470 3.7L Misfire

[Gawdtilla]

I’ve been trying to tackle this issue for the past month and can’t seem to find a solution. I have an 89 470 mercruiser 3.7 165hp. Cylinder 3 seems to be mis firing as it’s idling very rough and I can’t seem to pin point the issue. I’ve replaced the Ignition coil, the condenser and the points. I have spark to all 4 wires leading to the spark plugs and even the 3rd one that seems to be mis firing. Compression is strong on all 4 cylinders too and we even replaced the fuel pump. However when I pull the plug from the distributor on the 3rd cylinder there is no change. Spark plugs are brand new too and all voltage was measured so I’m not too low. Am I missing something ? We plan on changing the spark plug wires and distributor cap tomorrow as well, any help would be appreciated

 

[jhamblett]

Check firing order. 1-3-4-2 clockwise on the distributor

 

[Gawdtilla]

Citing

Check firing order. 1-3-4-2 clockwise on the distributor

the firing order is set correctly

 

[jhamblett]

Did you swap the wires to see if the misfire follows the wire

 

[Gawdtilla]

Yes I swapped all the wires multiple times ( within the correct firing order) and nothing has changed. The spark plug on number 3 is dry when I take it out, could it be a fuel delivery issue ? And if so should I clean and rebuild the carb ?

 

[Eric 3.7LX Alpha 1]

With the cap and new wires installed, you covered everything electrically. You could try an inline spark tester on the plug boot and run it. If sparks cut's out, you will see it as it happens.

 

[Gawdtilla]

With the cap and new wires installed, you covered everything electrically. You could try an inline spark tester on the plug boot and run it. If sparks cut's out, you will see it as it happens.

I’ll buy one tonight and give it a try !

 

[karayj]

So if the when you check for spark are you removing sparkplug re-attaching the plug wire and then grounding the plug and looking for spark on the tip of the plug? Also a dry plug would make me look to see if the lifter is working and if the intake valve is working.....

 

[Gawdtilla]

So if the when you check for spark are you removing sparkplug re-attaching the plug wire and then grounding the plug and looking for spark on the tip of the plug? Also a dry plug would make me look to see if the lifter is working and if the intake valve is working.....

I replaced all 4 spark plugs and wires and am getting strong spark all across. I think I’ve narrowed the issue down to a bad carb as when I poured fuel into the left side of the carb it sounded like it was running back on 4 again

 

[karayj]

So one of your Jets are clogged ? You should rebuild unless you to pay big bucks for a new one . Anything used has to be rebuilt unless it came of a recently blown engine or boat that was running fine and they are parting it out for other reasons. Find the serial number on the side of the carb and start searching. Before you do that have someone move the throttle back in forth and see if only one jet is spaying fuel or 2.

 

[nola mike]

I replaced all 4 spark plugs and wires and am getting strong spark all across. I think I’ve narrowed the issue down to a bad carb as when I poured fuel into the left side of the carb it sounded like it was running back on 4 again

No. Your carb dumps all the fuel into the intake. Wouldn't cause a problem on 1 cylinder. Agree that I'd pull the valve cover and check that you don't have any bent pushrods and the valves are opening correctly. I'd also reinstall plugs and put a timing light on it. Make sure you're getting a consistent spark over time

 

[Gawdtilla]

No. Your carb dumps all the fuel into the intake. Wouldn't cause a problem on 1 cylinder. Agree that I'd pull the valve cover and check that you don't have any bent pushrods and the valves are opening correctly. I'd also reinstall plugs and put a timing light on it. Make sure you're getting a consistent spark over time

We reinstalled brand new NGK spark plugs and checked if the valves are opening correctly and it all seems okay. I have ordered a leak down tester to check the cylinders again, also the carb was very dirty when we removed so we have a buddy of ours rebuilding it for us ( his family owns a carb rebuild shop). As for the timing light we bought one some time ago and adjusted the timing and it still misfired. At this point if the carb rebuild does not do it, i don't know where else to look.

 

[Gawdtilla]

So one of your Jets are clogged ? You should rebuild unless you to pay big bucks for a new one . Anything used has to be rebuilt unless it came of a recently blown engine or boat that was running fine and they are parting it out for other reasons. Find the serial number on the side of the carb and start searching. Before you do that have someone move the throttle back in forth and see if only one jet is spaying fuel or 2.

Thanks for the suggestion, i took it in today to get it rebuild so hopefully i can let you know if anything changes next week.

 

[Gawdtilla]

So if the when you check for spark are you removing sparkplug re-attaching the plug wire and then grounding the plug and looking for spark on the tip of the plug? Also a dry plug would make me look to see if the lifter is working and if the intake valve is working.....

the right hand side of the plugs are wet with fuel and the left hand size are bone dry. We poured some fuel into the left side of the carb that leads to the misfiring cylinder and it started idling much better. I took the carb in today to get it rebuilt.

 

[QBhoy]

I’d hazard a bet on either the crappy stator type charge system being to blame…or it has the usual cylinder head warping issue, showing up only when warm or under load. Obviously caused by the usual overheating problem that it’s undoubtedly had at least once in its life previously

 

[Gawdtilla]

I’d hazard a bet on either the crappy stator type charge system being to blame…or it has the usual cylinder head warping issue, showing up only when warm or under load. Obviously caused by the usual overheating problem that it’s undoubtedly had at least once in its life previously

the stator charging system has been disabled and we have done the alternator conversion. As for the cylinder head wrapping issue, it idles from a cold start on 3 cylinders so i would rule that out.

 

[QBhoy]

the stator charging system has been disabled and we have done the alternator conversion. As for the cylinder head wrapping issue, it idles from a cold start on 3 cylinders so i would rule that out.

Ok. Fair play. Have you done a warm compression test ? Just to shut me up, haha . No sign of gas bubbles in the coolant header tank ?

 

[Gawdtilla]

Ok. Fair play. Have you done a warm compression test ? Just to shut me up, haha . No sign of gas bubbles in the coolant header tank ?

compression is strong after running the engine for a bit as well, and we also checked the coolant and there was no bubbles either after running the engine for a bit. Sent the carb for a rebuild and changing the intake manifold gasket this week. If that does not fix it then we are going to do a leak down test to inspect the cylinders even further. Totally stumped !

 

[nola mike]

So originally it was just 1 cylinder, but really you're having limited fuel in 2 cylinders. I'll change my answer and say carb could do that. What did the plugs look like when you pulled them? And my suggestion previously was to put the timing light on the misbehaving cylinder wire and watch for a consistent spark