Suggestions sought for strange misfire problem

Hello Gentlemen and Ladies.

I'm after any helpful suggestions, since February's cross country rain festival, my bike has had an intermittent dead spot between 3500 and 4000rpm.

Below and above this range it generally runs quite well but if you crack open the throttle in this range it just seems to have no power for a second before picking up.

Thing is sometimes it goes fine and it seems fine if you roll on the throttle from below 3000rpm. I have been trying to analyse the problem so I could narrow down the area of the problem eg spark or fuelling.

It seems more like when the bike is running out of fuel and you need to switch to reserve rather than just dropping one cylinder.

I've got a new set of plugs and I think I'll drop the carbies out for a clean, so I'm after suggestions for things to check while I'm in there to save duplicating the effort.

It's always had a bit of a flat spot at that range so I'm wondering if it is an interaction of some other factor plus a bad mixture at that rev range.

Any helpful trouble shooting tips gratefully received.

Does it stutter in that range? Sounds like worn needle or seat.

sounds more like an air/ fuel problem to me than electrical (also are the inlet boots all tight & in good nick ? )

NZspokes wrote:Does it stutter in that range? Sounds like worn needle or seat.

Thanks for the quick reply.

I was trying to make out on Saturday whether it was running unevenly in that range at constant speed, not really sure.

I take it you mean the float chamber needle and seat, wouldn't that cause it to idle badly? I actually will pull away quite well from 1500rpm.

I'm not trying to nullify your advice just understand how it relates to my symptoms.

I know from other posts that some of you guys have done a lot more fooling with fuelling than I have.

paul wrote:sounds more like an air/ fuel problem to me than electrical (also are the inlet boots all tight & in good nick ? )

Good point, I'll be sure to check that too.

Re-Cycled wrote:

NZspokes wrote:Does it stutter in that range? Sounds like worn needle or seat.

Thanks for the quick reply.

I was trying to make out on Saturday whether it was running unevenly in that range at constant speed, not really sure.

I take it you mean the float chamber needle and seat, wouldn't that cause it to idle badly? I actually will pull away quite well from 1500rpm.

I'm not trying to nullify your advice just understand how it relates to my symptoms.

I know from other posts that some of you guys have done a lot more fooling with fuelling than I have.

No needle that is mounted in the slide that comes up with vacuum, if it or its seat is worn it makes the motor way to rich in that range.

How many ks on the bike?

NZspokes wrote:

No needle that is mounted in the slide that comes up with vacuum, if it or its seat is worn it makes the motor way to rich in that range.

How many ks on the bike?

That sounds more like it, yes the carbs have a lot of Ks on them.

Re-Cycled wrote:

NZspokes wrote:

No needle that is mounted in the slide that comes up with vacuum, if it or its seat is worn it makes the motor way to rich in that range.

How many ks on the bike?

That sounds more like it, yes the carbs have a lot of Ks on them.

Ok, if the carbs have a stack of ks on them I would give them a full rebuild. Kits, jets, slide needles and seats. Check the slides for wear etc.

Amazing how hard it can be to tune a motor with just worn jets, fuel flows badly over them and you get rich and lean spots.

Hi Peter,

your model has brass emulsion tubes, or spray tubes, that the slide needle goes down into. The needle vibrates "in the wind" and ovalises the tube. The mk2 carbies have a nickle plated tube that doesn't wear. I've read they can wear after only 10000km. Did these carbies come with the new motor?

If you're going to pull the carbies down then swap all of the phillips head screws that hold the float bowls on for better hex head screws. The original screws are made of cheese, and will turn to mush when you go to take them out.

This all sounds like good advice, anyone know of a good supplier of kits?

Yes Rolls still running MK1 carbies.

Re-Cycled wrote:Hello Gentlemen and Ladies.

I'm after any helpful suggestions, since February's cross country rain festival, my bike has had an intermittent dead spot between 3500 and 4000rpm.

Below and above this range it generally runs quite well but if you crack open the throttle in this range it just seems to have no power for a second before picking up.

Thing is sometimes it goes fine and it seems fine if you roll on the throttle from below 3000rpm. I have been trying to analyse the problem so I could narrow down the area of the problem eg spark or fuelling.
T
It seems more like when the bike is running out of fuel and you need to switch to reserve rather than just dropping one cylinder.

I've got a new set of plugs and I think I'll drop the carbies out for a clean, so I'm after suggestions for things to check while I'm in there to save duplicating the effort.

It's always had a bit of a flat spot at that range so I'm wondering if it is an interaction of some other factor plus a bad mixture at that rev range.

Any trouble shooting tips gratefully received.

I have a fix for your problem Peter and because I'm a good bloke I have included a pic to make it simple.

Cam chain slack ? Tensioner not tensioning? This can cause a hesitation when accelerating and rough engine when on a constant throttle at certain engine speeds. But you would probably hear the chain rattle more than normal.

Re-Cycled wrote:This all sounds like good advice, anyone know of a good supplier of kits?

Yes Rolls still running MK1 carbies.

http://www.smebike.com/about_us


This guy will send you all you need. Yes hes in NZ but he sends around the world. He is local to me and a great guy that knows heaps. His website is rubbish so e-mail him for what you need.

Um thanks Chris, I guess that would fix the problem, shame I don't like silver engines.

I don't thinks it's that sort of problem Phil, it's more like "dies" than "hesitates", thanks.

Spokes: I'm not seeing any links to your NZ guy

Woops, now added.

Should put down my rum when posting.

? Small hole in diaphram of one or more of the carbys. Are they diaphram type carbs?

Just an update on this problem, thanks to all the people who posted helpful suggestions.

I replaced the spark plugs and 2 rides now and the problem has not returned.

The plugs weren't that old and certainly not as old as the previous set that hadn't had any problems.

There is still the "normal" flat spot which obviously needs parts replaced in the carbs to fix, but the dieing is gone.

The confusing (for me) aspects of the problem were
1. the sudden onset of the problem
and
2. that it worked ok at lower and higher revs

In hindsight, when operating in the rich part of the carbys range one or more plugs must have been fouling and took a while to clear once out of that range.

Moral of the story, plugs are cheap and easy to change, try them first.

Thanks again guys for the input.

Check the fuel line route .We had a problem similar to this on the way to P.I. on the triumph .Tured out ,as the tank was dropped onto the frame

the line pinched a cable in the last 10mm .Of cause you couldnt see it .When you take the tank off and refit it the line was in a fractionally differant

spot so all was well .Eventually it kinked (0ne day ) and problem returned .Made the fuel line 20mm longer and it ran perfectly .

To fit plugs you would have removed tank .Triumph has carbs .

Funny thing (not at the time ) bike started shitting it sellf at DUNNEDOO

Yours sounds a bit differant but from memory the 1200 fuel line is a prick of a thing to attach to the tap .