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Mystery... any ideas?

S

SeaMoose

New member
Hello and Thanks for Reading

'86 Bayliner with AQ260. Engine starts right up, sounds great, burns no oil. Easily reaches plane, runs strong and cool (Temp gauge shows under 180 degrees all of the time). There's no hour meter on these but it can't be that many.

However... If you run it for a while (20 minutes is enough) at planing speeds - just under 1/2 throttle - and then run it at idle speed for 10 minutes it refuses to run at more than approximately 1500 RPM until it completely cools down again.

Throttle up, at about 1/4 throttle the engine misses, cuts out, (but not misfires) - and you get the smell of gas.

What I've done to try and solve the problem:

1) Checked the ignition system - replaced points, rotor, rotor cap, plug wires. Ran a little stronger (as expected) but problem exactly the same.

2) Checked the fuel system - Fuel vent clear, fuel lines good, fuel filter replaced. When it was acting up, removed the fuel cap, didn't change anything.

3) Cleaned the backfire preventer (it wasn't that dirty)

4) The ignition wiring seemed a bit hot, fuse panel was a bit dirty. Replaced the fuse panel, cleaned every electrical connection on the engine. Got a higher reading on the volt meter with the engine running (From 13 volts to about 14), and some intermittent circuits in the cabin work solid now, but it didn't change the problem.

If it wasn't for the smell of gas when this happens I'd think something sticking in the carburetor, but as soon as you throttle back below the point where it misses it runs normally. It's getting more fuel than it can burn.

The only thing I can figure at this point is that the exhaust must be constricted somehow. Is there a flapper valve inside the manifold? Might it be barely open on idle and then won't open again when hot?

Any other suggestions? My mechanic has no clue...
 
A few items to check:
1. The fuel pickup tube strainer in the fuel tank
2. The anti-siphon valve at the fuel tank
3. The fuel line filter, and the carburetor body fuel filter (if you have one?)
 
Thanks, I have an IR heat gun. Everything seems plenty cool - although I just got this boat this year so I don't know what normal is.
 
Thanks MikDee
This would certainly be the #1 reasons Engines won't run at higher RPM's alright. Trouble is, I can idle out to the lake and run the boat at 5/8 throttle for an hour with absolutely no problem. The only time I have trouble is when I run.. idle for 10 minutes.. and try to run again. I can idle down for a short period with zero effect.

I know, it's a real stumper. The only thing I can guess is that something is restricting the exhaust.
 
However... If you run it for a while (20 minutes is enough) at planing speeds - just under 1/2 throttle - and then run it at idle speed for 10 minutes it refuses to run at more than approximately 1500 RPM until it completely cools down again.
Click to expand...

Unless this engine uses an electric fuel pump, the fuel flow is not likely to change in 20 minutes run time.


However, have you considered that your ignition coil may be warming up and causing an issue during this 20 minute run-time?

Describe your ignition system.


.
 
RicardoMarine said:
Unless this engine uses an electric fuel pump, the fuel flow is not likely to change in 20 minutes run time.


However, have you considered that your ignition coil may be warming up and causing an issue during this 20 minute run-time?

Describe your ignition system.


.
Click to expand...

Thanks for the reply.

Hadn't thought about the coil having a thermal issue. I supposed that's possible. A coil is easy to replace :) Other than the points, plugs, wires, and rotor cap this is a bone stock 1985 everything - that sat un-used for many years, previous owner brought the boat back to life, used it two years, and sold it to me. Previous owner knew what they were doing.

The first time this happened we ran the boat on plane for 20,30 minutes, idled around for about 20 minutes, and then headed back to the marina. Problem appeared right away, but - and this is really strange - after 20, 25 minutes of running at reduced RPM the problem cleared up on it's own - which really baffled me.

Second time about the same run times, but the problem did not clear up on it's own.

Third time was a controlled test after I replaced the fuse block and cleaned all the electrical connections.

This is why I am thinking there is some valve somewhere that moves easy when cold, but gets sticky when hot....
 
Interesting theory. Next time I am at that boat I'll start the engine with the backfire preventer off and see if it's opening all the way.
 
Remove flame arrestor ( it's not a backfire PREVENTER), check state /movement of choke. REPLACE the flame arrestor, start engine, then remove arrestor after engine starts.
 
Indeed Captain Bob, Indeed. Sorry - my first GAS powered boat in 30 years.... Put myself through college working on engines similar to this, but they weren't in boats.
 
If you have replaced the contact points recently, did you set them via "gap" or "dwell".
Gap is an estimate only of correct dwell angle.

If this is the original ignition distributor, check for shaft wobble at the cam area. Wobble here will cause an erractic dwell angle.
If the dwell angle is causing over saturation, the coil may be over-heating.

.
 
Thanks Rick, very interesting. Had not heard of this before. I'm 90m from the Marina so will check this over the weekend, assuming I get down there (always the plan doesn't always come to pass)
 
If you do stay with the kettering system (points), you'll need a dwell meter in order to adjust the dwell angle.

Look up your OEM specs for the dwell angle.... probably near 30* or 31* or so.
This is the angle at which the contacts are closed (coil saturation) during a distributor shaft rotation of 45* per cylinder.

Check dwell at idle rpm, and then increase rpm slightly to see if the dwell angle remains the same.
 
Next time the problem appears, pull a couple of plugs and look at 'em. Sounds like they foul when you idle, then finally clear enough to get on with gettin on ...
 
Thanks Curmudgeon
The problem is repeatable - we proved that on the last test. Thing is, you let it cool down, the problem vanishes.

Right now a choke problem seems extremely likely, and a hot coil second. I have long suspected not enough fire, or not enough air, or a restricted exhaust - it really does run like it's choked, the smell of gas would confirm this.

Unfortunately the Admiral says we stay home this weekend so please, everyone, stay tuned!!
 
...............................

SeaMoose said:
Right now a choke problem seems extremely likely,
See post #1, second paragraph.


and a hot coil second.
Which is more likely to show up after 20 minutes run time....... a closed down choke chimney.... or an ignition coil that has warmed up and is failing?

I have long suspected not enough fire, or not enough air, or a restricted exhaust - it really does run like it's choked, the smell of gas would confirm this.
Good Call!
Look at the position of the choke plate while the engine is cold.
Make sure that the Helix "heating element" is providing heat and that it continues to provide heat.
Make sure that the warm Helix is opening the choke chimney as the engine warms up.
Click to expand...
 
BTW, a good SBC automotive mechanic should be able to adjust the helix choke plate tension, and check the function of the heating element. Favor the lean side.

Also, he should be able to set the dwell angle correctly, and check for upper distributor shaft play.
He'll also know how to look for an erratic dwell angle.

He can replace the ignition coil also.

If he sets ignition advance, make sure that he dynamically checks the progressive and total advance as per OEM specs.
This requires that the harmonic balancer be marked off up to approx 35*.

These specs are not same as Automotive.





.
 
Thanks Jeffnick
It's electrical - or it's a choke problem - of that I am convinced. Will check the choke operation next weekend (Admiral made me stay home this weekend and take care of the house LOL)
 
Update: Choke operation when engine is started cold is fine. Waiting for a helper to take the boat out, run it on plane for 10 minutes, someone holding the engine hatch open so we can monitor choke, and secondary barrel butterfly valves, then take it back in, idle around and see if choke closes.

This last weekend it was windy, and the Mayflies were ferocious, so the Marina was dead.
 
Update:
With the helper we determined that the choke was opening very slowly, and not 100% open either. Replaced the choke thermostat. Replaced the tach. For the first time EVER, the motor ran all the way up to 4,000 RPM - Quite a shocker when the back barrels opened for the first time.

Still have the run hard, idle around, won't run over 2100 RPM issue - However when I took the boat out this morning it did the same thing for a second, and then cleared right up and ran right to 4K RPM.

Clearly something in the carburetor related to the throttle is a little "sticky". Before I take it off and have it rebuilt going to try dousing it with carburetor cleaner - unless that's a no-no.
 
I don't believe that 4k RPM will be your WOT RPM.


Here's the issue with doing a WOT RPM test:
Engine must be in Full Tune....... good cylinder pressures, good valve operatioin, good valve timing, good spark and spark timing, good fuel delivery, and so on.
Hull must be clean and free from bottom growth.
Propeller must in near perfect condition and very close to the correct pitch.
Final drive ratio must be correct.
The hull must be loaded as you intend to use the boat.


If you are still unable to reach your specified WOT RPM, then one of several things is not quite right:
Engine not in full tune.
Incorrect propeller.
Incorrect final drive ratio.
Dirty hull.
Over loaded hull.
Stern heavy hull.



As for spraying your carburetor..... you'd need to force the chemical into the metering circuits in order to gain anything.

Somewhere near the venturi openings, you'll see several tiny brass orifices. These orifices introduce air into the metering circuit emulsion tubes.
Using caution..... spray the cleaner into these ports. With any luck, some of the chemical will reach areas of these metering circuits, and may clear them from any gum or debris.
It may be worth a try.


.
 
Thanks Ricardo
Up to this point the best I could do was 24 knots, after replacing the choke thermostat it's running 30 at WOT so I am definitely making progress!!

I know exactly where you're coming from with the orifices will see what I can do.

Alan
 
if you're familiar with Q-jet carbs, then maybe your primary power valve in the carb is sticking, or not opening fully. Make sure to check this if you take it apart, as well as the accelerator pump, float needle valve, level, & drop.
 
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