Leaking enrichment plunger? Fouling plug #4

Hi,

After cleaning the carburetor passages (without dissambling the whole rack and butterfly valves), plug #4 gets carbon-fouled after a few km's at full throttle. The others remain spotless. The fuell level in all carbs is equal and the enrichment plunger slides freely. A topic found via the the search function teached me that I should try to blow/suck air via a rubber tube through the brass tube of the enrichment circuit, in order to test for a leaking enrichment plunger.

Well...I can blow or suck air through the enrichment plunger quite easily. But on the other hand, it doesn't have a rubber tip or a precisely matching brass needle seat. So would it ever completely close? I could only test this at carb #1, because all 4 are still on the bike.

The plunger is assembled correctly. It has slight wear on the conical part. Would this be the problem, or may there be another reason why the system permits airflow?

By the way: Spark plug cap #1 has a resistance of 12K. So I should replace it. But could this also negatively affect #4?

Thanks!

 

From my own, and recent experience, I had completely cleaned and rebuilt m carbs, verified valve clearances in spec, vacuum sync with YICS blocked, and then colortuned the A/F mixture. Bike ran great for 400 miles and then I noticed a difference in starting, idling, and sound of the engine. #1 plug was fouled. After chasing ignition fault diagnoses and not finding any, I removed the carbs. I removed the enrichment circuit plunger from #1 carb and noticed a bit of build up on the end of the plunger. I polished the plunger and the inside of the carb where it sits. Since the carbs were off the bike, I also did the other three. Bike is back to running perfectly. It is amazing that if a little more fuel gets sucked in through the Enrichment Circuit that it will foul the plug and make the cylinder dead. As Fitz states, these carbs reward precision.

Hope this helps

 

BruceB, thanks for this real life example! May I ask how you polished it?
I can imagine it will still leake if the surface isn't absolutely flat, whereas polishing actually is rubbing a layer of material way. The polishing process often rounds edges and so on...

 

I used a soft bristle brush on a dremel tool. the Enrichment Plunger is what the "choke" assembly is connected to in the top front of the carbs. It is made from brass and has a conical shape to the end that fits into the carb. I used a narrow soft nylon tip brush on the dremel to clean up the bottom of the Enrichment Circuit where the plunger fits. Really didn't take a layer away. Also, so we are on the same page, it is not flat. The bottom of the Enrichment Circuit Plunger is conical and so is the receptacle it sits in.


Hope this helps

 

English isn't my native language. I meant 'flat conical', as in an even surface of the cone.

Do you have a link to the dremel brush you're talking about? Because if I'm right, it should be shaped at precisely the same angle as the seat/bottom of the enrichment circuit...

 

I didn't "hone" the seat/bottom, just cleaned it..

http://instrumentclinicusa.com/index.ph ... ts_id=2171

This just removes surface residue without removing the carb material.

 

OK, BruceB, all clear.

I did so, but still I can suck air through the tube.
Which is no surprise, because the circuit runs to the diaphragm area too, I noticed when blowing a liquid through it.

So how can a vacuum at the place before the butterfly valve, suck gas from the brass tube when the enrichement plunger is opened (or leaking), if the enrichement circuit also runs to the diaphragm area, which in turn is in connection with the airbox?

This hole in the diaphragm area is not supposed to ble closed, as far as I know. So what am I understanding wrong in this story?

I included 2 pictures to clarify...

 

Enrichment circuit-- the plunger with the small black cap at the back of the carb, right next to the mixture screw. If that is allowing you to push/suck air through..... Check a couple things: 1. Is the spring that winds around the shaft missing? That spring is normally pretty strong 2: has the plunger housing flange failed? I just had that happen for the first time 3: if you are sucking blowing air through the long thin brass tube on the bottom of the carb then you need to make sure to cover the very tiny hole on the side of the tube, otherwise you just suck/blow air in/ out that tiny port hole.

2. The jet you show from the top next to the piston bore is an air metering jet. Without the diaphragm/slide/needle assembly in place and the cap on, you will just have free-air motion through that. With the slide/diaphragm in place, you should be able to blow air through the kidney bean shaped hole at the carb front and the slide will lift up. When you stop, it should drop right back down. If it doesn't, then the main jet needle will hang open and cause a rich condition but should also try to race, while the other car s should try to pull down---resulting in a over rich condition in that carb. While you are looking at the air jets, make sure they aren't in the wrong order and that they are not mixed up with ones from the front of the carb.

I hope that helps...... But check that spring first of all---

Dave fox

 

The outlet for the choke/enrichment circuit is AFTER the butterfly valve (it is closer to the engine than the butterfly valve is) and air is drawn in thru the opening under the vac piston diaphragm, thru the carb body, interrupted by the choke plunger valve, and then dumps into the carb throat. The reduced air pressure inside this internal passage........as the air is drawn from the inlet diaphragm area to the output in the carb throat.......is, due to the speed of the air flowing thru the circuit, at a reduced pressure (compared to atmosphere). The fuel in the bowls is under atmospheric pressure, and thus the fuel is "pushed up" the brass "siphon" tube, into the airflow thru the internal passage, and output in front of the butterfly valve into the carb throat.

http://www.xj4ever.com/inside%20your%20carbs.pdf

 

Problem solved! I gently polished the plunger and seating. And annealed the copper ring under the main jet before reinstalling it, to soften the metal because it showed wear. No carbon-fouling plugs anymore!

For future readers of this topic:
I understand from Chacal's explanation that the enrichment circuit needs the air from the diaphragm area for a venturi mechanism that lifts the fuel up. I assumed the circuit lifts the fuel just through the vacuum after the butterfly.

Then I only don't understand how I could ever not suck air from the siphon tube, because the circuit is open at the diaphragm area. Never mind, the problem is solved now...

Thanks all!