Mercruiser 3.7/470 pistons 1 and 2 not firing.

[artificialreef]

Good evening folks. Trying to dial in my motor. Running rough. Heres what i found. Compression good 139-149 all 4. Runs rough. Removed plug wires on 1 and 2 and no change, still runs rough, no change in the sound. Removed 3 and 4 and engine stalls and shuts down. There is spark to all 4 cylinders. This leads me to think 1 and 2 are not firing (there is spark).The dizzy was changed, the firing order is correct. I put the number one at tdc and connected the number one to the one the rotor was pointing at and reattached the wires 1,3,4,2. If i am 180 degrees out is that just moving the number one to the other side of the dizzy (and arrange the rest accordingly)? Will it damage anything to run it like that for a few minutes because i tried that and heard a loud pop. Then froze in my tracks and went back the way it was. Dont want to damage anything like break a rod or something if am not 180 out and then i am.

Thank you.

 

[artificialreef]

Re: Mercruiser 3.7/470 pistons 1 and 2 not firing.

I did the wires one at a time. The question reads like i removed 2 at a time.

 

[artificialreef]

Re: Mercruiser 3.7/470 pistons 1 and 2 not firing.

Nope not 180 degrees out. I double checked. I had it right the first time. Any ideas on why pistons 1 and 2 will not combust with good compression and has spark?

 

[bonzoscott]

Re: Mercruiser 3.7/470 pistons 1 and 2 not firing.

Even though you have spark on all four, it may not be enough on 1 & 2. I have had similar problems on other types of motors. My first thought is Plugs / Plug wires / Dist cap / Rotor. I know it's not best to start throwing parts at a situation, but I have done the ohms test on wires which checked out good with no improvement. Don't know the age or condition of the items I mentioned on your boat, but if it is sketchy, I'd replace them so you know where you stand.

 

[artificialreef]

Re: Mercruiser 3.7/470 pistons 1 and 2 not firing.

Everything brand new but the coil. Even brand new distributor. I am getting spark and fuel. Compression good. Could it be not good enough spark? But then why would 3 and 4 be combusting. Another clue is the plugs looked a little rich. They had an odd half dark half light look to them. But that was all 4 again. And 3 and 4 are firing.

 

[bonzoscott]

Re: Mercruiser 3.7/470 pistons 1 and 2 not firing.

Re-check all of the procedures you mention. You can often overlook something simple. Bring number one to TDC, assuring it is the compression stroke by thumb in the spark plug whole. Then check it is at Zero on the timing dial and rotor pointing at your number one plug wire. If all your wires are correct to proper plug for the firing order, that should get it roughly started. If not, only thing I can think of is some bad valves on 1 & 2. Were those by chance the ones that were 139? Has the boat ever ran good for you? That Bi-color of the plugs sounds strange. You may need to go back to carb issues.

 

[artificialreef]

Re: Mercruiser 3.7/470 pistons 1 and 2 not firing.

All the tune up and re-wiring i have done is correct. This appears to be in the positive coil circuit to the choke. I followed the string "idles good but runs rough" and started looking at that circuit but that string took a detour. I am getting 12.3 at the battery (I know, i am charging it) Then 11.7 at the choke and + of the coil. When i disconnect the wires to the choke it goes up to 12.25 at the wires to the choke and the + of the coil. Looks like the choke has an issue but i can not run it with that connection floating. I will repair the choke but is there somewhere i can connect the red and purple at the choke just to test it? Bypass the choke.

Thank you.

 

[artificialreef]

Re: Mercruiser 3.7/470 pistons 1 and 2 not firing.

Update to pistons 1 and 2 not firing. I retrofitted a spare carb i obtained in a swap and she's purring like a kitten. I thought maybe because the boat is on a trailer and 1 and 2 are towards the bow the fuel was not getting up the slight incline through the intake manny. My hunch was correct. When i pulled the newly rebuilt carb i noticed it was all black on the bow butterfly wing and black up through the throttle body. Maybe when i was trying to dial in the timing all the backfiring fouled up the carb. Or maybe i just botched the carb rebuild. It was only my second rebuild. Now since the test carb is retrofitted and not coast guard spec i will have to try to rebuild the original again.
Quick question on that. Do i need another rebuild kit? Or since there are all new parts in there can i just disassemble clean the inside and reassemble. I probably already know that answer since i think i recall there were gaskets in there and i will not reuse fuel gaskets.

Anyhow. I will try to find a good youtube video and follow that step by step this time doing the carb. All mechanicals on this tub almost done and then on to the upholstery. Both motors running cool, idleing smooth and throttle responding crisply. Coming up on 4 years in dry dock but its getting close. Thank you all for your patience and professional opinions.

 

[Eric 3.7LX Alpha 1]

Re: Mercruiser 3.7/470 pistons 1 and 2 not firing.

You can always pull the carb apart and see how the gasket looks. If they're not too old you may luck out as far as getting another rebuild kit.

Here is a nice thread Alpheus posted on rebuilding his carb. There's a series of videos thru the whole process. Check it out:

http://forums.iboats.com/mercruiser...er-quadrajet-rebuild-video-update-362542.html