Mercruiser 5.7 wanting to stall at idle after driving around

[todhunter]

1987 Mercruiser 5.7 / 260 HP, SN 0B651656

Engine has a Rochester QJ carb that was freshly rebuilt with a Cliff's kit this past winter. TBIV ignition with new plugs, new wires, new cap, and new distributor sensor. Timing is set to 8 degrees BTDC at idle. Good compression on all 8 cylinder (178-185 psi...I know, gauge may be off, but it's consistent). Cold start it fires right up with throttle about 15-20%...if it's sat more than a couple of days, I give it one full throttle pump before cranking. I can let it run a few minutes then pull the throttle back to idle and it is happy at about 600 RPM. After driving for 30 minutes or so and pulling back to idle, the engine wants to die. If I get back into the throttle, it's fine and it will run for as long as I want it, but coming back down to idle, it wants to die again. If it dies, it becomes difficult to get started again - sometimes holding the key over for a long period (10 seconds) can get it to gradually catch and start running again, but I have to get off idle quickly to keep it from dying again. It did this yesterday when putting the boat back on the trailer and while attempting to re-crank it, it smelled like it was flooded (I did not pump the throttle during this attempt).

Today I put it on muffs in the driveway and started it with the spark arrestor off, and it started fine. While messing with it, I was moving the choke plate by hand and if I started closing it, I got the same symptom where the engine would want to die. When I did the QJ rebuild, I did not replace the divorced choke thermostat, and honestly it looks kind of rough. I'm thinking the choke is not opening when warm and the engine is flooding, causing the bad idle / hard start. Do you think this may be the issue?

I have zip-tied the choke linkage for the primaries open on the carb and plan to see how it does on the water today. If it runs better and I get positive feedback to my hunch from here, I will buy a new choke thermostat from Cliffs this week.

 

[dubs283]

When I did the QJ rebuild, I did not replace the divorced choke thermostat, and honestly it looks kind of rough. I'm thinking the choke is not opening when warm and the engine is flooding, causing the bad idle / hard start. Do you think this may be the issue?

I have zip-tied the choke linkage for the primaries open on the carb and plan to see how it does on the water today. If it runs better and I get positive feedback to my hunch from here, I will buy a new choke thermostat from Cliffs this week.

Could be the choke just needs adjusted. Whenever one rebuilds a carburetor it is imperative to ensure the choke is operating properly and the idle air mixture is set correctly. A vacuum gauge can help fine tune the idle air mixture but isn't completely necessary especially for a non performance setup.

Sounds like you are on the right track monitoring the choke but before you stick it open it's best to monitor the operation and re adjust if necessary

 

[Bt Doctur]

I was chasing a similar issue last year. Would run all day then decide to flood out.Did a rebuild, no luck, changed the carb to edelbrock, no luck
come to find that my mechanical fuel pump was putting out 15 psi when it wanted to and then would revert to 4.5 psi.
Switched to a electric pump and it never happened again

 

[tank1949]

If you are sure that fuel lines and tank are debris free, I suspect trash found its way into carb. This is an old mechanic's trick that some may not like, but it has worked several times for me. Start cold and disconnect throttle at carb. Remove air breather. Let motor warm up. Accelerate carb manually and with other hand, partially cover carb's throat. Engine will start to stall. Remove hand. Repeat a few times. The partial suction that you are creating will suck out small crap into motor.

 

[todhunter]

Boat maybe did better yesterday, but I wouldn't say locking the choke open fixed the issue - it still was struggling to idle as I was putting it on the trailer. I'll do some investigation over the coming weeks - I'll look into the choke adjustment, may go ahead and set up a fuel pressure gauge between the pump and carb, and may pull the carb off / apart to just make sure no trash has gotten in there and double check the float level.

This is a fresh restoration and I ran new fuel line from the tank to the pump as well as putting on a new water/fuel separator, but the tank is original and I didn't open it up.

 

[jimmbo]

deleted

 

[nola mike]

Boat maybe did better yesterday, but I wouldn't say locking the choke open fixed the issue - it still was struggling to idle as I was putting it on the trailer. I'll do some investigation over the coming weeks - I'll look into the choke adjustment, may go ahead and set up a fuel pressure gauge between the pump and carb, and may pull the carb off / apart to just make sure no trash has gotten in there and double check the float level.

This is a fresh restoration and I ran new fuel line from the tank to the pump as well as putting on a new water/fuel separator, but the tank is original and I didn't open it up.

Any fuel dribbling from the venturis at idle? What happens if you open the throttle all the way when trying a hard start?

 

[Lou C]

A common problem on the QJet is the idle air tubes get clogged, these can be carefully cleaned out with carb cleaner a very thin wire & compressed air. Cliff has instructions in his forum on his website on how to do this. Do you have any nozzle drip at idle from the venturis? Take a look.

 

[todhunter]

When I did the rebuild (per Cliff's book), I did pull out the idle air tubes and cleaned out underneath them.

Now that you guys mention it, yes, there was fuel dribbling from the center of the venturis at idle when I ran it on muffs yesterday morning with the spark arrestor off. Is that not a good thing?

 

[todhunter]

What happens if you open the throttle all the way when trying a hard start?

I'll have to try that next time - not sure.

 

[jimmbo]

When I did the rebuild (per Cliff's book), I did pull out the idle air tubes and cleaned out underneath them.

Now that you guys mention it, yes, there was fuel dribbling from the center of the venturis at idle when I ran it on muffs yesterday morning with the spark arrestor off. Is that not a good thing?

A sign of, a High Fuel Level. Float set too High, Float heavy from absorbing Fuel, ineffective Inlet Valve, Loose/Leaking Inlet seat, too high Fuel Pressure

 

[todhunter]

A sign of, a High Fuel Level. Float set too High, Float heavy from absorbing Fuel, ineffective Inlet Valve, Loose/Leaking Inlet seat, too high Fuel Pressure

OK, so we're probably onto something then. Seeing as the float is new, I doubt it's absorbed fuel. I'll plan to pull the carb and check the float level and also look at the valve and seat. I'll also see if I can start sourcing the parts to put a pressure gauge on the fuel line - @Scott Danforth , didn't you have a thread somewhere outlining what fittings were needed to add a gauge?

 

[nola mike]

OK, so we're probably onto something then. Seeing as the float is new, I doubt it's absorbed fuel. I'll plan to pull the carb and check the float level and also look at the valve and seat. I'll also see if I can start sourcing the parts to put a pressure gauge on the fuel line - @Scott Danforth , didn't you have a thread somewhere outlining what fittings were needed to add a gauge?

Is it a mechanical or electric pump? If electric, pretty easy to disconnect at the carb and jump the pump; the pressure gauge may already have the correct fitting

 

[todhunter]

Is it a mechanical or electric pump? If electric, pretty easy to disconnect at the carb and jump the pump; the pressure gauge may already have the correct fitting

It's mechanical. I'm guessing I'll have to fab up a hard line with the gauge between the pump and the carb.

 

[Lou C]

That can also be fuel boiling in the bowl from high temps. The older pre Vortec has a heated intake and the QJets used a thick insulator gasket. With that gasket I have not had a problem with fuel percolating in the carb

 

[jimmbo]

OK, so we're probably onto something then. Seeing as the float is new, I doubt it's absorbed fuel. I'll plan to pull the carb and check the float level and also look at the valve and seat. I'll also see if I can start sourcing the parts to put a pressure gauge on the fuel line - @Scott Danforth , didn't you have a thread somewhere outlining what fittings were needed to add a gauge?

That brings into question, is it the correct Float? There are several dozen if not Hundreds different Floats for the Q-jet(different Shapes, Mass, and Fulcrum length) and as well as Inlet Valves(Orifice-Needle Size, Height. An Inlet Valve that has a larger Orifice will need a different Float than one with a more restricted inlet. Fuel Pressure also affects Fuel Level in the Bowl.
Also there is a Gasket, that has to be between the Inlet Valve and Carb Body, some variation there too.

 

[Lou C]

Also when you install the needle valve in the float arm, make sure that little clip is not in one of the holes but on the side. If it’s in the hole, that can cause the needle valve to jam and flood the engine.
As far as the variations best bet is to use the Rochester carb # on the side of the carb when buying parts. There were MANY variations because the carbs were used from 1966 to 1986 in everything GM made that had a V8 across 5 divisions and was used in boats till 1990.

 

[todhunter]

That can also be fuel boiling in the bowl from high temps. The older pre Vortec has a heated intake and the QJets used a thick insulator gasket. With that gasket I have not had a problem with fuel percolating in the carb

The dribble? Possible, but I'd be surprised. When I ran it on muffs in the driveway Saturday morning, it was dribbling when I first fired it up from sitting cold all week. My rebuild kit from Cliffs came with 2 gaskets and he said to use the thick one, which I did.

That brings into question, is it the correct Float?

Possible it could be wrong, but I ordered the kit from Cliff's High Performance, gave him the part number off the carb, and made sure he knew it was for a marine application. Any way to check?

Also there is a Gasket, that has to be between the Inlet Valve and Carb Body, some variation there too.

Like an aluminum crush washer? Fairly certain that was there and installed.

Also when you install the needle valve in the float arm, make sure that little clip is not in one of the holes but on the side. If it’s in the hole, that can cause the needle valve to jam and flood the engine.

OK, not sure what I did with the little clip. I may take Friday off and pull the carb off / apart, and that's one thing I'll check.

 

[Lou C]

If you got it from Cliff with the # on the carb it is likely right

 

[jimmbo]

Possible it could be wrong, but I ordered the kit from Cliff's High Performance, gave him the part number off the carb, and made sure he knew it was for a marine application. Any way to check?

Frankly I have never heard of Cliff's High Performance. I would have obtained the Parts from a Mercrusier Dealer, as they certainly have the Parts Listings
The Number that was stamped on to the Carb Body outside the Secondaries?
Being an 87, anyway to tell if it is the Original Carb?