70hp idling characteristics?

[johnyrude200]

So as you’ve all probably figured out, I’m cutting my teeth on my first 70hp. This one is a 1993.

I just got back from my first trial run with the motor after having done all the usual stuff (clean/reset all carbs, new impeller, oil, etc). The motor is mounted on a 1968 17′ boston whaler montauk. It seems to sit pretty low in the water when trimmed all the way down, low enough where the exhaust relief passages submerged.

It took me the better half of an hour to get it to stop stalling out at idle, and it was running very rich/rough at anything below 1500RPM. The only thing adjusting the low-speed needle on the carbs did, was get it to sneeze back when too lean. Richening up didn’t seem to change anything.

I noticed that when trimmed up, the idle improved quite a bit and also RPM’s came up both in forward and in neutral. Fooling with the cam follower adjustment and timing didn’t seem to improve anything. I ended up maxing out the throttle cable trunion to adjust idle RPM, settling in at 900-1000 in neutral, and 700-900 in forward.

The service manual says you can get it down to 600RPM by adjusting the timing to 4°, and using a timing light I could not see any change when fooling with that adjustment screw. It seemed to be constant at -2°.

1. Am I missing something here with the timing adjustment? I went through the process to make sure the timing reference arrow is in the right spot prior to getting out on the boat.

2. Am I correct in concluding these motors are greatly affected by the depth of water they are sitting in? The motor seemed to idle much smoother in forward gear by me trimming it out of the water. I.E. back pressure/breathing better? Also when the boat is moving, it idles better.

3. I ran the motor at 25:1, so it seemed to be smoking quite a bit. Than again, most of the time I’m dealing with single carb motors (at least for now, next season will be servicing more 50-70hp range motors). So would this offer an explanation for a lot of smoke?

4. What type of fuel economy do these motors show…gas guzzler? I hit 30.5MPH with a 15 pitch prop.

If I could get it to idle a little smoother, than I think all would be fine with this motor. From 1200-2200 RPM, it is shaking badly, seemingly running rich. Above that is cleans up well and has a good smooth power curve, topping out at 5400-5500RPM.

I also found a 1994 service bulletin saying I should move the overboard water indicator to the top of the powerhead just behind the exhaust bypass gasket. I’ll have to get that done in the near future to avoid the risk of an overheat?

[fleetwin]

OK, well this engine has the "fastrak" trim system, which trims down in quite a bit to help plane off boats better. But, you are quite right, idle quality is surely affected by trim angle and exhaust reliefs being buried in the water. You could always move the trim pin out a hole or two, if all the negative trim is not needed to plane your rig.
This engine has the dreaded "dwell linkage", along with quikstart, which make sync set up tricky. Let’s begin at the beginning though, did you check TDC with the OMC piston stop tool? If not, trying to split hairs setting up idle timing is a waste of time. Rough running off idle is usually caused by poorly synced carbs. These engine usually idle pretty well, much smoother than the two cylinder 40-50s, but the sync steps must be followed exactly and in the prescribed order. Maybe you should back off my 25:1 fuel mix recommendation slightly while you are trying to dial in the idle. Make sure the engine is getting up to temp, or it will take a long time to drop out of quikstart and idle poorly.
I would definitely move that overboard elbow up to the top as prescribed by the bulletin. There was also a bulletin about increasing high speed jet size on these engines as well.

[dan-in-tn]

There were a host of things done to that motor from its introduction in 1986 (56 cu in) till the ultimate change in 1994. The early motors (’86 to ’93) had bridge ports (exhaust). Many changes (bulletins) were done to improve reliability. In 1994 the powerhead picked up oval exhaust ports which was the cure for cylinder scoring.
Now for the problem you have at the moment. Yes the motor is very sensitive to exhaust back pressure. You will quickly learn that trimming the motor up and putting it in gear is normal. After it is moving thru the water you can trim down and get on plane. Just part of learning to run this motor. Do you plan to keep this motor on 25/1? It doesn’t need that kind of oil. 50/1 is normal. If you don’t like that 40/1 would be more than enough.
Your idle timing problem can be cause by a center hub magnet that has moved. Remove the flywheel and look at the center hub. You can usually tell if it has moved. (Glued in place). Before you put it off too long you might pull the cylinder head and check for cylinder scoring? If you have scoring then of course anything else you do won’t help. There are modifications that can be done to the exhaust port when the motor is bored to prevent the same problem again. These motors called for a slightly fatter H.S. jet. .002 I believe from stock depending on what the model calls for. Moving the water elbow to top of the block insures the block is full of water before it pees. I think they dropped the top end timing about two degrees. I may be leaving something out. Been a long time ago since we were modifying all of these motors.

Dan in TN

[fleetwin]

quote t2stroke:

There were a host of things done to that motor from its introduction in 1986 (56 cu in) till the ultimate change in 1994. The early motors (’86 to ’93) had bridge ports (exhaust). Many changes (bulletins) were done to improve reliability. In 1994 the powerhead picked up oval exhaust ports which was the cure for cylinder scoring.
Now for the problem you have at the moment. Yes the motor is very sensitive to exhaust back pressure. You will quickly learn that trimming the motor up and putting it in gear is normal. After it is moving thru the water you can trim down and get on plane. Just part of learning to run this motor. Do you plan to keep this motor on 25/1? It doesn’t need that kind of oil. 50/1 is normal. If you don’t like that 40/1 would be more than enough.
Your idle timing problem can be cause by a center hub magnet that has moved. Remove the flywheel and look at the center hub. You can usually tell if it has moved. (Glued in place). Before you put it off too long you might pull the cylinder head and check for cylinder scoring? If you have scoring then of course anything else you do won’t help. There are modifications that can be done to the exhaust port when the motor is bored to prevent the same problem again. These motors called for a slightly fatter H.S. jet. .002 I believe from stock depending on what the model calls for. Moving the water elbow to top of the block insures the block is full of water before it pees. I think they dropped the top end timing about two degrees. I may be leaving something out. Been a long time ago since we were modifying all of these motors.

Dan in TN

I recommended the heavy oil mix in an effort to avoid one of those mystery powerhead failures, maybe I over did it a bit…Dan makes a good point about the pesky loose inner flywheel sensor magnets, and another good point about ensuring your powerhead is in good condition now…And yes, the jet update kit recommended backing off WOT timing a few degrees.

[dan-in-tn]

I had forgotten about TRAC midsection. That’s another 4 degrees of negative trim you didn’t need in this application. All the more reason to get going with trim up and then trim down before taking off. Bass boaters have been doing this forever.
TDC tool in this case won’t help you diagnosis the flywheel hub magnet. It’s locates the flywheel key and the markings on the flywheel rim, but can not tell you where the slipped hub is at. You just have to do that visually. Other things happen to the magnets in the hub, but we will hope just a loose hub could be the problem here?
More oil is not going to save you from scoring a cylinder if the bridge port is sawing the rings! Just part of the early design. When these engines were repaired (bored) the bridge is relieved so it doesn’t destroy the rings.

Dan in TN

[johnyrude200]

I’ll pull the flywheel and investigate. Compression on the motor after sitting since last fall was 102-107 even on all 3 cylinders prior to me running it, so I suspect the internals are still OK. I can retest compression now that it was run for a few hours continuously and surely everything is well oiled and wet. Might do a good de-carb too.

Thank you both for throwing so much good information at me, it will be a learning curve, and I’ll need to go through your questions/recommendations 1 step at a time.

To start, this motor is running cold. I have not seen it rise above 123° yet, even after running it at near WOT for a few minutes. Any recommendations on how to improve this? Water in the river is still quite warm up here (70° or better).

I don’t have a 1993 service manual; I’m using a 1994 copy. What is the default carb settings…1.5 turns? ’94 calls for this, BUT ’95 calls for 2.75 turns. I am assuming 1995 motors and newer have a lot of revisions? Also is the synchronization process the same? Could explain what I did wrong if I used the wrong manual.

Don I followed the service manual using the piston stop to establish the reference arrow at the top of the manifold is in the correct position. The rest of the manual basically says to run the motor at idle with timing light and use the adjustment screw located under the flywheel, behind the starter, to adjust timing. But when I changed this screw setting, it didn’t help anything and the timing light showed no change to when cylinder 1 was firing. What did I miss here?

I ended up playing with the carbs, trim setting, and throttle cable trunion to get a idle where the motor didn’t stall out anymore at ‘lower’ idle. I don’t think it’s set correct, because anything below 700-750 and it’s stalling out. It’s also somewhat pulsating at idle back and forth between 700-900 RPM, not steady. Runs rough up through 2000RPM than clears out and acts normal. If I trim UP at idle, the motor starts breathing more easily and may actually sneeze/miss running a little lean and RPMs go up noticeably, as much as 1100RPM.

What does the large high speed jets do to help this motor?

[fleetwin]

OK, the first thing you need to get familiar with on this engine is the "QUIKSTART" system…Quikstart automatically increases the timing when the engine is first started cold and is running below 1100RPM. This system is supposed to make the engine easier to start and less fussy during warm up. Quikstart disengages once the engine reaches about 100 degrees, or it can also "time out" after a few minutes (depending on pack vintage) in cases of thermostat problems or extreme cold conditions.
So, what does Quikstart sound like when activated….Well, it really sounds like the engine is misfiring at idle, because the timing is going back and forth between its advanced/cold setting and base setting. Why is this you ask? Why doesn’t the engine just have a high idle during quikstart operation? The quikstart is set to automatically turn off when RPM reaches 1100, this is so the engine can be shifted in and out of gear while quikstart is on without damaging the gearcase or ramming the dock. So, when the engine first starts cold (and is in idle position), quikstart will advance the timing bringing up the RPM, but the RPM will probably exceed 1100, so the quikstart will drop out…But, the engine is still cold, so quikstart will engage again, and this ping pong session will continue at idle until the engine reaches about 100 degrees or the powerpack times out.
The other thing to keep in mind is that quikstart will engage everytime the engine is restarted for about 5 seconds, regardless of engine temp.
So, you can see that it is very important that quikstart is off while you are trying to set base timing or you are going to see the timing light bouncing back and forth and get confused, I’m thinking this is what you are seeing now, which is why you are having trouble setting base timing.
The other confusing thing here is that we are dealing with base timing setting that are "after top dead center". I think the base timing setting for this engine is 4 degrees ATDC. Keep in mind that 6 degrees ATDC is retarding the timing, not advancing it… 2 degrees ATDC is advancing the timing from its base setting. So, with all this conflicting stuff going on, it is easy to get confused trying to set up sync/link on this engine.
I’m pretty sure you manual will talk about disabling quikstart and warming up the engine before trying to make the base timing adjustment. Quikstart is disabled by connecting the engine end of the white temp sender lead to ground and unplugging the yellow/red lead from the powerpack, which is what tells quikstart to come on (cranking the engine).
Next, you will need to straighten out that thermostat, the engine isn’t going to idle well running cold. Some of these later model thermostat designs are crazy complicated, so look at your manual first, and be ready for plenty of springs/cups/washer/grommets to come flying out.
Your 94 manual should be OK for the 93 model, not sure what the base needle settings are, I guess I would start at 2.5 turns out, and gradually lean them out. The pulsating you are seeing at idle is telling you quikstart is on, that is why it is idling rough and you can not set the base timing.
All that being said, these engines do idle quite nicely when set up properly, but it is very easy to get confused, you are not alone, trust me…
The larger high speed jets in these kits richen up the mixture a bit, giving a little extra cushion from detonation/preignition damage. These 56CI engines went through a few years of "mystery powerhead failures", before the design got straightened out. This is really a shame, because it is a nice running engine, and mechanically quiet.
Sounds like you have checked out TDC, so that is good. But, like Dan said, a loose inner hub magnet on the flywheel won’t show up on the TDC tool, but will show up on the timing light. I would go ahead and try to set up sync/link properly before removing the flywheel, I don’t think you have a flywheel issue, you are just seeing/getting confused by quikstart operation.
Like we have said, the trim system on this engine can give plenty of negative trim for better acceleration on short deep V hulls. But, the engine is going to idle poorly when trimmed all the way in, so I would make you adjustments at a "neutral trim setting", the follow Dan’s pointers about operating the engine.
The other thing that is going to mess with you mind here is what is referred to as the "dwell timing linkage", you will see the cheesy little spring loaded gizmo near the timer base. Now what the heck is this malarkey all about? Engineering on drugs? Well, perhaps they should have been…
Someone in the brain trust decided that these engines would run better off idle if timing advance was delayed a bit when the engine is operated just off idle. So, you will see that the engine stays at base timing for a bit while advancing the throttle. You will see this lovely feature on some of the older 90 degree V4-V6 models as well. So, you will not be able to set base timing properly unless you hold the throttle against its stop, or keep the throttle cable attached so the throttle will stay against its stop. Go ahead and just try putting the throttle against its idle stop with the cable off, you will see it won’t stay there because the dwell spring pushes it off.
OK, will get back to your other post and straighten out my mistakes there…

[johnyrude200]

Well that all makes sense and thanks so much for the explanation. I was wondering what that spring loaded contraption was. With the motor the way it sits, I cannot hold the throttle against the stop or it will stall out. I have the throttle cable maxed out to keep it at a higher idle to start up, but at least have it set up where it will idle down to 700-900 in gear, or about 900-1100 in neutral.

The quick start feature all makes sense with the pulsating now too.

On the topic of engine temperature, what should I do to help this motor warm up? I pulled the thermostat already and you sure are right. A lot of stuff seemingly going on there. Do these thermostats work similarly to the older style ones…just open up at 143°? This one appears closed, but than again these 40hp and above motors have much simpler cooling systems/water routing. I can see why they pump water like a fire hose because they are certainly more forgiving in terms of preventing clogs throughout the water path (basically just into the giant exhaust bypass cover, around the cylinders, through the T-stat orifice, than out the cylinder head and down through the exhaust housing…and not really any small areas for things to get clogged up in).

[fleetwin]

Does your thermostat set up look like this?
http://www.marineengine.com/parts/johns … +Crankcase
If so, what you have is a "vernatherm", along with all that other jazzy stuff. All those bits/pieces must be oriented properly, and the diaphragm must not be split, along with proper vernatherm operation, or the engine won’t warm up…
Higher RPMs bring more water pressure which can overcome the big spring’s ability to keep the diaphragm sealed on the vernatherm face, so seeing the temp gage go down a bit at higher RPMs is normal and a good thing. I would not try to heat up that vernatherm, or the pin might just pop out. I don’t have a dimension for closed/open lengths though…
PS, another glitch in the ointment is that some of the water pumps may develop too much pressure at idle, causing the diaphragm to blow off its seat…This is truly a pain, most of the water pump kits have extensive directions about this though. Moving the elbow to the top of the powerhead, and removing the restrictor nipple at the end of the overboard hose should lower the pressure enough to solve this situation…So, for now, just pull the plastic restrictor off the overboard hose, and hold the hose overboard to see if the engine warms up this way….
Good luck with this jig saw puzzle!

[johnyrude200]

yes that is the same one on this motor, and in fact the same model you picked (go play the lottery today!). I’ll give it all a try. Today I’m going to try to install the water indicator to the updated position.

Now to just find a tap that will match those threads! Or can I get one that is a little more ‘gross’ threaded and use a hardware store brass elbow?

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