OMC Cobra 4.3

[khickey446]

I have a 1988 starcraft with an omc cobra 4.3 that I purchased last year. It starts fine, idles fine and runs fine up to about 2800 rpm when it starts running rough and won't develope power above that point. I have rebuilt the carb (2 bbl), installed new spark plugs, points, rotor, condensor, cap, wires and even a new coil. Timing is set properly and compression is even on all cylinders (180 psi). Idle mixture was set with the best vacuum I could get at about 15". I don't know if that's normal for this engine but there are no apparent leaks. Fuel pump pressure and volume check ok. I also jumped the ballast wire with 12v directly to the coil - no help. I'm grasping at straws now. Could the electronic shift module be causing this problem? I need some help.

 

[Boatin Bob]

Re: OMC Cobra 4.3

It wouldn't be your ESA because you would notice that at the lower RPM's as well, since you have points I'm just wondering if your timing advance (the weights and springs in the base of your distributor) are functioning properly, they may be advancing only up to a certain rpm, they can get rusty and start to seize up, it's worth a look.

 

[a.d.mobile]

Re: OMC Cobra 4.3

What prop is on this thing how long and weight????????

 

[khickey446]

Re: OMC Cobra 4.3

To Boatin Bob - the distributor weights were one of the things I've already checked. They appear to be free and when you increase power, a timing light shows the ignition has advanced. But as you say, it might not be advancing are far as it should. It's worth another look. To A.D. Mobile - I installed a new Michigan Wheel prop (14 1/2 x 19) on the boat last year. This is the same size that was on it. Also please note, this rough engine problem occurs both when the drive is engaged or disengaged when I'm just running the engine at the dock.

 

[akriverrat]

Re: OMC Cobra 4.3

you don't say what carb. choke sticking, main jets plugged, plugged carb filter, water in gas, air leaks on connections between tank and pump, plugged tank vent, sticky anti-syphon valve, dirty flame arrestor, valves adjusted too tight. not getting rpms in neutral would lead me torward restriction in fuel or air first. i ran a 4.3L for several years with 6 degrees total advance and still got nearly normal wot rpm in my jet. also i have had 2 different engines with points that would float and restrict top rpm. one from too wide a gap the other weak spring on new set of points.

 

[khickey446]

Re: OMC Cobra 4.3

To Akriverrat - The carb is a Rochester 2bbl which I just rebuilt. All the jets and passages were cleaned as well as the venturi. A new carb filter was installed and the flame arrector was cleaned. At idle, the fuel pump pressure tested 6.5 psi and the flow was well within limits. I haven't checked the anti-syphon valve yet because I'm not sure extactly where it is. I checked my fuel line and I don't have an in-line one. I'm assuming the valve is in the fuel line outlet fitting itself on top of tank. When I pulled the plugs yesterday to change them, they appeared to running rich(black & sooty). As I said, I've adjusted the idle mixture, but there is no high speed adjustment on this simple carb.

 

[Boatin Bob]

Re: OMC Cobra 4.3

Your initial post said it started and idle'd fine up to 2800 RPM, now you say it runs rough at the dock even in neutral, based on that then I would check out the ESA, when in neutral you probably should be around 600-700 rpm, momentarily lift up the ESA interupt switch (the one with the metal arm attached) and see if the RPM drops a few hundred, if it does then it should speed up again when you release it, if it doesn't then it could already be activated or not working at all.

 

[khickey446]

Re: OMC Cobra 4.3

To Boatin Bob - I'm sorry if you mis-understood me but the engine does idle and run fine up to about 2800 rpm. When I'm running it at the dock, I've disengaged the shift linkage by pulling out the power lever (just as you do when you first start the engine)and accelerate it that way. I've just spent the last couple of hours on the lake tinkering with the boat. I put a timing light to each spark plug (one at a time)while the engine is running rough but none were mis-firing. As I said before, I can only get about 15" of vacuum at idle. It increases up to a steady 20" as I increase power but falls off to around 10" when the roughness starts. I'm leaning to fact that I have some sort of vacuum leak but I don't see anything apparent.

 

[mvaughn]

Re: OMC Cobra 4.3

Sounds like it could be too much total advance. Does your timing light have an advance dial? Where is the timing set at idle?

 

[khickey446]

Re: OMC Cobra 4.3

To mvaughn - My timing light is old and quite simple - no advance dial. The timing is set at 6 degrees BTDC per the maintenance manual.

 

[kenimpzoom]

Re: OMC Cobra 4.3

I'm grasping at straws here but who knows.<br /><br />Could it be something in the valves?<br /><br />Does the vacuum gauge fluctuate at 2800?<br /><br />Its timing chain sloppy?<br /><br />You can check total advance by making marks on your harmonic balancer. Use white out (correction fluid) to put marks on the balancer, use the timing tab as a guide to see how far apart to make marks.<br /><br />I dont know much about carb, does it have a power valve circuit?<br /><br />Can you rev it in neutral past 2800 rpm?<br /><br />Ken

 

[Lark40]

Re: OMC Cobra 4.3

You may have to borrow or buy more diagnostic tools.<br /><br />A leakdown tester and an advance timing light to start with. <br /><br />Not sure how safe this would be in a boat, but using an unlit propane torch can find vacuum leaks around a carb. The engine will speed up when it sucks in some propane through a leak.

 

[Boatist]

Re: OMC Cobra 4.3

The Hick<br />Most of the time high RPM problems turn out to be not enough fuel. Put a fuel pressure gauge on the engine and take a run on the lake. If pressure drops below 4 Lb may need a fuel pump. Are you sure there is no fuel filter on input to the Carb. Make sure the power valve is working and not stuck. Make sure engine does not have high elevation jets in carb.<br /><br />Too high a pitch prop can cause engine to bog down at high RPMS. At 2800 RPMS what is your speed?? What pitch prop are you running?? How many blades??<br /><br />Next I would change timing to a much later setting maybe 0 degrees and make a run and see if it makes any difference.<br /><br />Valve float can cause a high speed miss but not likely at 2800 rpms. If it were at 4000 rmps be very likely. Is the top of the engine getting good oil flow?? Rocker arms and lifters. Might be a good idea to adjusted and recenter your valve lifters or even leave a half of a turn loose for one run.<br /><br />Spark blow out also possible. Are your plugs gapped wider than .035??<br /><br />When you fix it come back and tell us what it was.

 

[mvaughn]

Re: OMC Cobra 4.3

TheHick,<br />For what it's worth, I have the same engine, same year. Mine runs through the rpm range the best set at 10 degrees BTDC. <br /><br />I could be wrong here, but if it is a fuel delivery problem, at the 2800 rpm i would think the engine would start to run very lean, before the stumbling, causing the carbon to burn off the spark plugs?

 

[QuadDriver]

Re: OMC Cobra 4.3

The ESA shorts pulses to 340 instantaneous RPM. it wont rev up. do you have a working condensor attached to the ign? do yourself a favor, drop $70 and remove the points and condensor for a new pertronix, also ditch the purple resistor wire going to the points, and add a blaster coil. this will eliminate the ign.<br /><br />also, is the choke butterfly opening up all the way?

 

[khickey446]

Re: OMC Cobra 4.3

To mvaughn - I advanced my timing to 10 degrees BTDC and the motor seems to run a little bit better, But this was in driveway with the garden hose hocked up so I didn't want to run the engine long. I'll see how it is on the lake tomorrow. I checked my total advance and it appears to be 20 degrees.<br />To Ed Graffius - I've been thinking of installing that electronic ign adapter but don't I also need to install the new ESA module with the timer in it? Were can I pick up this blaster coil and do you have a part number.

 

[QuadDriver]

Re: OMC Cobra 4.3

Not sure what you mean by a timer.....the ESA circuits simply count pulses and allow only so many per unit time, when active it shunts the pulse to ground thru a resistor so you dont fry the primary circuit - it wont hurt a pertronix (but will play hell on an MSD-6M - I have a special adaptor for that)<br /><br />MSD blaster coil - MSD8203 from summit racing - $35

 

[Lark40]

Re: OMC Cobra 4.3

Don't install an electronic ignition in an OMC Cobra engine without the exact instructions from the manufacturer on the Cobra installation.<br /><br />Pertronix had a diagram on their site showing how to wire some resistors to make their product work with the OMC ESA. Otherwise, the ESA will not work.

 

[Boatin Bob]

Re: OMC Cobra 4.3

If you do go for a Pertronix system go to this page down at the bottom to see their diode fix for the OMC/ESA problem.<br />Ed...there is a newer style OMC module that has a timer so that the ESA will only stumble the ignition for a max of 8 to 9 secs. It actually does away with the overstroke switch right out of the circuit. I don't believe you have to have this to work with Pertronix only the diode fix.

 

[mvaughn]

Re: OMC Cobra 4.3

If you have the extra $, the electronic ignition would be a nice upgrade. For now, double check your total advance, 20 is pretty low, should be about 32 degrees.