Revs wont go above 2600. Pse advise top 5 reasons

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RIBW

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We are desperate for some advice on what to check to diagnose our problem. All help would be gratefully received.

Our boat is a 23ft Falcon (23spc). She has the Volvo Penta 501 (V8) petrol engine and twin props (AQ290) This year she has been anti-fouled (hull & props), the fuel and filters cleaned, the carb serviced and distributor cap and rotor changed. Last year the HT leads were changed. Trim tabs work fine. Our norm is to cruise at 20kn using 3000rpm with some trim tab. This year we cannot get the revs above 2600 whatever we do.

Below 2600 she sounds sweeter than she has ever sounded.

What would the first 5 things a pair of amateurs (but technically capable) should check to diagnose the problem.

Thanks in advance
Bob
 
I assume this is competition, so my votes go for ...

1 Dirty bum
2 Barnacles on the drives or props
3 Blocked fuel filters or screens or strainers somewhere
4 Blocked air filter
The bonus ball
5 Dirt in the carbs

Not electrical IMHO.

Do I win !
 
superheat6k said:
I assume this is competition, so my votes go for ...

1 Dirty bum
2 Barnacles on the drives or props
3 Blocked fuel filters or screens or strainers somewhere
4 Blocked air filter
The bonus ball
5 Dirt in the carbs

Not electrical IMHO.

Do I win !
Click to expand...

1, 2 & 5 ruled out by OP as she appears to have a clean bum, drive & prop, and carb serviced.... My favourite would be exhaust bend restriction ... rust build-up where cooling water hits exhaust.... or something ignition based such as rotor or ignition leads...
 
:
.. My favourite would be exhaust bend restriction ... rust build-up where cooling water hits exhaust.... or something ignition based such as rotor or ignition leads...[/QUOTE said:
Na Bo44ocks
They got too much booze onboard
:D
Click to expand...
 
Divemaster1 said:
1, 2 & 5 ruled out by OP as she appears to have a clean bum, drive & prop, and carb serviced.... My favourite would be exhaust bend restriction ... rust build-up where cooling water hits exhaust.... or something ignition based such as rotor or ignition leads...
Click to expand...
Ok - so when this year were the bottom and props cleaned ? If done say Feb or March then by now She could be fouled again.

Ignition fault would likely show at lower revs, but you are right an exhaust restriction could be an issue.
 
Does it rev in neutral? Are there any other symptoms such as a missfire once it reaches its current max rpm? Divemaster could be on to something, when were the manifolds and risers last changed? On salt water they only have around a five year lifespan.
Could be fuel related. I would rig a temp separate supply and see if that cures the problem. At least that would rule fuel in or out. If not fuel or exhaust, then air management and electrics. Is the throttle lever actually operating correctly? You haven't changed props since last year have you? Could have been given the wrong size.
 
Thanks everyone!

Normally Kawasaki would have hit the bullseye but she is still at 'winter weight' i.e. completely de-stocked - that includes booze and a load of kit for servicing.

We've checked the bottom and props recently and she is still clean.

As to Divemaster's 3&4, these were both checked when the carb was serviced (professionally by a tusted local with 30 years experience).

Alf's exhaust corrosion suggestion is interesting - we have done nothing in this area since we bought her (3 and half years). The risers do settle at different temperatures, using the amateur's hand test.

Neale,
Yes, she revs freely in neutral and she sounds really smooth.
After a big bill for overhauling the fuel system earlier this year and my lack of confidence with temporary petrol supplies I would be inclined to recall the chap who serviced the carb.
The throttle seems to work OK - full range and no resistance spots.
The props are unchanged (and undamaged).


We'll clean and remake all the ignition connections, but It looks like an exhaust problem is the one area that has not been addressed by us.
Does removing and inspecting the risers tell us all we would need to know about this possibility or do the manifolds need to come off as well?

Thanks again
Bob
 
The top 5 reasons for an engine (petrol or diesel) running OK at low revs but not achieving full revs or power other than a dirty bottom or prop are

Fuel supply restriction
Fuel supply restriction
Fuel supply restriction
Fuel supply restriction
or er Fuel supply restriction

Check all lines, filters, etc and ensure all are clean and there is no air in the system.

Assuming of course that the carb etc has been properly re-assembled and all jets cleaned when overhauled. The fact that you have worked on the fuel system and now you have a problem would reinforce the above.

Edit: Just read your post again. Also check the ignition timing. A retarded spark will also cause lack of power....
 
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Thanks for the further suggestions.

The plan is to collect a box of exchange items and give her a trial next weekend and if that fails, she is going back under the care of the chap who serviced the fuel system.

Regards to all
Bob
 
What happens when you push the throttle past the point where the revs stop rising, do you get any backfiring from the carb, or does it just go flat and make strange sound until you pull the throttle back to where it influences the revs?
If it does either of these then its fuel starvation, do you have an electric or mechanical fuel pump, I have changed lots of electric ones as they give up long before mechanical ones do, they can't keep pace with demand so you get the symptons you have, have you looked down the carb barrels when driving to see if there is fuel being delivered to the larger secondary barrels which only open up as you get up to around 2500rpm, if the airflaps don't open it won't deliver any fuel just when you need it most as now the engine is breathing through 4 barrels instead of the two smaller ones which it would appear are working fine. Not sure which carb you have but most have a vacuum operated flap which opens as the revs rise and lift needles to let the extra fuel in. So check these are working, I don't mean the throttle plates at the bottom of the carb as these will open mechanically when you push the throttle, its the ones you see at the top of the barrels when you take the flame arrestor off, and they only open when the engine is under load so you won't see anything happen if you rev it in neutral unless you get up to around 4000rpm as they need vacuum to work.
I know this sounds confusing but a good carb mechanic will understand the principle.
 
Tank still full of last year's petrol? Power loss at top end with sweet running at lower revs fits with this.
 
MASH said:
Tank still full of last year's petrol? Power loss at top end with sweet running at lower revs fits with this.
Click to expand...

MASH,
As part of the service we had the old petrol taken out and replaced with fresh (and since topped up with more).

dingdongs said:
a carb rebuild required as the accelerator pump is not giving sufficient fuel for the extra ponies needed for full throttle with load.
Click to expand...

dingdongs,
The carb was removed and partly rebuilt by our experienced engine man. I don't know, but he seems to know the carb inside out. I'll check with him next weekend if the trial fails again.

SPANNERMAN,
There is no backfiring or stuttering - just flatness (no strange sounds). A couple of years ago we had a faulty HT lead and that was limiting revs but was accompanied by strange noises, so I think I know what you mean.
It is a mechanical pump. We haven't checked it.
I'm not 100% sure but I think we have a ROCHESTER 2 barrel 2G? - the one with the electric choke. The vacuum possibility sounds interesting. The chap who serviced the carb could not have put her under load (we are 1 hour from water where we can exceed 8kn). I don't know if he took her to 4000rpm to obtain a vacuum. This is yet another question for him.
If it is a vacuum problem, what would be the fix? A full carb rebuild?

All,
Thanks for the continued support! It is much appreciated. Weather is looking good for a trial run on ThursdayFriday next week.
Bob
 
I'm with the fuel and ignition line.

Carb rebuild not all right and ignition timing possibly off. What kind of ignition do you have?
If it revs fine (and I mean to 4800-5000 rpm without hesitation) in neutral chances are the advancing is not working, as mentioned.
 
Spi D said:
I'm with the fuel and ignition line.

Carb rebuild not all right and ignition timing possibly off. What kind of ignition do you have?
If it revs fine (and I mean to 4800-5000 rpm without hesitation) in neutral chances are the advancing is not working, as mentioned.
Click to expand...

Spi D,
The ignition is rotor and distributor (i.e. not electronic).
Regards
Bob
 
On the parts list for 501A and 501B the distributor has a sensor instread of breaker points. This suggests some sort of electronic control meaning that ignition adjustments are not done by turning the distributor.
Workshop manual here http://boatinfo.no/lib/volvo/manuals/aq211_572.html#/102

If, however, you are right we assume the setup comprises distributor, possibly vacuum and definately a mechanical advance.
Settings to check is the static (inital), dwell and advancing (vaccum and mechanically controlled) degrees. If these figures are not available you can check the initial and total advance at the given rpm.

Figures are on the owner manual pg. 25
http://vppneuapps.volvo.com/ww/PIE/...03&d=Owners Publication&s=38627248&lang=sv-SE

The vaccum from the inlet manifold controls a push/pull on the distributor. Check the hose..

Inside the distributor, below the breaker fitting you'll find these that advances timing according to rpm. Check movement and spring tension:
31042.jpg
 
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Found this. Explanatory although not with a vacuum regulator.

Your base engine supposedly is a GM (Chevy) also.

 
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