SU carburetter needle valves

[WLAcopilote]

Hello,
I have experienced very intermittent problems with the twin SU's on my Rover V8. It has been suggested (and makes sense) that one of the needle valve is sticking in the open position when the float chamber is empty (because the float moves in an arc and the needle moves in a straight line) and the tolerance / gap between the needle and it's guide / seat is too great.

It happened again this weekend; certainly the evidence points to this cause: plugs 1, 4, 6, 7 sooty and the left-hand carb had a wet overflow. Haven't had the carb off yet to look but car is OK again now...

Question is has anybody else had this problem and how did they cure it?

WLAcopilote

[love machine]

They do wear and stick. I'm not sure about RV8 carbs but I think they're a common fitting across the range, you can get super duper high flow ones which don't wear out from minispares. I would look into the feasibility of swapping them around before you fork out to a RV8 rip off specialist.

[nel]

Well as it's only intermittent the float must still be watertight, and the needle valve may just be hanging up as you say. It could also be the needle valve and its seat that are unevenly worn (unlikely), a failed spring in the needle valve (seem to remember that they have a sprung peg that the lever acts on), a bit of crud in the chamber too big to pass out but capable of blocking the correct seating, etc.

The lever mechanism actuated by the float is just a bent bit of inox plate, so can be "re-modelled" by hand to effectively set the float level. I'd take the tops off both the float chambers and, being careful not to mix the bits up between the carbs, compare the lever and needle valve on the dodgy one with the same items from the working side. You might find a difference in the set up that you can correct by bending the lever slightly. Obviously check for dirt whilst you're in there.

Best of luck.

[WLAcopilote]

Nel, Lovemachine, thanks for your contributions. Guess it's time to get the cut-down 1/2" spanner out and remove both carbs again...

[Pigeon]

The float level setting can be quite critical; don't reset it unless you know what it is supposed to be. Google will find you loads of stuff on setting SU float level.

[WLAcopilote]

Cheers Pigeon,
what baffles me is the fact that I've had no trouble wuth this for about eight months then..bop it misbehaves. Measured the needle valves I removed last year and found quite big differences between the I.D. of the seat and the O.D. of the needles themselves. The needle valves are the brass tricorn type.

WLAcopilote

[nel]

Out of curiosity - why should the float chamber end up completely empty? Is the car only used occasionally so the petrol in the chambers vaporises? If the float level is set correctly then, in normal use, the pressure from the petrol pump should maintain enough of a level to stop the float getting jammed at the bottom of its range. Having said that, if all is well with the set-up it shouldn't jam at the bottom of the range anyway.

As Pigeon said, the level in these chambers is critical as it obviously affects the flow rate drawn out by the low pressure at the venturi. Hence the idea of checking the one that is jamming against the one that is functioning well to ensure that they are set-up the same.

[kevinday]

I haven't played around with SUs since I had my 240Z, but let's see. I would take the carbs off and take off the top housing. It is possible that the piston that the needle is fitted in is sticking. Clean up all surfaces with a carb cleaner and refit. Check for free movement of the piston, don't forget the spring that returns the piston to the closed position.

Also check both needles for same profile (letter codes should be the same, also that they match the jet size.

When reassembling don't forget the oil to damp the movement, lack of this can give strange results.

By the way Haynes did a manual on SUs, I used to have a copy but gave it away when I went to injection.

[WLAcopilote]

I haven't played around with SUs since I had my 240Z, but let's see. I would take the carbs off and take off the top housing. It is possible that the piston that the needle is fitted in is sticking.

Wouldn't have thought this would cause the plugs to soot up nor the overflow to become wet with petrol.

Clean up all surfaces with a carb cleaner and refit. Check for free movement of the piston, don't forget the spring that returns the piston to the closed position.

Also check both needles for same profile (letter codes should be the same, also that they match the jet size.

Both needles are BDR; both jets are 0.1"

When reassembling don't forget the oil to damp the movement, lack of this can give strange results.

Oil in both dashpot dampers, changing weight of oil had no effect on symptoms - tried SU oil, 3 in 1, ATF, 15w40, 20w50 and EP90...now using 15w40

By the way Haynes did a manual on SUs, I used to have a copy but gave it away when I went to injection.

I will compare the float levels on both carburetters. Car used regularly - at least once a week. Fiddled with a pair of spare all brass needles; it is possible to get the needle to stick. Perhaps float level is a little too low; I set the level on the low side to be cautious.

Mxture is spot-on - car pulls very well across speed range (will pull 3rd gear from 15 to 70mph) with no bogging or hesitating. Spark plugs are NGK BPR6ES with 0.8mm gap.

Thanks again for suggestions...
WLAcopilote

[Pigeon]

nel said:

Out of curiosity - why should the float chamber end up completely empty? Is the car only used occasionally so the petrol in the chambers vaporises?

They seem to lose quite a bit of petrol by evaporation even using the car twice a day, especially in hot weather. The electric fuel pump in my Morris Minor used to play tunes when I switched the ignition on as it made up for the evaporative loss. Though I'd expect less of this on the RV8, as the exhaust manifold is not right under the carbs.

kevinday said:

I would take the carbs off and take off the top housing. It is possible that the piston that the needle is fitted in is sticking. Clean up all surfaces with a carb cleaner and refit. Check for free movement of the piston, don't forget the spring that returns the piston to the closed position.

Be very careful doing this, don't use any form of abrasive as the clearance between piston and dome is critical. Any metal removed at all will bugger it up. Solvent action is the key.

I don't think this is it, though; it might richen the mixture but it wouldn't cause the float bowl to overflow.

[nel]

Agreed - I think kevinday has got the wrong end of the stick. If the damper pots are working OK then, other than topping the oil up occasionally there should be no need to go there. 'Tis nowt to do with the overflowing float chamber.

[phil hill]

Evaporation ?? Yes !! Any RV8 gets pretty hot, aluminimum gives up it's heat easily and heat rises, so the fuel does evaporate from the float chambers. Does on my MG, don't see why it won't on others.

Float sunk ?? Yes !! Been there too !! The original floats in my SU's were metal, not plastic, and one had sprung a leak. Float sinks, needle valve wide open, fuel pump floods float chamber, fuel overflows, engine runs like cr@p.

[WLAcopilote]

Lots of useful suggestions there...I haven't had chance to remove and inspect the carburetters yet...


The fault occured last week after leaving the car stood for a few hours following a short run and on Saturday whilst cruising on an "A" road near Llangollen. Prior to this the car had been stood for a few hours in the sunshine - seems there could be something in this evaporated fuel idea. Curious why only one carb but as suggested previously I will remove both and compare setups.

Many thanks
WLAcopilote