1978 Yamaha 400XS one cylinder won’t run!


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Hi guys/gals. I have been using all the treads religiously and have found great help from everyone’s conversations but I’m defeated with my ‘78. I bought the bike as a project for myself throughout the offseason and was able to get the bike running but only with the right cylinder. I have had the carbs off lots and found a few issues that have been resolved (hopefully) and I still can’t get the left cylinder to run. When I mean I haven’t got it to run I mean that I have had the cylinder running but with the choke pulled out. It will not idle on its own. Here was my troubleshooting process:

1. First thing I did was check compression on both cylinders. The troubled cylinder is around 140psi as is the running cylinder. (From what I have read this checks out)

2. I checked spark and found no spark. Spark plug was changed and I now have spark.

3. I pulled the carbs off and started away at cleaning them. The bike has pod filters and aftermarket exhaust ( I understand that this is a recipe for a long tuning process). When I pulled the jets out of the carb they have already been upgraded to larger. My main jet is 147.5 and my pilot jet being 50 (from what I can see. It is very small numbers). Soo here’s the kicker when reading on this awesome forum I found that needles can break off for the fuel/air mixture screw.... well one of mine is broke off. To my luck I got it out by pushing it back the way it went in and ordered a new screw. After cleaning and finding that issue i still can’t get the cylinder to idle/stay running.
In my eyes it has to do with the pilot circuit in the carb. I was worried I ruined the pilot hole in the carb body but wouldn’t that indicate a rich mixture?? (I know it’s running lean cause if I keep it running with the throttle it gets very hot.

also just another thing. It seem my carbs are missing the pilot air jet that sits in the hole at the mouth of the carb (screw in one) both are missing but this doesn’t explain why I can run one cylinder and not the other with all the same parts?? Could it be a timing thing or a valve problem??

Any help would be much appreciated!! Thank you!


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You absolutely need those pilot air jets. They are kind of hard to find for these years carbs. They are a 170 jet. I have some if you can't get them. Your also missing the idle set screw. I would also go to a smaller pilot jet. Maybe 45 tops as a 50 is way to big. If you have pods that are not uni foam filters the bike will be very hard to tune and will not run as well. Using the stock air boxes and H-pipe is the best but with uni's and the stock H-pipe you can get a decent tune. Your exhaust setup will also cause some tuning issues.
No wonder the choke has to be pulled out, funny to hear of somebody chasing a super big air leak by bumping pilot up, it would have to go maybe to 100 to do anything there.

When you open air bleed up that much you have essentially a vacuum leak, the vacuum on the pilot fuel jet has dropped to nil because the airjet is big enough the vacuum to draw fuel has gone to air instead, air being easier to pull than liquid fuel.

Do like post #2, 'the expert' says, you have to fix that.
Once you fix those issues, make sure to sync the carbs with a manometer.
Thanks a lot guys! I’ll try doing the following recommended by XSchris. Yes I plan on syncing the carbs. I already read a thread from here and have one made up Like I said I’m just trying to get a grasp on what the guy before me was thinking and fixing all his mistakes and find missing parts. I would like to make sure it’s done properly. Another quick question. The unipods your talking about what is the difference between those and say a regular filter you would buy and fit on? Smaller micron filter? Restricts the air a little more?

I’m in Canada and just wondering where the most reliable source as well for new pilot fuel jets? I actually have been looking for the air jet ( because I figured that had something to do with it) I’ll do a little more digging but might have to contact xschris for his if that would be an option?
The OEM filter combines both carbs (using the H-pipe) to allow the CV carbs to share intake vacuum which opens the slides faster. Each side then sees two induction pulses instead of one, it keeps the air more active in carb to instantly work slide faster.
Make sure to use only real mikuni jets. They will have a square within a square stamped into them. The aftermarket ones are very poorly made and sizing can be very inaccurate.
Thanks for all the help from all of you. I’m getting the air jets and the idle adjustment set up!

kinda on the same lines of questioning but doing some reading it seems that if I want to rejet the pilot fuel jet I need to get ones specifically for my carb. It looks like the guy before me had put in the VM jets which (from my reading on the forum) will not work for the BS34 carb which I believe I have on the bike? I found some BS30/96 pilot fuel jets and I’m wondering if these are the ones I should be buying??

Here are more pictures of my carb.


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The VM pilot fuel jets are almost the same as BS ones, the only difference is 8 holes instead of 6 which means the emulsion will aerate a bit more. The two pilot types physically interchange, the 8 air hole ones will act a bit leaner than the 6 with the same value, i.e., an 8 hole 70 pilot will act closer to a correct 6 hole 60 give or take. You consider the total amount of hole area as the equivalent of a needle on the main jet, more air equals dropping needle down.
Your carb needs the bleed type pilot jets. If what is in there now is a non-bleed type (no holes in the sides) it will not work.
Okay. Thanks! The ones in there are the bleed types so I think I’m good. I was more or less concerned about the 6 holes Vs 8 and the little different designs that the VM and BS jets are. By designs I mean they obviously with both fit in the hole thread wise but just the BS seems to be tapered at the bottom a little more than the VM. If it doesn’t matter and it’s just depending on what will make the bike run better (as amc49 said with one being more lean) I’ll just order what I think will be alright to start with and see what happens.
These should look like this with 6 holes.
DSC03934.JPG Just make sure it's a real mikuni jet.