1971 70 hp Chrysler strange stalling issue

[Charlie75]

[FONT=&quot]OK, here is something for the experts! [/FONT]
[FONT=&quot]My 70 hp Chrysler (1971) has been converted to automotive coil and condenser ignition long since. It has worked out just fine, but recently it has somehow picked up this strange habit:[/FONT]
[FONT=&quot]Each day, when taken out the first time, it fires right up, idles perfectly, shift flawlessly and runs like a charm. It hits 5000 rpm with ease, no hesitation whatsoever. [/FONT]
[FONT=&quot]BUT? When we stop for coffee, let?s say about 15 mins or so? A good hot restart, perfect idling, shifts nicely into gear again, and then? when opening the throttle it just stalls and dies, no matter how slowly or quickly we try to run the boat up on plane. We can restart it over and over and same thing happens.[/FONT]
[FONT=&quot]NOW? just when starting to open the throttle, a very quick push on the key switch to slightly choke the motor when it?s about to stall, immediately cures the symptom and? wroooom the motor comes back to life and gains speed.[/FONT]
[FONT=&quot]SO? thought it would be a simple fuel starvation problem, and had the diaphragm renewed? no difference. The old one looked good. Tank vent, priming bulb, fuel lines, filters? everything seems to be in order.[/FONT]
[FONT=&quot]Why the hell this doesn?t happen the first time each day the boat is taken out? beats me![/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]ps. the carbs are cleaned, floats seems to be ok, timing is correct, the points are brand new and properly gaped, and so on. HELP!!!!!! :-D[/FONT]

 

[Frank Acampora]

Well, I have never had it happen to me but sometimes these engines will percolate the fuel out of the bowl on a hot shut down. Try pumping the bulb before the restart. Normally you should not need to do this but if the engine wants it then give it what it needs.

Also, remove the pump and impulse hose. be certain all orifices are clean and allowing the fuel pump to deliver its full rated capacity. Check the button valves to be certain that all three are functioning correctly.

 

[Charlie75]

Thanks Frank, the thought crossed my mind, but didn?t do the priming test ? silly me. Figured out that 1-2 mins of idling would anyway fill up the bowls?
The valves inside the pump appeared and felt ok, also renewed the gasket. No apparent cracking or flaws on the impulse hose.
Maybe I should hook up a t-hose and measure the fuel pressure and delivery rate to verify the operation of the pump? Back in the 80?s we had some unexplained piston failures on two of these motors? could have been a weak pump causing lean mixture at high RPMs? I don?t want to take that risk anymore.
And that fuel boiling away thing? Never happened to me before either. Did notice though that the head and exhaust cover plate is slightly hot to the touch at idling speeds, but they cool down at cruising speeds. I?ll renew the impeller next time and check the buzzer for proper function.
My old 120 hp has severe overheating problems at idling speeds, the buzzer sounds every now and then, but never do the carbs appear to boil dry?

 

[Charlie75]

Ok, I did some researching today. Tried priming the carbs after a hot shutdown, > no difference.*

So the problem is probably not in the fuelsystem.

The coil and condenser both appear to be pretty old. I would have thrown in new ones already, but didn't find any info on theese (ok, because its not Chrysler original)*
So how do I find a matching coil and condenser for this application??? Anybody have part numbers? How about buying a set for e.g. an old MerCruiser V8?

As we know temp and heat can cause erratical operation of theese components.

An other thing... I have a bunch of those old Motorola/magnapowerIV powerpack-coil packages. They were designed to work with the optical distributor. However, rumours tell they can be hooked up to a points dist?!? How can that be? Can they really be trigged using both points as well as the optical eyes??? That would mean I could simply switch to CDI, but I somehow like the simplicity of the all analog coil-condenser setup.

Cheers!

 

[Frank Acampora]

I forget the exact mechanics of it but yes, they can be triggered by points. Both the electric eye and points send a voltage to the internal gate transistor. I don't remember if the transistor is triggered to dump the capacitors into the coils at make current or break current though. Remember that the points still need to be accurately adjusted to specifications even though they do not need a capacitor with the CD box.

Remember that with the Motorola CD box you get the advantage of a much higher voltage spark. The electric eye is a bit more precise than points but, of course, the non-distributor Prestolite ignition is most accurate as far as timing.

 

[Nordin]

I agree with Frank. The MagnaPower CD will work with both preamp and points. The box dump to the coil when you break current to the transistor.

About the start issue when warm. Have you tried to pulled out the knobe for coldstart without choking, just to open up the throttle a bit and let it idle a bit higher just to fill up the carb bowls, if that is the problem.
As it comes back to life when hiting the choke, it seems to be a fuel or fuelmix related issue. How is the air/fuelmix screws setting? Should be about 1 1/8-1 1/4 out from seated.

BTW I always prime my engines when starting warm or cold

 

[Charlie75]

Yes, the carbs are primed and filled up, and I always start the motor with advanced throttle and let the idle stabilize. All adjustments are also set according to specs.

The whole problem really does not make any sense:?
The motor benefits from hitting the choke when warm. Too lean a mixture or starvation would be a problem BEFORE reaching normal operating temperature, right?

Therefore my conclusion is that it's not fuel related, even if the choke seems to help things out.

The next question: could a bit of extra fuel compensate for a weakened spark, for instance???

Here's a test I'm planning: since my ignition incorporates an external resistor (9V coil), I'll try bypassing that resistor using a remote switch just as the motor is about to stall, and see what happens.

 

[Nordin]

Well okey try that, but seems a "long shot" since the spark would not be weak after the starter bean off. It will put the voltage down when turning, but then !!!
Since choking solve the problem I think about a fuel issue. Try to go back to Motorola CD ign.system.
Try another carb clean and take notice about the lowspeed/idle circuit.

 

[Charlie75]

I do nothing but agree with you Nordin. Seems like a fuel issue allright. The only thing that dogs this is, why the problem isn't present when the motor is cold???

And since the analogue coil-capacitor ignition has worked out fine for years, there is a good reason to belive it can do so in the future as well.

Capacitor discharge ignition gives a stronger spark, true... but the only times I've actually benefited from CDI, was when the battery was so weak, that the fuel would not ignite on cranking. (could be solved with resistor bypass).

The Prestolite system has several benefits, but otherwise it's like a complete mess to me compared to dist.ignition, lots of components to fail, and spareparts are getting expensive. My 1975 120 HP had the Prestolite system retrofitted after the unreliable magnapowerII was removed.

Cheers!