Success

[Mickey Sergeant]

Finished installing my new mikuni carb today.Fired right up.Bike ran rear well.Need to get motor broke in,Then see how well it's jetted.The info and pictures were very helpfull.Havent gave up on the bing carb.Now if only it will stop raining.We get a lot of that in Oregon you know.

Mickey Sergeant
74 Berkshire

[joe novak]

It is best to check your spark plug (electrode) color to determine if the jetting is correct.  joe

[brian kirby]

Spark plug color is not an accurate way to jet with new pump fuel mixtures. With the new pump fuels a perfectly jetted bikes plug will look like a way too lean plug used to look. If you are using race fuel then you can go by plug color, if you are using pump fuel you have to jet by the sound of the engine and feel of the power delivery. I dont know the chemistry involved, but in the mid-late 90s pump fuel changed its formula and it no longer gives the plug readings it did before that.

Brian

'72 Berkshire

[Dave Mitchell]

Mickey, glad everything worked well.It's a sweet thing to do a swap give'er a kick and smell the bean.I'm in N. Idaho and we've had everthing you can imagine thrown at us this week.If it ain't the mud it's the dust not much in between here.
Dave

[firstturn]

Brian,
  Thanks for making this point as I see people trying to set their carbs with pump fuel and it just doesn't work as it did in the old days unless you are using Race Gas.  I too am not a chemists, but I think it has something to do with the alcohol in the gas burning hotter?  I have a good Friend who in is a chemical engineer and I will ask him.

Ron Carbaugh

[brian kirby]

Ron,

I cant claim to have figured the fuel thing out on my own. In the mid 90s I raced jet skis and got advice on building engines from Harry Klemm, I am sure you know who he is. He has a company called Group K which does high performance PWC engine modifications, and they have an excellent general technical section covering things like air leaks, compression readings, and piston siezures. It is geared to watercraft, but its general two stroke information you can apply to any two stroke engine.

http://www.groupk.com/docstech.htm

The two articles on fuel are here:

http://www.groupk.com/tec-gas96.htm

http://www.groupk.com/tec-gas97.htm

quote:To further complicate alcohol's jetting problems, the clean burning alcohol produces very little, if any, plug coloring. At Group K, we have never been big fans of fine tuning carburetion by "reading" spark plugs. The clean burning oxygenating chemicals in modern gasoline's provide even less plug coloring than earlier fuels. We believe that this makes "reading plugs" even less accurate and less meaningful than it ever has been.

Brian

'72 Berkshire

[brian kirby]

Here is an even better quote from Harry in the carb tuning section:

quote:After the full throttle running, and the plug chop, a combination flashlight/magnifying glass must be used to view the carbon deposit at the base of the porcelain (down inside the spark plug where the porcelain insulator and outer steel spark plug casing meet. A ring of dark brown at the base of the porcelain denotes ideal fuel mixture, light brown is lean, and a ring of black is over rich. This is the only area of the spark plug that accurately indicates fuel mixture. Furthermore, this reading only indicates full throttle fuel mixture. No part of the spark plug can indicate low speed or mid range fuel mixture. The upper part of the spark plug porcelain (by the electrodes) is often very light or white in color, however this coloring is mostly affected by additives in the gasoline and oil. The coloring of the end of the porcelain in no way indicates appropriate fuel mixtures of any throttle range. The cosmetic appearance of the spark plugs can defiantly help a pwc mechanic to quickly diagnose the symptoms of a major operational problem. But as far as carb fine tuning for personal water crafts is concerned...reading plugs qualifies as a very questionably accurate way to fine tune the carbs. Very few professional PWC engine builders recommend their customers to do carb fine tuning based on plug readings...and even fewer engine builders do it themselves.

From my experience I would say PWC and off road motorcycle racing throttle profiles are very similar, short full throttle bursts, with the majority in partial throttle.

Brian

'72 Berkshire

[Dave Mitchell]

Thanks Brian,keeping up w/technology seems like a 24/7 thing anymore any and all help is very much appreciated.
Dave

[Ernie Phillips]

Mickey,
Good to hear that your Berkshire is running well.   Remember to keep an eye on those hose clamps (if that is what you used).  Me, Christopher, and Bob "toocloseracing" have all had the carb back off and cause problems.
Another thing that people tend to forget is the Mikuni "choke"  uses an enrichening circuit.  With the "choke" on, keep the throttle completely closed.
As Brian and Ron pointed out, plug reading are different now.  To "perk up" our 28mm Mikuni Berkshire that Christopher raced last year, I kept leaning it out and it got better and better ... we had no melt down but plug readings were light tan.

Ernie P.
Chattanooga, TN

[Mick Milakovic]

Hi Ernie, if you guys are running a 28 on a Berkie, what would you recommend for a Piner?  I have a 34 on Ambr's, and it's jetted almost as small as I can go, yet it still feels rich.  I'm thinking a 32 and use the 34 on my Scrambler.  Thanks,

Mick

[Dwight Rudder]

quote:Originally posted by Mick Milakovic

Hi Ernie, if you guys are running a 28 on a Berkie, what would you recommend for a Piner?  I have a 34 on Ambr's, and it's jetted almost as small as I can go, yet it still feels rich.  I'm thinking a 32 and use the 34 on my Scrambler.  Thanks,

Mick

That would approxamate that velocity of a Bing.
Dwight

[Mick Milakovic]

Thanks Dwight.  My Bing has a huge "smilie" on the slide and just won't run right.  The cost of replacing it with a Mikuni is driving my decision.

Mick

[tomale]

The size of a carb seems to have be a personal choice...I have seen 125 Hodies with 38 mikuni's on them and a 380 Cz with a 34, I would have never thought either would have worked that well. I raced that Cz and it pulled like a Tractor on drugs.... my Mc 5 has a bored out 34 and it works really well, my 440 maico has a 38 on it and I am not happy with it...on the maico I am going back to a Bing....

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)