Mercury Black Max 175 1988 RPM Issues

[shankforce]

I have done all of the basics (Compresion, spark test and others rebuilt carbs). I cant seem to run this motor at low rpms 900 to 1000 is the lowest.
I have changed numerous parts the whole ignition system is replaced. I have had mechanics look at it. Still rpm problems then when it is low like 750 it stalls. I took the idle advance off and ajusted with the same results.

 

[CharlieB]

Re: Mercury Black Max 175 1988 RPM Issues

Crank seals not holding enough pressure, won't idle at low speed but not noticeable otherwise. Well, maybe an inch or two of prop pitch at WOT difference than when new.

What's your WOT RPM?

 

[shankforce]

Re: Mercury Black Max 175 1988 RPM Issues

What are the crankseals?
Well WOT is off right now becouse I was messing with it to get propper RPM.
It will only get about 4200 and running slow where I have it now so I will have to try to get it back where I had it. Confusing part is I have it timed where the book says 20 BTD full crank and 10 ATD at idle. So what should it read on the gun full throttle on the water?

 

[CharlieB]

Re: Mercury Black Max 175 1988 RPM Issues

Two-strokes are special creatures, each cyl is, in effect, a separate engine, all sharing a common case. The crankshaft has a seal, quite similar in appearance to a piston ring, around the crank in between each cyl to control 'blow-by', from one crankcase chamber to the next. These seals are very important for very low speed operation as the length of time the pressure/vacuum pulse is much greater than at higher RPM. An aging motor will not like to idle near as low as a fresh engine. Yet both may exhibit near equal WOT performance.

WOT timing can be set, spark plugs out, wires grounded, remote starter button, WOT in Forward gear. WATCH OUT for the spinning prop!!!!!

Some service manuals specify setting 2 degrees less than a WOT running setting.

How long has it been since those carbs have been thoroughly cleaned, floats set to a 'T'?

Fuel pump rebuilt?

Primer bulb replaced with an OEM part? It DOES make a difference on big HP engines.

Clean the fuel pick-up tube in the tank.

Since you've taken care of everything else, might as well replace the water pump impeller and change gear oil, that motor should be good to go the rest of the season!

 

[shankforce]

Re: Mercury Black Max 175 1988 RPM Issues

Impeller and gear oil good.
I have a 1998 campion 622 I do not know were the fuel pickup is on this.
Fuel filter new.
No Water seperator.
I have a old bulb on cause I bought a new one and it did not work as well. Not OEM on the new one.
Carbs have just been cleaned and rebuilt new rings and gaskets.
fuel pump not rebuilt yet.
Question if the moter is not going fast enough or not enough RPM at WOT is it the primary pickup to retarded or advanced or the High spark to retarded or advanced?

 

[j_martin]

Re: Mercury Black Max 175 1988 RPM Issues

Get a new mercury bulb (not aftermarket).

One sleeper that gets the idling is the bleed hoses. Those are the small hoses that octopus all over the engine.

3 items about them.
a. Condition: They carry fuel, and run through hiden places.
b. Routing: Routing is dependent on the engine. The biggest factor that changes routing is whether the reed cages are vertical or horizontal. Theory on this. A bleed hose routes from the reed block of a cylinder to a port in the cylinder opposite in timing. With vert reeds, the carb/reeds are directly in front of the cylinder. With hor reeds, the carb/reeds are across the block from the cylinder.
c. Check valves. They need to be operationan and they fail.

Another sleeper is small vacuum leaks in the intake or crankcase area. You can find them by spraying a fine mist of water (some use carb spray) at the possible bad gaskets and listening for an engine note change.

The v6's aren't real famous for a smooth slow idle, so don't get yer hopes too high.

Also, it looks like you've got it a little fuddled up. You need to do a formal link-n-sync to get to a known good starting place.

hope it helps
John

 

[shankforce]

Re: Mercury Black Max 175 1988 RPM Issues

Well a little update.
I live in Florida and I just picked up my boat from Siesta Key Marina in Sarasota.
The machanics there said they worked on the motor for 3 hours and could not find any thing wrong with it.
The service request was work on the Idle clean the carbs and replace the Impellar.
They cleaned the carbs and said there is nothing wrong with the impellar or water pump. They did offer to clean the bottom of my boat (20ft) for $500. So I know it needs it but that is a little much to scrape powerwash and sand.
When I headed back to my dock I noticed all of the nuts on the timing screws were left very loose and the throttle cable had a ton of preload locking my shifter up. On the way back I retimed the idle to 10 deg ATDC and set the High spark to 20 BTDC at full throttle. When I idled my boat to dock it the tell tale stoped peeing and was pushing out hot steam then the temp alarm went off.
Getting real tired of the sevice in my area also tired of working on my motor.
Seista Key Marina very poor workmanship.