Cold Running Motor…Revisited

[johnyrude200]

Last december I posted on the ‘green boards,’ and did again just now, but figured it might get more attention here. So this is the situation…

I have a ’76 johnson 25hp that is running cold. It has a new thermostat, and pumps tons of water.

So I went through this motor again today and set timing using the timing fixtures, and also made sure the link and sync was set properly on the timing cam attached to the carb/magneto.

The motor is running cold, and I have a laser thermometer to confirm. It is barely reaching 80°, perhaps 90°. The barrel water is about 44°. I can get it to idle fairly well. Have the low-speed needle set at 1 & 3/4 turns out, smokes like hell until it clears out, and at 3/4 range it is misfiring. At top range it seems to smooth out.

I am guessing this is simply because it is running so cold? It wasn’t misfiring at 3/4 throttle last fall or last summer when I ran it, but it was bogging down just above idle until it cleared out, then would run OK, but smoke like hell. It did not misfire at 3/4 throttle (right before it hits the timing cam at the sharp advancement).

I almost never see motors run this cold. I have seen some of the newer, single cylinder OMC motors (as in, 90’s), run at around 60-70°, and those run rough too until the temperature comes up.

I guess I’ll be telling my customers and explaining how to set the carb for anyone buying a motor I’ve calibrated in the winter, or wind up purchasing some sort of heating element to warm up the test tank moving forward.

[Mumbles]

Has the motor ever overheated or had the power head off? Maybe the 313435 plastic restrictor is missing or melted out of the exhaust housing. The water hole in it is only about 1/8" or so. If it’s missing it might be letting too much water bypass the thermostat.

[frankr]

OK, let me say right off, I have no experience with running motors in 44 degree water (here in FL). Having said that, I wonder if maybe mumbles got it. Check that restrictor.

If that isn’t the problem, I wonder if the problem really is abnormal. I mean, a lot of water goes through that powerhead even with the thermostat closed. Maybe you are expecting it to get warmer than it can?? I think it should run fine at 80 degrees anyway. Might have to enrich the idle needle a tad till summer arrives. And try to get that spark/carb sync as close as you can.

Are you aware of the problem the 25 has with the drain valve dumping into the upper cylinder, making it richer than the lower one? That makes adjusting the idle mixture difficult (I say almost impossible to get it right). That is a problem built into the motor. I used to do my possibly illegal modification on them.

[jerry-ahrens]

Maybe you have a hair line crack in one of the coils causing the misfire? I would do a coil power test say with the Stevens or Mercotronic machine to test them under a load. I ‘ve learned the hard way many times to not assume a componet is ”good” just because it’s new. Something to think about anyway, As far as the cooling issue, I wonder how the powerhead temperature is after a good full throttle warm up? Running in the test tank may not be an accurate test for the thermostat, since you can’t really put a good load on the motor. Just a few things to think about here..

[Mumbles]

Exactly what Jerry says about running in a test tank. When pushing a boat at full throttle and making 25 horse power, it’s also going to generate a lot of heat. A lot more heat than when it’s lightly loaded. Probably nothing to worry about if the restrictor is intact. I don’t think any of my non thermostat motors get over 100 F while running in a drum.

My old ’77 25 is coming back for servicing sometime so I’ll check it when it’s here.

[johnyrude200]

I’ll go through the motor again in the next little bit and report back. I do have a set of new coils to swap out, and will double check the flywheel woodruff key.

Frank – I read your post from yesteryear about the recirculating hose, however since the blue boards are now gone I have a question. Do I just reroute the hose by drilling a small hole in the pan, then the exhaust, and running it into there? Of course plug up the orifice on the intake bypass (temporarily with a toothpick, and permanently with the crankcase seal).

I have been toying with starting to do these conversions since I have been working on a lot of the post 1968 18-20-25’s, and they all idle like crap and blubber like hell until they clear out…plus a ton more smoke. I’d like to think it isn’t something obvious I’ve been missing on 4-5 different motors, as I’ve checked the low-speed bearing (packing), cleaned carbs, gone through the usual timing of the ignition, etc. These seem to be the only motors I have been working on that run this way so after reading what everyone has said, I guess it’s just the way they run

I have observed that on pretty much all of the 18-20-25’s from that ’68-76 period, the initial setting of 1 turn out for the low-speed needle doesn’t work. It will make it run, but they cough (lean), and generally about 1 & 3/4 turns to 2 turns seems to be what fixes this. At the very most, 2 &1/4 to 2 & 1/2 turns.

If it’s not coils, timing, or a lean setting, I’ll pull the powerhead to check the water restrictor.

Jerry – even with sustained 1/2 throttle, temperature wasn’t coming up. What has me scratching my head is it didn’t run like this when I put it away last fall. What is alarming/red flagging to me is that 3/4 throttle it is misfiring badly. The only thing I’ve changed since I put it away was I re-calibrated using the timing fixture last night. Previously, it was using a feeler gauge with the point rub bars on the ‘set’ mark on the crank cam. They are new points (well, used all last summer new). I also want to note I use test props in my 275 gallon tank; so hopefully that helps negate some of the ‘non-load calibration’ issues.

Frank – I set timing using the timing fixtures and a continuity meter and have it right on for 1/2 between the 2 timing marks on the magneto. When I put the motor away last fall, timing was set placing the rub bars on the the ‘set’ mark on the crank timing cam and a feeler gauge. Am I missing something here? I can go back and forth between the two settings if necessary for some trial and error before pulling the powerhead to investigate the water restrictor.

[frankr]

If you find anything new concerning the timing fixture, be sure to let me know. Meanwhile. here is the rerun on the mod.

From around 1950 thru 1967 the crankcase puddle drains led to a channel and passage in the front of the casting and were dumped into the exhaust housing and lake. And the motors ran very well. Then came the years of “smog control” in cars and they ran like crap. Outboards didn’t escape either. A mad rush was made to eliminate emissions into the water, and some of them ran like crap also.

OK, here is the plan.
Starting 1968 part of the drainage system was eliminated, and what was left was routed through a hose to a fitting in lower bypass cover….actually one of the bypass cover bolts which was a combo bolt and fitting. From there it went through the casting and into the exhaust area, where the heat was supposed to evaporate and "smoke off" some of the fuel & oil. The rest still went out the exhaust into the lake.

Starting 1973? They re-routed it into a fitting in the upper bypass cover so it would be drawn into the cylinder and burned. You know how well that worked out.

What we are going to do is revert your motor back to the 1968 system. It won’t run as well as the pre-1968 models, but the only way to do that is with a pre-1968 powerhead. To do this you will need a 3/32" aircraft (extra long) drill bit and a special fitting and gasket washer. The fitting is part number 315230 and washer is 303630. Try a dealer. A decent hardware store should have the drill bit.

Remove the bolt from the lower front corner of the lower bypass cover. Before you go any further, screw your fitting and gasket into the hole and attach a piece of hose. Can you blow through it? If you can, it already is drilled, so do NOT continue with the drilling step in the next paragraph. But if you cannot blow through it, it needs to be drilled, so remove the fitting and continue.

What we are going to do is drill a 3/32" hole through the bottom of the existing bolt hole, into the exhaust area for the drainage to flow through. Yes, I know, you can’t get a clear shot at that with the drill. That’s what the aircraft drill bit is for. Eyeball-align the bit as close as you can with the bolt hole and drill right through the lower motor cover pan, enter the bolt hole, and carefully drill through the bottom of the hole till it breaks through inside. You should be able to tell when it hits "air" inside. Then STOP drilling. You don’t want to drill on through the next wall.

Screw the combo fitting into the bolt hole and attach the drain hose. You are done.

[fleetwin]

Yeah, I’m thinking there isn’t too much wrong with this engine, unfortunately. I think your best bet is to follow Frank’s recirc advice, and try to get the engine to heat up more. Unlike the 9.9/15 we were discussing in a past thread, this engine DOES have a thermostat bypass in the head. Are you SURE the thermostat is sealing correctly? Did you use a new 310058 rubber thermostat seal? Is the head surface under the thermostat flat/clean/not corroded/eroded allowing water to bypass the thermostat? I know there were bulletins back in the day about plugging the drilled bypass in the head, and that really did help em heat up and run better. Unfortunately, it seemed to me they ran a little too hot in the heat of the summer, but the temp would be great any other time of year. I can’t remember the part number of the rubber plug that was used, but I’m sure you could come up with something. Certainly easy enough to try, you just have to pop the thermostat cover off.
Like Franks says, these engines ran crappy at low speeds for the most part. That recirc hose that is rerouted to the top cylinder seemed to create uneven running/fuel mix conditions in the two cylinders. The usual adjustment scenario was having to lean out the carb to clean up a rich idle, but then there was a lean spit. So, you usually had to leave the engine raggedly rich at idle in order to avoid the sneeze. I have often wondered about ways to correct this situation. Frank’s cure surely would work. I have wondered about putting another bypass nipple on the bottom cover and splitting the recirc hose so it runs to both cylinders. Perhaps this would create more even conditions, or maybe just a mess because of the opposing pulses between cylinders. Never really got a chance to try any of this out, just left em rich and made sure they were warming up.
You could certainly "test" the recirc theory by unplugging the hose and capping off the bypass nipple. Then extend the recirc hose and route it overboard. Now run the engine, clean it out a bit at WOT, then let it idle and see if idle quality improves…Go ahead with Frank’s fix if temporarily draining overboard improves things.

[johnyrude200]

Well, here’s what I found. The test tank water was up to a balmy 50° today, and the motor was able to get up to about 98°. Correction to my earlier posts…last night I was barely up to 72° even at sustained medium throttle.

I was able to lean the carb out 1/4 turn (so, about 1 & 1/2 turns out), and of course adjust the idle setting down to correspond. Last night I could not do this, the motor would spit terribly or just plain stop running. I did note both last night and today that when I leaned it out to 1 turn (as the manual describes), it did help it run smoother, but would spit and stall almost immediately at the low range (anything above a 1/3rd throttle seemed OK, but still had misfires at 3/4 range).

The misfire at mid range was no longer present today, but it was blubbering like hell all the way through, loading up with fuel/oil. At 3/4, after about 10 seconds, it would clear out and smooth, but there was a whole bunch of smoke and unburned fuel being thrown away. It was struggling back and forth between smooth and rich pretty much at all ranges except WOT, and I don’t run them in the test tank with test props at WOT not for more than 2-4 seconds due to overtaching (easily reach 7000RPM in a hurry = bad). Service manual recommends 4900 RPM for calibration.

My conclusion is that the ignition system is/was fine, and that it is just running cold and rich. I guess I’ll chalk this one up to yet another one of these ’68-76 18-20-25’s that behaves this way. The misfiring is what cued me to reach out to everyone – just more experience for a rookie I guess.

Don and Frank, I will take one of these motors and do the conversion to check out the difference. I’ve been going through a batch of these in the past couple of months and have a bunch more waiting in the wings, so I might as well learn about it and if it makes them run better make it part of the routine. Don I did check the thermostat seal and it was in good shape.

[dan-in-tn]

That restrictor is part #203390 I believe. It was called a bumper and actually came from a different part of the engine. As someone stated you put it in the bypass hole to warm the engine up. I can’t remember if you left it there or not, long time ago. Those motors did have a tendency to have an off idle problem. Like Frank has stated dumping the fuel was a problem inside and was problematic, but we were not allowed at that time to just dump it overboard. I can’t imagine doing it now! It would be nice to bring it around and put it back in front of the reeds with a check valve. The key would be for it not to be consumed until the motor was up at speed. Easier said than done. In ’77 OMC changed the midsection on the motor and that helped with the back pressure issues. Of course it was a different powerhead by then also. It wouldn’t be till 1985 before the thru hub exhaust would help,tune the motor even better. Don would probably remember more about the issues on these motors. Don’t be shy about fiddling with the pickup roller. I believe we would pickup the carb late to improve the blubber some.

Dan in TN

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