MYSTERY MISFIRE

[PAULJAC47]

Any advice on this would be most welcome.
1975 a4 misfire between 2-4k all gears.Tried following.
Coils.Leads.points,Plugs,condensors,Timing static and dynamic,carbs are spotless jetted correctly floats etc all correct height,new rubbers airbox to carbs using old carbs to head rubbers but seem good and no leaks when spray wd40 around when running.Bike starts on the button and revs cleanly on stand only breaks up under load 2-4 k over 4k ok.plenty of fuel getting through tank vent is clear.Have also balanced carbs and pulling same vacumn all 4 carbs,compression test shows no diff between cylinders,exhaust is new with new head collet seals.Only thing i can think of to check is valve clearances!!!!! is that likely?as have always thought bad starting would be a symptom of this..........

[DogsbolloxofZ1B]

Hi, need more info.
1. When did this first occur?
2. Have you had the engine apart recently?
3. You seem to have checked most things, are you sure its advancing
correctly, need a strobe on it.

[KWACKERZ1]

Incorrect valve clearances, or potentially Knackered valve seats, or a damaged valve can cause crap running at the revs you are talking about.
A friend of mine did all the stuff your talking about on a bike he bought and eventually took the head off to find one valve with a nibble out the edge, it compression tested about right but when it ran on load it was rubbish.

Check the timing like DogsBX says, check valve clearances and then consider pulling the head off!!!

Altenatively before you do, if its always done it what spec engine, carbs, filters and jets are you running?
Badly setup carbs could cause poor running at those revs as you are not yet on full load.

What is the history, just started to happen has always happened etc

[PAULJAC47]

Hi Guys thanks for tryng to help.bike is a us import with one family owned history 14k on clock had stood for 12 years in container covered in grease i have timed with strobe and is bang on i have not had engine to bits but the carbs have been stripped and re-built using standard bits original 115 main 17.5 pilot jet, they were stuck solid but really free now i am starting to think valves, and before i pull head may try and come up with leak down tester to see if any leakage through valves any more ideas gratefully accepted,Cheers PAULJAC47.

[Pigford]

As its been standing for YEARS the potential for stuck/damaged engine bits is high

Stuck rings, burned (marked) bores where the piston rings have been touching, corroded valve stems/valve seats.....

Do as they lads say and check clearances first.... then its just a case of working your way down thru the engine

[RALPHARAMA]

If you're not already running Nippon Denso plugs, it's a very cheap investment, as many bikes on this forum have been plagued by shit NGK plugs most of which are no longer made in Japan. http://www.spark-plugs.co.uk are the folk to buy from - I think they're 'bout £1-15 per plug or there abouts.

Could be partially blocked pilot jets. Have you tried borrowing a different set of coils?

[PAULJAC47]

Hi thanks for trying to help out have tried all items you suggest hope you found z1 manual useful on link i posted...Cheers pauljac47

[FER]

Have you tried moving the needle position ? This in the right area of the rev band for the needle position to maybe a problem . Try one up or one down and see what happens .







fer

[Rich]

Have you checked the emulsion tubes? Lean or rich missfire?

[Pigford]

Try a 'roll on' test....

Get he running in a high gear at 3000 rpm....

#1 Hold steady throttle.... what happens

#2 Try openning throttle... Get better or Worse.....

#3 Try closing throttle a bit... Get better or Worse.....

[toycollector10]

Are the jet needles Kawasaki/Mikuni originals or Keyster or other?

The profiles on the needles is critical. My "other" in-line four would barely run at all using Keyster needles. Then I sourced genuine H**** needles and the bike runs like a charm.

So maybe if you get genuine Mikuni needles..??

[DogsbolloxofZ1B]

As toycollector10 says. Changing needles on their own is in my oppinion a no no. The needle jet & jet needle need to be a matched set and the needle lowered such that it almost touches the back of the Main jet with the slides in their lowest position, this will ensure that the vacuum flowing through the carb cannot lift fuel up the main jet into the airstream, if it does you'll be rich and no amount of adjustment on the airscrew will correct it.

One way of confirming the above for yourselves is to compare how easy it is to suck through the main jet on a worn set of carbs via one thats had the right kit fitted, its significantly more restricted with the new, a it should be.

Carbs restored by me circa £330, includes full resto, clean polish as per original finish etc and full kit used per carb.

[Pigford]

Sorry Dogs.... But...

One way of confirming the above for yourselves is to compare how easy it is to suck through the main jet on a worn set of carbs via one thats had the right kit fitted, its significantly more restricted with the new, a it should be.

Is there a correlation/correction factor for people who smoke and those who don't

[DogsbolloxofZ1B]

With my six sigma hat on isn't it all relative? Point taken though

[Rich]

Mikuni needles are coded for length and the double taper. The taper code is the CN in 5CN and the DL in 5DL. 5 is the code for the length. Check what size needle jet you have as well as these also altered dependant on needle used. 3 hole and 5 hole emulsion tubes alter the fuel available when the throttle is opened. You also need to take into account air jet size and the difference a blocked air jet would make. The area you are having problems with is the junction of the pilot and main systems and both will make a difference.
I would check that all jets / needles are correct for the carbs / year take it back to std and start from there
If you want to learn more about them "John Robinson Motorcycle tuning 4 strokes" explains the coding and Wired George (USA) is very helpful and has a very good technical forum, but as has been said before make sure everything else is right before starting playing with the carbs.