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Geofftaylor

Carburettor sputtering.

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My front SU started to make what sounded to be a backfire under hard acceleration. I decided after a lot of messing about it could be due to valves so took off the head and gave it a decoke (it needed it) and ground in the valves. All looked good. All reassembled, tappers done, timing done, carbs balanced .... no difference. I still have the tractor sound on acceleration which drops right back once cruising. Any ideas.

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Backfiring and sputtering on acceleration denotes too much fuel building up in the venturi so check jet centering and piston and if there's oil in the dashpot which may raise too fast hence flooding the venturi on acceleration.

Edited by Geko

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Lack of damper oil will make the mixture lean, not rich. The purpose of the oil damper is to make the mixture richer by delaying the rise of the piston during acceleration and maximising the suction in the venturi. Could the piston be sticking down perhaps?

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Thanks for your input. I will check the needle centering. Can’t see why it would have moved since assembly quite some time back but you never know. Dashpots do have oil.

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1 minute ago, RobH said:

Lack of damper oil will make the mixture lean, not rich. The purpose of the oil damper is to make the mixture richer by delaying the rise of the piston during acceleration and maximising the suction in the venturi. Could the piston be sticking down perhaps?

I have been messing with the carbs this morning and looking at the oil level was one of the first things I checked. Both pistons are lifting on acceleration and, using the screwdriver lift method, seems to be pretty much near enough set to the correct mixture. The noise seems to emit from the front carburettor and sounds like a diesel when accelerating hard in low gears. Maybe the problem lies elsewhere but following a head off rebuild I am struggling now. The car is running very well when cruising.

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Perhaps there is an exhaust manifold blowing? Poor sealing of the gasket is possible.

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Sorry to be obtuse Geoff but how do you know it is the front carb if it only happens during hard acceleration (presumably that is whilst driving)?  You describe it first as a backfire , which to me means an irregular cough or splutter, but later you say a tractor-like sound so is it actually a continuous roar? 

Is the car standard or does it have a tubular exhaust? (There have been instances of the clamps not working properly on the latter because the the flange is not as thick as the original so that the clamp won't apply proper pressure. )

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Hi Rob,

Apologies for being a bit vague but I was slowly determining the source by a process of elimination. I initially sounded like it was an exhaust problem and consequently dismantled and reassembled the whole system carefully but the noise continued. I then checked the manifold to head gasket. All was good. 

The reason I was blaming the front (1&2) carb was, with the air cleaners off and revving the engine whilst looking into the carbs the front one seemed to be the source of the noise. The noise isn’t irregular and, on hard acceleration, sounds like an old tractor going up a hill or a raucous motor cycle engine. (It is a tractor engine but doesn’t normally sound like that!). It can be mimicked to a lesser extent when revving the engine whilst the vehicle is static. 

The exhaust is tubular stainless. 4 into 2 and the 2 into 1 and then onwards out the back. I am wondering if I am looking in the right place. I am normally pretty good at finding faults but this one is getting me quite frustrated. I did wonder if I was getting spark failure at one plug but haven’t taken that further yet. 

I have taken a short video but it is obviously more than the 4.9 mb allowed! All suggestions gratefully received. 

Geoff.

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Pinking ?  if so retard the ignition a tad.

Bob.

Edited by Lebro

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Yes that's what I was beginning to think Bob.

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Hi guys,

it is a bit more than pinking. I’ve had the timing set across a wide range with no difference.

Geoff

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Water pump pulley anyone?

Peter W

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Try swapping the dashpot damper rods, the one on the front SU might not be damping piston lift despite oil present.

Peter

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1 hour ago, BlueTR3A-5EKT said:

Water pump pulley anyone?

Peter W

Possibly - or dynamo bearing perhaps?  Worth trying with the fan-belt off?

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1 hour ago, RobH said:

Possibly - or dynamo bearing perhaps?  Worth trying with the fan-belt off?

 

2 hours ago, Peter Cobbold said:

Try swapping the dashpot damper rods, the one on the front SU might not be damping piston lift despite oil present.

Peter

Hi guys,

it is a bit more than pinking. I’ve had the timing set across a wide range with no difference.

Geoff

I’m contemplating putting some thicker oil in the damper as the piston does seem to be lifting significantly higher than the other carburettor. I will try swapping the dampers. I’ll try most things now! The odd thing is that I’ve run this car for years with nothing going wrong I couldn’t fathom until now. I am now also thinking that the carburettor is just where the noise is manifesting itself and not the source. It’s not  a bearing noise. 

Geoff

 

 

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Geoff, If the oil is too stiff ( eg EP80) it will slow the fall of the piston as well as slow the lifting. Ive tried !

The sleeve might simply be stuck in its lower position,failing to seal against the seat, so killing damping

SU%20Damper%20Valve%20markup.jpg

Peter

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10 hours ago, RobH said:

Possibly - or dynamo bearing perhaps?  Worth trying with the fan-belt off?

 

11 hours ago, Peter Cobbold said:

Try swapping the dashpot damper rods, the one on the front SU might not be damping piston lift despite oil present.

Peter

Hi guys,

it is a bit more than pinking. I’ve had the timing set across a wide range with no difference.

Geoff

I’m contemplating putting some thicker oil in the damper as the piston does seem to be lifting significantly higher than the other carburettor. I will try swapping the dampers. I’ll try most things now! The odd thing is that I’ve run this car for years with nothing going wrong I couldn’t fathom until now. I am now also thinking that the carburettor is just where the noise is manifesting itself and not the source. It’s not  a bearing noise. 

Geoff

 

Hi Peter,

thank you for taking so much trouble to expand your explanation with diagrams which are very comprehensive and interesting. I will have another go this morning and try changing the damper pistons as suggested. I am also contemplating changing the carburettors over to see it the problem moves but this is a fiddly job as all the linkages will need to be changed. It will however determine if the problem is elsewhere. I plod on. 

Geoff

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I'm delighted to see that Peter Cobbold is contributing once again - I am one of a great many who has missed his excellent and erudite contributions.

Ian Cornish

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3 hours ago, Geofftaylor said:

I’m contemplating putting some thicker oil in the damper as the piston does seem to be lifting significantly higher than the other carburettor. I

Bear in mind you may be being misled; because the noise seems to be coming from the front you are assuming the rear one is correct but perhaps its that one which isn't lifting enough....? 

A duff plug or misfire due to electrics should be evident in a loss of power.

Edited by RobH
further thought

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19 hours ago, roy53 said:

spark plug ?

My thought also. 50+ years ago one of my mentors told me that 90% of SU carburetor problems were electrical in nature, and instructed me to make absolutely sure the ignition system was perfect before even touching the carbs.

As for one piston lifting sooner or higher, that's usually a poorly adjusted link between the carbs.

Tom

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8 hours ago, ianc said:

I'm delighted to see that Peter Cobbold is contributing once again - I am one of a great many who has missed his excellent and erudite contributions.

Ian Cornish

Thank you Ian, kind words from you in particular are much appreciated. But I have another priority that takes a lot of my time, and the DC business I have not forgotten. The TR doesnt see a lot of me either. But there are topics eg SUs, supercharging, combustion, water injection that I still find time to post about, on occasion.

Peter

 

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On 6/21/2019 at 11:27 AM, RobH said:

Bear in mind you may be being misled; because the noise seems to be coming from the front you are assuming the rear one is correct but perhaps its that one which isn't lifting enough....? 

A duff plug or misfire due to electrics should be evident in a loss of power.

I am now leaning very much towards the problem being on the other side of the block involving the ignition system. I had all of the plugs out cleaned and out back and the chance that they all went back in the same cylinder is low but possible. I intend to start at the plugs and work backwards. I have attached a very short clip, 4.9mb doesn’t give you much, of the noise. 

Geoff

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 Inlet valve not closing correctly? That could be tight tappets re check the clearances. slightly bent valve? cheers, Bill.

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