1990 Merc 350 Alpha One - stalls out after running over 3500 RPM

[cwburkeva]

After having this problem, I threw up my hands and took to the shop. Was told they figured out the problem. Now it is back. Here are the symptoms, any help would be appreciated:

1. Runs fine, starts, etc when cruising at around 2500-3000 RPM.
2. When increasing to 3500-4000 RPM for a sustained run, I get a hesitation/fuel cutout, then all of a sudden, engine just chokes out.
3. I can start it and it will idle fine, but when put into forward, I cannot increase throttle and it cuts out. I can restart several times, and eventually it won't start at all.
4. When cooled down (usually overnight), it starts and runs perfectly.

I have rebuilt the carb (Rochester 4bbl), replaced coil, spark plugs, spark plug cables, rotor & cap, fuel pump. Shop said that it was battery cables that had bad crimps, so they re-crimped everything. Said ignition was getting inconsistent electrical power as a result.

I have three thoughts - all are costly, but not sure where to start or if I am on the right track.

1. Replace Thunderbolt IV ignition amplifier - it could be overheating.
2. Replace Shift cut-out switch; not sure if that is going bad or is only affected when running at higher RPMs (over heating to make bad contacts)
3. Replace Carburetor

It also acts like vapor lock or water in the fuel. But that doesn't make sense since I ran it for about 45 min to an hour at the lower RPM without hesitation or issues.

 

[Bondo]

It also acts like vapor lock or water in the fuel.

Ayuh,..... Pull the fuel filter, 'n examine it's Contents,......

 

[Bt Doctur]

would not be the first time a vent was clogging or the anti-syphon is acting up

 

[kulle]

After having this problem, I threw up my hands and took to the shop. Was told they figured out the problem. Now it is back. Here are the symptoms, any help would be appreciated:

1. Runs fine, starts, etc when cruising at around 2500-3000 RPM.
2. When increasing to 3500-4000 RPM for a sustained run, I get a hesitation/fuel cutout, then all of a sudden, engine just chokes out.
3. I can start it and it will idle fine, but when put into forward, I cannot increase throttle and it cuts out. I can restart several times, and eventually it won't start at all.
4. When cooled down (usually overnight), it starts and runs perfectly.

I have rebuilt the carb (Rochester 4bbl), replaced coil, spark plugs, spark plug cables, rotor & cap, fuel pump. Shop said that it was battery cables that had bad crimps, so they re-crimped everything. Said ignition was getting inconsistent electrical power as a result.

I have three thoughts - all are costly, but not sure where to start or if I am on the right track.

1. Replace Thunderbolt IV ignition amplifier - it could be overheating.
2. Replace Shift cut-out switch; not sure if that is going bad or is only affected when running at higher RPMs (over heating to make bad contacts)
3. Replace Carburetor

It also acts like vapor lock or water in the fuel. But that doesn't make sense since I ran it for about 45 min to an hour at the lower RPM without hesitation or issues.

sounds the exact problem i had/have with my 93 merc 4.3L. i checked everything and replaced the common stuff as you did. i thought same, ignition module overheating, etc. i ended up throwing my hands up and winterized it. during which, i found a tear in ujoint belows, so i took my drive off and bell housing, and my shutter valve was lodged in exhaust bellow. everythjng is replaced but now it seems my starter solenoid is dead, so i still havent had a chance to water test to see if that shutter was causing the shutoffs at higher rpm(load). would love to know what it is! if you find root cause, please update here.

 

[cwburkeva]

I have through the transom exhaust and I did change my flapper/shutters last year. No other shutters are there that I know of.

 

[cwburkeva]

Bondo - checked fuel filter - clean, no dirt, no water.

i did check the shift cutoff switch. I am suspecting it might be the culprit. It is very sensitive. I touched the button with the end of a flat screwdriver and barely depressed. It killed the motor with perhaps a 32nd of an inch. Seems like it shouldn’t kill without the lever depressing it about half or more before killing the motor. Thoughts?

 

[harringtondav]

I'm thinking not the shift cutout switch. The Y plate only activates the switch when shifting out of gear due to dog clutch resistance. Once in gear it doesn't care. But a quick way to check is to disconnect a bullet connector at the switch and see what happens. You'll be stressing your lower shift cable a bit for a few shifts to neutral, but no real harm.

Next time it dies for the night, pull the spark arrester and look at the accelerator jets with a helper pumping the throttle. Fuel squirt or none will help you narrow down the essential three: Fuel. Spark. Compression.

 

[cwburkeva]

Some additional info since last post.

After checking fuel filter and no issues with that, I went to start her up. After many key turns and no starting, I moved the throttle/shift control into the forward position forcing it into gear, then back to neutral. Tried to start and nothing. Then I moved the throttle/shift into reverse and back to neutral. After doing that, I got her started. I firmly believe that when doing that the shift cutt-off was the issue.

I have ordered both a used shift cut-off and TB IV ICM (both tested and guaranteed) I didn't want to dump $800 on two parts since they both have Service Bulletins with updated part numbers.

I get them by Monday and will swap the shift cut-off and if needed the ICM.

I have eliminated the Carb and vapor lock as the problem. I have owned this boat for 16 years. I never had this problem until last year, and vapor lock was never an issue and can't imagine that after this long it would finally do it. As for the Carb, it was rebuilt and it runs fine up to the point of this particular issue. (hours later sometimes)

Will update this post next week when I see if I solve the issue.

 

[QBhoy]

Hope you find the issue but off the cuff, I’d think this was or is a previous or current water in fuel issue or a tank pick up blockage issue.
Mid there was ever any amount of water in there, the carb tends to never recover from it...until it’s been off and cleaned properly. All the best.

 

[djmakko1]

you never mentioned "checked the fuelPump" perhaps gets hot after a while and won't deliver enough gas above 3500 rp's .
just a thought.

 

[cwburkeva]

Update:

Changed the cut-out switch, which improved my starting issue as the original switch was on its way out - since it sits above the valve cover breather tube, it gets a little oil. Need to change out the valve cover grommets and elbow connectors - they are original from 1990.

After suspecting that the cut out switch could be the issue and running for 20-25 min, the problem reared itself. I then replaced the ICM, ran it and no solution.

Considering I was running out of options, I went back to Fuel. In fact I wondered if maybe something in the carb may be wonky from the rebuild. So I ordered a rebuilt carb from a reputable place. I even upgraded to an electric choke. Installed on Sat, set the idle speed in driveway, then dropped her in water yesterday to set final idle speed.

Tested he at variable RPMs for the first 20-25 min and held my breath for the 15-20 beyond that. Total water time about 40-45 min with keeping engine running and really putting her through WOT to idle.

So far it appears that my issue is solved! New rebuilt Rochester QuadraJet seems to be the final solution.