Misfire at idle

[Blurredman]

Hi guys,

I'm having a bit of an idle problem with my '87 ETZ 250, wondering if there are people who have fixed this.


If the bike is idling for a few minutes, at the set speed of 1200-1300rpm, it will start to mis-fire slightly - and when pulling off feels a little wooly- but only when idling for several minutes.

The bike runs pretty great otherwise, revs nicely through the RPM, is smooth in the delivery and does not mis-fire or any other oddities when actually riding.


I have done the following to try and resolve the problem:
Changed spark plug,
Changed coil,
Cleaned carburettor thoroughly and adjusted settings for best tune.
Double checked timing and condition of contact breaker for gap/wear- 'looks' fine, but obviously could still be a problem.
Checked the bike is charging.
Checked for inlet air-leaks. None found.


Things I need to look at:
Replace the contact breakers?
Check electrical connections? - Fuse box, continuity. Fuses themselves.
Spark plug cap? Changed a few hundred miles ago..


As far as I can ascertain- when the mis-firing occurs, I can resolve this (?) - or rather, get around it from stalling completely, by keeping the revs up until I move off. Once I move off, and stop at the next traffic light- The same process of waiting for a couple of minutes before any sort of issue occurs.
When it does this 'mis-fire' the charge light/ignition lights do not flash - (giving an indication of poor connection).


Anyone else had similar symptoms??
I recall where I used to keep the pilot screw on the carb at 1 turn. Now it's at 3/4. Wondering if the idle circuit is lean? But trying to adjust these makes the idle a little lumpy. just 1/4 of a turn wouldn't effect the bike that much.. I really don't think it is carburettor related, at this point.

[Puffs]

No, I never had symptoms like these. Some conjecture:

Lacking cooling, the engine may warm up, and lacking flow through it, the warm engine may also warm the carburettor via the manifold. It will take some time after you start riding again before it cools down; could that be the 'wooly' period? (What is 'wooly' anyway?) I can imagine that something like that could be a cause, a warm carb may give a different mixture. How exactly I don't know, but fuel will evaporate more. Do you still have the plastic isolating flange between barrel & manifold? Regardless, it might still be a mixture issue.

The other thing that changes if you have the bike idle for a while, is that your voltage will drop. Do you have a volt meter in your system? Are your symptoms influenced if you switch off the lights (or on)?

[DAVID THOMPSON]

if its a 6 volt system try to check voltage when running ok and whats it down to when messing up

i never met a bike that ran very good on 6volts

my r27 was a great bike but it was wired with a coil and extra battery so it ran the coil as a 12v system
rest of bike was 6volts

[Blurredman]

12v system Dave.

[Blurredman]

Changed contact breaker and condenser.

Misfires happening all over the place. Those reproduction points are utter cack.

Will try with contactless ignition (Muller- but with different magnetic rotor) I have spare (tomorrow perhaps) and see what happens then.

[Puffs]

OK, but why blame the contact breakers for that? Do they not want to open, or do they not want to close & make contact? If the contact points are properly clean, I'd look for a problem elsewhere. As you apparently replaced both points & condenser at the same time, it's probably that condenser that's at fault.

Yes, let's see what happens with the Muller ignition. To me too the ignition seems most likely cause for your original problems. We've now learned that with the points it doesn't run backwards, but it has this slightly awkward idling problem.

[nice2day]

Could it be the float valve in the carburettor leaking very slightly?...Little fuel is used during idling so if the fuel level rises slightly over this time it will run rich as you accelerate off. Two strokes run badly when the mixture is rich but as you run the engine with more throttle, the level lowers back down to nearly the correct level and you don't notice anything wrong. As a quick test you could let the engine idle for a long period and turn the fuel off say 30 seconds before riding off...(turn back on the fuel as you do so) . If the effect disappears or is reduced it may point to what I have suggested.

Les

[Blurredman]

Good Idea.

I'm not entirely sure it is the contact breaker's per se. Which is why I will do the electronic ignition with one I have spare.

That said, If it still continues, I have a now spare carburettor which I could try, if I find the float in my original carburettor to still be 'floaty'. I did check the level only a month ago, to make sure it was up to spec. I also could take the carburettor off from my other bike- Swap them around even, to see if the problem changes bikes.

I notice however, that you only have to leave it in neutral (for the idle to increase by 100rpm or so) to 'get around' the misfire.

A very good shout however. I will check the float, I do have 5x brand new spares if it is at fault. I checked the valve itself last month and that was good. So it could be a leaking float.


But I need to do this:

ONE THING AT A TIME. I need to change one thing. Then use the bike for a while, then change another thing if the issue persists. Otherwise i'd bury myself in whats, and whys.

[Puffs]

Yes, fuel level in the float chamber; might be an explanation. Good call. When you came with this thread, I was thinking: what's happening when the engine idles? Carb might be heating up, voltage drops, and the other thing is: the engine vibrates up & down, or rather it shakes in the rubbers, with quite an amplitude. It's possible that under those conditions the float valve seals poorer, so that the level rises. (The float itself shouldn't be the cause: if that were leaking you would always have problems, not only after some idling.) It might be due to wear on that valve. That's easily recognised, so worth a look. Or test as nice2day suggests, that test will tell you without any work.

Another test that might help you diagnose the problem, without changing anything to the bike, is seeing if your headlight influences the problem. Particularly with the headlight on, the voltage to the ignition will drop while idling, particularly if your battery gets a bit weaker. You now say the problem goes away if you put it in neutral (something you should always do when stationary, IMHO, to save clutch plates & bearings), because it then idles some 100rpm faster. That's consistent with both possibilities: it then charges more, and it shakes less.

[Blurredman]

To-day:

Checked float, not leaking. Double checked valve- seals fully.

I did however change the spark plug. And the excessive misfiring went away that I had somehow enduced when last tinkering. I did however change the points back to the original (original MZ) ones. However I did leave the replacement condenser.

I rode 60 odd miles today on the bike, but they were mainly open road 50-60mph. The bike performed very well.
I did tune the bike slightly, allowing a richer idle (by half a turn) and raised the idle just a tad. I did a number of sitting in gear and it did mis-fire twice. However, I let them happen, and nothing really went wrong. It didn't stall, ignition light didn't come on and it continued to idle fine. I will keep my eye on this- For I normally commute in the city and have to sit for considerable time at lights- I likely wouldn't think there was any issue at all if I didn't commute in such low speed/standing conditions.

I did initialy try to install the contactless ignition- but the original factory rotor trigger is too large in diameter and doesn't clear items, specifically a heatsink, on the unit itself.


Thanks for your help guys, will keep you posted.

[nice2day]

The mystery continues but it's interesting to engage the mind for a solution. Ignoring all the ignition possibilities which can be eliminated by replacing items one at a time...Here's another off beat possibility... Could it be the oil metering is set too high on the initial delivery rate? If oil is collecting slightly by pooling due to the low flywheel speed not flinging the injected oil around, when you rev it, the larger amount of oil might upset the firing quality for a few seconds....It might be worth checking the pump setting.
I am happy to be proved wrong as I enjoy the mental engagement to solve problems like this....
Good luck keep the updates coming.

Les

[Blurredman]

I don't use the pump, Led.

The bike is premixed and has been so since 2013, when I introduced an infinate loop to the pump so that it doesn't run dry.

I do have a spare clutch side oil seal. I'm hesitant to replace it, but it's one of the only other things I can think of.

Once the bike settled down (at speed) there isn't excessive smoke. Full throttle there is a bit, but I would expect that in my Years of riding MZs. So I don't necessarily think the seal is at fault.

[nice2day]

Definitely not over oiling then....

Les

[Blurredman]

Definitely not over oiling then....

Les

Well.. we have yet to rule out the clutch side Oil seal...

[Blurredman]

I have solved this issue.


There seemed to be an air leak between manifold and carb. I always used to put ordinary grease on the mating surfaces, but one of the last time I re-installed the carburettor, I didn't do so....

I have now put some grease 'packing' in between the surfaces and around the inlet, and the bike idles lovely and power is back without bogging.

Thanks guys for your suggestions!