Voltage drops when cranking...

[CraigH]

Way back in reply #1 it was suggested that the compression release was probably the problem
Then in reply # 7 the cam was again suggested to be the villian .
Then the thread wandered off because of numbers on a DVM
I never use one apart to test the output from the alternator & diodes as numbers just confuse things as has happened here because people do not understand the significance of the numbers.
In your case the 2 minute test which you had already done is to get a pair of jumpers and a known good battery , ie your vehicle .
You then bypass your mowers wiring by jumping directly from the battery to the starter motor
If the starter fails to turn over the engine then you do the same from the known good battery
As that failed to turn over the engine then either the starter is bad or the cam is bad , which is why Illengine suggested the cam back in reply # 1

The 2 minute test for a bad cam / mal adjusted valve lash is to try cranking without the spark plug as you had done .
So cranks no plugs won't crank with plugs = reset valve lash carefully & try again.
Same results = stuffed cam

You verify the cam by removing the rocker cover , adjusting the valve lash then rotate the engine watching the rockers .
In your case the inlet should reopen a tiny amount just before the piston gets to the end of the cylinder .

To check the compression relief against wear, you close down the inlet valve lash.
You can go as tight as 0.002" without seriously affecting the performance of the engine, but that is of course a stop gap to get you by till the new cam arrives or till the end of the season

AS for dropping battery voltage under heavy discharge, this is quite normal and happens when the current draw exceeds the discharge rate of the battery
The discharge rate is a function of the plate geometry & metallurgy , the paste chemistry & grain size, the bonding agent used for the paste, the electrolyte & the busbars that connect all the plates together and the temperature of the battery & wires .
The main purpose of carbon pile load testers is to sell confused customers new batteries and I am yet to see an auto parts store use one correctly .
Most mowers use # 4 ( Top end ) # 6 ( bulk of brands ) or # 8 ( cheapies ) gauge wire, usually 25 to 42 strand .
The current capacity of that wire is roughly 45A, 35A or 25A respectively so shoving a 200 +A load from a carbon pile across the terminals is pointless because the wires can not carry that load and the starter will never draw those sort of amps unless there is a dead short .

So I can close the inlet valve clearance to 0.002" and this should have the same affect as what the cam decompression system does? If so, I'll try that later today.

Ill test the starter on the bench later aswell and report back. But as you've said, I've linled it all up to my van with a 2.2 diesel engine (running) and this had no effect on starting the mower which is why I've kept coming back to the cam decompression, or bad starter. I'm leaning towards cam decompression and if that's not the case, I'll get a new starter (starter is the most expensive item at £80-180 depending on brand).

 

[ILENGINE]

So I can close the inlet valve clearance to 0.002" and this should have the same affect as what the cam decompression system does? If so, I'll try that later today.

Ill test the starter on the bench later aswell and report back. But as you've said, I've linled it all up to my van with a 2.2 diesel engine (running) and this had no effect on starting the mower which is why I've kept coming back to the cam decompression, or bad starter. I'm leaning towards cam decompression and if that's not the case, I'll get a new starter (starter is the most expensive item at £80-180 depending on brand).

Just keep in mind that the issue could be a combination of both items. If the decompression has been broke for some time, or in in some cases the valves out of adjustment and the starter is forced to overcome that resistance over time will damage the starter.

 

[CraigH]

Just keep in mind that the issue could be a combination of both items. If the decompression has been broke for some time, or in in some cases the valves out of adjustment and the starter is forced to overcome that resistance over time will damage the starter.

Thanks for that, that's a good point, the valves were miles out of adjustment when I took the rocker cover off originally, so could be an overworked starter too.

This mower is turning into abit of a nightmare, but hopefully it'll be worth the hassle (and expense) when it's all running sweet soon.

 

[slomo]

the valves were miles out of adjustment when I took the rocker cover off originally

Ah ha.

Set the valves yearly for a good running engine. Only takes a few minutes. Good winter time project when you are not mowing.

 

[CraigH]

Ah ha.

Set the valves yearly for a good running engine. Only takes a few minutes. Good winter time project when you are not mowing.

Was going to fix this one up as a winter project but been off work for the last 10 days with COVID so figured, why not get on with it now as I've had plenty of time on my hands (well, I say plenty of time, I've been looking after little one aswell as trying to get a few mins spare to have a look at the mower and test bits).
I'll have a look at the valves later and see if I can get it to turn over properly if the 0.002" clearance trick is a good test to try?
Is there any other ways to test if the cam is bad?
Also, a bench test on the starter, what sort of voltage figure should I be aiming for? 12v?

 

[slomo]

Is there any other ways to test if the cam is bad?

Remove valve cover. Watch the valves as you rotate the engine. Both valves should move roughly the same at different times of course. Oh pull the plug out to turn the crank easier.

Also, a bench test on the starter, what sort of voltage figure should I be aiming for? 12v?

A starter in free air, unmounted, should measure full or darn near full battery voltage. As in "basically" no voltage drop. Battery should be 12vdc or higher while cranking in free air on the bench. Good battery and cables........

More explanation, I messed the above up. You should have full static battery voltage testing in free air. If you are voltage drop testing, should read close to 0vdc drop. Two ways of testing here. Voltage drop method ON the mower is the best method while under load, cranking the starter.

Don't forget to inspect your ground cable end to end. Polish the connectors up and mounting points.

 

[CraigH]

Thanks for the help/pointers above.

I set inlet valve to 0.002" with piston 1/4" down from tdc. The engine turned over marginally better than before but still wouldn't make a full revolution with the plug in and a fully charged battery.

Next was to bench test the starter.
I fully charged the battery again and was getting 12.8v on the multimeter with nothing connected to the battery at all.
I used some heavy jump leads and tested the starter, connecting the earth lead to the mounting plate at the top of the starter and the positive lead to the terminal on the side where the positive lead bolts to, the voltage when the starter was spinning (I tried it 3 times for 5 second bursts) was 12.2v across the terminals on the battery.

So, what would people recommend my next step should be?
Time for a new starter? If so, what would you recommend? Aftermarket or genuine?
Do you think the cam decompression system is broken?

 

[CraigH]

Forgot to mention, cam moves both valves around an equal amount (judging by eye) so the cam appears to be o.k, not sure how to test the decompression system on it further though.

 

[ILENGINE]

Forgot to mention, cam moves both valves around an equal amount (judging by eye) so the cam appears to be o.k, not sure how to test the decompression system on it further though.

With the plug out, turn the engine over by hand while observing the bottom valve which on the single OHV engine is the intake valve. After the intake valve closes and you turn the engine and the piston comes toward TDC you should see the intake valve bump off of the seat slightly and then close as the piston hits TDC. That bump is the decompressor. No bump which I suspect in this case will me time to open up the engine.

 

[sgkent]

THE ENGINE MUST BE ABLE TO SPIN FREELY 360 DEGREES MULTIPLE REVOLUTIONS WITH THE PLUGS OUT BOTH BY HAND AND STARTER BEFORE GOING FURTHER. There is no compression when the plugs are out.

to have 12.2V while spinning the starter, and 12.8V at rest, would be within a normal voltage drop I think. Why the engine can turn freely by hand when the plugs are out but load the battery so much when the starter is used with the plugs out is beyond me.

Until the engine spins freely with the plugs out, doing other adjustments, etc., taking an engine apart is just wasted time in my eyes.

ONE TEST might be to spin the engine by hand several revolutions with the plugs out. Disconnect the starter wire and tape it out of the way so not to short or cause another issue. Turn the key to start and see if the engine still spins freely by hand. Would a shorted alternator winding bind the engine enough and pull the voltage down? What if the PTO was wired to full on when the engine was starting and the belt was binding. Need to be sure the engine will spin freely by hand in the start position. The voltage should not fall much in the start position when the starter or starter wire is removed.