470 missing at speed

[nola mike]

Was running great, then started missing over 3k rpm while pulling a river. So far:
1. Did a half assed carb rebuild. New Accel pump, otherwise reused everything in the carb. Looked pretty clean. Filters did as well.
2. Broke a plug off in the head. Managed to get it out. New plugs (pic after about a mile of running) and distributor cap. Mallory electronic conversion.
3. Checked timing
4. Compression 120-160 (?)-120-100 immediately after getting the old plug out. After running that mile I was 140 in #1 and 120 in #4. Not good, but I don't think bad enough to make it miss?
5. Cleaned all engine grounds etc

Still guessing this is fuel related, but I have a coil from 4.3 that I want to swap in (one of the posts on the old one is loose).
1. Have a spare carb and bought a kit to rebuild
2. Want to check fuel pressure next. How do I plumb into my solid fuel line?
Anything else I need to be thinking of?

 

[achris]

Usually a fuel problem will lead to more surging than a miss. If it's a quick 'miss', like the key was turned off and back on really quickly, it's more likely ignition.

Easy to verify one way or the other. Take the boat out, with a friend, then with the flame arrester off the carb and the engine doing its thing, very slightly cover the carb air intake (or close the choke shutter). Be careful, that thing is pulling some serious air in, so it will give your hand a good tug! If the problem goes away, you have a fuel problem, if not, it's ignition...

Oh, and the chances of it being the coil.... about 0.3%....

Chris..........

 

[kenny nunez]

When I had my shop I had a propeller shaft dyno to load the engine which was a really big diagnostic tool.
I had a 3.0 with a high speed miss and after going through the ignition, the miss was still present. Just as Chris suggested we just barely closed the choke and the miss went away. The fuel pressure was correct and the carburetor was gone through. It turned out that the small power valve spring that is on the brass rod in the float chamber cover was slightly rusted and weak. That spring is about 1/2 the size of a ball point pen spring. I replaced the spring and that cured the whole problem. Of course I could have sold the customer a new carburetor but it was word of mouth that kept my shop busy. I never regretted what I paid for that tool after that.

 

[stonyloam]

Snow White plug says you are getting coolant in that cylinder.

 

[Scott Danforth]

agree with stony.... white plug would mean its leak down test time.

 

[nola mike]

stonyloam Scott Danforth , that's what I initially thought when I pulled #4, but the other one is from #1. Possible less likely to have the same leak in those 2i think.
​​​​​​​@achris, kenny nunez , I'll check that out. I can't tell from how it's running which it is. I keep going back and forth. Got spare carb and kit here. I'll rebuild and see if I can track down fittings to check fuel pressure for the weekend. Unfortunately once I get to the boat I'm a long way from any parts...

 

[nola mike]

Grr. Carb rebuild, replaced plugs, cap, etc, took it out again, no difference. I did try closing the choke while it was missing--no difference until the engine started to die. Also wiggled wires around while missing, no change. My tach has occasionally bounced around without issue for years, I'll disconnect it and try again. So what's the next step? I'd love to confirm that this is ignition related. Do they make something that can record spark output? Like an inductive timing light type sensor that can connect to your phone? That would be fantastic.

 

[achris]

Grr... Do they make something that can record spark output? Like an inductive timing light type sensor that can connect to your phone? That would be fantastic.

Grr. Why is everybody obsessed with connecting everything to a damn phone! Use old-fashion TROUBLESHOOTING techniques. They've been around a lot longer than phones with apps, and are far easier and more reliable.

How about this for an idea (after you throw the phone overboard, or leave it at home)... Get yourself an inductive timing light, connect it to #1 spark lead and run the engine. When the engine starts missing, look at the timing light (not the damn phone!). If that's the problem cylinder, you'll see it. Move it to the next cylinder and the next... Eventually you'll find the cylinder causing the problem. If it's on the first, try the rest as well. If it's on one cylinder only, then it's the rotor, cap, lead or plug. If it's on all, then the coil and points come into play....

Don't need a phone and app to do simple PROPER troubleshooting.

Chris........

 

[nola mike]

Problem with that is that the rpms are too high, light is blinking awfully quickly at 3k rpm. Don't know that I'd be able to pick it up esp if it's coming before the rotor. And esp since I'm basically a 1 man show. It would be easy and unmistakable if you could record it. Besides, don't you have a fancy widget to plug into your motor?
The timing light is sitting in the boat, I was planning on trying it again with a helper. But I still like my idea.
Also, if I threw my phone overboard I'd have no way to call for a tow. ..

 

[achris]

Problem with that is that the rpms are too high, light is blinking awfully quickly at 3k rpm.

Trust me, you'll see it!

Don't know that I'd be able to pick it up esp if it's coming before the rotor.

Before the rotor? No, I said put the timing light on each PLUG LEAD, not the lead between the coil and the distributor.

And esp since I'm basically a 1 man show.

So put the timing light on the floor so you can see it by just turning your head..

It would be easy and unmistakable if you could record it. Besides, don't you have a fancy widget to plug into your motor?

No, I don't have 'Vessel-view' or 'SmartCraft', or anything like that. I have 6 analog gauges (Tacho, oil pressure, water temp, volts, trim and fuel level) and an alarm buzzer. And I catch just as many fish as the guy with 'Smart-everything', probably more because I'm not obsessed with my smart-toys... The only thing I have that 'plugs in' to my engine is the diagnostics on the laptop. That gets done at service time to check for codes, and that's it....

The timing light is sitting in the boat, I was planning on trying it again with a helper. But I still like my idea.
Also, if I threw my phone overboard I'd have no way to call for a tow. ..

No marine-band transceiver (as required by law!)? (I have both Marine 27MHz and Marine VHF)

Chris........

 

[kenny nunez]

Do you have a Petronics ignition conversion? If you are still using points & condenser get your hands on a dwell meter which is the best way to set the points. One other thing to check which can be done on a flush out, start the engine and put it in gear then turn the engine off. Push the throttle all the way open then look to see if the roller is still centered in the “V” notch on the shift cable mounting plate. Your lower shift cable could be just a small amount out of adjustment and pushing the “V” notch just enough to intermittently cause the miss. Since you said the tachometer would bounce around the cable adjustment might be the cause since the tachometer reads from the negative side of the coil.

 

[QBhoy]

HI
with this particular engine and it’s horrible idea of two dissimilar metals with reference to the block and head...I’d be worried about that. Also it’s tendency to overheat after water pump issues.

 

[achris]

HI
with this particular engine and it’s horrible idea of two dissimilar metals with reference to the block and head...I’d be worried about that. Also it’s tendency to overheat after water pump issues.

Yes, not a great idea, but probably not the problem here. And he's not had 'water pump issues', it's missing at speed, not overheating.

Chris........

 

[QBhoy]

Yes, not a great idea, but probably not the problem here. And he's not had 'water pump issues', it's missing at speed, not overheating.

Chris........

Yeah. But a previous overheating issue usually leads on to a weak or blown head. Often followed by missing and compromised running. Seen it a few times on this engine.

 

[nola mike]

Ok, good and bad...
tl,dr: it was the f'in coil! 2nd time in 3 years (although this one was from my spare engine) Scott Danforth


@achris: you were right, the miss was clear as day on all cylinders. I mentioned putting the light on the coil wire to distinguish whether the problem was coming from the coil or the cap\rotor. Even without smart craft etc, you aren't plugging in your laptop to look for a code if the engine starts misbehaving?

​​​​​​​Anyway, found a Napa with a coil in stock, swapped it in and it ran great...for about 2 minutes. Then engine died, no restart. Swapped the old one back and made it home. Swapped the new one in and out 3 more times to make sure it was dead, because really? But yup, new one completely toast. Anything that could have done that other than a faulty new coil? Im running a ballast wire, didn't change the wiring, and the old coil still works...just seems fishy.

 

[Scott Danforth]

My guess is its not the coil, however the feed to the coil, or your tach is causing issues

Disconnect the tach lead and try

 

[nola mike]

Well, it's definitely the coil; the new one worked (until it didn't). I had disconnected the tach with no change. I don't know what would blow out the new one except for overvoltage (which isn't there), or the wrong coil? I think the coils are pretty standard though right? Going to slog 30 minutes back to Napa tomorrow and get another. If it blows again then I guess I have more work to do. I don't know if there quality of the Napa brand coils either. ..

 

[achris]

What ignition system do you have? Points? Petronix?

If it's the original setup, points, then I would be also changing the points and the capacitor (which should be changed annually, along with the points, but nobody does that :facepalm

By the Merc manual, you need a coil with a primary resistance of 1.5 ohms. (and if you have a choice of secondary, 9.4k to 11.7k ohms).

.... Even without smart craft etc, you aren't plugging in your laptop to look for a code if the engine starts misbehaving?....

It's an MPI, it doesn't misbehave... (and even if it did, it would sound the buzzer to let me know something was amiss. And yes, then, when I got home, I would plug the laptop in, just like you used a timing light, as a diagnostic aid... But as for engine monitoring apps on phones, nope, completely against that idea. :smash: it's just 'blinky lights'....

Chris...........

 

[nola mike]

It's a Mallory electronic ignition. Uses original externally ballast coil. The Napa coil is ic12sb, which Google tells me should work...

 

[nola mike]

It's an MPI, it doesn't misbehave...

Chris...........

tell that to tpenfield