Please Help w/ Mikuni Carb Settings

AussieCombat said:

Next down the line is the Needle and Seat, it lets ALL the fuel into that carby and just like a dirty Tap or gauze filter,
it must be clean. Has the Float level been checked, They don't often move but stranger things have happened.
Low float level can give difficult starting, and a general leanness right through.

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Well, here's an update. Last Friday night, I checked fuel flow (good), and adjusted the air mixture screw. The next morning, using full choke, she started the first kick. First time that has happened in a long time, so I was feeling optimistic. I rode it about 40 miles to the rally and she ran perfectly. After sitting about 3 hours, I started off and immediately got missing/popping during acceleration (damn). That night, I pulled the carb, removed the needles and jets and cleaned everything w/ carb cleaner. The next day, I rode 250 miles home, and it was the same deal. It ran great for about 225 miles, but then began to act up. At this point, I am convinced there's some trash in my fuel tank. . . BUT, here's a new twist. On more than one occasion, I have gone off and forgotten to turn off my fuel petcocks. Usually the result is coming back to a strong fuel smell, or even a fuel stain on the ground. It doesn't always do it (at least not right away). There is obviously some issue with the float needle or seat that I need to address (I also need to check the float level). Is it possible that a sometimes sticking or leaking float needle/seat is causing an over-rich condition/flooding that might explain this?

I still intend to dump the remaining gas from the tank into a bucket to see if there is any trash in there, but I sure didn't see anything in the carb when I had it apart. What is the consensus on the use of those small in-line fuel filters? Do they work, or do they create problems of their own.

AussieCombat said:

Also, some engines don't need to have the 'choke' lever pushed right down, or fully open, to work efficiently.
The cable adaptor can be a good addition to English bikes as then you can easily adjust to desired opening.
Once the engine has started and been run for a while then they can be started normally.
I hope I explained this properly and is some help. Good luck.
Get it cleaned, then ride it again, then start on one thing at a time.
AC.

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Thanks for some great information on how the "choke" works. I do in fact have the Mikuni cable adapter, using the stock Commando lever. I often wondered if I could use a half position for those times the engine isn't all that cold. As I said, the other day, it started on the first kick with full choke. Later though, after the bike had sat for a few hours, the engine wasn't cold enough to demand choke. In fact, it wouldn't start with it. I then resorted to having to turn in the idle screw one turn, after which it started. Not a very elegant solution since after riding a block or so, the engine is idling too fast. I'll give partial choke a try.

My sense is that this starting issue has nothing to do with my "missing under acceleration" problem, but I'll be glad to get it sorted out.

John

Well my bike allways starts first kick, Hot cold makes no differance. Only time I had trouble it was due to low battery voltage. How are your plug wires, caps and wire connections? If in fought I would replace them. Also make sure your battery voltage is high enough to keep the Boyer EI happy. Maybe your charging system is just a bit weak, so after a while riding the Battery getslow enough for your Boyer EI to get a bit cranky and act funny. This has happened to me running high watt bulbs & spending to much time at low RPM's around town. I had to leave the head light off and hit a bit of high speed freeway to recharge.

Howdy John,
Good talking to you this afternoon, do check the float level, and float seat/needle as we discussed, but also check (FIRST) to make sure that your choke (enrichener) has some play in the cable when in the off position (a snap down wouldn't hurt). If you are still having the issues send me the carb and I will give it a full examination and a going over. Torontonian and AussieCombat have raised the next level of concern, that being your ignition system...What do you have for ignition? I had a '79 Bonnie that mimicked these symptoms and it ended up being the stator plate.

RS

Bonwit said:

There is obviously some issue with the float needle or seat that I need to address (I also need to check the float level). Is it possible that a sometimes sticking or leaking float needle/seat is causing an over-rich condition/flooding that might explain this?.

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Yes. Recently a friend with a 750 using a VM34 had almost identical carb problems to yours. It turned out the float was signficantly out of spec. That's somewhat unusual on a Mikuni. Once re-set the bike starts and runs perfectly and consistently.

In regards to the in-line fuel filter, it certainly helps. I don't use one, but run the fuel lines down from the taps to a T junction and then up and over to the carb inlet. This lets any fine crud collect in the bottom of the hose junction. I slip off the hoses and blow them out once a year and that's seems to work fine.

Mick

RoadScholar said:

Howdy John,
Good talking to you this afternoon, do check the float level, and float seat/needle as we discussed, but also check (FIRST) to make sure that your choke (enrichener) has some play in the cable when in the off position (a snap down wouldn't hurt). If you are still having the issues send me the carb and I will give it a full examination and a going over. Torontonian and AussieCombat have raised the next level of concern, that being your ignition system...What do you have for ignition? I had a '79 Bonnie that mimicked these symptoms and it ended up being the stator plate.

RS

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Thanks for all your help. I'm looking forward to checking out the float assembly tomorrow. Regarding the choke cable, it does appear to me that there's plenty of slack there.

I'm using a Boyer ign. system. The battery's age is unknown to me, but I guess it's more than 3 or 4 years old. I did have it load tested about a year ago and all was okay.

The problem with under-voltaged EI systems is their tendency to drift to full advance. Speaking from my own experience, this shows up when the motor is under load, and the rpms drift downward below, say, 3500 RPM (I came home the last 5 miles last year on my Victor in first gear; undercharged battery). If you find yourself needing to downshift on even the most modest grades, check the battery. An undercharged battery degenerates remarkably when the headlight is switched on, say from 11.5 volts to below 10 volts, as gtsun has already stated. Measure the battery voltage at idle with the lights on.

Sounds like your getting close., Perfect time to fit fuel filter (s).
Mikuni floats have been known to stick but not often, usually crap related, not necessarily rust but
that gooey, almost varnish type stuff that comes free of charge with modern fuel.
Needle and seat OK ? The little brass hinge on the float can wear, but not likely.
Filters will pick up a fouling problem straight away.
Then you can start on.... Borrow a battery, coils getting hot...........................
Just bloody annoying isn't it ?
It's taken me 3 months too get everything together for my rebiuld, missing gaskets, etc,etc and now,
me ol mate, who has stated the bottom end for me thinks he's going on holiday. Pigs arse.
AC.

I have experience with three Nortons equipped with single Mikunis. My own 850, a freinds S type, and my friends 750 Combat. All of those bikes ran the needle with the clip on notch above center, leaning the mixture slightly, in the mid range. Actually with Amals I have found the needle clips are always on notch up too. I think this is a characteristic of the Norton engine (with stock cams anyway). Triumphs nearly always use the center groove. My bike has a 230 main I think. All these bikes ran very well. It must be true that the top end is ruined by the single carb, but on my 850 I don't notice much difference. I seem to be able to ride fast wihout ever taking the engine over 5000 RPMs. I know a lot of people probably hear this and think I am an old guy, but actually hardly anyone keeps up with me. My bike actually seems faster than some others, but it may just be the way I ride it.

The adjustment of the air screw makes a huge difference to part throttle acceleration, with the Mikuni, with the Amals it doesnt make much diference except to idle.

With the Mikunis I would gradually turn the screw in until I got nice acceleration at part throttle. The air screw made a huge difference to this area of running. It is an area that is often in use around town too.
On my friends S type I had to file the slide cut away a lot before the engine would stop fouling the plugs. I have forgotten what the corretct cutaway is. I didn't have to adjust that on the other two bikes.

Nigel