Blocked jet in HS6 carb?

[Van]

Hi All
I have a strange problem where my car sometimes lacks power and sounds rough as if only one carb is working (GT6 Mk2 with twin HS6 carbs).
The other day this started happening 8 1/2 miles into my 9 mile drive into work.
The next day it happened 8 miles into the journey. I stopped for a few minutes, checked a few things and then it was fine. It also happened briefly on the way home.
Over the weekend I checked that the pistons move up and down freely, there's oil in the dampers, jets are at the same height, butterflies opening together etc and everything seems fine.
Had the problem again today, intermittently, worse on the way home. If it happens, sometimes I can pull the choke out enough to make the jets come down slightly and then it runs fine at lowish revs/throttle openings (30 in 4th or 40 in OD) but complains if I put my foot down.
Other times I can make it go away for a while by pumping the choke in and out a few times (enough to raise and lower the jets). Other times it just goes away of its own accord.
As everything seems to be OK I am thinking that there must be something lurking in one of the jets that is getting in the way (should not have come in through fuel as there is a filter between the pump and float chambers).
Jets and needles have done 40,000 miles (no idea if this is a lot) but the way this has suddenly started coming and going does not seem to me like a wear related problem.
Do other people go along with this diagnosis or have you experienced another cause of this?

Cheers
Van

[nang]

Sounds as if the piston IS sticking occasionally. Happened to me once. Take it all apart, maybe a light rub with a bit of fine wet and dry paper to clean off carbon deposits will fix it.
Tony.

[Nick Jones]

Might be worth popping the dashpots and float chamber covers off and using a piece of tube to blow back through the jets and see what appears in the float chambers.

Might also be a simple case of fuel starvation where the pump is simply not delivering enough fuel to keep up with demand.  This could be a problem with the pump itself, a blocked filter or strainer (may the one in the pump) or something in the tank restricting the outlet (I have that T-shirt!)

Good luck,

Nick

[piman]

Hello Tony,

"maybe a light rub with a bit of fine wet and dry paper"

Not to be recommended, use only solvent cleaners on the piston and dashpot, their clearances are very fine so abrasive cleaners should be avoided.

Alec

[Van]

Thanks for the answers chaps. I had thought about blowing down the jets, will try that this weekend. Also I will give the relevant bits a clean, although the way moving the choke in and out a few times helps makes me point the finger at the jets more. Car ran a bit more consistently today and I was troubled more by big puddles than carbs.
Van

[Rich_s]

I have had similar problems on my 1500 spitfire, in my case it was the fuel pump filter and inline filter blocking up and restricting the fuel.

rich