Engine only starts when I spray gas into the carbs

[sgstepp]

I think I have a fuel starting problem. I can only get it to run when I spray gas into the air inlet of the carbs. Is this a fuel pump problem? I disconnected a fuel rail hose and gas pumps out about an ounce for every stroke of the fuel pump. I hold my finger over the hose end and there is absolutely no pressure. I have just rebuilt the carbs after a long storage time and I know they are clean and hopefully working ok. A continuous spray keeps the engine running but rough. Should I install a new pump. It appears to be my only option. I can't find a workshop manual that tells me how to adjust fuel pressure if I can.

[FI Spyder]

The pump is what I've read described as a low pressure high volume pump (be it OEM mechanical or electric). On the mechanical pump you can vary the pressure by the thickness of pump to block gasket (more or less mechanical stroke from the pump cam. If you have gas in the bowls it should start and run for a short period of time. You can undo the bowls with out taking off the carbs and seeing if there is gas in them.

[Hasbeen]

sgstepp your pump is obviously pumping a little, but not producing enough pressure. The pump is very simple, & only a few things can cause problems.

Firstly was it working when the car was parked? Has this occurred after some period of sitting with no use, or has it developed over time? Slow failure points towards a diaphragm problem.

Your pump has a diaphragm which does the sucking & pumping. This is lifted by an arm driven by the jack shaft, opening up the pump chamber, & this compresses a spring. The spring pushes the diaphragm back down, reducing the chamber & pumping the fuel.

The flow is controlled by 2 valves. The inlet is opened by the vacuum as the chamber is opened, allowing fuel to flow from the tank. The vacuum also closes outlet valve, preventing flow from the carbs. When the spring closes the diaphragm, pressurising the pump chamber, the inlet valve is closed by the pressure, preventing back flow, & the pressure opens the outlet, pumping fuel to the carbs.

The problems can be;
1/ restricted or blocked fuel line or filter reducing or preventing flow.

1 A/ clogged strainer in the top of the pump, or in the tank.

2/ dirt or wear causing either of the valves to not close sufficiently.

3/ split or holed diaphragm.

4/ Broken or week spring.

5/ broken worn or bent pump arm.

6/ leaking hoses.

I'd check for dirty/clogged filter, hoses or strainer first. Then the pump diaphragm or valves. These are not available as TR parts, but in Oz 50/60s Ford Major diesel tractor kits, [about $15] fit.

To increase the pump output you only have to reduce the thickness of any spacer/gasket by 50 thou, [1/20 inch] to make a big difference. Our carbs don't like much above 2 PSI fuel pressure.

Don't be frightened of the pump, they are not rocket science.

Oh, & good luck.

Hasbeen

[john 215]

Hi,

The only thing I would add to my too learned friends comments is...... how old is the fuel, its not like wine and gets better with age ! the opposite it actually goes off,

Have you got a a good fuel feed to the pump also ? A lot of owners install a inline filter and these by there nature do block up.

Cheers John

[sgstepp]

My thanks to all of you for your comments. I take it that pump volume and pressure are related. The more volume, the more pressure. Is there a range of pressure that will give good performance or do I shoot for the max of 2 pounds to operate the float in the carbs? Also I installed a new fuel filter when I took it out of storage. It has been stored for several years so I added fresh gas, after cleaning out the fuel line and putting a new filter in. I will remove the pump and see if I can take it apart to see if the rubber parts are ok and go from there. Is there a simple way to measure the fuel pressure? A new pump may be in order. Thanks again for the replies.

[Hasbeen]

Don't remove the pump as a first step. If it was supplying the carbs, & you had no flooding when you put the car away, the actual instillation should be fine. You can remove the top of the pump to check the strainer quite easily with it in place, & check & replace the diaphragm as well.

Many people have some trouble getting the pump back in, with the lever running on the cam, so start checking it in place. If you do remove it, be careful to keep the gaskets & any spacers between the pump & the block, as the thickness of those dictates the pump pressure.

Any workshop still doing carb work should have a low pressure gauge suitable to put in the hose to the carbs to check the pressure, if you can get the car to one of them. There are probably very cheap Asian gauges available at the larger auto accessories shops if you can't.

Yes pressure & flow are related, but anywhere between 1.8 & 2.2 PSI pressure will give adequate flow.

After that time stored the diaphragm could have hardened & cracked when asked to work. The fact that the pump will actually pump fuel, but not generate pressure indicates a simple problem, allowing the pressure to escape. It should be reasonably easy to find. Check the diaphragm spring is not rusted out or broken.

If you do remove the pump, you should be able to get it pumping by hand operation quite easily.

Hasbeen

[sgstepp]

I am following up with my problem with fuel flow. I opted to put in a new fuel pump to eliminate that as a problem with fuel flow. I was unable to check the rubber diagram in the original oem fuel pump because it is a sealed unit. I still have the same problem with the fuel flow, it runs good when I spray gas into the carbs but it dies shortly after it burns that fuel. It has to be the carbs that are causing the problem, I am thinking even though they are newly rebuilt recently. It has fairly new gas in the tank, but maybe I should try sucking gas out of a bottle to eliminate the fuel as a problem. Could the vacuum system have anything to do with the carbs sucking up the fuel out of the carb bowls? What else could I try? Any help is appreciated. Thanks to all.

[FI Spyder]

You could try sucking fresh gas out of a bottle. The vacuum in the intake doesn't suck gas out of the carbs (directly), it is the air rushing by the orifice that creates the low pressure (Bernoulli effect) that sucks the air.

The carbs are CV's (which means constant velocity) regardless of engine speed the air going through the carbs is at a constant velocity (unlike American type carburetors) with the volume changing as the slides go up or down and the moving needles supplying more or less gas as needed. You may want to dismantle the carbs and make sure they where overhauled properly, I only trust (usually) my own work and never assume what others have done is done correctly. If the orifices aren't blocked with crude/varsol from old gas etc. and diaphrams aren't cracked/torn if they are Zeniths (they should have been replaced).

As for old gas old gas I always go to Ice Pilots NWT were they were to pick up a DC3 that had sat for a year in Gimli to fly it back to Yellowknife in winter. It wouldn't start so after going through the regular stuff they poured some gas on the snow and threw a torch (American usage) on it and it wouldn't burn. They drained it, put new gas in it and took off.

[sgstepp]

Yes these are dual zenith carbs. I am the one that rebuilt the carbs thinking that I put in all of the parts that came with the rebuild kit including the needle valve and all of the gaskets and a thorough soaking of the bare carb body(s) in carb cleaner. I blew out all of the holes and ports that were there. Obviously I missed a port that causes the Bourelie effect and must be the problem. It sounds like this is a replaceable orfice and was missing from both of my rebuild kits. Thank you for pointing this out for me.
I also tried fuel directly from a bottle and no improvement was made which further points to this orfice that you are talking about. I am not sure if I will try the torch in a snow bank idea but it does show that clean fresh fuel is important.
I will dismantle the carbs and try to follow the flow of the fuel thru the carbs. I should be able to find this orfice. Thinking back to the rebuild process, is this the orfice (hole or port) that the metering needle sets into?
Thanks for your input. It is appreciated.

[Hasbeen]

If you have the wrong pump, one with a short arm, or too much thickness of packing between the pump & the block you will not get enough pressure/flow from an old or new pump.

To test your carbs get a container with a suitable outlet to take the fuel hose, & put it on the roof, or hood of the car, so it can gravity feed the carbs. I once drove my car 10 miles supplying fuel in this way.

If it runs happily, you'll know your problem is the pump. If it doesn't you will know it is your carbs.

One good test of the pump is after the car has been running. Turn it off, wait a minute, then pull the hose off the copper rail at the carbs. You should get a little squirt of fuel as you remove it, from residual pressure in the line. If this doesn't occur you have a supply problem or leaking needle & seat in one or both carbs.

Hasbeen

[sgstepp]

I filled a bottle with fuel and stuck a hose in it and fastened the other end to the suction side of the pump. The bottle was at the same level as the pump. I could see fuel coming out of the fuel rail next to the carb when the engine turned over. Then fastened the hose back onto the fuel rail. There seem to be plenty of fuel coming out of the hose so I tried the starting procedure and had no luck. Only ran when I sprayed fuel into the carb. So I think the pump is supplying fuel with pressure(did not measure pressure)to the carbs, the next thing to do today is remove the top of the carbs and work my way thru the carb as best I can without removing the intake manifold. Removing and installing the intake manifold is another problem that is for another forum, but anyway I am trying to do this without removing the intake. (I have trouble with sealing the water o-ring on the intake manifold.) Wish me luck.

[sgstepp]

As an update from my previous post, I want to ask some further questions about the carbs. Without removing the intake manifold, I have removed the upper caps of the carbs along with the metering needles(s). Also on one carb I was able to remove the bottom bowl. There was fuel in the bowl up to about .188" from the top of the bowl. There is two tubes that set in the bowl of gas, the center metering valve tube and another that is offset from the center and appears to have no thru opening to a port or anything. Just sets there. The gas must travel up the center tube along side of the metering needle. When the throttle is pushed the intake air disc opens causing more air to pass over the metering needle causing more fuel to be drawn into the cylinders. If this is true then my engine should be running. And it isn't. The tube that is offset and smaller must do something or it would not be there. Does it have something to do with fuel flow? Is this my problem area and this is the orfice that you are talking about? Where am I wrong in my thinking. Does any of this post help to figure out my problem. Thank again.

[Hasbeen]

Sorry sgstepp, I'm afraid I don't know much about ZS carbs. Mildred would be able to tell you, but she has disappeared with the change of format.

I think she probably got her knickers in a knot with the disappearance of the Workshop Manual section. She had put much work into providing a permanent record of problems like yours & their answers.

I must admit I can't understand how we could have abandoned that useful reference. I hope someone with US experience will be able to help you.

Just one point. My suggestion was to actually run the engine with fuel supplied by gravity from that fuel source, not put it through the pump. I suggest you try it again, connected direct to the carbs. Gravity from the top of the screen height will supply all the pressure you require, if your carbs are working.

Hasbeen

[rugbyref1]

Just for the record .... the Workshop Manual was transfered to the new format in full and unaltered.

Mildred decided not to join the new forum and also, like a child spitting out a dummy, requested removal of her 'contributions'.

[sgstepp]

I have come to the conclusion that I must need a new needle and orfice jet for each of the carbs. Does anyone know where I can purchase these items. I can see the Pistons go up and down when the throttle is operated up and down, I have fresh gas in the bowls, but No visible fuel is being sucked up the orfice. I can see the sprayed fuel from my spray bottle being sucked into the air inlet of the carb so there appears to plenty of air and the engine runs good when fuel is sprayed into the carb. What else could be wrong?