v8 wont run for longer than 10 seconds

[RetNM]

Here is one way to see if there is a leak between the carb and the fuel manifold.

Easier to show than to explain so click HERE.,

 

[Roger B]

Tracking across the spark plugs is a sign of a very weak mixture. As you have to have the choke full, jet in the lowest position, this is also indicative of a lack of fuel. Is there any simple way to check the fuel level in the float chamber? Can you attach a piece of clear pipe to the outlet?

 

[Mike1]

147cc V8, that's a powerful engine twin carburettors maybe the answer just a thought
Mike 1

 

[peterl95124]

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.

View attachment 143665

Roger, nice looking, would you be willing to share your SU carb design,
I think it would look better than an RC carb on my Duesenberg, Peter.

 

[Roger B]

This design was based on Ray Amsbury's twin choke SU carb for his 120cc V8 coupled with my road and competition experience with SUs. It is fitted to a 12cc twin which may be somewhat smaller than your engine. I am happy to share the DWG that I worked from, but I am not a designer. The build log with a lot of the thoughts and experiments is on MEM however you will have to sign up to see the pictures.

 

[daz59]

I have managed to get it running for longer and smoother now, I bypassed the float chamber as mentioned by the other member, I still have to have full choke on to keep it running though.

Still wont run very well wide open, im wondering if the carb is not sucking the fuel very well, that would make sense to why i have to run it with the choke fully open.

I am getting quite a bit of blue ish smoke coming from one of the exhaust manifolds, I may have to remove a head to investigate that.

 

[daz59]

I can run it now without it stopping, ive given the needle more taper so it now runs with the choke closed. But ive only got a very limited rev range, if i open the butterfly anymore than half, its coughs and splutters. It idles quite nice and low.

 

[daz59]

Here is another one as I go up and down in the rev range

 

[TonyM]

Try a thinner oil in the dashpot and make sure the piston in the dashpot is not sticking. Also can you change the spring to a weaker one.

 

[holmes_ca]

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.

Peter, SU carbs are very simple I used to tune 3 on a 1959 Healey 3000 Mk11 and 2 on a Healey sprite,

Edmund Alberta,

 

[TonyM]

It still seems like fuel air mixture is a problem for you(too lean).

Assuming you have made sure the dashpot and choke slide are working smoothly then you could try needles with different profiles.(You could buy these for the original SU's.)

First. Try lifting the piston up and down with no damper oil. It should lift very easily. The damper is to stop an inrush of air when the throttle is operated. Although too much air too quickly does not look like your problem. It will lean the mixture if it happens.

The problem may be in the air transfer hole in the piston that gives the dashpot the differential pressure required to give the lift. Make sure that is clear. It is a very important part of the SU carb. As is the damper.

Regarding the needle. You have a good starting point with the one you have so if you can make a couple with different profiles. Starting with the profile of one you have, Make one with a reduced the diameter near the tip to Very fine and only slightly increase the diameter up to about half the movement. Then taper towards the current end size at fully closed blending in to give a curved transition.
If that fails then try increasing the jet size very slightly with a needle sized to suit the increase.

 

[TonyM]

Found this on youtube for you. It gives a good description of how the SU works. Well worth watching. Start at 2.30 to see the How it works.

 

[daz59]

Found this on youtube for you. It gives a good description of how the SU works. Well worth watching. Start at 2.30 to see the How it works.

Thanks for the link, very informative.

I got hold of a small carb from a 130cc water blaster.

I fitted it but im still having similar problems, I just cant get it to rev without coughing and spluttering, so im starting to wonder if its a spark issue. What sort of gap should the plugs have?

 

[daz59]

My spark plug gaps are around 0.5mm

Possibly the coil, see picture, no idea what its out of. Could possibly try a chain saw coil?

 

[TonyM]

0.5/0.6mm should be fine. Would love to know what the coil is too if anyone recognizes it.

 

[daz59]

0.5/0.6mm should be fine. Would love to know what the coil is too if anyone recognizes it.

https://www.model-engine-ignition.com/model-engine-ignition/coils/ss-model-engine-coil
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