Hello and my 1st post after my 250/1 sezied.

[DAVIDGUEST123]

Hi.
Firstly I have to say what a great source of infomation this site has been.
I've been in the process of trying to get a '78 250/1 running correctly and finally I seized the thing. I have fitted the contactless optical 6v system (good bit of kit) and it is set correctly. The problem I've been having is with a replacement carb which ws fitted to the bike when I bought it. The carb is a Amal concentric mk2 (L2932-303)with a 107 needle, main of 230, and a slide of 2928. Fine around town but when running at high speed for more than 20mins the plug would foul. Over rich you would think ( Plug ring black). Run again with 220 main and ......bugger.
Two bits of advice/ info that I'm after are , rebore/piston are they available and is it worth me swapping out the Amal for a bing or the orginal bvf. I can get a barrel and piston for about £160 but that could turn out to be be a very expensive 20min thrash.

Thanks for any help/advice

[Puffs]

Hi David,

I'm not sure if Amal's are common on the 250 2T's, so I do not know if it's worth to replace for a more common carb. At any rate, mine's a '89 251 with the original BVF, and it never seized.

Probably jetting contributed (as it seized after you went down 1 size in main jet), but the first question that comes to mind is: what piston did you use, and with what clearance?

And also: could the plug fouling be caused by too much oil, or the wrong oil?

Good luck!

[DAVIDGUEST123]

Thanks Puffs.
I think your right about keeping the carb a standard BVF. This bike has been quite a challenge. I bought it from an old Aussie Hell’s Angel who had given it to his son to ‘play with’. One of the more interesting fabrications the youngster had engaged in was to manufacture a pannier rack. The rack was attached to the upper AND the lower rear suspension. This made the handling somewhat ‘Sporty’
Back to the seized barrel/piston. I think I have come up with a diagnosis. The Amal was running really rich as the correct 210 main jet had been drilled out to about 250. This was done to compensate for a incorrectly fitting head gasket. When I tried to lean it off. It was fine until a constant high rev run.
I’v enclosed some pics of the damage and would been interested in anyone’s second opinion. The spark plug was new old stock Champion. Opinion on the flakey insulator will be very welcome.
Many thanks Dave

[DAVIDGUEST123]

Couple more pics

[DAVID THOMPSON]

spark plug was new old stock Champion.

never had any thing but trouble from Champion plugs

get that out of there

Dave

buy non running VW remove heads fix destroyed threads replace with Bosch or ngk plugs

test drive a few days and sell it =$$$$
also a few bmw motorcycles heads destroyed by Champion over the years

[DAVIDGUEST123]

Thank for the ‘Heads up’ re Champion plugs. I bought them in haste when I was having fouling problems.
Turn out to be a bit of a false economy.

[Blurredman]

I don't think there's anything wrong with Champions. Some applications they work better than others. I prefer Denso over NGK in the situation of the MZ.

[Puffs]

On the plugs: I can't remember ever had any problems with Champion plugs, but I only used them in cars, as I can't remember their rating system. A plug with a rating that is to warm could give problems (and too cold may foul), NGK 7 or 8 should be OK.
For about a year now I use Brisk plugs in both my MZ & Jawa; these are quite cheap & work fine too. For engines like this, IMO there is not much in plugs, beyond getting the heat rating about right. But Blurredman has far more experience with plugs in MZ's than me.

Comments on your pics:
- I do not recognise the piston from what I see, but the size (68.96) suggests it's a piston fitted in the original bore, with 0.04mm recommended clearance. Looks like something (a small bolt????) left a mark on the crest, do you see similar in the head?
Your 2nd piston pic shows it was properly seized. There's no pic of the barrel, but you probably need a re-bore. Buy a good piston.
- The Champion L-3G should be equivalent to the NGK B8HS, so that should be fine. Bit on the cool side maybe. The colour of the insulator might suggest it was run a bit lean (consistent with your diagnosis), but it's not all that clear to me.
- You might want to use a new head gasket, and when installing it first properly fit it around the ridge on the barrel. However, it looks like the head properly carried onto the gasket, so I don't really see a problem in it leaking, and I see no signs of that either.
When re-assembling w/ a new piston & gaskets, also have a look at the clearance between piston (in TDC) & head, the squish band.

A final question: what fuel/oil mixture were you using, and which oil?

[DAVIDGUEST123]

Thanks for the info Puffs.
The oil I've been using is the Fushs Silkoline Super 2 At 40:1ration. The mark on the crown have got me puzzled ( no marks on the head). Needle bearings ok
and I've not found anything stuck in the head...maybe a bit of piston? Also to me the colour of the spark plug looks ok, half brown along the lower conductor.

Any input greatfully accepted

[Puffs]

That oil should be excellent & 1:40 should also work fine. Look in the book though, MZ may also for this bike prescribe 1:50.

That mark on the crown looks like something a small bolt could have left, but then there should also be a similar imprint in the head - or it happened in another head. That's also possible, that this piston was previously used in another engine, and possibly incurred some damage there.

Only other suggestion is trying to measure the clearance between piston & barrel (feeler gauge, measure at skirt), but that's probably not going to work well/ be reliable because of the damage.

Seriously lean should be closer to whitish in colour, but normally a 2T looks a bit darker than that; it does appear to be a bit on the lean side IMO.

[DAVIDGUEST123]

The mystery is solved. Rebore, piston gaskets and a couple of hours in the garage working on the bike , might help jog my memory when filling with petrol that oil needs to be added.
Pint on left from bike. Pint on right premixed on the bench. Bloke behind the camera ...muppet.

[Blurredman]

Oh dear oh dear

[Puffs]

Good that you found it!

That's the advantage of using coloured 2T oil, as apparently that Fuchs Silkolene is. With lighter honey-coloured oils, the only way to find out if you added oil is to put some on a clean dish & let the petrol evaporate (outside, in the sunshine). If the dish feels greasy after that, you have, if not, you haven't.

How are the small end, big end, main bearings? Bearing failure can sometimes also necessitate yet another rebore, so when in doubt, best bite the bullet now.

[DAVIDGUEST123]

Yep, well all the bearings look ok. The gugdeon pin hasn't blued and theres no play in any of the bearings. The sezie wasn't complete and I rode the bike back home, so ther was some oil in the fuel.
Hopefully I'll get the bore done before the weekend and get it back on the road. The engineer who is doing the rebore for me used to do a few bits for me before and won't do a rebore unless he has the
replacement piston in his hand.
Total cost of my stupidity will hopefully be fairly low. Age and memory ..like cranks weights and pistons ..when one gets higher the other gets lower

[Puffs]

OK, good. Roller bearings don't need much oil and there probably was some oil in the fuel from before.

Yes, a rebore + piston shouldn't brake the bank, but best buy a decent piston; the very cheapest are not necessarily a good buy.

Please keep us posted!