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Blowback through carb - Andeh - 12-03-2019
Hi all,
Interesting problem. My special has developed a miss and blowback through the carb (1.25" SU) under very certain conditions. If I'm travelling uphill, in 4th at full throttle the engine will begin to miss and blowback repeatedly through the carb until I stop and start again. If I take the same hill easy, at half throttle, it will travel up it without fault.
It idles fine, and runs well under all other conditions, and can pull 4th on the level, it's just when it gets load in 4th at full throttle uphill that it falters. It performs faultlessly at full chat in 1st on a trials hill. It's getting annoying between sections though!
I've a new fuel pump on the way as I'm not certain it isn't just running out of fuel under those conditions, but I'd be interested to know on other people's thoughts. Has anyone had similar symptoms?
RE: Blowback through carb - David Stepney - 12-03-2019
I recently had a similar problem on my RP saloon. My car is perfectly standard with a Zenith 26VA carburettor, so i cannot guarantee that what i found will be you problem. My car also spat back occasionally when pulling hard. I eventually traced it to a worn distributor top bush which was allowing the points gap to vary from 12 thou to virtually nothing. I cured the problem temporarily by opening the points gap a bit before obtaining a refurbished distributor.
RE: Blowback through carb - Oxford Jack - 12-03-2019
This sounds like a typical symptom of lean running, spitting back through the carb. I would suspect first an air leak, quite possibly from having a bend in the face flange, carb to manifold. Alternatively incorrect fuel level in the float chamber. The later best checked with an appropriate drill bit under the shut off mechanism. Also check the end of the butterfly spindle for a leak. A dob of grease on a suspect leak can be helpful.
RE: Blowback through carb - Nick Turley - 12-03-2019
Could be a sticking Valve ?
Hope this is helpful.
RE: Blowback through carb - David Stepney - 12-03-2019
Tight tappets can also give a similar problem. Bill Williams excellent book 'Austin Seven Specials' recommends 4 thou inlet and exhaust but I think this is a bit too tight. Doug Woodrow in the 'Austin Seven Manual' recommends 6 thou inlet and 7 thou exhaust. I set mine to 6 thou inlet and 8 thou exhaust just to be on the safe side.
RE: Blowback through carb - Dave Mann - 12-03-2019
I've only ever set the tappets to 0.004" since getting my first Seven in 1962 and the only handbook I had then was Bill Williams 'Austin Seven Specials'
RE: Blowback through carb - john williams - 13-03-2019
Next time it happens try pulling the choke out slightly, that will confirm or reject mixture problems. If it does not work, follow the other suggestions made regards valve clearance and ignition.
RE: Blowback through carb - Bob Culver - 13-03-2019
In the absence of any other info always ran .004 valves. Life was short but that was normal on old materials.
If not fully closing should be evident on the handle hot. Moderns overlook that marvellouse diagnostic tool.
Dunno what modern valves made of, but most modern materials have higher expansion. Does not matter if stems of separate material. Clearance has a surprising effect on valve duration.
Fuel starving can show in various ways. On another make due restricted float movement car would abruptly cut at medium speed on the flat but OK uphill! Try blowing back thru the pipe.
RE: Blowback through carb - Robert Leigh - 13-03-2019
Apart from the other suggestions raised, I think it could be a partially blocked fuel filter. If you are using an SU pump particularly check the inlet filter in the bottom. It has a very fine cylindrical strainer and does not need a much muck to reduce its throughput.
Robert Leigh
RE: Blowback through carb - Andeh - 13-03-2019
Thanks for all of the advice. Replaced all the inlet/exhaust gaskets to no avail, brand new throttle spindle fitted, carb and float check over and cleaned, dizzy checked for wear (okay at the moment), tappets set very recently at .004 and .006. None of this has helped so far, but I am eliminating things...
However, did find a crushed fuel pipe - it has much less than half it's proper diameter, so it would restrict things somewhat. Didn't have time to test today, but I have cut that section out, so fingers crossed. New SU pump and fuel filter fitted for good measure, and the rest of the fuel system seems good.
Do we think a car would be using more fuel in 4th uphill labouring than 1st on a trials hill at full chat?