Stock Skorpion carb modification

[omrra91]

FACT:
The Dyno does not lie. At sustained high revs the stock Skorpion carbs cannot supply enough fuel to the bigger carb. I hesitate to use the word sustained because the bike runs lean off the chart in just seconds. Most of the people on this forum understand that running this lean leads to burnt valves and burnt valves don't form a proper seal in the valve seats and now you are losing precious horse power. I'm no superstar mechanic but I get the concept.

OPINION:
For street use only, stock carbs are okay. The only mods to my Skorpion Sport Cup are the removal of that micro screen filter, a clip moved on one of the needles and an in line filter.

For street use and the occasional track day. I feel you have to make the fuel delivery mod that I have pictured. The extra transfer tube has proven itself on three different Skorpions on the long front straight at P.I.R. I'm not even sure this is an original idea. I think if you ask some of the guys who used to run the Yamaha SRX600 at the track they'd say that they have done the same type of mod.

For the track, you know the answer. Flatslides are great for creating top speed when jetted correctly. Having said that I'll admit that I don't use them. I like my Keihin 33mm smooth bores. They are beautifully made. They are a big jump in performance over and stock carb, modified or not. And I wouldn't be surprised if they work better than Flatslides on a tighter track with shorter straightaways. I have no experience on that type of course.

BANG FOR THE BUCK:
If my Skorpion was going to do double duty as street bike and track day bike I'd go with the modified stock carb. I just can't justify the extra expense of an after market carb for track days. Even if you did go high dollar on your street bike and put Flatslides on it would they work very good at low revs? I don't think so.

PARTS FOR MOD:

(1) 1/4 by 28 Tap

(1) 7/32d rill bit for tap hole. Opinions vary, make your own call on bit size.

(1) Drill bit for opening up the unused spigot on the carb bowl. I'm sorry I have no notes on the size I used. Make your own call.

(1) Tube of two part epoxy suitable to exposure to race gas.

(1) Napa Auto part #1410 brass elbow fitting. I hope they still have these listed under this number.

(1) Short length of appropriately-sized fuel line.

(2) Small hose clamps.


In my experience this modification goes a lot smoother with beer, a good friend and the optional bike race playing on the DVD. Time required is directly related to beer consumption. Private message me if you prefer to have me do the work. You can send me your carbs for mod but I'd rather sell you a modified carb complete. That way you don't have to go without your bike while you wait for me.

Regards, Jeffrey
[/b]

[hb7]

Where is this DYNO Chart that doesn't lie....?

[Bill Jurgenson]

Even if you did go high dollar on your street bike and put Flatslides on it would they work very good at low revs? I don't think so.

Not if put too big ones on. A stock or nearly stock motor cannot handle anything bigger than the dual 34mm, especially if the exhaust is road legal. I can readily imagine that your 33mm CR Keihins are good. I have a pair of them too. For the stock or nearly stock eingine, an ideal setup. But they are hard to get nowadays and not exactly cheap, either.
As to drivability, I run variously a TM34-65 or that Bikeworx kit with TM42 and I can adjust my idle down to 800rpm, not that there is any virtue in that, but you can't do it with the stock carb. And I can loiter around town in 5th gear at 2400 if I am very careful AND accelerate. That is definitely not possible with the OEM carb. Not recommended either, as a general habit but it does keep the noise down when necessary.
I firmly believe that that modern flatslides like the TM34 from Mikuni or similar FCR from Keihin is the only way to go, regardless of usage. Mikunis are cheaper, better made, but harder to set up. Over here, there are those racing that prefer the Mikunis to the Keihins.
With the proper setup, you get both more performance and better mileage at the same time.
You do have to learn to control that right wrist, something the CV carbs cannot let you do.

[hb7]

Upon further thought, the 165 main jet in the secondary is much smaller than the ~2mm fuel feed pipe. How is fuel starvation possible ?

[keithcross]

Upon further thought, the 165 main jet in the secondary is much smaller than the ~2mm fuel feed pipe. How is fuel starvation possible ?

Not to sure, but maybe because the fuel feed pipe is flowing fuel under garvity, the main jet is flowing fuel under suction, at least I think this idea may be right.

Keith

[hb7]

Suction yes, but the size of a 165 jet vs. the ~2mm fitting
is huge. I would suspect that the stock carb is properly sized to run all day up to the stock red line ~6750 rpm without
running lean.... which is why I would like to see the
DYNO chart. If he was running an aftermarket exhaust, then it would lean out some...but valve burning lean ? There would be detonation (pinging) ....at the high revs.

[omrra91]

Okay,

Henri is saying "red line" which is very specific.

I'm saying high revs. Which is not specific.

I said the stock carbs were fine for street. Sounds like that's what Henri is saying.

The stock carbs are not good enough for on track, pushing to the rev limit, go fast like you don't care, no matter what exhaust you use. This is not a shocking statement to anyone and it is certainly not what my post is about. Some people on the forum asked me what I had done to the stock carbs so I just put the information out there. If I was interested in pointless banter I would post a claim that I was going to win the next MotoGP on my stock MZ Skorpion street bike.

Jeffrey

[basser23]

Jeffrey,
The valentino ran Sunday..in places...you might have done very well on the Skorp...

[Chayanne]

In the motorcycle photo I saw a the model like a skorpion and have a "Belly pan cover) under the base of the motor. Will you please send me information about who sells this type of belly pan for MZ Skorpions ?

[omrra91]

Hello Chayanne,

Thanks for the message. I raced four full seasons with the oil retaining lower you see pictured. It is from a Honda Interceptor 500. I had a local Fiberglass Company (Glass From the Past) fill in the bottom to meet oil retention rules. I spent several hours making beautiful, strong aluminum brackets. The extra work payed off. I threw down my Skorpion on the left side at about 105mph and the following year I had a much less violent crash on the right side at 95mph. The lower was uneffected in both crashes.

Regards, Jeffrey

[Phil Krix]

Hi ,

I will be getting putting TM34-B65 carbs on my stock Skorpian Replica and sticking with the standard airbox/filter arrangement. Anyone have recommendation re jets as a starting for tuning? I have Ignitech igntion already. Have received a lot of good advice from Bill Jurgenson.

Regards
Phil Krix

[Bill Jurgenson]

fuel starvation comes from the much too small petcock, not from any carb connectors or that 165 mainjet.
Change that petcock or get rid of it entirely.

Problems gone.

[Linegeist]

Succinct as always Bill. Thanks - my very next winter job!

[hb7]

You can ride flat on with the stock pet cock....all day.
Any crap on the needle valve filter, it will starve.
The older version pipe with the sediment bowl can be drilled out a little larger....
If you feel the need. The newer petcock has a little more flow...
If you fit other than stock carbs, I'm sure it will be marginal.

[djsbriscoe]

Bob,

Succinct as always Bill. Thanks - my very next winter job!

Will you be taking pictures as well so that you can add it to your excellent swing arm pictorial?
How's things down in Anglesey by the way, not heard from you in a while.

David.