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MG MG Y Type - Carburettor icing

I have a recurring problem of a "rattle" in the engine accompanied by an immediate loss of power. Dip clutch and lift off gas pedal and the noise goes and power can be resumed.Occurs infrequently but when travelling at steady 55-60 mph in wet or damp/cold weather conditions.Todays experience of problem prompted me to stop and lift the o/s bonnet to discover that the carburettor was dripping wet and icy cold.After a matter of moments the engine was running perfectly and completed the next 35miles at speeds up to 65mph without the slightest indication of the problem recurring(until next time!)The carb is standard YB, but fitted with a spacer(MG Midget) between it and manifold together with a heat shield to the petrol bowl.
If it is icing up, what are the effects and how does this create a "rattle"
Keith D Herkes

Following my plea for help yesterday about the icing up, I think this was a "red herring". This morning I removed the heat shield and the spacer block to return it to standard. Took car out on a run and on the local byepass at about 55/60 mph, yea, guess what, the rattle set up and power died. Usual proceedure, and power resumed. Pulled into first laybye and checked carb- OK no sign of moisture, just warm. On return trip I edged the speed up to 65mph and not a trace of the problem. In fact performance seemed a little better if anything. HELP.
Keith
Keith D Herkes

Heat sheilds come in handy in Oz to avoid fuel vapourising in the lines. Not sure what the rattle is, but check your fuel pump and filters, sounds like a fuel starvation issue to me.
Martin Cutler

Keith and I have been conversing on the 'net over this icing problem for a while. I suspect it is not the carburettor that is the problem, but a sticking valve. But there is a huge quantity of experience out there, so Keith 'went to press' over it.

My reason for quoting the valve/valve guide problem is the loss of power consistent with an open valve, as compression would be lost. The rattle would be the huge clearance at the tappet-rocker end. There is a danger the push-rod could jump out as well. There have been other ideas, such as a sticking rocker holding down a valve, and even a sticking cam-follower.

The last idea I had which I hope it is not, is an exhaust valve seat insert,( ie for leadfree fuel,) if pressed into the head, perhaps coming loose and moving a little.

Over to you all.

Neil.
Neil Cairns

I assume there is enough clearance for the valve to stick open and not hit anything? This would explain the loss of power. Has the motor recently been done up? New guides? If the valve was tight in the guide, may be a problem. If new stainless valves where used, they have a greater expansion rate than the original valves, so if the clearances where set too tight, as the motor warmed up, the valve would nip up in the guide. This would lead to disasterous results in an engine where there is little clearance, as the piston and or other valve would come into contact with the stuck valve. If there is no danger of touching other components, then this may be your issue. If you are discussing whether it is an unleaded seat floating loose, I assume you have recently had the head converted?
Martin Cutler

Thank you all so far for your thoughts.The suggestion by Neil about a loose valve seat has also just been put forward by my "engine man"(he did not do the head when it was converted for lead free) I see this as another possibility but how can this be checked as presumably it would only loosen when hot and would seem perfectly tight when cold.A compression test when cold shows, 145, 144, 150, 150 psi. Any further thoughts?
Keith
Keith D Herkes

Only way to check if a valve seat is coming loose is strip the head. Might be some signs of damage to the valve and seat if it has been hitting at a bad angle. The compression tests are well within sight of each other, so this would seem to suggest no damage to the valve/seat and thus sealing capabilities.
Martin Cutler

After further work to eliminate faulty fuel pump or sticking starter I now feel there can be only one answer, as Neil said at the outset, a sticking valve. I can now induce the "fault" with some consistency by travelling at a steady speed between 55 and 60 mph for a distance of approx 1 to 1.1/2 miles. I intend to remove the head in the next few days and will feed back on what I find.
Keith.
Keith D Herkes

Hi Guys, As I said, the icing carb was a false trail as I was clutching at straws to avoid removing the head.Over the weekend I removed head to discover that two of the exhaust valves were sticky and one particularly tight.A reamer indicated that there were no high spots in the guides so I have relieved the valve stems around the area that is exposed to the direct exhaust gases and that part of the guide in the exhaust tract. Whilst reassembling the top end I also discoved that the rocker shaft had been built up as per the later 14.75" length,with all the extra washers, and not the 14" shaft as on my engine,with washers at the very ends only. This meant that the rockers were unnecessarily tight and the centre spring was in fact coil-bound. Test driving over 20 + miles at constant speeds of 60 and 65 mph have not seen any recurrence of the "rattle". Keep your fingers crossed for me. Thanks everybody for your thoughts.
Keith
Keith D Herkes

This thread was discussed between 15/10/2001 and 23/10/2001

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