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spitting back through Carbs

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Kbeal
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Joined: Mon Feb 15, 2016 9:01 pm

spitting back through Carbs

Post by Kbeal »

Last summer my SP was running well. Over the winter with very limited use due to the wet weather, it has developed an annoying fault: hesitating under load around 2000 rpm and spitting back through the carbs. It also spits back while revving the engine hard in neutral. It starts and idles fine.

I have checked and replaced the vacuum advance (it was completely knackered) and re set the timing. Whilst I had the distributor base plate out I checked the bob weighs, they moved freely so unless I've missed something the advance mechanisms are ok.

Therefore I think that the cause must be a fuel problem. As the car was running fine back in October I doubt that the fuel mix has altered so i think it must probably be either leaky jet diaphragms (ethanol in the fuel / age) or spindle bushes. Other options include needle jets in the fuel bowls but they seemed ok when I inspected them. Fuel pump works fine - it was renewed last year.

Coil, plugs, plug leads and extensions, rotor arm and dizzy cap were all new last year from reputable sources.

Before I remove the carbs and strip them down is there anything else obvious that I should be looking for?

Kevin

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John-B
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Re: spitting back through Carbs

Post by John-B »

Water (condensation) in petrol or just old petrol? I suggest that you siphon it out before doing anything drastic.

Sydsmith
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Joined: Sun Feb 14, 2016 11:15 pm
Location: Aberystwyth Wales

Re: spitting back through Carbs

Post by Sydsmith »

Interesting problem Kevin and assuming the obvious, the contacts are correctly set and the capacitor is good, I agree perhaps a fuel problem or fuel starvation.

Too lean mixture will not fire properly, unspent mixture is blown into the exhaust and is ignited by hot exhaust gases from the next stroke and the unspent fuel explodes in the exhaust. Check the plugs in both banks, white plugs weak mixture black plugs rich, straw mixture about right.

Is it possible the fuel is old or a mixture of old and not so old? Modern fuels deteriorate from day one and by a year old they become more or less useless. Try using two stroke mixture in a chain saw that is more than six months old and you will see what I mean, by six months starting is difficult by a year old the saw will just not start.

Hope that helps and no doubt there will be more input for you from others shortly. Syd

Flinty
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Joined: Mon Sep 24, 2018 7:34 pm
Location: Scarborough

Re: spitting back through Carbs

Post by Flinty »

Kevin Just a long shot but have you checked for oil in the carb dashpots.
Shortage of oil can have that effect.

Steve.

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Kbeal
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Re: spitting back through Carbs

Post by Kbeal »

Thanks for the suggestions.

Fuel is fresh - I was down to 1/4 tank so filled up last week in case stale fuel was the issue - but I haven’t checked for water
Carb dash pots ok for oil
I’ll check the plugs again but now we’re on lockdown I can’t do a road test!

Cheers
Kevin

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John-B
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Re: spitting back through Carbs

Post by John-B »

Water will settle at the bottom under petrol, so you should replace it all. You may have to run the starter a long tome to get past the water in the pipes and carbs into fresh petrol.

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Kbeal
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Re: spitting back through Carbs

Post by Kbeal »

I’ve got some water finding paste so will dip the tank
Thx
Kevin

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JohnM
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Joined: Thu Feb 11, 2016 1:31 pm
Location: N W London

Re: spitting back through Carbs

Post by JohnM »

Hi Kevin,
it does sound like a fuel supply problem - three suggestions that you may already have tried:

first, what happens if you pull the choke out when this happens? If it gets better, it could be a mixture problem. If it doesn't get better or change, the bowls could be almost empty.
next
shut off the fuel at the bowl filter tap, disconnect the feed pipe to the carbs and connect it to a fuel can (carefully!) to give a simple gravity feed to the carbs. If the engine runs cleanly from that, the problem's in your pump & feed lines somewhere.
so,
have you tried actually checking the fuel flow at the carbs? I'd take off the feed pipe to the o/s carb and stick it in a container to see what the flow is like when you run the pump. If the flow is poor, the feed pipe from the bowl filter to the carbs may be deteriorating and swollen inside, restricting the flow. There's obviously enough flow to fill the bowls and start the engine, but there may not be enough to keep the bowls full under high demand.

Hope that might give you a few ideas, keep us posted with the results (we're not going anywhere, after all!)

Cheers, John
John M in Middlesex, NW London
1962 SP250 "B" Spec

Chris_R
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Joined: Wed Feb 10, 2016 12:48 pm
Location: Twickenham

Re: spitting back through Carbs

Post by Chris_R »

At last year's Woodley Carnival a TR2 (also using SU carburettors) came in spitting, popping and backfiring. Displaying exactly the symptoms you describe, it would start and idle fine but as soon as any throttle applied it would spit and pop and backfire. Quite spectacularly too, it was almost a better attraction than the rest of the carnival! According to the owner it started after he filled up with petrol on the way to the show. He dismantled both carbs several times while on site, reset the points, checked the timing, none of which fixed it and was concluding valve timing had slipped somehow. He did all the things you've just descibed. We'd wandered over for a general laying on of hands as is the wont of classic car owners when someone else is having problems. The only thing left was the condensor which one of our group had a spare of. Replaced the condensor and the engine ran smoothly once again. That's all it was. Before you start dismantling all kinds of things that is an easy thing to try.

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JohnM
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Re: spitting back through Carbs

Post by JohnM »

Good thinking, Chris - I forgot to ask Kevin if he had standard or electronic ignition.

John
John M in Middlesex, NW London
1962 SP250 "B" Spec

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