carb Issues

[Scotty]

Bit of a mystery this one folks.

New carb fitted in June (new old stock) from the carb hospital chappie. Last one he had in the 1275 marina dept! Its a later HS4 but with the waxstat thingmy. Anyway, fitted and colourtuned and idle to midrange fairly spot on and flat out it picks up well and runs cleanly, but seems to run incredibly lean.

Midrange at about 50, but when putting power in, it is spluttering and misfiring like a bad one! If you ease off things smooth out again as things do if you turn the tap on fully. Today, in an effort to figure it all out, I opened (closed) the choke just when the misfire occurred and I was instantly rewarded with power and smooth running again. So probably a jet issue I think. Engine seems fairly standard with the exception of a 4 branch exhaust linking to a side exiting silencer.

May use the little programme that is available to aid SU jet selection, but any similar experiences out there guys?

Scotty

[philcoyle]

Re: carb Issues

If it runs better with the choke out I would be looking for manifold gasket leaks and /or leaking servo or servo pipe. Followed by filters and fuel pump and pickup pipe in the tank. Well that's the weekend taken care of.
Phil

[dbazdavies]

Re: carb Issues

have you checked the tappets - points gap - plugs - ignition timing at 3000 revs ?? should always do this first before even touching the carbs - make sure the carb is free in the dashpot !!

[Gareth]

Re: carb Issues

Checked there's oil in the dashpot???

[b_caswell]

Re: carb Issues

The waxstat unit on the jet is designed to compensate for the air temperature so as the air gets hotter the air density decreases so the waxstat leans off to balance the mixture. Marlin engine bays tend to run hot so possibly whats happening is the mixture is leaning off too much. I would be inclined to get a standard non waxstat jet unit to replace the waxstat.

[greyV8pete]

Re: carb Issues

Conversion kit for waxstat to non wax available:

http://www.minimania.com/part/CSK65A/Hs4-Carb-Conversion-Kit--Upgrade-For-Waxstat-Style

Until I read this forum post I never knew that such a thing existed. Google to the rescue! Peter.

[Scotty]

Re: carb Issues

Yes indeed Guys, a few weekends tinkering shall now be had. Thanks to all for your replies. I will replace the waxstat, check for induction leaks, although my brakes are without servo.
Something tells me its jetted wrong, although I never considered fuel pressure. When I fitted the new carb, it ran perfectly and cut out after around 30 secs. On removing the float chamber top, there was no fuel. Primed fine on the starter, cover on then ran for 30 secs etc.

Pondered for a day then a close examination revealed there was no vent hole on the inlet chamber top at all. This was found after a close exam of the old worn out HS4, which had a 1mm vent hole on side of cast body almost where inlet fitting was. New carb had a strange plastic ball bearing on top, which i drilled clear. This resulted in starting and ok running but is obviously a larger vent than normal. I would have replaced with the old top but this is worn out around the float hinge area.

Scotty

[lil_red_roadster]

Re: carb Issues

Also check the needle valves in the float chamber...these are particularly sensitive to dirt/debris.
Waxstats generally are the work of the devil.

[stevejgreen]

Re: carb Issues

likes machine gun.jpg

Also check the needle valves in the float chamber...these are particularly sensitive to dirt/debris.
Waxstats generally are the work of the devil.

[john cole77]

Re: carb Issues

I was watching an interesting video that was explaining the basic principles of how an SU carburettor works.
What happens when you accelerate is explained.
The symptoms you describe tend to suggest that the piston assembly is rising too fast when you accelerate, the oil in the dashpot is supposed to slow the rise of the piston to give the engine an extra burst of fuel.
You indicate that the problem stopped when you pulled out the choke which has the effect of delivering more fuel.

This is the link to the you tube videoa https://www.youtube.com/watch?v=60Bj...3F91CA3FEE0297

It's by John Twist from University Motors in America, they specialise in MG's

Have a look it may point you in the right direction