1979 XS400 build with Mikunis -- carb advice needed

Hey all. I picked up a low mileage but long-neglected 1979 XS400 for a song that the PO had started chopping up into a cafe build. The bike had been sitting for a long time and was not running. The cafe style is what I was going for anyway and I'm at the stage where I'd like to get her running consistently before starting on the aesthetic stuff. She did not come with the original carbs; instead, there were a set of Mikuni VM34s in need of a serious cleaning, and they'd never been jetted for that engine.

Here's what's been done:
- carburetor cleaning/rebuild with new gaskets and rings
- new plugs (one hole retapped) and good spark verified
- gas tank re-sealed and petcock rebuilt
- new gas lines from petcock to carbs including fuel filter
- new carb boots
- timing checked, valve clearance within spec
- starter motor removed and hole plugged, so... kick start only
... and lots of other things unrelated to fuel, combustion, compression, etc.

When I kick, I can get her to turn over, usually with some throttle applied, but she won't run for longer than a second or two, rarely more. She doesn't seem to like the choke applied, and the idle and air screws have minimal effect. The plugs indicate she's running really rich. I counted on having to re-jet (pilot jets are VM22 style), but the pilot jets currently installed have no markings on them so I have no idea what my starting point is. Moreover, I can't verify that they're actual Mikuni jets (despite having the same measurements).

TBH, I'm not married to the Mikuni carbs and, for the same money, I'd rather just get a set of stock carbs for this bike than buy a few dozen jets in hopes of making the current carburetors work. Unfortunately, there are very few carb sets for the XS400 from '79 and I've seen them anywhere from $100 to $300.

What do you think my best option is? Did the 79s use the same carbs as the 78s or 80s? What about the XS650s?

And thanks in advance. I've gleaned a ton of REALLY useful information from this site before ever posting a question of my own.

FWIW, the motorcycle is a XS400G (3F8).

XS Chris has a set of carbs for sale in the classified section for a very reasonable price. In fact, almost prefer you don't get them because depending on how my carb clean goes I'll be interested in them hahaha

In meantime, you said mixture screw doing nothing? How many turns out? Did you synch them? There are websites to calculate correct jet size, but using a totally unrelated carb set seems problematic...

3F8 is a 1980 xs400 g model. So yours carbs would be the same a 80-82 sohc xs400 bikes. The 77-79 xs400 bikes carbs will work but use different parts. I would avoid xs650 carbs as they would need to be modified to work/fit. Post some pics of your bike.

Roger all. @buz: The air mixture screw has minimal effect. I've tried it all the way tightened and +4 turns out. Unfortunately the bike is inconsistent when starting so it's difficult to compare different settings. The two carbs are synced in as much as the throttle cables pull the plunger the same amount.

@xschris: That's good to know re: the model years. I'd wondered if mine was a December '79 model, but I don't remember verifying online. I was told by the PO it was a 1979 and that's what I was going with. It wouldn't surprise me if the bike were an '80. There's a set of '80 carbs on ebay right now for $80 shipped but are listed for a 3F9 model, so I was hesitant. If you can verify these will work I'll snatch them up (unless you're selling as as well, as buz said).

As far as photos, I haven't taken many. But here are a few.

You have the 80 g model. These bikes build dates are usually a year before the actual model year. My 79 for example was made on 6/78. Make sure to get a factory intake H-pipe as it will make the bike run much better. VM34 carb are much too big for the 400 from what I have heard. The 30 or 28's are better but still can be a pain. To bad the bikes all cut up as it will make getting things working right much harder. Make sure you get the sohc xs400 carbs as the dohc ones will not fit/work the same.

Thanks. Stock carbs are likely the route I'll go. The Mikunis are just too much of an unknown right now (both time and expense).

I'm not sure what the factory intake H-pipes look like, but I'll do some research. I've got some disk-style air filters that came with the bike that I'll probably try to reuse. The air box is long gone and I'm doing away with what's left of the battery box. That whole "triangle" in the frame will remain mostly open.

Use black foam uni filters if you must do a "pod" filter. The stock bs34 mikuni cv carbs don't like turbulent air and any pleated or screen type will cause a lot of it. Using the stock intake H-pipe will help smooth a lot out. Post pics of your filters.

Here are those "disc" style pod filters that came with the bike from the PO. Sounds like they're gonna present exactly the kind of turbulent air issues you're describing.

Here's a photo of what I'm presuming is the intake H-pipe. I just ordered one.

And lastly, here's an example photo of a UNI filter -- is this what you're talking about?

Yes that's the H-pipe. Yes those disc filters are junk. I would look for the black type foam filter without any cap on the end. Longer is better. A 6" is better than a 4". 50mm-52mm inlet with work to fit the h-pipe.

6" Uni's on the way along with stock carbs and H-pipe. Hopefully I'll be able to get all installed this weekend or early next week.

I'll keep you all updated with the progress. Thanks again, Chris.

Awesome!

Yep, stock carbs from a 1980G/SG.

xschris said:

Yes that's the H-pipe. Yes those disc filters are junk. I would look for the black type foam filter without any cap on the end. Longer is better. A 6" is better than a 4". 50mm-52mm inlet with work to fit the h-pipe.

Click to expand...

Do you have a part number for the uni filters you would recommend? They fit with the stock battery box on the H pipe? I saw on another post you cut the flange out of the a stock pair of filters.

I used the flange from stock filters to help them fit better. No I don't have the part number as I had them many years ago. Them where 50-52mm 6" full foam black uni's. I think I paid $35 for the pair.
In the first pic I had a filter on the crank case as you can see. I would recommend using the stock hose to the H-pipe instead.

Also using the stock boxes will be your best option and give you the best overall performance.

xschris said:

I used the flange from stock filters to help them fit better. No I don't have the part number as I had them many years ago. Them where 50-52mm 6" full foam black uni's. I think I paid $35 for the pair.
View attachment 34914 View attachment 34915 In the first pic I had a filter on the crank case as you can see. I would recommend using the stock hose to the H-pipe instead.

Also using the stock boxes will be your best option and give you the best overall performance.

Click to expand...

The paper inside is starting to tear. My question is also, when I got the bike the pilots seized so I had to destroy them to get them. I replaced them with a size bigger and I feel like the bike doesn't pull as hard as my other Xs400. Do you think using uni pods without the H pipe might justify the bigger jet to make it pull harder ?

Stock setups work best from what I have found. I would use a stock pilot jets ( real mikunis) and adjust the pilot mix screws to at least 3 turn out to start. OEM filters can still be bought. Yamaha still has them and hiflo makes them. Search by the oem part number. The hiflo are not as well made but are cheaper and work ok.

https://www.ebay.com/itm/Hiflo-Air-...835551&hash=item35e0bd3f43:g:eDQAAOSwtYddGSdd