Engine stuttering and racing

Now, I'm hoping I've got to the bottom of this, but just in case I haven't I thought I'd ask here too.

When I took my bike out on the motorway this evening, I found that the engine was stuttering. That is, the revs would go down to zero for just a moment, then back to where they were before. The reason for the ride on the motorway is because I also found that as the engine warms up, the revs creep up to as much as 6k, and sit there as an "idle" speed until I shut the engine off and let it cool, if I shut off and restart, it goes back to the high revs it had before.

Ok, so those are the symptoms. Now I resynced the carbs this evening after readjusting the idle mix. Both are now spot on, I did find as I was doing plug number 3, that it was black, suggesting over rich, confirmed by the Colortune. Also, colortune suggested that plug 1 was running lean. I tweaked both until they were spot on, then balanced the carbs so that they're all sitting spot on the 14 mark, I've never had them so level.

Now the engine sounds sweet, but I haven't been able to take her out tonight to check, and I've got to get her ready for the roadworthy inspection on Thursday, so I'ma bit pressed for time. Can any of you think of anything else that it might be? I can't check the float levels, because the drain screws are jammed and sheered off. Unless someone can tell me how to do it by looking directly at the floats? For my model, the ideal height should be 3mm +-1

Anything else?

 

Check your choke cable as well.... if it stick open it can cause the bog and rise effect

 

Just a thought.. Could this indicate an air leak round the carbs.. I've had them on and off so much, I wouldn't be surprised.. I'll test for leaks tonight if it's worth it.

Is there anyway to fix such leaks? Gasket compound or something? I only want a short term fix to get it through the test on Thursday, after that I'll make a permenant fix if one is possible or buy new rubbers.

 

The sputter was prob due to the rich plug while the high rev indicates
a lean (poss air leak )condition.
You may have already solved it with the pilot screw adj and sync.
Take her out and see if you've solved it then come back with
the results.

 

Thanks for that, I was going to check it, I just wanted to know if I was on the right track with a leak if it's still at it.. I'm a bit pressed for time at home, I have 3 hours working time tonight when I can run the engine, 3 hours tomorrow and then that's it.. I'm off to the testing centre on Thursday, 7.30am if it misbehaves then, it's off the road 'til I can fix it. So I wanna know what I should look for if it's not one thing or another, so I can get as much done without firing up the PC and asking questions and waiting for the answer.

 

Run her on the lean side for the test....get all the cyl the same-
real lean and you sould have no probs at the test site.
Just don't go too lean or you can burn the valves/cyl crowns.

 

Ok, so I've got the test tomorrow morning.. The bike is still misbehaving, but it's a lot better. The over revving seems to have gone away, but I took the bike down the motorway, and after 10 mins at 5k+ revs, I get the same stuttering. Drop below 5k and the symptoms go away.

So I'm pretty sure the bike will get through the test, They won't run it up that high, but I can't sell this bike with this going on.

My thoughts are that this happens because not enough fuel is getting to the engine when the revs are high. Which either means that the hose is gummed up or the float height is wrong, because I cleaned everything else. The only other thing I can think is that maybe the added vacuum causes a leak that isn't present at low revs..

Does anyone have any thoughts on this?

 

Sure. I think you answered your own question(s.) You may have hit both nails on the head, actually.

1. Float levels. If you've got a float or two that are way off either way, the symptoms you describe would fit.

OR

2. Throttle shaft seals. (Vacuum leak that doesn't happen at low revs.) Actually, it's probably more the position of the throttle than the RPMs/vacuum. Throttle shaft seals can break down in such a way that they seal fine until the throttle shaft is turned more than a few degrees, then they leak. I've seen this as have other members.

But before you go through all the grief of breaking the rack, filing the butterfly screws, etc; CHECK THE FLOAT LEVELS!!!

Yes, there is pain involved. But there is also no way to "tune around" it if that is the problem.

We don't just make stuff up here; we've BEEN THERE. Get the bowls off, get one drain screw fixed, and "rotate" that good bowl down the rack if you have to but check the blarsted floats.

Before you fret about throttle shaft seals.

 

Thank you very much for the advice Fitz..

I'm still a noob at this so despite the countless warnings, I still (until recently) thought I could get away with doing the minimum, but of course, my bike has other ideas, I guess I've learned stuff as much through not doing what I should have done than through doing what I was told..

So the bike passed the test this morning, thanks to the excellent advice I've had here. I'm not going to take the carbs off again for a little while, as I have to work on my XJR which now needs a full service. I am going to sell the XJ when I'm happy that the engine is running properly, I bought her to learn on and learn is what I have done, so now I need another challenge.. I've a hankering for an old Norton Commando, like yours which I want to rebuild, I love their shape, I think they're proper bikers bikes.

So looking at what's at issue here, the floats are probably set too high, (I'm hoping) so that there's not enough petrol in the float chambers when the needle drops and allows more petrol to enter and this starves the engine.

If this isn't the problem, then it's possibly the throttle shaft seals.. Since I'm a noob but a quick learner, I'm assuming that these are the seals around the shaft that holds the butterfly valves?? Hmm.. That's going to be a real pain to fix, you're right.. Oh well, I'm not pressed for time any more, so if the floats don't turn out to be the problem, I'll take the carbs out and set about rebuilding them and replacing those seals over the winter in the comfort of my kitchen (Something I'm sure my wife will be delighted about )

Once again, thanks for your help and advice.. Like most, I'm really pleased I found this site.

 

Well, I just wanted to put a close to this topic.

I took the carbs apart yesterday after getting an unexpected day off. Before doing so I managed to get one of those drain screws undone, so I did the float height measuring, and the particular carb I chose was spot on, which was useful.

Anyway's, using that float as a reference, I measured the others, found one more that was spot on, and one which was 3mm less, and one which was 2mm more. So I sorted them out. While I had the carbs off, I decided that maybe it wouldn't hurt to clean the plungers. So I took them apart and took them to my polishing machine until they gleamed.

Puttng the carbs back together, the idle mix was close but needed a tweak, same with the balance.. But riding the bike down the road was like riding a new bike. She started without the choke for the first time ever, and no more racing, no more stuttering.

Now if I could just get rid of the vibration at 6k+ I'd be a happy man.. I've a feeling that's the cam chain.. The valves are within tolerence.

 

Arrrggghhh...

Story not quite over..

Well the stuttering has gone.. Yipeeeee...

But, after an hour on the motorway with the bike at 5k+, when I finally reach a town, I have found (both on the way to work and the way back) that the bike sticks at high revs. Up to 6000. Not good when you've got cars around you.

I DON'T think this is the idle circuit, but I gues it could be.. There's two other circuits, the cold start enrichment circuit, I read somewhere of the enrichment circuits sticking open, but I can't see how, when I switched it off over an hour before and it was behaving normally.

What I have found is that blipping the throttle a few times will eventually clear this, and the bike then behaves normally unless I take it on the motorway again.

My feeling is that the slides or butterfly valves are getting stuck.. But equally this could be a vacuum leak I suppose?? I've checked for leaks.. The rubbers all seem in good condition, and I get no change in tone with butane.

Help!?!?!??