Today is a very lucky day - Carburettor inspection.

[Its been a long time]

I can't believe my luck this morning. Opened up a set of carbs from a XS650D with completely unknown history. The carbs had fresh fuel in the float bowls and everything looks almost as new. I was thinking I was going to find a total disaster in there. It's my lucky day.

One question, the float needles are steel tipped, should they have a rubber tip? The needle valve assembly looks new.

I'm thinking I'll give these carbs a few squirts of cleaner, blow through with a can of air and fit them right back together again.

 

[motormike]

Double check the diaphragms .... pull sightly... look for small tears and pin holes... small hole bottom of float.. I run a wire thru it.... air and spray cleaner doesn't always work.... set float level...

 

[Its been a long time]

Double check the diaphragms .... pull sightly... look for small tears and pin holes... small hole bottom of float.. I run a wire thru it.... air and spray cleaner doesn't always work.... set float level...

Thanks, understood. The carb cleaner is dripping pale yellow from various places, seems to be working. Can of air spray too but I'll make sure I give that a poke if I can find wire small enough. Diaphragms look OK but subject to more scrutiny. Meantime the jetting. It's a US spec 1977 XS650D and this is what is in the carbs -

Pilot jet 27
Needle jet 8ZL, one is stuck, one can be removed
Needle 4M1 on centre groove of 5
Main jet 135
Air screw 2.5 turns out, no markings, bent tip on both carbs
Slide has no visible marking

Anyone care to hazard a guess how that's been running, standard pipes and air filters.

For a laugh see picture below.

 

[JOHNDADDY]

A bit rich I'd guess lol

 

[Its been a long time]

A bit rich I'd guess lol

Yes, I think main jet needs dropping to 125 (manual says 122.5). I guess pilot screw was out 2.5 turns on account of the 27 pilot jet (25 in the manual. I think the needle and needle jet look stock.

 

[mrtwowheel]

You may have a big problem if the tip of that brass is broken off and stuck in the carb body.

 

[mrtwowheel]

You may have a big problem if the tip of that brass is broken off and stuck in the carb body.

 

[Its been a long time]

You may have a big problem if the tip of that brass is broken off and stuck in the carb body.

The good news is that I've just come in from checking the diaphragms and successful extraction of the other needle jet after soaking it a few hours with WD40.

I reckon I've been incredibly lucky with these carburettors. I was expecting far worse from reading all the problems people have with old BS38's. Every orifice seems to check out OK with carb cleaner and spray can air.

First thing in the New Year I'll order jets, gaskets, O rings and put them back together.

 

[5twins]

Never heard of an 8ZL needle jet, maybe from an aftermarket rebuild kit? The jet sizes you found certainly seem to be. The rebuild kits for the '76-'77 carbs come with those sizes. They're wrong, of course, but the kits have been sold that way for years, lol. The stock needle jet should be a Z-8, and repops are available if you wanted to replace them. I'm not sure where those carbs spent most of their life, but if it was somewhere hot and dry, like Arizona for instance, there's a good chance the throttle butterfly shaft seals are all hardened up and in need of replacement. You can remove the left seal on either carb for inspection without having to remove the butterfly plate or throttle shaft. Just take the nut and parts off the end of the shaft. Pull the seal out and give it the "pinch" test to see how hard it is. It should be very soft and flexible, easily pinched oval between your thumb and finger .....

Here's an old one that's all hardened up. In the second pic, I'm pinching really hard and it ain't compressing much at all, lol .....

 

[Its been a long time]

Never heard of an 8ZL needle jet, maybe from an aftermarket rebuild kit? The jet sizes you found certainly seem to be. The rebuild kits for the '76-'77 carbs come with those sizes. They're wrong, of course, but the kits have been sold that way for years, lol. The stock needle jet should be a Z-8, and repops are available if you wanted to replace them. I'm not sure where those carbs spent most of their life, but if it was somewhere hot and dry, like Arizona for instance, there's a good chance the throttle butterfly shaft seals are all hardened up and in need for replacement. You can remove the left seal on either carb for inspection without having to remove the butterfly plate or throttle shaft. Just take the nut and parts off the end of the shaft. Pull the seal out and give it the "pinch" test to see how hard it is. It should be very soft and flexible, easily pinched oval between your thumb and finger .....

View attachment 257557

Here's an old one that's all hardened up. In the second pic, I'm pinching really hard and it ain't compressing much at all, lol .....

View attachment 257558

View attachment 257559

Thanks very much for the comments. What you say makes sense. Funnily enough, all the parts apart from the bent primary air screws bear the square within a square Mikuni logo. I viewed all the parts under a 10x glass and there's no mistaking the logo.

I guess today there's little chance of getting original Mikuni parts to refresh the carbs. But I'll try. I aim to put it back to Yamaha standard spec, but I'm tempted to go 125 on the main jet versus 122.5 standard. The exhaust system is standard and the air filters are the stock filter hardware with new foam fitted by me.

The bike came from Minnesota. It has ~16500 miles on the clock and I can believe it's genuine going by the overall condition of the bike.

 

[5twins]

Sometimes repops imitate the Mikuni label too, lol. Like I said, I never heard of a needle jet with that number-letter code before.

 

[Melnic]

One day I'll design and 3d Print a tip to go onto my carb cleaner w/ a cone shape on the end to connect and seal up better to carb passages so I can spray into these float bowl holes to verify passages are good.

If you have not had carb spray come back at your face while cleaning a carb, you have not experienced your right of passage (or you read enough forum posts to know to not do that to yourself)

 

[Its been a long time]

Sometimes repops imitate the Mikuni label too, lol. Like I said, I never heard of a needle jet with that number-letter code before.

I suspected as much........

I'm going to call Allens Performance who are the official UK Mikuni agency. I will see what they can do for me.

Thanks a lot for the interest, it's appreciated. Main thing for me today was expecting a whole load of mess in the carbs. And they're pretty darn clean. Big bonus for me.

Next job is check out the electrics for problems........

 

[Its been a long time]

One day I'll design and 3d Print a tip to go onto my carb cleaner w/ a cone shape on the end to connect and seal up better to carb passages so I can spray into these float bowl holes to verify passages are good.

If you have not had carb spray come back at your face while cleaning a carb, you have not experienced your right of passage (or you read enough forum posts to know to not do that to yourself)

Oh, I have. Bl**dy stung like crazy in my eye this afternoon!

 

[gggGary]

Steel float needle tips is correct for BS38's. So far trying to use various rubber tip "will fit" needles hasn't gone well for me.
I make a brass drift to tap stuck needle jets down and out from the top. The oring must be replaced. I always do the throttle shaft seals, 5twins found a Suzuki part # for them that's less expensive than Yamaha's.
Also a quick dip in HOT water of every carb float I have out, looking for bubbles (leaks).

 

[Its been a long time]

Steel float needle tips is correct for BS38's. So far trying to use various rubber tip "will fit" needles has nt o gone well for me.
I make a brass drift to tap stuck needle jets down and out from the top. the oring must be replaced I always do the throttle shaft seals, 5twins found a Suzuki part # for them that's less expensive than Yamaha's'
Also a quick dip in HOT water of every carb float I have out, looking for bubbles (leaks).

Thanks very much. Appreciated.