Stutttering over 4K revs

[Fentible]

Hi 16VGTIdave, I did wonder about that but compressions are 135 L/H and 130 R/H............tappet gaps are right in the middle of the adjustment range.

 

[16VGTIDave]

Then you are back to the carb or ignition system. Have you checked the o-ring on the enricher (starter, choke, whatever you want to call it) plunger? Are the air jets clear? Is the needle properly seated in the piston? Have you tried moving ignition components from left to right to see if the problem follows?

 

[Fentible]

I'm fairly certain its the carb, all the ignition components are new. You have made me wonder about the O ring on the choke mechanism. The mechanism moves freely but I did remove them to clean everything out...................I'll need to check that. Thanks

George

 

[bcware]

I would consider replacing ignition components. I had all kinds of problems when I first started that went away once i replaced the entire ignition system. If all the basic adjustments are not working it's time to pony up and spend some money. Don't assume anything works right just because it looks new. If it cannot be objectively tested or measured replace it.

In my opinion a very small leak somewhere will not cause issues at 4,000 rpms and above; only at idle. To affect 4,000+ rpms you would need a major leak; like an entire air filter not being attached or something like that. A little tiny seal somewhere will not matter at high rpms. Maybe a clogged air filter? That sort of thing.

If you have a nice, low, stable idle I would pretty much rule out a leak.

You said all the ignition components are new. Did you replace them yourself? What exactly did you replace?

 

[Fentible]

Bcware...sorry for the delayed response...weekend away.

Ignition components replaced are points, coils, condensors and spark plugs. I swapped coils and plugs over, no difference.

Air filters were replaced using proprietary filter foam over the original metal carcases and have done less than 50 miles.

Having rebuilt/renovated a variety of motorcycles and always managed to get them running correctly, You can probably get a sense as to why I am struggling with this one.

I would love to try a carb swap with a set of known good carbs to see if that resolves the problem.

 

[xschris]

My guess is the air filter foam you used is causing part or most of the issue.

 

[Fentible]

It was air filter specified foam and as stated the R/H pot runs fine but I will try a run without the filter fitted to see if it makes any difference.

To all I really appreciate your thoughts and taking the time to input. Thank you

 

[bcware]

I had just as much trouble to be honest. The xs400 carbs are a pain; one little modification from stock can keep it from running correctly or one minor adjustment, etc. It's like all or nothing with these things.

My friend's cb360 would run no matter what; we even had main jets fall out and the bike still ran fine! Ridiculous!

The xs400 needs surgically clean carbs and little tweaks to the air intake often have a big impact. The best I can say is to try everything systematically and test everything you can to rule out causes. Compared to other bikes, however, this one can be a bit trickier in my experience.

If you replaced the points make absolutely sure your timing isn't off. That setting has the highest risk of a mistake in my opinion.

 

[Fentible]

Well I'm truly stumped with this bike, I have no idea what is going on with it or what is causing the sooted plug on the left hand pot and the inability to rev cleanly beyond 4K revs. Every ignition component replaced and where possible swapped with the right hand pot to no effect. I still end up with a sooted plug.

Is there something obvious I am missing??

Ignition Side

Renewed Plugs (several times), Points, Condensors, Coils, HT leads and HT caps

Carb Side

Renewed: Float valves, and all jets in the L/H carb. Diaphragms are perfect, both slides operate cleanly and in harmony and the carbs have been balanced. Air filters renewed (even tried running without them to no effect), Choke mechanism checked and functioning

Additionally

Points gaps, spot on at 14 thou, timing (bulb lights at LF and RF point), valve gaps, perfect, compressions both sides indicating 150psi.

I should also mention that the bike starts on the button, ticks over nicely and runs well below 4K revs, but hit that spot and it just coughs and splutters up the rest of the rev range.


Where else do I go, is this bike just cursed??

 

[xschris]

Did you remove and clean your emulsion tubes (needle jets)? Make sure they are in good condition and all the holes are clear.

 

[Fentible]

Yes, replaced the emulsion tube and needle with new in L/H carb, even though they looked fine.

 

[Fentible]

After exhausting all the possibilities regarding a misfire on my 1980 XS400SE (4G5) with regard to ignition and carbs. I am still left with a bike that stutters and misfires from around 4K revs and struggles to rev beyond when on the move. Still starts on the button and ticks over at exactly 1200 revs without a stumble, revs up on the stand and on the move, pulls quite strongly until the 4K barrier.

The Right plug shows a perfect mixture while the left looks black and sooty. Decided to try a hotter plug today so went and bought a BP6ES, new plug in and 400 yards onto a straight I opened her up to the now annoying 4K stutter, took it about half a mile and with the new plug in, it did drive through the 4k barrier, also pulled better in the higher gears but still misfiring. Once back on the drive I took the new plug out to see what looked like wet oily deposits around the end and nose of the plug.

That got me thinking, wet oil on the plug would surely cause tracking and a misfire?? Source of the oil ingestion can only be damaged oil rings, worn valve guides or the valve stem oil seals aren't sealing. This bike had stood for 18 years prior to me getting it running and has an indicated 18K miles on the clocks.

 

[Fentible]

As in dedicated post, it turned out to be the valve stem oil seals. Replaced and misfire gone.

Thanks to all who contributed.

Fent

 

[xschris]

Glad you got it. Not typical thing but possible. The bike should have been smoking a lot if they where bad enough to foul a plug.

 

[Fentible]

That's the strange part, there was the odd puff of smoke when revving up to about 7k on the stand, but not at any rev range below 4K revs. My theory is that the VSOS weren't allowing copious amounts of oil through that would 'drown' the plug, I believe that a small quantity of oil directed toward the plug was causing the spark to track away from the central electrode, enough to cause the misfire and which then gradually caused the plug to soot up. Had I not used new spark plugs on a very short run I wouldn't have noticed the oil around it.

It's been an odd one but I'm so pleased its resolved.

 

[clumax]

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.

On a stand or under load ?