Possible SU float level problem

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GSR 54D
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#1 Possible SU float level problem

Post by GSR 54D » Fri Dec 14, 2018 3:15 pm

Hi

Triple SU carbs on 4.2, under heavy breaking and particularly after releasing brake pedal pressure the engine revs drop very low/stall.

Now initially thought fuel may have been coming through needle and seat valve in float chamber prematurely whilst float was returning to normal position, so put a fuel supply line pressure regulator in to reduce pressure on needle but no change. Tuning good at all other times although I do get a little bit of back firing whilst foot off accelerator from high revs and also when engine hot split second overrun when switching off engine.
I’m wondering if the vacuum from using brakes has anything to do with it.?? but when stationary applying brakes on and off has no affect on revs.

Any other ideas please.

Thanks

John H.

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cactusman
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#2 Re: Possible SU float level problem

Post by cactusman » Fri Dec 14, 2018 4:31 pm

Easy way to see if the floats are leaking....take em out and shake them. Any shloshing and they need replacement. Exact details of how to set the float heights are in the workshop manual of Haynes manual. :bigrin:
Julian the E-type man
1962 FHC
1966 MGB....fab little car too

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#3 Re: Possible SU float level problem

Post by GSR 54D » Fri Dec 14, 2018 4:39 pm

Thanks Julian Have done that, fuel level in floats all ok and shut off clearance adjusted using 11mm pipe! There is a chance one of the 3 shut off valves are not perfectly closing under braking hence i put in the regulator, but its something deeper!

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#4 Re: Possible SU float level problem

Post by mgcjag » Fri Dec 14, 2018 4:52 pm

Hi John....what is your regulator actually doing(what pressure set to)..you shouldnt need one....what fuel pump are you useing....if you think you have a faulty valve replace it.....if its not closing it will overflow...Steve
Steve
1969 S2 2+2 & Building a C type replica

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#5 Re: Possible SU float level problem

Post by GSR 54D » Fri Dec 14, 2018 5:28 pm

Hi Steve

The regulator was a recommendation by a tuning company Airey Tuning in Hampshire and admittingly hasn’t changed anything. I presume the thought was less pressure on valves when the float was reseating the needles. I’ve checked all 3 valves and they appear to function fine however it will be a job on the list to replace. To be honest I don’t think this is my problem as I have no other signs of flooding at any time.
I’m wondering the change of vacuum or fuel too high in the jet and leaks more fuel when braking but I’ve checked settings as manual describes.

I’m hoping someone else may have had same issue and knows something else I can try!

John H.

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#6 Re: Possible SU float level problem

Post by johnetype » Sat Dec 15, 2018 7:37 am

I'd try replacing the non return valve on the braking vacuum tank. If you don't want to disturb that, just add an additional non return valve in the vacuum line from the manifold. Readily available for around £5 and removable afterwards.
John

1969 Series 2 FHC

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#7 Re: Possible SU float level problem

Post by GSR 54D » Sat Dec 15, 2018 12:32 pm

Thanks John

This is an area I don't know anything about so will do a little research to understand the system.

John H

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#8 Re: Possible SU float level problem

Post by PeterCrespin » Sat Dec 15, 2018 1:08 pm

The mixture sounds weak under high vacuum, hence popping on overrun and stalling under heavy braking. Try richening idle and and check throttle spindles for leaks. Just clamp off the brake vacuum take off before spending money on new parts. As it doesn’t stall when braking while stationary I doubt it is related.
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 79 S2 XJ12L; 97 XJ6L

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#9 Re: Possible SU float level problem

Post by GSR 54D » Sat Dec 15, 2018 3:06 pm

Hi Peter thanks for info, this is another area where I"m learning. Will check throttle spindle glands/bushes/seals, and adjust mixture. Question please, regarding 2 breather holes on the carburettor mounting surface to manifold - are these the openings which vacuum system flows through? attempting to understand how the vacuum system from brakes have an effect!

I will double check again on test drive but to confirm, what you've picked up on Peter, when quickly decelerating the revs drop slowing through 1000rpm and then suddenly to stall when foot is taken off brake. Seconds later further foot brake applications, when stationary, have very little fluctuations in revs...maybe 50rpm.. if any. (The last point is what I want to double check)

John - for what its worth I've bought a valve for £3 after your reply and so definitely worth a try thanks.

John H.

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#10 Re: Possible SU float level problem

Post by PeterCrespin » Mon Dec 17, 2018 12:16 am

The holes are to allow atmospheric pressure to act on fuel in the float bowl and push fuel through the jets. There is a casting rib either side but only one side is drilled (to allow for handed carbs). The central rectangular passage at the top is to allow air under the diaphragm Both ports vent from the air filter to prevent ram effect scenarios, where the pressure above the fuel or either side of the diaphragm could become unbalancedswing too far from atmospheric..

I think the brake connection is unlikely because you don’t report brake problems thacan be associated with a leak. With an intact (i. e. closed) system there is no sustained flow of air into the manifold, just a small ‘tidal’ effect as the various hoses and spaces are partially evacuated to establish a vacuum. If there was a split diaphragm or other leak, air could enter the .manifold under braking and weaken the mixture. This could be worse during high-speed/hard braking if it is associated with longer pedal presses rather than brake effort per se.

Pete
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 79 S2 XJ12L; 97 XJ6L

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#11 Re: Possible SU float level problem

Post by GSR 54D » Mon Dec 17, 2018 8:55 am

Thanks Peter

I think this is an oppotunity to take the carburettors off and get a complete understanding whilst going through the service components.
I have also just remembered that this issue was worse on an occasion when braking slowly downhill fowards on a steep drive.

Much appreciated

John H

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