Some help required to diagnose a problem with starting a mower

[jpadie]

Hi

first post here - be gentle! I know precious little about motors, and particularly not the words used for various bits.

I was asked to take a look at a neighbour's Stiga mower (combi 55 with a Honda engine) which was not starting. Engine had compression, spark was ok and air filter was clean. Exhaust was not blocked (which is a common problem here with bees filling anything that looks like a hole). So I concluded it was the carburettor and disassembled it. The float valve was ok but the little pipe from the fuel bowl in to the carburettor body was blocked. Probably with old fuel that had varnished over the winter.

I cleaned this out in an ultrasound bath and then blasted with contact cleaner. The motor then started and worked for a few hours before the revs got lower and lower and then died.

I was not able to start it again. So disassembled the carb again and made sure that the little pipe/tube into the carburettor body was still clear. It was. but still not joy with getting it to start.

I asked a friend to keep trying to start the motor with the pull chord so I could see what was happening with the two baffles in the carburettor (I don't know what they are called but there is one on the engine side and one on the air side, throttle valve and choke valve perhaps?).

I found that the motor would start every time cleanly if i manually moved the engine side baffle so that it was shut (parallel to the engine). But try as I might with adjusting the various springs and screws, I don't seem to be able to get the machine to start without manual intervention. in removing the carburettor the only thing that I changed to start with was the clamp on the choke cable which I initially loosened. I'm fairly sure that I replaced it within 2-3mm of it's initial position. Obviously since then I've wiggled just about everything!

Can anyone point me in the right direction please? happy to provide photos if that is useful.

many thanks, in advance.

Justin

 

[Rocky J]

Closing the throttle would increase the vacuum, which in this case might make me suspicious of a vacuum leak . If taking the fuel hose off the carb does the fuel run freely?

 

[jpadie]

I changed the fuel pipe as the previous one seemed perished. it tore easily when removing from the fuel inlet.

in replacing the fuel pipe I used one with a much greater thickness but the same inner diameter. The fuel still flows just fine and the tank empties quickly. The fuel bowl seems to fill fine. so not a clogged inlet valve.


If it helps, if the choke is on or the throttle too far forward, the motor turns over and then there is a high metallic clank when the starter cable is pulled.


I am feeling the characteristic compression cycle when pulling the starter chord though. Is that not relevant to vacuum?

 

[Rocky J]

That would be compression in the cylinder you feel, the vacuum lose I am thinking would be starting from the intake valve all the way out to the chock, or your air cleaner, Maybe the carb gasket or if it has a plastic insulator in between the carb and the engine something that makes it easy to get air instead of building vacuum on the inside of the carb which would draw fuel air mixture,

 

[jpadie]

i see. the gaskets are all designed to be reusable and only one has actually been removed (which is between the carb and the air cleaner. It's a fairly simple design. but worth stripping them down and looking for anything wrong in those joints.

thanks. will post back.

 

[jpadie]

gaskets all check out fine. no tears, no obvious wear.

i tried turning the engine over a couple of dozen times then checking the spark plug. Dry as a bone. So the fuel isn't getting in to the cylinder I guess. Either suction issue or a blockage in the fuel nozzle.

If the gaskets are ok where else might I check for issues? I will strip the carb again and this time fully dismantle the fuel tube (i think it unscrews from the inside, but are there air supply channels that I should be looking out of (that create the suction perhaps)?

 

[keakar]

I found that the motor would start every time cleanly if i manually moved the engine side baffle so that it was shut (parallel to the engine). But try as I might with adjusting the various springs and screws, I don't seem to be able to get the machine to start without manual intervention. in removing the carburettor the only thing that I changed to start with was the clamp on the choke cable which I initially loosened. I'm fairly sure that I replaced it within 2-3mm of it's initial position. Obviously since then I've wiggled just about everything!

Can anyone point me in the right direction please? happy to provide photos if that is useful.

many thanks, in advance.

Justin

this sounds like your choke isnt engaging, at full throttle its supposed to push that butterfly flap closed so it creates enough vacuum and sucks gas up for starting. if the flap isnt closing then it isnt being choked and thats your whole problem right there. something is out of adjustment or the linkage is out of alignment when you put the carb back on

 

[Rocky J]

I would go back to the bottom of the tube , that will be your main jet , most do unscrew out and make sure if your screw driver is to wide of a blade you grind it narrower or file it but those threads in that hole need to be undamaged for reassembly. All the trash,rubber hose dust, water end up at the bottom of the bowl right where the main jet is.

 

[jpadie]

this sounds like your choke isnt engaging

the choke seems to engage only the external baffle. but I might well have cocked up the linkages. here are photos if they help point out my errors!

with the governor linkages remove the baffle on the engine side is pulled fully closed by the spring. I don't see the governor moving much whatever the choke position. and certainly it does not appear to close the baffles (which would need the governor to tilt counter-clockwise. (p.s. i hope i'm using the right term here: when I say governor I'm meaning the inverse L shaped bit of metal on the right of this last photo).

I would go back to the bottom of the tube , that will be your main jet , most do unscrew out and make sure if your screw driver is to wide of a blade you grind it narrower or file it but those threads in that hole need to be undamaged for reassembly.

it's next on the list; thanks. I need to rule out this before mucking around too much more.

All the trash,rubber hose dust, water end up at the bottom of the bowl right where the main jet is.

when I first took a look at the machine there was just varnish in the bottom of the bowl. no crud.

 

[Rocky J]

The throttle cable when pushed full on needs to close the choke full shut ,the butterfly by the air cleaner, push the throttle lever all the way on the handle , look at carb , loosen the cable clamp, push or pull the cable through the clamp until you end up full shut, tighten the cable clamp and try to start in that full choke position .