Problem with alternate engine cut off at high rpms

[martines]

Hi everyone,

new boat owner of old bayliner capri cuddy 1999 with mercruiser 4.3l v6 (0L329748) with 2 barrel carb (807764); registrations says 205 HP, looking for help where after the first few hours problems had started:

-the engine has a little rough idle
-revs ok (on the move) up to nearly 5000 rpm with the new prop (4 blades, 19'' pitch)
- however at high rpm over 3-4k, the engine is kind of cutting out and stalling for few seconds only, the rpms are going down to 1,5-2k, boat gets off plane and then recovers by itself and gets back to high rpm and on plane for 10-15 seconds, and this keeps going on intervals like running fine 10-15 seconds/ cutting out for 4-5 seconds. Below 3k rpm it rarely happens, but it does. (often the tacho is going wild - up too high or down to zero. Same with the fuel gauge - if it might be connected with the issue)

what is done:
- new battery
- new manifolds (original were 1 piece ones and left bank hydrolocked, now new types with risers are installed)
-cylinders copression test done - all between 10-11.5 bars
-new oil
-new spark plugs
-new fuel filter
-cap and rotor seems in very good shape
- syphon valve checked - was clean and functional
-screen on the carb was clean, no junk
-tank seems clean
-tank vent pipe is ok
-electrical fuel pump is pumping ok into external bucket (though the flow/volume seems not too much, but stable)
-oil level switch is jumped over

-during running and exposing of the above problem, the voltmeter to the fuel pump still shows stable 11.4 V
-looking directly into carb chambers the right one seems to give more mist than the left one / when problem occurs seems fuel is reducing in both, but not sure as it migh be effect of temporary reduced revs or which comes first?...)

-I just removed the carb for to check and clean it at least and will report later.(if needed, a rebuild kit will be ordered, but deliveries to Bulgaria are taking 4-6 weeks at least at double cost than US )

There is no even decent car service in the area, not to mention boat or merc mehcanic and I'm forced to do the things myself, on top of the slow and expensive deliveries I'd like to find the root cause without trowing bunch of new spares where not needed....

So, I'm looking for good advice where and how to proceed.

Thanks in advance for any help

Best regards
Martin

 

[alldodge]

:welcome: to iboats

Remove the gray wire off the coil. The gray wire goes to the tach and the tach may be causing the issue with cutting out.

Get a vacuum fuel pressure gauge and Tee it in between the fuel pump and carb. Fuel pressure should be between 4 to 7 psi.

If you have good pressure and the coil wire removal did help, then put some clear tubing inline between fuel pump and carb. With it running check for air

 

[martines]

Thanks for the welcome and the reply ALLDodge ,

here is the progress from yesterday:

1. I disconect the tacho from the connector behind the gauge

2. Not having t- piece, I connected the fuel pump directly to a pressure gauge (know that it is not fully correct, as there could be good pressure and still low flow) and it shows round 5.5 psi.

3.after cleaning the carburetor (it looked quite clean, but I soaked it with IPA, then wd40, then carb cleaner, then blew all jets and nozzles with compressed air) and the idle revs gets significantly better, scared to say perfect. Then try it out at WOT and the problem was still there, but the engine sounds better and the drop outs get shorter in timing only 2-3 seconds, and in sequence - only 1-2 times per minute.(still drops with some thousands rpm)

4. Cleaned most electrical connections in the engine bay. Then I got to the fuseboard under the dashboard and I found that the 20 AMP fuse was very loose - nearly falling out of place. I tightened it up and gave a short try at WOT for like 5 minutes (storm was coming) - The problem still appear 2 times in a how to say a much lighter way: the rpms were still temporary dropping down with duration 3-4 seconds BUT with only 2-3-400 rpm (not with thousands as before). With the connected tacho the drop outs were so minor it could barely catch them, but I hear them.

Is it possible that the problem is with the advance timing of the ignition? When the cut offs/rev drops appear, it sounds as it is not fuel strvation any more, but changes in the angle of firing (not sure if these are the correct english terms). I know the thunderbolt has the feature to autoadjust the timing, but I have not yet researched it how it works. Why would the thunderbolt try to adjust the angle when running on stable 4k rpms for example ?


Thaks again for your help

Best regards
​Martin

 

[Fun Times]

Hi there! If at one point you were able to reach almost 5000 RPM at full throttle without cutout then your Rev limiter doesn't sound like it's going to be a problem at 3-4000 RPM.

On your next test, you may want to connect a timing light to the coil wire and check to see if you are getting a continuous spark during the surging to help narrow down what is happing whether it be fuel or spark related.

 

[martines]

Hi guys,

Long story short:
I mentioned new fuel filter/separator (OEM Quicksilver one) was changed in March and it had only like 4 hours of operation till problem appear.
Yesterday in despair, I just bypassed it and VOILA - engine revs up and run couple of hours at WOT perfectly stable, no sign of stalling . (for to make sure this was the reason I set the filter in and offline 3 times and when was inline the problem showed up immediately). So the moral is dont trust even in new spares

And next 2 issues:

1. Mentioned that I took off and cleaned the carb. Now the air/fuel mixture seems rich, as the spark plugs are coal black, so how I can adjust it? My carb looks like this and the only setting option I could see is this jet...:

2.With the brand new manifolds, I've noticed that the starboard one is much warmer than the port one. (I can keep my hands on -no problem, so I assume the hot one is not more than 50-60 celcius, the port one is nearly cold.....) . Disconnected both water pipes (coming from the thermostat housing) that are feeding the manifolds and let them in the bildge - the port one had large volumes of water coming out. The starboard one very little flow. I ha™ve opened the thermostat housing- all seems fine and both channels are interconnected/ pipes are clean so I see no reason for different flow. (if I squeeze or tap the port one, the complete volume goes through the other)
Next thought was that could be some restriction in the Y-exhaust pipe- took off the starboard riser and dig in with a wire, but nothing; I poured a bucket of water in and seems there are no restrictions. So is it a problem at all (what concerns me is that I changed both manifolds due to failed starboard one (is it possible it had burnt out due to higher temperature? Or was just coincidence as the old were the original 1 piece manifolds- 15 years old)


Thanks for your help and this website which is a great source for information.

Best regards
Martin

 

[NHGuy]

Good going, you are making progress.

Yes, the engine has spark advance management, when it senses knock it reduces timing advance. That's not very easy to feel though. Others with similar boats may be able to tell you if they have experienced it as you did.

Yes, the mixture screw will need adjusting. Get your engine idling, and fully warm. Turn the mixture down, clockwise, til the engine stumbles. That's called lean mixture, it's when there's not enough fuel. Then back it out til it runs poorly. That's too rich a mixture.
Then find the place between where it feels smooth and still accelerates from idle well. That's supposed to be halfway, but on many carbs it's actually closer to the clockwise (lean) stumble setting.
If you have a vacuum gauge you can also adjust the mix for maximum vacuum, both ways work.

Yes, it's normal for exhaust manifolds to have different temperatures at the dock or while idling. When underway there's a much greater flow and pressure. Things equalize well when you get moving. If either manifold is normal the engine has water available to the thermostat.

 

[alldodge]

As NHGuy mentioned the exhaust will always be different temps, it's just the nature of hydraulics in that water flows easier to the port then the starboard side. So far as the rich mixture, your idle jet is just for that, it's for idle only. If you are running rich at idle and not at higher speeds the carbon would be burned off when running. My guess would be the main jets have erroed over time allowing to much fuel to enter when under load, or more likely the float level may need to be lowered.

As for the fuel filter issue, the filter should not cause a restriction check the lines feeding it. Also the filter should be placed between the tank and pump. Also check your anti-siphon valve on the tank

 

[martines]

Hi guys,

I was bothered with the temp difference at risers, so yesterday I've made a temp log. The result is temp differense at the risers of round 15 degrees celcius (60 F). I was scared by handfeeling it and thought it is more, but here is the reading of an expensive thermologger gauge (green versus blue). Seems not that bad.

I bag for advice over the latest problem: the boat can not swith to neutral gear while the engine running, When the lever is to neutral the prop is still engaged. I need to switch the engine off and on again and it gets to neutral. Can this be fixed with adjusting the wires or is it something else ?

 

[alldodge]

Alpha drives use interrupter switches which causes the engine to misfire momentarily. This reduces the load on the drive so it can be shifted into neutral. Below is from The adults only section at the top of this area which explains it at length. What is needed, is to adjust the shift cable so the interrupter switch is in the correct position. Info on how to adjust the shift cable is also in the adults only section, in the Merc section, and Chris's how to videos. If after adjusting the drive remains hard to shift, you then need to look at replacing the lower shift cable or replacing/fixing the interrupter switch.

http://forums.iboats.com/forum/engi...3-how-to-how-the-shift-interrupt-system-works

 

[martines]

Hi guys,

Checked the interrupt switch and it functions for itself, so I started playing with the shift cables where all seems had been fine before I messed up with them, when I got this idea to test:
while the engine was running in neutral, I pressed the switch but nothing happens (it suppose to shut down the engine, isn’t it?).
Followed the wires from the interrupt switch to a (strange purple/white wire - isnt't that one for base time settings??) at the socket of the ECU (or whatever is the name of the electronic module).. here how it looks:



Then with info from the above links just for testing, i made a jump wire from the switch directly to the minus of the coil, and here the system works again and the boat is shifting out of gear not bad .

What bothers me is that the electronic module is not working correct in the way its suppose to do (connecting the negative terminal of coil to ground when interrupt switch is pressed is job of the modulde, right ??...)


Any ideas and suggestions ?

Is theer a way (without stroboscope) to check if the advance timing is still functioning ?

Best regards
Martin

 

[alldodge]

The Pur/Wht wire is used to put the engine in base timing mode for setting ignition timing. The interrupt switch should cause the engine to start to die, and since you pushed the switch and it did not, I suspect your switch is bad or a connection between the switch and module is the issue. Take a jumper and cross the 2 terminals on the switch and see if it works.

Did you use a timing light as Fun Times mentioned on your coil wire to see if you are loosing spark?

 

[martines]

Hi ALLDodge,

The switch is ok and functions when pressed but does not affect the engine. Even shortening the wires - still nothing -the engine keeps running.

One of the switch wires seems to be ground; measure the other one if there is connection to all terminals of the the plug in the ECU and it showed connection to purple/white one, which is strange........as there is connection but the ECU does not act. Is it the right connection (how did it work till week ago and now it doesn't)

Do you know to which colour wire at ECU the interrupt switch should go to ?


Refer timing light I don't have one, but I ordered the needed stuff and I'll build one next week....

 

[alldodge]

Hi ALLDodge,

The switch is ok and functions when pressed but does not affect the engine. Even shortening the wires - still nothing -the engine keeps running.

One of the switch wires seems to be ground; measure the other one if there is connection to all terminals of the the plug in the ECU and it showed connection to purple/white one, which is strange........as there is connection but the ECU does not act. Is it the right connection (how did it work till week ago and now it doesn't)

Do you know to which colour wire at ECU the interrupt switch should go to ?


Refer timing light I don't have one, but I ordered the needed stuff and I'll build one next week....

The switch grounds a circuit in the module which in turns grounds the neg side of the coil. One side of the switch is connected to a Blk wire which goes to ground. The other is connected to a Wht/Grn wire which also goes to the distributer wht/grn from inside the module. This is why when you ground the coil the shifting is easy. Appears to be something inside the ignition module has gone out, and also may be the problem with your missing at higher rpms.

 

[martines]

Hi ALLDodge,

problem is found: next to the switch there was a not connected black/white wire not connected anywhere on this end and tapped with rubber plug, with the other end going into the module. I diconnect the end of the switch so far plugged into the purple/white and stuck it into the black/white one and now the interruption works

Tried to set up the shifting cables (according to Chris how to video) and it get adjusted and fine switching off gears, but not with the mentioned setting of 6'' of the lower cable between the brass bit and the hole. It took a bit more then 7'' to get running properly .. Should I worry about that?

For some other issues I'll open new post as this one gets long.

Gratitudes for your time and help over my newbbie questions

 

[alldodge]

If you did exactly like the video shows and cannot make it work, then you probably need a new lower shift cable.