Hunting at idle

Hi looking for some advice I have a volvo penta 250a 4cly petrol twin carby, model approx 1990 .
I can no seen to fix this problem I have checked carbys for vacumn leaks, new points , new plugs, changed fuel filter, cleaned out fuel tank and rest timing but at 2000+ rpm runs fine when coming off from high revs the engine seems to take a long time to idle back down then starts hunting from almost stalling to 1800+ rpms then back continusly

perhaps it is time to overhaul the carb's...

If not carburetor related, look at your ignition's mechanical advancing system.
Make sure that the flyweight return springs are fully pulling the flyweights back into the "no advance" mode (i.e., your BASE advance only).

We can change RPM by simply changing the ignition advance while making no change to the throttle plate position and/or low speed fuel metering.
Rusty or compromised flyweight return springs may be causing an erratic advance (beyond BASE advance) while near idle RPM.

Simple method:
Throw your timing light on it while in the RPM range where this occurs.
You should see a very consistent spark event as per each RPM range.
If the spark event is not consistent, then perhaps that's what's causing the hunting.



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Hi.

I would first synchronize the carbs. Try a Uni-Syn gauge for that, it works way better thatn the system recommended in the VP owners' manual.

You said "New points"???? I suggest that you install an electronic ignition kit (Pertronix or Hot Spark) and you will most likely experience a smoother idling and better starting. Been there, done that.

Good luck.

Great idea, and I did preface my suggestion by saying "if not carburetor related....... "


I agree...... try replacing the points system with the Pertronix kit. This is our only reasonable choice for this distributor.
Keep in mind that this does ZERO to correct any issues with the flyweight system, so I would still suggest looking into that.


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hi yes also overhaul carbys about 2 years ago.
Like the suggestion of electronic ignition where can i buy that i live in oz.
Is there any information on how to dismantle the distrubtor so to look for corrosion on parts .

tyson1 said:

Is there any information on how to dismantle the distrubtor so to look for corrosion on parts.

Click to expand...

This should be self explanatory once you get into it.

Meanwhile, with the distributor cap removed, see how well the rotor moves, and more importantly, see how well it returns.

BASE advance is a result of where the distributor housing is positioned (no progressive advance at all).
Progressive and total advance are results of the mechanical advancing mechanism beyond and in addition to BASE.

If BASE advance is 10*, then lets say that the progressive would be 20* (making a total of 30* at the crankshaft and at the specified RPM)
20* at the crankshaft is only 10* at the distributor.
IOW, the rotor should move to the tune of approximately 10*, and then fully return without a hiccup.

Rusty or compromised flyweight return springs will make for an erratic advance at idle RPM, and an overly aggressive advance above idle RPM range.
Either way.... it's not good for your engine.

When all is working correctly, you should see a fairly linear advance curve through the RPM range.



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Hi guys just an update were i am at . I have removed distributor and taken in to be reconditioned by expert ignition guy here in south oz, should be back me next week.
If this does not fix the hunting it needed to be recond. as i have had since new 20 + years.
Thanks for the advice.

Just an FYI for you.
Most or many of these shops who own a distributor machine, are more so geared towards the Hot Rod, HP or classic car guys.
Your Marine ignition curve will be quite different from the automotive.
The curve should not ramp up quickly, and will work better if it's more linear.
You may need to take your OEM specs to him.



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Hi just an update , i have installed the recondition distributor as per instruction in workshop manual and all went well the motor idles perfectly now.
Thanks to all advice it has been great help.
Tyson

Good for you.

Just make certain that when you adjust the BASE advance, that you also look at the total advance, and see if it's as per the OEM specs.