Stutter/Misfire at high RPM

[ben]

This follows on from my post on cylinders 3&4 not firing. Managed to get those working by replacing the plug caps. Now I have a new problem!

When riding for a bit at high RPM/speed (5-10 mins at 70+ MPH), the bike starts to stutter, which will turn into a misfire if I carry on. Then after a few mins at low RPM/speed the misfire/stutter goes away.

I previouly took off the carbs and cleaned a load of gunk out of the float bowls and jets (didn't touch anything else).

Things I have tried/checked:

• Checked for kinks in tank breathers and fuel line.
• New fuel filter - previous one was installed backwards so couldn't see if it was blocked. It wasn't.
• New sparks.
• New spark caps (NGK straight) - Cut 10mm of the HT leads before installing
• New (2nd hand) regulator/rectifier.
• Checked coils - primary resistance out by .5 ohm, secondary resistance well in spec.

Anyone got any ideas?

[BBROWN1664]

Is this after 5-10 mins of riding?
Does it seem like all cylinders failing?
When it happens, turn the engine off and open the fuel cap gently and listen to see if there is a very gentle hiss. If my thinking is right, when you turn on the ignition again, the fuel pump will run (it wouldnt normally do more than a single click after just being turned off). If you get the hiss and pump running, time to check the fuel pipes / breather hoses are not kinked.

[darrsi]

Check the carb rubbers are on properly and sealed.
Had my carbs fitted back on years ago and one edge of carb rubber #2 had folded in on itself leaving an air leak.
Because it was #2 and hidden it was only spotted on removal again by a different mechanic.

[unfazed]

Reading back through your original post, You said you cleaned the jets. Did you remove the main jets to clean them??

[ben]

It is after 5-10 mins of riding. It seems to be a little intermittent. Sometimes banging the throttle open will stop it a bit, but it will start again when going off the throttle. Sometimes it will just sit at 85ish and not go any faster because of the stuttering.

I tried opening the fuel cap - no hiss. The fuel pump relay didn't tick when turning it back on. I even lifted up the tank to check the fuel filter and it was full of fuel, so I don't think it's a tank breathing issue.

Checked the carb rubbers. They're all on securely. They look a bit cracked but I think the cracking is just on the surface.

When i removed the carbs I cleaned the main and pilot jets.

Even tried balancing the carbs, but they were fine.

I've run out of ideas!

[unfazed]

When you removed the main jets did the nozzles by any chance fall out. They can fall out without being noticed and the lack of one will cause high speed over fueling. Sometimes they are put in upside down which also casues issues as the flow flow is effected when the needle is raised at high revs

[ben]

Would that be the needle jets? I will confess, I didn't take them out. Just took out the Pilot jet, main jet holder, and main jet.

So over the weekend, I thought I'd balance the carbs. Turns out they only needed minor tweaks, they didn't really need balancing. I also moved a breather tube as it was getting stuck on the carbs when lifting the tank, and checked to see if the coolant needed topping up (it didn't).

Someone suggested pulling out the choke when the stuttering starts. If it stops, then it confirms it's a fuel issue. So I went out for a ride and there was no stuttering. Absolutely ragged it for a good 20 mins and it was absolutely fine. I'm pretty sure none of the things I did would fix the problem. So who knows whats happened there. Maybe some crap got dislodged.


I don't like magically disappearing problems because I don't know when they're going to magically appear again. I'll try and test it out again a few times over the next week.

[unfazed]

It is where the needle from the diaphragm  slides  go into the carb body. In  there is a nozzle sometime called the needle jet, which is held in place by the emulsifier tube which to add to the confusion is also called the main jet holder or the  main needle jet.
.
The main jet screws into this emulsifier tube.
.
The main jet has an O ring which needs to be in good condition or fuel will seep through giving a richer mixture on 1/4 to full throttle. .
These O rings are best checked carefully whenever the main jet is unscrewed as they get flattened and crack at the edges.Unfortunately nozzle is called needle jet in one part of the service manual the not shown in the Service manual check list and has a different name in the parts catalogue which is one of the reasons it can often be missed/ Add to this it fit one way but can be installed both ways.
.The Concave side goes in the hole first and the flat side is held the emulsifier tube..Once you get through the confusion it will make sense  You can check that they didn't fall out by removing the Diaphragm cover, lifting out the diaphragm carefully and looking don into the hole you should see a smooth concave part around the hole where the needle goes

[basher]

any update?