Toyota Hiace 2RZ carburettor problem

[petergoudie]

I have a 1991 2RZ Toyota Hiace with 2.4litre petrol engine and carburettor.

It has a problem that when powered up a hill, possibly opening the second venturi barrel, it then goes rich and the engine is rough and stays that way until cold.

If it's left overnight or for say 8 hours and the engine goes cold, it runs perfectly again.

It is not the choke flap valve closing. I checked this.

It took me a long time to realise but when it goes rough and blows black smoke, the fuel level seen through the site glass goes low or disappears altogether. I think therefore the power valve is sticking open or whatever operates the power valve does not return to the unloaded position. If not that, what else is there that can drain the fuel bowl?

It happened again today. After it is parked, the fuel drains out of the bowl and I can see the side of the float. When I start it, the fuel level goes up to the correct height but the engine runs rough and blows black smoke if I haven't let it cool long enough.

Can anyone help me diagnose the problem?

[Dazza]

It does sound like the power valve is sticking open. The power valve should close when the throttle is only partly opened because it is vacuum operated. It should open when you open the throttle so you have low manifold vacuum. If you take the top off the carby there will be a rod with a spring on that should be attached to the top part of the carby. This rod pushes down on the power valve that screws into the bottom of the fuel bowl. You could try unscrewing the power valve and and checking to make sure there is nothing stuck in it to hold it open.
A vacuum leak on the carby can also cause the power valve to stay open, so check that the screws holding the top of the carby on are tight. The rod that pushes on the power valve to open it might have fallen down too. It is only held in place by a split washer that is a tight fit in the top part of the carby.

[petergoudie]

Thanks for your help. I plan to open the carby in a week or so. I've had the van since it was 11 months old and it now has about 240,000km on it.

The problem has been happening for a very long time but as I don't drive it hard it's only sporadic. When it runs normally it performs well and gives good fuel economy.

The carb has never been opened and I will check the circlip on the piston rod although I'm not so sure that's the problem as the fault is only induced up a steep hill that might require the second venturi to come in (that is, low vacuum I guess). Also, the fault goes away overnight when the engine cools down making me think it is a sticking valve or clogged power valve that slowly returns to the correct position.

Another person said it could be the auxilliary diaphram which I will check when I work out exactly where it is.

[petergoudie]

I just took the air inlet pipe off the top of the carb and started the engine. I left it to warm to normal operating temperature and the choke butterfly was fully open.

When looking down the secondary venturi at idle revs, there is a stream of fuel coming out of a port (or hole) just above the butterfly valve. The port is on the outside of the engine that is, towards the passenger door.

I thought the idle jet port was below the butterfly so this leads me back to the power valve. Does anyone know where the power valve injects its fuel into? Is it from that port where the fuel is coming from. Also, I doubt if anything should be coming from the secondary at idle.

[Dazza]

Idle jet holes are above the throttle butterfly. Fuel should only get sucked through these idle jet holes if the throttle butterfly is partly open. The secondary butterfly should be fully closed when your engine is idling. I would try to close the secondary butterfly by hand while the engine is running and see if the fuel stops coming through the idle jet. You might have some throttle shaft wear or the return stopper screw may have vibrated out a bit so it stops the secondary throttle butterfly from shutting fully.
Another thing to check would be to see if the breather hole from the secondary actuator diaphram is blocked with carbon. Vacuum on one side of the diaphram opens the secondary butterfly when the throttle is opened past half throttle and there is a breather hole that runs into the throat of the carby from the other side of the secondary diaphram, that is there to allow the secondary throttle butterfly to close. It is only a small hole about 1 mm diameter which can easily block off with carbon and can be hard to find when the carby is dirty.