Completely Stumped....

[Kurt]

I've posted a couple of times in the forum about my lack of power issues and have tried everything I know to resolve this strange problem. Basically, the engine starts up immediately, idles great and runs pretty good up to about 1400 rpm. In neutral, if the throttle is advanced, I can't get over about 2,100 rpm. After that, additional throttle advance results in bogging and no additional rpm. No backfiring or other strange noises - just bogging. In gear, engine will throttle up to about 1400 rpm max - additional throttle advance makes no difference - just bogging, but never stalls. I've read everything in here on such subjects and my issue seems to defy logic. Here's what I have done to no avail:
1) New carburetor (no time on it yet except tests/adjustment at the dock)
2) New plug wires
3) New cap, rotor -- no avail. Pulled old distributor as I was concerned that I was not getting sufficient advance. Old distributor shaft had about an inch of play up and down. Didn't think that could be good and I thought I had found my smoking gun. Purchased rebuilt distributor and installed (electronic ignition). Timed engine. No change.
4) New wet exhaust (was concerned about backpressure issues and mine was original)
5) Pulled manifold and hot exhaust. No blockage. Carbon everywhere, but, hey, it's exhaust. Looked fine. Replaced exhaust manifold and flange with new gaskets.
6) Pulled plugs that were new approx. 10 hours (or less) of operating time ago. All fouled with lots of carbon evenly across all plugs (all looked just as dirty) - Not good so I replaced all plugs. No change in condition.
**Compression is 120-130 across all cylinders.

I'm at a loss. Any ideas out there. I'm tired of working on the engine and just want to go summer cruising. I'm about to pull the thing and have it completely rebuilt, but I know that's just out of frustration as with high compression values, easy starts and great idle like I have I can't see how the engine could need a rebuild. The engine has been very well maintained and looks showroom clean - no rust or annoying leaks.

I'm about to hire a mechanic to check things out cause I'm out of ideas. The only 2 additional thoughts I have are the following and I would appreciate any input on the likelihood of these causing my symptoms:
1) Possible elevated exhaust backpressure originating from the water lift muffler. Only problem with this idea is that there is great water flow out of the back of the boat - in neutral at about 1800 rpm water shoots about 10 feet behind the boat!
2) Valve clearances - could out of adjustment valve clearances cause these symptoms (fouling plugs and lack of power)? I know nothing about valve clearances or their symptoms. I have owned this engine for 1.5 years and never had issues until a few months ago. I just ignored the power loss symptoms as I usually just go sailing. It seems symptoms started shortly after a relatively long cruise (15+ hours of constant motoring each way), for whatever that's worth.

Thanks in advance for any advice! I'm glad to have new carb, distributor and wet exhaust (not to mention fresh water cooling) for peace of mind, but there are no more parts I can buy that I haven't already installed.

[Jack Crutchfield]

Sounds like a fuel problem to me. Since you have a new and assumed properly adjusted carb, have you replace your fuel filter(s)? Other than that, I would check the fuel pump.

[High Hopes]

What is bogging?

[Kurt]

I initially thought it was a fuel problem also. So, I cleaned the carb. I then bought a new carb because the threaded area of the carb housing where the idle mixture screw sits was totally stripped out. I drained and inspected the Racor filter. I also drained the smaller in-line fuel filter. I didn't change the smaller filter or the Racor as they are only 1 year old and don't look bad and I am getting proper fuel flow when I manually pump fuel through the system and through the carb - fuel looks good. Performed a thorough manual fuel pump check and it is working properly. So, I moved on to think timing/ignition issues. I must be missing something, but hopefully you can see why I am stumped. Since in-line filters are cheap, maybe I should just go ahead and replace that as well.

Steve -- I have become so accustomed to using the term bogging from my days of tinkering around with 2 stroke motorcycles and old muscle cars so it might be hard for me to describe. To me, it's just where additional throttle results in no increase in rpm beyond a certain point and power loss as well - severe bogging can result in decreased rpm as throttle is increased, although I am not experiencing that. A definite sound and feel to it.

[blackdove]

Bogging Engine

Hi Kurt,
Seems like you have tried everything. I'm certainly no expert in engines, but I know a little about hydrocarbons. If this problem has only been associated with the tank of fuel that you are using (didn't occur before the last fill up), I think you may have adulterated fuel. It may not have water in it, but it may be a combination of things, i.e., varnishing, etc. Before a rebuild or hiring a mechanic, why not try some new fuel in a portable tank and see how that works. Good luck.

Blackdove

[High Hopes]

Thanks for the explanation Kurt.

I don't think it will be a fuel filter as the carburetor bowl will have enough gas to rev up the engine initially. It wouldn't bog down until the bowl emptied.

Probably not bad gas as you can get to 1800 rpm, but if you have filled up recently and things started going bad then, check into it.

The only thing you haven't mentioned is the coil and igniter. Can you measure the dwell on the igniter? Can you check for spark at higher revs (might be a shocking experience)? I'd get a timing light out also, just to make sure the spark is advancing. Should be 17 degrees at 1600 rpm.

Take the air filter off and re-check? Fouled plugs sounds like too rich a mixture.

Just guesses. Good luck.

Steve

[rigspelt]

I'm no expert, but I sympathsize so can't resist putting my oar in.

1. Fuel - This seems most likely. Could it be a "fuel starvation" problem? See the fuel troubleshooting guide chapter 4 in MMI's manual, steps 1 and 2. I happened to read the fuel starvation posts in this forum over the past couple of days, and this seems reminiscent. One had a loose spring in a priming pump bulb. Sounds like you ruled out water in the fuel and the filters. Antisiphon valves can go bad. Fuel line clogged? Fuel pump issues? Sounds like you may have ruled out carb, though some of the posts had some unique solutions there. Perhaps search the Moyer forums using "fuel starvation".
2. Compression - you report good compression, but I don't think that rules out trouble inside the engine that only becomes noticeable under load. Any backfiring at higher loads? Did the motor overheat during that longish cruise when this started? I once met a motor with a cracked head that bogged under load. A mechanic diagnosed it with a simple test.
3. Spark - Doesn't sound electrical, though I guess it's not impossible.
4. Air - Is the engine getting enough air at higher RPMs? Flame arrester clean?
5. Prop shaft - There is a fascinating post in here about someone chasing a problem all over the engine. Turned out to be a thin line wrapped around the prop shaft that wasn't visible on quick initial check (they were looking for rope, I guess).

[Marian Claire]

Bogging

Is the chock cable adjusted properly? Maybe it is restricting the air supply at high rpms. Dan

[Baltimore Sailor]

I have the same problem with my A4: can't get above 1300 rpm or so even after a new carb and coil, full tune-up, hull and prop cleaning, and timing adjustment. However, at that rpm I'm moving close to hull speed for my boat (7.5 mph). I have an Indigo 450 10 X 7.4 3-blade prop, and if I'm getting near-hull speed at those rpms, is it even conceivable that I'd ever get 2000 rpm or more?

Wouldn't the rpms one gets out of the prop be dependent on the theoretical hull speed of the boat and the efficiency of the prop?

[Don Moyer]

I'm going to copy my good friend Tom Stevens on this thread for his comments. Meanwhile, I'm thinking that if an Indigo prop moves any normal sized sail boat to a hull speed of 7.5 MPH (6.5 knots) at 1300 RPM, I'd suspect a tachometer error (like it may be set for the wrong number of cylinders). It also bothers me that you can't get more than 1300 RPM out of your engine. As a practical matter, the fact that your engine can move your boat to hull speed (with any prop) would indicate that it does not have a general power issue, therefore it ought to be able to spin an Indigo prop faster than 1300 RPM and calls into question your tachometer indication.

Don

[Baltimore Sailor]

Good point. It's the original tach, so there may be a problem with it that I hadn't accounted for. I will check out the settings to make sure they're correct. Maybe I can also borrow a tune-up tack from the local auto parts store to double-check the reading.

[Kurt]

Thanks for all the suggestions.
1) I'll first check my timing advance with a timing light again. I did this once, but the roll pin is very hard to see cause the water life muffler is kind of in the way. I'll remove the flywheel cover and make a mark where it should be at 1600 rpm (about 17 degrees I believe).
2) If that checks out, I'll hook up a portable gas supply and see if that makes a difference. I don't think the tank has been removed for at least 10 years, although I don't see how bad gas or junk in the tank could result in timing-like symptoms of reduced rpm/horsepower (engine never hesitates or quits, just stops increasing in rpm with additional throttle) and quickly fouling spark plugs.
3) Prop and flame arrestor are clean - thanks for those suggestions, though. The anecdote of someone having a cracked head which resulted in bogging is an interesting idea. I will be having a mechanic come on down to check out mystery stuff like that if these other things don't help (which I believe is going to happen). Could you have good and even compression with a cracked head - a head cracked enough to cause bogging under load?

[sunnnnseeeker]

A small crack may not be noticable in a compression test but for sure it would leak water. A small crack ,especially if you don't know is there, would probably not effect performance, unless the water is getting into a cylinder or 2.

[hd78half]

Bogging

The fouled plugs after 10 hrs. use indicate a problem with the fuel mixture. Try leaning out the mixture screw and/or changing the main jet size. Don can probably help you with the main jet.

Al

[Kurt]

I think I now have a diagnosis for my mystery problem described in this post! If you look at the start of this thread, you see that I did initially think that my loss of power issues had to do with an exhaust restriction and hence elevated exhaust backpressure. So, I replaced all the original wet exhaust hose and removed both the manifold and hot exhaust for inspection. Manifold look fine - no rust. Hot exhaust, however, with the 90 degree bend is impossible to see through. I just blew through it with my mouth. Yesterday, I finally had a reputable mechanic look at my engine. After checking for good spark (which I had as I replaced my ENTIRE ignition system!), he had me again remove the hot exhaust from the manifold and fire the engine up. Lots of noise and smoke in the cabin, but, low and behold, the engine accelerated explosively like it has not in a while. Apparently, just blowing through the exhaust is not enough as it needs to allow quite a bit more forced air through than just the small amount from my breath. So, I will be replacing the hot exhaust and making it all proper with an anti-siphon valve (which I don't currently have) and all. I'm keeping my fingers crossed, but I'm pretty sure there must have been a restriction somewhere around where the cooling water is introduced into the hot exhaust as if it was some other issue, the engine would perform badly still with the hot exhaust removed. At least I now my ignition is new, including distributor, and carb is new. I'll report back on this issue once the exhaust is replaced to add to the Moyer Marine library of solutions to power loss problems.