Stutttering over 4K revs

[Fentible]

Hi

After seemingly getting this 1980 XS400 running well, its now struggling to get over 4000 rpm under load but will rev cleanly on the stand. It starts easily and ticks over beautifully, in riding it accelerates nicely up to the 4K point after which it starts to stutter and miss. Plugs indicate a lean (ish) mixture on the R/H side and a rich mixture on the left. I would have put it down to fuel starvation but the fuel is flowing nicely via the vacuum tap.

Points gapped as per manual, timing set as per manual, valves adjusted and set. New float and needle valves, float (rubber) height set at 23mm and using the clear plastic hose method on the drain bowl stub gives a fuel level roughly consistent with the join between float bowl and carb. ATU is free and working, carbs have been thoroughly cleaned and are as per factory spec in terms of needle heights and jets. Additionally the carbs have been balanced using vacuum gauges

Ive worked my way through the hints and tips on a previous link but am at a loss as to where to go next. Still feels like fuel starvation, running on prime makes minimal difference. I did have an external filter fitted which I removed and replaced with straight hose, to no avail.

Thoughts, previous experience??

Cheers

fent

 

[bcware]

I run an inline fuel filter and most other people do too.

Have you inspected the diaphragms recently? When you move the slides up and down manually, do they fall at roughly equal rates?

Look into that float height on the rich side and that carb in general. If you're too lean the engine usually has no trouble revving to redline like a banshee and staying there, but running too rich will bog the rpms down and keep them low.

4,000 rpms is about when your needle/diaphragm is starting to open up to allow the main jet to start providing fuel. You could be flooding a bit on the one side. I'd verify that float height you've chosen with data from this site. It sounds like an off number to me, but I have the older brass floats. If you have brass floats the height should be closer to 26mm. Plastic floats are a different story. I believe you want to be like 2mm below the joining seem of the float bowl.

 

[Fentible]

Thanks for the input bcware, I did thoroughly inspect these carbs prior to refit. The diaphragms certainly looked okay no rips or tears that I could see and they are seated nicely under the carb top (after 2 goes). The slides move evenly although the right side was gummed solid when I got them, even to the point of preventing the butterfly spindle from rotating. However after much cleaning they move nicely and all the internal passages seem clear.
The floats as fitted are neither plastic or brass, they are of a solid black rubber type material. The bike has been in possession of the original owner from new so they are definitely original (UK spec). The Haynes manual cites 32 mm as the correct height but I would have to bend the tang to a ridiculous angle to achieve that so I'm going with the misprint theory. I'll try raising the heights to get 2mm under the carb/float chamber join to see if it makes any difference. Probably need about 26mm on the float height to achieve that

Would an airlock in the tank create a similar symptom?? I don't think I've ever checked the fuel cap vent.

 

[16VGTIDave]

Is the bike stock or modified? Could the stuttering be better described as surging? Is it happening at full throttle, part throttle, or with the throttle barely open? Does it get better or go away if you close the throttle a bit?

 

[Scorpio1963]

I know the type of floats you are talking about,if they are old they may get soaked and not work properly.Also how is the petcock on your gas tank,that would be something else worth looking into.

 

[bcware]

they are of a solid black rubber type material. The bike has been in possession of the original owner from new so they are definitely original (UK spec). The Haynes manual cites 32 mm as the correct height but I would have to bend the tang to a ridiculous angle to achieve that so I'm going with the misprint theory. I'll try raising the heights to get 2mm under the carb/float chamber join to see if it makes any difference. Probably need about 26mm on the float height to achieve that

Would an airlock in the tank create a similar symptom?? I don't think I've ever checked the fuel cap vent.

I believe I have read about others with your type of float having the same required float height as the brass ones. The Haynes does have a misprint for float height in their book; there is a correct description in the back of the book somewhere, but I think it is wrong is the carb chapter. For the brass and yours 26mm is right. This is what I use and the fuel level sits about 2mm below the lip of the float bowl where it attaches to the body of the carb. Be sure to take into account the height of the gasket if you are measuring over it with a ruler and measure both sides of each float.

I think your symptoms of more indicative of excess fuel rather than too little. In my experience whenever these bikes are lean they surge and rpms hang very high up. When you're lean you won't have a problem hitting redline. If you're rich you'll hit a "wall" and the bike will not want to climb rpms any longer.

 

[Recreateme]

This could potentially be a vacuum leak somewhere.. If you get going the engine creates a vacuum that when open, the butterfly valves sucks more fuel from the jets, but if you have a small leak when the vacuum is strong enough (in your case 4k ish) rather then suck extra fuel it sucks air causing it to go lean and stutter..

Just my guess as i think i had this issue haha My bike would only rev to 3K under load, on the stand she would redline then i would hop on put it into first and it would max out at 3k regardless of how much i turned i the throttle.. After cleaning my carbs multiple times and replacing my stock petcock with a non-vacuum one it seemed to get fixed.. Not sure exactly what fixed it but it went away lol

I would clean your petcock and make sure it flows freely, double check the boots for cracks, and lines for an air leak, before taking apart the carbs again.. if that doesn't fix it my next guess would be clogged needle jet, or floats not keeping enough fuel in the bowls.

 

[bcware]

Typically vacuum leaks only become significant at idle. At higher rpms so much air is being drawn in that a vacuum leak doesn't make a difference.

A lean state will cause rpms to stay high, and redline will be reached easily. A rich state will cause bogging and will keep rpms low or prevent them from rising.

He is therefore showing signs of being too rich, not too lean.

 

[Fentible]

Many thanks for all the suggestions. Been back at work so limited opportunity to get in the garage. I'm going to recheck the float levels and will report back once done.

Cheers

Fent

 

[Sandrew]

I'd point out something different.
As you said, you got it decently running, so I wouldn't dig into the carbs just yet. Check clear ground between engine and frame, also fuse connectors for ignition. If the connection is bad, it might idle perfectly but with higher revs it needs stronger spark and with some bad wires problems might arise.

Last summer I encounter the same issue, after taking wheels off the ground and busting my balls with my stiff-ass cafe bike. The result was, shuttering over 4 krpm and top speed therefor about 70 km/h, didn't know where to look for the problem and didn't have the means on the side of the road, so kept driving, it got worse and worse until it stalled. To my surprise after the stalling the starter button wasn't working either. Only then I realized what was going on and could easily attend to it. The wire's on these old harnesses are brittle and may brake inside the isolation.

Funny thing was, after making good contact, the bike was running better than before, and it lost the shuttering it had when reaching max speed. Previously thought of being caused by poor carb tuning.

Just sayin' cause there are too many topics on these forums concerning carb jetting. People tend to mix it with rocket science and expect it to be pain in the arse. Just take the time and think of what you're doing (a bit OT but what concerns freeflowing exhaust and pods, I didn't need to make any changes in the carbs (and these badboys had seen better days), just went up one size on the mains to get to spend more money on gas).

On the other hand, I may be completely wrong. Haha, cheers!

 

[Fentible]

Thanks sandrew, I did go through most of the connections at the time but its entirely possible I missed a few.

One of the things that probably won't help and which I hadn't realised was using resistor plugs (BPR7ES) with suppressor caps. If the ignition system is marginal the additional resistance can't be helping. Will change to BP7ES asap.

I have ALSO been exploring fuel levels in the carbs. By attaching a clear plastic tube from the stub on the float bowl, running it up the side of the carb and opening the drain screws I can check the fuel level. What I found is that the LH side with the rich plug has a fuel level roughly with the mating join between carb and float bowl whilst the leaner LH side has a fuel level about 1cm lower.

I then started the engine and blipped the throttle noting what the fuel level in the clear pipe did. On the leaner side the fuel level dropped almost to the bottom of the float bowl before steadying out about midway up the float bowl if the throttle was held steady at 4-5K.

On the richer side, when blipped the fuel level didn't drop as far as the LH side and held higher about three quarters of the way up the float bowl when held steady.

So Im going to lower the static fuel level in the LH carb and raise it in the rH carb by bending the float tangs as required. Now then, looking at the attached photo, at what point do I measure my float height. Is it at the highest point on the edge of the heel, at the back edge where the pivot is or on the level part of the float before it curves away??

All theories equally considered

 

[Scorpio1963]

You want to turn your carb upside down in order to check your float level,make sure you measure from the top of the float to the gasket on the bowl for your measurement.Since you have those lastic or foam floats I would try 24-5 mm on the height.

 

[Fentible]

Scorpio, not sure what you mean by upside down, as in the photo??

Every carb Ive ever done is measured with the float assembly tang resting on the needle of the float valve without depressing it. I'm also sure that this orientation is how the manual describes the process.

My issue is at what point on the upturned float do you take the measurement? as you can see from the shape of it, depending where you take the measurement, could vary it by several mm.

In any event I've used the clear pipe method and now have the float chamber fuel level sitting at exactly 3 mm below the carb/float bowl join on both carbs. Interestingly in order to achieve that, the heights of the floats differ between the carbs by 2mm. I also noted that one of the floats had been bent inward slightly and was just catching the side of the inlet valve chamber.

I'll refit the carbs tomorrow and let you know what happens.

Cheers all Fent

 

[Scorpio1963]

Yes that's what I meant,I also wonder if you could get brass floats for your carbs they are definitely easy to measure and set.Having rebuilt many carbs on autos I have dealth with those type of floats many times.They are difficult to adjust.

 

[Fentible]

Thanks Scorpio and you are right these were terrible to set up but at least I can hopefully rule them out of the equation now.

 

[Tombo]

You can always take a "pre-mount" wet measurement of the *fuel level. (*because this is NOT the float height). In pretty much any motorcycle carb, you want the fuel to be about 1-2.5mm below the bottom of the bowl. The closer to 1mm the better in my experience (with carbs in general). Wet set is the only truly accurate way to know whether your carb is able to deliver fuel as quickly as is needed (though the xs400 carbs seem more forgiving than others I've worked on)

Before you get started, find a clear tube that is the same size as the bowl drain screw (I forget what it is, probably 5mm). Make sure it fits snuggly in the hole left when you remove the drain screw. Now, insert this tube into the bottom of the drain screw on the carb you are testing. (I recommend connecting both carbs together for this measurement, choose one for the tube, then do the other one next.) Use a piece of tape or something to attach clear tube to the side of the carb, making sure it goes higher than the top of the fuel bowl.

Now, set your carbs up so that the top of the fuel bowl is level. Now attach your fuel tank and set to PRI. wait a few minutes for carbs to fill. The clear tube will show you where the top of the fuel is in the bowl. Make sure it is within 2.5mm of the top of the bowl. Now turn your tank to "On". Remove clear tube (with a cup or something to catch the fuel that will leak). Replace drain screw. Now remove the drain screw on the other carb, repeat. If they don't meet the 1mm-2.5mm tolerance, then I'd recommend adjusting float height.

Cheers

 

[Fentible]

Hi All

well its been a while but still having trouble with this XS400 at the prescribed revs.

The L/H pot is running way too rich and the R/H pot is spot on. The bike starts on the button, ticks over nicely and runs pretty well in any gear below 4K revs. If i rev it through the gears it will start to miss at 4k revs and really struggle to reach 6-7 k revs. Black smoke out of the exhaust.

If I swap the plugs over the nice clean one will soot up in less than a mile.

I've just replaced the coils, plug leads and plugs. Fuel height checked, diaphragm and carb slide okay, choke not sticking on.

I've had these carbs on and off so many times, they are spotless inside and the jets have done less than 18K miles

Worn jets or should I simply try and rectify by lowering the needle on that side if that is possible on these carbs??

 

[bcware]

Don't modify the one side to compensate; there is something wrong. A bandaid fix is not a good idea.

Synch the carbs with a manometer.

Check the diaphragms for pin holes.

Check the emulsion tube and emulsion needle for roundness; sometimes they get egg-shaped over time and the needle doesn't plug the hole.

Check the float on the rich side for pin holes if it is brass.

Replace the float needle and seat.

Check your float height on both sides of the float; it can get bent on just one side.

 

[Fentible]

Thanks for that bcware. I do agree that modifying one side isn't great practice but I really am at a loss here to understand why the L/H pot is running rich. The bike stood for 20+ years with fuel in the system and the carbs were so gummed up that it wasn't even possible to move the butterflies. The carbs were stripped, soaked for weeks in carb cleaner then ultrasonically cleaned. The bike starts on the button and ticks over beautifully....................just won't run over 4K revs without stuttering.

The rich running carb has been systematically dismantled and inspected. I can detect no perforation in the diaphragm, the floats are a rubberised type of material and the fuel level in both carbs is at 3mm below the float bowl-carb join. Float valves were renewed. The idle mix screws in the top of the carbs (not blanked off on UK models) are 1.5 turns out and have been adjusted to no effect to the issue and the carbs balanced using vacuum gauges. The air filters are both new.

I would not have anticipated worn jets with less than 18k miles on the clock and besides the R/H pot is perfect. I may try swapping the diaphragm/slide assembly to see if that makes any difference but if that doesn't work, I am at a loss

 

[16VGTIDave]

Has the engine been checked? What's the compression on the left cylinder? How are the valve adjustments?