Spit mk3 front carb rich but no adjustment

[Andy Moss]

Setting up spitfire mk3 with adu257 carbs.

Colour tune in front and rear carbs.

Rear carb has Bunsen blue, front carb is yellow.

I have wound the jet up to the top and still yellow colour on front carb.

Checked metering needle centred has click when piston falls

Checked float chamber to ensure needle valve seals ok and float level is correct.

Changed the front carb for a previouly known good one.

Same problem!

Baffled!

Could it be the after marker fuel pump forcing petrol part the needle valves.

Everything is new and barely covered 100 miles since overhaul.

Should I try changing the fuel pump?

[Andy Moss]

Changed the fuel pump. Could I be miss reading the colour tune.

 

Yellow rich and I need to see Bunsen blue.

 

Baffled!

[Pete Lewis]

yes  youre correct yellow is rich blue about right

 

is there something on the air filters blocking the front face ports??   do a test with the filters Off.

 

as you have swapped the carb its looking like an external problem

 

if fuel pump psi was a problem both carbs would probably show yellow

 

check the rocker  breathers are clear.

 

Pete

[Andy Moss]

Rocker breather diafram was very tight and not moving on the overrun. I freed it off and it seems to have helped. I can get Bunsen blue on both, but the exhaust still smells rich. Are all of these cars like this. I've had this one for 15 years and I don't remdmber it not smelling a bit of unburned fuel.

 

Air filters were off so I could balance the carbs.

 

Do you have any other ideas?

[Pete Lewis]

jets sticking and not returning after you have used the choke,   you push the control in but the jets stay 'down'  

 

test.... just push choke in and pop round the bonnet a finger the base of the jet it  will pop back up if the levers are holding it open.

 

its a pretty common SU fault  

 

Pete

[Andy Moss]

Jets are returning ok and I have checked them.

 

I don't actually know what it was, but seems to have gone away. Mixture always ends up stinking at idle.

 

Thinking of pulling the metre needles a little further out of the floats inside the SU.

 

Cheers for you help

 

Andy

[Pete Lewis]

Do yo have the right needles fitted ??

whilst these cars are pre any real emmision controls they should not be a stinker

 

Is it rich or unburnt fuel due to other factors ,, any misfiring ???

[Andy Moss]

BO needles fitted.

 

The mixture seems ok at higher revs, but at idle the mixture quickly becomes rich, which seems to worsen with the duration that it sits at idle.

As I say above. I think a combination of not reading the colortune well and other factors did not help.

 

The engine runs well and I have advanced the timing to about 7degs BTDC from the factor spec of 6. This just avoids pinking and responds better on acceleration during road tests.

 

Has anybody experimented with the needle positioning in the pistons to reduce fuel metring at the idle point? The pistons do not sit on the metal bridge at idle, so further adjustment could be achieved by moving the needles.

They were originally set with the needle shank at the same level as the inner piston area, I have had some success with repositinging the needle slightly.

[daverclasper]

Hi

 

I have heard of people moving the needles as a get out of jail method, though no personal experience. I assume this would also alter mixture at higher revs?.

 

Dave

[Pete Lewis]

If you move the needle then you just adjust the jet to compensate for the change

 

its not doing much,

 

have you the correct jet sizes fitted ??

 

Pete

[gollane]

I had a similar problem with my GT6 with HIF6 carbs ,I cured mine by getting the float height absolutely correct,amazing the difference 25 thou makes to the available mixture adjustment .