Service bulletin for improving Robin engine starting

[ernemats]

My 422 which is now 12 years old starts without any problems as low as 20 degrees, at lower temperatures I might have to try it several times but it has always started and it still has the original battery and very seldom have I put it on charge.
Now my 1845 will start at the same temperature ranges but it struggles at the low temperatures and when it does start it idles very roughly for several minutes and smokes a lot.

 

[BobRip]

My 422 which is now 12 years old starts without any problems as low as 20 degrees, at lower temperatures I might have to try it several times but it has always started and it still has the original battery and very seldom have I put it on charge.
Now my 1845 will start at the same temperature ranges but it struggles at the low temperatures and when it does start it idles very roughly for several minutes and smokes a lot.

Any idea what is different about you machine? Yours is about the same age as mine.

 

[ernemats]

I have no idea. Several years after I purchased mine sometimes it was very difficult to start in cold weather, then I changed spark plugs to the ones Jack Robin suggested and that seemed to help.

 

[BobRip]

I agree. One of the first things that I did after getting my PT was to up the battery cable sizes, and add a central ground point on the engine, with extra ground wires to the front and to the tub itself. I also sanded the ground point on the engine for the best electrical connection. (In hindsight, I probably should have run one over to the ground screw on the the electrical interconnect, but that was before I had taken it apart.)

As always, YMMV...

All the best,

Peter

In order to get an idea of the losses in the battery cables I measured the voltage drop on the cables while cranking. I then calculated the resistance assuming 90 amps of cranking current. If you have a better number let me know.
Positive cable. 0.023 volts 0.00253 ohms
Negative cable. 0.018 volts 0.002 ohms
Negative cable connection to starter motor. 0.05 volts. 0.0005 ohms.

The total resistance is 0.005 ohms. This is about 4% of the starter motor resistance. The test was done with the engine warm. During cold weather and a cold engine the starter current will go much higher and the cable losses will increase. With fatter cable the losses could be cut in half. Will it help to use fatter cables? The calculations don't rule that out. I don't plan to do this, but if a cable fails, I will replace it with fatter cables.

 

[Gravy]

In order to get an idea of the losses in the battery cables I measured the voltage drop on the cables while cranking. I then calculated the resistance assuming 90 amps of cranking current. If you have a better number let me know.
Positive cable. 0.023 volts 0.00253 ohms
Negative cable. 0.018 volts 0.002 ohms
Negative cable connection to starter motor. 0.05 volts. 0.0005 ohms.

The total resistance is 0.005 ohms. This is about 4% of the starter motor resistance. The test was done with the engine warm. During cold weather and a cold engine the starter current will go much higher and the cable losses will increase. With fatter cable the losses could be cut in half. Will it help to use fatter cables? The calculations don't rule that out. I don't plan to do this, but if a cable fails, I will replace it with fatter cables.

.02V isn't much voltage drop at all in a starter circuit. On mine, I would expect at least .2V and up to 2.0V in really cold weather when the starter is really working hard.

 

[BobRip]

.02V isn't much voltage drop at all in a starter circuit. On mine, I would expect at least .2V and up to 2.0V in really cold weather when the starter is really working hard.

This is just across the wires. The battery voltage will drop as you stated. My experience is that you do the math, but the actual affect is much greater than calculated. We are getting into strange territory. Edison stated that whenever you talk about batteries you are lying. What I think that means is that batteries are too complex in their behavior to describe that behavior and be completely accurate. I have found this statement to be accurate, but I am probably lying.

 

[ponytug]

Well, it is a 1445, and it starts fine at 40. 35 is getting a bit slow, but it always starts on the first crank. The thick oil in the hydraulic pumps is clearly the issue for the starter. Nonetheless, a glow plug is on my list. I can check the cable size; I think they are 2ga.

I try to let the machine warm up the hydraulic oil before trying to do much; I'm not wild about having any cavitation in the intake oil filter from the tank.

All the best,

Peter

How does your machine start below 50 deg and 40?

 

[ponytug]

Dear Bob,

It is always great to have real numbers! With those sorts of resistance numbers, any gain at all might be solely in the contact resistance at the cable ends. Yet another reason to take them apart every so often and clean them.

As I said, I did it preemptively, not because I had a problem, but because it had been a common problem for me on other engines. It might be one of those chromed under hood items- looks nice, but not functional...

Thanks again for measuring!

All the best,

Peter

In order to get an idea of the losses in the battery cables I measured the voltage drop on the cables while cranking. I then calculated the resistance assuming 90 amps of cranking current. If you have a better number let me know.
Positive cable. 0.023 volts 0.00253 ohms
Negative cable. 0.018 volts 0.002 ohms
Negative cable connection to starter motor. 0.05 volts. 0.0005 ohms.

The total resistance is 0.005 ohms. This is about 4% of the starter motor resistance. The test was done with the engine warm. During cold weather and a cold engine the starter current will go much higher and the cable losses will increase. With fatter cable the losses could be cut in half. Will it help to use fatter cables? The calculations don't rule that out. I don't plan to do this, but if a cable fails, I will replace it with fatter cables.