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Collection, Preservation and Display of Old Lawn Mowers

Dennis z type carburettor

Hello, Any help or advice will be greatly appreciated. I have recently aquired an old Dennis z type (1949) and I am having difficulty with starting the engine. I can get it to run by putting a small amount of petrol into the cylinder via the spark plug appeture when it will start and run ok for about 30 seconds and then dies out. I suspect this is a carburettor issue. The carb fitted is an Amal 418 and as I understand this is not the original which I have read should be either an Amal 49/002 or a 47/022. Does anybody know if this carb is suitable or not, if so how the settings should be? Or where I can obtain the correct carb for the Dennis? Otherwise the mower seems in very good shape and appears to have been well looked after. Spark is good and compression is fine.


wristpin Tue, 21/01/2014

Will leave the detail of what carb it should have to others but the need to "dope" the engine to get it started suggests a weak mixture.
First check for air leaks in the inlet manifold joints - failed gaskets, warped flanges etc. With the engine running dribble some oil around the joint or squirt with carb cleaner and see if it's sucked in. Or wipe some heavy grease around the joints as a temporary seal and see if it makes a difference .
Second check, is the air cleaner correct and supplying enough resistance to "suck" to drag sufficient mixture through the system for starting ? Not a very scientific way of putting it but I expect that you'll get the picture. Try holding your hand over the intake while cranking it over.
Fuel level in the carb float chamber . Probably determined by a spring clip on the float engaging a notch in the needle but worth a check.
Starting technique . You need to find out what your engine likes and "catch it by surprise! " I suggest closing the choke, closing the throttle , using the "tickler" to flood the carb, cranking it over a couple of times , opening the throttle about a third and opening the choke and cranking again.
The manual for my Dennis suggests that the initial cranking should be done with the engine decompressed but I've never been able "to get my head around that" as to my mind no compression also means no suck.
Good luck!

Jon Tue, 21/01/2014

In reply to by wristpin

Thank you for the advice. With some fettling of the carburettor I managed to get the engine sustained running for the first time today and what a sound indeed. The thottle lifter was not reacting correctly when moving the thottle lever but got that working  and has done the trick. Also I  had put a new fuel hose on which was a little too long and was hanging down. I think the engine was struggling to draw up the fuel into the carb. I will check the inlet manifold gasket and make a new one if needed. The air cleaner in non original also and is only a small unit on the end of the carb rather than u-bending round and coming out at the front.

I have been using the top starting dog as I find the lower one too agressive. It threw the starter handle out of the shed, you really have to be careful when they decide to fire up.

When I originaly got the Dennis it would not turn over. On examination this turned out to be the Wico magneto holding everything fast as the main centre spindle had somehow expanded and frozen inside the housing. I managed to take it apart and knock it out with a hammer and luckily managed to find a replacement rotor to get it working again. Getting the impulse back together was no easy task as I had little idea of how it was suposed to work. When replacing back onto the mower from what I can make out there are only 2 possible positions for it to be installed 180 degrees from each other as the timing cam on the back of the magneto is fixed by a small key in the shaft.

The Dennis lives.

wristpin Wed, 22/01/2014

In reply to by Jon

Good that you are up and running. Not sure about your theory re the fuel pipe; so long as the tank is at the correct level in relation to the carb the fuel should find its own level. Air filter - sounds as though yours is similar to mine which I know to be original as the machine has been in the family all its (and my!) life, see. my avatar!