lack of top end power

[rambo]

Hi

So after many years i got my mz etz 251 rolling again. needed lots of setting work

i have a bing 84 carb
slow jet is a 0.5
main jet is 130
and needle sits in the midle ( 2th groove)
spark plug is a bosh wr 7 bc+ and the ring on the treads is nice brown
it has one of those expansion pipe's http://www.bsabantamclub.com/wp-content ... mage-4.jpg
it runs well from 2000 all the way to 7000 rpm but......
when it puls its seems to pul stop pulling pul stop pulling in a fast rate. you do not notice it while driving but can here it from the engine.
also from 0 to 90 km/h all go's well but it doesnt go any faster. once it hits 90 in 4th it stays there. shifting to 5th makes no difference.

is it the pipe?
is it the needle?
is it?????????

[DAVID THOMPSON]

try a stock pipe //// and most pipes do not very often improve things// over a stock pipe..except add noise

and if the man with the badge hears you coming and sees you going he knows who to chase

[rambo]

hate to admid it but put on stock pipe yesterday. wasnt all that great. than cut out about 20 cm ( between the 2 seems from mufler and cone to have a faster impuls return) and it seems to be much better. all i adjusted for now is the air screw so no jetting done jet.

but it already seems more responsive in low revs ( couldnt really drive it yet since it was rainfloods yesterday)

[Blurredman]

I wonder if the spark plug is hot enough ?

Recently I went on a trip to Turkey on my MZ, so because I knew I would be doing faster droning for hours and hours I bought two new NGK BH7 and two NGH BH8, and I fitted the 8, in order to improve cooling because I was worried about overheating at that speed.

I found that the bike would not go above 4.5k RPM in top. I assumed it was because there just was too much wind (it was a windy/rainy trip) and that there was a lot of weight on the bike and thus it was the reason it couldn't get past 60MPH.

However, one day whilst having problems with the bike (I think the rain was getting into spark plug cap and it was constantly cutting out, (later on I greased it up- which may or may not have solved the problem.. unknown.. ) however I replaced the spark plug with the hotter grade and I found that the bike could indeed get to 5k RPM and produce 70MPH when not battling the winds or resistence against lorry 'wind wall' when trying to overtake.

And then I rode for 800miles with no overheating or any issue concerning power output either..


I've looked at comparison tables for that spark plug you have mentioned but cannot see that it is compatable with NGK B7HS or B8HS... Infact, that spark plug has a huge reach on it..

You need to try a different spark plug which..

Threaddiameter: 14mm
Threadreach: 12.7mm
Seattype: flat
Hexsize: 21mm
Tipconfiguration: non projected

[rambo]

plug is a 7 grade. grts

[Skorpion]

As Blurredman said you seem to be using a totally wrong spark plug, Just because its a 7 grade has nothing to do with it, Use a NGK B7HS.

[Guesi]

Originally there should be a B8 HS...

And 130 main jet is far to big.
Try a smaller one. Most of customers use between 120 and 125.

[rambo]

looked some further. plug is a bosch w7 so compared to a ngk b6 so i should ride a warmer spark plug? also the 130 seems to run fine ( do note the compression has been raised) however ill try a 125 and 120 just because i have them to compare.

thnx for the advise.

greets

[breakwellmz]

Have you ever done a full throttle `plug chop`?
Use a new plug and find yourself a long bit of clear road.

[rambo]

yes and mixed result. the porcelain part was dark grey but the ring of the threaded part was shiny brown coffee color ( that was with previous exhaust not yet tried with the new one because off the rains and the 5 year old daughter that's with me this week)

[rambo]

So. Here i am so far original pipe on. And had a play with the jets. Was a 135!! Went down to a 125. Had a 120 in but it ran slower. So back to a 125. It now tops out at 117 kmh on gps with the boch wr7bc+ and has a nice brown color to it. have put in a ngk br8hs but then i top out at 110 and when shifting to 5th it slows down again. Did notice that the chain is prettyz tight ( thinck it gets its run no run feeling from there) so still need to loosen that one up but ran out of time. Would the chain hold the engine back so much?

It used to run 140 with ease as I remember.

[DAVID THOMPSON]

that loose then tight chain does not help
hold bike so wheels are off the ground and spin them by hand
make sure bearing are free and no brake drums dragging
no shining spots under fenders where tire is rubbing any thing

and proper size tires my old BMW 1963 R50 S runs faster with 350x 18 front and rear
400x18 on rear slows it down a bunch

[rambo]

Was plan from second i have time. It has skinny race tyres and no fenders at all. Just the bare needs. Wheels an engine and a steering wheel. Thnx

[rambo]

Dos strike me... if the chain tensions and loosens it means the gears it runs on are not perfect round...

And there is no end to it. Wanted to put back the bosh plug... sat loose in the hole. As it appears... the plug hole has been sleeved once... and the sleeve is stuck on the ngk plug. I could go and sit in a bar with mates instead of working on cars and bikes... o well at least my time and money doesn't get spend on something i pis out evaporatas and becomes rain at some time.

[breakwellmz]

Sorted out your plug or still in the bar?
Might this inspire you?-