Rough running

[RobertGrieve]

Hi all,

Had the car out today and ran it for a bit as I near the end of my restoration and am experiencing a little rough running, here’s what I have found/know

1) starts perfect
2) idles well for first 5 mins or so
3) begins to sounds like a misfire, runs rough
4) fuel drains out of the front carb drain hose
5) upon blipping the throttle the car settles back into a good idle but then problem re occurs

Does anyone think this is related to fuel float heights? I was concerned it could have been compression related as it warms up but I think the fact the car sorts itself out without being cooled/turned off means it’s not that! Must be fuel delivery?

Any help appreciated

Robert

[cactusman]

If fuel is running out of the overflow then either the float height is hopelessly out or the valve in the float chamber is leaking and over filling the chamber. Will lead to over fuelling of which ever carb is affected and cause rough running and probably a very black pair of plugs. Check the float height...dimensions for the two arms it operates are in the work shop manual. Peer closely at the float chamber valve plunger. Most are Delrin tipped...kind of synthetic rubber. Gets hard over the years and cracks and can be damaged by ethanol in 95 unleaded. I'd bet on the valve plunger and replace them all. They are only a few quid for three. Another distinct possibility is that the float has sunk. Fish it out and shake. Any hint of fluid inside and it is no good. New brass ones can be had or you can now get unsinkable plastic ones... Either way you don't want expensive fuel draining on to the road!!

[christopher storey]

I second everything Julian has said. It sounds like a faulty needle valve to me, or possibly a sinking float, although I would have expected that to cause trouble from the very start. The other point to make is that people often change to a newer style fuel pump ( facet/bendix /etc) , which can lead to over-pressure on the delivery side. The SUs require no more than about 2.5 to at most 2.8 lbs /sq.in to assure satisfactory delivery , and if the pressure is significantly more than this you will get flooding of the very sort you are experiencing

[RobertGrieve]

Thanks both - I’ve remembered something else which is that when the key is first turned and the pump turns on, a few seconds later the fuel drains from the drain hose also so think that’s probably suggesting dodgy valve also. Plan to investigate tomorrow once I’ve sorted ride height.. not enjoying tortion bar adjustment

[abowie]

Just firmly tap the top of the offending fuel bowl a few times.