My Mercruisers won't rev out

[Flybridge 31]

If got two 255Hp PETROL Mercruisers. We cruise around at about 1200RPM with no problem.

The boat has been just antifouled and I've had the props retuned. I was told they were a little out of spec.

The other day we opened them out and all I got was 2500 RPM. They wouldn't rev out. The boat was doing 15kts.

When I got the boat 7 years ago we ran it flat out and got 25kts but I can't remember the revs.

My Prop man says she should be doing 32kts at 4200 RPM.

I think the speed is over the top but I do think the 4200 RPM is right.

Now the strange thing is BOTH motors are doing the same.

I could understand if one was performing and the other not.

The boat is used to cruise and as I stated we run around at 1200 to 1500 RPM at 6 to 7 kts and all is good, but I want to be able rev out when needed.

Any Ideas?

[ocean40]

If I recall correctly the 255s originally had Rochester 4-barrel Quadrajets on them, but the same engines with 2-barrel carbs were rated at 233 hp. An owner may have changed out the 4-bbl carbs for 2-bbl.

Since I always operated my 233s in the lower rpm ranges I changed the carbs for little Holley 2300s, they gave a better idle and better fuel consumption. Smaller carbs or over-pitched props or both would give you the lower rpm at the top end but might give you a lower fuel consumption at that 6-7 knot speed.

[Flybridge 31]

These engines are running the Rochester 4 barrell Quadrajets carbies and it appears that the secondries aren't opening due to corrosion.

Which would explain the fact that they won't rev out.


I've done a bit of reading and it is suggested that it is better to replace the carbies instead of rebuilding them due to the corrosion factor.

These carbies would be the standard ones and the boat is nearly always run at displacement speeds of about 6 to 7 knts.

I'm keen to improve economy and not to concerned about top end speed.

The boat is suppose to do 25 to 30 Knts which is over the top for me. If changing carbies improved my fuel range and my top speed dropped to 18 knts I would be happy.

What do you guys suggest as to what size carbies would achieve this?

[Boracay]

Just out of curiosity I ran a search on your keywords:-Mercruisers, Rochester 4 barrell Quadrajets.

Lots of replacement parts out there. Given the skilled workers available in Oz and the amount they charge I can see how you could be better off with new of the plane from America rather than trying to fix what you have.

I did come across a site with a discussion on a similar sounding problem to yours so there may be some good tips there.

I'd pay close attention to all safety aspects of running old petrol engines

The suggestion to buy the manual/s is well worth considering.

[ocean40]

Depending on year, the 233s and 255s were either Ford 351 blocks or Chev 350s. Both Fords and Chevs had Rochester carbs, 2-bbl or 4-bbl depending on horsepower.

The Ford-based 233s had an adapter to mount the Rochester 2-bbl but a Holley 2300 will bolt on to the Ford manifold directly. You can add a spacer for the PCV hose connection and you will need new flame arrestors. Carb linkages are pretty straightforward but will require some bracket fabrication.

Holley markets 2 marine versions of the 2300, one jetted for the Ford 302 and one "universal." Hit their website and do some comparisons, they also have a "remanufactured" department with lower prices. Summit Racing has a good collection of adapters and spacers and fuel line connectors.

Some interpretations of USCG Regs for gasoline power require solid or shielded fuel lines and double hose clamps on hose barbs, and flexible fuel line has to meet A-1 specifications. Fire-shield on the hose may satisfy this requirement.

A rebuild on both carbs will almost certainly be cheaper than switching. If you operate the boat so that the secondaries never open, and they have become corroded closed, you have turned the 4-bbls into 2-bbls anyway. If they run well the way you operate them, might want to wait for the rebuilds until you sell the boat.

[Opie91]

Originally Posted by Flybridge 31

These engines are running the Rochester 4 barrell Quadrajets carbies and it appears that the secondries aren't opening due to corrosion.

Are you sure the secondaries are not opening? I am pretty sure that the quadrajet has mechanical secondaries and they might not open by just poking them with your finger. Try pushing the throttle all the way up with the engine off and see what they look like. If the corrosion you are talking about is on the secondary blocking plates on the top of the carburetor, a bit of corrosion is not unusual to see on those.

Will the motors go above 2500 in neutral?

You could have a simple (or not so simple) fuel restriction issue that is not related to the carburetors or just have way too much crap on the boat. What is your fuel/water/holding tank load like? Have you added a bunch of weight to the boat? Chain maybe?

Planning hulls do not like a lot of extra weight.

[Flybridge 31]

Hi thanks for your replies, The engines are 350 Chev Mercruisers.

The engines will rev way past 2500 in neutral. My mechanic has told me the secondaries aren't opening.

As for fuel restriction the two engines have two fuel tanks (one for each engine) as for the weight I'm carrying water tanks are full and the fuel tanks vary from 3/4 full to 1/4 full.

[wmcruiser]

I had 5.0 v8 and went down refurb carb route.was cost effective to replace complete carb.£185 from merc dealer.if you can rev in neutral to 3000+ is boat bottom/drive clean.problems i had also was broken valve no7 lost loads of hp.get or do compression test on engine before buying anything.

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