Finding my standard...

So today I spent some time buttoning up some loose ends. Tightening fasteners, making sure everything is ready to ride. I also made sure I was getting a good ground on the top motor mounts. Used the dremel to clean off the powder coating and ran a separate ground wire to the coils. The coil mount is the Home Depot grounding clamp, but I am unsure it is actually getting grounded through the powder coating. I was also out in my storage shed when I saw my levers on my eternal tracker project and thought those might look nice on my 75. They came in a box of spare parts from an old Yam ty250.


They were a direct bolt on part and mated perfectly to my stock controls. They have a more aggressive look to them. They were very raw finish, but a few minutes with some Blue Magic and they shined right up.


After all this I gave the bike a nice bath, and since it’s a beautiful day I took her out for a short maiden test ride to a few local spots I had scouted for some potential calendar photo shots. She still has an intermittent miss on the right cylinder, but runs well under acceleration. Almost felt like an intake leak, but once I got home I checked with brake cleaner but no indication of leaks. Weather is beautiful here this weekend. Sunny and mid 60’s. I love it. Hope to get a few more miles on her and work out some bugs. Overall I am pretty happy with the way it rides, other than that miss issue and a rear brake that needs some attention.

Still battling this misfire on the right side. The bike runs good under acceleration, but at idle and deceleration i can hear the miss. It is getting fuel as you can smell the unburnt fuel through the exhaust at idle. The Pamco and coil seem to be working properly, although I likely need to fine tune the advance setting. I tested for an intake leak on a hot motor with brake cleaner but saw no indication of a leak. I will be testing it again on a cold start up to see if i can get a different result. Cam chain is adjusted. Valves are adjusted at 6/9. And I still have this "squeeking" noise coming from the right exhaust when the misfire is occurring. Am I mis-diagnosing this and possibly it is a carburation issue? The right plug is fairly sooty, but I would assume that is being caused by the extra fuel not being fired. Left side is running well, but possibly a little lean based on the plug condition. These plugs are after about 60 miles of riding.

Have you tried swapping plugs side to side?

Double-checked the float height on the affected cylinder?

Yes, i forgot to mention that. Swapped the plug wires and the condition did not follow the wires. It is consistently on the right cylinder. Also compression reading are good at 140/145.

I have not checked the float heights. But now that you mention that, I have noticed after I close the petcocks and come back the next day that the fuel line is empty. I had sent these carbs out to Rick West at OldSkoolCarbs. I never bothered to check that when I got them back from him before installing.

Yes, do check your float level. I don't know what brand parts Rick uses but original Mikuni or Yamaha float needle and seat assemblies are best. The aftermarket ones can be hit and miss, sometimes working OK, other times leaking right out of the box. My dealer told me he quit using them because too many bikes came back leaking.

Do you have the clear tube tool for checking float fuel levels externally? It works really well .....

https://www.ebay.com/itm/Carb-Float...905378?hash=item421702dde2:g:-AwAAOxyeR9TKGci

Thanks for the link to that tool, I just purchased one. I'll report back on the findings when it arrives next week. Fortunately I do have a spare set of parts carbs if any parts are needed. In the meantime i'll examine for any intake leaks.

It may be worth checking the starter jet on the sooty side. It means unscrewing it from the top to make sure there are no foreign objects allowing fuel to dribble through when it’s supposed to be closed.

Clear tube is a great method but be sure there are no bubbles in the line, an air bubble will throw the apparent level off.

Also level the bike side to side as best you can. I place a torpedo level on the front motor mount and stick some shims under one of the centerstand legs if need be.

With float level tool in hand I proceeded to the shop with a short amount of free time tonight. Bike is very close to level and both float heights measured out right about gasket level on the bowls. So I decided to pull the right carb and have a look around.


Everything looks very clean. No debris was found anywhere. Mixture screw was out 3/4 turns as it should and jets were clear. I pulled the float and needle, all looked in great shape too. The only thing that looked suspicious was the fuel crossover hose. As you can see it has a serious kink in it. I remember having a hell of a time getting it on and clamped. But I wouldn’t think this would cause a problem since both petcocks are operating as they should.

Next up is to pull the intake boot and give a good inspection before reinstalling the carb. Also giving the carb guide another read for good measure.

Intake boot looked good and is pliable with no cracking. I pulled all the jets again (right carb only) and was a little surprised that it is running a 125 main / *45pilot. From what I have read, the stock main is a 127.5. Since I am running stock airboxes and free flowing exhaust I would guess that I should be running at least 127.5 or 130's. Does this sound right? Curburation is a bit of an Achilles heel for me, but I am learning. Going to pull the other bowl and check those jets also to be sure they are the same.

*edited to correct jet size.

willis said:

Intake boot looked good and is pliable with no cracking. I pulled all the jets again (right carb only) and was a little surprised that it is running a 125 main / 145 pilot. From what I have read, the stock main is a 127.5. Since I am running stock airboxes and free flowing exhaust I would guess that I should be running at least 127.5 or 130's. Does this sound right? Curburation is a bit of an Achilles heel for me, but I am learning. Going to pull the other bowl and check those jets also to be sure they are the same.
View attachment 176262 View attachment 176263

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I'm not an expert on the 38's Willis, but I think that's the wrong type of pilot. That looks like a VM22/210. If those are the stock carbs for the 75 they should be the BS30/96.
Maybe someone with more 38 experience will correct me.

From what I have researched my pilot looks like the BS30/96 style. As you can see, the holes are offset/staggered like the pics you attached. The VM22 style has dual holes that are not horizontally staggered.

willis said:

From what I have researched my pilot looks like the BS30/96 style. As you can see, the holes are offset/staggered like the pics you attached. The VM22 style has dual holes that are not horizontally staggered.

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The "tell" is the metering orifice. Is it at the top where the threads are or at the bottom?

I'll pull it again on my lunch break and have another look here shortly and report back. I overlooked that part of the diagram.

After lookin at your pic again, looks like it is the BS30/96. Might be worth looking and double checking the orifice.... but it looks like I was wrong. Mea culpa.

Nice jet pics, and they reveal a few things. First, your pilot is a #45. What you're thinking is the #1 is simply the right side of the Mikuni "square-in-square" emblem stamped before the actual size number .....



As Jim said, to determine the type, you need to look at the ends. I think yours are the correct type because of the more tapered end they have, but check to be sure .....



But, notice the textured appearance your pilot jet has. That's usually an indication that the jet sat for a long period of time all gummed up. The fuel residue and varnish has actually eaten away at the jet or "corroded" it. It's possible this has happened to the inside too, to the metering orifice. So, that jet may be larger now, flowing more fuel than it originally did. I'd be replacing them.

Now let's talk about the main. The metering orifice through yours looks quite large for a #125 and I'm thinking maybe someone drilled them out. The sides of the hole are usually smooth but yours seem to show some "tooling" marks. I'll have to pull out my jet stash and have a look. I think I may have some new 125's I can check. In the mean time, here's some much larger 145's. Notice the metering orifice size looks very close to yours and that it's sides are smooth .....



Yes, with your freer flowing pipes, you'll probably need one or two sizes above stock for the mains. I can't say exactly what size, experimenting is the only way to figure it out for sure. So, I'd get some 130 and 132.5 mains and start playing around. The 45 pilot may be fine, as long as you don't have the needles set leaner than stock. Many times when increasing the mains, you incur upper midrange break-up due to those larger mains bleeding over into the upper midrange and making it too rich. You "fix" that by leaning the needles a step. But this also leans the upper part of the idle circuit and that can cause a flat spot just off idle. If that happens, then a larger pilot (47.5) would be needed.

5twins said:

The metering orifice through yours looks quite large for a #125 and I'm thinking maybe someone drilled them out. The sides of the hole are usually smooth but yours seem to show some "tooling" marks.

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Yeah... definite signs of being "modified."