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Briggs and Stratton Magnetronic 3.5hp engine for lawnmower. Serial number indicates built in 1992. Unused for a number of years - was running OK before being put into storage. Generally cleaned up; sump and petrol tank cleaned out; air and fuel filters cleaned; carburettor dismantled and cleaned out - diaphragm appears in serviceable condition; jet clear; Choke-a-Matic flap free and fully closed when not running.

Engine will not start cold. If I take the air filter off and put my hand over the air inlet (ie. really choke it) and pull the start it will start after a couple of pulls then I take my hand off. Chole-a-Matic flap fully open. If it doesn't keep running it will restart with another pull. Engine runs very well. If stopped the engine can be restarted easily -- until it is cold again.

Any ideas?

Post model xxxxxx, type xxxx, trim xx from engine blower housing, cover over engine.

I'd suspect the diaphragm is the culprit. Seems like you covered all the bases with the other stuff.

mechanic mark said:

Post model xxxxxx, type xxxx, trim xx from engine blower housing, cover over engine.

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Engine serial number is 92908 5175 01

Check the top of the tank for being flat. Those were bad for the tank top not being flat causing an air leak under the carb and being hard to start. The diaphragm has fuel pump valves on it. If they are not sealing can be hard starting. If the diaphragm is old i would replace it.

Hammermechanicman said:

Check the top of the tank for being flat. Those were bad for the tank top not being flat causing an air leak under the carb and being hard to start. The diaphragm has fuel pump valves on it. If they are not sealing can be hard starting. If the diaphragm is old i would replace it.

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Many thanks. There is no distortion of the tank top as far as I can see. In the process of getting a diaphragm.
Regards.

Hammermechanicman said:

Check the top of the tank for being flat. Those were bad for the tank top not being flat causing an air leak under the carb and being hard to start. The diaphragm has fuel pump valves on it. If they are not sealing can be hard starting. If the diaphragm is old i would replace it.

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Hi Hammermechanicman, sorry for the delay in reply but been busy. New diaphragm obtained and fitted. Same result - will not start when cold. Opened the jet in increments of quarter turn attempting to start each time. Got to 1 and a half turns open and still no joy. Took air cleaner off and put hand over inlet, less than one turn and it fired up but ran roughly as you might imagine. Closed jet back to its original point and engine ran smoothly. Stopped it and restarted it - one pull only needed.
It just doesn't seem to like being cold!
Tank top flatness: Previously I put a steel rule straight-edge on the carburettor seat (tank top) and it seemd fine to me. How much deformation would be necessary to cause this problem - if it is indeed the problem?
Regards, Grumpy Boy

Have you checked for a leaking intake manifold? Some of those carbs need to have the diaphragm preloaded? Could you have installed the diaphragm and gasket in the correct order? Just some things to check. Section 3 of this manual may help. https://drive.google.com/file/d/0B6NaqjIxWV1ycG8wd0s3Z2Q2X00/view

See page 10 in Operators Manual that I posted above & adjust valves. Let us know how it goes, thanks, Mark Intake valve .005 Exhaust valve .008
If valves are out of adjustment compression release on camshaft will not assist in cranking.

Grumpy boy said:

It just doesn't seem to like being cold!

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My 2 Honda-engined walk behinds hadn't been started in a month. I was thinking this will be easy because they ALWAYS start on the first yank. Although it was 50°F and sunny, it had been in the 30's the previous night. 6 pulls later on full manual choke they started coughing, but wouldn't run unless the auto-off choke was held halfway for 20 seconds.

If your shop is unheated and your engine is cold-soaked, then that may be completely normal operation for December.