Carbureted Engine Tips

A friend of mine from the UK sent this e-mail to me.

Hi Larry, Long time no speak! I have a contribution for your RAF builders site. As you know a friend and myself built G-YRAF last year, and have been flying it since January. Something happened to me whilst taking off one day was which was very worrying - and could have been serious - and the same thing might happen to other builders, possibly with more serious consequences!

On take off, at about 10 feet, the engine suddenly lost power and the take-off had to be aborted. Fortunately, there was enough power left on full throttle to fly it along the runway as if it was on the back of the power curve, and I was able to put it down without mishap 20 to 30 yards further on. On taxying back to the hangar, we suddenly had problems with the throttle sticking open, almost pushing us through the perimeter fence at one point - all very strange! We were baffled by what might be causing this odd combination of loss of power and sticking throttle, and carried out checks on all the usual suspects; throttle return spring, plugs, distributor, ignition leads, fuel etc.

Everything seemed to be in order, until we took the air-box off the carburettor, when we found, to our horror, a large piece of a strange, black, fibrous material jamming the butterfly open. For a moment we were at a loss to know how this enormous piece of material had got into the carburettor! Suddenly, it dawned on us that it was a piece of (well-baked!) Scotchbrite from the carb. heat tube!

The RAF build video recommended that a sheet of Scotchbrite be rolled up and pushed into the flexible tube as a filter to prevent any rust particles getting from the heat exchanger into the engine via the carburettor. But, in our case, the cure was more serious than the ailment!

What seems to have happened was that the Scotchbrite had initially slipped down the tube until it was closer to the exhaust, where it had got slowly baked, until it became fibrous and LIGHTER! This meant that on using carb. heat (which we had that day) bits of it had been sucked into the carb. causing the loss of power. Thinking the power loss on take-off was due to carb.icing, I quickly applied carb, heat, which caused the huge lump to be sucked into the carb.; eventually causing the throttle to jam partly open.

We removed the carburettor , inlet manifold, and carb. heat tube, and made sure that there were no particles of the offending substance anywhere in the system. We then replaced the carb. heat tube, needless to say without the benefit of a Scotchbrite filter! The engine is now running fine. Ours was a 1997 kit, and RAF may have got wise to this in the interim and stopped recommending it - but just in case there is someone out there who is tempted to put a roll of Scotchbrite in their carb. heat tube, my advice is: DON'T - it could be very bad for your health! Hope you are well! Very best regards, Vic