A Few Running Anomalies - Help Please

[Swami]

Got my TC-102 onto the boat (12' alum dinghy) and onto the water at last.
She starts fine: 1 or 2 pulls (unless I flood, or over choke it).
I may have it a bit too low in the water at present, but will raise it next trip.
It has gallons of water at the outlet tell tale after my recent head de-rusting sessions.
The water temp at outlet is not hotter than my hand can stand, but the head/cylinder and exahust pipe are far too hot to touch. Is this normal?
It runs (in my newbie assessment) well at idle, well at half speed, and well at full speed.
It blows not too much smoke.

But ..... after about 5 - 10 mins it seems to run slower and more labouriously. I swear the speed of motor and boat has dropped 20 - 30% after half a mile or so. Also, it seems to occassionally ... lurch ...slip... miss power ..... cavitate a little .... sort of like hitting a "heavy water patch" or something. I look around to see if I hit weeds or something, but no.

I have checked the plug gap - it is OK at 20 thou.
I have checked the points gap - it is OK at 20 thou.
I have correct fuel mix at 25:1 (previous owner said so).
I just gave the carbie a full strip and clean.

After a break or 5 - 10 mins, it starts fine and has full power and speed again, only to slow about 20% after 5 mins.

Hmmmm? What might cause this "tiring", this "labouring" of the engine when warm?
Could it be too deep in the water, too much exhaust back pressure?
Could it be compression rings/gasget/housing leakage when hot?
Could it be head cracks opening up when hotter?
I have a NKG A7 plug. Should I try a Champion D16 for a comparision test?

Also, the prop I have is a 9 and 1/4" 4 blade which seems like overkill in the power department. Might a 2 blade be more suited to a 4hp on such a small boat and for better speed?

Swami.

[RickUK]

Hello Swami - could be a number of things, but I think the first suspect is the weak oil ratio you are using and the motor may be trying to seize.
Go to a 15:1 mix first and see if that cures things. Rick

[chris]

I agree with you rick it does sound like it is seizing
you should run 102s on 10/1
does it pump water at the idle as the 102 does at very low speed but the other models with the square barrells don't.
It is normal for the exhaust to be hot. You should be able to touch the barrell with your hand.

[Swami]

Thanks Rick and Chris.

I will remix some fuel and try again. But Chris, are you saying 10:1 is better - per se - for the 102, rather than 25:1 even WITH the smaller carbie jet? It is true, as I have read, that recuding oil will make less smoke and just as happily feed tolerable combustion, but oil reduction = lube reduction in my book, which is not the manufacturers specs anyways, so why have people bothered to do it and advise it.

Yes, it certainly pumps water at idle, and it is only warm at the outlet, for both slow or fast running - yet the barrel seems too hot at all times. This might, as you suspect, indicate tightness in the piston and rings due to under-lubrication.

[RickUK]

Hi Swami - 10:1 or 15:1 as you prefer. 10:1 was for the old oil - 30 grade engine oil, but with modern oils and especially synthetic oils 15:1 is fine - that's how I run all mine. People like to use the higher ratios to reduce smoke, but synthetics smoke less anyway!
Discharged water will just normally be a little warm. the head area around the spark plug becomes a little hotter than you would comfortably like to hold, and the rest of the barrel should be warm but which you can hold.
The upper part of the exhaust is the hottest area as it is taking the dishcharged exhaust direct from the cylinder at that point.
Different spark plugs won't have any effect on engine temperature. Rick

[bigoink]

A further point to the 10:1 25:1 debate........years ago when we were tweaking 2 stroke motorcycles for racing purposes it was a generally held opinion that if more oil was added to the mixture ,the fuel /oil/air mix would run leaner and hotter if the carb jet size wasnt compensated for.