Hello, I have a Briggs & Stratton 128m02 on a CM push mower. Purchased last spring. Engine may have 20 hours on it.

Complaint: No start using rope crank when engine is hot.

Engine starts when cold, and can be restarted when hot with starting fluid. Pulled air cleaner and noted the choke was in the open position. When restarted using starting fluid engine runs fine. Does not stall, no loss of power noticed (1st cut of year, grass isn't too long).

When engine is allowed to cool, engine can be restarted with rope crank. Starts easily unaided with first crank when engine is cold or cool. Engine will start if I stop and restart the engine before it warms up.

This engine did sit over winter with gas, I did add an appropriate amount of Stabil last fall. Emptied tank while mowing and added fresh gas to complete lawn, issue remains.

Using the handy, dandy search function led me to check spark. Apparently B&S has a quality issue with their ignition coils? Pulled spark plug when engine was hot, grounded against casting and saw a spark. Allowed engine to cool and repeated, did not notice an apparent change in spark intensity however I am not sure if there was if I would be able to tell. Spark plug does not appear fouled.

Did not notice a difference in effort required to crank engine when hot vs. cold, did not use a compression tester. Have not checked valve lash.

Thoughts? Suggestions?

 

When you try to start it hot do you use a normal starting procedure? If so you may have the choke kicked in and it floods. Try opening the throttle to the "fast setting" and see where it takes you.

 

Just inspected engine, there does not appear to be any knobs/levers to adjust the choke position. This has always been just a pull 'n go mower.

 

There's a lot of different models and some of those have automatic chokes, although I couldn't identify them. But as a rule a choke position would be in a "start" position of the throttle and the faster speed setting of the throttle would be a run only with no choke and open throttle plate. With the latter, if flooding was an issue, the engine would have the chance to flush out the extra fuel and start.

In the models where you could control the choke you would usually have a "start" and "run" position on the throttle.

Sometimes the flooding occurs because of hot fuel in the carburetor which sort of boils up into the throat and helps to make starting difficult.

 

When you give the brand as CM is that for Craftsman?

 

Yes, CM for craftsman. I believe this mower has the "Smooth Start Plus" automatic choke feature.

 

Yep. They call it a no choke, but the parts diagram shows a choke plate and shaft. The actuation of it is apparently automatic. I don't know if a throttle position would kick that thing off or not. On the older carburetor fed cars you could with full throttle.

Having the choke fully off and the throttle fully open would be the key to starting with a flooded engine.

Let's see if some of the tech guys here know some tricks to working with that one.

 

Right, I know it has a choke just not one with a convenient lever. More stuff to go wrong, right?

Out of curiosity I went and checked the choke position. It was closed, and of course started right up. Let it run for about run for a couple minutes until it would no longer restart, checked the choke position and it was wide open. I understand for a engine at temp no choke is necessary, but for starting a warm engine shouldn't the choke be closed during crank?

 

An engine at temp only lasts maybe a minute after shutoff so the choke is likely to come into play fairly quickly. I'm not sure what switches it on. I doubt if it's thermostatically controlled. If that's the case it probably should be on every start.

The problem with auto chokes is they have to meet the demands of something with a lot of variables in the process of starting. You have the temp of the engine, the air, the moisture content of the air = air density and how hard someone is able to pull the rope.

Starting an engine is a little harder than keeping it running because you have no impetus to work with - you have to create it.

 

Anyone else have any thoughts?

 

This style carb/engine combo is notorious for your problem. The choke is connected to a linkage that holds it open when the muffler gets hot. So, the choke does not close again until things cool way down. Sometimes a warm engine needs a little choke anyway. The simplest solution that works is to tilt the mower up on it's side a little with the carb up in the air for a second or two before starting. This dumps a little fuel from the bowl into the throat so you can start it. It's a design problem, so without retro-fitting to the old style with a primer bulb, you have to work around it.

 

Just my two cents as you have gotten great advice already. On any Briggs ready-start that comes into the shop for service or repair, I set the spark plug gap to .020 in. It seems to work. Of course, this assumes that the choke linkage is moving properly.