ETZ251 won't cruise smoothly

[alexxx]

Hi Folks,

Got my MZ ETZ251 mot'd today and took it for a first ride, first impression, very little torque but it picks up and zooms through the gears fine if you let it rev out.
One problem I noticed was when trying to maintain a steady speed say 30 or 50 mph the bike felt like it was stuttering along, it was smoother to accelerate than cruising smoothly. Also noticed a lot of surging on overrun on downhills.
Bike starts easily, has good compression, electronic ignition is fitted as standard.
This is my first ride on an MZ, maybe this is how they are?
Any advice appreciated

[DAVID THOMPSON]

make sure you have the proper plug installed and gaped

[Skorpion]

Hi alexxx.

Yes these engine's can seem to be 4 stroking between 3000-3500 rev's on a steady throttle, I think it's as the carburetion is between pilot & main jets.

If you spend time getting a reliable tickover 1000 rpm or less, you will find that the surging on the overrun greatly reduced.

[alexxx]

Thanks folks,

Will check plug and try to get a reliable tickover.
Think you are correct regarding the jet issues, it seems to be hunting between the pilot and main jet.
Are there any carb swaps that can improve, something from a Japanese two stroke?
Never had this issue on any japanese 2 stoke, I had a CZ 125 mx bike and a Maico 490 MX bike all run better on steady throttle than this MZ and MX bikes are designed to be on or off throttle yet I could ride around casually off road easily.

Thanks

[Blurredman]

The big question.... Do I want surging, or a low and possibly unreliable tickover?

I've kinda learnt to live with the surging and realise that it's just one of those characteristics of the bike.

I found my surging was greatly reduced when I checked the timing and adjusted it. Of course, this was because the bike in question was on a points system and the points had worn, however. It is still worth checking that it is timed correctly...

Unsure what year it is you've got. If it is an early variation then it could be a BVF, otherwise a Bing.

[Skorpion]

Yes you can also reset the ignition timing to between 2.5 mm & the recommended 3.0 mm BTDC,
you should see an improvement in running, every engine is different, past 3.0 mm BTDC you get vicious kickback when starting.

[Andy_C]

Hi Folks,

Got my MZ ETZ251 mot'd today and took it for a first ride, first impression, very little torque but it picks up and zooms through the gears fine if you let it rev out.
One problem I noticed was when trying to maintain a steady speed say 30 or 50 mph the bike felt like it was stuttering along, it was smoother to accelerate than cruising smoothly. Also noticed a lot of surging on overrun on downhills.
Bike starts easily, has good compression, electronic ignition is fitted as standard.
This is my first ride on an MZ, maybe this is how they are?
Any advice appreciated

This sounds like it is running rich - over rich will make it stutter at a steady speed - try dropping the needle a notch.

The surging is mainly caused by being over rich at the bottom end of the carb setting - aim for a very low tickover speed (no more than 1000RPM) and adjust the air screw for the lowest reliable tickover.

The low speed surging can be just about tuned out but can be a compromise between surging and pickup from low revs.

They do not need to surge or stutter, it just takes time and patience to get it sorted. My ETZ feels pretty good and does not surge too much, oh yes and one other thing, check your chain adjustment, if too loose it will make is surge even more - dont ask why it just does.

Take a look at this video I did last year on my ETZ - seems to go pretty well.

Good luck https://youtu.be/2oxyDO83aw0

[alexxx]

Thanks all for all the helpful advice.
The Carb is a BVS, standard I think, I have never opened it up but will do so to check the settings.
The surging on over run I can live with but I was hoping for a smooth cruising machine.
Checked out a few youtube videos and all seem to cruise along ok so my bike must have something amiss.
Will change for a new plug, check/clean carb, it always needs the choke to start unless hot already so I assumed it was more lean than rich?
I don't think I have the tools needed to check the timing on this electronic ign bike but will check online for a how to.

[Andy_C]

Carb should be a BVF and should be a 30N 2-5, although a BVF 30N 3-1 will also work - thats what I have on mine.

If you fancy spending around £160 you can get a 30mm Mikuni from Allens Performance that is reputed to be very good.

Indeed they should cruise comfortably and smoothly.

For timing just whip the head off and set the piston to (think it's 3mm BTDC) - or whatever the standard timing is (that's what I use on mine) - if the points gap is out that will also make it run badly so check the gap.

Once you have the BTDC setting check that the points are opening at that point and you have it set.

I made some marks with white paint on mine on the rotor under the points so I can easily check / re set points if needed. I have leccy ignition on mine (an old Sparkrite unit) and that has the luxury of an LED that goes out when the points open so it's easy to set timing. Sorry, having re read your email it looks like you do not have points ign - still easy enough to do you just use a multimeter to check the point at which it fires - I used to have a 301 that had the leccy ignition. Wish I never got rid of it as it never surged and had a lot more bottom end that the 251 - you live and learn.

You will get there in the end.

P.S. If it is an original carb the needle jet and needle may be worn making it run rich on the standard settings, also worth checking fuel height as that can also make it run rich. There is a manual at https://mzriders.files.wordpress.com/20 ... manual.pdf which is worth looking at and covers most things - quality is a bit poor but still readable.

[alexxx]

Thanks to all,

I will be giving the carb a strip down this weekend, checking the timing and fitting a new plug.

Regards

[alexxx]

Thanks Andy C for the youtube and manual links.
Yep..your bike rides much smoother than mine

[Andy_C]

No problem Alex.

I think the watchword is patience, I found with mine that once the timing was set as per the book it took a lot of fiddling with the carb ( The BVF 30N 3-1 fitted on mine has 2 screws that need adjustment - a slow running mixture screw & an air bypass screw, unlike the 30N 2-5 which has only a slow running screw) - remember aim for the lowest consistent tickover 1000RPM or therabouts.

They dont have much grunt below around 3000 RPM so do not be suprised if not a lot goes on before 3000RPM, if you set the slow running mixture to week it does tend to rob the engine of what little torque that it does have below 3000RPM so it a bit of a compromise.

One other thing comes to mind and that is the state of the silencer - if it is coked up / gunged up internally you will find that it robs power from the engine at higher revs, I give mine the caustic soda treatment once in a while to remove build up, and it does make a difference to the running.

Good luck and keep us all posted

[DAVID THOMPSON]

after market silencers not made to original MZ spec can be a problem
mz was the king of making hot air flow in a 2 stroke engine
the Japanese learned from them..

[alexxx]

Update:
in preparing to strip and clean the carb I first took off the fuel pipe...the fuel looked light green! it must have been quite old, it did not smell off but certainly looked it, so I drained the tank flushed it with clean fueland topped it up with fresh.
Turned the carb ( I think it is a BVF 30N 3-1) on it's mounting as found it difficult to remove...do you have to remove the filter box? could not get enough rearward space to remove from front mounting.
There was some sediment? in the float bowl but all else looked clean, took out the main jet but could not see any holes in the side to clean (like japanese carbs I am used to)
The needle was almost at the highest level, just one groove left below so I set it to the middle (what is the suggested standard position?)
There seems to be only one external adjustment screw on the RHS, what is that screw? an air feed screw or tickover screw?
Anyway, it started easily, I took it out for a ride, seemed to take a while to warm up but did run much smoother than before, I expect it was the foosty fuel the was the main issue.
I did find it would not pull up inclines with ease unless you changed down and let it rip through the gears, it even bogged down in taking away from the traffic lights one time, do I need to lift the needle and richen it a bit more? Will take out the plug tomorrow and check, it rode fine on flats but if you tried to cruise up small inclines it bogged down, you could hear the open throttle but no action.
Overall, very much smoother but bogging down if faced with any incline

[Andy_C]

The old fuel certainly would not have helped !

If you have only 1 adjustment screw on your carb it will be a 30N 2-5 a BVF 30N 3-1 the has 2 adjustment screws.

Believe that for the 30N 2-5 you should start at 1.5 turns out and then adjust either in or out for best tickover, the 30N 3-1 adjustment is different - 2.5 turns out for the air screw and 4 turns for the bypass screw, and then adjust them for best tickover / slow running.

Whatever carb you have it should be marked either 30N 2-5 or 30N 3-1, if not marked you may have a "cheapo" reporduction which spells trouble.

Needle position for the 30N 3-1 is 3rd groove from the top, dont know about the other - but mid point is usually a good bet, it set in the topmost groove it will be running weak.

If it bogs down it dosent sound right, you do need to dial in around 3000RPM to get away reasonably, and if going up an incline if revs drop below 3000RPM you need to go down a gear - have you checked the timing yet? sounds like it may not be right - it has a dramatic impact on running.

Hope this may help - keep at it you will sort it out.