Help with 1999 Mercruiser 5.0 305 Thunderbolt V Ignition!! Backfire through carb

[precool26]

Hello guys! After months of frustration and wasted money on this engine I am hoping you can help me. I have a 1999 Maxum 1900sr with the 5.0 Mercruiser 305 and the thunderbolt V ignition...

At first when this problem happened, it was during a WOT run. I start having backfiring/popping coming out of my carb along with loss of power. After putting it in two shops, the problem is only worse. I have replaced dist cap and rotor, plugs, plug and coil wires. I have had a so called "complete carb job" done at the last place for 400 something dollars, to no avail. I can tell you that my oil pressure switch is bypassed. Evidentally has been since I have owned it (bought last year). I dont know what else to do. The shop I have it in now has just called saying I need a new fuel pump and carb to get her running again. 1300$$$. I am so hesitant to do this seeing as how everyone I have had look at this thing has come up with good ways for me to spend money with them, but with no effect on performance.

She will run somewhat decent until she gets hot, then the backfiring starts to occur. I have messed with idle mixture screws and disconnected my electric choke in feeble attempts to find something.. I am hopeless in this thing. When I pull my wires off my dist cap one at a time with engine running, some make a noticeable difference in the purr of the engine, while some do not. I can hear the popping of the spark when i do so, but it is in no way synchronized, more erratic and kind of al l over the place.

The mechanic says everything is fine as far as compression in my cylinders. I just dont know and honestly cannot afford another attempt at fixing it.

Please help, please. Thank you guys

 

[precool26]

I forgot to mention that she has never really performed amazingly since ive purchased her, but for the first 4-5 months she was doing everything I needed her to mechanically, with no hesitation. It just started one day and has never been the same since..

 

[Scott Danforth]

Backfiring through the carburetor is either two wires crossed, crosstalk in the distributor, or your ignition timing is off.

 

[precool26]

So, what should I try doing next? I need some suggestions. I do feel like there is an ignition issue but why would it not run rough until she gets heated? Wouldnt there be a backfiring/missing situation from the time it cranks to the time it dies? It will run until operating temp, then start this...

 

[Bt Doctur]

There are so many reasons why, you just need to fine yours.
Poor fuel/phase seperation
Restriction in the anti-syphon valve
Engine timing
Detonation damage
Coil failing
coil tower failing(arcing)
Leaking manifold/ riser fouling a plug with water

 

[alldodge]

Sorry to hear but I don't think your shops are that good at what their doing. They rebuild the carb and that doesn't work so now they want to replace it and change the fuel pump. Checking a fuel pump is real simple, and they should have done that before the carb was rebuilt. You don't rebuild a carb if the pump is not working correctly. Hoping the replaced the inline fuel filter in the carb while it was apart, also hope they actually took it apart

I'm guessing your running lean at higher rpm's and that's why your back firing. If the plugs are pulled I would guess they are white and maybe even have some spots on them.

Since you haven't had the boat long, I will guess some more and think the prior owner started having some problems and may have replaced the carb with a used one and maybe the wrong jet size installed. Either that or your anti-siphon valve is sticking, tank vent is clogged or another fuel line restriction. If the boat is running lean it won't take long to burn a piston

 

[thumpar]

I would start with the ignition side. Cap, rotor and check the plug wires. Check the timing when you are done. I believe those need to be set to base mode to set the timing.

 

[Rick Stephens]

Start by checking wire order. That's free.

Then replace cap and rotor and check timing.


You can find step by step diagnostics in the service manual as well. Including firing order and timing.
http://boatinfo.no/lib/mercruiser/manuals/mercruiser24.html

 

[precool26]

I replaced cap and rotor last fall, and Have replaced plugs, plug wires, and coil wire two weeks ago. According to the manual, I have my firing order correct. What is next? And to clear things up, I brought it to one shop, they did the carb cleaning. Got it out and the same problem was present. After bringing it to a new shop I found out he says I need the new pump and carb. He said that shop either must be dumbasses or they didnt do what they say they did. He wants to send it to Houston to get a professional job done on it. I dont know what to do, Im trying to get this done myself without having to pay out the butt. Can I get some more in depth processes for me to go through on this from you guys, so that I can test some different things? I have removed and checked the anti syphone valve. It looked good and I could blow through one end, but not the other, so Im assuming it is operating properly.

Thanks guys!

 

[thumpar]

Was the timing ever checked in base mode?

 

[precool26]

My mechanic claims it was, I never laid eyes on it.

 

[alldodge]

Go to the local auto store and pick up and vacuum fuel pressure tester. Something like this, also have them at harbor freight.
http://www.amazon.com/Actron-CP7803-Vacuum-Pressure-Tester/dp/B0006V2BS2

Also remove the fuel line between the fuel pump and carb. Reason for this is you want a fuel line fitting to screw into the carb with a end the resembles this or barb fitting. This one is bent 90 degrees but you want one which is straight


Next get a foot of 3/8 fuel line and 3 hose clamps.

Cut the 3/8 fuel line in half. Reinstall the original motors fuel line on the pump and connect one piece of the 3/8 fuel line to the other end with a hose clamp. Attach the fuel pressure tee to one end of the line. Install the fitting in the carb and with the other half of the 3/8 fuel line, the to the tee fitting. Attach the vac/fuel pressure tester gauge and start the motor. Should read 5 to 7 psi.

 

[Fun Times]

Are you to the point of that you're going to start working on the engine yourself vs having a shop do it? If so we're here to help guide you as we can talk you through doing things and there are some videos online that may help give you a better visual too.

For testing, you are going to have buy and/or rent some tools such a fuel pressure test gauge along with Merc part number 91-806901 (http://ww2.justanswer.com/uploads/th...el_adapter.jpg) , a vacuum test gauge in order to test the fuel pump to fuel tank vacuum readings and also test what the engine intake manifold vacuum readings are.
Also you'll want to perform an engine cylinder leak down test to see if excessive air is escaping out at the wrong times. In the following link is a long boat video with a Mercruiser engine but he started the leak down test at the 27 minute mark.... https://www.youtube.com/watch?v=fEbYw0P0Y5k

http://www.justanswer.com/boat/6wy42...ercruiser.html

Do you or a friend own a ignition timing light to check timing?
What was the max RPM you were ever able to get out of the boat at full speed with a little bit of trim up added to the stern drive?

Bear in mind that if you or the service shop is just going to throw parts at it until it's fixed then doing so one at a time is better than two at a time or you will never know the real cause of the problem.

Edit: Just saw AllDodge's way of testing fuel PSI which will work too vs my suggestion above.

 

[precool26]

Thank you guys. I just picked my boat up from the shop and brought it home. While at the shop, my mechanic (which is a knowledgeable guy, the most trusted outboard guy around here, he knows his stuff just refuses to work on i/o anymore) broke it down for me. He said that my carb needs a complete prof rebuild (as he was going to send it off to Houston) its running too rich and that my fuel pump was losing pressure... I am currently in the hospital as my fiance and I have just had a baby girl, who might i add is my first child and was born on my birthday!!!!!. As soon as she is released I am def going to take your suggestions and test it. My question is with him saying that it is losing pressure, would a pressure test not show a failure until its at higher rpms or under a load? Also I have a friend here who owns a reputable mechanic shop and Im sure he would let me use his light. The problem is that my tach has not been working since I bought the boat so I have never been able to read my RPMs. I just drove it cautiously, never FULL throttle as I didnt want to risk cruising over 4500.. I kinda listened to the sound to gauge the revolutions... I hope I am not writing too much, I just know that I cannot afford to continue to pay mechanics for this kind of business... Thank you for all your answers!! I will also post pics if needed. As far as a leakdown, I have learned how to do it by watching that same video, I just havent performed one myself!

 

[alldodge]

Congrats on the new addition, best to all ya'll

The fuel pump pressure will in most cases be more at idle then any other rpm. If the pressure is to much for the carb it will cause it to run rich at lower rpm. If it is running rich then a lean condition would not happen unless there is clogged jets in the carb. Either way if its running rich I don't see a fuel pump issue

 

[precool26]

ok! I will test and get back to you on that probably sunday. As for my next question, What should I do about the carb? Should I get a reman, or have mine rebuilt, or can I simply test it myself or what? I have no experience with carburetors, but I am a competent person who is capable of it, If I knew how or was shown. I do know that I bent my fuel line going into the carb at the fitting trying to take it off. it was leaking, but I tightened it super tight and got it to stop. I didnt know much about it so I tried to use channel locks to tighten/untighten it and stripped out that brass fitting pretty bad on top of twisting the metal line itself...

Newbie over here...

 

[alldodge]

If the carb is flooding, I would suggest getting a reman. It's not overly complicated to rebuild one, but does take (in most cases) a tank to clean out all the tiny ports. The fuel line can be replaced with USCG approved A1 fuel line (and barb fittings) or buy a new line from Merc.

Note: always best to use line wrenches on fuel line nuts

 

[precool26]

ok! Can I ask a question. I did alot of tweaking with the carb the last time I tried to run it. I put new plugs and wires on it and started it. It was hard to start when it used to start right up. I kept having to advanced throttle and prime it, then it would run and die. So i started messing with the idle choke screw and the mixture screw. I did exactly what the manual said but it wasnt running right. So then I disconnected my electric choke and voila, it ran. It is running higher than it used to on the idle though. So could you please explain to me how I can set those all back the way it needs to be and get it right, before I start checking my pressure on fuel. Thank you so much.

Ps; is my butterfly supposed to be fully closed when throttle is in neutral? like fully, because there is a slight gap in it..

 

[alldodge]

is my butterfly supposed to be fully closed when throttle is in neutral? like fully, because there is a slight gap in it..

Only if the motor is cold. If the motor has been started or a warm day it will not be closed. Actually never is totally closed, always has an 1/8 gap at the top.

Disconnecting the electric choke would cause the choke to stay more closed. If its a bit more closed it runs more rich. I think you do need the carb adjusted correctly, what adjustments need to be made is unknown from here, but it may also need to be cleaned inside.

The higher then normal idle means something is not adjusted correctly

 

[precool26]

Ok, so how do I make the appropriate adjustments?