1989 4.3 V6 RPM's falling off

[achris]

I think a fuel pressure guage would be a good idea here. Find an old pump to carb hard line, cut a section out (about 3/4") at a convenient place for a gauge fitting. Put the guage in and go for another run. Monitor the guage. See if you are losing fuel pressure at high speed. Another question, carb needles, sprung or solid?

 

[jasoutside]

I don't remember, were there times the engine continued to act badly or was it just for a short time at WOT?

RPM's first drop at WOT range. Then, it's kinda 50/50 at lower RPM's after that. Sometimes it wants to stall out and sometimes (like last night) it runs greatfor the rest of the night at 4000.

If you have spark knock it may show in your spar plugs. Which engine & ignition is yours based from? Thunderbolt IV? TB V?

I'm pretty sure I have Thunderbolt IV

Completely out of left field but do you have power steering?

Yah, I can't think that would be the case. Res is full and I'm not turning when it stumbles.

 

[jasoutside]

I think a fuel pressure guage would be a good idea here. Find an old pump to carb hard line, cut a section out (about 3/4") at a convenient place for a gauge fitting. Put the guage in and go for another run. Monitor the guage. See if you are losing fuel pressure at high speed. Another question, carb needles, sprung or solid?

I don't have a hard line laying around. I might need to try and cobble something together here. Let me see what I can pull together.

Sprung or solid needles? Really sorry, no idea on that one. I have the 4.3 V6 version here: http://www.i-5automotive.com/marine/

 

[NHGuy]

Is it the 4 barrel or 2? And what Chris was asking is if it has a spring assist needle valve or is it only seated by the float?

 

[NHGuy]

Also it's probably time for the fuel line that has a hand bulb. If a pump of that can refire the upper rpms you'd have isolated the problem to fuel.
Sometime when this is happening shoot your coil with an infrared temp sensor gun. I wonder if it's cooking?
Is your ignition rotor snug on the distributor post? And is the shaft snug in it's bushing, no side or vertical play?
Remove all the fuel fittings from the pump to the carb and make sure they are clear and open. Especially the on-carb inlet screen and whatever is just inside of it. Don't use teflon tape on them when refitting, thread seal is ok though.
Your boat looks newly built so don't be insulted if I ask, but is there any water under the engine that can be picked up by spinning parts and thrown around?
Are you getting any lake water in through the sides at wide open? You are probably trucking right along over 50 MPH. And with no doghouse over the engine perhaps it's getting water in the carb.
If enough got on wires or in the carb it could cause "things."
Where is your timing set? 8 degrees BTDC? Is your Thunderbolt IV module the right one? You want it to read V6, It may also say something like V6-14 or V6 22 on it. That would designate 14 degrees advance or 22 degrees total advance. If it came off a V8 it might put too much timing into the spark.

 

[Patfromny]

Having a new carb and then having it gone through a week later as well as them checking the fuel before and after the pump doesn't rule out fuel problems but more than probably, your dealing with a problem elsewhere. I found this diagnostic flow chart from iboats posted by DonS for a thunderbolt 4 problem. This being intermittent it may not help you but I thought it would be good to go through the the steps and save the info. It seems that there are a few people having troubles with this system while under load and it runs great in the driveway. Once again, not being familiar with your set up it is hard to understand where everything is but maybe check an ignition sensor. It sounds like there are several parts there that need to be checked out. Maybe even a neutral saftey switch in shifter? Just giving you some ideas. Hope you find this problem soon bud.

 

[jasoutside]

Is it the 4 barrel or 2? And what Chris was asking is if it has a spring assist needle valve or is it only seated by the float?

This is a 2 bbl. Really sorry, I still don't know that 2nd part. Sorry I'm so dum!

Also it's probably time for the fuel line that has a hand bulb. If a pump of that can refire the upper rpms you'd have isolated the problem to fuel.
Sometime when this is happening shoot your coil with an infrared temp sensor gun. I wonder if it's cooking?
Is your ignition rotor snug on the distributor post? And is the shaft snug in it's bushing, no side or vertical play?
Remove all the fuel fittings from the pump to the carb and make sure they are clear and open. Especially the on-carb inlet screen and whatever is just inside of it. Don't use teflon tape on them when refitting, thread seal is ok though.
Your boat looks newly built so don't be insulted if I ask, but is there any water under the engine that can be picked up by spinning parts and thrown around?
Are you getting any lake water in through the sides at wide open? You are probably trucking right along over 50 MPH. And with no doghouse over the engine perhaps it's getting water in the carb.
If enough got on wires or in the carb it could cause "things."
Where is your timing set? 8 degrees BTDC? Is your Thunderbolt IV module the right one? You want it to read V6, It may also say something like V6-14 or V6 22 on it. That would designate 14 degrees advance or 22 degrees total advance. If it came off a V8 it might put too much timing into the spark.

Allrighty, let's see if I can get all your q's here...

I'll run the gauge in between the pump and carb first I suppose. Then go from there.

Swapping out the coil was suggested, I don't mind putting a new one on to eliminate that.

I'm pretty sure the distributer is all good but I'll double check that.

Everything is open on the fuel line between the pump and carb, no screen as I have a spin on fuel filter.

Nope, no water in the bilge, no water spraying in. I'm only going about 35 mph.

I don't know how to set timing but the marine tech did check it to be good.

TB IV is original to the engine so that hasn't been changed out for another.

Having a new carb and then having it gone through a week later as well as them checking the fuel before and after the pump doesn't rule out fuel problems but more than probably, your dealing with a problem elsewhere. I found this diagnostic flow chart from iboats posted by DonS for a thunderbolt 4 problem. This being intermittent it may not help you but I thought it would be good to go through the the steps and save the info. It seems that there are a few people having troubles with this system while under load and it runs great in the driveway. Once again, not being familiar with your set up it is hard to understand where everything is but maybe check an ignition sensor. It sounds like there are several parts there that need to be checked out. Maybe even a neutral saftey switch in shifter? Just giving you some ideas. Hope you find this problem soon bud.

I'll need to digest this flow chart a bit. Hmmmm......

 

[jasoutside]

OK, here's your spark plug read...

And here's how compression came out:

#1 135

#2 131

#3 135

#4 140

#5 133

#6 136

 

[zdeyonker]

Those compression reading are all within 15%, so they should be good. Is it just the angle, or does cylinder 3 appear to be running leaner than the rest? I also prefer running Delco's in all my GM's. They tend to last longer and put out a better spark. I run NGK's in my lawn mower... You've also got a varied set of gaps there, what did you gap them to? Do you have any pictures of your distributor and cap? Check for carbon tracking to make sure spark is staying at the right cylinders.

 

[Watermann]

Healthy motor by the looks of it. :thumb:

The new carb that was leaking gas, makes me wonder. Have you thought about putting the old one back on and giving it a run?

 

[pckeen]

At this point, I'm thinking Father Damien Karras might be your best bet.

 

[Patfromny]

The plugs look ok. Maybe a tad bit rich but not enough to cause a problem. Nothing to worry about. If they were wet, I'd be concerned. The gaps do look different but is really hard to say because of the different angles you held each plug in the separate pictures. Some of the ground electrodes seem to be at a 90 degree angle and some seem to be bent down more. I would check the gaps. If some are too wide and your coil is weak the spark may have trouble jumping across the large gap. If the gaps are all correct, you might try making each gap a bit tighter than factory specs. This too could help out a weak coil. Good luck.

 

[achris]

When you rebuilt the engine, did you put new lifters in? If so, did you buy genuine or aftermarket (automotive)? If aftermarket, we're they specifically 'anti pump up'. How far have you set the rockers? 1 turn down from zero lash? How did you determine zero lash? Turning the push rod or lifting it?

Chris.....

 

[Watermann]

The motor was a complete donor relieved from another boat that was brutalized after being gutted. I think it's about 300 pages back in his rebuild thread. :lol:

 

[jasoutside]

Healthy motor by the looks of it. :thumb:

The new carb that was leaking gas, makes me wonder. Have you thought about putting the old one back on and giving it a run?

Nope, I think I may have seen a crack in the internal housing on that old carb. I didn't look at it close though.

But, I have been thinking more about this new carb though. Once I get the fuel pressure gauge on, and so long as it gives good readings, I may need to turn my attention to that again.

At this point, I'm thinking Father Damien Karras might be your best bet.

Sheesh, for cryen out loud, it sure seems like I've done about everything else eh!

The plugs look ok. Maybe a tad bit rich but not enough to cause a problem. Nothing to worry about. If they were wet, I'd be concerned. The gaps do look different but is really hard to say because of the different angles you held each plug in the separate pictures. Some of the ground electrodes seem to be at a 90 degree angle and some seem to be bent down more. I would check the gaps. If some are too wide and your coil is weak the spark may have trouble jumping across the large gap. If the gaps are all correct, you might try making each gap a bit tighter than factory specs. This too could help out a weak coil. Good luck.

I haven't checked the gaps on these. I can do that soon.

The new carb was running very rich until I adjusted it correctly.

I did pick up and install a new coil today. We'll see what happens on the next run though I'm not holding out high hopes that will do the trick.

When you rebuilt the engine, did you put new lifters in? If so, did you buy genuine or aftermarket (automotive)? If aftermarket, we're they specifically 'anti pump up'. How far have you set the rockers? 1 turn down from zero lash? How did you determine zero lash? Turning the push rod or lifting it?

Chris.....

Oh gosh, I didn't rebuild this engine. This came out of a donor boat. Thumper the Baja.

 

[Rick Stephens]

The new carb was running very rich until I adjusted it correctly.

Adjusting a carb. The idle mixture screws set the mixture at idle only. Once the throttle plates open a little bit the idle mixture screws no longer effect anything. The only two simple things you can do to adjust richness throughout the range of airflows through your carb is to check the float level and set to specification, and set the travel on the throttle pump. ALL other adjustments are done by changing jets, needles, springs, linkages and ports.

I mention this just so you know that changing your carburetor from the factory one to a rebuilt can be a HUGE chore. It is often easier to buy a new carb and follow the process to adjust it from scratch than to assume a given rebuilt carburetor is going to come from a rebuilder set to work on your 4.3L and not a 5.0L or 5.7L motor. And just the 4.3L has a very wide range of possible calibrations based on which heads, intake and cam are installed. You have absolutely no idea what engine a given carburetor came off of and it may require a lot of adjustment to tune to your engine.

Rick

 

[Patfromny]

Check the gaps for sure. Never believe that plugs come pregapped. If they get dropped during shipping or during stocking they can be pushed closed or close to it. With the new coil, just gap them to factory specs.

 

[jasoutside]

Another lake test tonight.....

Coil? Nope, not the coil.

Just on a whim I checked the air cleaner too. Nope, not the air cleaner.

My patience is quite long, but it's starting to grow thin.

 

[Watermann]

Don't feel bad about the coil, every home mechanic I know has a perfectly good used coil on a shelf, including me... :lol:

 

[jasoutside]

Yah, kinda figured it wouldn't be the coil, but, now I know for sure. Oh well.

Whelp, my fuel pressure gauge was supposed to be in today, didn't happen, probably tomorrow. We will get that on and see what happens there.

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.