The prop that is on the boat 13 1/4" X 17 which is actually the towing prop.
I haven’t looked up your combo and verified what prop matches your application. If what you say is true, that this is the “towing” prop, then that may be your whole problem with reaching your desired WOT RPM. A “towing” prop will be geared low to deliver a “hole shot” to lift skiers etc out of the water and achieve a good towing speed. If you want top end speed, you need to gear the prop for it. One sacrifices the other. I have two props for one of my boats for this exact reason. If everything is checking out normal, and you want better top end speed at the expense of hole shot, then you need to re-prop the boat.
Speed per se isn’t the issue. Even with this prop, the engine is bogging down.
Check float level in the carb as well as fuel pump output. If the throttle is opened and fuel can’t be delivered fast enough, the engine will lean out, and stumble until you back off on the throttle. Also check the ignition coil.
I towing prop(ie, lower pitch prop) will allow the motor to rev faster with a lower boat speed. The propeller on my boat should be able to over-rev the motor.
The propeller on the boat is within the factory recommended sizes.
I’m leaning toward the timing chain due to the fact the the motor runs so much better when the timing is 20-30 degrees above TDC. The problem is I don’t know for sure, or how to know for sure.
An idea occurred to me the other day. Does this engine have a carburator? Some of the 4 cylinder GM truck engines had a two barrel progressive Rochester carburator. The engine runs on the smaller primary throttle valve until higher power is needed then the secondary valve opens quickly. If the main jet of the secondary throat is plugged, the engine would rev well without load using only the primary throat but bog down as you describe when under load as fuel was not added to the secondary throat air flow.
Also if this has a mechanical fuel pump, check its flow and pressure as modern gasoline warps the fuel pump valves over time. This causes lack of flow and pressure problems.
Let us know where you stand with this problem and if you have found the problem.
I dealt with this exact problem on a 1985 Ski Nautique boat once. The point system and electrics were completely at fault. Part of the trouble was the older wiring wasn’t delivering as much voltage as it should to the ignition. On top of this the point ignition was horrible at higher RPM and was seriously impacting performance. I simply installed an electronic ignition upgrade, which is very simple and easy to do. I also ran the ignition directly off the battery-it runs and accelerates just like a new boat now. I had to run some wiring hidden under deck, but the problem was solved immediately. The owner is quite happy and that was 3 years ago.
Using a vacuum gauge might help you narrow down the problem. With the engine idling and under no load, the vacuum should be ~25 inches of mercury (less allowing for elevation). As the engine is throttled up no load, the vaccuum should stay approximately the same or rise. With the engine under load, the vacuum should drop, i.e. go toward zero as the throttle is advanced. If the vacuum is near zero when the ‘bog’ happens either the engine is overpropped, the ignition system is timed incorrectly, or valve timing is off. If the vacuum is nowhere near zero when the ‘bog’ happens and continues to move toward zero as you advance the throttle even though the rpm drops, you have a mixture problem or the spark cannot jump the spark plug gap at the higher cylinder pressure.
Let us know what you find when you do the vacuum gauge test.
The wire from the coil to my points fell off(maybe the problem?) when I when taking it apart. I’m on my way to the store to get a Pertronix electronic ignition conversion and a new coil. Hopefully that will put an end to this. I will post results.
Metal249- That was the kit I installed, but make absolutely sure you have the required voltage going to it or it won’t work properly. I ran into trouble on the boat I worked on because I thought I would just be swapping the points for the ignition upgrade, but the old wiring wasn’t providing enough voltage for the new electronic ignition. The result was that out on the water the boat worked alright for 10 minutes then bogged down and started acting funny.
Measure voltage at the battery and then confirm if you lose voltage by the time you get to the points. If you lose to much-run the electronic ignition directly off the battery. Just remember to install a toggle switch if you do this though to turn off the electronic ignition when the boat is not running- otherwise you’ll damage the electronic ignition if you leave the power running to it! Hope this helps.
Does it FEEL like the engine starts mis-firing or does it just bog down smoothly? If mis-fire is suspected, change the plug WIRES. If a smooth bog-down, check fuel flow. You might try setting the timing to produce maximum RPM at full throttle…And 17" of prop pitch is a lot of pitch for your boat…
I put on new coil, pertronix electronic igniton, and new plug wires. I checked TDC on the number one cylinder and the mark is correct. Runs much better. I set the timing and tuned it up (5 degress above TDC, idle at 550rpm), but when I give it a little rev, it stays at 2000rpm for a few seconds, before slowly dropping down to about 1000 rpm. I recheck the timing and it jumped up to 20 degrees above TDC. I also put a fuel pressure gauge on it. It reads 3.5 psi at 1000rpm and 3.5 to 4 psi at 2000rpm. Is my pump bad? Also, the recommended pitch is 19 or 21.
Take the distributer cap off and with your hands, first rotate the engine forward while you observe the rotor turn forward also. Then rotate the engine backwards and see how far you can turn the front pulley (balancer) before the distributor rotor begins to turn backwards too. This will give you some idea of the slop in the timing chain…Any more than 10 degrees is too much slop and some will argue that 10 is too much…
Try this. With the boat out on the water, set the distributor timing to obtain maximum RPM. Then, back at the dock, check the timing and see where you have it set… And yes, a vacuum gauge WILL detect jumped valve timing. The readings will be very low. A normal idle should produce 14-15" of vacuum. If the best you can get is 8-9", the timing chain/gear has jumped time.
I wonder about the issue that you have to advance the spark nearly 30 degrees over spec, even with mechanical advance working. I think the problem is valve timing. This is a pushrod valvetrain right?
Here’s a quick check:
First, pull the valve cover. Turn the crank to TDC with the distributor rotor pointed AWAY from plug #1. This is the end of the exhaust stroke and the beginning of the intake stroke for cylinder #1. The intake and exhaust valve rockers for #1 should be slightly (depending on valve overlap) and equally depressed. If one is pushed down further than the other the valve timing is probably off. I’ve seen over advanced cam timing give the symptoms you see. In that case the intake valve rocker would be further down than the exhaust. Time to uncover the timing gears and look at the marks on the cam gear.