200t running problem.

[Riggerbear]

What was the 'mod' you done, changing the pump piston or blanking it off ?

I've changed pump pistons and it hasn't helped. I wonder if the walls of the 'hole' that the pump piston sits in wear ?

Just my 5 pence's worth

[Gerbutt]

Barrie forgot to mention a leaking welch plug and faulty check valve! Have you checked the pumping diaphragm?

 

 

The Welch plug looks fine I think. What's a check valve and is the pumping diaphragm the black one behind the plate with the four screws, that one looks ok, has no holes but not sure if it's working as it should?

[GardenKit]

I did say "try these for starters" Steve:big grin:

I did not want to overload the poor guy, besides, I had to leave something for you to chip in:001_smile:

[Gerbutt]

What was the 'mod' you done, changing the pump piston or blanking it off ?

I've changed pump pistons and it hasn't helped. I wonder if the walls of the 'hole' that the pump piston sits in wear ?

Just my 5 pence's worth

 

 

I did the blanking off which has worked on others I've done. The way it runs isn't as if the pump was amiss, just thought I'd cover that base. Cheers though 👍

[GardenKit]

The Welch plug looks fine I think. What's a check valve and is the pumping diaphragm the black one behind the plate with the four screws, that one looks ok, has no holes but not sure if it's working as it should?

 

The check valve sits between the metering chamber and the main jet. Its function is to allow fuel out, but not allow air in. Can be tested with a pice of fuel hose pushed over it and blowing/sucking through it.

 

The pump diaphragm is under the cover with one central screw. The one under the 4 screw plate is the metering diaphragm. Its function is to depress the metering arm and open the needle to allow more fuel in from the fuel pump as fuel in in the metering chamber is depleted by being drawn into the carb body via the one way valve. It is commonly believed that the diaphragm is drawn down by vacuum, but in reality it is pushed down by atmospheric pressure entering via the little hole in the 4 screw plate which is why this hole must be clean.

The diaphragm must be in good flat order to function. If it is too soft and stretched it will go down too easily and cause flooding. Too flat and hard and it will not respond quickly causing a lack of fuel and weak running.

[Gerbutt]

The check valve sits between the metering chamber and the main jet. Its function is to allow fuel out, but not allow air in. Can be tested with a pice of fuel hose pushed over it and blowing/sucking through it.

 

 

 

The pump diaphragm is under the cover with one central screw. The one under the 4 screw plate is the metering diaphragm. Its function is to depress the metering arm and open the needle to allow more fuel in from the fuel pump as fuel in in the metering chamber is depleted by being drawn into the carb body via the one way valve. It is commonly believed that the diaphragm is drawn down by vacuum, but in reality it is pushed down by atmospheric pressure entering via the little hole in the 4 screw plate which is why this hole must be clean.

 

The diaphragm must be in good flat order to function. If it is too soft and stretched it will go down too easily and cause flooding. Too flat and hard and it will not respond quickly causing a lack of fuel and weak running.

 

 

Thanks for that gardenkit. I'll check the hole in the plate to see if it's blocked. The diaphragm does seems very floppy so I'll try replacing that before I get a new carb. Do you sell the carb kits?

[spudulike]

I did say "try these for starters" Steve:big grin:

I did not want to overload the poor guy, besides, I had to leave something for you to chip in:001_smile:

 

Top man Barrie, think we should do a duet of that Elaine Paige and Barbara Dickson classic "I know him so well".....reckon it would go viral:lol:

[spudulike]

You can't visually inspect the welch plug but if it leaks, it tends to give issues similar to a faulty accelerator pump.

 

If you forced too much epoxy in to the welch plug on the back off the carb when sealing the pump, it can flow in to the check valve but this usually kills the high speed revs leaving just the idle.

[GardenKit]

I did the blanking off which has worked on others I've done. The way it runs isn't as if the pump was amiss, just thought I'd cover that base. Cheers though 👍

I don't want to be too critical, or damning of others methods, but, in my opinion blanking off the accelerator pump is not so much a 'mod', as a 'bodge'. (I must admit I too have done this bodge in the past)

 

The pump is there for a reason, and that is too overcome the 'flat spot' which occurs in the transition from running on the idle jet, to running on the main jet as the butterfly is opened.

At idle the flow through the carburetor is low. Some of the fuel is evaporated and some remains liquid at the bottom of the intake pipe.

Suddenly opening the throttle will give an immediate air flow increase to the cylinder. The evaporated fuel follows the air flow, but the liquid fuel takes longer to increase its speed. This means that the first second after opening the throttle, the engine will get a lot of air but very little fuel and the response is bad.

This is compensated for by the acceleration pump in the carburettor.

Remove this pump and you need to compensate to some degree by richening up the idle L screw, which sort of works but cannot be done in production because of emission regs.

Why not simply repair the accelerator pump?

[GardenKit]

Top man Barrie, think we should do a duet of that Elaine Paige and Barbara Dickson classic "I know him so well".....reckon it would go viral:lol:

Its a good job we are here to moderate each others advice Steve, but strangely we normally concur.

But never ask me to sing with you again:scared1: