1989 4.3l OMC Cobra - No top end power

[msdegraaf]

Purchased the boat last year and found that when you punched to the throttle it would want to stall but you could work it through and get speed. Started looking into the problem and found the the dwell on the points was way off and the timing was advanced 22 degrees. The first thought was that the timing chain had jumped a tooth or maybe the carburator. Took the engine out and checked everything out - everything looked good. Rebilt the carburator. Things ended up being electrical - new cap/rotor/points/resistance wire was only at 6.3 volts and replaced that. Coil is putting out 40000. Distributer bearings were replaced because they were worn out. New ignition wires/plugs/condenser/starter as well as all the usual startup items like fuel filter, oil. The engine starts and idles great - punch it and it takes off but it doesn't go past mid range - 25mph. Running out of options which might be causing this problem.

 

[Lou C]

Re: 1989 4.3l OMC Cobra - No top end power

Check for fuel system restrictions, anti siphon valve on the gas tank and the pick up tube in the tank. The anti siphon on mine was corroded and the engine would do the same as yours, start and idle fine, it would rev fine in neutral, but put a load on it and it had no power. New anti siphon fixed that.

 

[msdegraaf]

Re: 1989 4.3l OMC Cobra - No top end power

Thanks for the advice - took the AS valve right out the tank - still no difference. When we tested the fuel pump volume we got 1.5 cups for 1 min of running - the OMC manual indicates that it should be 2.5 cups - Another question - this is a two BBL carb so the high rpm jets are not adjustable. Any ideas what the OEM jet size should be? Wondering if the jets have been replaced with the incorrect ones. As well could the fuel pump need to have a pump kit installed to increase the fuel volume?

 

[danond]

Re: 1989 4.3l OMC Cobra - No top end power

Check to see if your exhaust flappers are still where they're supposed to be. You might be dealing with a significant restriction as when they fail they block the exhaust at the point where the Y comes together.

 

[MikDee]

Re: 1989 4.3l OMC Cobra - No top end power

Is your fuel tank vent clear? By the way, The resistance wire is supposed to be less then 12volts! Thats why its there, or you'll fry your points in no time

 

[freak007]

Re: 1989 4.3l OMC Cobra - No top end power

If your fuel pump is only working at 60% of its required capacity you need to find out why... look for restrictions in the fuel system... you need to get that fixed before trying to troubleshoot anything else

Rubber lines sometimes deteriorate and look good from the outside, but are collapsing on the inside... I would replace all the rubber lines (fairly cheap and many consider this "normal" maintenance)

Check your metal lines for any kinks... any holes BEFORE the fuel pump will result in the fuel aerating and reduce your pumps efficiency...

make sure your fuel filter isn't installed backwards (it happens fairly often on inline filters)...


If everything above checks good you might need a new fuel pump

 

[msdegraaf]

Re: 1989 4.3l OMC Cobra - No top end power

Is your fuel tank vent clear? By the way, The resistance wire is supposed to be less then 12volts! Thats why its there, or you'll fry your points in no time

Yes it is supposed to be less than 12 volts but not less than 9 volts. I ended up putting 1.3 ohm resister in and new wire to replace the resister wire. Didn't feel like spending 1700.00 for a new wiring harness.

 

[MikDee]

Re: 1989 4.3l OMC Cobra - No top end power

Yes it is supposed to be less than 12 volts but not less than 9 volts. I ended up putting 1.3 ohm resister in and new wire to replace the resister wire. Didn't feel like spending 1700.00 for a new wiring harness.

I don't know where you got your information from? As I recall it should be around 6volts, but do you really think it's that critical, that you'd have to replace it?

Anyway, is your centrifugal advance (under the breaker plate) working correctly? can you turn the rotor some by hand?

 

[Maclin]

Re: 1989 4.3l OMC Cobra - No top end power

Yes it is supposed to be less than 12 volts but not less than 9 volts. I ended up putting 1.3 ohm resister in and new wire to replace the resister wire. Didn't feel like spending 1700.00 for a new wiring harness.

The reading at the positive terminal on the coil should be about 9 volts. I am very sure you should not need both the resistance wire and a resistor. The resistor wire alone should be all the ballast resistance needed. Is the 1.3 ohm resistor a stock component or something you added in?

The problem of not getting past 2,500 rpm also sounds a lot like a points/dwell problem. Are you sure the dwell is correct now? Can you run it on the trailer with muffs and get a dwell meter on it and put it up to 2,500-3000 briefly and watch the dwell to make sure it does not fluctuate thru the rpm ranges?

It also could be the fuel pump. If your test is valid then it is not at capacity and could be limiting the rpm's. If it is a fuel delivery issue then I think it would be surging though and not just get to a certain rpm and stay there acting like it is governed.

 

[Maclin]

Re: 1989 4.3l OMC Cobra - No top end power

I reread your last post and realized that you could have meant that you got rid of the resistance wire and used standard wire in it's place.

I think verifying the distributor advance is good advice, see if the rotor moves some and then snaps back as MikDee mentioned...