HS6 jet bridge fuel issue (or not)

[mike3md]

Hello all,

 

I am having a problem with my HS6/4a inlet manifold set up, which was fully rebuilt and set up on a rolling road last year. Since then the car has run really well, until that is I (as it turns out) foolishly removed them to allow access to the starter.

 

They were removed on the inlet manifold, the only disconnections being choke cable, throttle crank rod and fuel line.

 

On replacing them on the car I now find that although it starts immediately as before,I have a constant bad misfire, and it will not run below 2000rpm. All the ignition side was untouched, but I have progressively replaced it without any improvement. The rear plugs look very lean, so I looked at the jet bridges and found that while the front carb bridge was slightly wet with petrol, the rear was bone dry - and raising the rear carb piston slightly causes the engine to slow. So I have checked the fuel supply (new pump last year) at the rear carb and there is flow, petrol in the float chamber. The float is plastic and the needle and seat are clear and working. I have blown through the jet assembly, and to be sure replaced it with a new one, but still the bridge is dry. Putting a fine plastic tube down the jet and sealing the top shows there is some fuel lower in the jet, and I can blow bubbles into the float chamber from the jet, using a carb cleaner aerosol.

 

The chokes seem to be working correctly - without any improvement. Air cleaners removed to no effect. Fuel level in the float chamber is about 3/4 full without the float. Tank is over 1/4 full.

 

A couple of questions (apart from the obvious one!)

 

1. Should the bridge be slightly wet (engine off)?

2. There appears no means of adjusting the plastic float height, although SU advise in their tech notes that it is not critical - is this correct?

 

The car is parked slightly uphill, in the sun, and I am getting very hot and bothered.........

 

Thanks in advance.

 

Mike

[Richard V]

Mike,

 

Check that the manifold is seated correctly on the head. A freind of mine had similar issues and finaly came to the conclusion that air was getting in, down stream of the carbs. He checked the nuts for tightness, and as all was fine, ticked that off the list and continued his troubleshooting. What he had missed was that the small pin in the head had not located correctly in the corresponding hole in the manifold, resulting in tight nuts but a big air leak. Have a real good look underneath to be sure that you dont have the same issue.

 

Good luck.

 

Richard.

[mike3md]

Richard,

 

Thanks, I will look at that tomorrow - enjoying the cool of the evening in garden tonight, a break from the car!

 

Mike

[mike3md]

Thanks to Richard, I loosened off the inlet manifold and noticed that part of the gasket surface wasn't mating fully with the manifold face, where it is narrowest under the inlets. A smear of gasket seal and careful attention to the clamps and nuts and hey presto - success!

 

That's the obvious question answered, any ideas on 1. and 2. ??

 

Mike

[Peter Cobbold]

The depth of the petrol meniscus in the jet is not critical as long as its not overflowing. Usually its a few mm say 5 below the bridge - and the pressure difference needed to pull the petrol up that 5mm is around 0.005 psi. The 'constant depression' that pulls the fuel into the airstream is provided by the piston can be calculated from the weight of the piston plus the spring down-force divided by piston area- it'll be in the ballpark of 0.25 psi maybe more. So its much more than 0.005psi.

Wet or dry - a bit of moisture wont matter as long as there's not an actual flow, petrol will wick up between the needle and jet. But it might be evaporating so fast the flow's not obvious, just a wet film left. Your piston-lifting test would show a rich mixture if its an issue.

Peter

[Richard V]

Hey, good news Mike,

 

You need to thank a bloke called Jacky in the Alsace for the pain and suffering he went through to find the same problem. I just remembered the funny story

 

Cheers,

 

Richard.