no rpm drop when spark connector removed

[madcat302]

Hi all! I I am trying to fix my boat motor. I have a 1987 mercury force 75 horsepower. It is an outboard 3 cylinders 1 carburetor. I have done a compression test on all cylinders they are Each running at 120 psi thereabouts... I ran some seafoam through the carburetor to try to clean off the carbon buildup. I changed all the sparks today and Pulled the hood of each spark while the motor was running... I noticed an RPM drop on the top and bottom cylinder but not the middle cylinder... I did however I notice that it was still sparking any ideas?

Cheers!

 

[Big Fish Billy]

Re: no rpm drop when spark connector removed

Sounds like either the spark plug, the wire, the connector, or the coil itself may be defective...or the plug might have gotten fouled again by a loose piece of carbon....so start by looking at the plug, switching it with another, and then the other components...

 

[Jiggz]

Re: no rpm drop when spark connector removed

Well, if it was still sparking that means the spark could not be strong enough or it could also be a reed problem. To isolate you need to switch around the coils or CDM's and see if the problem moves with the switching. If it doesn't then it is time to re-verify your compression numbers. Remember, switching coils and CDM means literally moving the components and not just wires.

 

[pnwboat]

Re: no rpm drop when spark connector removed

So exactly what kind of symptoms are you experiencing that you're trying to fix?

 

[madcat302]

Re: no rpm drop when spark connector removed

So exactly what kind of symptoms are you experiencing that you're trying to fix?

When the engine is started it stalls out in idle. Once it is going if thrown into gear it stays going but if I increase the rpms too much it sputters and kinda drops out... When put back in idle it stalls out.

 

[madcat302]

Re: no rpm drop when spark connector removed

Thanks Big fish Billy and Jiggz! I will switch the CDMs next.

 

[jerryjerry05]

Re: no rpm drop when spark connector removed

They didn't make a 75 in 87.
The 87s were Force, Mercury wasn't involved until they bought the co. in the early 90's
Model#serial#?
Any water on the plugs?
One plug cleaner than the others?
Swap coils.
Outboard ignition . com or CDI ignition has test procedures for outboards.
Post the correct year and model?

 

[madcat302]

Re: no rpm drop when spark connector removed

Sorry the year is actually a 1997 75 elpt.

I switched the spark plug out and switched hoods (which I'm hoping also switched the CDM and retested by pulling the hood of the spark plug. Again no rpm drop and the motor sounded better when the hood was removed. It almost sounds like the engine is backfiring when I have the hood on that plug... I'm going to make a video so you can see what's going on...

 

[madcat302]

Re: no rpm drop when spark connector removed

Well, if it was still sparking that means the spark could not be strong enough or it could also be a reed problem. To isolate you need to switch around the coils or CDM's and see if the problem moves with the switching. If it doesn't then it is time to re-verify your compression numbers. Remember, switching coils and CDM means literally moving the components and not just wires.

I have uploaded a video to better help trouble shoot... I switched spark plugs out an still nothing... Maybe someone will notice something obvious that someone like me (no experience) is missing...

 

[Jiggz]

Re: no rpm drop when spark connector removed

When you mentioned hood you actually meant boot. But switching boot does not switch the CDM's. From the sound of the engine it is obviously misfiring on one cylinder hence the no change in rpm when #2 plug boot is pulled. You need to switch the CDM's, i.e. between #1 and #2. The CDM is where the other end of the plug boot and wires connects to. If the misfiring moves to #1 cylinder, then you can surmise the original CDM on #2 cylinder is or firing or working correctly. However, if the problem of misfiring stays at #2 cylinder, then it means the problem is not CDM.

 

[Big Fish Billy]

Re: no rpm drop when spark connector removed

Did you take a compression test with all the spark plugs removed? That cylinder certainly seems dead. And you got the same result when you switched out plugs and coil from another cylinder to that one and visa versa? Another less likely thing could be a stuck or damaged reed valve for that cylinder.

 

[madcat302]

Re: no rpm drop when spark connector removed

When you mentioned hood you actually meant boot. But switching boot does not switch the CDM's. From the sound of the engine it is obviously misfiring on one cylinder hence the no change in rpm when #2 plug boot is pulled. You need to switch the CDM's, i.e. between #1 and #2. The CDM is where the other end of the plug boot and wires connects to. If the misfiring moves to #1 cylinder, then you can surmise the original CDM on #2 cylinder is or firing or working correctly. However, if the problem of misfiring stays at #2 cylinder, then it means the problem is not CDM.

Got it. I will move the CDMs tomorrow and see what happens... Fingers crossed that's what it is...

I did remove all the spark plugs when I did the compression test. I will recheck tomorrow.

 

[submariner1980]

Re: no rpm drop when spark connector removed

WATER!!!!!! Cooling water Doyou have any water to cool motor and lube impeller connected??
Hopefully you got my post before you start testing CDMs.
And my IMHO is you have problem with reed valve cyl. #2. if compression test numbers you mentioned before are accurate. Check spark plug after same test you have filmed, if plug is dry reed valve is cause.
CDMs are good, what I have seen so far.
Don't forget a water. Good luck.

 

[jerryjerry05]

Re: no rpm drop when spark connector removed


Auto Zone sells a spark checker.
When it's hooked up it measures the strength of the spark.
It should jump at least 7/16 "
The spark is there,but how strong.
After switching coils/cdms measure the spark.
It's rare that the coils/cdm go bad.
The compression? What were the exact readings?
Remove all the plugs when doing the test?
Water on the plugs?
I had a head gasket go bad and it still had good compression.
The reeds? It's easy to pull the carb and inspect them.
The video didn't show any water hooked up??
Profile? Location? You might be close to someone who can help.

 

[madcat302]

Re: no rpm drop when spark connector removed

I am located in the Vancouver Canada area (Delta)... I just ran the PSI check again and I am sitting at about 119 Psi on that cylinder.I switched the CDM around and still nothing. I guess I will go get a spark checker from the auto parts store... do you think if there were some damage to the Reed that there would be a dead cylinder? I do see moisture inside however that could be from the carb cleaner I sprayed in there?

 

[madcat302]

Re: no rpm drop when spark connector removed

oh yeah, I would normally hook up the earsbut I figured as I was only running the engine for a couple of seconds I could get away with it?

 

[Jiggz]

Re: no rpm drop when spark connector removed

oh yeah, I would normally hook up the earsbut I figured as I was only running the engine for a couple of seconds I could get away with it?

It could take as short as 15 seconds to totally ruin the water impeller if ran without water regardless of rpm. Hopefully yours survived. If the CDM switching did not change anything, then the next option, since the compression number is acceptable, is to look into the reeds. If you have NOT done this before, search the forum for "reed" or "piston removal" to get an idea how to do it. You can still pursue checking the spark with a spark checker although after the CDM/coil switch, it will be a moot test at this point.

 

[Big Fish Billy]

Re: no rpm drop when spark connector removed

If you changed the whole shebang, spark plug and coil from say number 1 to number 2, experienced the same results, lowering of rpm and 1 & 3, nothing on 2, compression is within 15% on all three cylinders, then you're not getting fuel to 2, there's a problem with the reeds for that cylinder.

 

[Jiggz]

Re: no rpm drop when spark connector removed

Had this been a three-cylinder-three carb motor like the 85HP, the symptoms of a broken reed will be easily detected through blow backs in the affected carb and fuel spitting back from the carb's opening. However, it is a single carb which makes it very hard.

 

[jerryjerry05]

Re: no rpm drop when spark connector removed

Post the other comp #s.
If it's all about 119/120 then you should be OK.
Any more than 10# and you need to check into it.
The old plugs,all burning the same?
Moisture on the plug?
19453 is the part # for a new impeller.