HS6 Fuelling Problem

[RCHunt]

Hi, I have a rally prepared Volvo 142 fitted with twin HS6 carburettors. I had some difficulty getting these set up but once sorted they have performed magnificently. Until... I removed the carbs & manifolds to access the oil cooler pipes and on replacing the carbs the forward carb is spewing fuel out of the overflow. I suspected the float needle valve was dislodged or sticking but this does not seem to be the case. The float appears to float quite happily and the needle valve moves freely. However there is a ring worn around the viton tip. The needle valve, housing and float were recently purchased from Burlen. Am I missing something?

[grumpydad]

Hi
yes probably a tiny pit of dirt
that is so small you cant see it on the seat
have you got a in line filter or just the one in the pump

[RCHunt]

I had thought that but I have now dismantled it 3 times and flushed it through but it still persists, I will try again tomorrow! Thanks. It just seems odd that it has only happened after I have removed and replaced the carbs.

It has a Malpassi Filter King pressure regulator and filter.

[Burdekin]

Have you set the float height also check the float for holes (put it in a bowl of water).

[RCHunt]

Thanks, I've tried all that.

[142 Guy]

I haven't lived with this on an SU; but, I did encounter this problem on a motorcycle. In my case, after a 5 month storage period with no fuel in the carbs the viton tip on the needle would not seal reliably. Presumably the rubber had hardened leading to a seal sometimes, pour gas on the floor sometimes situation. A new needle fixed the problem. Interestingly the new needle and the old needle looked identical with no obvious signs of wear on the original needle.

With the bowl and float removed from the carb, rig a hose up to the fuel inlet so that you can blow through the hose. Use your finger to gently lift the needle up to the seat - the float cannot exert a huge amount of force so you should not have to either. Blow through the hose and if the needle is not sealing, then you still have a needle valve problem. Either damage to the seat or a problem with the viton tip.

You said you purchased (and presumably installed) a new needle, float and housing. Is that before the leak or after? Does 'housing' include the seat for the needle? It is possible that although the replacement needle is new to you, the viton may have hardened a bit while sitting on the vendor's shelf leading to a fail to seal problem. When I did my repair, I initially tried to rehabilitate the original needle by scuffing the viton with some 600 SiC paper which did nothing.

Try the hose and puff test. That should determine whether it is a needle valve problem. Whether it is the actual needle or the valve seat requires further investigation.

[RCHunt]

Thanks for your responses, I thought I had solved it yesterday after I had dismantled it for the 4th. time flushed it through, rechecked the float height etc. It started up fine but then started doing it after ticking over for a minute or so. I'm suspecting maybe dirt in the petrol, but wouldn't that affect both carbs?

[Burdekin]

Are you using a electric fuel pump? If so try removing the pressure regulator and hook up a mechanical pump and ensure you have the spacer fitted.

[142 Guy]

Based upon my motorcycle experience, the defective needle would sometimes seal and sometimes not. I have never owned a four wheeled vehicle with carbs. My 1971 142 had D jet so its been einspritzen since day 1. But, I have had numerous friends with SU equipped vehicles. Non sealing needle jets was a rather common theme on older SUs and in the '70s, the Grose-jet was the preferred solution to needle jet problems - amongst my friends. I see they still make them:

https://mossmotors.com/386-390-grose...or-needle-seat

I have no hands on experience with the Grose-jet.

If you have a paper element style fuel filter it is unlikely that dirt is causing the problem. Mesh fuel filters are less effective. As suggested by Burdekin, too much fuel pressure can be a problem. The front needle valve may be the weaker link and overflows first to relieve excessive pressure. Check with a fuel pressure test gauge. The running pressure should be less than 3.5 psi on SUs.

[RCHunt]

I had the bright idea of swapping the components within the float chambers around but again after running for a few minutes it started chucking fuel out of the forward carb overflow. I think that shows that the fault is not with the components.
Like the idea of excess fuel pressure, no time at present but will check this out next week.
It just seems odd that everything worked fine before removing and refitting the carbs without doing anything to them.