Mercruiser 4.3LX 205hp, Misfires Under Load Won't Accelerate past 2200rpm

[TruDaddy]

Hello All,

I have an issue as stated in the topic: My engine will not accelerate past 2200 rpm, it also begins to misfire when idling under load. The misfire is that obvious intermittent shutter that can be felt. Here is my stats and progress so far:

Boat Specs:
- 1989 4.3LX 205hp Thunderbolt, V6, On muffs and on water boat fires up and idles at dock like a champ. Last year the motor would bog at 2300-2500 then blast through onto plane.

Prop:
- New 4 blade from 3 blade: all specs are the same except extra blade

Fuel Related Work:
- Rebuilt Carb last week made adjustments to specs and double checked to the best of my ability
- Idles at around 700 rpm and 55-60 lbs oil pressure
- Replaced Fuel Pump Filter and Carb Filter (including springs and gaskets)
- Checked fuel for water - Good to Go
- Checked fuel lines for blockage - nothing I could see
- Removed fuel pickup screen is fine

Cooling System Work:
- New Impeller (Last Year)
- Thermostat opens at 143-145 degrees

Electrical (All new this year):
- Cap and Rotors
- Spark Plugs (Carbon fouled when removed, not surprised)
- Checked Wires visually but not electrically. They are newer and in great shape but??
- Batteries Charged and Tested
- Tested coil with key in run - 12v

I obviously need to compression test and timing at this point. I am assuming this is spark

 

[alldodge]

Hello All,

I have an issue as stated in the topic: My engine will not accelerate past 2200 rpm, it also begins to misfire when idling under load. The misfire is that obvious intermittent shutter that can be felt. Here is my stats and progress so far:

Boat Specs:
- 1989 4.3LX 205hp Thunderbolt, V6, On muffs and on water boat fires up and idles at dock like a champ. Last year the motor would bog at 2300-2500 then blast through onto plane.

Prop:
- New 4 blade from 3 blade: all specs are the same except extra blade

Fuel Related Work:
- Rebuilt Carb last week made adjustments to specs and double checked to the best of my ability
- Idles at around 700 rpm and 55-60 lbs oil pressure
- Replaced Fuel Pump Filter and Carb Filter (including springs and gaskets)
- Checked fuel for water - Good to Go
- Checked fuel lines for blockage - nothing I could see
- Removed fuel pickup screen is fine

Cooling System Work:
- New Impeller (Last Year)
- Thermostat opens at 143-145 degrees

Electrical (All new this year):
- Cap and Rotors
- Spark Plugs (Carbon fouled when removed, not surprised)
- Checked Wires visually but not electrically. They are newer and in great shape but??
- Batteries Charged and Tested
- Tested coil with key in run - 12v

I obviously need to compression test and timing at this point. I am assuming this is spark

Last year would bog down but then take off. This year after rebuilding the carb it's worst.
I think you still have a carb issue. The missing under load and not getting above 2200 sound like a lean condition. If it was electrical the engine would not idle correctly but you said it idles like a champ, meaning no issues. If it was a fuel pump issue, it would take right off then bog down. Your TB4 ignition is fairly simple and I don't see an issue which pops out.

Need to take the carb apart and soak it for a long time in cleaner to get into the ports. Your float level I believe is set to low and your idle jets are to closed down. Main jets are probably clogged up

 

[TruDaddy]

As far as the comparison to last year and this year, I had the current issue before the carb rebuild. I rebuilt it in response to current issue and the assumption that last year's bog simply got worse. I appreciate the response. I understand your inclination and diagnosis. As far as the main jets and idle jests are concerned. I see no way, short of simply buying new ones that they could be cleaner. Soaked for hours. They are perfect in appearance, run through with appropriate tools, and have been blown out with air. As far as float is concerned, I agree in theory that it is possible but it is set spot on to specs, 15/64", soooo what should I set it to? The misfire under load is the smoking gun for me. I have rebuilt several carbs and though accidents do happen and I will check again, I still think electrical.

 

[TruDaddy]

Soooo, no one else has any info on the matter?

 

[nateccnn]

A misfire could be caused by blown intake gasket or leaking vacuum hose. Possibly even a sticky valve.

 

[alldodge]

Good point nate,put a vacuum gauge on it to see if something shows up. Check compression?

 

[Bondo]

Soooo, no one else has any info on the matter?

Ayuh,.... Welcome Aboard,..... Is it the Qudra-Jet carb, or the Merc/ Weber carb,..??
I agree with AllDodge, ya still got a fuel delivery issue,... Probably the Carb,...

Yer spark plugs, ya say, were sayin' they're runnin' Rich, yet yer symptoms says it's runnin' Lean,...

Ya need to do some Testin',.... Like a compression test,...
Then run it at low speeds for awhile, 'n re-check the spark plug Colors,... Run it somemore, at hi-speeds, 'n re-check the spark plug Colors,...
On the Hi-speed test, ya gotta shut-down reasonably Quickly,... Don't idle back to the dock,....

You can try runnin' a remote tank to the fuel filter, to see if yer tank system is workin',...
The fuel pump can be pressure tested,...

 

[TruDaddy]

Thanks for the advice folks. Bondo I was confused by the counter intuitive evidence of the fouled plugs as well but I have only had this boat since last year and did not do the plugs as I was told "they had been done." There is no knowing how old they were. I did a timing check today. The timing was off by 4 degrees. I set it to 8 degrees at idle and then checked the advanced specs at 2500 rpms. It is now in spec range for the "advanced curve" per the manual. I just got a compression tester and am going to check that right now. I will check the condition of the plugs at that time as I have run the motor several times for troubleshooting since I installed them. I will return with more info.

Bondo - I did rebuild the carburetor (4 bbl Quadrajet) but no one checked my work. I did set the idle screws to 2.5 turns and the float bowl to 15/64". I did not adjust/check anything with the accelerator pump as I did not have the specs on that. Assuming all was done correctly is it possible there is something I am missing that I need to adjust that would keep it lean upon acceleration? The timing adjustment may having taken care of the misfire feeling when under load (haven't put on the water yet) but doubt it would fix the apparent lean issue.

 

[Bondo]

Thanks for the advice folks. Bondo I was confused by the counter intuitive evidence of the fouled plugs as well but I have only had this boat since last year and did not do the plugs as I was told "they had been done." There is no knowing how old they were. I did a timing check today. The timing was off by 4 degrees. I set it to 8 degrees at idle and then checked the advanced specs at 2500 rpms. It is now in spec range for the "advanced curve" per the manual. I just got a compression tester and am going to check that right now. I will check the condition of the plugs at that time as I have run the motor several times for troubleshooting since I installed them. I will return with more info.

Bondo - I did rebuild the carburetor (4 bbl Quadrajet) but no one checked my work. I did set the idle screws to 2.5 turns and the float bowl to 15/64". I did not adjust/check anything with the accelerator pump as I did not have the specs on that. Assuming all was done correctly is it possible there is something I am missing that I need to adjust that would keep it lean upon acceleration? The timing adjustment may having taken care of the misfire feeling when under load (haven't put on the water yet) but doubt it would fix the apparent lean issue.

Ayuh,..... Unfortunately, I'm a Merc/ Weber/ Edlebrock/ Carter Clone kinda guy,....
There are some Qudra-Jet fellas in here, I just ain't 1 of 'em,...
Yer plugs will tell ya what the carbs doin',...
Ya wanta be very careful with a lean condition, you can hole a piston, Quickly if detonation sets in,...

 

[TruDaddy]

Bondo - I have heard Quadrajets tend to have some nay sayers.

Okay so here is what i have going on so far fellas. Removed all of the plugs. All of the new plugs had black carbon on them. Compression test findings read as follows:

1 - 180#
2 - 180#
3 - 180#
4 - 140 #
5 - 135# then 155# second go with a little oil
6 - 155# then nothing with a little oil.

I think I may have messed up the oil test as my compression tester stopped holding and reading on all cylinders after I did a couple of secondary checks.

So I tried the air test thing. I think I did it right but here is what I did. Set the crank to 0 tdc. then rotated 120 degrees to get it to cyl 6. I attached an air hose to cyl adapter and put in air, 30 ish lbs, waited and listened. I heard nothing at all. I stopped and came back to you guys with my results. So, what do you think? Head Gasket, Valves, Rings, just fine or what? Technically all is within the 70% range vis a vis this bulletin http://www.boatfix.com/merc/Bullet/97/97_25.pdf

 

[alldodge]

Since your gauge is in question I lean toward things are ok for now. The timing correction mentioned previously should help but still think you issue is with the carb. Do the plug check as Bondo mentioned, it should provide some good insight

 

[TruDaddy]

AllDodge - I did idle on muffs and in the water twice since rebuild of carb and replace of plugs. Today the plugs were black. This suggests the mixture is rich though. Failure to accelerate suggests lean. I am confused.

 

[Bondo]

AllDodge - I did idle on muffs and in the water twice since rebuild of carb and replace of plugs. Today the plugs were black. This suggests the mixture is rich though. Failure to accelerate suggests lean. I am confused.

Ayuh,.... A carb has an idle circuit, a low speed circuit, a hi-speed circuit, a accelerator pump circuit, 'n a Choke,......

'n they All gotta play nicely together,....

 

[TruDaddy]

I may have discovered the problem. The problem I discovered certainly could have caused my issues. The 4 and 5 wires were on the wrong cylinders. This error had occurred after changing cap/rotor but before first water test. I will put on the water tomorrow but upon firing it up on muffins it certainly sounded stronger and smoother. I can't believe I didn't double check this earlier.

 

[NHGuy]

Hi, I did the learning on quadrajets last winter.
To set the idle mixture. With correct idle speed and timing warm the motor til the choke stays open. It's ok to help it open as long as it stays open when you let go. Turn your mix screws in one at a time til the motor stumbles, then back out around half or 2/3 turn. My motor works best that way. A smaller amount of idle fuel won't use as much gas, won't blacken plugs and won't dirty your transom as fast. If it won't holeshot well, add a little more fuel to the idle mix.
The idle mix won't do anything for the 2200 rpm "wall" but it will help with keeping your plugs operable.
If your secondary air spring is not tensioned right the boat won't go fast, it can give you the dreaded quadrabog. The spring that closes the secondary flaps should be set 3/4 turn beyond slack.
To adjust it on the motor you need teeny tools and patience. The tensioner bolt is turned with a torx bit. I think it's a 1.5. You can buy a set of security bits at Harbor Freight for $9 or something. That tensioner bolt head faces straight down on the starboard side of the carb at the rear. The tension is adjusted by turning a slotted cylinder in the starboard side of the carb. It looks like a straight blade screw that's sunk in the side of the air horn. Clockwise tightens it, CCW loosens. You loosen the tensioner bolt while setting the spring tension on the slotted pivot with a small straight blade driver.
Here's where it gets sticky, You MUST put the small screwdriver in the adjustment slot BEFORE loosening the tensioner bolt because if the adjuster goes slack the spring will fall away from it and will cause you to need to remove the carb and flip it over to re catch the spring on the adjuster. Don't ask how I know!
Also, I'd say to put on a set of wires. They are about $60 or $80. I had a pop that was solved by wires a few years ago. I used 8 mm stainless core marine. Plus mine are red and look good with my red and black Mercruiser stuff!

And yes the crossed wires could be your issue. Also, separate the wires, and keep them away from the motor so they can't cross fire or arc to ground.
You are almost done, I'm feeling it!

 

[Fastatv]

Hi, I did the learning on quadrajets last winter.

To adjust it on the motor you need teeny tools and patience. The tensioner bolt is turned with a torx bit. I think it's a 1.5. You can buy a set of security bits at Harbor Freight for $9 or something. That tensioner bolt head faces straight down on the starboard side of the carb at the rear. The tension is adjusted by turning a slotted cylinder in the starboard side of the carb. It looks like a straight blade screw that's sunk in the side of the air horn. Clockwise tightens it, CCW loosens. You loosen the tensioner bolt while setting the spring tension on the slotted pivot with a small straight blade driver.
Here's where it gets sticky, You MUST put the small screwdriver in the adjustment slot BEFORE loosening the tensioner bolt because if the adjuster goes slack the spring will fall away from it and will cause you to need to remove the carb and flip it over to re catch the spring on the adjuster. Don't ask how I know!
Also, I'd say to put on a set of wires. They are about $60 or $80. I had a pop that was solved by wires a few years ago. I used 8 mm stainless core marine. Plus mine are red and look good with my red and black Mercruiser stuff!

^^^^^^^^^Yep, after reading the entire thread, I also thought that maybe the secondary might be mis adjusted....and you described the process well. I always found it very difficult to adjust on the vehicle, but I have done that as well. But if he had two plug wires crossed, he may be good to go. I hope he reports back on his findings.

 

[TruDaddy]

So time to report gentlemen...I went out this morning and fired her up on the water. Got past the wake zone and gave a little, then a little more then all. She went up on plane and got up to 4300 rpms. She definitely needs a little fine tuning on the carb. I will use NHGuy's advice for that but now a new issue.

SHE OVERHEATED ALMOST IMMEDIATELY. I shut her down immediately in the middle of the lake. Sat for a while and waited for the gauge to retreat. Nothing. I got towed in by a nice man and his son doing some fishing. I brought her home and now time to look at stuff. The impeller is new from last year. I am hoping there is just an obstruction but who knows?

 

[Bondo]

So time to report gentlemen...I went out this morning and fired her up on the water. Got past the wake zone and gave a little, then a little more then all. She went up on plane and got up to 4300 rpms. She definitely needs a little fine tuning on the carb. I will use NHGuy's advice for that but now a new issue.

SHE OVERHEATED ALMOST IMMEDIATELY. I shut her down immediately in the middle of the lake. Sat for a while and waited for the gauge to retreat. Nothing. I got towed in by a nice man and his son doing some fishing. I brought her home and now time to look at stuff. The impeller is new from last year. I am hoping there is just an obstruction but who knows?

Ayuh,..... Drop the lower unit, 'n back-flush the hose at the T-stat housin' comin' from the impeller,...

 

[TruDaddy]

Ayuh,..... Drop the lower unit, 'n back-flush the hose at the T-stat housin' comin' from the impeller,...

I already replaced the impeller. I remembered yesterday that I accidentally ran the motor with the hose off for at least 45 secs. There were barely any splines left on the impeller. I replaced everything I could with the rebuild kit and fired her up with water on. I noticed there were two decent streams of water coming out both sides of bell housing area but not the center of prop. Is this normal? I never had cause to look before.

Also, she started to overheat. Either the thermostat is shot now or there is an obstruction. Temp climbed to 155 and she did not back down. It is possible there is some shards of rubber in the system but were do I start to look? How do I back flush?

 

[TruDaddy]

I am confused. The temp gauge after hours is still at 145 degrees. Is this normal? Where is the temp gauge sensor located? I can't find it on the schematics i have been looking at.