How to speak "Carburetor"?

[Fuzzlewump]

Ok, so I'm having some serious carburetor issues...welcome to the club, right?  :laugh:

A couple of members have offered to help me tune them, and I will be taking them up on this kind offer. In the meantime...

QBS asked me about some characteristics the bike is showing, and after much thought and observation, I think I've found a way to describe them. I'd really appreciate any opinions or advice that anyone could offer!

Here are some general behavior issues I'm having with the bike...

Fluctuating idle- Sometimes she revs up spontaneously, sometimes she dies if I don't hold the throttle on. Sometimes she'll do one for a few seconds, then suddenly jump into the other. Sometimes this problem goes away for a few days, then returns.


Misfiring and Backfiring- It misfires at idle regardless of how I've adjusted the idle mix, jets, airbox, etc. When the bike is tuned so that it's running lean (hot and "wound tight"), the misfire is not as bad. At this setting, there is a significant loss of power over the entire rpm range. It's like the engine is working really hard but the drive train isn't tapping into any of that power. When the bike is running like this, coolant spews from the radiator strong enough to wet my pants leg, and sometimes strong enough to squirt up into my face.

When I've tuned her to run rich (cooler and not "wound tight"), the misfire is very bad at idle and she backfires like a cannon during upshifting and downshifting, and when revving the engine while in neutral. At this setting, there is some significant power upon acceleration, but only at the low end of the rpm range, or only at the high end. Sometimes the power is at the high end but moves to the low end after a few days. Sometimes it's at the low end but moves into the high end later on.

I've read that you should adjust idle mix screws to the right until it runs rough, then to the left until it runs rough, and set it somewhere in the middle. With my carbs there doesn't seem to be a middle. It's either too lean or too rich.


Idle Sync is off- I start out by getting the throttle plates to crack open equally by sight, and lock in this setting with the sync rod nut. They are set to be equal with the plates almost completely closed, just barely cracked open. When I start with this setting, the slightest adjustment of idle mixture screws creates either a super high revving of the engine, or bogs the engine until it dies. No middle ground.

-If the engine starts revving, I reduce idle speed, then try to adjust idle mixture screws until running smoothest. If the engine starts dying, I increase idle speed, then adjust idle mix screws until running smoothest. Regardless of which one I do, the sync goes waaay off and the bike runs with a hard rattle or vibration. Bringing the carbs back into syn at this point starts the whole process all over again until I eventually reach a point where the bike will not start (way too rich) or idles at about 5,000 rpms (way too lean).

These are the basic problems I'm encountering with getting the bike to idle correctly. If I could figure out how to solve these, I think I could figure out the issues related to the accelerator pump.

[Fuzzlewump]

I'm sorry for such long posts, my brain doesn't always work with me and I have difficulty describing things in a concise manner.  ::)

[Rick G]

You are concise enough !  Its a complicated matter.

[pinholenz]

Really useful description. From the symptoms you describe, my best guess would be that there are intermittent air leaks into the carb system. However, I know that you are a thorough and methodical worker. At the risk of sounding patronising, can you confirm that you have checked the following:

1. Removed YICS and blanked off YICS ports on the carb boots.
2. Sprayed carb cleaner on to every possible leak joint where air might enter the system below the venturi while the bike is running. Any change at all in the rpm indicates a leak. Don't forget all the vacuum pipe joints such as at each end of  the petcock and the fuel pump vacuum lines.
3. Can you itemise what you have done to the carbs? Throttle valve seals? Float heights? Gaskets etc. Part or whole rebuild?
4. Have you checked and adjusted the valve shim clearances?.
Cheers

[QBS]

Obviously, you've got some fuel related issues.  As suggested, an exhaustive search for vacuum leaks would seem to be in order. 

Your comments about spewing coolant are rather worrisome.  The cooling system is only very distantly related to the fuel system, if at all.  A bike can have serious cooling system problems, but a well sorted fuel system, and it will run very well until it overheats.  Also, a bike can have terrible fuel problems, but a solid cooling system, and it won't overheat or show any other signs of heat related distress.  To have signs of cooling system distress, while simultaneously exhibiting fuel system issues, would seem to me to indicate two separate and most likely unrelated issues.  I'd focus on the fuel system first.  Never the less, the cooling problem could be indicative of a failed head gasket.

Hopefully, some of the  much more experienced wrenches here will shed more light on these mysteries and speculations.

[Fuzzlewump]

Thank you for the prompt responses, gents! As Rick has said, diagnosis over the internet is difficult at best....I appreciate the help you offer.

To answer Pinhole...

- The YICS was leaking badly when I first put everything together, so I removed it and plugged the ports. I've been running without it from the start, just to eliminate another potential problem area.

- I've sprayed around with brake cleaner to try and find any vacuum leaks, and so far none discovered. But if the leak is intermittent, that might explain why I haven't found it with my tests...yet.

- Valve clearances haven't been checked yet, but when I removed the carb boots it was evident that the valves are really filthy. This is another motivator to do the top end cleanup.

- The carbs have been dipped once, but afterwards I've disassembled and sprayed through the channels with brake cleaner about three times. I've had them off the bike and completely taken apart a total of five or six times, and they always look squeaky clean. The inline fuel filter shows no garbage being caught, the fuel coming out of the fuel pump and out of float drains is crystal clear when drained into a glass.

I've got my floats set so that the fuel height is about 21mm below the gasket on rear carb, and 20mm on the front. When I first fill the bowls after putting them back on the bike, the rear float has a tendency to stick until I knock on the carb a little.

Work done on the carbs includes...
NOS needle valve sets with new o-rings
throttle valve shaft seals replaced
all new jets (all but one of them from Mikuni)
new gaskets
new idle mix screws with new springs/o-rings/washers
one new drain screw (old one matches it in shape and size)
new accelerator pump diaphragm
fuel pump rebuilt with keyster kit
petcock rebuilt with keyster kit
throttle plates reattached with new loctited screws
all fuel and vacuum lines replaced with new, if the old proved to be worn or leaking

Accelerator pump nozzles and emulsion tubes look to be in great shape, and are a snug fit.

The carb boots have some cracking on the outside, but none on the inside. They also passed the vacuum leak test.


For the first couple of months the bike was holding up well with the exception of my challenges in getting the carbs tuned. In those months I was having the carb problems that I described in my post above.

In the past three weeks or so everything has kind of gone haywire, for lack of a better word... during this period all gaskets on the cylinder heads have begun seeping oil, as well as the shift shaft seal, the stator cover, and both forks. The accelerator pump began leaking lots of gas, the petcock also began leaking fuel and no longer works with vacuum, I was losing the rear cylinder from time to time, brakes started screeching, and she started running hotter than normal. The accelerator pump nozzle in the front carb started sputtering instead of squirting a steady stream (couldn't find any blockage). The 2nd venturi in the front carb also became loose and was rattling around inside the throat, and I found a 5-inch nail in my new rear Avon. A couple of days ago the stator and r/r both died.

I've been tackling each of these issues one by one. Accelerator pump is repaired with new diaphragm (still trying to set it correctly), which has eliminated loss of rear cylinder for the most part. Petcock is left on prime with the vacuum port plugged until I can rebuild it. Tire has been plugged and is holding air nicely. New gaskets are on the way, and I'm shopping for a charging system.

@QBS... The coolant only spews when she runs really hot, which happens when I've got her tuned lean. If I tune her rich, she hasn't been overheating. But your comment about a failed head gasket makes a lot of sense, considering that all my gaskets are leaking badly and since this started, the overheating seems more frequent.

I should note that whenever I put a manometer to the bike, the front cylinder seems to display "slower" vacuum. The fluid rises and falls considerably slower than the fluid in the gauge for the rear cylinder. Adjusting sync sometimes changes this.

[pinholenz]

Like I said, you are a thorough worker!

I can only guess at a couple of other things to try. (Bummer about the stator and r/r)

1. Disconnect the small air filter on the front carb. It could be blocked. This feeds air in to the emulsion tube and seems to compensate for the loss of vacuum from the flapper mechanism. Any improvement?
2. Do symptoms improve when you run without the main air filter?
3. Although I have yet to tackle this myself, I am told that valve clearances adjustments are critical to correct tuning. Its next on my to do list
4. My bike idle improved with some Upper Cylinder Lubricant in the petrol. This helps remove varnish and gunk from valve stems, valve seats and rings. It used to be marketed as REDEX in the old days (when I was a boy).

All the best