Motor trouble - ever hear of this? (for the gearheads and I;m not one)

[So_I_Ski]

I own a 350 chevy block PCM, 4 barrel holly carb so not EFI. About a week ago the motor started dogging at anything over 2600 rpm. I can drive all day at 2600 or lower but anything above and about 7 - 10 seconds later it will stall out unless I pull back on the throttle. I can pin it and again it runs flat out but only for 7 - 10 seconds. I've changed the fuel filter, fuel pump, secondary rotor, checked and cleaned contacts in the distributor and rotor, cleaned the heat exchange, checked the impeller, put in a fuel conditioner (sea foam), sprayed in a carb cleaner and my buddy noticed that it was still misting gas into the carb as it was stalling. Seems like only some kind of ignition issue or maybe a faulty sensor shutting down the engine over 2600. Got a date with a mechanic sometime next week but just wondering if someone has seen this problem before or if anyone has any ideas. Thanks

[Intheday]

Condenser if it has one caused this on an old boat of mine

[Rednucleus]

Your description sounds like fuel starvation to me - but that's all I got, sorry

[jimski]

Accelerator pump In the carb

 

[HPskier1]

+1 for accelerator pump, did that on an old boat of mine. Rebuilt the carb and good to go or you can just do the pump.

[Orlando76]

350 PCM 4 barrel? Year? Although they do exist, rarely seen in the wild. Sounds like fuel, most likely the accelerator pump otherwise bowls are shutting off too soon.... but... you have fueling misting which means it is getting something.

 

Now if you meant to say around about a 1992 351 PCM then it’s the Pro Tec Ignition going out. But I’ve never seen a PCM 350 from the days of the Pro Tec ignition to know what they used for spark on the 350. I’m not even 100% certain bow tie flavor was available in those short few Pro Tec years.

[So_I_Ski]

Accelerator pump has two votes so that sounds promising so thanks for the input. It certainly acts like it's starving for fuel and if I let it stall out then it takes a minute or two to start again. And the year is 2006. It's in a Gekko that I bought new.

[Orlando76]

Really? Got a picture of the motor? Still can’t rule out ignition. Do you have a timing light and know how to use it? Have you taken distributor cap off? Ought to check timing and make sure it’s fully advancing and not a rusty spring.

[TEL]

Pull the pickup tube out of the gas tank clean the small screen on the end of the tube.

[503Kento]

Check your fuel lines, you might be sucking in air through a crack or loose connection.

[PacMan]

I would also say carb. Sounds similar to a problem with an 86 mc prostar my family had. Rebuilt the carb and a little tuning afterwards and everything's great.

[BraceMaker]

Go with @TEL doesn't sound like accelerator pump to me at all.

 

You said you can floor it and it acts normal for 7-10 seconds. The start of that would be accelerator pump then power valve as the vacuum dropped the secondaries should be mechanical on that carb.

 

Then after 7 or so seconds the fuel pump starves and the level in the floats drops

 

My old prostar would do this and every time in was crap in the spring inside the fuel tank pick up tube

[Windsurfnut]

There was a 5.7 HO PCM carb 315 around the mid 2000’s

 

Sounds fuel. Anti siphon valve in tank clogged/restricted?

[waterbeat]

Sounds like a fuel supply issue. Besides what others have mentioned above, there may be an additional filter concealed somewhere between the tank and the engine. Ran into this once on a Malibu where a filter was concealed behind the bulkhead in front of the tank.

[LeonL]

First off, if the engine stalls at cruising speed it ain't the accelerator pump. (Note: acceleration). Some carbs have a filter at the fuel inlet. You didn't mention that.

[DW]

Won't be the accel pump as noted, that should have dumped well before 7-10 seconds. Anti siphon valve stuck shut would cause a vacuum and restrict fuel flow, you can clean it with carb cleaner or brake clean. Dirty fuel filter, a restriction in the fuel line, clogged fuel tank vent (if a sealed fuel cap), are likely suspects but sounds like to checked those. Is there anything specific you could point to, did you do anything to the boat just before the problem started (maintenance, run out of gas, etc.).

 

Debris in carb a likely scenario, you might also want to check the float level. You might have something on the secondary carb system not allowing fuel to be fed when the secondaries open up thus the mixture goes lean, the visual fuel mist test won't tell you that. So verify both bowls have the same amount of fuel in them. 2600 RPM seems a bit low for them to open but worth checking. Holleys should have a filter right at the inlet IIRC.

 

What do you mean by contacts in the distributor, points or hall sensor?

[Jetsetr]

Fuel filter, collapsed fuel line from tank not venting, dirty, plugged or restricted tank pick up filter, restricted fuel line...

Easy tank venting check trick is to run with the fuel cap off...

Not accelerator pump.

[ReallyGottaSki]

I was 'arguing' with a guy who said the tank and fuel delivery wasn't he problem, said it was ignition because it fell on its face and had similar symptoms to yours at a higher rpm, and he didn't want me to take the tank out and dip tube apart.

 

I squirted compressed air backwards the fuel line back into the tank, the rpm and duration the symptom reappeared became higher and took much longer to occur.

 

So the tank got accessed, and the dip tube, screen and such finally got the proper attention

 

Another troubleshooting method is to run the engine off a temporary/portable fuel source, like an outboard tank, but it has to have good flow available through its QD.

[vtmecheng]

Agree with everyone saying it isn't the accelerator pump, when that goes your engine will bog or die when you throttle up more quickly. The suggestions made from @BraceMaker down are good things to check. Keep us posted.

[Keith_Menard]

@Orlando76 I have an 2002 Gekko GTO fuel injected 350. When I was looking I noticed some out there that were newer than mine, but had carburated engines. Odd...but they exist.

 

I had similar issues on a Camaro I had...it wound up being the screen filter IN the carb. No one ever checks that one...myself included. The other thing it can be is a collapsed bowl. Sometimes over the years the bowl will get a pin hole or something in it, so it will fill up with fuel and cause a lean out. If I had to guess in this situation...it would be that one.

[2Valve]

I vote for a fuel tank venting/pickup tube issue. One quick test that's been around for years on old motorcycles (non-fuel injected) is when the engine won't maintain a certain rpm, to open the fuel tank. If there's a sudden rush of air, you know you have a venting problem. Hope you get it solved.

[Keith_Menard]

@2Valve Yup...VERY easy test...just be careful not to take on water when you stop :)

[BraceMaker]

@2Valve - bees always the friggin bees.

[So_I_Ski]

What a great site and what a great bunch of guys taking the time to offer possible solutions. Too many questions to answer directly but I did check that screen at the carb inlet last week and it was clean. There is only one fuel filter and a friend installed that a couple of years ago because it had no filter from the factory (odd). And I didn't do anything that I know of to precipitate this problem such as run low on fuel. Plus it came on in three stages. Started my holiday and all good for first three days. Then day four just up on plane and it died completely. Took a minute to start, then ran fine for two more days of skiing. Then cruising down the lake for 5 minutes and a quick brief drop in rpm and back up to speed without touching the throttle. Then I went home from our place on the lake for two days and when I came back, immediately headed out but only got about 2 minutes down the lake when it started dropping rpm and surging back up on it's own. This continued as I limped back to my dock. From then on as I tried different solutions it settled into that I described initially where I could drive it only at 2600 or lower which I did for the last 5 days of my holidays just to cruise out to where my buddies gather to ski on the second lake. Fortunately, half dozen of us all take our holidays at the lake at the same time and we all have tournament boats so my skiing was not interrupted.

[vtmecheng]

Been thinking about this one. If the tank isn't venting at all then wouldn't it eventually stall even going slower than 2600 rpm? If it were venting some then he should be able to get it up to full throttle for a short time before causing a stumble and that time would likely increase as fuel in the tank decreases because the air is more compressible/expandable. From the description, the stumble happens more quickly. The vent thing is easy to test so he might as well but I have a feeling that won't be it.

 

The carb float could be getting stuck such that it can't replenish fast enough or it has a collapsed bowl. Also, did you replace the fuel filter that your friend installed or are you assuming the one he put on is still good after two years? If it's been a couple years you may want to pull it and see how things look in the tank while there.

 

If all of these are good it's always possible that your ignition coil is starting to get weak but I feel they don't tend to go in this manner.

[eleeski]

Not sure what kind of boat you have but some boats have a check valve at the top of the fuel tank where it connects to the fuel line. That can fail and restrict fuel flow causing your problems.

 

Some fuel tanks have a sock filter on the fuel pickup. If this is plugged you can get the same symptoms.

 

My old American Skier had both a sticky check valve and an inlet that would intermittently plug up. Similar issues even with the new ZO engine. Running on outboard style 5 gallon tanks cured the problem. I removed the stock tank and will clean the tank and bypass the sock filter in the tank (unless I can figure out how to service it) - someday. Blowing out the sock filter didn't solve the problem but something popped for sure indicating that there was a filter there and it was plugged. Bad design when one can't service a filter - but the boat is 40 years old.

 

New boats sure are nice...

 

Eric

[BraceMaker]

@eleeski I still wish we had those in ski boats. Hauling the gas in cans anyway!

[teammalibu]

Try running on a clean fuel supply directly to the carb. Eliminate fuel supply issues!

[So_I_Ski]

@vtmecheng yes the first thing I did was replace the fuel filter that my buddy had installed plus checked the thumb sized screen entering the carb. So I was too busy today to try anything but tomorrow morning I will go with Bruce's suggestion and pull the fuel tank pickup tube and check it for debris. Spoke to a merc tech today who had also stopped by to visit a mutual friend and he suggested the primary coil as the possible culprit. So replacing that might be my next trick. After venting or fuel supply hiccups and then the coil, it sounds from you guys that I might be into carb issues but those are beyond my pay grade so I will wait for my appointment with the monkey wrench mid next week. Unless one of guys wants to fly up to beautiful BC and go skiing on a remote lake that Andy Mapple described as one of most beautiful settings he had ever skied on when we had him up here coaching about 6 years ago. He skied at noon each of the three days and ran from 28 thru 39 off at 34 and never missed a ball. He went out on his road bike at the end of each day and said he was in the best shape of his entire life. All lean muscle and I think he said he weighed 178. What a loss.

[Jetsetr]

A float in a carb that sinks cause the engine to run rich, not lean...when it sinks it holds the needle and seat open flooding the engine...usually fuel will run into the engine thru the bowl vent.

 

 

[So_I_Ski]

@Jetsetr thanks for that explanation. So if I understand you that would align with my symptom when I let it die instead of pulling back on the throttle. Then it doesn't want to start for a minute or so because it's flooded.

[Orlando76]

Meh, I still think it’s worth while, instead of (I’m assuming) pull back seat, fight a filler hose, pull a tank, etc to just slap on timing light that doesn’t require any tools and see that the timing isn’t retarded. At that 2600 rpm mark you should be getting 30* advance?

[Jetsetr]

@So_I_Ski unfortunately no...it will run like a raped ape with all that extra fuel at high power settings with a stuck/sunk float...however when you come off full throttle or close to that and get below 1500rpm(ish) it will run EXCESSIVELY rich and quit at idle...plus, it will probably blow black smoke, you will see fuel dripping into the carb from the vent tube and you will smell the excessive fuel in the exhaust....not to mention the engine will not sound happy.

 

What we think (the Collective) is you’re basically running out of fuel (lean) at higher RPM due to a lack of fuel. Hence, a fuel delivery issue. Could be a fuel pump failing as well but those mechanical pumps are usually pretty bulletproof.

 

The GOOD news is this should be an easy issue to solve with the old school carb and fuel system. I LOVE my carbed 351 and points ignition...a screwdriver and a matchbook cover will get me home!!

[RichardDoane]

It stalls because it is either out of fuel or has lost spark. Do a better inspection of the fuel line. There must be another filter from the factory somewhere. Sounds like loss of fuel flow.

[Rednucleus]

An interesting test would be to have a a helper CAREFULLY pour fuel into the carb as it starts to stall out and see if it comes back to life. That will tell you that fuel flow is the problem. Please have fire extinguisher handy for this test.

BTW, does it stall out if brought up to these RPM's in neutral, or only under load??

[Jetsetr]

delete

[So_I_Ski]

@Rednucleus as far as I can tell it only stalls under load. I ran it at 3000 in neutral and no stall but truth be known I may not have let it run long enough. So I guess I should hook up the fake a lake and try it in my driveway. Assuming it only does it under a load does that change what I should be looking for? And @Jetsetr, it's an electric fuel pump but that was one of the first things I replaced and no change. Unless I got a bad pump but wouldn't it be very odd that a bad pump would have identical symptoms? @RichardDoane there is definitely not another fuel filter hidden somewhere. The boat came with none so my buddy installed one for me which I have just replaced. @Orlando76 while you are correct that a timing light would be an easy test, unfortunately I don't have one. Also if that was the problem, it wouldn't run great above 3000 for about 7 seconds, would it?

[BraceMaker]

Wait what boat do you have and which engine

[TEL]

Again ,Pull the pickup tube out of the gas tank clean the small screen on the end of the tube.

It's starving for fuel, if the screen is not on the end some boats have it inside the tube. You will have to unscrew it from the 90 degree elbow and pull it out and clean. Dead revving it on the trailer it will probably run fine under load it needs more fuel.

[Orlando76]

It could.

Electric fuel pump!? Hmmm.... again, never played with a 350 carb pcm but doubt they used electric fuel pump. PSI is probably too high. Is it a 4160 Holley carb? Carbs typically like 6 psi, electric pumps vary, usually 36 psi. But that should cause an adverse affect.

[BraceMaker]

@Orlando76 that's what's surprising me. Most carbed boats have mechanical fuel pumps.

 

 

Sort of sounds like a fuelie boat that got reconverted to a carb.

[Orlando76]

Got pictures?

[Orlando76]

Have a serial number? What’s the 11th letter in the serial number on engine?

[TEL]

Not common to see an electric fuel pump on a boat. @Orlando76 they have been making electric fuel pumps for carburetors for over 50 years, that are self regulated under 7psi

[BraceMaker]

@TEL while true I'm not used to that on boats.

[Orlando76]

I’m aware of that Tom. But again, rare on boats, and it’ll be a first for my PCM experience with a carb. Hence why I said “electric fuel pumps vary, typically 36 psi”

[vtmecheng]

In neutral it doesn't take much fuel to hit 3,000 rpm. It's only in gear and propelling the boat that the engine starves for fuel.

[Rednucleus]

My buddy's carbed 350 97 Sportster came stock with an electric pump. EFI was a more expensive option. If the pump pressure was too high it would be overwhelming the floats and running rich. Would be easy to check fuel pressure and wondering if you have a fuel regulator - the Sportster has a low pressure electric pump, no regulator.

[Orlando76]

@Rednucleus by any chance was that Sportster a Mercruiser 350?

[Rednucleus]

@Orlando76 not mercruiser for sure; I think PCM - I will verify

Update it's a 350 chevy, 310 hp Indmar package