Intermittent Miss / Rough idle / Wont Rev - fuel related? UPDATED!!

ShawnQ

Chief Petty Officer
Joined
Jan 13, 2003
Messages
529
1995 88SPL

Ok guys,

If you have been following my previous posts, I had an intermittent miss when idling for any period of time. I thought I fixed it after having the carbs rebuilt, and the plugs/plug wires changed. It would idle great on the hose for any length of time.

I then had a problem with my starter drive spring, replaced it, and took it to the lake in hopes of a good runnning engine.

As soon as I pulled it off the trailer, I backed into the marina, and idled over to the dock. While idling over, it began acting up! It was light outside, so I couldn't see any arching on the plugs/plug wires like I had previously seen. However, I did reach down and touch them and they did not shock me like they did when they used to arc.

My brother parked the truck, and came to the boat. I tried to motor out of the marina, giving it a good amount of throttle, trying to get it to 'jump' back into power like it usually does. After about 7-8 seconds of full throttle, it kicked in and took off like a rocket, purring like a kitten just like it always has at high RPM's. During those 7-8 seconds, it sounded as if it wasn't running on all cylinders, or was just getting choked.

From there, I did a 5min long steady full throttle run over to the area where I catch bait. I then proceeded to idle in the area for 15-20mins while cast-netting, without even a hiccup. I again took-off to head to my fishing grounds on another 5 min full throttle run. When I pulled into my fishing area, dropped down to idle, and eased into my spot, it began running rough again.

Just out of curiousity, I reached down while it was running rough, about to die, and pumped the primer bulb. It was semi-deflated. When I primed it, it jumped back to life and sounded fine again. If I would've done this earlier at the boat ramp, would it have worked? I'm not sure.,,wish I would've tried.

Again that evening, when setting up my drifts, the same thing would happen. As long as I pumped the primer bulb, it would idle correctly.

FYI: I previously had a similar issue, which was probably the same issue which I THOUGHT I Had fixed. Last year I had the same thing going on, I would have to pump the primer bulb to keep the engine running at idle, and sometimes even at throttle. I replaced all of the items below, and took it to the lake and had '0' problems. I only ran it once, and then put it away for the winter thinking that issue was fixed. I could be wrong.

-All Fuel Lines have been replaced. from tank to engine.
-Fuel lines on carbs were replaced when carbs were rebuilt.
-Check valves behind carbs are operational (pressure tested them with a syringe(sp?).)
-All hoses and hose clamps are tight. Hoses on fuel tank, filler, and vent are double clamped
-Fuel Filter is clean and free of trash
-Fuel is new with 50:1 mix (pre mix, non VRO)
-OMC Primer Bulb is new (had a new one on it, bought another one and tried it also, just in case)

Any ideas?
I'm thinking fuel pump?
Do these things have regulators?

What stumps me is that at full throttle it is running great. I would think that a fuel problem would be the opposite. I would think it would idle fine, and then full throttle would be pathetic. But, I'm not familiar with mechanical pumps. If I don't prime the bulb, and then give it full throttle...it runs rough but eventually kicks in...like it is starving of fuel but finally gets enough to get going. I assume this isn't good for the engine, so now that I know it is again a fuel issue, I will make sure the bulb is fully primed.

Does the pump work on vaccum? At low RPM's, could it be suffering, but able to keep up at high RPM's?

Any help is appreciated. Unfortunately, I still do not have a manual.

Thanks,
Shawn
 

ezeke

Supreme Mariner
Joined
Sep 19, 2003
Messages
12,532
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Start with the anti-siphon valve at top of the tank. Better yet, run it on a portable tank with new hose, fuel and oil. Just eliminate the possibilities.
 

ShawnQ

Chief Petty Officer
Joined
Jan 13, 2003
Messages
529
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Anti-Siphon valve?

My tank has 4 holes in it.

One is an elbow coming off of it that has a 2" fuel fill hose.

One is a 3/4" 'vent' hose that is clamped to a plastic Perko fuel tank vent.

One is a hole cut into the tank for the gauge's sending unit.

The other is a brass elbow for the fuel supply line.

I've been told before that my vent may be clogged, but it works fine.

My tank is above deck, under my console, and easily accessible if this helps any type of testing.

My concern with a temporary portable tank is the fact that I can't always get the problem to repeat, it is intermittent. Some times it happens, some times it doesn't. Seems more prone to happen when I'm idling. I've never had it die at high RPM (except when I had a big air leak in my old fuel lines).

Thanks
Shawn
 

ob

Admiral
Joined
Aug 16, 2002
Messages
6,992
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Also insure that the pulse hose from the crankcase to fuel pump is secure with no leaks.Before starting engine ,make sure that the fuel bulb will pump up completely firm and hold.This will confirm that there are no leaks between it and the carburetors and that the needle valves are holding.Another item to check is the fuel pickup tube in the fuel tank.Insure that it has no fractures and that the strainer end is not restricted.
 

Solittle

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Joined
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Messages
7,518
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

If it runs fine at WOT for a time then begins to run rough I doubt that the problem is with the fuel pump. Your symptoms point to trash in the carbs. I would start by thoroughly cleaning them and installing new carb kits.
 

ezeke

Supreme Mariner
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Messages
12,532
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Coast Guard regulations require that tanks have anti-siphon valves on the outgoing fuel fitting. This is to prevent the fuel from being siphoned from the tank if there is a leak at the hose or engine.

The anti siphon valves are mostly of the type that have a ball bearing with a spring. The older ones are designed for inboard type installations, not outboards, and they are not designed for today's fuels. In addition, they are prone to sticking, causing poor fuel flow.

Regardless, the fuel system has to overcome the pressure of the valve in addition to lifting the fuel.

The bottom line is that if all of the outgoing fuel lines and the engine are higher than the top of the tank, the anti-siphon valve can legally be removed. If not, it is good practice to replace them with the newer ones.

If you take the tank fitting off and find that there is no valve inside of it, you can move on to the fuel line and bulb and so on.
 

ShawnQ

Chief Petty Officer
Joined
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Messages
529
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Ezeke,

My tank is lower than the outboard, but the fuel lines run below deck and are lower than the tank...so from what you say, it should have an anti-siphon valve.

The outgoing line appears to be nothing more than a brass plumbing type fitting, 90* angle coming out of the tank. It is made by a well known manufacturer (I don't remember the name, but found it online easily previously when trying to figure the capacity). I'll have to look at it closer. I wish I hadn't just put all that fuel into it, sure would be easier to move around!

SoLittle,

I thought the same thing. I just had the carbs rebuilt with new kits. I was there the whole time watching the mechanic do it. They came apart clean, and went together cleaner...the only way there is trash in the carbs is if there is trash in the fuel tank, which would go to the fuel line. The fuel line is new and the filter is clean and clear (it's transparent)...so theoretically, the carbs are still clean. They haven't sat more than 2 days since being rebuilt and re-installed with new gaskets and hoses.

OB,
How long should the bulb remain firm before it releases pressure, without running the engine? What is a suitable amount of time?
I will attempt to check the fuel pickup line, it should be easy to access.

Is there anyway to pressurize the fuel system without hurting anything? I've got a compressor, adjustable regulator, and could probably pull the hose off of the back side of the Primer bulb and push air through the main supply line and into the tank. If I plugged the fuel-fill fitting and the vent fitting, I should have a 'sealed' environment, correct? Everything is double clamped and the correct sized hose, all new...it should be tight.

Thanks again guys. You'll make an OB mechanic out of me sooner or later! Give me a 5.0L Ford and I'm all over it! Throw a V4 Carbed OMC at me and I'm lost!

SQ
 

ezeke

Supreme Mariner
Joined
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Messages
12,532
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Again, the quickest way is to beg, borrow or steal a portable unit, bypass everthing and see if the problem is still there.

If you put too much pressure on the system you will blow the membrane on the fuel pump and the motor will not run correctly, for sure.

If you want to see if there is an anti-siphon valve without taking anything apart, try to siphon the fuel through your fuel hose to a receptacle that is no more than say, 10 or 12 inches below the bottom of the fuel tank. If there is an anti-siphon valve, the flow will not sustain itself.
 

ShawnQ

Chief Petty Officer
Joined
Jan 13, 2003
Messages
529
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Try siphoning through the supply line or through the fill hose?

If I was to pressure check anything, I would start from the tank side of the primer bulb, and check my fittings between the primer bulb and the fuel tank. I wouldn't pressurize anything on the engine.

Thanks,
SQ
 

ezeke

Supreme Mariner
Joined
Sep 19, 2003
Messages
12,532
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Nothing that I have said on this thread has anything to do with the fill hose, the vent hose, or the gauge.
 

mikesea

Lieutenant Commander
Joined
Oct 1, 2006
Messages
1,830
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

you say the ball was semi deflated,this could be a restriction between the fuel pick up in the tank and the ball,the fitting that your fuel hose connects to at the tank should be the anti siphon,but, often they are removed due to problems you are having,look at that area of the fuel system,you might have something floating in the tank ?? that at times gets caught at pick up,or if there is an anti s. it might be acting up,you can get a new fuel pick up assy,and replace what you have,usually there is a anti s. with the pick up tupe ,but Ive seen everything from all kinds of home made applications,I agree with the fuel pump theory,runs bettr at high speed,maybe as your getting on plane,the object??? in tank moves,or the restriction becomes over come with increased vacuum.Its all a theory,but when a ball is getting sucked closed .a restriction upstream is probable,like sucking on a straw and putting your finger at the bottom,if your vent was bad,simply loosening the cap on tank would fix that temporarily.the fuel pick up dont have a screen on it,does putting more fuel in the tank help,the pick up is often near the bottom ,maybe 1/2 inch higher,but that can be adjusted when installing,the higher in the tank it is the less fuel you can use,so its good to keep it up some to avoid sucking the bottom of the tank,if yours is very close just a piece of plastic or??? could get sucked onto the tube.worth looking at,and the portable tank ,though intermintant,could run trouble free giving some idea
 

ShawnQ

Chief Petty Officer
Joined
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Messages
529
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Mike,

Thanks for the info.

When I re-finished the interior of my boat, I siphoned all of the fuel out. It all appeared clean, but there could very well be something clogging the pickup as I didn't remove it and have never inspected it. I don't even know what it looks like. As far as I know, it could just be a piece of aluminum tubing stuck down into the tank.

I wouldn't think there is a valve in the 90* fitting that connects to my fuel supply line, but there may be.

As for the fuel 'cap'. Mine doesn't have a cap. It has a plastic 'flip open' fuel filler neck attached to the fuel filler hose that then goes about 3' to the tank. It has a fairly soft high density foam O-ring around the cap. If this is leaking, would it cause trouble?

I'm having trouble understanding the 'air tight' idea when my tank has a vent on it. Is this vent only supposed to let air out, and not in? If so, it could be dysfunctional (I'd have to test it again).

I'll probably end up buying a cheap 3gal portable tank and give it a go, see what happens.

Thanks again for the patience and input fellas. It will hopefully save me some cash and down time through a dealer.

SQ
 

mikesea

Lieutenant Commander
Joined
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Messages
1,830
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

you dont want the tank to be ''air tight"it has to allow air in because the engine is sucking the fuel,if it didnt vent the engine would be sucking a vaccum that if it didnt stop running for lack of fuel,it could actually suck the sides of the tank in,if your vent which ,yes allows air in,and is supposed to keep fuel from coming out.the vent is what allows the engine to suck gas without causing a vaccum.often,bugs get in the vent,or corrosion causes it to not vent,that will allow a vacuum situation to happen,if you suspected the vent to be the problem,you could simply open the fuel fill,that would then vent the tank,of course you cant leave it open to the elements.and either the vaccum situation or a restriction will cause the ball to get sucked ,when the problem arises,you can open the fuel cap,if the vent is the problem then the vaccum will stop,I thought you mentioned the vent was ok,(I read alot of stuff allday)so look into the vent and or the fuel pickup for trouble
 

ShawnQ

Chief Petty Officer
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Messages
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Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

The vent is fine. Air passes through it with ease.

I'll check the fuel pick up as soon as I get the chance.

Thanks guys.
SQ
 

ShawnQ

Chief Petty Officer
Joined
Jan 13, 2003
Messages
529
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Ok guys, I went out there and successfully removed my fuel pickup. It was as clean as a whistle.

As suspected, i found no type of anti-siphon valve. Just a simple 90* brass fitting, a piece of aluminum pipe with a piece of rubber fuel line attached to the end and a piece of screen in the end of the rubber line.

The only problem that MAY be between my tank and the engine is the fuel line could possibly be smashed inside the rigging tubes by the steering cable? Doubtful...but possible? Not sure how I would test that.

I still think it is something in the engine's fuel system, or possibly a power pack. BUT, the one thing that keeps me from looking at electrical items is the fact that I can get rid of the rough idle by pumping the fuel bulb.

Here's pics of how my boat's fuel system is setup:

Here is the fuel pickup assembly:
img0201gx4.jpg


Here is a close up of the submerged end of the fuel pickup:
img0205bn4.jpg


Here is a close up of the fuel supply line end of the fuel pickup:
img0206tm1.jpg


Here is the type of fuel filler cap I've got:
img0213rm8.jpg


And the type of vent I've got:
img0216qb3.jpg


Here is the fuel fill hose and the vent hose going to the vent and the fuel cap:
img0226pt9.jpg


Here is what I believe to be the choke mechanism with a built in fogging/winterize port: (?)
img0221vs2.jpg


Here is the fuel pump region on my engine:
img0222kq9.jpg


Here is where the fuel pickup comes out of the tank, and into the fuel supply line:
img0224cx7.jpg


Here is a view of the fuel supply line coming from the tank, straight into the rigging tube:
img0228kz0.jpg


Here is the fuel supply line coming out of the rigging tube, into the fuel filter, and out to the engine:
img0229dw1.jpg


img0233yz3.jpg


It's a fairly simple system.

Any further ideas? I pulled the round cover off the front of the fuel pump, and inspected the first layer of the diaphragm and it appeared fine. I didn't want to get into it any further as I've never dealt with one before and don't have the parts if I mess something up.

Thanks again.
SQ
 

Solittle

Fleet Admiral
Joined
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Messages
7,518
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

I have nothing more to contribute to the problem. At some point I would replace that dinky in line fuel filter with a filter/water separator type. They look like an automotive oil filter.
 

ShawnQ

Chief Petty Officer
Joined
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Messages
529
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Solittle,

I was planning on adding a water seprator filter some time this season. However, with the current problems I am having, I am more eager to make it run correctly than I am to add more pieces to it. That is definitely in my plans though, if I don't end up selling it first out of frustration. :devil:

I didn't get a chance to check the crankcase line or prime the bulb to see if the carbs will hold the pressure. I will try that tomorrow for sure.

Where is the crankcase line located on this engine?

SQ
 

ShawnQ

Chief Petty Officer
Joined
Jan 13, 2003
Messages
529
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Hey guys,

One thing I DO want to clear up. The primer bulb is not 'sucked flat'. It just is not full of fuel like it should be. It acts as if it has air in it, and I have to compress it a few times to get fuel back into the engine.

I was reading through an old post from when I first bought the boat. I was having similar problems, and I thought i had fixed them when I replaced my fuel lines and filter. I haven't run the boat much since then, so I may have never actually fixed it. I could make it run fine then by just pumping the fuel bulb, similar to what is happening now:

http://forums.iboats.com/bbBoard.cgi?a=viewthread;fid=28;gtid=678580;gpid=678580#gpid678580

Thought I'd throw that out there for more ammo to any of you who are trying to help.

Thanks again.
SQ
 

ob

Admiral
Joined
Aug 16, 2002
Messages
6,992
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

If pumping the fuel bulb allows the engine to settle out ,it is most definately a fuel delivery issue.Only problem I'm having assisting in the diagnosis is the fact that you say it will run at full throttle without a hitch for 5 minutes at a time until you slow to trolling speed.Otherwise,I'd suggest that your fuel pump is either failing or there is an air/fuel leak somewhere between the primer bulb and the carburetor inlets.Even a small leak is a problem.The fuel primer bulb should pump up firm and hold medium hand pressure with absolutely no signs of colapsing before starting the engine.If you can feel it beginning to colapse in your hand ,even after several minutes of holding pressure on it,then fuel is being pushed past the carb bowl needle seats or leaking out somewhere between the bulb and the carb inlets.

Did you inspect the pulse hose that leads from the crankcase to the fuel pump?If there is even a small leak on it ,it will not actuate the fuel pump diaphragm efficiently.Particulary at low rpms when the pulses are weaker.

If you test run the engine again and it acts up ,try pushing in intermittedly on the key primer and see if it will settle the engine out.This will help rule out a slow speed carburetor circuit issue.If everthing is working right and there are no idle orifice restrictions ,pushing in on the key primer on a warm engine at trolling speeds should flood the engine.If the engine picks up ,then your issue lies within the carburetors.If there is no effect,then the primer valve isn't getting fuel supply.
 

iwombat

Captain
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Jul 12, 2006
Messages
3,767
Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Re: Intermittent Miss / Rough idle / Wont Rev - fuel related?

Reading back a bit, you say you watched the mechanic rebuild the carbs from disassembly to assembly. This implies that the mechanic didn't soak the carbs.

You may still have gunk in 'em.
 
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