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Do carbureted engines have a steady idle?

Fishmaster23

Regular Contributor
There may have been something wrong with my engine, but my old carbed gm350 volvo I/O always had a little bit of a surge +/- 50rpm and it drove me nuts, then of course when you put it in gear it would drop about 200rpm, so It always idled a little high in neutral to prevent stalling. After we rebuilt the carb I think it only dropped 100rpm, but anyways, I replaced the engine with a MPI engine and the rpm is as steady as can be. When I shift gears I think it drops just a tad for a couple seconds, then it's back to sitting at a steady 700rpm, just like a car and boy am I a happy camper. So was there something wrong with my old engine or is that just nature of the beast? I love the fact that I can maneuver at the dock and not worry about the engine stalling. :)
 
I have had carbureted engines idle very smoothly. Likely carb or ignition items were an issue but if you get the need to replace the engine any high wear engine where compression is significantly different cylinder to cylinder or valance wear is different could also contribute to idle varying.
 
When they are 'right' they are 'steady'...

On the MPI engines, the ECU recognizes the RPM drop when shifting into gear and opens the IAC valve a bit, to compensate....
 
Ayuh,..... My carbed motors have always idle smoothly,...... do alotta that freshwater trollin',......
 
You mentioned "my old carbed gm 350 Volvo I/O".
May we assume that this engine is equipped with a mechanically advancing ignition system?

If not a carburetor issue, please read on.

(disclaimer.... this is a long shot!)

.... at idle speed, there is no mechanical advance being added to BASE advance.
.... BASE advance should be constant at either 8 or 10 degrees BTDC.
.... your system uses a flyweight/cam affair (2 of these).
.... the flyweights are controlled by two small springs that pull them both back into (and keep them in) BASE mode at idle speed.
.... the spring value allows the flyweights to
progressively advance the spark event as RPM increase above idle speed.
.... when/if the flyweight return springs have become rust compromised, they loose their spring value.
.... when/if they have lost their spring value, they will not consistently pull and hold the flyweights back into BASE ADVANCE mode.
.... if this was to occur, the spark "lead" at idle may be somewhat erratic or pulsating, so to speak.
.... even with a good fuel delivery system, a pulsating BASE advance spark event (of let's say 2 degrees or so) will present itself as engine RPM "surging".


Connect up your standard mode timing light.
With the engine at idle speed, see if the BASE advance is steady or if it is erratic.
If the BASE advance is erratic, your flyweight system is not holding BASE advance at idle speed, of which may indicate weak springs.


CAUTION:
Do not haphazardly replace the flyweight return springs (no matter what the supplier says) without the aid of a shop who owns/operates a Sun, Allen or King distributor machine.

These little springs not only hold BASE advance while at idle speed, their spring value also controls the very important progressive advance.


Again, this would only apply if all else is OK.


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The old one had a prestolite ignition which I believe was electronic.

This is an 89 GM350 volvo aq271c 275hp

1989 pre-dates Volvo Penta's use of the EST system. (EST = electronic spark timing, of which does not use a mechanically advancing system)

Even with a Prestolite electronic triggering system, it will still incorporate a mechanical advance.

Try what I suggest in post #6.


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In first post, OP stated he replaced this engine with an MPI engine, so doubt he can check the advance mechanism anymore
 
Dieter said:
In first post, OP stated he replaced this engine with an MPI engine, so doubt he can check the advance mechanism anymore

Yes, the MPI engine would use an EST ignition system that would not incorporate any mechanical advancing.


Original thread title: "Do carbureted engines have a steady idle?"

In post #1 Fishmaster asks about his old carbureted engine regarding a bit of an RPM surge.


There may have been something wrong with my engine, but my old carbed gm350 volvo I/O always had a little bit of a surge +/- 50rpm and it drove me nuts, then of course when you put it in gear it would drop about 200rpm, so It always idled a little high in neutral to prevent stalling. After we rebuilt the carb I think it only dropped 100rpm, .................................






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I think the old engine had a bad power valve too. Found out the adjustment screws on the carb were only .50 and .75 turns out yet it still ran.
 
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