Stuttering at 50+

Standard & electronic conversions
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bornfree
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Stuttering at 50+

Post by bornfree »

We got a 1967 traveller a couple of months ago. It’d not been doing much for around 3 years. Once it’s warmed up with choke off at around 45+ it starts to stutter and lose power. Once we’ve pulled over we can then start up again. It’s a 1098cc with a recent unleaded head, electronic ignition, alternator conversion. This is what we’ve done to date;

Filled up with fresh fuel, second tank with super.
Loosened fuel cap
Replaced coil
Cleaned rotor
Cleaned carb float chamber and checking clear
Cleaned fuel pump filter
Tightened up fuel pipe feed to carb
Blown down supply pipe and heard reassuring bubbling.

After all this it went really well on a couple of runs. But the latest one was not good, lots of stuttering! So,
Checked the float chamber again, and a very very small amount of muck at the bottom. Cleaned that out.
Checked the fuel filter in the fuel pump again, and struggled to see any muck.

Tried another run out, but again get to 50-55Mph then same behaviour recurs. Again needed to slow and stop and then off I went again.

So now I’m wondering if I should drain the fuel tank and clean it. My thinking is that as I pick up speed maybe muck is being sucked up in the tank, and the faster the pumping the more muck. This then reduces the flow of fuel. When I slow down the muck is stick stuck on the inlet. Then when I stop after it’s started stuttering all the muck falls away and off we can go again smoothly. Does that sound logical?

So is draining the tank by using the nut underneath sufficient to drain, and see what comes out? And then wash it through with some clean petrol. Or do I need to remove the tank and put some tank cleaner in and shake it around? (Yes I’d rather not have to take the tank out!)

What are your thoughts, please?



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Panky
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Re: Stuttering at 50+

Post by Panky »

It could be the fuel pump itself struggling to keep up with demand. Pull the fuel pipe off the carb and poke it into a clean container then turn the ignition on, you should get a healthy flow of fuel into the container of about a litre a minute. If that's OK try removing the air filter, start the engine, rev it and then clamp the palm of your hand over the carb inlet. This will super choke the engine and will, hopefully, shift any bits of crud lodged in there. Do this a couple of times but be prepared for the suction - it will be quite strong.


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Custard
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Re: Stuttering at 50+

Post by Custard »

Unless your tank fittings are in good condition trying to undo the drain plug or remove the fuel pipe may cause more problems so I wouldn't rush into that.

It could be that the fuel pump isn't giving you enough fuel so at 50 you are emptying the float chamber as you suspect. Just that it is the fuel pump struggling to keep up rather than the pipe being blocked.

If you take the pipe of the float chamber and run it into a jug (only do it when the engine and exhaust are cold and the vehicle outside) the pump should run at full tilt so you can see what the flow looks like. Don't run it too long as you don't want to overheat the pump. It normally only runs intermittently.

Also is the float height correct. If the fuel level is too low it can run ok at smaller throttle openings but the float could be shutting off the fuel pump too soon.

It is also worth checking there is oil in the top of the carb, and that the needle is located correctly with the shoulder flush with the bottom of the piston.

It can be tempting to push it fully home if it has been removed while cleaning the piston.

Sean.

oops. Panky types quicker than me :D



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bornfree
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Re: Stuttering at 50+

Post by bornfree »

Those are good ideas, thank you. I will try them first!



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Mecanglais
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Re: Stuttering at 50+

Post by Mecanglais »

I used to replace the carburettor float bowl gaskets with one of the new SU ones, which are a very dense card, with a dab of grease (and perhaps a spare gasket in the glovebox just in case but I never needed it), I could remove the three screws and peer in the float chamber with ease to check that the float chamber is filling properly.

It could be fuel evaporation, though this is generally accompanied by furious ticking from the electric pump, which you haven't mentioned? The muck you found in the float chamber could be the flexible fuel line between the carburettor and fuel pump breaking down internally (particularly if steel braided outer) which could be restricting flow, worth carrying a spare.

Might also be worth checking that the carburettor piston moves freely, for the length of time it takes to check that.

Another easy check is the low tension wire on the distributor baseplate, these can get degraded over time, become much higher resistance and struggle with increased speeds.

Also worth checking the condition of the air filter, with the standard setup with the breather in the filter housing, they can often get choked up and cause air flow problems with high demand.

Don't get rid of the old coil, it's probably better than the new coil you have put on!



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bornfree
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Re: Stuttering at 50+

Post by bornfree »

Thanks, I’ve replaced the air filter and that didn’t change things. Yes no clattering of fuel pump.

I’ve just got a new fuel pipe between carb and pump, so will replace that tomorrow.

The rest of the ideas I will work my way through, and report back!



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