Hints and Tips - Carburettor Problems

From: Nick Wright
Sent: 17 May 2000
To: 'Pre-War Cars List'
Subject: Carburettor problems

Dear listers - can anyone help me?

I have just completely refurbished the twin 1 inch SU carbs on my MG P-type, but there are still some problems:

1. The (brand new) float needle seems to be incapable of preventing the float chamber flooding. When the petrol reaches the level of the overflow pipe it pours out in short pulses, separated by one or two seconds. I have tried replacing the float needles again with the old fashioned solid metal type, but this has not improved things. It would seem that petrol pressure is pushing down on the needle more strongly than the float is able to push up. Could there be a problem with the (standard SU) pump? This is the most serious problem and I would be very grateful for an answer...

2. The left hand carb whistles loudly when idling. I guess there is a tiny air leak somewhere - any ideas?

3. The engine runs too fast (about 1500 rpm) when both carbs are fully shut off. They have new spindles, bushes, butterflies - everything, and the pistons are both seating properly...

Many thanks in advance for your help

Nick Wright


From: j.holland
Sent: 17 May 2000
To: NickWright british-cars-pre-war
Subject: Re: Carburettor problems

With reference to item 3 on Nick Wright's list of woes I have been experiencing exactly the same problem on my Lagonda Rapier. The carbs have been flow tested and the manifolds have been levelled on a surface grinder but still the high idling persists. Needless to say the carbs have been fully rebuilt save for the pistons for which replacements are not available. The only clue I have is that although the pistons seat nicely without the engine running they are slightly off their seats when the engine is running.The problem is worse when the engine is warm and sometimes runs on when switched off. That can be aleviated if it in gear and the clutch pedal is depressed. Any thoughts?

Rgds James H


From: Lawrie Alexander
Sent: 17 May 2000
To: NickWright
CC: british-cars-pre-war
Subject: Re: Carburettor problems

Nick,

The first thing I'd do would be take a pressure reading on the fuel line where it enters the float bowl. If you read more than 2.5lbs, your pump is overcoming the capability of the float valves. Then make sure your floats are, indeed, floating and not punctured/partially filled with petrol. (Hold them in a can of very hot water and see if any bubbles escape.) As for the fast idle, make sure the throttle plates are properly centered in the venturi, so that they can close all the way, then that the interconnecting linkage is not holding one carb open when the other is shut.

Hope this helps!

Lawrie
British Sportscar Center


From: Alan Fairless
Sent: 18 May 2000
To: british-cars-pre-war
Subject: Re: Carburettor problems

Reference the high idling speeds. The fact that the pistons are off their seats indicates that there is some airflow through the carb.Assuming the linkages are not holding the thing open, then the only way this can happen is by wear in the throttle spindles ( unlikely in a rebuilt carb ) or the butterfly valve not seating properly. The way to fix this is to unscrew the throttle stop so that the valve is completely closed . Next loosen the screws that attach the butterfly to the spindle, the butterfly is designed to have some movement, so if you hold the throttle shut firmly and retighten the screws the valve should centralise.

Now comes the fly in the ointment. The butterfly valves are matched to the carb bodies, and were checked at the factory on a flowmeter to see that they were OK ( I know this because I saw them doing it). If a replacement butterfly has been fitted and does not match correctly , or there is wear in the carb body the valve will never seat correctly. The only option might be to try a different butterfly. However, SU carbs do not seem to be that critical, I am sure recentralising the butterfly will fix the problem.

Regards etc

Alan


From: paul
To:j holland; NickWright; british-cars-pre-war
Sent: 17 May 2000
Subject: Carburettor problems

James,

the answer is obvious - sell the car and buy a Nash!! :-)

Only thought I have and its so improbable/silly I hesitate to suggest it, but I wll anyway.

In my simple mind it seems to me that if the carb butterflies are fully shut it will cannot be possible for mixture to get past and the engine will stop. ie. if you were to stick a piece of gasket paper between carb and manifold it would be impossible for the engine to run - I hope!

So something is getting past, I just wondered - if the carbs have been completely dismantled and rebuilt is it possible that the butterflies themselves are the wrong way round. They are chamfered to ensure they seat well in the carb cylinder, but if they are backwards they won't seat properly and there will always be a gap.

Like I said, silly suggestion so I'll revert to my original one!! Chains, glorious chains.........

Paul


From: Nick Wright
Sent: 18 May 2000
To: 'Pre-War Cars List'
Subject: My carburettor problems

Well, the butterflies are definitely the right way round, both of them shut off fully and as far as I can see both pistons are seating properly - they fall with a very satisfying 'clunk' (this may be worth checking though). The floats were quite empty when I took them out and if they had any sort of leak I am sure they would, by now, have sunk altogether. I will check then again though. The whistle is a disappointing high-pitched monotone I am afraid...

I will investigate the pump further I think - seems to me that problem number one could well be that that the pump is a high pressure one. As it is only an inch away from the carbs, I think I may have found my gremlin. ???

I shall try and track down the source of the whistle this weekend

Over the years the butterflies have worn small grooves in the top and bottom of the venturi. When fully closed there is room for movement, left and right - tiny gaps all the time. Maybe my whistle comes from here (and this could perhaps be part of the source of the fast idle?) But, all these problems were worse before the rebuild, and there was no whistle then. But of course whistles mean small holes...

Sorry guys, I am waffling!!! I shall look forward to any more pearls of wisdom..

Thanks
Nick


From: paul
To:j holland; NickWright; british-cars-pre-war
Sent: 17 May 2000
Subject: Carburettor problems

James and Nick,

I just sent a reply to your carb problem to yourself and British-cars-pre-war (duplicated below) and the mail delivery system has bounced it, saying it couldn't be delivered to all recipients. Did you get it??

An additional thought, concerning the flooding carb. SU carbs come in high and low pressure versions, depending on where they are mounted (under bonnet or at rear near tank). I've never had a problem but if you had a high pressure one and it was mounted close to the carb with big pipes the pressure might be too much for the needle valve, possibly, perhaps.... I'll go and get my coat!

Paul


From: j.holland
Sent: 18 May 2000
To: paul; Nick Wright; british-cars-pre-war
Subject: Re: Carburettor problems

Paul,

Yes I agree it would be far better to have a Nash but nobody is going to fall over themselves to relieve me of a Rapier that idles at 15 thou! I am sure the butterflies are the right way round-they have been examined by Burlen's several times.There is one other bit of info I neglected to give;before the carbs were rebuilt there was a homemade lead weight on the top of each piston!

Sorry for the late reply but I'm only on half throttle today thanks to a visit to Colchester Beer Festival.

James


From: Clive Sherriff
Sent: 18 May 2000
To: j.holland
CC: british-cars-pre-war
Subject: Re: Carburettor problems

Message text written by j.holland
>There is one other bit of info I neglected to give;before the carbs were rebuilt there was a homemade lead weight on the top of each piston!<

Ok = possible that you now have lightweight alloy pistons instead of early heavy brass/bronze ones - the alloy ones should have a spring above them to force them down - i.e. increase the "weight" - if they dont have these you're in a bit of trouble - and so are Burlen for not noticing it!

Clive Sherriff


From: j.holland
Sent: 18 May 2000
To: british-cars-pre-war
Subject: Re: Carburettor problems

Interesting. The pistons are made of some sort of alloy-they look similar to mazak to my untrained eye. I have tried 3 different grades of spring, all to no avail. The carbs by the way are 1 1/4" HV3.

James H


From: paul.weston
Sent: 18 May 2000
To: Reply requested; Reply requested
Subject: Re: Carburettor problems
>Paul,
>I didn't get your original message-I had the same problem replying to Nick's
>original message.

Strange, I didn't get it at work either. It seems whilst I receive mail from brit-cars at home I can't reply to it. I shall see what happens from work!


>Yes I agree it would be far better to have a Nash but nobody is going to
>fall over themselves to relieve me of a Rapier that idles at 15 thou!

Can't agree, if you can get a Rapier engine that idles at 15thou!!! the racing boys will be falling over themselves for the technology :-))


>I am sure the butterflies are the right way round-they have been examined by
>Burlen's several times.There is one other bit of info I neglected to
>give;before the carbs were rebuilt there was a homemade lead weight on the
>top of each piston!

That sounds like a bodge!!

As someone else has already suggested, most likely butterflies aren't seating properly. Have you tried a bigger hammer?


>Sorry for the late reply but I'm only on half throttle today thanks to a
>visit to Colchester Beer Festival.

I'm sure you only went out of a spirit of enquiry and didn't enjoy it in the slightest!

Paul


From: j.holland
Sent: 18 May 2000
To: paul.weston; british-cars-pre-war
Subject: Re: Carburettor problems

>Can't agree, if you can get a Rapier engine that idles at 15thou!!! the racing boys will be falling over themselves for the technology

It still won't compensate for all the extra weight not to mention transmission loss through the ENV box!


From: Nick Wright
Sent: 18 May 2000
To: 'Alan Fairless'; 'Pre-War Cars List'
Subject: RE: Carburettor problems

There are grooves worn into the venturis by the butterflies having been attached to severely worn spindles in the past (ie before I rebuilt them). New spindles and bushes mean that the butterflies are now fluttering in the right place again, but perhaps air is getting through underneth them via the groove. I know this sounds like there might be a bodge coming on, but can anything be done to restore the venturi to its original state??? Also, I have alloy pistons, but I had no idea these were wrong. Can anybody else verify this. Do I really need to convert them to spring return??? (the MG carbs are semi-downdraught)

Cheers

Nick



From: Alan Fairless
Sent: 18 May 2000
To: british-cars-pre-war
Subject: RE: Carburettor problems

re the grooves-- you could try filling them in with some epoxy/metal paste.

re springs-- I have some pre war SU's which have (albeit, quite heavy) alloy pistons and no springs, so don't rush into things.

The only way to avoid using springs, if indeed you should have them, would be to restore the pistons to their previous weight by adding some lead or something. You should weight the piston until it reaches the top of its travel at about 2/3 engine max revs, and under some load. Without a rolling road, this might be more difficult than it sounds. because you need to do it without restricting piston movement or adding weight. In the past I have removed the bonnet and glued a needle to the top of the piston so I could see what was happening, and remembered to compensate for the weight of the needle, when unglued.I must admit to not having used this method to compenstate for removed springs, but I have modified piston weights to match a carb to an engine and it worked, whether it arrived at the optimum setting is a different matter. However, I don't think incorrect piston weight would affect tick over speed. The normal effect would be inability of the engine to pick up speed cleanly, rich running for too heavy, weak for too light, or vice versa, SU's are tricky things and don't always do what you expect.

Good luck

Alan


From: John Hardy
Sent: 18 May 2000
To: Nick Wright
Subject: Re: Carburettor problems

>
>Dear listers - can anyone help me?
>
>I have just completely refurbished the twin 1 inch SU carbs on my MG P-type,
>but there are still some problems:
>
>1. The (brand new) float needle seems to be incapable of preventing the
>float chamber flooding. When the petrol reaches the level of the overflow
>pipe it pours out in short pulses, separated by one or two seconds. I have
>tried replacing the float needles again with the old fashioned solid metal
>type, but this has not improved things. It would seem that petrol pressure
>is pushing down on the needle more strongly than the float is able to push
>up. Could there be a problem with the (standard SU) pump?? This is the most
>serious problem and I would be very grateful for an answer...
>

I think that probably dirt is in there somewhere. This has happened to me a few times

>>
>2. The left hand carb whistles loudly when idling. I guess there is a tiny
>air leak somewhere - any ideas?
>

That depends.......is a recognisable melody. Broadway show tunes perhaps?

>
>3. The engine runs too fast (about 1500 rpm) when both carbs are fully shut
>off. They have new spindles, bushes, butterflies - everything, and the
>pistons are both seating properly...
>

I think that this may be a result of your warbling carb. Air getting in. Is the piston sitting too high? I've come across carbs with the mounting flange warped before now.

May the motoring deities guide your contrivances.

John Hardy

www.electract.co.uk
www.brakedrum.co.uk


From: John Hardy
Sent: 18 May 2000
To: Nick Wright
CC: british-cars-pre-war
Subject: Re: Carburettor problems


>Also, I have alloy pistons, but I had no idea these were wrong. Can anybody
>else verify this. Do I really need to convert them to spring return??? (the
>MG carbs are semi-downdraught)

IMHO I don't think that this is a problem. My early Riley 9 carb pistons are alloy so are most of the others I've come across. I rebuilt a pair of 1.25" SU's at vast expense myself last year. Got them on the car and the idling was through the roof. Upon examination I found that the bridge across the jet had had a spacer peice rivetted on thus lifting the piston. Although to all intents and purposes it was seated it was seated too high. Look for anything non standard and remove or renovate it to remove. When you're back to how the carb body left SU then you've got the basis for a rebuild. Unless the scoring in the venturi goes past the butterfly closed position thus causing a leak it shouldn't cause this problem and it could be polished out later(possibly as part of a gas flowing exercise). If it is at the butterfly perhaps it could be repaired with a good quality filler such as Belzona. (possibly).

May the motoring deities guide your contrivances.

JH


From: Nick Wright
Sent: 22 May 2000
To: 'Pre-War Cars List'
Subject: Carburettor, and radiator problems

Dear Listers,

Thank you very much for all your help and suggestions last week regarding my various carburettor woes.

I investigated the system at the weekend and, sure enough, the car was fitted with a high pressure fuel pump from an MGA or some-such. I swapped it for a correct low pressure pump and the flooding problem has been solved. Naturally the fuel level in the jet is now much lower too, so I am hopeful that this may have some affect on the fast idle, but I have not been able to try that yet as I also removed the radiator this weekend...

... which brings me on to my next question ... There is a large hole in the steel bottom pipe stub of the radiator which had been very crudely 'repaired' with filler. Being steel, I don't think it will be possible to make a soldered repair, and welding is, needless to say, out of the question. Can anyone suggest a good 'bodge' that will effectively and neatly patch up the hole until I can get the pipe stub removed and replaced when I win the lottery? I wondered about fibreglass...

Cheers

Nick Wright
1935 MG PA


From: Clive Sherriff
Sent: 22 May 2000
To: Nick Wright
CC: 'Pre-War Cars List'
Subject: Carburettor, and radiator problems

Message text written by Nick Wright
>Being steel, I don't think it will be possible to make a soldered repair<

Not so - and no Bodge either. The secret is to clean back totally to bare steel, degrease thoroughly, tin the bare steel with a good fluxed solder, Not too hot or you melt the rest of the radiator!, then solder on the similarly prepared and tinned plate of steel, copper, brass or whatever.

Clive Sherriff

Oxford UK


From: Adrian Roger Twelvetrees
Sent: 22 May 2000
To: Clive Sherriff; Nick Wright
CC: 'Pre-War Cars List'
Subject: Re: Carburettor, and radiator problems

Nick,

I recommend that you use either "Bakers Fluid" or some other such aggressive flux on the steel. A trip to the hardware shop and a read of the jar labels may help. The flux in cored solder is usually fairly benign so that it can be used for electronic circuit boards without risking corrosion of the components. The secret is in the cleaning and then fluxing all over the area to be bonded to keep it clean before you tin it like Clive says. Early Landrover radiators are brass with steel brackets soldered on to hold the unit in place, so your fix should last indefinitely.

Best wishes to all,

Roger


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