Cranking speed

[first230sl]

I thought I recalled a "rule of thumb" battery test (measured by observing cranking RPM when cranking for some number of seconds) posted by Karl some time ago - but after a lot of searching, I could not find it. So my sincere apologies if this is still available and I am asking it unnecessarily.

Two questions:
1) what kind of cranking speed can I expect / do I want on an 8V71TI? I live in Vancouver where is can easily get down to 50 degrees overnight in the summer.

2) What kind of dropoff in cranking speed should I expect after some seconds (10 seconds / 15 seconds - whatever) of cranking?

I am just trying to determine whether I am getting reasonable cranking speeds and whether my batteries still have the ability to dig deep when need be.

BTW - I have 4 8V golf cart batterins in series for the port engine, and 8 of them in series/parallel for the stbd engine and some house loads.

Thanks!

Murray

 

[REBrueckner]

I can't answer your specific question about cranking RPM's, but to make starting instantaneous and a lot easier on your engine and starters: Install block heaters or oil pan heaters and pre heat the engine before starting, or keep the engine room heated.....As soon as I realized how fast my 8V71TI's start when pre heated I ALWAYS warm them first when shorepower and time permit....

 

[first230sl]

pre heat the engine before starting, .

I do the same (I use space heaters in the engine room to keep the temperature around 70 or 75) - and the engines fire up instantly and run smoothly without smoke when I do so.

I do find, however, that when I don't use the heaters and the temperature is in the 50s, starting the engines requires at least 3 or 4 tries, and then the engines hunt for 30 seconds or so before settling in.

That is part of the reason I was asking about cranking speed. I was wondering if my somewhat difficult cold weather starting might be due to under-speed cranking. But since I don't know what is an acceptable cranking speed.... I though I would ask.

Thanks!

Murray

 

[REBrueckner]

My 1972 48 YF is the same vintage as yours..and my 8V71TI's behave exactly as you described...cranky below about 70 degrees. When getting a tune up, there is a buffer screw that can be adjusted which may better control cold start hunting. It's also possible a valve adjustment MIGHT assist cold starting if the valves haven't been checked in a thousand hours or so...

Depressing the fuel shut off button while cranking for the initial three cranks of a few seconds each for cold starting helps control initial smoking by limiting extra fuel injection...they'll still smoke but not as much...

I had my engines "tuned" ( valve adjustment, racks,etc), several years ago...one engine seemed a bit better, the other a bit worse after the work...Had the mechanic come back and recheck the "other"....it was properly set up...so I left it alone....Same guy has done my engines for 20 years, so I trust him....

 

[Genesis]

Rack balance is CRITICAL and this is as much an art as a science.

The buffer screw is a crutch. You should need zero or very close to zero of it in order to control hunting. The maximum RPM rise allowed is very small anyway, but if the rack is balanced correctly then you should be able to run with zero or very close to zero buffer screw and no hunting problems. Its real purpose is to dampen undershoot by the governor when returning to idle; ideally it should have zero impact on idle RPM, but a small increase is allowed. Note that if you change idle RPM you must back off the buffer screw before setting the idle and then re-set the buffer screw once the base idle is where you want it.

The other thing to check is the injector holddown bolt torque. More is not better, as it will bind the racks. Everything under the valve cover has a torque spec and it is important that they are obeyed.

Most of the time if you don't have a binding injector rack the problem with excessive hunting is found in the rack balance. This can be checked by a competent mechanic (or you) without disturbing anything except the throttle delay, and is the first place to start if you're running into this sort of problem.

 

[REBrueckner]

Ditto to Genesis' first paragraph on the idle speed/buffer adjustment sequence..
and I'll take his word on the rest...

 

[krush]

I wouldn't use cranking speed as a very informative test. It's one of those things that it's either too slow, or not good enough...and there are way to many variables involved.