Pulsating 15hp detuned down to a 9.9 question
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Usually I’m going in the other direction, but this time I’m downgrading from a 15 to a 9.9. I have a ’75 OMC 15hp which I’ve gone through and tuned up. I’ve swapped out the carbs to a 9.9 carb.
Issue I’m having is it is pulsating at mid range, and it appears that the fuel/air mixture is wrong. I’ve done a link and sync several times, double checked fuel pump/filter (A-OK), checked the lines (OK), and also rechecked the carb twice. When I set the pickup to happen earlier, the motor runs lean at the low end and blubbers at the top end, still crappy in between.
It is OK at the top end and at idle, but is bogging down/surging/pulsating at mid throttle. I’m wondering if perhaps the carb jets are clogged (going to pull it apart a 3rd time and clean again to be safe). One thing I noticed which seems strange, is that I have the low throttle stop set so high that the pickup arm is past the index mark on the timing plate to keep the motor from stalling (it does idle down pretty low, and smoothly, though).
I’ve tested it on my test boat and in the tank with a test prop, same results in both scenarios. Ideas…? Ignition system is strong (1/2 to 3/4" spark, timing set with the timing fixture).
I am not 100 percent on this but think there was more than just a carb difference on these motors, at least on later models. Are you sure this is the only difference on the 1975 models?
Have you tried it with the original 15 hp carb to see if it is present? Or do you have it, hence the de-rate?
I have the original carb, but a fuel supply issue is something I wouldn’t expect when de-tuning, if anything, I would expect a flooding/blubbering issue. This is why i am reaching out to the powers-that-be on this one.
What compounds this head-scratcher is the changing over generations of the carb follower arm diameter. In the dozen or so of these I’ve worked on, they’ve all been 9.9 -> 15 conversions, and they’ve all been pretty straight forward.
I know the ’74-76 models are known to be, basically, the worst motor OMC ever made, so this is why I am so curious. Am I missing something about the ignition system that would account for this situation?
The carbs on these motors aren’t exactly high tech and they do have issues in the mid range. It seems the motors were designed with two speeds in mind, fast or slow. You’ll probably just have to live with it.
I thought the reeds were different from the 9.9 to the 15.
I think there are three main parts between the 9.9 and 15, The Carburetor, the tuned pipe under the powerhead, and the carb advance cam on the mag plate.
I’m with Mumbles, many of these engines exhibit the symptoms you are speaking of, especially the older ones. If you dial in the idle mix for smoothness, the engine falls on its face during acceleration, or it may even bog out completely when slowly decelerating through this trouble range.
There were several updates on these engines that included carbs, throttle cams, and cylinder heads. Usually, I have to leave the idle mixture a tad rich to avoid the bogging/lean midrange. You might try retarding the cam pick up as well.
The 9.9/15 differences for the 1975 models were a carburetor, cam, and reed shims. Both engines used the sealed exhaust tube in 1975. I usually found the symptom you describe was worse on the 15hp models, so I would have thought the 9.9hp carb would have improved it. Tell us why your customer wants to detune the engine….I figured out the issue. After tracing my way back all the way to the end of the fuel system, I found the leaf plate (not the leaf valves) actually had wear spots where the end of the leaves flutter against that plate. It was allowing about a 1/2mm gap to stay open in 3 of the 4 leaf valves. Instead of the plate being perfectly square where the leaf valve flaps lay flat and rest, it was rounded off. This would explain the irratic operation at mid range. Either they were allowing enough fuel to idle OK, or they were wide open for WOT. But in between, FORGET IT!
After checking the carb 3 times, checking the pickup 5-7 times, double checking fuel lines and pump, I had no other choice than to get more invasive. Thankfully this solved the issue.
The motor still has a little bit of flutter at the mid range sometimes (no rhyme or reason for it have a small RPM fluctuation, even despite holding the throttle steady), but it is running at an acceptable level.
The customer wanted a 9.9 for a 10 foot inflatable for his kids, and also the option to switch back to a 15hp in the future if necessary. This 15hp manifold did not have shims on the leaf plate. The donor motor I took for a new leaf plate from was a mid 80’s 15, and had the shims.
If interested, I can post some pictures of the worn reed plate next to a known good one.
The next time you get the flywheel off look down at the armature plate and with your hands on both sides of it, give it a lateral move back and forth and see if it moves around the center boss a little. These motors tend to develop this problem of a wobbly armature plate after 40 years of use. If yours does move you can easily see that the point gaps are also changing. This I believe causes your mid-range miss. Why in the mid-range I do not know, but in any event if it moves it can be corrected.
To correct this, remove the 5 screws holding down the armature plate and pull it off of the support ring. Remove the plastic bearing and set it aside for now. Put the amature plate back on and move it from side to side to observe the lateral movement. Turn it a little and do it again. It is supposed to not have any lateral movement in all throttle ranges, since any movement will cause the point gaps, and consequently the spark timing to change. If it has movement, remove the armature plate and look at the center boss (the metal material surrounding the drive shaft that is about 1/4" in height, maybe 2" in diameter) that the armature plate’s center opening surrounds. On the sides of this boss, about 1/2 way up its height strike a center punch or nail or sharp screwdriver, with a hammer, at the 12,3,6,and 9 O’clock positions. Do it lightly at first. Replace the amature plate and see if the lateral movement is fixed. Turn the armature plate and check for lateral movement again. If there is still lateral movement, strike the very same positions of the boss again. What you are doing is creating a build up of metal material on the boss to increase it’s outer diameter and keep the armature plate from moving laterally. You don’t want to create too much build up or it will create throttle resistance when in operation but a little resistance is OK. Eventually the lateral movement will be gone. Replace the plastic bearing and put the armature plate back on and connect it down to the timing plate with the 5 screws. Reset your point gaps and check them again at various throttle positions to verify that they do not change (especially at mid range). They shouldn’t change anymore.
With that in mind, if you know the throttle position that you were getting your miss, you could check your point gaps first, then move the throttle to that position and check them again. My guess is one of them or both have now changed at that mid range and that was your problem. If they don’t or you have no movement then perhaps it is another issue. If they do change, that lateral movement should be fixed if you want to have a smooth running motor.
Very, very helpful advise. I will do this if I end up seeing the motor again. The customer already took it away, said it ran fine the first time out, but had troubles the next day. I may just give the guy his money back and tinker with the motor a little longer using this tip you provided.
Obviously I need the motor to be pretty reliable in terms of performance. I already have it to the point where it’s a half pull starter, but it is behaving strangely and I hadn’t even thought to look for wobble on the armature plate. Makes sense though – I’ve seen this in 9.5’s, 18-20-25’s, 10’s, etc…just hadn’t even thought to look for this in one of these 9.9/15’s yet.
See, it’s always good to get a 2nd, 3rd, or 4th opinion from others here because it is nice to have a fresh perspective offer insight on an issue.
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