these are better pipes I will stick with them.
CB750 K1 restored
1979 CB750Fz - original except for exhaust, 14000km
1984 VF45F Interceptor - all original, 12000 km
1968 S90 - all original, 2100 miles
1973 H2a, Restored
1973 H1D, next project
CB750K1 (sold)
1976 KZ900 (sold)
1981 CB900F (under restoration)
2015 Yamaha FJ09, my appliance rider

Offline CycleRanger

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Re: Anatomy of a HM341
« Reply #10 on: November 21, 2013, 08:38:23 pm »
Neat project!  Yeah, it would be fascinating to see 300's and the other 341's.
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