number 4 no fuel???

[Jamescenterconsole]

Ok so I think I have my RPM issue fixed. I replaced all four Ing. coils and took it out and it ran fine 5200 on the boat tac and 5000 on the service tac. Speed was better also. All this however has raised another question.
When I got back I pulled the plugs and found that number four still does not look like it has burned as much as the the other three. 1-3 all have a dark brown to black spot and 4 is very light. Is it possible that 4 is not getting the fuel it should? If im looking at things correctly 3 and 4 both run off the same carb. I know it has spark becaused I checked it it after I replaced the coils. 4 is on the right.

 

[Chris1956]

It could be water infiltration, washing the spark plug clean. Of course, with water infiltration, you would not normally see 5000 RPM. What prop, motor and kind of boat do you have?

 

[Chris1956]

If the boat is that 18 foot CC, you would either need to have a small pitch prop or a bad tach to get to 5000RPM.

 

[MH Hawker]

looks more like no fire

 

[Jamescenterconsole]

Chris1956 your correct it is the same boat you helped me with on my last post.
1981 18' Cobia center console
1982 Mercury 80 Hp tach on boat and service tach were with in 1-200 rpms of each other, 4900 on the service and 5100 on the boat
#4 has spark I saw it, and felt it
prop was seized took a puller and kroil a week to get it off number is unreadable I will pull the prop again, measure it and calculate pitch
sorry for the lack of info

 

[racerone]

Possible reed valve problem or lower crankshaft seal problem.

 

[Chris1956]

Check the fuel pump(s), for some leakage past the diaghram(s). Maybe a small amount of extra fuel at idle speed helps wash the plug clean, but doesn't affect the firing at high speed? Does she hesitate a little when accelerating off idle? It is just a guess.

 

[Jamescenterconsole]

Fuel pump was also rebuilt and does not appear to be leaking. As far as running it seems to run perfect. No hesitation no miss at any speed. I also thought of reed valves, maybe stuck shut??? I have pulled a 2 stroke 4 wheeler down and know what they look like but not sure about outboards. What would the lower crank shaft have to do with it?

 

[racerone]

????????-Well if the lower seals are leaking you would suck in exhaust on the piston up stroke and lose crankcase compression on the piston downstroke.---This would result in poor scavenging / inefficient operation of the bottom cylinder.There is more than one petal on the reed valve bearing block and they simply do not get stuck.----Could be a broken petal or debris stuck in one petal.

 

[Chris1956]

Leaking lower crank seals can allow water into the cylinder, causing the cylinder to misfire. However since your RPMs are good, I do not think that is happening. Your motor will have a doughnut-shaped reed valve body under the carb, that wraps around the crankshaft. It will have reed valves on each side, the top side feeds cylinder 3 and the bottom feeds cylinder 4.

 

[Jamescenterconsole]

One thing I did notice the last time I ran it on the hose, after it ran at idle for awhile there was oily gunk coming out of the exhaust top and bottom. I didn't think much of it because I've seen other motors do this when they idle for long periods. Is this correct? This was before I took it out for the test run in which case it ran as described above=good

 

[Faztbullet]

Lower Crank seals.......

 

[Jamescenterconsole]

if it is crank seals is it worth fixing??????

 

[Chris1956]

Crank seals are fairly easy to fix, but you need to pull the powerhead. So remove upper cowl support, starter, wiring, lower cowling (4 screws), and about 10-12 powerhead mounting nuts. Two of the nuts are likely in the front, flanking the steering arm. I always remove the gearcase as well to make getting the powerhead off easier.

Now loosen the crankcase bolts near the lower end cap, remove the lower endcap bolts and then pull the lower end cap. Replace the two seals with 3 seals and an oring. Install the end cap and re-torque the crankcase bolts. Now you need a powerhead base gasket and reinstall everything.

Piece of cake...

 

[enginepower]

Have you checked the compression on this? Low compression can cause some black plugs and oily output. Do you premix? My opinion on the plugs is #4 is running lean. I can see the spark traces on insulator so looks like you are getting fire. #2 looks slightly rich but not too bad. #3 looks pretty good but #1 looks real rich or compression isn't very good. I would check compression and make sure damage isn't already done.

 

[Chris1956]

A compression check is a good idea. However, I expect the compression to be good, as you are reaching high RPMs, so the cylinder is likely firing at high speed.

I do not think cylinder #4 is running lean, as it is fed from the same carb as cylinder #3, which is normal/rich. I still think it is a slight leak in the inner exhaust cover, exhaust baffle or lower crank seal. The slight leak of water could wash the plug clean. It would need to be a small leak, so the plug can fire at high speed.

 

[Jamescenterconsole]

I did check the compression befor I started and if I remember correctly they ranged from 95 to 115. which I thought was low. I then pulled all plugs and filled each cylinder with ATF. Rotated by hand and refilled, did this for a week and rechecked at the end they were all 100 to 115. I'd have to check my notes but I think that is what I remember. Ill recheck them and see what I have now, it may have changed since Ive run it a few times. ****The ATF did run out of the exhaust through the prop dont know if that means anything.

 

[Jamescenterconsole]

Bump

 

[enginepower]

So did you recheck compression? Curious of numbers since you have been running it a few times. You said carb feeding 2 cylinders but does it have individual jets for each cylinder? That plug is too white and you need to address it before you end up with serious damage. It could be crank seals as suggested but if the cylinder is running lean, it will run hotter than normal.

 

[racerone]

???-----One carburetor throat and one jet in the carburetor !-----The air / fuel splits at the reed block some goes to #3 and some to #4 cylinder.