SP engine screams and dies

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Fredo
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SP engine screams and dies

I have just been building a nice SP 35s out of spares. The engine is a 1989 alloy with the delorto carb. I have rebuild the carb using a service kit. When started, the engine runs madly at high revs and then stalls. What are the likely causes? Have I missed something in the carb, or is it to do with the governor mech? I thought I would seek some advice from you guys before investigating further.

 

Thanks in anticipation.

 

John

wristpin
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Joined: 23/05/2012 - 22:09
From the clues that you've

From the clues that you've given  it will have a mechanical governor so there is the possibility of an internal malady. Possibly disconnect the throttle cable and operate the throttle by hand and see if you can detect the governor trying to counteract the open throttle.

A couple of images of the throttle and governor linkages will be useful.

 

Fredo
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Hi

Hi

Thanks for your thoughts. A bit of progress this morning: I decided to ignore the Qualcast book and stay on full choke for a few seconds longer. It ran fast, but it ran. As soon as I move to half choke, it screams and dies. Perhaps a carb problem after all? Don't be fooled by the old cowling around the starter (for zenith carb). I simply put this on to keep the old look. The engine originally had a green cowling.

wristpin
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Sounds as though it is not

Sounds as though it is not getting enough fuel through the main jet. 

Unscrew the nut on the bottom of the float bowl , The nut includes a hollow fuel well so clean any crud out of that. You can then try squirting carb cleaner up through the main jet / emulsion tube, and see if that helps. Failing that it will be necessary to remove the jet for cleaning. To do this safely and avoid doing damage you need a good fitting parallel sided screw driver.  A, not to maul the slot in the jet and B not to damage the internal threads of the carb stack that the jet and bowl nut screw into. Then thoroughly clean the jet including the tiny cross drillings.

gtc
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Sounds as though it is not

Sounds as though it is not getting enough fuel through the main jet.

That's my suspicion, too. The symptom sounds to me like what happens when you shut off the petrol on a two-stroke and let the carb run out of fuel. The engine eventually accelerates madly in a last struggle to keep alive, then suddenly dies.

 

Fredo
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Well thank you both.

Well thank you both. Certainly an interesting afternoon: I took the carb off and removed the main jet assembly. Then I gave it death with carb cleaner, but it all seemed clear. However, or re-assembly I realised that I had the return spring for the throttle wrongly fitted and the adjusting mech for the governor miles out. As a result, the throttle and governor were not working together. However, I wasn't able to test my theory, as the mover refused to start. An extensive and careful investigation led to the conclusion that the electronic ignition had failed (no spark). I tested with a tool that illuminates when a spark goes through: nothing. I tested the tool on a working mower: glowed nicely. I changes the plug: nothing. I looked intently at the plug as the engine was turned: nothing. I even held the plug in the hope of a shock: nothing. New 3 pole coil ordered from Gateshead. We will see what happens when it arrives. At least getting the flywheel away was easy, despite the reverse thread.

wristpin
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Looking at the back of the

Looking at the back of the dead coil is the hard plastic case showing signs of separating from the potted innards? Usually the sign of failure or impending failure.

Fredo
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Well, new coil fitted and

Well, new coil fitted and mower running nicely!! Hooray!! Many thanks for all of your input. The reason for the motor deciding to scream and die, when taken off full choke, seemed to be caused by two things. One was that I was taking her off full choke too quickly (as advised in the instruction book). Seems ok if you give her longer on full choke. Secondly, the flow from the fuel tank seemed very slow. All better now that I have changed the tap.

I have replaced the bottom blade and sharpened up the cylinder with the kit you can get off amazon etc.

Thank you both again. John