I am thinking about ordering new Mikuni jets for my '75 RD350 and I have a few questions:
Is the baffled air box a handicap on my stock (except for Bassani chambers) '75 RD350? Is there any real advantage to changing to the earlier style box? I may know where to get one if it is worth the effort.
Do you normally jet up for chambers? If so, how much?
Do you jet a combo like mine as was original given that today's fuel is so much different than 50 years ago? My right hand plug looks good as is. But I am fighting a fouling left cylinder and am just throwing EVERYTHING I can think of at the problem.
How do I identify the baffled from the non baffled? I tried the search, but didn't have any luck.
It currently has the yellow foam air filter, UNI I think. Is that one okay? If not, please recommend one.
Thanks guys! :thumbs:
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its not the box, its the filter itself
the later box breathes just fine
earlier ones w an unbaffled filter run 140 mains stock so, its a better option if you still have the baffled air filter in it. I would imagine the stock af is long gone though
Wasn't there a factory notice showing how to make the boxes breath better?
I seem to remember that.
Quote from: m in sc on January 26, 2026, 09:04:29 PMits not the box, its the filter itself
the later box breathes just fine
earlier ones w an unbaffled filter run 140 mains stock so, its a better option if you still have the baffled air filter in it. I would imagine the stock af is long gone though
So, just to be clear; my bike has a yellow UNI filter. So that means that I am not dealing with anything baffled and I need to run the larger main jet. Do I have that right?
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that was an early tsb, yes. the lid front gets opened up. the later lids have an 'overbite' of sorts. I 'think' it was for r5s though
yes. correct.
Thank you very much! That takes the guess work out of much of what I am trying to do.
Do you jet up when any when you install chambers when all else is stock? I will monitor the plugs, but I just wanted to get a sense of what to expect.
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yes. start at 150 -160 mains all else stock except pipes.
:thumbs: Thank you............AGAIN!
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Still dealing that left cylinder?
Not directly relevant as such, but:
Did the 73 RD perform better in period tests than the 74/75?
"Restricted" air flow and #120 MJ vs more open air filter and #140 MJ.
Also, my A model (74), although sold as a 75 in Norway, as no B models were imported, had a #105 MJ installed when I first worked on it. Apparently installed by the dealer at a service way back in the 70s. Possibly supposed to be a retrofit of baffled filter and #120 MJ, but somehow smaller jets were installed.
Quote from: quocle603 on February 04, 2026, 08:17:01 PMStill dealing that left cylinder?
Yep. I am trying a number of things at once right now. That is not the way I normally approach these things, but I am ready to put this issue to bed.
I am swapping a right carb to that left cylinder to definitively eliminate that carb as the problem. I have completely disassembled all of the right carbs that I have and selected the best pieces from them, cleaned them and am now waiting on new Mikuni main jets so that I can reassemble it.
I also am trying one step hotter plug on that cylinder, a BR7HIX. The Iridiums do not foul as quickly as the copper and I can clean them several more times than the coppers.
I also installed a new battery to make sure it has all the voltage it needs.
I cranked it over with a spark checker and I have good blue spark to over 1/4 inch on both cylinders at 12.5v. Although, the right seems to carry the spark to just a bit farther gap than the left, but not much. Also, that was static, without the engine putting power into the battery. So running, it should have a bit more spark. I also checked how far the spark would jump with different voltages in the battery by draining and charging the battery between 12v and 13v and the differences were dramatic. Not surprisingly.
I also checked the timing AGAIN and set it at 1.9 this time rather than 1.8. I always run 93 octane and it is cold here so I don't think I am on dangerous ground. I also don't think it will matter. But I am trying everything I can think of.
And all this is on top of the extensive changes that I have already made. If this actually addresses the issue, I plan to back track and start changing one item at a time back to see if I can isolate what it was. That is IF I can get that left cylinder to run as clean as the right.
Sounds like something is missing seems like youre trying everything to fix it. Yeah, the one controlled variable is the best method to truly understand correlation and causality. Seems like people have been providing you a ton of support and suggestions.