v8 wont run for longer than 10 seconds

[daz59]

Hi All,

I recently purchased a 147cc v8, free lance engine, from a deceased estate. The chap that built its name is Allan Foster, located in New Zealand but from the UK I believe.

It has an su carburetor (built by Allan) I can start it on full choke and half throttle, I cant get it to run on anything but full choke, if I reduce the choke once running it dies, giving it full throttle it seems to cough and splutter but not gain much revs, once it dies its quite easy to start again, but again dies in a few seconds. The fuel supply to the carburetor is fine. The plugs seem wet, I dont understand that given it wont run on anything but full choke.

I have disassembled the carburetor multiple times, all is clean.
Apparently it has had about 4 hours run time by the builder.

See attached video and pictures.

Any help would be much appreciated.

 

[RetNM]

I can't give you a direct answer to your problem, but there are a couple of things in the photo's posted that puzzle me.

1: The black box on the lower right in pictures 1&2 seems to have a cable that is not plugged in to anything? Don't believe it means anything though.
2: What looks to be a fuel filter or filter/pump combination. I am sure you have checked that already.

It sounds like it is starving for fuel. The thing is it shuts off like the ignition is suddenly turned off. There appears to be a group of wires that are connected to the engine. Perhaps the builder incorporated a oil pressure/flow safety sensor to protect the engine.

 

[daz59]

I can't give you a direct answer to your problem, but there are a couple of things in the photo's posted that puzzle me.

1: The black box on the lower right in pictures 1&2 seems to have a cable that is not plugged in to anything? Don't believe it means anything though.

That box is a hall sensor I believe, there is a magnet on the crank shaft it reads to set the spark timing, you can adjust the timing by rotating that sensor around the crank

2: What looks to be a fuel filter or filter/pump combination. I am sure you have checked that already.

There is no fuel filter, the air filter is just a simple brass gauze that is currently removed.

It sounds like it is starving for fuel. The thing is it shuts off like the ignition is suddenly turned off. There appears to be a group of wires that are connected to the engine. Perhaps the builder incorporated a oil pressure/flow safety sensor to protect the engine.

It does give the impression that its starving for fuel, but why are the plugs wet? When running the piston in the carburettor is only half open even when giving it full throttle.

The only wire connected to the engine are to that timing sensor.

Thanks for your reply.

 

[RetNM]

This is the fuel filter/pump that I was talking about. Just trying to see if we are both on the same page.

 

[daz59]

This is the fuel filter/pump that I was talking about. Just trying to see if we are both on the same page.

View attachment 143646

Thats the float chamber. Its separate to the carburetor.

 

[RetNM]

Gotcha, I was thinking some type of a sensor that might shut down the system. It sounds good until it quits, but only starting up at fuel choke certainly does point to a fuel system problem.

Any possibility that there may be an air leak around the base of the carburetor?

 

[peterl95124]

I remember reading about SU carbs a while ago (because originally on early RR Merlin engines IIRC), anyway that was a long time ago, all I remember now is that SU are complicated and possibly finicky, you might want to start by reading up on them. I'm sure they've been modeled before, but I don't remember ever seeing any in for example BAEMClub.com, and searching this website for "SU carb" or "SU carburetor" doesn't turn up anything useful. You might want to start a new thread with the title "SU carburetor help needed" to see if anyone can assist.

start with the easiest possible fixes and work your way through "all the usual suspects" list, compression, ignition and timing, carburetion.

firstly if your battery voltage is low then at more open carb you get more air which means higher compression which means the spark might not be strong enough, try a slightly higher voltage battery and see what happens.

you probably have already done this, but just in case, make sure the SU piston moves freely, make sure the needle it controls is straight and clean, etc, etc, etc. And was it really designed to be in a non level installation. Also give this a try, remove the float valve and reservoir and just position your fuel tank so its fuel level is at the carb level, if it runs then the problem is the float valve is too slow and you're starving for fuel.

if all else fails, put a plain old RC model airplane engine carb on it (between 7.5 and 9.5 mm bore, I'm guessing, for engines with near 1" cylinders, that's what I've been using) and give that a whirl. I've never had a problem with them. if it does run then you've eliminated a lot of other potential problems.

 

[Shopgeezer]

I would try another carb. The symptoms you describe are typical of a carb that has sat for a long time without use. I have these problems frequently with older engines that have been stored for some time.

 

[Zeb]

The SU website has a lot of interesting reading and diagrams. I could see any variance in friction, stiction, and clearances on that piston really messing with the main metering.
The S.U. Carburetter - SU Carburetters

 

[daz59]

Gotcha, I was thinking some type of a sensor that might shut down the system. It sounds good until it quits, but only starting up at fuel choke certainly does point to a fuel system problem.

Any possibility that there may be an air leak around the base of the carburetor?

No air leaks that I can see.

 

[daz59]

The SU website has a lot of interesting reading and diagrams. I could see any variance in friction, stiction, and clearances on that piston really messing with the main metering.
The S.U. Carburetter - SU Carburetters

I remember reading about SU carbs a while ago (because originally on early RR Merlin engines IIRC), anyway that was a long time ago, all I remember now is that SU are complicated and possibly finicky, you might want to start by reading up on them. I'm sure they've been modeled before, but I don't remember ever seeing any in for example BAEMClub.com, and searching this website for "SU carb" or "SU carburetor" doesn't turn up anything useful. You might want to start a new thread with the title "SU carburetor help needed" to see if anyone can assist.

start with the easiest possible fixes and work your way through "all the usual suspects" list, compression, ignition and timing, carburetion.

firstly if your battery voltage is low then at more open carb you get more air which means higher compression which means the spark might not be strong enough, try a slightly higher voltage battery and see what happens.

Thats a point I noticed, when trying to start when cold and the carb fully open, I am getting sparking externally, like the spark plug is shorted out and its jumping from the spark plug connector to the head. Its probably doing this when running. Ill try and insulate the plugs better. I dont quite understand why its doing that, is it the rich fuel shorting out the spark plugs?

The battery is fine, its a 6v lead acid that ive been keeping a close eye on.

you probably have already done this, but just in case, make sure the SU piston moves freely, make sure the needle it controls is straight and clean, etc, etc, etc. And was it really designed to be in a non level installation. Also give this a try, remove the float valve and reservoir and just position your fuel tank so its fuel level is at the carb level, if it runs then the problem is the float valve is too slow and you're starving for fuel.

Everything is free and moving correctly in the carb, not being level is a good point. I will try a fuel tank directly to the carb

if all else fails, put a plain old RC model airplane engine carb on it (between 7.5 and 9.5 mm bore, I'm guessing, for engines with near 1" cylinders, that's what I've been using) and give that a whirl. I've never had a problem with them. if it does run then you've eliminated a lot of other potential problems.

 

[daz59]

I would try another carb. The symptoms you describe are typical of a carb that has sat for a long time without use. I have these problems frequently with older engines that have been stored for some time.

That is top of my to do list, just need to find one of suitable size.

 

[TonyM]

All depending on how this carb has been made. The SU carb has an idle jet which as it suggests allows enough fuel for idling. This is taken over by the main jet when the piston and needle are raised simoultaneuosly. As the piston is raised it also raises the needle which is tapered allowing more fuel at the same time as more air. It could simply be that the needle needs lifting a bit in the piston or the seat needs lowering a bit. In the SU's I worked on the needle had two or three circlip grooves so it could be adjusted. The other main problem associated with most carbs is the fuel level in the float chamber. If the float is sticking, or the float valve sticking, or the fuel pressure is too low to replace fuel as it is being used the engine will misfire and stop.

 

[daz59]

All depending on how this carb has been made. The SU carb has an idle jet which as it suggests allows enough fuel for idling. This is taken over by the main jet when the piston and needle are raised simoultaneuosly. As the piston is raised it also raises the needle which is tapered allowing more fuel at the same time as more air. It could simply be that the needle needs lifting a bit in the piston or the seat needs lowering a bit. In the SU's I worked on the needle had two or three circlip grooves so it could be adjusted. The other main problem associated with most carbs is the fuel level in the float chamber. If the float is sticking, or the float valve sticking, or the fuel pressure is too low to replace fuel as it is being used the engine will misfire and stop.

I cant adjust the needle height relative to the piston, but I can lower the jet quite a bit (exactly what the choke does)
I need to lower the jet nearly as far as possible to get it to run.

 

[TonyM]

Where is the fuel tank? If it's the bit to the right of the float chamber that looks too small to allow the float chamber to replenish much as the fuel is used. If not it must have a pump somewhere to fill the float chamber from a tank below.

 

[TonyM]

You could try different needles. Thinner at the bottom half. You could buy different needles in my day. Different profiles for higher performance or economy

 

[daz59]

Where is the fuel tank? If it's the bit to the right of the float chamber that looks too small to allow the float chamber to replenish much as the fuel is used. If not it must have a pump somewhere to fill the float chamber from a tank below.

Thats bronze colored thing? No thats the air filter. I have a temporary tank a foot up to allow a bit of pressure to the float chamber, its not very big but given the height I am getting enough flow.

 

[daz59]

You could try different needles. Thinner at the bottom half.

The current needle is about 20mm long and only has about 0.25mm taper on it, a lot less than I was expecting, I could give it a bit more taper.

 

[Weldsol]

One thing you could check (if the carb is as a full size one ) SU's used thin oil as a damper in the top of the piston guide and if the wrong oil (to thick) was used it would slow the reaction time of the piston and give poor performance

Paul

 

[Roger B]

I have made a fairly successful 6mm bore SU type carb. At this size an overflow tank was easier than a float chamber. The fuel level is quite critical, it should be a bit below the top of the jet. The orientation of the piston is also important so that the air connections to the top side of the piston are on the engine side, not the air intake side. There is usually a guide slot to ensure this. Finally the piston must be free to rise and fall with the jet in every position. My needle has a fairly shallow taper, difficult to measure accurately.