1978 Carb identification

[Stefw]

Hey!

Long time lurker, first time poster - only just getting around to sorting out my 1978 after having it for a year.

It’s a 2F0 model - I’m in the UK but it’s a US import.

I’ve just been stripping the carbs and ultrasonic cleaning them but I’ve hit a bit of a bump on the jets. Someone has been in here before and torn them all up (nightmare to remove) but I’ve got them out now.

The pilots are 27.5’s but for these carbs I was expecting the vm22 style jet but it looks like these ar the older style as there are no holes? So I’m confused if these are the incorrect jets or what I need to replace with. The carbs have the correct 502 needle as I would expect for the 78 model.

The mains are so chewed up I have no idea what they are - I was assuming they’d be a 135 but now I’m not so sure - can’t see any markings on them, is there any other way to tell?

Thanks

 

[Stefw]

Some more pics of the carbs here

 

[5twins]

The pilot jets are original and are a "no hole" type of VM22/210. Yamaha must have contracted with Mikuni to have them made without the holes. They used them in several of their CV carbs at the time. You can replace them with the normal VM22/210 pilot jets with the bleed holes. They won't hurt anything and will work just fine. Yes, the mains should be #135's, best you replace them as well.

It can be difficult to find a screwdriver that fits the jets properly because they are metric sized screwdriver slots. So, I've taken to custom grinding 1/4" insert bits to get a perfect fit. For the pilots, I start out with a 4mm 2" long power bit, which fits pretty good, but can be made better with a little grinding .....

For the small round mains, I start out with a Bosch #8-#10 slotted power bit. It has to be narrowed down and shortened slightly for that perfect fit .....

 

[Stefw]

Thanks 5twins,

I have ordered the type with holes and some mains as well, all genuine. The ultrasonic made the needles go a bit funny with the coating so I’ve ordered some replacement needles as well, should see me right.

I’ve got the right screwdrivers, just a shame whoever was in there before didn’t!

Only thing I need to find now is o rings for the mixture screws as they are perished. I saw a post of your actually stating 1.3 x 2.5 ID - the ID seems a fair bit bigger than 2.5 but I guess what’s in there has shrunk over the last 4 decades. Is there likely to be different options or is 1.3 x 2.5 the go to?

Here in the UK I can only find 1.3 x 2.6 but assume that would work fine.

Cheers

 

[5twins]

Yes, that should be fine. I've used a 1.5mm thick o-ring in the past but it was a bit big. It filled the whole width of the o-ring groove on the screw and shaved a little off the O.D. as I installed the screw, lol, but it worked. Eventually I found this better size. Yamaha never sold this o-ring separately so it took a little experimenting to find it. Here's a pic of a new mix screw w/ the o-ring installed. Notice how the o-ring doesn't fill the groove in the screw completely .....

Now here's that 1.3 x 2.5mm replacement, pretty good match I'd say .....

 

[Stefw]

That’s a great help, thank you!

At least that’s one job done - I’ve got the whole bike to go over and replace all bearings/swingarm bushes etc. I’ll be doing your grease zerk mod on the swing arm for the bronze bushes.

I used to have a highly modified special and had thoughts about modding this one, but just the other day I came to my senses and it’ll be staying standard. Originally from Memphis with 2k on the clocks, struggle to believe that’s genuine but I’ll take it.

One this one is sorted I think I might gift it to my dad and then find a special I can mess about with.