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I think I'm missing something obvious, so could someone point me in the right direction, please...

I have an 11hp Briggs Powerbuilt OHV in a small ride-on mower. It's become tricky to start, whether hot or cold. The engine cranks slowly - generally too slow to fire up.

The battery is fine, and the engine sounds fine when/if I eventually get it to run. Sometimes that will involve jump-starting and trying several times.

If I remove the spark plug, it cranks over very quickly. Voltage at the battery drops to 9.5V.

But if I replace the spark plug, cranking is very slow, and voltage at the battery drops to 4-5V.

I've checked the valve clearances, and they seem fine.

I've also connected a battery directly to the starter motor - it spins quickly, with no voltage drop.

So, are we looking at a bad earth or ignition switch?

Thanks.

So, engine starts/runs quickly with battery cable directly to the starter lead? If so, gotta look upstream in the +12v power circuit. Or was this test on a bench?
Good contacts everywhere?
Solenoid partially working? The internal bar that connects the 2 big poles could be making only minimal contact and not moving enough amperage.
Ignition switch not sending consistent +12v power to small solenoid terminal? (I kinda doubt this one)
Starter Motor weak?
Decomp lobe on camshaft worn and not ticking open the valve? Try turning flywheel by hand to past TDC.

dougand3 said:

So, engine starts/runs quickly with battery cable directly to the starter lead? If so, gotta look upstream in the +12v power circuit. Or was this test on a bench?
Good contacts everywhere?
Solenoid partially working? The internal bar that connects the 2 big poles could be making only minimal contact and not moving enough amperage.
Ignition switch not sending consistent +12v power to small solenoid terminal? (I kinda doubt this one)
Starter Motor weak?
Decomp lobe on camshaft worn and not ticking open the valve? Try turning flywheel by hand to past TDC.

Click to expand...

Yes, the engine turns very quickly with a battery directly to the starter. This is while still in the mower. In fact, since posting I've started it up and used the mower for half an hour - all good.

After running the engine, I switched it off and started it up again - without a hitch. But leave it for a minute, and again it's cranking slowly. Maybe this is a symptom of a weak starter?

I haven't checked the solenoid - I was kind of thinking it would either work or it wouldn't.

The flywheel feels pretty normal by hand, although I'm not sure what I'm checking for...?

"A" battery?

Is it "the" battery on the mower, or one from elsewhere?

What is the voltage of the battery on the mower while just sitting there?

If it is from the battery that is on the mower, undo and clean all of the terminals and ground etc...

Fish said:

"A" battery?

Is it "the" battery on the mower, or one from elsewhere?

What is the voltage of the battery on the mower while just sitting there?

Click to expand...

I've been using a jump starter pack alongside the mower's own battery. It was the jump starter I used to connect directly to the starter motor.

I've also used the jump pack to each of the battery terminals, and the engine turns just as slowly - I'd kind of ruled out the battery connections because of that.

The mower's own battery is reading about 12.8v just sitting there. I'll recheck.

Try jumping it with the ground cable on the engine base instead of the battery.

Easy way to test the mowers heavy load circuits is to bypass the mower all together.
So try running a jumper from the battery - to one of the starter motor mounting bolts.
See how the engine cranks then.
Similarly test the ground by hooking a jumper from the battery + ,directly to the + on the starter motor.
A stronger cranking with either jumper connected reveals a bad connection somewhere.

In the same vein, hook the starter directly to the mowers battery.
A slow crank reveals a bad battery.
Batteries can show 14V but still not have any Amps available to crank the engine.

Thanks for your help, guys - you were right. :smile::thumbsup:

With the battery disconnected, the mower starts fine from my jump-starter pack. But it turns slowly using jump leads directly from the battery to the starter/engine.

Oh, and with each side of the battery jumped to the engine, it was still slow.

Like I said at first, I knew I was missing something obvious. Still a bit puzzled why it wouldn't crank from my jump pack with the battery connected, though...

Find where the battery ground cable connects to the frame, remove cable and clean both frame and ground cable contact areas thoroughly.

Walt Conner

I would disconnect the battery and check it with a stand alone load tester (local auto parts store can check too if you don't have a tester.) If bad, replace and retest. You'll never be able to resolve with certainly if your battery drops voltage under load.

I just had a similar experience with my 16 hp Briggs garden tractor and what I found to be the problem defies logic. The battery read 7 volts cranking. I boosted the machine with a larger battery (car). Still very slow crank. I then boosted directly to my starter and it cranked well. I then thought bad connections, solenoid, etc. Remembering the 7 volts cranking, I put in a new battery and problem solved. The dilemma is why didn't it work when I first boosted it? I kept my old battery as I want to test it for a short.
Again I have to logically explain this.
I guess my recommendation is to use a known good battery with the old one disconnected.

You may have a bad battery and a poor connector somewhere. The 7 volt drop and increased resistance within the battery is not sufficient to push enough amps into the starter circuit.

When u jump the starter with the old battery hooked up you’re benefitting by the bad wiring connection upstream to the old battery. The new battery then drives most of the current through the starter and a little current back into the old battery. But because the jumped battery is ‘good’, it can push amps into both loads with a negligible voltage drop.

Some takeaways:

1). Only troubleshoot electrical problems with a good battery in the circuit.
2). If in doubt, test a questionable battery using a battery load tester. A voltmeter alone is insufficient.
3). Often times a good battery will not turn over a starter with a really bad battery left in the circuit. You can often jump ‘through’ a weak battery but a low resistance ‘bad’ battery will scavenge too much current.

I replace my batteries in my cars every five years if they need it or not.

You may have a bad battery and a poor connector somewhere. The voltage drop and decreased resistance within the old battery is not sufficient to push enough amps into the starter circuit.

When u jump the starter with the old battery hooked up you’re benefitting by the bad wiring connection upstream to the old battery. The new battery then drives most of the current through the starter and a little current back into the old battery. But because the jumped battery is ‘good’, it can push amps into both loads with a negligible voltage drop.

Some takeaways:

1). Only troubleshoot electrical problems with a good battery in the circuit.
2). If in doubt, test a questionable battery using a battery load tester. A voltmeter alone is insufficient.
3). Often times a good battery will not turn over a starter with a really bad battery left in the circuit. You can often jump ‘through’ a weak battery but a low resistance ‘bad’ battery will scavenge too much current.

I replace my batteries in my cars every five years if they need it or not.

Well, I've finally found the reason for the slow starting - the compression release mechanism had failed (or was in the process of failing).