New 4.3L Mercruiser longblock won't accelerate but will idle

[Catfish Tish]

First time posting so I'm sorry if I am doing this the wrong way. I've read a lot of threads and still stumped.

2005 Tahoe Q6 Fish and Ski Inboard 4.3L Mercruiser.

We bought this boat from a dealer a few years ago who lied to us and replaced the engine with a Jasper. Either way it ran great for years and this memorial day we had been tubing as always and the engine acted up and almost died. I checked the engine bay and there was a water oil mixture all over the top of the engine by the carb. I checked the oil and still milky so I knew it was bad and later saw it was a cracked block but still was able to get it on the trailer.

I bought a new engine from Fraser engines with a 3 year warranty and I installed it with a friend. It starts and runs at idle but wont accelerate. I had a local shop double check the timing and still doesnt run right and will pop once you try and go faster. I replaced starter, cap and rotor, battery, battery terminal, ignition coil and fuel filter. The shop said it might be the ignition module? The part is on backorder. I dont want to mess up the new engine and dont mind replacing parts but what's next carb or fuel pump? I've read a lot of threads and figured it was time to ask what's the next step? Drain fuel? Rebuild carb? Ignition module? We love the boat and just want it running I'm pretty handy and dont want to wait until August when the shops have time to look at it. Thank you in advance for your your help

 

[achris]

Sounds like a fuel problem, but to check the ignition, use a timing light and with the engine in neutral, accelerate and see that the timing advances.

 

[Scott Danforth]

that "pop" is called a lean sneeze. your motor is starving for fuel......you have a fuel system problem. and since it runs, you do NOT have an ignition module problem

 

[Catfish Tish]

Thank you for the reply I feel the same like its starving for fuel. The mechanic I had set the base timing and check with a light said the timing is right so the distributor sensor is usually the issue.

I will take the carb off and clean it and see if that helps. Some people have told me to just replace the carb if I put a new engine in. I am at the point I dont mind replacing some parts since it's a older boat.

I will update and see how it helps cleaning the carb.

Thanks again

 

[Lou C]

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anti siphon valve on the gas tank where the fuel line connects
make sure fuel vent is open and not clogged

when I first had this boat we had the same problem, it was a corroded sticking anti siphon valve. replaced it with a better quality Evinrude/Johnson one 15+ years ago and still fine...

 

[Catfish Tish]

Thank you Lou C I will try the anti siphon valve.

I replaced the wires and ignition module just in case and took the top half of the carb off cleaned it and replaced a few parts but didnt totally rebuild the carb. It runs better but cant get above 2000 rpms on a hose. Made a wake but wont get on plane on the water. If it's not a anti siphon valve my next step is rebuild the car . Drained 25 gallons of gas out of it and put fresh gas and sea foam in it.

Thank you all for your help . I'm no mechanic but I feel like it's a fuel problem.

 

[Rick Stephens]

Absolutely do the carb rebuild the right way. Every passageway needs to be open and perfectly clear.

Also dump the contents of your water separator into a clear glass container and look at it closely, just to make sure you still aren't pumping crap into the carb.

 

[Catfish Tish]

I dont have a water separator I just have a fuel filter that I believe separates the water. I took the carb apart again and am soaking the jets in carb cleaner overnight. I didnt replace that gasket so I'll be doing that also tomorrow. I've got a lot of different feedback. Some say it's a new engine and a valve may be stuck and just needs ran to break in. I let it idle for about 30 mins today and put a timing light on it and looks good at idle. Seems like it's missing at higher rpms above 2000. Only other option I feel is the denso ignition module. Took it off one bolt broke off so I'm wondering if maybe that's a sign the unit might be bad. Not cheap 500-600 so hopefully soaking in carb cleaner overnight maybe will do the trick. Any input is greatly appreciated.

 

[Rick Stephens]

Watch a few videos on YouTube on how to rebuild a carburetor. It is just a little more than soaking the carb, needs the passageways clean and blown out. The cleaner you get everything, the better the rebuild job.

In general, electronic ignition either works or it doesn't. Is Nippon-Denso the original OEM Mercruiser ignition system? You don't want electrical components that are not marine rated.

 

[Catfish Tish]

It is the original Denso Marine ignition system.

Question

Do need to have a tks carb or can I get a different carb to see if that's the issue?

 

[Rick Stephens]

Someone else will have to answer that. I have never worked on a TKS system and don't know how they are wired up. The TKS carb has a richen solenoid for cold starting instead of a choke. I don't know what happens if it is replaced with something else.

Rebuilding a TKS is no different than any other carb, simple really, except for the TKS solenoid being different. Disassemble and use the rebuild kit to replace wear parts and gaskets.

See if alldodge can answer your questions better than I.

 

[alldodge]

No need for TKS unless that is the type of carb you want. A regular Rochester or Merc 2bbl carb with a choke will work as well

 

[Catfish Tish]

Thank you all dodge I think I am going to order a Rochester and just see if that's the issue.

Thanks again for all your help I will update what I find out

 

[Catfish Tish]

So I got a new denso ignition module and that did not fix the issue. The timing light is not right when giving throttle but looks good at idle. Looks like it has to be electric. I'm going to try and swap ignition coils again maybe the new one was bad. I will update

 

[Catfish Tish]

Update Boat is fixed!!! I ended up dropping it off at the boat shop and it only took them 2 hours to turn the distributor 180 degrees and adjust the carb. Boat runs better than it ever has. Should have checked the distributor before I bought all the parts but at least it's done and running. Thanks for your help!!