Slow to plane but runs good?

acuraguy81 · Aug 11, 2010

I am helping a friend to figure out why his 1989 merc 350 with a 4 barral rochester qjet is so slow to come out of the hole and is shy of getting to top speed but the boat runs good and starts and idles good. Boat ran good few years ago, got the carb rebuilt and was worked on but now it does not run as well. Im guessing the reubild kit had the wrong jets and power valve in it. can some one add some idea's??????

PS this boat is a 20 foot open bow invader.

JustJason · Aug 11, 2010

Re: Slow to plane but runs good?

start with a compression test and see if you have a dead cylinder

Cuyose · Aug 12, 2010

Re: Slow to plane but runs good?

Also check what your WOT rpm are and how that number compares to what it should be in the specs. If you aren't getting the correct WOT and everything else is working right, bottom clean, good compression, no excess abnormal weight, etc. Check your prop pitch and take a look in the prop section of the forums.

If you are most concerned about acceleration and hole shot, dropping an inch or 2 on the prop pitch will help you out and also ease up the load on the engine, however you will need to make sure you don't over-rev per the engine specs.

Beefer · Aug 12, 2010

Re: Slow to plane but runs good?

I'm feeling a prop issue on this. I'd go to the prop section an repost this. Include the prop information (size, pitch, etc.).

acuraguy81 · Aug 12, 2010

Re: Slow to plane but runs good?

The boat ran fine when friend bought it few years ago still same prop. only thing done was rebuilt carb and some other maintenance, throttle linkage, shift linkage and some fuel pick ups things like that and since it has not ran as string. PS if it had a dead cylinder it would not idle so smooth.

Cuyose · Aug 12, 2010

Re: Slow to plane but runs good?

You would be surprised how well a V8 will run on 7 cylinders under no load. You really can't tell by idling. after idling for a bit, pull each plug and see if one is wet. Only takes a few minutes and could solve your issue.

I had the same issue a while back even after doing tune-up and replacing plugs. There was a post here a while back where someone was tearing his hair out trying to figure out symptoms very much like this and after double and triple checking he figured out during a tune up he had switched a couple ignition wires.

Cuyose · Aug 12, 2010

Re: Slow to plane but runs good?

one other thing to check is your ignition advance. if your ignition is not advancing as you bring up the rpm, it is going to rob you off all kinds of power.

Huron Angler · Aug 12, 2010

Re: Slow to plane but runs good?

I'm guessing that the hull is also an '89...have you checked for wet foam under the deck? If you are hauling extra weight down there it would result in what you are describing.

Easier to check the I/O for issues first rather than tear up the deck, but just something to consider. Good luck!

acuraguy81 · Aug 12, 2010

Re: Slow to plane but runs good?

How can I check the advance? with an advance timeng light? Also the boat will run near 50mph just takes for ever to get there for ever it so odd it would jump hard before and run up 55-60

Cuyose · Aug 12, 2010

Re: Slow to plane but runs good?

There are quick and dirty ways with a regular timing light as long as you have degree marks on your flywheel. You would have to check the specs for your engine on what the advance would be, but as you get up to 300o-3500 rpm, you should be getting max advance, and the timing mark will move.

If you have mechanical advance, under the plate in the distributor where your condenser, etc is screwed on, will be some springs and weights. Many times people overlook this and find that they are pretty rusted up

acuraguy81 · Aug 12, 2010

Re: Slow to plane but runs good?

good call with the deck but she does it after being out for 10 months plenty of time to dry out. and i realy think the shop a few years ago did something to the carb to get it out of wack wrong jets or power valve some thing. Prob is he got the boat worked on and did not use it the rest of the year till the year later and then did not want to take it back to the shop and the shop did not water test the boat!

acuraguy81 · Aug 12, 2010

Re: Slow to plane but runs good?

ok sounds good. but would the advance affect the out of the hole problem?

Cuyose · Aug 12, 2010

Re: Slow to plane but runs good?

It could, but the advance really doesn't come into play until the rpms increase. I would be more interested to find out how your spark plugs are reading. If the weight and everything checks out, it really sounds like you are running on 6 or 7 cylinders or a couple wires are crossed, you have lack of power everywhere, slow hole shot and you are missing almost 20% power on the top end compared to how it used to run.

acuraguy81 · Aug 12, 2010

Re: Slow to plane but runs good?

Ok well I disagree I am very good at hearing a dead cylinder the motor runs too good and it is timed with the shop doing the work so. I may do a compression check and check the plugs. Does any one know how to find out what jets and internals the carb should have compared to what is maybee in it?

Challenger84 · Aug 12, 2010

Re: Slow to plane but runs good?

acuraguy81 said:
good call with the deck but she does it after being out for 10 months plenty of time to dry out. and i realy think the shop a few years ago did something to the carb to get it out of wack wrong jets or power valve some thing. Prob is he got the boat worked on and did not use it the rest of the year till the year later and then did not want to take it back to the shop and the shop did not water test the boat!

It will take forever and a day to dry out a water logged boat!
Only way is to tear it out and redo.
Hope that's not the problem but don't dismiss it because it's been out to dry for 10 months

Challenger84 · Aug 12, 2010

Re: Slow to plane but runs good?

Check for spark and compression.
If they plan out then recheck the carbs.

Cuyose · Aug 12, 2010

Re: Slow to plane but runs good?

I am not sure why you are so hell bent on tearing apart the carb and digging around before doing any of the easy stuff.

Someone mentions you might have taken on water and the boat is much heavier than it was before and your response is, that's not it, it had plenty of time to dry.

I stated check the plugs for spark and the advance, and you say it's not that cause you would be able to hear it..

Really, people are trying to help you here. A lot of these people had these same issues and are giving you insight into what it was. You seem to be fixated on what you think the problem is (carb), but haven't really ruled anything else out.

It helps everyone out when you systematically go through some of these things people are recommending before jumping to other ideas. If you don't, then you have a bunch of people trying to chime in on adjusting a carb that had nothing wrong with it to begin with and no one wins.

EddiePetty · Aug 12, 2010

Re: Slow to plane but runs good?

acuraguy81 said:
...... PS if it had a dead cylinder it would not idle so smooth.

.......for a SBC, wrong answer!!!!!!

Cylinders 5 and 7 are adjacent and fire sequientially. Believe me, any Chevy will run quite nicely with those two plug wires reversed....and I dare you to hear it !!!

Challenger84 · Aug 12, 2010

Re: Slow to plane but runs good?

EddiePetty said:
.......for a SBC, wrong answer!!!!!!

Cylinders 5 and 7 are adjacent and fire sequentially. Believe me, any Chevy will run quite nicely with those two plug wires reversed....and I dare you to hear it !!!

You got that right!
I had a spark plug wire loose and the engine run fine.
I put the wire back on with not much different in sound, but there was a little difference.

bigdan1 · Aug 12, 2010

Re: Slow to plane but runs good?

if problem started after carb rebuild, maybe the cable adjustment aint right

if the secondaries dont open at all , it will be out of power sooner .

seen this many times ...... sometimes the little things ,we miss !

Slow to plane but runs good?

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