Mk2 Spitfire Carbs Flooding ?

[KevinB]

Hi,

I have an issue with the carbs on my 1966 Mk 2 Spitfire flooding and fuel flowing through the overflow holes on the top of the Float Chambers. 

This happened now and again and I would open the float chamber remove the needles make sure clean and refit, all OK for another period of time when it would happen again. Not always the same float chamber sometimes front sometimes rear?

To cut a long story short, it became so bad I've slowly worked through replacing items one by one to try and solve the problem, I've also spoken to "Burlen's" the SU carbs specialist and taken their advice too. 

When I say so bad I mean the chambers now flood in seconds from starting. 

So far I've replaced needles, seats (well I've done this twice in case the first set was not to best quality?)  and floats in both chambers, set float levels as per manual and also adjusted so that the float would shut off sooner rather than later as a test, no difference (just delayed flooding by a few seconds) fitted new rubber hoses, just because, fitted an in line fuel filter after the pump and new fuel pump. Checked for fuel line blockages to tank and carbs, none apparent. 

So now I have lots of nice parts with the same flooding. The guys at Burlen's said that the carbs run up to 3psi, the pump should output at about 1.5psi. We tried a pressure tester on the new pump and it went off the scale !!! Didn't test the old pump but the issue was the same with both?

My next test is to try an electric fuel pump  as as test to see what happens, just a temp set up. 

So why would things be fine, then get worse ,then replacing everything we still have the same issue ?  and then the new pump, seemingly, with more power that Fireman Sam's  Fire Engine !!!

I would welcome your thoughts and comments please.

Thank you,

Cheers, Kevin

 

[Pete Lewis]

replacement pumps from the never know who makes it have been a subject of excessive pressure for a long time 

you could cut the internal spring down to reduce the high pressure  ,it is only the spring that developes the output pressure 

some have (if it fits)used the spring from their old pump to reduce output 

there are regulators around , some dont seem to work that well

the trouble  with changing so many part s is you loose track of which one is the culprit 

its unfortuneate  that many new parts sold are much worse than what you are removing 

and one sneaky problem is when you mess with fuel lines the hose gets cut inside to make the dreaded rubber slivers that flaot about and jam the float needle 

next time flush fuel into a jar see if you can catch some , much of these come from fake nasty fuel hose  if unsure what you have get some Gates Barricade from club shop 

Pete

 

[johny]

You must get the fuel pressure within limits before anything so on here search the thread 'Excessive fuel pump pressure' in the title as it gives the solution for a mechanical pump....

[KevinB]

Thank you both for your help on this. I’m getting an original AC pump from Dave Saunders on Thursday so hopefully we should a little better with that, will let you know. Thank you. Kevin 

[Unkel Kunkel]

It’s peeing down; All my plans for today are thwarted ..

- so pull up a chair and I’ll tell you what I’ve learnt about flooding SUs

- a subject about I which I knew absolutely nothing a year ago…

 I see 2 main causes; sometimes a combination of both:

1. Needle valve problems:

Poor sealing due to:

a. wear

b.debris under the valve seating.

This can be fuel tank debris but often the  “slivers” problem  caused by fuel rubber pipe debris. This has been  high- lighted by Pete who has coined the term.

( The” Pete slivers ” can be really tiny) 

2.Excessive fuel pump pressure.

The needle valves in good condition,  and not compromised by debris, should hold up to the pressure as per manual to 3.5 psi

The pump pressure is determined by the return movement  of a diaphragm by a spring.The spring largely determines it and it can also be adjusted by altering the the “stroke” of the diaphragm by different thickness gaskets between the pump body and engine block.

Unfortunately…

New pumps are being supplied that develop a much  higher pressure.

I bought one which produced nearly  twice the upper range pressure ( and both carbs leaked ++++)

The cause is the spring under the diaphragm which  is very noticeably stronger than the original spec.

The solutions  (in the absence of suppliers  selling us equipment inside the required spec.* ) :

1. Salvage the spring from you old pump and use that.Tried that - it works. 

2.Add spacer gaskets between pump and  engine block, as per manual - ( tried that too and it also works)

3.Add a fuel pressure regulator -  like the Malpassi  Filter King:

I ordered  one of these. It was while I was waiting for it to arrive I fiddled around with options 1. and 2, which were successful.

Having bought the thing and as it has a rather nice filter, I fitted it.:

( transmission tunnel is out ..)

The pressure was OK then anyway but

End result = psi in range:

 

but more importantly.. No flooding.

 

* 1.The suppliers surely must be aware.

I  emailed.I was very quickly offered  a full refund but the bland , “ No other reports of problems ..”I found rather disappointing and  dismissive. I declined.

2.Not only are they outside spec., it is arguable that the result of this -ie  spewing fuel can be viewed  as a safety issue.

3. There is another issue with some  of these pumps-the actuating arm/lever which is operated by the camshaft is rough as ..the proverbial , needing finishing before installation to prevent cam damage.

Good Luck!

 

 

 

[Pete Lewis]

thats and  good covers most eventualities we see on here 

Pete