Bike dies below 3k


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Thanks to CaptChrome I was able to get the bike fired up and running (see video). Had to use a car battery, but it worked and a new battery is on its way.

The goal is to transplant this engine into a go kart and have some fun. I have the go kart, now I just need to get the engine running consistently. It does not have to purr like a kitten, but I need it to at least idle and run. I’m trying to do this as budget friendly as possible. For example, I will be welding the motorcycle frame to the go kart frame so I can reuse as many parts as needed. Now, on to issue number 2.

I need help. I cannot get the engine to run below 3k.

The carbs are Mikuni bs34. These carbs were just trashed from sitting for so long (around 15 years, see pics). Everything was stuck, nothing moved. I used heat, pressure, and a lot of patience and finally got everything out and have now cleaned the carbs 4 times in my attempts to get this thing to idle. In the process of doing so I have broken a float bowl mount, stripped the top of a pilot jet and dented the top of the main jet washer that goes to the nozzle jet that holds the needle. I was able to remount the float bowl post with jb weld. I got the pilot jet out with an extractor and used a dremel to recut a little slit so I could use a flathead to screw it back in. The washer still seems to be working okay. And the bike will sing even with all of these issues, just not below 3k.

I’ve been running it without the air filter just to see if I can get it to stay running. The old air filter literally crumbled into dust when I touched it.

The cv boots are good. I did the quick check and placed my thumb over the oblong orifice on the outside and the slide goes down very slow, so there are now air leaks there.

I bench synced the carbs because I don’t have the tools to do it the proper way.

But, I cannot get it to stay running below 3k. Once it goes below 3k it will cut out. When that happens, if you give it full throttle it may pick back up but usually bogs out.

I tried cleaning everything but, I still cannot get the needle valve set out. I think that’s what it’s called. This is the brass fitting that the float bowl pushes the needle into to stop the inflow of fuel. It is a press in kind not the screw in kind. I have sprayed chemicals, used heat and anything else I can think of but it’s still stuck. However I did clean the center, and it does run so it is getting fuel.

I have taken everything apart for cleaning except for the butterfly stet up and the float needle brass piece that I can’t get out.

It won’t start without starting fluid or squirting gas inside the carb. Once it starts though it’s good as long as it’s over 3k.

I’m using 91 octane.

I have a 1981 xs400, special 2

So, here are my questions:

—1. Can I get a cheap set of Amazon carbs and make this work? If so, what type do I need or can I use? Meaning, can I use a non-cv and just rejet it? Do I use the same size carb or would another size be better or would another size work? I saw a post by cpazambrana that he used Amazon carbs that worked. I tried reaching out to him, but he hasn’t posted since April 2020.

—2. If I keep the bs34 and they aren’t trashed beyond repair, does anyone know the carb jet sizes for all of the brass? The websites I’ve seen seem to mention some of the sizes but not all. For example what is the size of the needle jet that the cv needle goes into? And what is the size of the needle float bowl jet? Also is there a cheaper Amazon type carb kit out there? Again, it does not have to be perfect, I just want it to idle and rev.

—3. The pilot screws are good and the tips are not broken. I’ve turned them the standard 2-3.5 turns out, nearly all the way out, and all the way in. It seemed to stay running the longest when it was screwed all the way in to about one turn out. Would that mean I have some serious air leaks? Does that mean it is running lean?

—4. Any other recommendations or ideas?
Those are nasty. Nice detailed post. I don't know how much help I can be with your 3 specific questions, but I would try to find a decent set of used carburetors on ebay (or post in the classifieds here) and try to clean them up. I've struggled with carbs for too long that just weren't going to get clean (usually due to corrosion from ethanol blend fuels that absorb water). The $40-$60 spend on another set to play with was worth it to me. It's a gamble though.

I've had a set of carbs on an XS400 that didn't have the right o-ring and washers under the pilot screws, so they needed to be turned out about 6 turns for it run right. You never know what someone else put in there for parts and brass.
Nice gardening project you had growing in the carbs!

Sounds like an issue with the idle circuit here. Since adjusting the pilot air screw does pretty much nothing and it runs best at idle with the screw all of the way in, I do think you have an air leak. The pilot circuit is used with the throttle butterfly completely closed or just slightly open (the operating range that you are having trouble with). If you are certain that the holders between the carbs and the engine don't leak, I am thinking that your butterfly seals are leaking.
Thanks Travis and CaptChrome for your help. I sealed up some of the leaks, cleaned the carbs yet again, and adjusted the pilot jet on top. Ended up backing it out to 4 turns and it idled at 1,200! Now it wants to bog when you crack the throttle so it’s back to tuning again. lol. I’ve got new intake mounts on the way and am going to try to find a rebuild kit now that I know the carbs aren’t completely trashed. Guess I should also do a proper carb synch too.
Always use new real mikuni jets when jets get damaged. With any old and very dirty carbs all the internal rubber bits must be replaced. Pilot jet plugs, float valve o-rings and needles, pilot mix screw o-rings and butterfly shaft seals. I search the net with OEM part numbers as a few other bikes from the time use the same stuff.