Mercruiser 350 260 thunderbolt iv having fuel and or ignition problem

[Amitchell001]

So I am in need of some help. Motor is 87 350 260 with thunderbolt iv and alpha one. in neutral the boat idles fine and can rev up as far as
You want no problem. Under load different story. Here is the history
-Boat dies under more than half throttle. Keep it under half throttle and its fine. Wont turn more than 3000 rpm
-change fuel filter boat is worse. Wont even plane out.
-pulled carb off. Had someone rebuild it last year....... They didn't do anything to it.... At least in convinced because if they did they would have saw the mangled primary metering rod...... Put a new rod in and cleaned carb
-runs now but not over half throttle. Try to go over half throttle now results in sputtering (I believe because its starving for fuel) and no power. It will stall if I don't back off throttle.
-checked cap it had some corrosion inside, changed cap and fuel filter again.
-yake the boat out and is ok full throttle but turning 3200rpm max. Back off the throttle a tiny bit and boat wants to stall. Bump up to full throttle and its fine. Bring throttle back to half just under where it loose power and it fine. Its not sputtering any more either. Take off again and wont go full throttle only half and n
ow only turns 2600 rpm.
-checked anti siphon vale to find its not there someone changed to just a barb fitting. I did look in the carb while it was running and the secondaries open and are getting fuel but thats when the boat loose power.
-checked fuel vent line and is ok
HELP is greatly appreciated!!!!

 

[achris]

Re: Mercruiser 350 260 thunderbolt iv having fuel and or ignition problem

I'm going to suggest what I suggest to everybody with these sorts of problems... I've had my hand bitten off a few times because people think I'm calling them an idiot... I'm not, it's just a very good, solid, sound troubleshooting method.... If you don't like what I say, don't be rude about it....

Engines require certain things to operate properly... Basically they need the correct fuel/air mixture to be compressed and then fired at the right time.

So, in this order.

1. Compression tests.
2. Check spark plugs, replace if needed.
3. If points, replace, along with condenser and rotor. Check and adjust, if needed, points cam dwell angle.
4. Inspect distributor cap and leads, replace if necessary.
5. Check spark quality (needs to be a nice BLUE spark drawn over 1/2").
6. Set timing. Check timing advance is working correctly.

When all of these items have been done, THEN move to the fuel system....

You can have the best, most clean carburetor in the world, but if the compression's low and/or the ignition system's not doing its job, the engine will still run like crap.....

Chris.........

BTW - Welcome to iBoats....

 

[Reel Poor]

Re: Mercruiser 350 260 thunderbolt iv having fuel and or ignition problem

Do as Chris suggested and if you still have problems there is a fuel pressure test in the first Sticky by DonS. There may also be a filter screen on the bottom of you fuel pick-up tube in the fuel tank that could be restricted.

 

[Amitchell001]

Now what? Compression numbers

Now what? Compression numbers

So I checked the compression..... 110,110,80,100,110,0,110,75...I pulled the valve cover and the valve towards the back of the motor (I believe the inlet) is stuck. I stopped there for today

 

[achris]

Re: Now what? Compression numbers

Re: Now what? Compression numbers

...so, in this order.

1. Compression tests....

so i checked the compression..... 110,110,80,100,110,[size=+1]0[/size],110,[size=+1]75[/size]...i pulled the valve cover and the valve towards the back of the motor (i believe the inlet) is stuck. I stopped there for today

That might be a good place to start...

 

[Bondo]

Re: Now what? Compression numbers

Re: Now what? Compression numbers

So I checked the compression..... 110,110,80,100,110,0,110,75...I pulled the valve cover and the valve towards the back of the motor (I believe the inlet) is stuck. I stopped there for today

Ayuh,.... That motor is a rebuildable core,.... The compression is to low to be called a Running motor...

 

[mainshipfred]

Re: Mercruiser 350 260 thunderbolt iv having fuel and or ignition problem

Don't know about the compression, the 0 cylinder is questionable. Did you happen to remove all of the plugs while taking the reading, if not you will get a lower reading. All year long I was having the same problem with cutting out while getting on plane. did everything rebuilt carbs, installed new fuel pumps, removed about 20 gal of gas from the bottom of the tank, changed all (3) filters, replaced plastic return lines. Finally I cut my deck to get to the pick-up tubes to check if they had a screen which they didn't. The problem after all that and not being able to use my boat all year was clogged check valves. I found aluminum shavings, fabric strands and plastic shavings clogging the valves. I cleaned them thoroughly put everything back together and she runs as well if not better than she used to. I read a lot of forums and did extensive internet research and never came upon anything telling me to check the check valves.
Good luck,
Mainshipfred

 

[achris]

Re: Mercruiser 350 260 thunderbolt iv having fuel and or ignition problem

By 'check valve' I assume you mean anti-siphons valve... However, a clogged anti-siphon valve will not cause the compressions to be low.... That engine needs to be pulled out, stripped down and rebuilt from the crank up...

Chris.........

 

[Amitchell001]

Re: Mercruiser 350 260 thunderbolt iv having fuel and or ignition problem

The anti siphon valve is not there. It is just a barb fitting. So I found a mercruiser 454 near by for $500. I'm thinking I might buy it. If I'm gonna rubiuld something why not put a big block in. It's out of an 81 seray and the guy told me he took it out and re powered with a straight 6.this seems like a strange concept to me...... He says he filled the motor with oil......any thing I should be wary of? Anything I should check besides compression on it? What do you think would be acceptable numbers? Thanks everyone for the feed back

 

[achris]

Re: Mercruiser 350 260 thunderbolt iv having fuel and or ignition problem

If you put a 454 in front of an Alpha drive you need to do one of 2 things...

Either

A/ Buy lots of spare drives.... or

B/ Be VERY VERY ginger with your right hand, prop it light, keep the drive oil changed regularly and NO quick take-offs....

Chris......

 

[mainshipfred]

Re: Mercruiser 350 260 thunderbolt iv having fuel and or ignition problem

Very good advice. Before undertaking such a huge expense it's always good to explore all options.