Volvo Penta 4.3 carb revving high after a run

[QBhoy]

A friend of mine has an early 90's 4.3 205hp carb engine.
After a run at speed for more than 5 minutes, when then slowing down and taking out of gear, the revs shoot up to around 2000rpm. A real nightmare when coming into dock !
First off, I was thinking it's perhaps a sticking cable...but I'm not so sure. The only way to stop it revving like this is then to turn it off then on...and it sorts itself out instantly and returns to normal tickover. This is the thing that throws me.
I haven't put much thought to it (not being my boat) but would like to sort it out. I have perhaps thought about cone clutches, overheating drive oil and over fuelling, but perhaps can't understand the returning to normal when turning off and on.
Anyone else experienced this and what is the problem ?
Thanks.

 

[Scott Danforth]

Check your cable

 

[QBhoy]

Scott
Do you think it is a cable ? After reading what I have written ? It could be, but why would the cable move when turning ignition off and on ?

 

[Horigan]

If you're revving then your throttle isn't closing, either due to a sticky cable or sticky carb linkage. I'd gain access to the carb, replicate the issue, and see what isn't allowing the throttle to fully close at the carb.

 

[QBhoy]

Thanks Horgan.
I can totally understand what you are saying and your reasoning. I haven't tried looking at this, but I still don't understand why turning the ignition on and off would have a mechanical effect on this. I.e. Why would it change the linkage or butterfly move to normal after turning engine off and on ?
Thinking about it now (and after a few beverages);
I used to have the exact same engine as this for many years. It didn't have an electric fuel booster pump. This one does. Do we think this might have something to do with it ?
It's the only variable I can think of that might be ignition related.
Thanks again.

 

[harleyman1975]

Is it fuel injected? IAC?

 

[QBhoy]

It's a Carb (4 brll) harleyman, as stated in title of post.
Thanks

 

[GA_Boater]

Check the choke and idle hold-off if it has one.

 

[QBhoy]

Thanks GA. I have disconnected the choke. Pretty sure it isn't that. Only thing I have done though.
Struggling to figure how to get suggestions now. Thought that knowledge might be shared between VP and Merc guys. Being the same base engine and all that.
Thanks.

 

[QBhoy]

What is the idle hold off ? The idle adjustment screw/spring mechanism ?

 

[alldodge]

My guess is the cable has some slop in it just from time. Move the throttle control handle to neutral, remove the barrel and adjust until it just slides back in, then remove and back it out one more turn. Backing out one more turn will apply a slight pressure in the idle position.

Another issue is the carb has corrosion on it and the plate will not close easy without some help. Have a look at it and see how easy it opens and closes without the cable attached.

 

[QBhoy]

Alldodge
First thing I'll look at, come the season. Totally see your reasoning and a few have suggested similar. It's all hypothetical at the moment because the boat is now winterised.
I just can't get my head round why turning the key off and on would have a mechanical effect on such things as controls, cables and carbs. Am I missing something ?
I will physically look next time it happens and see if the throttle linkage is actually returning to the idle position and resting against the set screw.
Any idea why someone would have fitted an electric fuel pump to this engine ?

 

[alldodge]

I just can't get my head round why turning the key off and on would have a mechanical effect on such things as controls, cables and carbs. Am I missing something ?

There is air being drawn through the carb, if its sloppy, I can see how the vacuum could keep the plate open a tad. You can go from idle to max rpm with very little opening of the throttle plate with no load.

Any idea why someone would have fitted an electric fuel pump to this engine ?

Most reasons is the mechanical pump cost more then the electric. Hope it was installed correctly by using a switch to kill the pump if oil pressure is lost

 

[Scott Danforth]

Starting the motor provides just enough vibration to help the bit of slip in the cable.

On a carb with your symptoms, the issue may be one of the following:
Throttle cable corroded
Throttle cable stretched
Throttle cable mis-adjusted
Choke stuck
Weak throttle return spring
Corroded throttle shaft
Debris in idle circuit

 

[QBhoy]

Guys
Just caught up on this. Superb advice and I'll defo be trying the above in the new year. The vacuum theory in particular makes sense. Also, thanks again Scott for this and other post. A testament to the value of such a forum !

 

[QBhoy]

All
An update...
We were out a run in the boat today, before pulling it out for some maintenance.
Managed to replicate the problem...this time whilst I was down the hatch having a look.
The following is occurring;
When the throttle is opened partly without the 3/4 barrels opening, then returned to tick over position at the control...all is normal.
When the throttle is opened wide enough for the back 3/4 barrels to be opened, it then continues to hold high rpm when throttle control pulled back.
I can slow the engine down by pulling (helping the spring) 3/4 lever back. It only down to around 1000rpm. I
As some of you have rightly said, the vacuum of the engine appears to be overcoming the return force of the spring.
It won't return to normal rpm of 700 or so by doing this. Only about 1000. When switched off and started again, the rpm will return to normal tickover.
Is this simply the return spring weak or is could it possibly be something else ? I can't see that it would be the spring, if I can't get the rpm to return to totally normal by physically pulling it to the max.....?

 

[alldodge]

Weak spring or linkage and/or carb shaft is binding

 

[QBhoy]

Thanks again All dodge.
Can't figure why it wouldn't totally return to tick over revs when I manually hold it right back with my hand though.
The carb shaft binding...? What's this ? Can you explain a little more about this ?
Thanks again.

 

[alldodge]

There are two throttle plates (front 2 and rear 2 barrels) and the shaft should rotate freely inside its bores. If the shaft which goes thru the housing is sticking it may hang, or if the plates that cover the bores are loose, this to may cause it to not close all the way

 

[QBhoy]

Ah. Understand. Yeah, it's the back (aft) two barrels that are causing the problems. Suppose I could test that by holding the shaft rigid and trying to push down on the two butterfly flaps and see if there is play ?