87 NS25C2 Rough Idle, Rough Mid Range, Fine @ WOT

[MrTotty]

I've got an 87 model 25HP Nissan that I'm trying to figure out why it runs rough at idle through 3/4 throttle. The motor runs great wide open, and does not miss a beat. Here is what i have done so far.

1. Cleaned Carb (three times), there is not a spot of anything in it.
2. Checked for air leaks by spraying carb cleaner around the carb. It quits (stalls) when I spray around the butterfly spindle. There is very little play around it. I was told this was normal, even if the carb was new.
3. Checked Compression, it's 125 on both cylinders. I am in the process of running Yamaha Ring Free through it to see if it has some carbon, or a stuck ring but compression seems to be ok.
4. Change plugs, both are a light - Med brown.

Primer bulb stays hard at all times, pumping it does nothing when it's running rough. When you are running WOT and pull back it seems to be missing on a cylinder (or both) it cuts in and out, but never quits (even if left idling for a long period of time.

What else can i check

-Chris

 

[Sea Rider]

Some things to check :

1-Is carb's needle and float in top conditions, that's needle with no end deformations and float parallel to carb's body when turned up side down ?

2-Are you using NGK BR7HS-10 plugs correctly gapped tight to 1.0 mm with gauge ?

3-Is butterfly closing and opening properly when choke is pulled/pushed all the way in, does it get stuck at any choke position ? If so, lube shaft at both sides untill works properly.

4-Are internal fuel connectors o'rings in good state ? are not dried, worn, as letting air in fuel hose ?

5-If engine is old spry 1/2 can of carbon remover through carb butterfly, follow can instructions.

6-Had a similar situation, the culprit was a bad Ignition Coil overheating and running extrangely throughout the entire throttle range.

Happy Boating

 

[pvanv]

What are the jet numbers?

 

[MrTotty]

1. Float is level, Needle appears to be in good shape
2. Plugs are NGK B7HS-10, I'm not sure what the difference between those and BR7HS is but will order the other plug. I will check the Gap tomorrow, but I could swear I checked those before I installed them
3. Choke Butterfly works properly
4. Fuel fittings are in good shape, no leaks. Every time I remove the Carb the bowl is full.
5. Motor is an 87 model, I tried the Power Tune today, lots of smoke, but no change in the way it ran.
6. I would suspect the CDI if it had the problem at top end, it runs great @ WOT.

 

[MrTotty]

How would I find the Jet numbers, are they written on the Jets?

 

[pvanv]

NGK plug numbering, example BPR7HS-10:
B describes the thread, etc.
P is for projected tip (stays cleaner)
R is for resistor type (required in Canada)
7 is the heat range (higher number is cooler)
-10 is factory gapped at 1.0 mm

Yes, the jet sizes are on the jets. Tiny numbers.

Did you run the ohm tests on the ignition components, and verify ignition timing sync?

 

[MrTotty]

Will the plugs I have work as good as the Resistor type? Don't mind getting the correct plug if it will make any difference. I will check the Jet numbers. I did run an OHM Check and one side is .033 Ohms and the other is .036. I have not checked timing, I did pull the link off of it and manually move it a bit to see if it would get any better. It didn't, just sped up.

 

[Sea Rider]

-Both mentioned NGK plugs works same being the one with an R a resistor type, gap them tight as the gap recommendation is 0.9-1.0 mm. Most comes factory gapped to 1.0 but slightly loose. Shim must stay held to electrode tight at 1.0 mm

-Carb's needle tip must be sharp as a pencil, if tip is deformed or with slight neck change it.

-You have that engine from new or it's a second hand as to know if probably someone has fiddled with timing sync.

-Asssume you're achieving a 180 deg open butterrfly at wot, right ?

-When having air leaks on fuel hose will not necessarily experience fuel leaks through connector o'rings specially when engine is running. But could be sucking air in and engine missing. Is it possble to try out any other complete fuel hose that's working well with other Tohatsu engine ?

Happy Boating

 

[MrTotty]

I'll put a new set of plugs in it. Can't hurt.

I'll check the needle, i'll get a picture of it as well.

This engine is second (or more) hand. I bought it with a blown head gasket (was basically a free motor with the boat). I'll have to get a light, but will need some guidance on the settings, I see the markers on the upper end of the motor, I would guess that center is 0 deg. Where should it idle?

Interestingly enough, no the throttle does not open fully. it opens about 1/2 way. It almost seems like the arm that goes up against the roller has been ground, or is the wrong one. Even more interesting that it runs so good wide open. Now to see if I can find the correct arm.

All of the fuel line, connectors tanks etc are new, The bowl seems to stay full after running and disconnecting it will sit and idle for about 2-3 minutes after the fuel line is removed. I have tried two lines from the tank (both new) with the same result. There seem to be very few Nissans/Tohatsu's around here, have to order parts online.

 

[MrTotty]

No Title

So after doing some more digging, the only thing I can thing of is the throttle cam was ground down to stop the motor from going past a certain throttle. The cam does not look like the other ones I am seeing in pictures.

 

[Sea Rider]

The carb does not open fully due to being a restricted 25 HP, Is it possible to post 2 side pics of engine. Don't work with C2 previous model, it's been replaced by the C3, which should have slight min differences. Do you happen to have metalic air restrictor on the carb or it's a full opened standard gasket ?

If you are reffering to the cam with a middle goove, that's the new cam found on newer 25 C3 models. You have a 27 year oldie, electric parts don't last that long, so probably will be a good idea to check the cost of a new Ignition Coil, are relative cheap, don't worry, will not get pawned.

Happy Boating

 

[MrTotty]

No this motor has no restriction plate, it has a wide open gasket. So it's normal to only open up to half throttle? That seems a bit silly on the restriction. Can this motor be converted to a 30? My only reservation on the coil is why would it work fine running wide open but not below wide open? I'll get some pics tomorrow.

 

[pvanv]

Go through the link and sync procedure in the Factory service manual, so that you have full WOT of the carb when the handle is turned all the way, and that both idle and WOT ignition advance are correct. The 25C2 and 30A3 used the same carb, no restrictors in either; The 25 was limited by adjusting the link and sync so the carb did not open all the way. If you adjust it per 30A3 specs, you will get 30 hp from it.

 

[MrTotty]

Did they use a different throttle cam lever? Once the motor hits the indention on that cam it will go no further, and since there is no way to adjust the linkage between it and the Advancer Arm, the only thing you can adjust is timing. Without a different cam, I can't get the motor to open up no matter what I do. I did modify the edge of the link which allows it to open fully, however I can't get the timing to sync due to the cam not being in the right place (if it opened the throttle sooner, it would be fine). I reference the pictures in the post below. The Cam on both these motors is smooth (one has a divet one does not), mine was not smooth.

Also you mentioned Ignition Coils, are you meaning the CDI unit, or the Exciter or Trigger coils under the fly wheel (which both are no longer available).

I have looked and can't find a service manual for this motor, only later models.

 

[pvanv]

Both the 25C2 AND 30a3 use the same

346637180M

THROTTLE CAM

$1.91

 

[Sea Rider]

No need to change the throtttle cam if both models shares same. What you need to do is play with upper stopper located at 3 o'clock right of arm. The 25 is timing/carb restricted as both 25-30 are same CC. It's a cheap way of restriction. You need to place grip at idle, move gear level to forward and full throttle. You need to back upper A stopper nut & screw CCW some mm, you'll see how butterfly starts to open up. 25C2 comes factory adjusted to 20 deg, 30 HP to 25 deg. Once finish adjusting timing to 30 will see timing mark has advanced + 5 deg.








A good adjustment as not to wear throtttle components is : With left hand finger raise left carb pawl untill stops, with right hand rotate grip right, left several times, the cam should slightly bang carb roller, once this cond is achieved, adjust A stopper nut to lock the new adjustmnent. Apply small oil droplets on each rotating and sliding throttle parts for smooth operation.

The Ignition Coil is a separate electric component, is the round one that has 2 cables exiting for the spark plugs. I know sounds strange for engine to work well at wot and awful at idle and medium throtttle, have gone through this exact issue before, but my engine was in top mechanical working order. Don't like electrical troubleshooting as this prrocedure could take very long to diagnose specialy the ones with overheat related issues, that's why like stocking general electrical components and swapping them for fast spot on trouble detection, but that's me...

Anyway check if there's still available a IC unit, if with heat realated electrical issues and part being NLA and if 30 A4 is not compatible will have a summer grounded engine.

Happy Boating

 

[MrTotty]

So one of the hundred things seems to have made a huge difference. I took it to the water yesterday and noticed instantly that something had changed when I started it. It was smooth. I let it warm up and headed up river advancing the throttle slowly all the way to top end. It ran almost like it is supposed to. It no longer misses but seems to lag every now and then for a short second (Before it did it constantly). I made me a throttle arm (while I wait for a factory one) that does not have the cutout, It now opens fully (180) and I set the timing to 25 DEG advance at WOT (Took me a while to realize there is no marks on the flywheel, and you had to do it by a line on the case). Here are the things I did (at least the ones I can remember) since I ran it last.

1. Changed the plugs to the resistor type (this for some reason seems to have made a huge difference).
2. Cleaned the Carb about 5 times.
3. Made a new throttle bracket and advanced timing 5deg
4. added an O-Ring in place of a nylon washer on the left side of the butterfly on the Carb (it would die when spraying carb cleaner on it on both sides). A guy from Nissan said most would do that, and not be concerned about it. did seem to make it idle better.
5. Ran 1/2 can of PowerTune through it (per directions)

I ran it for a couple of hours, I'm thinking if I have a problem at this point it's carb related. I still think it may be sucking some air from somewhere. The only seal I have not replaced is the upper crank seal may need to look at checking that (I've got the seal)

 

[Sea Rider]

Good to read it's working much better now. The timing marks are located on Ignition plate, not on flywheel as per second posted pic, at least on the C3-A4 model are so. Have you tried running high octane fuel, the highest gas station available ? Did you changed carb bowl gasket ? could be sucking air there ?

It's normal when spraying carb cleaner or power tuner for engine to die if too much amount is squirted through carb, not an issue. Is it possible with a torque meter to adjust the cylinder head evenly to 30 NM (Newton Meter) if it was changed form original, need a re torque adjustment after several hours as to sit evenly against crankcase.

Happy Boating

 

[MrTotty]

On the timing, I was figuring it was like everything else and needed to be set with a light, it took a while to realize that's not the case. The C2 model is the same as the C3 on the timing marks. I have not tried high octane fuel, non ethanol is hard enough to find in 87, I will give that a try though. I did change the carb bowl gasket, even though I've had it about about 10 times, no fuel leaks from it, don't think that's leaking air.

I was meaning that spraying carb cleaner on the outside butterfly post (the rod that attaches to the butterfly and sticks outside of the carb). It would die when sprayed on both sides, I fixed one with an o-ring, that may have had something to do with the running better. A guy from Nissan said all of them would do that, which tells me that I may have compensated for a leak elsewhere. The other place I can't spray anything to test is the upper and lower crank seals. I replaced the lower when I replaced the power head gasket, but never looked at the upper one (didn't remove the flywheel).

The throttle arm that was on the motor (see pic on page 1) had a sharp point at the left side (upper) of the dip, the throttle would not advance past that no matter how much force you used (I did readjust the stops), that is why I ended up making a new one while the factory one is shipped, I wonder if the curves in those will affect the mid-range timing. I still think I'm sucking a little air from somewhere, but at this point it's usable.

 

[Sea Rider]

With a wrench adjust reed valves assembly nuts (9) tight, will be surprised on how loose all could be after 27 years of cold-hot cycles, could be sucking air through gasket too. Engine being an oldie, will be a good idea to change upper crankshaft seal & o'ring, could be dried, hardened ?

Happy Boating