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Varied RPMS on 350 crusader

S

steelyexprt

New member
OK, guys more questions, hope you can help. Twin 350 crusaders on a 1979 Pacemaker. Replaced ignitions this spring and the boat had been running great. On the way in from fishing on Saturday the port mortor started jumping rpms. Were cruising at 3500 rmps and it would drop to 3200 then up to 3800. Any thoughts on where to start looking for this gremlin?
 
What you may have going on is sometimes referred distributor "hunting" whereby the cam/weight springs are compromised, and may not be pulling the cam/weights back into BASE or Initial mode.

When this occurs, the ignition advance at idle fluctuates, causing a sporadic idle rpm.
This may also affect high rpm running as well..... although not as likely, IMO.

Best put a light on this, and see what's happening re; curve/limit.
We should see no mechanic advance until above 900 or so RPM.
Only then should we see our progressive advance curve begin.

If you see an idle advance that is jumping around.... this is most likely what's going on.

Refurbishing old distributors may not be cost effective today.
You will no doubt have some shaft and bushing wear.
There are some Mallory YLM 624 AV being offered via an e-Bay seller for around $170 if you catch one of his auctions.
If you have counter rotating enignes, I believe that this seller also has a few BV's.

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I forgot to mention, brand new YLM 624 AV in that motor this spring. Ran great for the last couple weeks, now it fluctuating at only high RPM. We troll on it all day with no issues from 800 to 1200 RPM. I believe, and will have to double check, but timing was 8 deg at idle and 26 at 3000 RPM when we retimed the engine 3 weeks ago.
 
Sounds like fuel pressure problems to me. Check to see if you have an anti-syphon valve and if it's dirty and sticking.

I assume you changed filters already?

Jeff
 
Jeff, are all anti syphon valves located in a similar place? I can follow my fuel lines to the top of the tank, and I am assuming that the fitting at the top is the valve? How do you make sure that they are working properly?
 
The anti-siphon vavle is a spring/ball check vavle, so to speak, but is used non-conventially in terms of a normal application check valve.

The mild ball pressure is to prevent a siphon in the event that a fuel line would become compromised and be at a lower elevation than the fuel supply.
In essence... it requires a certain amount of suction in order to pull fuel past one.... so they prevent a siphon from occuring!
Gravity, such as a "siphon", will usually not be enough!
Fuel going in reverse direction, would be "checked".... but this is not the intent.

These can become clogged and restrictive..... hence Jeff's suggestion of the valve and fuel filter!
Either of these need to be capable of enough fuel delivery to the engine.

If your carburetors have a screen at the inlet fitting, these also must be clean and free to flow enough fuel.

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If it was only an RPM drop, I'd check the fuel supply. Being as there is a rise in rpm as well as a drop, its likely to be the ignition timing. Did you install the YLM's "out of the box" or did you modify the advance curve?

Could also be the plug wires being intermittent...you aren't likely to notice the misses at idle (trolling), especially if the bite is ON.

Another remote possibility would be the throttle shafts leaking intermittently.
 
How fast do the rpms change? If very fast, I suspect ignition, plugs/wires. If "slower", but still smooth, then fuel restriction. And the engine really is changing, right; this is not a tach issue?
I'm not thinking springs, etc. Over 3200, seems like the springs could not even exist and you wouldn't see an effect, until you throttle back down.
 
YLM was installed out of the box, but ran perfectly over several long trips in May. Just did a valve job on that motor and everything was running great until Saturday. RPM's will run up to 3500 and sync with the Starbord motor then over a period of 2-3 seconds drop to 3200. Add more throttle and it want to race to 3800+ pretty easy over a couple seconds. Pull the throttle back a little and it will sync again for a short period of time and start over again.

Mark, What do you mean the throttle shafts leaking?

Filters are new, I plan on checking the screen in the carb and anti-syphon valve making sure its clean. I still suspect timing and/or ignition though.
 
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It's also starting to sound like beginning detonation.
Detonation that hasn't quite yet reached the destructive range.... and believe me, it can and will reach a destructive range!
This will lower RPM, cylinder temps will cool down, and it may repeat itself.
Long shot.... but worth investigating, IMO.

Best mark off your harmonic balancer, use a standard old school strobe type timing light, and see what your Ignition Advance is doing in Real Time/Real Degrees.
Jot the numbers down at varying RPM... up to 3.5K rpm.

Post the numbers for us.


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Right on, Rick. Like a boob I assumed someone had timed the motor properly after having so much trouble--the first thing I would have done. If that poor thing is running way too advanced, that would cause this problem and--if not corrected--holes appearing in the pistons.

Jeff
 
Yep, it's nothing that you will want to risk, particularily if fitted with the full dished pistons.


piston_detonation_damage.jpg
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However, as much of a NAZI as I am re; a Q/E built engine, even these can suffer from detonation damage if the ignition timing is not correct.

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so it is only an rpm drop w/o any rise...little bit different situation than described originally.

throttle shafts leaking - where the shafts exit the throttle body's casting - any excess wear and they can 'leak' which may lean out the charge entering the cylinders. I'd concentrate on the liquid side first.
 
Harmonic balancer is already marked, thats how we timed it originally. I will have to run through the whole curve next week and post it for you. I know at 3000 RPM it was 26 deg.
 
Follow UP: Turned out to be bad wiring on both the ground and the positive from the batteries. Both had breaks almost through. Fixed the wires and poof, problem solved. Still don't know how the breaks occured though???
 
I see that the problem has been solved! Excellent!:D

I'll make just two more comments for the on-lookers that may pertain to similar cases;
diver_dave said:
............. I'm not thinking springs, etc. Over 3200, seems like the springs could not even exist and you wouldn't see an effect, until you throttle back down.
Click to expand...
By suggesting; "springs could not even exist"..........., if this were to mean not much mechanical advance remaining, I would agree.
With mechanical advance (and for that matter, EST), once we reach this RPM range, the advance should be close to "full in", leaving only a degree or two on the table..... if even that!

steelyexprt said:
............ I know at 3000 RPM it was 26 deg.
Click to expand...
This would appear to be a good number for an engine using the full dished pistons. This may be a bit conservative, but better this than too aggressive!


Glad that you got it resolved. :D

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