Mercruiser 5.7 V8 (1997) jumping RPM

[illskills]

Hello, since I renewed my motor, I'm having problems with jumping RPM's. This is happening only when driving above 1700RPM until I reach around 3000RPM. RPM jumps every few seconds for about 300-400RPM (but not in the same time range). When going above 3000RPM or below 1700RPM or full throttle (5200RPM) it works great, except when full throttle the temperature goes a little higher, up to 100 degrees celsius (SALT water).

The things we changed (new):
thermostat, raisers and elbows, cap and rotor including cables, pistons, piston rings, spark candles, impeller, valves. Motor has 500 working hours, water pump is ok.
Ignition was also set but jumper wire was not put to an engine "ground".
Mechanic put carburetor into some fluid, so it would get "clean".., but I'm not sure if this could be the problem that it actually didn't get clean but dirty? Another thing that comes to my mind is that SENSOR, Hall Effect, Camshaft Position.., is that possible? I'm just guessing the possibilities.. any idea from your side would be really helpful.

Thank you very much, I appreciate every help I get from you guys.

Best Regards,
Manuel

 

[alldodge]

Howdy Manuel
Need your motor serial number?

Assuming your watching the boat Tachometer change rpm. Unless your hearing the motor change rpm alog with watching the Tach it may just be a Tach issue and not the motor.

Ignition was also set but jumper wire was not put to an engine "ground"

If you have TBV then the wire needs to go to ground to set timing.
If you have EST then the wire needs to go to 12V

 

[Scott Danforth]

welcome aboard.

what did you do to "renew" your motor?

your motor getting that warm is an issue it should never ever ever get over 160-170F (71-76C)

going to 100C / 212 F means you have a cooling water flow issue (you dont have enough flow)

with a carburetor, the only "sensor" in the system is the ignition trigger

if your RPM's are hunting, its most likely fuel related. however as AD stated, your tachometer could simply be flaky

if you are truly getting a 3-400 RPM surge, you would feel it, and hear it.

 

[illskills]

Hello and thanks!

Yes, I can hear the motor and feel it, the tachometer is showing correctly and while RPM jump the boat shakes.
From what I read on the forums, they say the max temperature should be around 85 degrees celsius. Flow should be good, since everything that includes water-cooling has been replaced with new parts. Probably is somehow connected with jumping RPM issue.

When you say ignition trigger do you mean that sensor in the attached image? I'm actually guessing it could be only that sensor's fault, or carburetor?

I also thought it could be fuel problem but I have tried changing fuel 2 times but with no difference. Also fuel filter was changed, forgot to mention above.

Engine serial number: 0F797531

Thank you for your answers.

BR, Manuel

 

[alldodge]

Thanks for the serial number

Your motor has TBV ignition and does not use the cam sensor in the pic. With it missing its might be the distributor sensor (item 6). Is their 3 wires or 2 wires coming from the distributor. If only 2 then I would replace the with the new 3 wire which has an extra ground

https://www.mercruiserparts.com/bam/subassembly/31770/1939/50

 

[illskills]

Hey,

the sensor does not look like the one you linked me: https://www.mercruiserparts.com/892150q02-sensor-assy-ign but it looks like one on the picture I posted before (I will take a picture of it tomorrow, maybe I'm mistaken).
Still it doesn't sound logical to me, why it would be running great below 1700 or above 3000RPM if it is sensor's fault - is there any connection?

BR, Manuel

 

[alldodge]

Then the motor doesn't match the serial number. A cam sensor is a EFI motor.

 

[illskills]

Sorry, I will take picture of it tomorrow, when I go on a boat to clean it. Probably I'm mistaken yes. This is the picture of the motor serial numbers which I took last time.
So the most possible solution would be a failure of a distribution sensor? And of course, the wire needs to go to the ground to set timing correctly.
Is there anything else, which would cause those problems?

 

[alldodge]

It can be a carb issue but rpm's usually don't jump around, they would bog and lag and then start picking up speed again

 

[Drcoffee]

On a carbureted engine, if its running hot, it may be a timing issue and/or running lean. An engine will speed up as it gets lean, then overheat and slow down.

 

[illskills]

Hey,

those are the pictures I took today. Not sure if it looks the same as on the parts page you gave me?
As far as I know, this one has been there from ages ago, when I bought the boat and everything worked fine until now.

BR, Manuel

 

[alldodge]

That's the TBV module and connector

 

[illskills]

Sorry to bother you, I'm not a mechanic Could you explain a bit more? Where is the sensor located then - on the inside of the distributor cap? So this module and connector are not the problem?

 

[alldodge]

The sensor is under the distributor cap
https://www.amazon.com/B-Ignition-M...84MSV3P/ref=pd_lpo_2?pd_rd_i=B0784MSV3P&psc=1

 

[illskills]

Thank you very very much for now! I own you a beer or 2..
BR