Logo

Very Odd Stalling problem

cheezomlet

New member
"I have an 1984 Glassport with

"I have an 1984 Glassport with a 120hp Mercruiser I/O.
I have owned this boat for 7 years trouble free, I keep up on the maintenance and never had a problem until recently, so heregoes!

The motor starts right up great when cold, every time, but it seems when I travel for a period of time at higher speeds, when I slow down to idle it will just sputter and stall. Then it will not start back up. Cranks over good, just will not fire. Then if I let it sit for about 10 mins it will start like it is loaded up (flooded, missing) and then idle back perfectly in seconds.
It seems the plenum under the carb is pretty hot, it vaporizes the gas when it hits it, and if I pull a plug it dont seem to be getting fuel.

The temps stay below 140 deg and it is getting plenty of fuel and spark.

Here is what I did sofar and still does the same thing:
-New impellor
-New starter
-Compression good
-Spark good (pertronix)
-Fuel supply good, cleaned filters
-Checked the Risers and manifolds, they are clear
-Took the Tstat out.
-Changed gas in tank (for possible water)
-Checked timing
-Carb was reman about 3 years ago, still clean.

Any ideas or similar problems other people have had like this? I am really running out of ideas that could cause this.

Thanks in advance
"
 
Your remark about the phlem be

Your remark about the phlem being hot strikes a cord. It would seem that the cooling jacket in the intake manifold are pluged with scale. More than likely at the gasket joints. To confirm a simple test would be to use a heat probe to check the various temps of all areas of the engines coolant jacket as opposed to that of the phlems.
best luck
bj
 
"Hi BJ,
Thank you for the fas


"Hi BJ,
Thank you for the fast response!
I actualy thought it may be that.
The manifold area under the carb bow side gets hot enough that you cannot hold you hand on it, the stern side is much cooler. I was not sure if this was normal or not.
Which gasket joints would accumulate sediment? If I pull the end caps off could I clean it from there or do I have to totally remove the manifold from the block?"
 
what about his shift controls?

what about his shift controls? if his controls are a little off they could be choking the engine out couldnt they?
 
can you explain the "plenu

can you explain the "plenum " thing? im a little new to this with the same problem...how do i fix it?
 
"The Plenum is the intake mani

"The Plenum is the intake manifold just under the carb. Mine gets hot and it vaporizes the fuel after running WOT. (wide open throttle) then back off to an idle, then to shift.

Once it sits for a few minutes it will start up again, usually acting like it loaded up (missing etc) then clears up and runs smooth again.

I pull the plugs and they are bone dry, even if I pump the accelerator many times during cranking.. or hold wide open trying it.

This is what leads me to believe it is overheating in the manifold, and there is blockage.

Will get to it this week, I have been working on another boat."
 
"The plenum is actually the ch

"The plenum is actually the chamber above the runners.

The fuel is supposed to vaporize when it's in the intake. You'll have much better combustion if it does this, compared to larger droplets. Has the motor been disassembled in the past? If it has, it's possible that the wrong gasket was used for the intake (the holes for the coolant going to the intake may be too small) or, as you said, there may be some kind of blockage. I think I would try back-flushing it first, then remove the thermostat housing again to see if there's something in it, restricting water flow to the upper hose.

Another thing to check in the anti-siphon valve where the fuel line attaches to the tank. It's a spring loaded ball that can collect debris over time."
 
"Hello,
Yes, right under the


"Hello,
Yes, right under the carb, to the runners.

The fuel should vaporize due to air fuel mixtures. What I am describing is when the boat stalls, and the accelerator pump is hit, it vaporizes on the intake plenum, the plugs are dry even if I try and start it, pumping the accelerator verifying plenty of fuel going in, but vaporizing.

It has perfect compression, great spark and fuel delivery is good.

I will check the check valve, but verified plenty of fuel directly from the pump. also the vents are clear to the tank.

The motor has never been dissasembled, and has always run perfectly. and maintained well. It is my personal boat."
 
"Have you tried going WOT whil

"Have you tried going WOT while trying to start it (I didn't see a reference to this), to clear it if it's flooded? That would be a good test of whether it's actually flooding or not."
 
"When it does this, have you c

"When it does this, have you checked for spark or checked only when it's cold?"
 
"Yes,
I check for spark, fuel


"Yes,
I check for spark, fuel and compression at the time it stalls and will not start. Its all there! LOL, but will not start. It cranks over fast enough too.

The only odd thing as I stated earlier is when I have someone pump the gas, when it hits the plenum it looks like its so hot it evaporates.... bun the engine itself is cool (tried w/ and w/o without "T"stat)"
 
"For diagnosing things like th

"For diagnosing things like this, I have a mustard bottle with gas in it and if I can't get it to start by clearing a flood, I squirt some gas into the carb, then close the bottle and crank it. This will tell you what you need to know.

If you have compression, spark and air/fuel, it should run. These are all you need but if the carb gets too hot, the fuel will boil and/or you can have vapor lock issues.

I use the mustard bottle because the top closes tight and I squeeze the air out before closing it so air expansion won't cause the gas to come out and be a fire hazard. I also make sure to have a fire extinguisher nearby.

I didn't see anything about it but you made sure the flame arrestor is clean, right? That's the single most neglected thing I see on customer boats and causes a lot of issues that seem to be something else."
 
"Thats a good idea a mustard b

"Thats a good idea a mustard bottle!!

I know i tried that last year when it started acting up, I cannot remember if it fired off or not, will try that when I get out again, great idea.

You mentioned backflushing the manifold... whats the best way to do this?

Bill"
 
"I have a Q on the water exiti

"I have a Q on the water exiting the boat also...
I also have a 87 Sea Ray with a 260 HP.. when I rev the engine I see the water gushing from the prop.. on the troubled boat I never seem to see allot of water from there, it exits from the side of the gimbal housing.

Would this tell me anything?

Thanks"
 
"The logic in adding gas is th

"The logic in adding gas is that if the gas in the bowl boiled, not enough new gas replaced what went out the accelerator pump and the bowl will be dry, more/less or, rather, it was at high RPM and the air/fuel ratio was a bit rich but wasn't noticeable.

Remove the hoses from the water circulating pump and attach a hose to the hoses that go to the exhaust manifold(s) and flush it out. Or, you can remove all of the hoses and use a replacement hose, going to a bucket so you can see if anything comes out. Flush the exhaust manifolds, connecting a hose to them and make sure the water exits normally. If you meet any resistance to the water pressure, it could be the risers or the riser gaskets causing this. Remove the petcock body and make sure water flows out easily. Don't just open these since a lot of sand, mud or other debris can become lodged in the water jacket if it's allowed to settle. If the hole is clogged, stick a thin wire in and clear it out till the water flows quickly."
 
"Thanks,
I will try flushing


"Thanks,
I will try flushing it out today.. I did take the riser off and it is clear and still in good shape. replaced with new gaskets.

I will let you konw the results.

This forum is great!! thanks for all the help sofar."
 
"OK,
I hooked the hose up the


"OK,
I hooked the hose up the the manifold where the Tstat housing connects... the water runs through free of obstructions, no rust out of drains etc. I saw the water pour out both sides of the gimbal housing, nothing from the prop. (it was tilted up)

Then I hooked it up with the Tstat disconnected thru the Tstat housing and nothing came through the manifold, are there ports that follow the runners on the intake to the block like a car? it came out where the impellor is only."
 
"Pull the circulating pump hos

"Pull the circulating pump hoses, too. The intake manifold has a couple of passages on each side and should pass some water but not at the same flow rate as the block and heads, which get a lot hotter than the intake.

If you have a large enough container that's deep enough for the water pickup slots on the lower gearcase to be submerged, run it on that instead of using a muff. I like having the water drawn up by the impeller because I can see how well it's working, not how well it works with pressureforcing the water into the raw water pump. I do the same thing with inboards, too. That way, if the raw water pump has a leak or a bad impeller, it's obvious."
 
"I only see the one hose to th

"I only see the one hose to the front of the engine from the Tstat that comes off the manifold. There is nothing coming off the rear.
I tested the boat in the water, there is plenty of volume coming from the impellor.

I mentioned that the water does not flow out of the prop like it does on my other boat.. is this normal? I do not remember if it ever did or not. Plenty comes out of the side ports off the gimbal housing."
 
"You should be fine- Alpha One

"You should be fine- Alpha One style drives have a separate cooling water outlet and yours is similar, so as long as there's plenty of water coming out, yours is working.

If you briefly place your hand on the intake manifold, it shouldn't be so hot that you burn your hand. It should be able to maintain the cooling ability at idle."
 
"Did you try adding gas with t

"Did you try adding gas with the bottle? It could be the idle mixture screw needs to be backed out, too. When it's cold, what is the idle speed? 650-700 should be about right for your motor. If it's lower, it'll stumble or die when you run it hard and try to idle hot."
 
"The carb mixture is set good,

"The carb mixture is set good, idle is at 800.. I did not get to try the mustard bottle yet, it ran great idling for 20 mins in the water, I didnt get to take it out.

Will do tomorrow hopefully."
 
Back
Top