84 Merc 150... everything works... nope

[Faststroker383]

I have an 84' Merc 150hp.
serial# 6132423

A friend brought me this boat to fix a "skip"... Here is what I have verified. The motor does run and does have a skip. The reason for the skip... no "combustion" on cyl 5 or 6.

1) Fresh gas getting to all carbs (premixed)
2) All cylinders have spark at the plug
3) all plugs are new
4) compression between 105/110 psi on all cyl
5) bleed lines are new
6) fuel lines are new
7) plenty of air being sucked in at carb opening all cyl
8) swapped carbs from 3/4 and 5/6
(No change in symptoms, no combustion on 5/6)

The above is what I KNOW to be true because I verified all of that myself. It is possible that gas is getting to the carb but not into the cylinders but WHY?

What would cause gas to be in the carb but not get to the cylinder if the carb itself is KNOWN to be good?

Theory 1... Dirty Carb
Dont have a rebuild kit but since 3/4 cylinders are working I could swap the carbs and see if the problem follows the carb. Since fuel is getting to the carb but possibly it's not coming out?

Theory 1 results: swapped carbs but symptoms remained on cyl 5/6 and 3/4 run fine... theory busted!

Theory 2... REED valves
I still have not isolated this theory. What perplexes me though is that there is plenty of air flow into the carb and NO fuel is being "spit" back out which is typical of reed valve failure. When I had the carbs off to swap them I looked at the REED valves through the opening but could see anything broken or any gaps. This doesn't mean that there isnt a problem but not something readily visible.

Theory 2 Results: inconclusive

Theory 3... Bad Lower main seal
This has not been checked as I really do not wish to remove the power head unless I'm almost positive on this possibility. Is it possible that the bottom main seal could be bad causing the crankcase compression / vacuum to leak out therefore not pushing the fuel into the cylinders?

Theory 3 results: N/A


This issue has been a head scratcher for me. Am I missing something simple or complex that would keep fuel from getting from the carb into the cylinder?

Any other thoughts on this?

 

[racerone]

Run at night and see if spark is leaking out of coils / plug wires.

 

[Faztbullet]

Pull plugs to see if has milkly look or water droppletts

 

[Faststroker383]

Electrical system is fully functional and spark is happening at the plug as stated above.

There is no water in the cylinder.

 

[Chris1956]

If you spray 50::1 fuel mix into carb 5+6, does the cylinder fire?

If so, you have some kind of fuel supply problem. Maybe air leak at intake manifold or carb base?

If still no fire, either timing for those cylinders is wrong, or you do have water getting into them.

Verify wires from switchbox to #5+6 coils are proper, clean and on correct connectors. You might be able to figure out when those cylinders are supposed to fire. Since #1 fires at about 6* ATDC at idle,, and all cylinders fire 60* apart, in order, cylinder 5 * 60* + 6* is 306*.

 

[Faststroker383]

Spraying fuel does not add any power. There could not be water getting into the cylinders if there is no water being applied so when testing that nothing changed.

The trigger wiring is connected correctly at the coil but I did not check it at the other end. This could explain the issue. I will look at this issue tomorrow.

 

[Faststroker383]

Checked everything out and all looks good. The wiring is in fact and correct as well as the timing.

Any other ideas?

What about the possibility of a bad lower main seal?

 

[Faststroker383]

Anyone?

 

[Dukedog]

never seen one "that bad"... pull kill switch, pull 5/6 plugs.. wide open throttle and spin it over ta see if its gettin' fuel ta those two???

if ya got fuel and tha plugs are firin' i don't know tha answer....

 

[Faztbullet]

How about a broke seal ring on crank??/ The 2 cylinder could be crosstalkings