Carburetor Identification

[fabricgator]

I have been wrestling with carburetors for the past few weeks.

With all the information available here, I was confident that I would be able to clean and adjust my carbs to perfection... Operative word: WAS

In the past, I had not removed and cleaned the pilot jets as they were stuck good, and I thought I could just give em a good squirt. Now that I know better, thanks to the info, I know that they were the reason I could never get the machine to run great.

So, with the determination of a sophomore on prom night, I began disassembly and cleaning, determined to remove and replace the pilot jets. I removed two with a customized screw driver and they were all junked up. PB Blaster soaked, heated, torqued, repeat until I skinned the slots off of the other two, then screw extractor, PB blaster soak, heat em, repeat until I finally drilled the #2 one out.

Not thrilled with my skills and prowse, then I broke the screw extractor off in the #3 carb pilot jet, then cracked the jet tower.... what a mess. I used to be a good wrench.......

1) Could I ask you guys to help me understand the different variations of these Mikuni 34mm carbs and the model years they came installed on.

2) How can I identify by visual inspection or placarding the different models of carbs.

3)How messed up would it be if I were to assemble a mis matched rack of carbs and could I ever get them to run right?

4)Could the carb bodies be used in different positions on the rack if I change over the butterfly linkage to make it all work? Say a #1 carb body used in the #3 position... I see that the #1 and #4 carbs are machined for the enrichment circuit shaft ball and spring, are there any other caveats?

I ask, because ideally, I would like to get additional carbs that could be used to complete this project AND have some spare parts for future repairs. I am talking about an ebay sale, or we have a motorcycle recycle junk yard. I would prefer to get a matching set to compliment my current inventory.

How much is my bike worth in parts?

[DGXSER]

Lets take these one at a time.

1) Could I ask you guys to help me understand the different variations of these Mikuni 34mm carbs and the model years they came installed on.

There are two different styles of carbs (basically). 1) The older style which have three holes in the inlet bells. Two Ts, and upper and a lower between each set of carbs. The pilot jet is capped off so fuel will flow from the main jet to the pilot jet. The idle set screw is on the airbox or inlet side of the carbs. And the idel mixture screw extends above the tower it is in and is visible. 2) the later model carbs which have 4 holes in the inlet bells. One T for fuel only between the carb sets. The pilot jet is open to the fuel bowl and no crossover between the main jet tower and the pilot jet tower (usually, sometimes it is there on the early 80 model carbs). The idel adjustment screw is on the engine side of the carbs. And the mixture screw is down inside a tower where it is not very visible.

2) How can I identify by visual inspection or placarding the different models of carbs.

See answer above

3)How messed up would it be if I were to assemble a mis matched rack of carbs and could I ever get them to run right?

Not sure it makes any difference if they are all of the same types, early or late or the bastardized early 80s listed above.

4)Could the carb bodies be used in different positions on the rack if I change over the butterfly linkage to make it all work? Say a #1 carb body used in the #3 position... I see that the #1 and #4 carbs are machined for the enrichment circuit shaft ball and spring, are there any other caveats?

Well, there are plugs and openings to accept the fuel and or fuel and vent Ts. So No 2 could work in no 4 position, but no 4 could not work for two as it would not have the vacuum nipple for the vac advance. No 1 might work for No 3, but No 3 would not work for No 1 as it would not have the mounting set up for the choke lever. Then as you mentioned the hole for the ball and spring to lock the choke lever in place.

How much is my bike worth in parts?

Sadly, probably more than if it runs well.

[crazy steve]

4)Could the carb bodies be used in different positions on the rack if I change over the butterfly linkage to make it all work? Say a #1 carb body used in the #3 position... I see that the #1 and #4 carbs are machined for the enrichment circuit shaft ball and spring, are there any other caveats?

I can answer this, as I'm in the middle of rebuilding mine and broke off the lower mounting boss for the choke, so I looked at this....

With your '79, I think you're pretty much restricted to '78-79 carbs for swapping carb bodies unless you go newer on all of them. The only difference in the '78-79 is jetting I believe. As to swapping positions, that's doable with some work.

The '78 castings are all the same (and I assume the '79s are too), just the position machining is slightly different. As you noted, the detent for the choke will need to be drilled in a swap, but I don't see that as a big deal. As far as swapping the linkage, you'll also have to move the butterfly shafts, washers, etc as these are differing lengths. The choke mount and idle speed screw bracket mounts are the same, but #1 isn't drilled/tapped for the idle speed screw; only #3 is. This would be the tough one to fix; because of the length of the screw, getting this hole aligned right with the support bracket would be fairly critical. And finding the right tap may be a bit tough too. If you were willing to go with a shorter screw and lose the support bracket, then I'd say no problem. Even swapping 'left' and 'right' could be done (i.e. 1 for a 2 or 4) if you tapped and plugged the existing fuel and vent holes then drilled out the other side; the bosses are already there.

One mod I'm making is I'm retapping the choke/idle screw bracket mounts to a 8/32 screw; those little bitty phillips they used were a b***h to get out. Swap those puppies to allen screws.

Oh yeah... the broken choke mount. I fixed that by epoxying a 6/32 nut into the unused vent hole, then drilled/tapped it to a 8/32 all the way into the carb body. Rework the choke lever mount to match and I'm home free.

'78E original owner

[fabricgator]

Thanks Don and Steve,

I just want to have this completed and done right.

I learned how to fix cars as a kid, because I could not afford to pay someone to do it for me. Now that I can afford it, I am usually not thrilled with the work I get, probably because I know what it should be.

I prefer to keep things stock.... Yamaha for Yamaha.
Its my parachute..... my life depends on it. I'll pack my own.

When I became an aircraft mechanic, it made me meticulous, aware, thorough.

The weather here has been beautiful. I envy those riding by on the weekends.

Thanks again for your assistance!

[crazy steve]

I learned how to fix cars as a kid, because I could not afford to pay someone to do it for me. Now that I can afford it, I am usually not thrilled with the work I get, probably because I know what it should be....

Thanks again for your assistance!

Boy, I know all about that. That's why I usually do all my own work; paying good money for semi-competent repairs isn't good for my blood pressure.

I'm pretty much self-taught and if I had all the stuff I broke over the years in my quest for knowledge in a pile.... well, it'd be a hellava big pile!!

Good luck!

'78E original owner