1989 4.3 V6 RPM's falling off

[achris]

Have you confirmed you have the correct ignition module? It should have "V6-14" on it. Then confirm it is advancing the timing correctly. Then move back to the fuel system. Either buy a new fuel line (http://www.iboats.com/FUEL-LINE/dm/view_id.1629567 don't forget to mention you're a forum member, you get a discount. ) to cut and add a fuel pressure gauge or some bits to add an outboard type primer to the line before the pump.

 

[jasoutside]

Have you confirmed you have the correct ignition module? It should have "V6-14" on it. Then confirm it is advancing the timing correctly. Then move back to the fuel system. Either buy a new fuel line (http://www.iboats.com/FUEL-LINE/dm/view_id.1629567 don't forget to mention you're a forum member, you get a discount. ) to cut and add a fuel pressure gauge or some bits to add an outboard type primer to the line before the pump.

I think we may have been posting at the same tim there Chris.

Yup, V6-14 TB IV

And yup, my fuel pressure gauge should be here tomorrow and I'll install that to see if anything shows there.

Thanks for chipping in buddy.

 

[achris]

...

For the run tonight, same as the others, at about 4400 it starts to miss/stumble/fall. I did notice a knock of sorts. I don't know if that is a legitimate knock as if something is loose and it wants to break. Or, is it the engine wanting to stall out and then knocks back to life. Either way, it's concerning.

That sort of knock can also be caused by a very lean fuel mixture taking too long to ignite.

Are you able to have someone else out on the boat with you? If so, have them drive. When the engine starts to fall off, carefully start closing the top of the carb with your hand. Be careful, that engine will be pulling a lot of air in and it will try to do the same with your hand.

Yes, we we're posting at the same time. Sorry about that.

Chris....

 

[jasoutside]

Have somebody else drive my boat???!!!

Oh wow, you are pushing me out of my comfort zone here Chris! You know I built this boat right, I didn't buy it:ambivalence::ambivalence::ambivalence:

OK, yah, I can do that. I have an old friend I can work with next week.

 

[NHGuy]

I feel for you about somebody driving my boat. We need the t shirt "built not bought".

But to go back a step. I really think you ought to get a cheapo timing light at Harborfreight, get the 2nd cheapest one because it has the adjustable advance knob. And check the timing. If it's too far advanced it can cause high rpm knock, which is ruinous.

To check timing with TB IV connect the timing pickup sensor around the #1 plug wire with the arrow pointed toward the spark plug, connect the light's clamps to 12volts, run the engine til it's warm enough to be off choke idling, and shoot at the mark on the harmonic balancer/damper while moving the adjustable knob to get the mark in view. That's the current timing. If it's off here's how to set it.

With a 9/16 combination wrench loosen the bolt under the distributor then softly snug it back up. You will loosen it again shortly so you want to be able to do that without much drama.
Now restart the engine, loosen the bolt a little, shoot the light at your mark while confirming the advance knob is still at 8 degrees or whatever you are using. Now rotate the distributor to the right attitude and snug it.

This can and should be done in the driveway on muffs.

The reason I kept repeating to confirm the adjusting knob position is cause mine turns really easily and I have knocked it out of position before.

If it's too advanced it could be what's going on. but with this issue there are no guarantees.

And be glad for the rich looking spark plugs, rich fuel can save your bacon when on the edge of detonation.

 

[Patfromny]

I agree with NH guy on the slightly rich plugs. They are fine. Rather slightly rich then lean.

 

[jasoutside]

AOK guys, I just spent a week at South Manistique Lake with the family and had a blast. The V6 ran perfect all week. Cold start, warm start, tubing, fishing......ran great without a single issue. Of course I kept it under 4K for the whole time except for one 5 minute run to check that fuel pressure gauge.

So, fuel pressure started at about 6.5# at idle and as I increased throttle made it's way to 5# by the time I hit about 4200 RPM's....but never dropped below 5#. So, non issue right?

I didn't end up trying to choke it at full throttle.

NH, thanks for the info on timing. That stuff seems out of my knowledge base and out of my league. Plus the marine techs checked that and they were certain it was good. I don't know if I should be trusting that or not really. I suppose after a bit of study I could figure it out like most everything else I've figured so far eh.

Another fella suggested a exhaust back pressure issue possibly?

Man, I do feel like I am chasing my tail on this engine....

 

[Bondo]

AOK guys, I just spent a week at South Manistique Lake with the family and had a blast. The V6 ran perfect all week. Cold start, warm start, tubing, fishing......ran great without a single issue.

Ayuh,.... Sure am Glad to hear that,....

5 psi is plenty,....

Another fella suggested a exhaust back pressure issue possibly?

If the motor was ever ran without water flow, the shutters coulda gotten burnt up,...
The pieces tend to fall to the bottom of the y-pipe,....
Sometimes they're blown out the prop, many times they get jammed up 'tween the y-pipe, 'n prop,....

 

[GA_Boater]

Ayuh,.... Sure am Glad to hear that,....

5 psi is plenty,....


If the motor was ever ran without water flow, the shutters coulda gotten burnt up,...
The pieces tend to fall to the bottom of the y-pipe,....
Sometimes they're blown out the prop, many times they get jammed up 'tween the y-pipe, 'n prop,....

That did happen. http://forums.iboats.com/forum/boat...raft-islander-rebuild?p=10226501#post10226501

 

[jasoutside]

Well for cripes sake. For cryen out loud. I didn't even think about those shutters.

Sheesh.

Well, couple things...

The engine was only run for a few minutes. It was certainly enough to burn up my pump (which was fun) and enough get the engine warm, didn't peg the thermostat though. Would that be enough to burn up the shutters?

Now, the only way to figure out whether the shutters are there or burned up is to pull that engine huh?

 

[Rick Stephens]

Don't need to pull motor - usually, s'long as you have clearance. Take the exhaust elbow off and then you can pull the lower rubber coupler off the Y pipe and the shutters are right there. Be careful, if there is anything there they can drop into the Y pipe and cause all kinds of grief getting them back out. You will need Manifold to Elbow gaskets - get the OEM Mercruiser ones.

Rick

 

[Bondo]

Now, the only way to figure out whether the shutters are there or burned up is to pull that engine huh?

Naw,..... Loosen all the hose clamps on the risers, elbows, 'n y-pipe,....
With a hose pick, loosen the rubber boots, 'n slide 'em up, or down, 'n pull the elbow outa the way to see the shutters are missin' in the top of the y-pipe,....

It ain't 'bout how hot the motor got, without water flow, the exhaust gases run 'bout 1200? anyways,....
The shutters burn up, 'n the inside of the rubber boots get singed,....

Pull the drive, ya might find the bit's, 'n pieces in the exhaust bellows or in the drive,....

 

[pckeen]

Really glad to hear you had a good week. You deserve it after all that.

 

[dmoriarty51]

I kinda doubt this is exhaust back pressure, it should be lethargic gaining rpm. Also correct me if I am wrong as I have honestly worked on one I/O but I believe Jas has the exhaust tube on this boat not a full bellows. As such shouldn't it have just blown the burned up bits out into the atmosphere?

I would be interested in seeing him try a different ignition module in that distributor, I would also be interested in what the alternator is putting out. Standard GM modules get weird when they start to fail on the road, high rpm issues are one of the early signs. They are also often picky about having enough power to perform properly.

It may not hurt to verify RPM with a different tach, looking at the thought of a rev limiter coming into play. Do they have one built into the module?

 

[third times the charm]

I'm going to say it's in the ignition system. If you have verified the plug type, and that your wires and coil is in good shape, then it may be the module dropping out on ya. 5 psi is plenty on the mechanical pump and carb setup. I didn't check and see, is the carb new or rebuilt?( can't remember what you said) you will get Jas, you have splashed her......that's the first hurdle, lol. Good luck.

 

[NHGuy]

Jas, does the voltage drop during your stumble? Take a look at the volt guage when she falters.

 

[jasoutside]

Whelp, not the exhaust shutters, they were right where they were supposed to be...

I do believe I have a solution though.....

 

[pckeen]

Do tell.....

 

[achris]

Does it involves a tin of petrol and a box of matches? :facepalm:

 

[GA_Boater]

Does it involves a tin of petrol and a box of matches? :facepalm:

Don't forget the paid up boat insurance policy. LOL