Petrol Engine Flat Spot

[syd]

Hi All,
I'm in the process of buying a Bayliner 2556 which has a 250hp Mercruiser V8 petrol fitted. The sea trial was done by a surveyer who found this.
It starts fine, runs smooth, but after about ten minutes at between 2500 and 3000rpm it hits a flat spot and hesitates, when lowering the revs it goes back to running fine.
It had a new carb fitted last year.
Any productive advice will be greatly appreciated.
Cheers All
Syd

 

[ChrisP]

Does it pick up above the stated range?
Check the air intake for blockages
Is the HT system breaking down, check leads and distributor
Is fuel clean, Carb and fuel filter clean.
Check Ignition advance and retard working correctly.
Air leak on vacumm side causing mixture to lean out.
Choke (if fitted) not opening properly causing starving.
If fuel injected. Clean injectors and check mamagement system.
If multichoke carb is the second choke opening correctly

that's a start any way.

 

[KevL]

Firstly may I say what a nice boat your looking at.

When I went on the sea trial in my Bayliner 2556 the Mercruiser was fine until we opened her up and she coughed, spluttered and lost power just like you're experiencing. I had a word with the surveyor who suggested that this was most likely due to a lack of use causing gunk to collect in the Carb which would either sort it self out or may just need flushing. He was right, all I have done is use the boat a bit and the problem has gone away. I guess a lot of people sell their boats because they have either lost interest in boating or it has become too expensive due to a change in circumstance. It'd be a good idea to ask the present owner what he is buying next, if the answer is nothing then he probably hasn't been using the boat much. Could try negotiating a price reduction though, I did ;-)

BTW Where is the boat? I may well have been to see her.

Anything else you need to know about the 2556?

Kev

 

[DepSol]

American engines are set up to use a slightly different fuel spec to our british equiv. Try using (and dare I recommen the same product with a thousand uses) Soltron. Ask the owner to treat his fuel and then go out for a slightly longer run at least an hour. If this does not solve it then you have a bigger problem.

 

[Col]

I think this one has a Holley 4bbl carb,Make sure it has been set up correctly Float Levels etc:I have had probs on these carbs caused by the secondary throttle butterflies sticking -they are vacuum operated.the secondaries are on the rear most of the 2 thottle spindles.
To check them, fully open the thottle (engine off!!) the secondary spindle should move ( with some resistance ) against a spring and a diaphagm, with no sign of stickyness.hope this may help.

 

[ArthurWood]

If it runs unevenly at 2500 - 3000rpm, check the ignition system first. Look at the plugs and go from there. Earlier this year I had a problem on one of my 7.4l Mercs where it suddenly lost power with a jolt at 23kt and I thought we had hit something. After checking the stern gear and finding it to appear OK, I then tried to rev the engine under load and at 2500rpm it started to shake. I won't bore you with the details of fruitlessly changing a shaft only to subsequently find that the problem was corroded contacts in a thing attached to the side of the distributor. We were in the Bahamas at the time and didn't care about being delayed for a week, but I'm not supposed to mention anything to do with fine weather. Good Luck

 

[syd]

FOUND IT!
Thanks for your help everyone, I went through everything you suggested and found water in the fuel filter. Drained the lot , found the seal on the filler cap missing, fitted new one, new filter, put some juice in the tank and YYYYEEEEEHHHHHAAAAARRRRR.
Cheers everyone.
Syd

 

[Guest]

Re: Not soltron then?

Hmm. well done to chrisP and a nomination for handheld vhf!

But a rather black mark for Depsol, recommending homebrewed snake oil, followed by possible engine rebuild.

 

[DepSol]

Re: Not soltron then?

If you cared to do your homework before mouthing off you will see that Soltron removes up to 13% of water in the fuel!!

Homebrewed snakeoil! you dont have a clue do you? And who said anything about a possible engine rebuild?

Black mark against matts for making foolish comments!

 

[Guest]

Re: Mouthing off

Forgive me. Not sure how your beloved soltron could have worked in this instance, as the water was in the fuel filter. Treating the fuel tank, or even filling the entire tank with soltron, would still have left water in the filter. True, you didn't mention rebuilds, but you did say that if soltron didn't work, then he had "a bigger problem", boat engineering speak for spending 500 quid minimum. Around 60% of your posts recommend soltron, so I'm afraid that you will get some stick.

 

[Guest]

Re: 1984

DepSol = NewSpeak = Department of Soltron

Before anyone flames me, I'd love this stuff to work, but like matts says, too many posts on one topic does make one suspicious...

 

[Guest]

Re: Seems it DOES work

Dave Steward has used it as an additive and it has reduced fuel consumption on his Turbo 36, so it must be tru. Not cos it's dave, but becos he is a T36 owner, and I always believe them. They are very sound chaps cos their boat is very sound and sensible, not in the least tarty or whimsy.