Overhauled Carbs VM28SC - Z1A issues

picard wrote:

Patton wrote: Are all 4 choke plungers fully closed -- as they are supposed to be -- when the choke lever is OFF?

Good Fortune!

Yeppers. all chokes work as they should. ~

Reason I (think I) do know this is when I first started the bike it needed the choke (it's in the 50s here right now) and they worked as they should.

picard wrote:

picard wrote:

Patton wrote: Are all 4 choke plungers fully closed -- as they are supposed to be -- when the choke lever is OFF?

Good Fortune!

Yeppers. all chokes work as they should. ~

Reason I (think I) do know this is when I first started the bike it needed the choke (it's in the 50s here right now) and they worked as they should.

I can definitively say the chokes work perfectly.

picard wrote:

mtbspeedfreak wrote: Just thought of this, what are your intake boots like? Any chance they'd be cracked or the plates are warped?

Plates?

The intake boots (if you mean what's often referred to as "the carb holders", i.e between the carbs and the cylinders) are new-ish and the bike passed a WD40 leak test with the old carbs (and they pulled about 7-8 psi vacuum as they should) but I can take the intake boots off and inspect them.

Yes, carb holders (I call them intake boots usually, for some reason...). There's a steel plate which is how they can be screwed into the head. The plates can be bent by over-tightening the mounting bolts, which could potentially create a vacuum leak. But if they're relatively new (as in not the original ones that are 40 years old), chances are they're fine.

Did you work your way through the tuning document I sent? I'm wondering if it's time to start "screwing" with the air screws.

Would you need a smaller pilot jet for your altitude? Even at 1/2 throttle, the pilot jet is still a big part of the mixture.

But I feel like the jets aren't a problem, the fuel level is. If they've passed the clear tube test, and they aren't overflowing, what would that be? Would the float valve and seat be the issue?

Trying to think of all possible situations, you don't have any kind of fuel pump do you? Just the gravity-fed (manual) stock petcock?

smaller jets all around for altitude would make sense. Depending on how lean or rich these were from the factory. Typically you gotta lean out as you go up in altitude, i'd say if you're regularly riding above 5k it's definitely worth considering.

I would think if the float valve or seat was an issue, that it would show up in the clear tube test. i.e. if they aren't seating right, it'll overflow, even if just a little, so you'll show too high on clear tube. And if they're somehow getting stuck, your bowl wouldn't fill right, which should also show in clear tube, but as a low level.

Internally, the plungers need to completely close off the starter (enrichener) circuit when the choke lever is pushed all the way down.

When cold, application of choke may seem normal while the plungers are pulled up, allowing the additional fuel mixture into the carb bore.
The issue presents as the engine warms and then requires zero mixture from the enrichener circuit.

Failure of choke plungers to completely close the enrichener circuit isn't so common as other prospective faults, but in this case where so many other possibilities have already been ruled out, it may be at least worth checking.








If not already done, would also assure that nothing stored under the seat is obstructing the air filter grill, such as a shop rag, riding gloves, etc.

Good Fortune!

Sounds as though the Fuel levels are just a tad high if at the gasket surface, as it should be a few mm below. These should be adjusted accordingly before re-jetting etc. Then using your, what vacuum gauges?, set the carbs so they are even. I like this simple tool, www.kzrider.com/filebase/doc_download/215-simplevacsynctool , it really shows when one is pulling more than another.
You Will have to adjust the Air Screws, for Idle and Off Choke warm up, low speed running. A small change must be tried and test ridden. Moving the Needles so they lower 1 notch might be a good Idea up in altitude, but only testing will determine that. I used to live up in Colo Springs, and would only change the Idle Mix for different seasons. No other Jets winter or summer, even with altitude changes, but they are CV carbs..

CV carbs are completely different at altitude changes, they actually work very well.

I would start by setting them for your altitude and then go from there. Sounds like they are a bit on the rich side and could use lower fuel levels. Patton may be on to something with the choke circuits.
Also, don't take this the wrong way. I am not a big fan of media blasting carb bodies. It is very hard to get all the blasting media from all the tiny circuits and threads in a motorcycle carb body.
Soda blasting is the only way to blast a carb and assure all the media is cleaned out.
I bought a spare set of carbs off of eBay that I didn't realize had been bead blasted until I got them here. Even after the owner had cleaned them and run them on his bike for several thousand miles, I am still getting blasting media out of the pilot circuits. These have never been on my bike, and won't go on my bike until I'm satisfied all the blasting media is completely purged from them. :pinch:

BlackZ1R wrote: CV carbs are completely different at altitude changes, they actually work very well.

I would start by setting them for your altitude and then go from there. Sounds like they are a bit on the rich side and could use lower fuel levels. Patton may be on to something with the choke circuits.
Also, don't take this the wrong way. I am not a big fan of media blasting carb bodies. It is very hard to get all the blasting media from all the tiny circuits and threads in a motorcycle carb body.
Soda blasting is the only way to blast a carb and assure all the media is cleaned out.
I bought a spare set of carbs off of eBay that I didn't realize had been bead blasted until I got them here. Even after the owner had cleaned them and run them on his bike for several thousand miles, I am still getting blasting media out of the pilot circuits. These have never been on my bike, and won't go on my bike until I'm satisfied all the blasting media is completely purged from them. :pinch:

Never Media Blast those poor little drillings. :ohmy:

Yes the carbs were bead blasted. I did so myself. I also took an extra month to ensure that all the blasting media was completely removed from the carburetor passageways and threads because I am well aware of the danger of not doing so. I spent two months rebuilding these, double and triple checking everything and in doing so, realized that I have a bad case of OCD. They are cherry.

Turns out, I had made a mistake, the main jets were not the stock 112.5 as I had remembered and insisted that they were, they were 122.5. I've since ordered brand new 110 mains and 15 pilots in a way to make good on my oversight.

Any other comments or criticism?

mtbspeedfreak wrote: Yes the carbs were bead blasted. I did so myself. I also took an extra month to ensure that all the blasting media was completely removed from the carburetor passageways and threads because I am well aware of the danger of not doing so. I spent two months rebuilding these, double and triple checking everything and in doing so, realized that I have a bad case of OCD. They are cherry.

Turns out, I had made a mistake, the main jets were not the stock 112.5 as I had remembered and insisted that they were, they were 122.5. I've since ordered brand new 110 mains and 15 pilots in a way to make good on my oversight.

Any other comments or criticism?

Uppity, uppity.. No criticism meant mtb. I am just not a fan of media blasting and was offering my experiences with blasted carbs. Pilot circuits catch it worst.
I think we all have a bit of OCD in our old(er) age, and if y'all are like me it can get you into more trouble at times it seems :pinch: :laugh:

I will report back when I have the appropriate jets tat mtn sent. Only one of the mains was readable and of course I had not taken that one out to check until yesterday. This strategy is essentially following Black's suggestion - jet for altitude (one size down from stock for pilots and mains and needle to clip 2) and see from there.

Last thing I wanted is start a discussion about cleaning methods (though I do find it interesting ) I have the carbs apart quite far and they do look very clean and nice inside.

picard wrote: Last thing I wanted is start a discussion about cleaning methods (though I do find it interesting ) I have the carbs apart quite far and they do look very clean and nice inside.

Haha yeah I was hoping that topic would be avoided. Not only did I clean out the jets with water and air, several times after media blasting, I also wiped down the insides with a clean rag with a few drops of 3-in-1 oil to make sure any residue was wiped off.

I only blasted the carburetor bodies with the top cap and float bowl assembled. I did not blast the interior of the float bowl area, nor try to blast inside any fuel or air passages.