1979 XS400F Carb and Jetting identification and replacement

[MISSIONmoto Mike]

Hello, this is Michael from MISSIONmoto.org I'm a new member on the forum.

I lead a WA state nonprofit 501c3 that takes donated bikes, we fix and sell them to support missions and youth programs. MISSIONmoto.org. Before posting, I'm pretty sure I read all the threads I thought my have an answer for me.

We’re finishing a donated 1979 XS400F project. I've already spent money on new jets, and I think they are wrong...I should have reached out for help earlier because PO may have had wrong types of jets to begin with and I just ordered bigger sizes of the wrong ones. You never know what surprises await you with abandoned, donated projects.

Bike has pod air filters and 2 into 2 short exhaust- no crossover.
We've got 160 compression on #2, 155 on #1
Set the valve lash correctly
Set points gap and timing correctly
Intake manifolds look rough on outside, but no cracks on inside
New, correct NGKs
Set float height to around 26.5 to 27 per manual but I did see another manual say 30+. No gas leaks now, but what is the correct measurement?
Carbs were meticulously cleaned with cleaner, ultrasonic, and compressed air.
Has Mikuni carbs, but can't confirm the exact type. Takes the round main jets.
Bike would fire right up, but only run with full choke.

1. #2 carb has the air/fuel mixture screw broken off in it. It’s the little screw that sits on top in between carb and intake. I was reading a post where it mentioned these can break if over-tightened. Sure enough, when I backed out #2 the little sharp internal end did not move at all. I’ve got to get it out and replace it. Any advice and what’s it called exactly and where I can buy one?

2. When pulling the main jet, there was a thin metal washer pressure fitted on top of the main jet. All parts diagrams I've seen show a smaller washer going in first, then the main jet going in through the middle of it for seating. Our washer only clicks in on the top, won't seat in the tube. The Main Jet was a 132.5 Big Round type. Legit?

2. Removed the metal screw plug and removed the 42.5 idle jet

3. Thought I'd better go two up on both the mains and pilots. So after looking at all Google images and ebay listing images, paying attention to large round vs. small round mains, and looking at the tips of the idle jets paying attention to tapered vs. flat ends
I ordered 137.5 mains and 47.5 idles from Niche-cycle off ebay. Had identical look and feel to my old ones.

4. Popped all jets in, mounted carbs and filters, bike ran worse than before! Could not even get it to stay running. Pulled carb and jets and the weird thing is after closer inspection the hole passages on the new main and idle jets look noticeably SMALLER than the old ones even though the size markings say they are two sizes bigger!!!

We are lost. Can you help us? I can send you pictures. I don't know XS models well and can't tell if PO put different carbs on or wrong jets in, or maybe I ordered the wrong jets or the new jets got marked with the wrong sizes at the factory?

Real bummer because we just finished a custom paint job and got the seat fabrication done and most electrical done. Can't sell the bike now; everyone loses until we can.

Thanks, Michael 206.427.1955 in West Seattle

 

[Greasey Fingers]

Hi, first you need to get the broken mixture screw tip out or it wont ever be right. I just had the same issue and removed it with success im glad to say. Heres a link to how I did it: http://www.xs400.com/forum/showthread.php?t=13979

Im new to rejetting myself and doing it right now, so ill let someone else hop in on that side of things.

Oh yeah in regards to jets sizes, someone did mention the following to me the other day: Jets differ on how they are marked. Simply stated, some measure the actual diameter of the hole i.e.: 175 =1.75mm. Some measure by the actual flow of the Jet i.e.: 175=175cc of fuel per minute.

Im not sure how relevant that is with our bikes, like I said im a newby to rejetting.

Btw those original jet sizes are probably correct ive heard thos numbers thrown around before. My original jets were 142.5 and 42.5 (Im going to try 150 and 47.5 with the E clip on the needle down one notch towards the pointy end)

 

[MISSIONmoto Mike]

Great, I'll work on getting the mixture screw out this weekend. Know of where I can buy a replacement? Also, are these carbs BS34s?

 

[Greasey Fingers]

Yeah they are bs34's, I got my mixture screw out of a kit: http://www.ebay.com.au/itm/Yamaha-X...orcycle_Parts_Accessories&hash=item3cf05c0ec9

I needed the jets too and the seller was happy to swap the standard jets in the kit for the sizes I wanted. It may be good to replace the mixture screws on both carbs so they are the same, probably doesn't matter though. I didn't use anything else in the kit just the jets and mixture screws, so your probably better off buying them individually. You could try mikes xs, there are different mixture screw types you will see a visual difference. The one in the kit in the link is the type you want, just reference your original one with the pictures anyway.

But before you buy anything get the broken tip out. Many have had to buy new carb bodies over broken tips that's why I was so happy to get mine out. Get the heat onto it and get someone with good patience and a steady hand to do it. An audience would probably not be a good thing, there may be a bit of cussing going on, and possibly a carb body being thrown against the wall lol

 

[MISSIONmoto Mike]

Thanks so much!!! Yep, will make getting the tip out a priority per your instructions. I can still see the very sharp end poking through, so I might have an easier time. I love your homemade tool!

I did send MikeXs.com an email to see what I could get individually. Going by pictures on their site, I could not confirm the mixture screws. If I don't hear back from them soon, I'll contact the supplier from your link.

Last question, do your main jets have that thin washer clicked in on top of them that you need to remove first to get them out?

Thanks again for the info on parts and carb. More thanks for giving me a feeling that I'm not lost anymore.

 

[ScottLuke]

To answer question #3 I would go to any dirtbike dealer and compare your jet/s to that of other Mikuni jets that they carry. Coming from a dirtbike background, I've always purchased Mikuni and keihin jets from my local dirtbike shop.

 

[MISSIONmoto Mike]

Good idea about visiting a dealer for jets. So, so many Japanese bike dealers and shops have gone out of business around the Seattle area. I'll need to find one...

 

[ScottLuke]

Hinshaws Motorcycle, Lake City powersports? are the ones I can think of off the top of my head that should have Mikuni jets. If all else, ProCaliber in Vancouver, Wa. Goodluck.

 

[MISSIONmoto Mike]

Yeah they are bs34's, I got my mixture screw out of a kit: http://www.ebay.com.au/itm/Yamaha-X...orcycle_Parts_Accessories&hash=item3cf05c0ec9

I needed the jets too and the seller was happy to swap the standard jets in the kit for the sizes I wanted. It may be good to replace the mixture screws on both carbs so they are the same, probably doesn't matter though. I didn't use anything else in the kit just the jets and mixture screws, so your probably better off buying them individually. You could try mikes xs, there are different mixture screw types you will see a visual difference. The one in the kit in the link is the type you want, just reference your original one with the pictures anyway.

But before you buy anything get the broken tip out. Many have had to buy new carb bodies over broken tips that's why I was so happy to get mine out. Get the heat onto it and get someone with good patience and a steady hand to do it. An audience would probably not be a good thing, there may be a bit of cussing going on, and possibly a carb body being thrown against the wall lol

Got the broken needle out tonight

 

[Greasey Fingers]

Nice, how did you get it out in the end, was it an easy one?

 

[MISSIONmoto Mike]

Took a dental pick with a 90 degree bend, filed it down to the size of the broken tip, then applied some heat with a torch, and then squeezed the dental pick with some channel locks. BINGO!