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View Full Version : 351 HO wonít idle


gimmemoedmb
06-14-2011, 02:29 PM
I bought the boat two years ago and it has been running great with the 4010 carb, no problems at all. When I started it for the first time this year I could not keep it idling without giving it some gas to keep it at at least 1000 rpm. Anytime I let off the throttle it would stay running for 5-10 seconds and slowly kill. As soon as the rpmís get above 1000 or so everything is fine. I was doing 45 mph on the lake no problem, but as soon as I let it idle (either hot or cold) it just wants to die.

One thought I had is it could be bad gas. This summer has been crazy busy (getting married in two months) so Friday was the first time I was able to get the boat out. I filled it up in October with non-oxy 93 and a can of seafoam. This is putting the gas at over 8 months old, which I realize is pretty old even with a stabilizer. I havenít changed the plugs yet this year because I like to take it out for the first time with the old plugs but I will be doing that tomorrow night. I did change the fuel filter and fuel/water separator this spring. I bought the fuel filter at Napa and it came with two gaskets, one being slightly smaller than the other. The one I pulled off (which I bought from skidim last year) was the slightly larger one, but the smaller one seemed to fit better so I went with that one. Could that be an issue? Could it not be sealing completely and I have a slight air leak or something? Another thought I had is I poured the gas from the old fuel separator into the new one. After I had it on I realized this maybe wasnít a good idea and I shouldíve put fresh gas in it, so I am going to do that tomorrow night as well. I also plan on draining the fuel tank and putting in fresh gas and Iím hoping one of those solves my problem.

Iíve also thought about the rotor, cap, and plug wires. They are all 3 years old with about 80 hours on them and all look good. Sadly I will be lucky to put 20 hours on the boat this year so I was going to put that off until next year, but if that may be my problem I have no problems replacing them now. Iím heading up for a family cabin weekend on Thursday and really want to have the boat running well. Any thoughts on what I need to do or am I on the right track?

thatsmrmastercraft
06-14-2011, 04:10 PM
I bought the boat two years ago and it has been running great with the 4010 carb, no problems at all. When I started it for the first time this year I could not keep it idling without giving it some gas to keep it at at least 1000 rpm. Anytime I let off the throttle it would stay running for 5-10 seconds and slowly kill. As soon as the rpmís get above 1000 or so everything is fine. I was doing 45 mph on the lake no problem, but as soon as I let it idle (either hot or cold) it just wants to die.

One thought I had is it could be bad gas. This summer has been crazy busy (getting married in two months) so Friday was the first time I was able to get the boat out. I filled it up in October with non-oxy 93 and a can of seafoam. This is putting the gas at over 8 months old, which I realize is pretty old even with a stabilizer. I havenít changed the plugs yet this year because I like to take it out for the first time with the old plugs but I will be doing that tomorrow night. I did change the fuel filter and fuel/water separator this spring. I bought the fuel filter at Napa and it came with two gaskets, one being slightly smaller than the other. The one I pulled off (which I bought from skidim last year) was the slightly larger one, but the smaller one seemed to fit better so I went with that one. Could that be an issue? Could it not be sealing completely and I have a slight air leak or something? Another thought I had is I poured the gas from the old fuel separator into the new one. After I had it on I realized this maybe wasnít a good idea and I shouldíve put fresh gas in it, so I am going to do that tomorrow night as well. I also plan on draining the fuel tank and putting in fresh gas and Iím hoping one of those solves my problem.

Iíve also thought about the rotor, cap, and plug wires. They are all 3 years old with about 80 hours on them and all look good. Sadly I will be lucky to put 20 hours on the boat this year so I was going to put that off until next year, but if that may be my problem I have no problems replacing them now. Iím heading up for a family cabin weekend on Thursday and really want to have the boat running well. Any thoughts on what I need to do or am I on the right track?

Replacing the old fuel is a good plan. Pop the dist. cap off and use a small flat bladed screwdriver to clean the crud from the electrical contacts. Do the same for the rotor - that will buy you some time. Remove and reconnect the plug wires from the dist cap and the coil to make sure you have a good connection. A light coating of dielectric grease is best for both ends of the plug and coil wires.

You may have some gunk in your idle circuit of the carb. For each mixture screw, turn clockwise until lightly seated and count the exact number of turns and write down on a piece of paper. Remove the screws and insert the straw from a can of carb cleaner and give it a good shot of carb cleaner - then replace the mixture screws. Turn them in until lightly seated, then back out the exact number of turns.

gimmemoedmb
06-21-2011, 05:08 PM
Thanks for the advice Peter, I really thought that was going to fix it. Unfortunately now it’s just surging at idle and in gear at low rpms (below 1200 or so). I changed the fuel filter gasket, spark plugs, and put fresh fuel in the fuel/water separator. I also sprayed carb cleaner in the mixture screw holes. I then started it up and it at least was able to idle for me without dying like it did before. This time it was just surging between 600 and 800 rpm. I sprayed some more carb cleaner in the mixture screw holes and that was all I had time to do. I wasn’t able to drain the fuel and put in fresh fuel although I no longer think that’s my problem anyway.

I got to the cabin and put it in the lake. It started up nicely and was idling perfectly. I thought the problem was fixed. That was the only time it ran perfect all weekend. Whether it was hot or cold it would surge about 200 rpm. I adjusted the idle but it was still surging. I burned off over half of the tank of gas and put a fresh 10 gallons of non-oxy in and that didn’t help. It only killed on me a few times all weekend so that part improved, but I couldn’t stop the surging. I did notice it was smoking more than I remember in the past and there was quite a bit of soot on the transom which leads me to believe it’s running a little rich. Both of my mixture screws are at 2-1/4 turns out and I tried turning them both in a full turn but that didn’t have any affect on the idling.

I’ve done some more reading on here and I’m thinking it may be my power valve. It sounds like this is maybe more of an issue with 92 and prior years but my symptoms seem the same. I’ve had the boat for 2 years and never had a backfire. I can only think it has to be the power valve or it needs a carb rebuild. Is the power valve easy to replace? Can I do it without taking the carb off the engine? Unfortunately the boat is at the cabin and I won’t have much time to work on it before my friends arrive on Friday so it has to be an easy fix or I’ll have to deal with it for another weekend. Otherwise could it still be my cap, rotor, and/or plug wires? Possibly the pcv valve?

east tx skier
06-21-2011, 05:29 PM
Sounds like my old 4010. I tried seafoam, distributor cap, rotor, plug wires, pcv valve, mixture screws, high octane, accelerator pump springs, power valve, and, ultimately, a complete professional rebuild. Ultimately, what worked perfectly was removing the 010 and adding a 160 after the 4. I'm not saying there isn't a fix. But neither I nor my certified all his working life inboard mechanic could find it (other than to put a 4160 on there).

Same boat. Money well spent.

thatsmrmastercraft
06-21-2011, 05:47 PM
The power valve is a simple replacement while on the boat. You just need to remove the primary fuel bowl to get to it. To test it, simply try to turn the small end while holding the larger end. If it turns it is bad. when they go bad, they go full rich so that could be your problem. One good backfire through the carb will do it. Usually when the power valve is blown they run terrible.

Your mixture screws should be roughly 1 1/2 turns out. If you have the mixture screws out too far, you end up compensating with the idle speed screw and you can end up in the main metering portion of the carb. This will get you way too rich at idle, but OK at most any speed. I would set both screws to 1 1/2 and try working from there.

You may also have developed a vacuum leak around the carb or intake manifold. With the engine running, and using extreme caution, spray some carb cleaner (using the included straw) around the base of the carb and around the intake manifold. If you find a lead, you will see a momentary increase in idle speed.

How did the plugs and dist cap look?

A bad PCV valve can cause havoc with your idle speed. With the engine off, you should be able to remove the valve from the valve cover and shake it to hear the check ball rattle around. With the engine running, you should be able to remove the valve from the valve cover and place your thumb over the end of the valve and feel the vacuum.

Any or all of these could be your problem. You may have an obstruction in the needle valve causing a too rich condition. The carb likely is in need of a rebuild, in which case it is time to switch to a 4160, or the Edelbrock marine carb may be an option. Most automotive performance guys don't like the Edelbrock because there isn't much to be done with them. That may be a plus for the boat. Resale would be adversely affected with the Edelbrock.

gimmemoedmb
06-22-2011, 12:07 AM
Sounds like my old 4010. I tried seafoam, distributor cap, rotor, plug wires, high octane, professional rebuild. Ultimately, what worked perfectly was removing the 010 and adding a 160 after the 4. I'm not saying there isn't a fix. But neither I nor my certified all his working life inboard mechanic could find it (other than to put a 4160 on there).

Same boat. Money well spent.

I was wondering when you would chime in. :-) I may end up going with the 4160. I want to try to see if I can get this one working though since it's worked pretty much flawlessly for me for the past year and a half ever since I've owned the boat. I also have this wedding thing in August that apparently costs a lot of money so dropping $500 on a new carb right now isn't very appealing. I had planned on a new cap, rotor, and plug wires next year anyway so that's not money wasted. I can rebuild the carb myself so I won't be putting more than $100 into this one.

gimmemoedmb
06-22-2011, 12:16 AM
The power valve is a simple replacement while on the boat. You just need to remove the primary fuel bowl to get to it. To test it, simply try to turn the small end while holding the larger end. If it turns it is bad. when they go bad, they go full rich so that could be your problem. One good backfire through the carb will do it. Usually when the power valve is blown they run terrible.

Your mixture screws should be roughly 1 1/2 turns out. If you have the mixture screws out too far, you end up compensating with the idle speed screw and you can end up in the main metering portion of the carb. This will get you way too rich at idle, but OK at most any speed. I would set both screws to 1 1/2 and try working from there.

You may also have developed a vacuum leak around the carb or intake manifold. With the engine running, and using extreme caution, spray some carb cleaner (using the included straw) around the base of the carb and around the intake manifold. If you find a lead, you will see a momentary increase in idle speed.

How did the plugs and dist cap look?

A bad PCV valve can cause havoc with your idle speed. With the engine off, you should be able to remove the valve from the valve cover and shake it to hear the check ball rattle around. With the engine running, you should be able to remove the valve from the valve cover and place your thumb over the end of the valve and feel the vacuum.

Any or all of these could be your problem. You may have an obstruction in the needle valve causing a too rich condition. The carb likely is in need of a rebuild, in which case it is time to switch to a 4160, or the Edelbrock marine carb may be an option. Most automotive performance guys don't like the Edelbrock because there isn't much to be done with them. That may be a plus for the boat. Resale would be adversely affected with the Edelbrock.

Thanks for the thorough response! For $10 and a simple replacement I'm going to get a new power valve in case that's the problem. From a few other threads I've read that may be the case.

I did turn my mixture screws in to 1-1/2 turns out and it didn't seem to run any differently than 2-1/4 out. They have also been set at 2-1/4 out since I've had the boat with no problems until now so I was hesitant to mess with that too much. If nothing else works I will try that again. I wish I would've turned them all of the way in with the engine running to see if it would've run any differently.

I will check for a vacuum leak.

The dist. cap looked clean. The plugs all looked good but were all a little wet except for one.

I'm also going to pick up a new pcv valve since they are cheap and I have no idea if/when this one was replaced.

Thanks again for the help. I'll let you know after this weekend if I made any progress and if not, then I'll let you know when my shiny new 4160 arrives.

oldairboater
06-22-2011, 03:03 AM
We have a problem with gas going bad if it sits for a couple of months. We actually end up with three layers in the tank. Old gas, gunk, and water. Hard to get out of a gas tank without pumping it as much out as possible. I doubt your power valve is out----power valve being out stays out and boat runs like crap. You are doing something I don't recommend and that is multi changes. Work on one thing at a time and fully eliminate that problem before doing something else. I would eliminate the gas problem first. All gas out and start over with new filter. If that doesn't fix it---then I would check ignition system for corrosion. If that doesn't work then I would check carb for vacuum leaks and worn bushings. Then I would pull the carb and do a through carb job---or get a good carburetor man to do it.

thatsmrmastercraft
06-22-2011, 09:56 AM
I was wondering when you would chime in. :-) I may end up going with the 4160. I want to try to see if I can get this one working though since it's worked pretty much flawlessly for me for the past year and a half ever since I've owned the boat. I also have this wedding thing in August that apparently costs a lot of money so dropping $500 on a new carb right now isn't very appealing. I had planned on a new cap, rotor, and plug wires next year anyway so that's not money wasted. I can rebuild the carb myself so I won't be putting more than $100 into this one.

Your Wedding Registry doesn't include SKIDIM?

east tx skier
06-22-2011, 04:25 PM
I was wondering when you would chime in. :-) I may end up going with the 4160. I want to try to see if I can get this one working though since it's worked pretty much flawlessly for me for the past year and a half ever since I've owned the boat. I also have this wedding thing in August that apparently costs a lot of money so dropping $500 on a new carb right now isn't very appealing. I had planned on a new cap, rotor, and plug wires next year anyway so that's not money wasted. I can rebuild the carb myself so I won't be putting more than $100 into this one.

I understand. I think the rebuild cost me $175 and I sold the 4010 for $80. So not a total loss. I had the guy doing the rebuild try to talk me out of it, too. I decided to see if I could get it working for less. Didn't bear fruit, but I hope it works out differently for you.

gimmemoedmb
07-27-2011, 02:11 PM
Thanks for the help everyone. I did figure out the problem several weeks ago. It was in fact the power valve. After putting the new one in it now idles like a dream. Guess I'll keep on chugging along with this 4010...it's been good to me so far...

thatsmrmastercraft
07-27-2011, 04:05 PM
Glad to hear you resolved the issue. It's amazing how bad they can run with a ruptured power valve.

Note to self: buy spare power valve.

Dino Don
07-27-2011, 08:42 PM
Holley part number 125-500 Power Valve Check Ball Kit from Holley or from Amazon. I put one of these on my 87 ProStar 351 and it had never run as good as it does now and I don't have near the worry about blowing the power valve. It is promoted as the cure for blowing power valves. It makes the old carbs like the newer ones as far as blow out protection. $14.38 is listed as cost--detailed instructions and wasn't hard to do--

My 2 cents worth!

Kyle
07-27-2011, 09:21 PM
I also have this wedding thing in August that apparently costs a lot of money so dropping $500 on a new carb right now isn't very appealing.

Don't get too excited or anything there buddy. You still have plenty of time to run for the hills. Times ticking just a few days away. I can send you a really nice pair of running shoes if you need to start high stepping it. :D :D :D

Just was reading this thread for the first time. Glad you got your boat fixed and at a low price. Now you can keep spending more money on her wedding.

gimmemoedmb
07-28-2011, 02:33 PM
Don't get too excited or anything there buddy. You still have plenty of time to run for the hills. Times ticking just a few days away. I can send you a really nice pair of running shoes if you need to start high stepping it. :D :D :D

Just was reading this thread for the first time. Glad you got your boat fixed and at a low price. Now you can keep spending more money on her wedding.

Ha! I am excited actually...she's a keeper!

TxsRiverRat
07-28-2011, 02:37 PM
Ha! I am excited actually...she's a keeper!

If you feel you have to, be smart and head to the Carribean to do it... Just sayin...

mayo93prostar
07-28-2011, 02:56 PM
Good info to know that power valve helped idle problem similar to what I have. I have a full rebuild kit sitting in the work bench drawer so maybe I will replace the power valve and see if it helps.

thatsmrmastercraft
07-28-2011, 04:29 PM
Good info to know that power valve helped idle problem similar to what I have. I have a full rebuild kit sitting in the work bench drawer so maybe I will replace the power valve and see if it helps.

To test the power valve in your existing carb, remove it and while holding the large end, try to turn the smaller end. If it turns the diaphragm has failed - if it doesn't turn it's OK.