Main Jet Washer

[mkj]

Happy New year to all. This is my first go on this forum, I have high hopes of the technical expertise I am about to encounter so here goes.

I have a 1979 R80/7 with the standard 32mm bing flat top carbs. I have been having trouble with my right hand carb which will not run on the main jet. I have changed all the jets and o rings , the diaphragm, the float, everything I can think of but nothing makes the slightest improvement. I have also changed all the ignition parts with no effect. Starts well runs fine until you open the throttle past 1/4 turn and then its like the right carb ran out of fuel, close the throttle runs fine no backfiring. It actually runs better without the diaphragms . I have also spent many hours adjusting the float level which is spot on. So why is there no fuel going through the main jet.
I am beginning to suspect that the washer on the main jet is the cause but cannot understand how, or what its actual function is. So for now I have removed it and will give it a go around the block on Saturday if its not snowing or raining.
So does anybody know what the function of that washer is

[Kurt in S.A.]

Welcome to the forum! I can't imagine how a washer could affect things unless the main jet is so near the bottom of the float bowl that when the washer is used it pushes it to the point of touching the bottom of the bowl. The main jet doesn't become fully operational until about 60% of throttle...at that point, it totally controls fuel to the carbs. Before that, the needle jet and tapered jet needle do the metering...fuel does flow through the main though. If you can't get the bike to run on 1/4 throttle, I would look elsewhere for the issue.

Kurt in S.A.

[gspd]

Best Bing info here:
https://forum.boxerworks.com/viewtopic. ... 15#p130515

Starts well runs fine until you open the throttle past 1/4 turn and then its like the right carb ran out of fuel, close the throttle runs fine no backfiring.

Sounds like textbook diaphragm perforation. Hold it up to a light and stretch it every which way to ensure there is no pinhole.

It actually runs better without the diaphragms .

Hmmm... never tried running one with no diaphragms. The diaphragm (vacuum) is what lifts the slide when opening the throttle.

So why is there no fuel going through the main jet.

If there is fuel in the bowl and the main jet and all the parts above it are clear and the slide and needle are lifting, engine vacuum should suck fuel from the main jet.

So does anybody know what the function of that washer is

I've never tried to run without the washer and never encountered a carb where it was missing.
My guess is that it's a sort of baffle to assure some fuel remains in the bowl recess on bumps and curves.

Keep in mind...99% of carb problems are ignition related and vice-versa.
Before seeking elaborate solutions and spending too much time on it, I'd switch the carbs from side to side and see if the problem switches sides.


Best Bing info here:
https://forum.boxerworks.com/viewtopic. ... 15#p130515

[RPGR90s]

how did you determine that no fuel is flowing through the main?

if you remove the right side air tube, does the slide rise as you increase rpm? If not, that would indicate a bad diaphragm and consequent problem with the slide not rising. The needle is attached to the slide on the carb, which in turn is attached to the diaphragm. Fuel only flows through the main jet IF the slide rises.

If the slide does rise as RPM's increase, verify that the needle also rises with the slide.
Did you verify that the needle is in the correct position on the slide?

[Airbear]

The main jet washer has an interesting history.

Tilly.jpeg (110.2 KiB) Viewed 1671 times

https://en.m.wikipedia.org/wiki/Miss_Sh ... 7s_orifice

[gspd]

The main jet washer has an interesting history

and legacy?

[mkj]

thanks for the interest , diaphragm is new, the slide moves with rpm as does the needle, jets are clean and there is plenty of fuel in the bowl I have tried various needle positions with out effect i can no longer remember what position its is in. Ignition strobes fine and advances. I am totally stumped but good to know what the washer is for wonder why other carbs don't have them.

[gspd]

I am totally stumped....

It only takes a few minutes to put the 'bad' carb on the other side and see if it works.
This will rule out any ignition or engine issues that may be causing your problem.
You don't even have to hook up all the cables, gas hoses, intake tubes, etc.
Just be sure it has some gas in the bowl.

[Rob Frankham]

I'm not at all sure that this is a 'Miss Shillings Orifice' AKA Tilly's Orifice' or RAE restrictor. The restrictor was in the form of a washer with a set and carefully calculated diameter hole set in the fuel supply line to the carb on the Merlin engine aero engine. It's purpose was to restrict the flow of fuel to the float bowl under negative G conditions and prevent loss of power when the aircraft was set into a dive. There's no way that the washer in the bing carb meets that description... and, in any case, why would anyone fit a precaution against negative G in a bike carb... so what's it all about then?

I believe the washer is there to help prevent air entering the main jet as a result of repid fluctuations in the fuel level due to vibration and uneven road surfaces. This is just my take and I could well be wrong but I can't see any other reason for it.

Rob

[Kurt in S.A.]

Does the main and the washer sit within the small detent in the bottom of the bowl? Been a while since I looked at the bottom of the bowl but thought there was a ringed area in the bottom. Could the jet and this bowl area work together in some way?

Kurt in S.A.