Petrol problems...

Hi,
My David Brown 25 petrol wont run for more than 5 seconds without dying, but I can keep it running by a quick choke each time. What is the problem? I have been dealing with this for months! I replaced the carb with a more reliable one, have installed new - leads, coil, points, condenser - Tank was dry, just put in fresh fuel, so no water in fuel
When I bought the tractor, it had been bought by him as dead for who knows how many years! When running, it runs well
Any help appreciated!
Many thanks - Joshua

This is the tractor before I bought it

The engine is not getting enough fuel. The quick choke reduces the air supply, so it becomes in balance with the restricted fuel supply. You could probably get it to run smoothly by adjusting the choke, but that only treats the symptom, not solving the problem. You probably have a restriction at the point where the fuel comes out of the tank, caused by rust or crud in the tank.

check your fuel flow at the carb should flow to fill a pint jar in no time. If not it is above there. Check sediment bowl and screen pull bowl and shut off out of tank flush tank. And above all buy clean fuel and keep it clean. Use clean containers for transfer too.

I put a 1 inch stand pipe in the tank so any thing in the bottom of tank can not drop in the outlet. I do this to all my tractors Saves me a lot of problems

As others said check the fuel flow but it also could be some leak in the intake system such as a leak between the carb and intake,leak around the intake where it mounts to the block or a small hole in the intake.

Did you check the timing?

Some carbs have a drain plug on bottom of bowl. If yours has one, remove it and see how fast the fuel comes out.
The carb on my Jubilee has two adjustments, turn the idle adjustment in (the smaller one) and it will richen the mix. Turn the bigger one in and it will lean out the mix, even shut off the fuel to the smaller jet too.

Others have mentioned the most likely causes. Choking the engine will help keep an engine running when there is a weak spark. Even though you have replaced all of the ignition parts you might be getting a weak spark because of corroded or frayed wire or corroded ignition switch.

Having a clogged air filter can sometimes keep an engine from running unless it is choked.

Some carbs will have a screen at the fuel inlet, it will be attached to a fitting going into the carb remove and clean the screen if there.

How long has it been since the fuel filter been changed.

I would pull the sediment bowl assembly and look for an obstruction there.

I believe having a clogged air filter would have the same effect as choking, restricting the air intake!

Rig up a temporary petrol tank (bottle/mower/boat can) and gravity feed it straight into the carb. If it runs then you?ll know it?s a fuel supply issue.

G'day. My wife is one of you Kiwi's. Grew up in Christchurch. Family was from the Gore area, spent a little time down there 3 years ago, love S' Otago area.

I had a very similar problem with my petrol Deere just last summer. Would die when under heavy loads or when "revving" on a downhill. Applying choke would bring her back to life .

To me the choke is the key here. Almost certainly a fuel restriction issue. Somewhere you have a chunk of rust or other crap hung up in the fuel line.

First off, if you have a drain screw on the bottom of the float bowl, do as suggested and run a flow test. Use a watch to time the flow for a minute so you have a baseline to see if you improved anything later on. When I finally did this on my Deere, it just a weak dribbling stream and I only got about a half litre a minute. The dribbling or turbulence is also a clue. If fuel system is not restricted it should give a nice even, smooth flow.

I made the mistake of trying to take shortcuts and guess where the restriction was. Don't do it. Take the whole piping apart, every fitting, etc. Find all "catch points" and don't stop until you've chased the whole line from the carb to the tank.

In my case, my Deere had been "farmerized" and converted (or is it perverted?) to a rubber fuel hose rather than the original metal fuel lines. I ended up finding multiple chunks of broken-down rubber hose and tank junk lodged in the intake from the tank, and the worst was the fuel shutoff valve was just full of junk.

I replaced all hose and fittings including the valve, cleaned the tank as best I could, put her back together and the flow improved by over 4X the original flow. Which is why you want to time the flow when you initially test it.

Fired 'er up and she ran like a champ.

BTW, I also added one of those nifty see-through inline filters and those are REALLY handy. It is now obvious that I still have some junk in the tank that I did not clean out, but for the price of these little inline Fram filters, I just keep changing them when I start to see too much grit inside. Really nice to be able to visually inspect this filter every time I check the oil and I feel like I'm heading off problems by just changing it out before it's really full of junk.

Good luck.

Grouse

Russ, not necessarily so...

If you look at a flow circuit diagram of a carb, you'll see that there is a bowl vent tube that extends out beyond the choke plate. What that tube does is equalize the atmospheric pressure on the fuel in the bowl with the pressure inside the air filter.

Closing the choke reduces the air flow behind the plate but leaves the vent open, therefore causing the intake vacuum to draw directly from the main jet, giving a full rich mixture.

Once the choke is opened, the pressures equalize and the carb returns to normal idle mix/ventura draw regardless of the air filter restriction, up to a point...

If the air filter becomes so clogged it can't move enough air across the ventura, then the siphon effect is reduced to the point little fuel can be drawn up from the bowl.

Thanks, I hadn't thought of that! So, then putting your hand over the throat of the carburetor is not the same as choking? Must be true on small 2-strokes with diaphragm carburetors, as many of them just have a plate over the intake for a choke

Yes, the little diaphragm carbs are different.

Not quite as sophisticated as a bowl type carb.

A restricted air filter will somewhat enrich the mixture, but it still depends on the air flow ventura effect to draw up fuel.