Problems with WOT

[robfogle]

I have had this boat about 2 months now.<br /><br />It is a: 1998 Bryant 196<br />Length: 19'6"<br />Weight: Dry 2750 Wet 3500 est.<br />Engine: 1998 Mercruiser 4.3L Carb<br />I/O: 1998 Alfa 1 Gen 2, Gear Ratio 1.62<br /><br />The engine starts and runs good, however I’m not getting over 4200 rpm at WOT. I have tried a 14.25x19 and 14.25x21 props. I get the same max RPM with both props. The speed that I’m seeing with these props is 19 pitch – 39mph and 21 pitch – 42mph<br /><br />I have searched this and other boards and the only one that is similar (that I have found) is where they had problems with the anti-knock sensor. Since mine does not have this I’m not sure where to go from here. <br /><br />Here is what I have done so far.<br />Took to Marina for tune-up, They did the normal tune-up stuff. <br /> Compression test all cylinders were min of 152psi and max of 155<br /> They set the timing 10Deg BTDC<br /> Changed all oils<br /> Changed oil filter and fuel water separator<br /> Changed plugs<br /> Checked wires, cap and rotor<br />I verified that the tach will go above 4200 I took it to 5000rpm for a very brief instant.<br />I verified that the tach is accurate with in 100rpm at 4500rpm<br />Ran two different pitch props 19 and 21 no change in WOT rpm is 4200<br /><br />I did notice that there is a high rpm stumble above 4000rpm. Will miss several times as you go over 4000 then seems to run fine . However this is running on muffs not under load. A second issue that might be related is it will diesel when you try to shut it off after being run at a moderate to high speed. I have been told that this is a known issue with the 4.3L.

 

[Mahoney]

Re: Problems with WOT

Did you check your advance timing?

 

[robfogle]

Re: Problems with WOT

I am not sure if the Marina checked the advance timing. What is the easiest way to do this as this is not covered in the maintance manual.<br /><br />Thanks

 

[cslaufer]

Re: Problems with WOT

I agree with Mahoney, sounds like a distributor/module issue

 

[Mahoney]

Re: Problems with WOT

If you know the advance curve of your engine, you can use a timing light with advance timing. For a quick and dirty way, if you don't klnow the advance curve specifically or dont have a timing gun that will shoot advance, just watch the timing mark with a regular gun as you increase RPM. The mark should move fairly quickly up to what you can eyeball to about 30 degrees before TDC at ~3000 rpm<br /><br />If its not moving smoothly up to that degree advance or not holding once it is there, there is definitely an issue.<br /><br />You can loose a lot of power and your engine can run very inneficiently(rich) if you are not getting enough advance. You can also check the plugs too noww that you have run with the problem for a while. A sooty black plug would support this.

 

[Jeff Quigley]

Re: Problems with WOT

I also agree with Mahoney. Sounds like a distributor issue. Save yourself some money and adress this as soon as you can. You should be able to find a mechanic that can help you out. Trailer the boat if you have to and bring a hose and earmuffs. Or, talk to your marine mechanic and let him/her know you "might know the problem".<br /><br />Although not entirely the same, when we were lacking the horses in our race car, the first thing to come out was the a couple of wrenches and the timing light. <br /><br />Keep us posted.<br /><br />Jeff

 

[robfogle]

Re: Problems with WOT

It runs good no problems starting, idling, or accelerating up to 4000-4200 rpm. But under load will not go above that.<br />I have already pulled the plugs and they are clean. No sooty black plugs. I have picked up a set of new plug wires and a new dist cap and rotor. I know that they might not be the problem but it will eliminate these as an issue. Since the boat is new to me and I don't know how old they are it will give me a base point to work from in the future. I will put a timing light on her in the morning and check for the timing advance.<br /><br />I believe that I have the Thunderbolt IV Ignition as the Ignition module is mounted on the exhaust manifold and I cannot find an anti-knock sensor. Is there any way to test the ICM module to see if it is ok?

 

[robfogle]

Re: Problems with WOT

The Timing is set at 10 degrees. I'm seeing 15 degrees of advance from idle to 3000rpm. The timing advance is smooth and stable. I'm still seeing a miss above 4000. If I rev the engine fairly slow it will rev up fine. If I rev it up several times quickly I see the miss above 4000. I'm not sure if this is because there is no load on engine or not. I should be able to put her in the water tomorrow and check the WOT and see if the new parts had any effect. I suspect not.<br /><br />One of the things that is confusing is the type of ignition system I have. The book (SELOC) says that from 1998 to 2001 it should have the Thunderbolt V system. My engine is listed as a 1998 and when I look at the engine the ICM is mounted on the exhaust manifold and there is no knock sensor. This leads me to believe that it has the Thunderbolt IV ignition system. Other things that don't look like the manual are: <br />One piece exhaust manifolds<br />Valve covers are plastic and labeled Vortec<br />Timing markings are on the vibration dampener not on the timing chain cover<br />Timing chain cover looks plastic and only has a notch for the timing mark.