1988 350 Stumble Problem

[Bnewton530]

Hi guys,

I have a 1988 350 Merc with the QJet and points ignition. I am having trouble getting above 2500rpm under load. Hard to believe, but it is intermittent. It has a rebuilt carburetor and the gas tank has just been thoroughly cleaned. The fuel lines, pump, lines, separator, and filter are new.

I also did a tune up with new plugs, cap and rotor, and points and condenser. The idle is set at 675 and the dwell is 30 degrees. 8 degrees BTDC.

I can rev to redline in neutral just fine. I can troll around at idle all day and I can walk it slowly up to 3000 rpm under load. I would say 60 percent of the time I can punch it from idle and get to 4K rpm on plane, but I cannot throttle up moderately to anything past 2500. When I am cruising along at 2500 rpm and roughly 35 mph, throttling up causes it to bog slow as if it is running out of gas. If I throttle back it will recover to 2500rpm.

Any suggestions help before I tear into this professionally rebuilt carb.

 

[nola mike]

Hi guys,

I have a 1988 350 Merc with the QJet and points ignition. I am having trouble getting above 2500rpm under load. Hard to believe, but it is intermittent. It has a rebuilt carburetor and the gas tank has just been thoroughly cleaned. The fuel lines, pump, lines, separator, and filter are new.

I also did a tune up with new plugs, cap and rotor, and points and condenser. The idle is set at 675 and the dwell is 30 degrees. 8 degrees BTDC.

I can rev to redline in neutral just fine. I can troll around at idle all day and I can walk it slowly up to 3000 rpm under load. I would say 60 percent of the time I can punch it from idle and get to 4K rpm on plane, but I cannot throttle up moderately to anything past 2500. When I am cruising along at 2500 rpm and roughly 35 mph, throttling up causes it to bog slow as if it is running out of gas. If I throttle back it will recover to 2500rpm.

Any suggestions help before I tear into this professionally rebuilt carb.

Check fuel pressure. Put a timing light on the coil wire and make sure you're getting consistent firing.

 

[Bnewton530]

Check fuel pressure. Put a timing light on the coil wire and make sure you're getting consistent firing.

I was thinking it could have something to do with the ignition coil. I bought a new coil but forgot to bring it with me to the water so I couldn’t switch it out on the fly. I will check fuel pressure and coil this weekend.

 

[Bondo]

I was thinking it could have something to do with the ignition coil. I bought a new coil but forgot to bring it with me to the water so I couldn’t switch it out on the fly. I will check fuel pressure and coil this weekend.

Ayuh,...... Welcome Aboard,...... It ain't the coil,.....

 

[kenny nunez]

The Qjet that is rebuilt, is it the original carburetor? I hope so. When you punch it to get 4k rpm on plane does it hold or does it feel like it is surging and slowing? If so, if you gradually pull back the throttle from full does it momentarily surge ahead?
When did this problem begin?
When you were working on the fuel tank did you replace the suction tube or remove the fine mesh screen from inside the suction tube?

 

[Bnewton530]

The Qjet that is rebuilt, is it the original carburetor? I hope so. When you punch it to get 4k rpm on plane does it hold or does it feel like it is surging and slowing? If so, if you gradually pull back the throttle from full does it momentarily surge ahead?
When did this problem begin?
When you were working on the fuel tank did you replace the suction tube or remove the fine mesh screen from inside the suction tube?

The carb is not the original to the boat. I’m not confident that the carb I replaced is original either. When I punch the throttle and get up onto plane, it does not surge or lose power. It will ride along at 4K for miles. I can pull back the throttle and ride at whatever rpm that I want at that time, but if I throttle back from 4K, I cannot throttle back up to 4K without coming down to an idle and punching it.

The tank is a 75 gal belly unit. The suction tube is welded into the tank and it does a 90 degree turn through a baffle. There is not an anti siphon valve on it either. I was able to suck the tank dry through a 1/4” siphon hose which suggests that there is not a mesh at the bottom of the tube. The vent was verified clean too at that time. The tank and fuel system was JUST redone. This was the first time out on the water since. The original carb would run well and plane out but would clog up with debris from the tank very quickly and it developed a leak at the throttle rod, so I cannot 100 percent say that the new carb doesn’t perform the same.

 

[Bnewton530]

Ayuh,...... Welcome Aboard,...... It ain't the coil,.....

I have a new coil that I will likely install anyways. If it wasn’t the problem, then I still have a spare. I am guessing that everyone is going to get caught up on the gas tank. Fuel pressure can be tested with the boat on the hose. If I have pressure from idle to redline in the driveway, the same could be said for under load, yes?

 

[Scott06]

When it happens check spark strength with a spark gap tester to rule it out.

I suspect you have a fuel delivery or carb issue. Prob a good carb cleaning will help, but would verify fuel pressure while its acting up

 

[Lou C]

With the QJet 2 issues to check:
Secondary air doors opening too early
Not using the correct accel pump for marine use
Both when corrected will make a big difference
Also if you can make sure your distributor is giving proper total timing advance at the specified rpm. Do you have a Thunderbolt ignition or a Prestolite points unit? Ah just saw its points. That mechanical advance unit can stick if not lubticated it is very simple to take apart & check. There’s a lubricating wick in the dist shaft under where the rotor goes that should get a few drops of motor oil each season. Have you checked the dwell?
I have the same Prestolite unit on my V-6.
Points seem to last at least 5 seasons in my use. If you can get a hold of an advance timing light you can check your total timing advance (base timing + centrifugal advance) @ Merc’s spec for the 5.7 V8. Prob 28-30* at 3200-3500 rpm I’d guess. But check to be sure.

 

[kenny nunez]

If you can pass a stiff wire down and out the suction tube that will rule out any mesh filter, believe me I was once fooled by one and after using a 6 gallon outboard tank the engine went to 5k and held. We then switched back and the problem came back.
You need to locate the numbers that are stamped on the main body of the Qjet. Contact Cliff’s high performance Quadrajets. This guy really knows these things. I am pretty sure you are leaning out really bad. Eventually you will hurt the engine.

 

[Bnewton530]

With the QJet 2 issues to check:
Secondary air doors opening too early
Not using the correct accel pump for marine use
Both when corrected will make a big difference
Also if you can make sure your distributor is giving proper total timing advance at the specified rpm. Do you have a Thunderbolt ignition or a Prestolite points unit? Ah just saw its points. That mechanical advance unit can stick if not lubticated it is very simple to take apart & check. There’s a lubricating wick in the dist shaft under where the rotor goes that should get a few drops of motor oil each season. Have you checked the dwell?
I have the same Prestolite unit on my V-6.
Points seem to last at least 5 seasons in my use. If you can get a hold of an advance timing light you can check your total timing advance (base timing + centrifugal advance) @ Merc’s spec for the 5.7 V8. Prob 28-30* at 3200-3500 rpm I’d guess. But check to be sure.

My dwell is currently at 30* and the manual calls for 29* +-2* and I have not checked the mechanical advance. That could be part of my problem. It seemed that as the day went on it freed up. Wishful thinking. I feel like at 4K rpm the boat was ripping and sounded great. I would assume that when I could get it to 4K, the advance must have been pretty close. I’ll check the advance mechanism after work.

How would you recommend that I check the secondary operation in the carb? It is a reman unit from a reputable place so I assume it was rebuilt with marine components

 

[Bnewton530]

If you can pass a stiff wire down and out the suction tube that will rule out any mesh filter, believe me I was once fooled by one and after using a 6 gallon outboard tank the engine went to 5k and held. We then switched back and the problem came back.
You need to locate the numbers that are stamped on the main body of the Qjet. Contact Cliff’s high performance Quadrajets. This guy really knows these things. I am pretty sure you are leaning out really bad. Eventually you will hurt the engine.

1795-9286 was on the boat when I bought it and the replacement matches.

 

[kenny nunez]

To really check the centrifugal advance is to remove the breaker plate and inspect the springs and weights for free movement. Sometime where the distributor is located water drips down and will rust the insides of the distributor. Of course checking the movement with a timing light needs to tested.
I still feel you should check with Cliff.

 

[Lou C]

Got my rebuild kit & accel pump from Cliff

 

[Bondo]

If I have pressure from idle to redline in the driveway, the same could be said for under load, yes?

Nope,.... Without a load, even screaming away in neutral uses almost no fuel,.....

 

[Bnewton530]

Nope,.... Without a load, even screaming away in neutral uses almost no fuel,.....

I agree with that. Doesn’t the fuel pump produce a set fuel pressure at any given rpm whether it is under load or not?

 

[Lou C]

All the fuel pump has to do is keep the fuel bowl in the carb full.

 

[Lou C]

All you have to do to check & clean & lube the centrifugal weights is remove the plate the points are mounted to I think it’s like 2 screws…

 

[Bnewton530]

I thought I would update everybody.

I replaced the coil. I noticed with the timing light that cylinder 1 would skip every few rotations. I replaced the coil and that issue went away.

I pulled the carb and checked the float level and checked the choke pull off, secondary, and accelerator linkages per the manual for my drive unit. To the best of my knowledge everything is correct and the fuel in the bowl was 100 percent clean.

I pulled the distributor and cleaned the advance weights and springs. It “appears” to be more free. We will see. It is back in time with 30 deg dwell and 8 deg BTDC.

The other thing that I looked into was the ESA linkages. I noticed that the interrupt switch was not popping back into the center notch in the claw shaped bracket when in forward gear and that the interrupt cancel switch was not made each time when in the forward gear. The switch would make sometimes and sometimes not. Keep in mind that I can shift my boat with one finger and that everything is adjusted via the Stuart instructions (it’s an OMC unit). I adjusted the striker on the bracket such that the interrupt cancel switch is made sufficiently when the bracket moves into forward position.

It does everything that it’s supposed to do on the hose.

I plan to hit the water tomorrow.