my 2002 Skorpion won't rev past 3500 rpm

[bill h]

some background, the bike was running fine when I put it away in October last year, due to medical issues I wasn't able to ride the bike until August.
Before putting it away I filled the tank and added Stabil.

Now the bike starts right up and idles normally but it won't accelerate past 3500 rpm under load, until that point it runs fine: no hesitation, sputtering etc.
When I back off the throttle it runs fine.
The bike will rev to 5000+ rpm sanding still in neutral.
When I use the enricher(choke) it has a fast idle and when I take the choke off it idles normally.

Thing I have checked and done:
drained the gas and added fresh gas
checked that gas is flowing from the tank to the carb (I do run a fuel filter)
removed the drain screw on the main carb to see that fuel drains freely, it does.
on the secondary carb I checked that the diaphragm assembly is free and that the diaphragm is intact.
also removed the main jet on the secondary carb and it is clean and gas is flowing to the secondary carb.
checked the valve clearances
new spark plug
ran it with out an air filter, still won't go past 3500 rpm
I am stumped what do I check next.

thanks for any help
Bill

[DAVID THOMPSON]

2 screws on left side in the brass butterfly may have came loose
or the second carb may not be opening the brass piece at correct time

the brass piece controls the vacuum if it not working or out of time things will not work

i hate this type carb it always seems to have some kind of problem

[samandkimberly]

Any luck?

It could be a plug wire going bad - I'd pull the plug cover off and makes sure everything is nice and dry in the spark plug hole; maybe pull the plug and check it too. Wet /bad plug wires can act exactly like this, only shorting to ground when theres' enough air/fuel/compression in the cylinder to make the path of lowest resistance somewhere other than the plug gap.

[DAVID THOMPSON]

its got to the point there so many things can go wrong its a miracle any thing starts and runs anymore

the choke enrichment plunger is stuck in my rt125 carb

and i even run fuel with no ethanol

dave

[bill h]

thanks Dave
I have checked the screws holding the butterfly and they were tight.
I checked that the carb rubber manifolds had no cracks and that they were tight.
I checked the operation of the primary carb slide and secondary butterfly when the primary slide is at the top, the secondary butterfly is wide open.
The carb throats are surprisingly clean .
The secondary butterfly operates smoothly.
However, when I put every thing back together and using auxiliary gas tank started the bike in neutral to watch the carb operation.
I noticed that when revving the bike past 5000 rpm, the secondary carb diaphragm assembly did not open.
Should it open ? and what would prevent it from opening?
As I said initially, the diaphragm assembly opens with finger pressure and closes smoothly.
I again checked for cracks in the rubber around the diaphragm assembly piston.
Also should the square notched portion of the diaphragm assembly piston face the butterfly?
I put my hand in front of the secondary bell mouth and felt suction when I revved it but I don't think the piston moved and the engine died when I released the throttle.
and lastly is there anywhere on the web I can about the operation of the carbs. I have only been able to find parts diagrams.
Thanks in advance for any help.
Bill

[DAVID THOMPSON]

one of the big problem with this set up is an almost total lack of any articles on how they really work

if i had had more info on these carb before hand there would not be a 660 powered MZ in my bike storage area

i hate cv carbs

and look at bike in a very dark place and see if spark is jumping any where when it not running correct

[bill h]

thanks again Dave: I fear my Skorpion will suffer a similar fate.

samandkimberly: one of the first things I did was put in a new Iridium spark plug.
The bike is garaged and neither the plug or plug wire appeared wet with moisture or oil.
It has been very dry here.
and what is really driving me crazy is how well it was running just before I put it away and what could have failed while it was sitting.

I will try running it in the dark garage and see if I see any arcing.
I will also spray the wire and plug cap with wd40.
Can I use a raptor 660 coil, I hate to spend money chasing an unknown problem but I may just do it.
I may also try wiring in a Norton Commando 12v coil (because I have a few) and see if that makes any difference.
and samandkimberly thanks for replying.

Bill

[DAVID THOMPSON]

being that my rt 125 is sick i will be wondering off to see if i can get my 660 traveller running
if i can it will just need a state inspection sticker and it should be good to go
as the battery i have on the rt 125 is same size
till i can sort out the enrichment valve in the 125 carb
dave

but its so hot and humid here that hugging the air conditioner is the only fun were having,,,

OH and i made homemade apple pie a couple days ago ...mmmmm!

[edfmaniac]

Is your butterfly opening when you actuate the throttle linkage? If yes, you can check the integrity of the diaphragm by blowing on the big pipe coming off the lower part of the diaphragm housing. You should easily be able to actuate it with air from your lungs.

[Henry74]

Hi, Im a newbie to MZ.

My advice on your problem is to hook a small tank(plastic) and feed it directly to the carbs. Obviously empty the tank or shut it off so it will not leak.
This is just to make sure that you have enough flow of fuel coming into the carbs.

let us know
Henry.

[DAVID THOMPSON]

above is a good thought .... i have heard that fuel flow via the petcock can be restricted

and has the fuel pump that came on some of the motors been bypassed

it is not really needed on the MZ bikes and can be a problem
feed fuel to motor after the pump and see how it goes if yours still has the pump
dave

[bill h]

edfmaniac: yes the secondary butterfly opens with the throttle and it is wide open when the primary carb slide is fully open. I tried your suggestion and the diaphragm assembly went right to the top with a little breath, so the diaphragm is intact.
While stationary I have run the engine to 5000+rpm and that secondary slide/piston/assembly does not move at all.
.
samandkimberly:I have run the bike in the darkened garage and didn't see any arcing.

Henry74: I am using an auxiliary tank to run the bike while stationary. I will try a run on the street using the auxiliary tank.
I don't have a fuel pump.
I have drained the tank twice now and taken the petcock assembly out to check for blockage. The fuel drains with a strong stream and there was no debris on the tank screen.
I have run a fuel filter for the 3 years I have had the bike.

Does the ECU have a rev limiter?
Where are the ignition pickups located.
Could it be the ignition is not advancing?

My gut tells me it is in the secondary carb but the rule is when you think its the carb it electrical.

The question I keep asking myself is what could have failed in the 10 months the bike was not in use.
Corrosion in the carbs is the first suspect but I have found no corrosion and it starts right up idles and runs fine to 3500 rpm.
Maddening!.

things to try next:
use a Lucas 12v coil I have and know is good.
I plan to mark the twist grip so I can measure 1/4 throttle 1/2 throttle etc. so I can see at what throttle opening the stumble occurs.
I will also do a plug chop at the same time to try to read the plug.
thanks
Bill

[edfmaniac]

If the CV carb slide isn't opening, you aren't getting vacuum through to the top of the diaphragm chamber. Maybe the slide is in backwards if that is possible? Or maybe there is something plugging the pathway in the slide? If you can operate the valve by blowing into the vent, and it doesn't open when the engine is running and the butterfly is open, you have a vacuum problem.

Yes, the stock ignition has a cutoff but it is above 7000rpm.

[edfmaniac]

My old stock carbs are for sale if you want to throw a $100 at me and bolt them on to see if that fixes the problem. They have the carb kit installed that improves response of the CV and the plunger style choke knob instead of the cable. Also have a mod to the bowl but it is done well, although unnecessary in hindsight.

[dandywarhol]

I have a Yamaha SZR660 - same engine. Check that the engine breather pipe isn't choked. The engine breather pipe (item 51) allows the crankcase to "breathe" (otherwise the oil seals would seep) and oddly feeds the secondary carb diaphragm operation. If choked in the wire gauze then the effect you have when you blow into the tube won't happen. I'm writing this assuming the MZ uses the same breather system.

https://www.yamahamotorcyclespares.co.uk/genuineparts/10717/10/yamaha%20szr660/carburetor?uid=0

The CV or constant velocity carb does exactly as it says on the tin - it allows the airspeed over the fuel pickup (main jet) to run at a constant speed, irrespective of the engine revs. The further down the piston is in the carb bore (venturi), the faster the airspeed - the further up the piston is, the airspeed slows; even though the revs are higher.
The reasoning behind this is that the air/fuel mixture stays more or less pretty constant whereas the other carb requires some mixture compensation device to keep the mixture right. CV carbs have been around for light years on automobiles and up until fuel injection became popular on bikes in the 90s, most Jap 4 cylinder bikes used CV carbs for simplicity.