1985 Mercury 85hp (4 cyl) runs great for 20 minutes then boggs down

[Markslund]

Thank you guys for your all inputs. I just wish I knew someone who either has my same motor or has actually had it in their hands and could definitively say yes, you have a thermostat, yes you have a thermal sensor, and yes there is or is not a “limp mode” on this model. I can’t physically see it and don’t have the experience to rely on. Thanks again…even

If post #46 shows your motor there is no thermostat shown !!----It does have a temperature switch and a wise owner would test and see if there is a warning horn.-----If none is found install one !

I appreciate that, and as much as I should have a warning horn if available…I’m still trying to track down possible reasons why my motor is bogging down after running great for 20 minutes. If there actually is a Limp Mode and associated parts it would be very helpful.

 

[racerone]

Hurry----Visit your local friendly boat shop.----Ask questions there.

 

[chris.olson]

Your engine does not have a thermostat. And there is no limp mode other than having a broken reed block on a four-cylinder, in which case it will "limp" back into the dock on two cylinders. The reed blocks are on the crankshaft in a Merc. There is two of them, one between cyl's 1&2 and the other between cyl's 3&4.

 

[Markslund]

Thank you guys for your all inputs. I just wish I knew someone who either has my same motor or has actually had it in their hands and could definitively say yes, you have a thermostat, yes you have a thermal sensor, and yes there is or is not a “limp mode” on this model. I can’t physically see it and don’t have the experience to rely on. Thanks again.

If post #46 shows your motor there is no thermostat shown !!----It does have a temperature switch and a wise owner would test and see if there is a warning horn.-----If none is found install one !

I appreciate that, and as much as I should have a warning horn if available…I’m still trying to track down possible reasons why my motor is bogging down after running great for 20 minutes. If there actually is a Limp Mode and associated parts it would be very helpful

 

[Markslund]

Your engine does not have a thermostat. And there is no limp mode other than having a broken reed block on a four-cylinder, in which case it will "limp" back into the dock on two cylinders. The reed blocks are on the crankshaft in a Merc. There is two of them, one between cyl's 1&2 and the other between cyl's 3&4.

View attachment 358140

 

[Markslund]

Thanks Chris Olson: This is very interesting. I have not heard or read about this. Would you mind explaining how the reed block works? Is it a thermal break that somehow resets when cooled? Or is a reed block something that is just mechanical (meaning nothing to do with overheating)? Could it have anything to do with my motor to run fine for 20 minutes every time and then bogging down? Thanks

 

[Markslund]

Hurry----Visit your local friendly boat shop.----Ask questions there.

Racerone: If it were not for others on this forum that truly want to help without the overtones and sarcasm, I would not be wasting my time here. FYI: I have spoken with my local Mercury dealer, and had another experienced mechanic look at it. I’m just a newbie (seaman apprentice) to this form, but I thought it was meant to be a format for helpful discussion.

 

[racerone]

I state clear facts.----Encourage folks to do simple trouble shooting !-----Some folks can not accept that they have to do a bit of work.----If you know nothing about motors than a new motor is in your future.-----All you have to do is turn the key.

 

[chris.olson]

Or is a reed block something that is just mechanical (meaning nothing to do with overheating)? Could it have anything to do with my motor to run fine for 20 minutes every time and then bogging down?

The reed valves allow the air/fuel charge to be pulled into the crankcase with vacuum when the displacement of the crankcase changes because the pistons are going upwards on their compression stroke. Then, when the cylinder fires and the piston comes back down the reed valves close, so it compresses the air/fuel charge in the crankcase until the piston opens the transfer ports, and the air/fuel charge enters the cylinder and scavenges it, forcing burnt exhaust gases out the exhaust port.

Yes, it is entirely possible to have weak reed valves that don't seat properly when the engine warms up, and it will drop one or more cylinders. When this happens it's possible to tell which cylinder(s) it is by putting your hand over the carbs and see which one is spitting back and doesn't have proper vacuum. This also causes the fuel pumps in the carbs to not work right because it is the crankcase pressure pulses that operate the fuel pumps.

Those reed blocks are also the #2 and #4 main bearings - they are an aluminum casting and they have grooves in them that hold oil to seal between cyl's 1&2 and 3&4. There is reed valves and stops on both sides of each casting for their respective cylinders. I have seen those reed blocks break before and have to replace the whole thing.

To get 'em out you have to pull the crankcase cover off, lift the crank up to free the blocks, split them in two to get them off the crank journal and then you can inspect them. If there's one or more that you can see light thru they are no good and have to replace the reeds.

 

[racerone]

With cold fuel / air mix going through them it is hard for me to accept that reeds will warm up. ----Total engine tear down in order to inspect / replace reeds.-----Unless dirt got in them ( mouse nest / seagull feathers ) the reeds will not be the issue.----Or if motor was propped wrong , then over revving can crack the reeds.------Find the real issue with this motor.

 

[Markslund]

The reed valves allow the air/fuel charge to be pulled into the crankcase with vacuum when the displacement of the crankcase changes because the pistons are going upwards on their compression stroke. Then, when the cylinder fires and the piston comes back down the reed valves close, so it compresses the air/fuel charge in the crankcase until the piston opens the transfer ports, and the air/fuel charge enters the cylinder and scavenges it, forcing burnt exhaust gases out the exhaust port.

Yes, it is entirely possible to have weak reed valves that don't seat properly when the engine warms up, and it will drop one or more cylinders. When this happens it's possible to tell which cylinder(s) it is by putting your hand over the carbs and see which one is spitting back and doesn't have proper vacuum. This also causes the fuel pumps in the carbs to not work right because it is the crankcase pressure pulses that operate the fuel pumps.

Those reed blocks are also the #2 and #4 main bearings - they are an aluminum casting and they have grooves in them that hold oil to seal between cyl's 1&2 and 3&4. There is reed valves and stops on both sides of each casting for their respective cylinders. I have seen those reed blocks break before and have to replace the whole thing.

To get 'em out you have to pull the crankcase cover off, lift the crank up to free the blocks, split them in two to get them off the crank journal and then you can inspect them. If there's one or more that you can see light thru they are no good and have to replace the reeds.

Thanks for the detailed info. I appreciate it much.

 

[chris.olson]

the reeds will not be the issue.----Or if motor was propped wrong , then over revving can crack the reeds.

Doesn't take much to eliminate reeds. Simple hand over the carbs check when it acts up. If it's not spitting back thru one carb and has decent vacuum then it's likely not a reed.

It does not require a complete teardown to do reeds - pistons don't even have to come out of the bores. Just set the powerhead on the bench, pull the case cover, lift the crank a tiny bit so you can rotate the reed blocks, and pull 'em out. If you have a bad reed block, Mercury Marine was out of those at last check, but a few dealers have them in stock from aftermarket. The only reason to replace a block is if it's broken - they break in the spider between the bearing and the reed housing. If the block is not cracked or broken and the reed seat is worn I lap 'em with 600 grit, then 1,000 using WD40 as a lubricant on the lapping table.

They get hotter than you think. The reed itself absorbs the heat from that bearing. Only the very tip of it is cooled by the intake charge. A weak one warps right at the base of the reed stop due to the temp differential and it won't seat. I put carbons in 'em and don't even mess with stock metal reeds anymore. Some guys flip 'em over if they find one that's not seating - that's a mistake that will result in a broken reed petal flying around in the crankcase.

 

[chris.olson]

For the OP trying to figure out how these engines work, this is your crankcase cover. The oval holes is where the air/fuel charge is admitted to the reed block. There is no seal between the cover (lower crankcase half) and the reed block - it is a precision machined fit. You can consider the reed valves to be pretty much the same thing as the intake valves in a four-stroke. Only difference is, they are operated by pressure and vacuum pulses in the crankcase instead of a camshaft.

When you put these back together do not use RTV sealant on the case halves - use an anaerobic flange sealant like Loctite 515. It's important to be meticulous during assembly. If you have any air leaks in the crankcase it will run a cylinder lean when it gets hot and burn a piston down.

 

[racerone]

The reed valves are not the issue with this motor !!!

 

[Faztbullet]

It is not a reed problem and no such thing as reeds weaken with temp. You have a fuel issue in one of the carbs. There is a piece of rubber debris floating around that either is plugging the high speed jet or inlet needle. Choking does not help as it effects the working carb also. If in my shop carbs would come off, cleaned and rebuilt with new gaskets, needle and float, all fuel lines replaced from fuel attachment to fuel pump to carbs and then giving back so you can enjoy fun on the water.

 

[Texasmark]

It is not a reed problem and no such thing as reeds weaken with temp. You have a fuel issue in one of the carbs. There is a piece of rubber debris floating around that either is plugging the high speed jet or inlet needle. Choking does not help as it effects the working carb also. If in my shop carbs would come off, cleaned and rebuilt with new gaskets, needle and float, all fuel lines replaced from fuel attachment to fuel pump to carbs and then giving back so you can enjoy fun on the water.

I had this problem with a 2002 90 HP 2 stroke at 10 years of age running E10 whether that matters or not I now have a 115 of the same year run on gasoline and the Q tip wipe produces zero specks.

The problem was in fact a black speck of deterioriated fuel line located in the cavity where the high speed jet is located on one of the carbs. I had done a thorough (I thought) job of cleaning the carbs when I did the final shot with 100ish PSI compressed air. Lo and behold this little speck comes out from hiding, just an accident that I was able to catch it.

With that I checked the lines with a Q tip and found particles. I removed and cut open the fuel filter and it was full of black specks and tan chips from the grey entrance fuel line. Changed out all that and no more problems.

 

[chris.olson]

It is not a reed problem and no such thing as reeds weaken with temp.

You are correct, reed valves don't weaken with temp, they weaken with age. Thermal cycling and fatigue causes a reed petal to warp. Diagnosing whether this is the case is very simple.

I sent the OP a message and he can call me if he wants - I'll be happy to walk him thru diagnosing it without spending a single dime on stuff he doesn't need.

 

[racerone]

Liquids need heat when evaporating.----Reeds valves are constantly washed by incoming fresh fuel.----You might say they stay cold during engine operation.----Hot exhaust does not come anywhere near the reed valves.

 

[chris.olson]

Yes, Kiekhaefer chose to put the reeds on the crank instead of an external reed plate like other designs use. And it's a very good design, especially when you want to build D Mod hydroplane engines that turn 10,000 rpm. But they have a few quirks.

Again, if the OP can't get it running properly he is more than welcome to call me up. I'll walk him thru how to properly diagnose it to identify the problem, and how to fix it. I can cover way more in 15 minutes on the phone than I can typing for hours on here.

 

[Faztbullet]

It takes years of use to warp a reed. Thats why it has a spec if its open a tad. If its a reed it does it all the time or it dont..they will not come and go on this model engine. Your lil motor is a tad different than posters as I have seen the reed stop alignment pin break and allow reed stop to shift around.