Runs at high idle with choke, Idles but dies with throttle

I have a Yamaha xs400 Maxim that runs very strong, starts within 2 seconds of turning the starter, and doesn't muck up plugs. The only problem is that I have to have the choke on to get it to run at all. Once I do start it with the choke, it idles really high (not sure an exact number as I don't have a tach but I'd hazard a guess at around 3-4k). After it warms up, I can take off the choke and it will idle at what sounds like a perfect idle speed (once again, no tach) but as soon as I touch the throttle. I'm at a loss for what's going on here.
Bike has around 30k miles and runs perfectly with no leaks or white smoke at all. The pipes are cut very short and it has pods. I replaced the jets with what others have used for cut pipes and pods so I assume those to be okay. Please help. :[

 

-Be sure your valve clearances are in spec.
-Be sure your float levels are accurately wet-set.
-Do as accurate of a bench sync as possible.

Then do a running vacuum sync. Your symptoms are consistent with being way out of sync or having a wonky float level.

 

I'm going to have to do some research on that. I'm sure there's a tutorial already typed up somewhere. I'll look and report back. Thanks Fitz

 

def sounds like float problem, if its idleing with or without choke but dies when you open the throttle it usually means your carbs aren't allowing the additional fuel flow to keep up with the air. so could be float or something clogging the jets. Another thing where the vacuum carb sync would be good is that the shorty pipes could be adversely affecting engine operating vacuum (lack of back pressure) so some more research might be in order.

 

The main jets are clogged, you're running just on the enricher circuit and the idle jets.

dave F

 

Okay so the general consensus is that my carbs are clogged or out of sync. I can say with some certainty that they're not clogged as I broke them down and soaked them in some pretty powerful carb cleaner for a few days and then sprayed out every single port, seam, crack, hole, and any other suspicious-looking area. They also had this issue when the intake and exhaust was 100% stock but assumed that there was a vacuum leak. That being said, what Hogfiddles is saying makes sense so maybe there's some crap in some port somewhere... Does anyone have a link to a reliable tut for doing all of the thing Fitz mentioned or an info-graphic on the different parts of a carburetor? I work on modern engines and carburetors are like some kind of voodoo rocket science to me.

 

What carbs are on that bike? Mikunis?

 

Yeah, I forget the size.

 

Also, anyone know some up- and down-sides to converting to a single carb? I have a jet kit and I can find some shims to make it work as long as it's not going to cut my power in half or something crazy. I'm down for the idea of ONE carb to give me troubles instead of two.

 

The downside is you'd have to fab a 2-1 intake manifold that would actually work. And the stock carb would probably not be "big" enough to supply fuel to both cylinders.

Mikuni carb breakdown: http://xjbikes.com/Forums/viewtopic/t=31061.html

Note that this only covers a basic teardown for service. FULLY disassembling Mikunis requires filing the peined-over butt ends off the butterfly screws so you can remove the butterflies and throttle shafts. You only do this if you're replacing the throttle shaft seals.

Which is why we don't submerge assembled carbs. The throttle shaft seals are trapped in the carb body and you can wreck them if your carb cleaner of choice attacks them.

Wet-setting float levels: http://www.xj4ever.com/setting%20fuel%20levels.pdf Read carefully, this covers both Mikunia and Hitachis. While the procedures are similar, the specs are different.

Be sure your valve clearances are in spec or you won't have much luck with a vacuum sync.

Valve clearances: http://xjbikes.com/Forums/viewtopic/t=14827.html while this was done on a 4-cyl, your bike has a similar valve train and the procedure is basically the same. Part Deux: http://xjbikes.com/Forums/viewtopic/t=29209.html

 

I may try to find a set of wrecked boots one day and try it out. Fabbing the 2-1 would not be very difficult, but it may be more trouble than the outcome would be worth.

 

Also, thanks for all of the help guys. I'll keep you posted if anyone cares.

 

Please do.....we DO care.

Best wishes and when you get stuck don't be afraid to ask. That's why we're here.

dave

 

When a Bike runs (on enrichment system) but dies when Throttles are opened indicates the Carbs are in desperate need of a cleaning and tuning.

It means Main Jet Supplied Fuel is not getting supplied to the Intake Air Volume.

Usually, this symptom indicates that the Carbs "Main Nozzles" (Emulsion Tubes) Air Bleeding Orifices are clogged, impeding the Main Air Volume from passing through the tiny orifices, which enable the siphoning and atomization of the Main Jet Supplied Fuel up and out into the Intake Air Stream.

::: MIKUNI Main Nozzle (Emulsion Tube) with Clogged Air Bleed Orifices :::

 

Seriously. That's a lot of bloody typing; we wouldn't bother if we didn't care.

We only hope you actually take the advice given; because we like to see people triumph over their issues not fight with them for a year or more (I'm not naming any names...)

 

After all of the work I've done building this damn bike, I'm not going to stop just because the carbs are acting a little funny. I bought a new can of carb cleaner and found my air nozzle for the compressor. I'm going to try cleaning every single little thing and then move on through the more difficult if needed. Will update as soon as I have something to update with!

 

Before you tear into those Carbs for their needed cleaning, ... buy a set of (JIS) Japanese Industrial Standard Screwdrivers.

They will make life a lot easier for you, ... and save you from wrecking the Jets.

 

CHECK THE VALVE CLEARANCES. Don't avoid it because it seems difficult.

Then run a compression test.

Just to be sure we aren't pissing into the wind.

Look at the Mikuni breakdown link I posted; there are pictures and what everything is. Just squirting carb cleaner at it won't do; and the cleanest, most well serviced and adjusted carbs in the world won't work for squat if your valve clearances are not in spec.

 

I tested the compression first thing when I bought the bike. It's perfect. Rick, I have a set. ;]

 

I like the look of your bike so far. As for the carbs, since you only have 2 carbs, I would replace them with some Mikuni round slides. You can find them pretty cheap on ebay these days and they aren't as difficult to dial in like the CV's you have, especially running short pipes and pods.

 

Orange-n-Black, I just read in your build thread about using pinesol for cleaning carbs. How did it work? After reading about the throttle shaft seals not being removable (technically speaking) and most carb cleaner demolishes plastics and rubbers, I am looking for a less toxic and less destructive method for cleaning this aluminum pain in the ass.

 

>After reading about the throttle shaft seals not being removable
>(technically speaking) ?

That's so not true....

The throttle shaft seals are most certainly removeable......that's how you replace them with ones that aren't worn out......you remove the throttle-shafts, pull the old wornout seals out, and stick the new ones in. Re-assemble, and then as you connect the bodies back together, you put new fuel rail o-rings in, as well.

THEN you'll have a rack that you can be sure of. Until YOU KNOW that the seals a rings have been done, you'll ALWAYS be wondering if there is a leak there.

DON'T be afraid to do that. I was for the longest time, and finally decided to tear a rack all apart. I wish someone had convinced me to do it earlier. Now I automatically do it to any bike project I bring in.

Do it. Doit... you KNOW you wanna......

Dave F

 

And, tfish, you know you wanna measure your valve clearances also. I noticed that you commented that your compression is good, but may have overlooked the previous step mentioned... check valve clearances. Do it. Do it. You should KNOW you have to.

These guys know what they're talking about. It truly affects how these bikes run.

 

:[ I really really don't wanna. I know I have to... Would there be any symptoms of the clearances being off?

 

Yeah you won't be able to get a decent carb sync.

And as a bonus, if you put it off long enough you'll burn a valve (and there goes the compression.) Repairing that will be a whole lot more expensive and complex than maintaining them.

Also, remember it's not a one-time thing. They need to be checked every 5000 miles; if you plan to actually ride the bike it's something you'll need to learn to do.

 

Fiiiiiine. I'll get out the ol' feelers, take the engine off of the bike, and have a go at it.

 

Re: Runs at high idle with choke, Idles but dies with thrott

Never did valve lash on a 400 but it'd be VERY unusual if you had to take the engine off the bike.

It is routine maintenance, not an overhaul.

 

Check out the engine mounts that hang the engine from the goofy frame on a 400 maxim/seca and then look at the valve cover. Now that I think about it, I could just take one engine mount off but I might as well just take the engine off anyway and do a much needed cleaning.

 

There is a lot more holding the motor than just those two front mounts. (I believe you only need to pull one to extract the valve cover.)

If you need a manual, send me a PM with your mailing address. I have a PDF of the "Multilingual" world-release manual for those bikes but it's too big to email. I can burn you a CD and drop it in the mail.

Being the "international" version (in like, five languages including English) it's a bit "interesting" to navigate; but it's better than nothing.

You don't want to pull the motor every 5000 miles.

 

I've got the manual printed and I've referred to it once or twice. Seeing as the bike is stripped down to nothing, I can have the engine off in 10 minutes. That's about the only nice thing about this weird frame.

 

Design Engineers wouldn't make it necessary to remove the Engine to adjust the Valves.
Sure, ... they make it difficult, sometimes, to pull routine maintenance.
Burying a Oil Filter beneath exhaust pipes.
Having to remove several panels to expose fasteners and components.

But, I have never heard of having to remove the Plant to do Valve Adjusting.

But, it you are going to pull the Engine to paint the frame of clean-up the Mill, ... consider having the whole Plant "Media Blasted" with Walnut Shell as the Media.

Walnut Shell removes grime, paint, oxidation, and cleans the Aluminum so nicely that it shines.

•••This Plant was Walnut Blasted•••
::: Note Carbs & Velocity Stacks :::

 

Re: Runs at high idle with choke, Idles but dies with thrott

That engine is stunning. 8O I'll try looking up someone that does media blasting and try to find a good price.

 

Never got to the carb cleaning on mine because I changed the carbs out for the round slide carbs. My son-in-laws bike needs it now so I guess I'll find out.

 

>Seeing as the bike is stripped down to nothing, I can have the engine off in >10 minutes.

So you're quibling about 10 minutes??? LOL

>Fiiiine............have a go at it

AHHHH!!!!! THERE you go.....SEE? We KNEW you wanna!

Dave

 

Re: Runs at high idle with choke, Idles but dies with thrott

Don't give me a hard time, Dave. :roll: I'm trying any excuse to not do it. The guy who had it before me bought it new and rode it all the time and had it taken care of by the same shop the whole time. Thing is, I don't know how many miles I put on it before I cut it up or when he last got the valves checked. I will start working on the valves and the carbs tomorrow night now that I got my Shure Bros tail light finished (looks sick, btw). I'll put up a picture in my profile that shows the HIDs, bar-end signals, apes, bags, fender, tank, and the mic tail light.

 

Hey, I'm not giving you a hard time. I'm trying to ghet you to where you WANT to do the work yourself. That way you will learn YOUR bike inside and out. THAT way when something does happen that's not quite right you will sense it almost immediately, be able to diagnose what is happening, and even if you can't fix it yourself at that moment you will be able to either tell us exactly what IS happening and we can give you the correct way to proceed, or if you (heaven-forbid) have to take it to a shop you can tell them what DOES need to be done AND you can checkto make sure that they do it the RIGHT way.

Now, regarding your comment about the shop servicing it since new.....

If they indeed did, they should have records of what was done and when. Too many shops have gotten to the point where they:
1. Don't want to work on older bikes
2. Won't work on older bikes
3. Don't know HOW to work on older bikes
4. do a quick repair hoping that it lasts long enough to get past the warranty period so they don't have to be responsible for anything

So here's what often happens:

1. They charge a very high amount for the repairs so that they make a lot of money off of you

2. charge a lot so hopefully you don't come back

3. do a shoddy or cheap repair, hoping it lasts long enough so that the warranty period is up, and then you won't go back to them--and they really don't care because they would rather work on the newer more expensive computer-diagnosed bikes because that's what they know how to do......read what the computer says, rather than actually listen to what the BIKE tells them they need to do.

etc........ etc........ etc.......................

We've all been at the point you're at. Some have said, "I'm gonna jump into the pool", others haven't. You're standing on the edge, man Jump on in.....the water's fine.

dave F
a. so that you make it waaay worth their time for a repair that they

 

I think this shop is one of the ones that does quality work on older metric bikes but charges premium prices. Their front office is completely full to the brim with 60s-90s metrics. It's beautiful. Either way; I want to do these things and for the most part I know how to do these things, I just need to convince myself that I HAVE to do these things. Thanks again for all of the info xj gurus. And thanks for putting up with my not-really-an-xj-kinda-is-but-really-isn't xs400 maxim!

 

It's half an XJ. And you need to learn this stuff if you plan to actually ride the bike.

 

Yes sir.

 

Get a Factory Workshop Manual.

You get all this:

• Workshop Manual
> Trouble shooting
> Maintenance
> Illustrated Repair Sections
> Diagrams
> Testing procedures

Plus: The Complete Owners Manual

 

Rick; he has the book.

What he is lacking is a "Roundtuit."

Once he GETS a roundtuit, he'll be fine.

A good price on Roundtuits (and they also carry Blinker Fluid) http://kalecoauto.com/index.php?main_pa ... round+tuit

 

I'm good on blinker fluid, but I can't find the old style alternator filters.

 

Go to AutoZone. They are usually kept right next to the Rebobulators.

Dave f

 

Re: Runs at high idle with choke, Idles but dies with thrott

DEAD THREAD? No updates on this thread. Was this issue ever fixed? Was it something as simple as a clogged petcock? Petcock turned halway between prime and on? I must know. Runs at high idle with choke? Tish400?

 

Re: Runs at high idle with choke, Idles but dies with thrott

Sorry all, got really busy with a Vulcan. The issue is not yet resolved but I have a really good idea of what it is. It has gotten better after a complete teardown and then a bench sync. I may have the jets wrong as well so I'm going to try some different sizes since I have the engine and wire loom off the bike and on the bench. Still needs the choke on to run but will idle now without it after it warms up. Will keep everyone posted really soon.

 

Re: Runs at high idle with choke, Idles but dies with thrott

I'll ask the question this time Can we have an update please

I was wondering if the diaphragms were leaking as I read through this....