Point gap .022 but needs to be .012 to run ??? Need new coil?

[RickW]

I'm helping my father to get his Mercruiser 488 to run better but we've run into a few problems. The last couple of times he had it out it started to run rough/miss/lose power at higher rpm. The problem worsened until it started to idle rough as well. Cleaned the fuel filters and replaced the water separator. We took the carb apart soaked it and used a rebuild kit. A little varnish, here and there, but nothing seemed to be plugged. When I went to adjust it using the idle screws, the rpm didn't seem to change when turning them in or out as per the service manual. From searching around the net a bit I found that it could just be a problem with the ignition.<br /><br />The plugs were just recently replaced. Replaced the wires and set the timing. Still not idling quite right. Checked the point gap and it was a little bit more closed than it should be so set it to .022 per the manual. Tried to check the timing but couldn't get the light to fire. Tried the timing light on my truck and it was fine. Came to the conclusion that #1 cylinder wasn't firing. Replaced the cap, rotor, condenser and points. Couldn't get the engine to start at all. Tried switching things back with the old parts but no change. Eventually, changed the point gap to .012 to get it running smooth. Reset the timing and was generally able to adjust the carburetor. One of the idle mixture screws didn't seem to change the rpm when I turned it out, only when I closed it. This screw has also been slightly bent somehow, not at the needle but along the threads. Could this affect the mixture?<br /><br />Anyway, took the boat out and about 5 minutes out into the canal, it started to idle up and down again. My question is whether this indicates a bad coil? My father just put $1200 into a rebuilt outdrive plus the money in the ignition parts carb kit and he really isn't too keen on spending the $45 on a coil unless he's fairly confident that's the problem. Any suggestions would be much appreciated.<br /><br />- Rick

 

[JB]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

Howdy, Rick. Running better with a reduced gap is a strong indication. The reason more modern engines are set with a dwell meter is that it responds to the percentage of time that the points are closed and open. By closing the gap you increased the dwell period in which the coil is "charged" and decreased the dwell of open points, when the magnetic field in the coil collapses and produces the high 15 to-18000 volt pulse to the plug, via the rotor and cap. That suggests a weak coil or incorrect plugs. You changed the plugs and the condenser (which might have been a suspect), so the coil remains suspect.<br /><br />Before you go out and buy a coil: reset the points to .022, pull a plug wire and see if it will jump a 1/2" gap to the block when you turn it over. If not, you have a weak coil. Then try it with the points at .012. If you get a significant increase in spark strength that will verify the first conclusion.<br /><br />If the budget can stand it, consider converting to pointless electronic ignition. It's much more reliable, starts easier and your plugs will last up to 1000 hours.<br /><br />Good luck.<br />JB

 

[RickW]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

I wasn't sure if I should have the plug in the wire or not, so I used a screw to extend the wire's contact. Should I have used the plug? The first time we turned it over at .022 there was a very light spark but the second and third times I got a good strong spark, the same as when the gap was .012. It's getting dark out so I may have been a little closer than a half inch on the second and third sparks, I'm not really sure.

 

[JB]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

Hmmm. That sounds like your coil is okay. One other thing changing the gap does: it changes the spark timing. Recheck your timing, with particular attention to advance of timing as you rev it up a bit. If the timing doesn't advance smoothly as revs increase, pull the points/condenser plate and eyeball the mechanical advance mechanism under there for a broken spring or other mechanical failure. It's a bit of a long shot, but doesn't cost you anything unless and until you find the actual culprit.<br /><br />Good luck.<br />JB

 

[RickW]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

Following some advice that I'm getting from someone on another forum I've checked the voltage to the + side of the coil with the engine running. With it not warmed up and idleing at ~ 850 rpm I got 10.5v and at ~ 1500 rpm it read about 13.5v. I would guess that this reinforces your oppinion that<br />the coil is probably good, yes or no?

 

[RickW]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

Also, one guy I emailed about the problem originally thought that there could be play in the distributor cam, either the bushings or the shaft are worn. He said that the fact that I got one weak spark at .022 tends to support this reasoning. I'd like to eliminate any other possibilities before I move on to pulling or replacing the distributor, but this sounds reasonable to me. What do you think?

 

[JB]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

Yes, Rick, That does reinforce my opinion on the coil. You don't need to pull the distributor to check the shaft for wobble or the advance mechanism for breakage. The advance mech. can be repaired or replaced without pulling it, also. If the bearings are worn (wobble) you'll need to pull and replace it. If your timing light now works, wobble in the shaft should cause the timing to jump around a bit.<br /><br />Good luck.<br />JB

 

[RickW]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

The engine seemed to run worse today so we dropped the point gap down to .010 which helped a little but not much. Before we changed it we weren't getting the timing light to fire which as I said before I assume means that #1 wasn't firing. Readjusted the timing and checked that it advances which it does. The timing is jumping all over the place but I don't know if that's just because the engine is running as smooth as it should be.<br /><br />Didn't check under the points plate as you said because the advance seemed fine. <br /><br />Do you know of anything besides the coil or distributor that could be causing this. I don't know if there are any exhaust sensors etc.. that might do the same thing.

 

[JB]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

Yo, Rick. With your timing jumping "all over the place" that strongly suggests bad distributor bearings or a bad advance mechanism. It could even be a distributor drive gear problem. If I misread your post, and you think it is because of vibration, peace. I think it's time to pull the points plate and, if the advance mechanism looks okay, the distributor. Inspect the gear on the end of the shaft for badly worn teeth and inspect the shaft for play in it's bearings. As a last resort, a well equipped dealer of the company that made the engine may still have a distributor testing machine, which can diagnose it in a few seconds. By the way, if your distributor has a vacuum advance pot, disconnect it and plug the hose. Then run your checks again. Do this before any of the above.<br /><br />One final thought: the "if it aint broke, don't fix it" motto means don't replace a part (except for routine maintenance) until you KNOW it needs replacing. Some unethical dealers encourage techs to troubleshoot by replacing and some techs don't know how to do it any other way. A real pro troubleshoots with logic and testing, not a parts bin.<br /><br />Good Luck.<br />JB

 

[RickW]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

A couple questions. When you say to plug the vacuum advance pot and rerun the tests, do you mean to see if the timing is steady and advances properly? When I take the distributor out, don't I need to mark somehow to assure that I put it back in properly and what is the procedure for doing that? What kind of play in the bushings is acceptable? Any?<br /><br />Thanks<br />-Rick

 

[RickW]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

Never mind about the vacuum advance, there isn't any.

 

[JB]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

Hey, Rick. Before you pull the distributor, pull the cap and make a mark on the points plate showing exactly where the rotor is pointing. Don't move the crank while the distributor is out. When you replace it, point the rotor to the exact same spot. Allow for a few degrees of rotation going in and going out of mesh with the drive gear on the crank.<br />Good luck.<br />JB

 

[RickW]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

I ended up finding the section in our manual about removing and disassembling the distributor so we got it out no problem. Our manual is in several incomplete sections due to some water damage while stored on the boat.

It ends up that there is a fair amount of play after all. If I had to guess, based on the opening of the points when pushing the shaft towards and away from them, I'd say the amount of play is probably .020. The manual says the tolerance is .002 so I'd say we found our problem. The main shaft is worn pretty bad.<br /><br />Do you think there's any point in trying to rebuild it ourselves? It seems like we'd have to replace the main shaft which they probably don't sell without the weights and we don't see how to get the bushings and it's housing off of the base of the distributor so I think we'd need that unit too. That's almost the whole thing so I guess a rebuilt is the way to go.<br /><br />Do you have any websites that you like to purchase from. Whoever's got the best price is probably going to get our order.<br /><br />Thanks again.

 

[JB]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

Congratulations, Rick. You found the cause without replacing a lot of crap that didn't need replacing. Good show.<br /><br />I don't buy online, so I can't recommend anyone, but I'll bet our buddies on this board can.<br /><br />Good luck and give us an update when she runs right again.<br />JB

 

[RickW]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

One suggestion I got was to go to a junk yard, explain what we're doing, and get the parts for ~ $50. If we did this I assume we could also convert to electronic ignition fairly cheap. What do you think about that idea and what should we look for to make sure we get the right parts?

 

[RickW]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

Another question for you JB.<br /><br />If we converted to electronic ignition, would that solve the problem? Would the play in the shaft, as it is now, affect the rotor and the contacts in the cap? I'm sure that it's not nearly as precise as the points but if we leave that shaft in are we just asking for problems down the road?<br /><br />What do you think?

 

[JB]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

Hey, Rick. Sorry to say, but you have to have a sound distributor to start with. If an electronic conversion uses a Hall-effect trigger (magnetic) it would be destroyed quickly. If it uses an optical trigger it would ignore the wobble for a while, until the worn shaft galled and froze up. Then you would have more trouble than you want. If it uses points you don't want it.<br /><br />Investigate a complete MSD conversion, including distributor. It will cost more than a used or rebuilt unit, but it will greatly improve starting, reliability and plug life.<br /><br />As for the used unit: it would save you a bunch at the front end, and if you get a good one, last well. On the other hand the vehicle may have made it to the wrecking yard because some tech couldn't find a trouble that was in the distributor. Check the shaft for wobble and use your original advance mechanism, points, condenser, rotor and cap. If your budget is limited it is worth the risk. Take your distributor with you when you go shopping, so you can duplicate it as close as possible.<br /><br />Good luck.<br />JB

 

[ODDD1]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

Rick, i realize your distributor has some wear in it but...i have seen these 3.7 liter distributor out last the engines themselves many times. all i have read says you have tried air gapping these points...that is a fine place to start but you set the points with a dwell meter while runnig...between 28 and 34 degrees....any slop in the distributor will be accounted for and you still have the coil saturaion time you need.electronic conversion a possibility if dist not too wiped out......a junkyard peice probably not an option..that dist. was never used anywhere but by merc in that motor...it looks like just a delco but it has a longer shaft on it. and 1 last thing..check the decal on you valve cover most had timing spec of 10 degrees...merc put out bulletin saying to time all 3.7's to 8 degrees.

 

[RickW]

Re: Point gap .022 but needs to be .012 to run ??? Need new coil?

Well we replaced the distributor with another used one and, what do you know, problem solved. The idle is still aliitle off but no problem with it and high RPM. No eltronic ignition but since I'm not the one footing the bill, that wasn't my decision to make.<br /><br />Thanks for your help JB.