Need help diagnosing rough running engine

[TitanTn]

My engine is running rough. Starts very easy, but the idle slightly shakes e whole boat and it doesn't get better with more rpms. I just rebuilt the carb and my starts are easier as a result. I'm fairly confident the carb rebuild went well, but I'm not as equally confident in the tuning part. I can't really tell if it's running rich or lean, or if that's even the problem. It's almost like what you'd expect when you're running on 7 cylinders. But it will get up to WOT and run almost 40. It's just rough the whole time. It has new plugs. Where do I start? I'm really tired of wrenching on this thing, as it's summer and I'm ready to use it.

[wiatowski]

I'd start with the timing.

[TitanTn]

I'd start with the timing.

That was my thought too, and of course I didn't have a timing light on the boat today, so I just adjusted it by ear to see what impact it would have. I ended up leaving it where it was as it really didn't seem to smooth anything out.

[wiatowski]

That was my thought too, and of course I didn't have a timing light on the boat today, so I just adjusted it by ear to see what impact it would have. I ended up leaving it where it was as it really didn't seem to smooth anything out.
bad wire maybe.... If you think you're down a cylinder... pull a wire off see if it makes a difference. if it shuts off... try a different one... unless you have a wire checker.

[Rusty]

Mine did that when fuel was flooding one of the cylinders from the carberator. Check/ replace the choke housing in the carb. I just went ahead and bought a new 4160. The carb that was on it was a cheap automobile carb that got me laughed at when I took it to a carb shop for a rebuild.

[michael hunter]

To find the dead cylinder.
1- With the engine off pull the spark plug wires off the distributor cap one at a time and push them back into the correct hole so they are not fully seated this will make it easier to do step 2

2- Start the engine and with insulated pliers or a spark plug wire puller if you have them. One at a time pull each wire off the cap and look for a change in the way the engine is running . If it runs rougher or the RPMs go down that cyl is working . If you get to one that has no effect on the engine that is the dead one .

3- Once you find the dead cyl pull the plug and inspect it . Ohm the wire for continuity if all look good do a compression test . If you suspect the plug or wire exchange them one at a time with the cyl next to it that is known good and see if the dead cyl follows the part exchange.

[DAFF]

Pull all the plugs and inspect the colour of the burn. If they are all black then look towards the float. If just one or two make sure the firing order is correct and swap the plugs and wires with a good running cylinders. This way you can determine the culprite of the miss fire. Was the miss there before or after the carb rebuild??

[TitanTn]

To find the dead cylinder.
1- With the engine off pull the spark plug wires off the distributor cap one at a time and push them back into the correct hole so they are not fully seated this will make it easier to do step 2

2- Start the engine and with insulated pliers or a spark plug wire puller if you have them. One at a time pull each wire off the cap and look for a change in the way the engine is running . If it runs rougher or the RPMs go down that cyl is working . If you get to one that has no effect on the engine that is the dead one .

3- Once you find the dead cyl pull the plug and inspect it . Ohm the wire for continuity if all look good do a compression test . If you suspect the plug or wire exchange them one at a time with the cyl next to it that is known good and see if the dead cyl follows the part exchange.

Thanks Michael. I really wanted to work on this today but it rained all day. I'll be trying this test as soon as I can this week.

[TitanTn]

Pull all the plugs and inspect the colour of the burn. If they are all black then look towards the float. If just one or two make sure the firing order is correct and swap the plugs and wires with a good running cylinders. This way you can determine the culprite of the miss fire. Was the miss there before or after the carb rebuild??

Thanks Daff. I'll pull some plugs and see what they look like. I'm sure the firing order is correct. It ran incredibly smooth early this spring, but because I needed to replace the accelerator pump, and because I knew the carb hadn't see any attention in years, I ordered a rebuild kit. The rough running came after the carb rebuild. I can reasonably assume that the carb rebuild has something to do with the rough running, but I usually get it wrong when I assume.

[DAFF]

I think the float setting would be on the top of my to check list expecially if the plugs look wet or blackened with carbon. Also the choke might be sticking closed partially causing the run rich situation. Never the less, if to much fuel was the issue after the problem is solved plan on a new set of plugs just to be safe. Washing them out with fuel can cause spark issues later in time.....

Rule of thunb, only touch one component at a time when trouble shooting. This way you can eliminate probile causes one by one. By changing a bunch of things at once you never know the true culprate or worse. How to fix, if things go from bad to worse.

[CornRickey]

I agree with sticking with the carb. Another issue is leaks. If you have a bad seal between the carb, spacer and intake you can have a cylinder lean. Is there fuel dripping from the primary jets? Pull the plugs and see what they look lime.

[TitanTn]

I might still have carb issues, but I think I found the rough running issue. A spark plug wire got loose from the holding clip and feel against the manifold. It burned heated the sheathing, it flaked off, and then burnt the cord wrapping in the wire. I was running on 7 cylinders.

Michael - thanks for the note on listening for the rpm drop. That's how a found it. It was a VERY slight drop (maybe 50 rpms) when pulling the wires from the other plugs, but then nothing when pulling the bad wire. I would have eventually found it when I removed all the plugs, but this helped.

I still think I might need to adjust the carb though. We'll see.

[CornRickey]

Pick up a holley manual from a autoparts store.

[kvand347]

Just food for thought...I had a situation where the negative wire going to the electric choke slid off and the boat ran like crap. Replaced the connection and ran great. Could be as easy as that? I'll keep my fingers crossed for you!

[TitanTn]

Just food for thought...I had a situation where the negative wire going to the electric choke slid off and the boat ran like crap. Replaced the connection and ran great. Could be as easy as that? I'll keep my fingers crossed for you!

Thanks Kevin, I appreciate it. Both of my choke wires are on, and I've checked all the simple things. The choke isn't actually doing a good job when the engine is cold, but once warm, the choke is off and and doesn't seem to be affecting the performance.