I must be doing something wrong...

[Chito]

So... 81. Bored 1 over. Stock everything. I was one up on the pilot jet. It had been wrecked. Put it back together. It Ran great for 4 or 5 years, but the exhaust was cracked and at some point that changed into an exhaust leak.

I bought Mikes Commando system.

After that it was really lean. Really hard to start. No power.

I stepped the pilot jets up and ended up at a 50 to get it to start decently. I can get it to start on the 47.5 with 5 turns or lower with a little bit of gas. That's pretty crazy right?

So I was up 2 mains too. It ran okay. Seemed lean up top. Stumbles under hard acceleration above 3.5k

Valves were fine.

Right hand plug: Silverish electrode with a tiny blackish on the circle.
Left hand plug: brownish electrode with really sooty circle

Now... a while back I had the cam chain guide go out, so I thought I'd try my spare motor. I've got that going... but it's the same. Super sooty on the left and needs to be jetted really big.

I pulled the carbs this weekend and cleaned them really well, set the float heights, got them synced up using a the yard stick manometer. Runs exactly the same. Stumbles over 3.5k. Still sooty on the left.

I had checked for leaks at the boots and didn't find any.

So question is... why do I have to jet so high? Not normal, right? Think it's an air leak on the right?

 

[hooser]

1st 3.5 is not at the top, it is mid range, you are on the needle mostly not full on the main yet. Try this when is starts to stumble back off a 1/4 if it picks up you are too rich.

 

[Chito]

I should have said it starts stumbling at 3.5 and continues until the top of the range. I want to say it's not a problem, at least a whole lot less noticeable, if I'm not that hard on the gas. Problem is, if I go down a jet size on the main, all the way to stock, it's still there... just with less power.

I thought it might be the diaphram, but they both look alright and slides seem to fall at the same rate with same sound... though faster than I expect.

It's got to be an air leak somewhere. I'll recheck.

 

[YamadudeXS650C]

You might have more than one issue.

The stumbling at 3.5 and above might be a degraded (poor) connection along the positive power line leading to your ignition. I had a similar issue and found a weak connection in the headlight bucket at the multiple connector. Could also be a degraded connection in the ignition switch or the kill switch.

 

[Paul Sutton]

There is a test for the diaphragms somewhere on this forum. It involves pushing the slides up and then blocking the air vent and releasing the slide. In a perfect world the slides should stay up until you remove your finger of the air vent on the carburetor. I must have a search - perhaps it is in the famous carburetor write-up! Shall start searching......

 

[Paul Sutton]

Diaphragm Drop Test - Here is a Youtube video showing how it is done:

I must say this is a very well done presentation - Thank you to the Authors.

Edit: Here is the link to the Carburetor Guide as given in the Tech Section:

http://www.amckayltd.com/carbguide.pdf

Another excellent piece of work. This guide discusses the Drop Test on Page 6. Worth printing a copy of this guide, it helped me a lot.

 

[5twins]

Do you have the original fixed needles or one of the aftermarket adjustable ones? Have you verified proper fuel level in the bowls using the clear tube test? Low fuel levels can cause difficult starting.

 

[Jim]

You might have more than one issue.

The stumbling at 3.5 and above might be a degraded (poor) connection along the positive power line leading to your ignition. I had a similar issue and found a weak connection in the headlight bucket at the multiple connector. Could also be a degraded connection in the ignition switch or the kill switch.

Let me add to that "more than one issue" thought. A bike I had a long time age got to be a hard starter and over about the same period of time started stuttering real bad at higher revs when I laid into the throttle real hard. Couldn't find any air leaks and tore the carb apart about a gazillion times. Finally, out of frustration I fessed up for a new carb (it was a one lunger). That fixed the hard starting but it still had the miss under hard throttle. When I got to the point of taking a match to it, this old dude at the bike shop said (matter of factly) "put a new coil in it." I did and that fixed it.

What's happening (or possibly in your case) is that when cylinder pressures are real high (large throttle opening) the insulation in the secondary winding's would break down and arc to ground because that was less resistance than the plug gap with high cylinder pressures. So I'll go along with Yamadudes thought that it could be multiple problems.... and and a tip the old guy gave me: "Never underestimate an ignition systems ability to fool you into blaming the carb."

Of course, that don't explain all your symptoms but maybe there is more than one fault.

 

[TwoManyXS1Bs]

Good one, Jim. A cheep/simple test would be to temporarily reduce the plugs' gaps to 0.015"-0.020", and note any running differences...

 

[jonesey]

JimD 54.
I had something along those lines too. Years ago a very good friend of mine had a 550cc Kawasaki 4' the twin shock one, almost new and in great condition. He came to me and asked if i'd balance the carbs for him as it had a missfire when it idled. Ran great but had a poor idle.
Well we cleaned and balanced those carbs until we were sick. No dice. Then one morning from cold I started it and let it idle, no revs, after about 1 minute switched it off and felt the four pipes. 1&4 were hot 2&3 were just warm. Ahah the only thing common is the coil. So i asked him to get another coil.

When we started removing the suspect coil I pulled off the 2 spade connections first and they just didn't feel right, too loose, not off but just not as tight as they should be. So before we stripped something else i said, lets tighten these first.

Cleaned the spades with wet & dry and closed the females up with a pair of pliers, put them back on and started the bike,Idled perfectly, No Missfire. Could not belive it.

Never underestimate the power of a loose connection.