spark but not firing

[fch2cxr]

My motor (1150 Merc Thunderbolt) dies when I put it on gear? I check ?Sync? and I got it to idle good, but as soon as I try to put it on gear it dies, did more and more checks and ended up discovering that #5 and #6 are not firing (they do have spark) but I know they are not firing because I pulled both spark plug cables off and engine kept running the same:redface:, I do have good compression ~115-120 on 1 thru 5(I was not able to check #6 I could not fit the gage in to the hole because of the base of the motor)
Any ideas???

 

[hamit59]

Re: spark but not firing

I would think it might be a carb issue. It's running lean. I would pull the carb look at the low speed jet sounds like it could be gummed up, a good cleaning would be in order.

Hamit
Bt1 USN ret

 

[hamit59]

Re: spark but not firing

One other thought you can spray premix into the bottom carb and see if it picks up on all 6 . that will tell you for sure it's a fuel problem

Hamit

 

[Chris1956]

Re: spark but not firing

You might post the year of that motor, so we know it's ingnition type. I will presume it is a distributor model.

Check the plug wires for arcing to ground. If they are replace them (Oldmercs.com)

Remove the spark plugs and look to see if they are real clean. If they are, you likely have water intrusion. Water will come from either the lower crankshaft seal leaking or the exhaust cover leaking or having a hole.

The other possibility is that the fuel pump diaghram is leaking raw fuel into the crankcase.

if all these check out, make sure the bottom carb is getting fuel and the mixture is set properly.

 

[fch2cxr]

Re: spark but not firing

Today I took #3 carburetor apart (at least the bowl area) it appears as it the needle was sticking, replaced the needle and seat, the screen/filter and respective gaskets, but did not fixed the problem. Went ahead and crank the motor, got it running and screw in the idle screw all the way in with zero response (the 3rd carburetor the one for 5 and 6 cylinders), again took the spark plug wires off on both 5 and 6 with no response engine keep running w/o incident, I did try that for all other spark plug wires (1, 2, 3, 4 cylinders) and as soon as the wire came off the engine bogged down.
I also took the spark plugs off on 5 and 6 to check for water intrusion, but was not able to tell?the plugs were wet, but it seem to be a gas/oil mixture, how can I tell if water is getting in to the combustion chambers?
reeds are my next step? Any ideas on how check them??? I don?t want to give up on my motors, I feel that they still got life in them, but they are also taking lots of my free time?

Ohh one more thing I tried... I sprayed a 1/50 mix of gas/oil(two stoke of course) directly in to the throat of carburetor # 3 and the motor did no responded at all...

 

[James R]

Re: spark but not firing

Gotta clean the carbs . That means removing them and stripping them and soaking in Gunk products Carb Cleaner. Thats the only stuff I have found to be successful in removing fuel deposits. If you dont have a manual for your motor then now is a good time to get one.

 

[j_martin]

Re: spark but not firing

Gotta clean the carbs . That means removing them and stripping them and soaking in Gunk products Carb Cleaner. Thats the only stuff I have found to be successful in removing fuel deposits. If you dont have a manual for your motor then now is a good time to get one.

James,
He said he dismantled the carb and fixed the stuck needle. He also said that fuel directly induced into carb 3 does not pick up the dead cylinders, and that the spark plugs were wet with fuel.

Iy has compression, at least for sure on #5. So the question is, what would be the common problem with 5 and 6.

Be sure it's not now flooding with carb 3.

Then the question is, what is the common ignition component for 5 and 6.

A plug that will fire in free air will not necessarily fire under compression. The discharge voltage goes way up, and if the plug is bad, or if there's another fault in the system, the juice will take the easiest path to ground instead of jumping the plug gap.

More detailed help would be forthcoming if we knew exactly which engine you were working on. Year, and/or serial number.

Sorry for rambling
hope it helps
John

 

[fch2cxr]

Re: spark but not firing

John, the spark plugs (all) are new, I think the motor is a 73(115 merc), I will chem my wires for arcing tomorrow...