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Vida

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Hi,

I have a Spitfire Mk4 (1972) 1300cc with running problems.

The engine has been replaced with a recon unit ( Ivor Searle ) with a stage 2 conversion.

The engine has had very little use, as its undrivable due to very limited throttle response.

The engine will start and idle without any issues , but when opening the throttle it will not rev / no power .

I have been through the basic's and there no induction air leaks , and the dwell and timing settings are all correct 

The timing will advance up when you operate the throttle so I'm confident the ignition side is ok.

I have carried the carb balance and fuel mixture set ups but this has no negative or positive effect.

If I close the carb orifices, for example by 70% the engine will respond to throttle openings and run through the rev range without any issues , so I'm confident my issue is fuel metering / supply .

I have stripped the carbs to a basic level and its evident that someone prior to my involvement has replaced the needles and jets on both carbs + both throttle shafts and butter fly's.

I have noted the needles are ANN (HS2 SUs) which I understand is the basic needle for this engine.

Has anyone upgraded to a stage 2 engine, and if so what needles have you fitted .

I options to upgrade to AAQ , AAM or ABY  and what colour dash pot springs are required. 

On further inspection of the carbs (SU) I have located the AUC part number (AUC 870) when cross refencing this number  carbs are for a Riley 1.5 ?.

Thanks in advance

Phil

 

 

 

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Quoted from glang-

Not sure what you mean by 'close the carb orifices' but are the pistons free to rise and fall and do they when operating the throttle? If they don't that might give the symptoms you describe...

The Pistons rise and fall when the throttle is operated 

If I reduce the carb inlets so they are a smaller size , the engine speed will increase with throttle operation and will operated correctly through out the all rev ranges 

 

Thank in-advance

Phil.

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Quoted from Vida-

If I reduce the carb inlets so they are a smaller size , the engine speed will increase with throttle operation and will operated correctly through out the all rev ranges 

You mean it works if you blank off half the air filter opening? That sounds like the mixture is way too lean, possibly wrong needles, possibly missing the dashpot springs.

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Quoted from RobPearce-
Quoted from Vida-

If I reduce the carb inlets so they are a smaller size , the engine speed will increase with throttle operation and will operated correctly through out the all rev ranges 

You mean it works if you blank off half the air filter opening? That sounds like the mixture is way too lean, possibly wrong needles, possibly missing the dashpot springs.

Yep reduce the carb inlets , 

he carb springs are there .

I think its the wrong needles as its had a stage 2 conversion, just not sure about the carbs , are the correct one AUC 870 

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The needle profile wouldn't have such a pronounced effect, you would still be able to drive it if the carbs were set up approximately right. Generally you want to go a bit richer than normal but it's non-trivial finding which needle & spring combination is best! I'm currently experimenting myself. 

Blocking the carb mouth has the same effect as choke, and both apparently resolve the problem so sounds very much like it's chronically weak. Does the engine stumble or spit-back when you open the throttle? Sorry for stupid question but does it improve when it's warmed up? How long are running it on choke? 

Have you double checked that jet and needles are fitted correctly? If not I would go back through the Haynes guide for setting up the HS2s (centering the jets and everything). As a quick check if your airfilters are off, the jets should be slightly below the carb bridge. The needles should be perfectly straight, smooth and clean. 

Pete

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