BF 15A will not run full power cold

"I have a Honda BF 15A that wi

"I have a Honda BF 15A that will only run at about ¾ power until it is warmed up then it will surge a bit and then jumps up on plane. The strange thing is I have taken it out 8 times and it gets up on plain in the same spot (about 10 min of run time) every time. If you shut it off and start it immediately it will run perfectly again. If it cools down for an hour or more you go though the same thing again. Both cylinders have spark, I pulled and cleaned the carb (it was spotless), new spark plugs, the fuel pump is strong, tested the trigger looks good, the kill switch checked out, the gas is new, thermostat is within spec, no crap in the fuel filter and throttle linkage is good. I did the test for the CDI unit that was in my book and some of the pins were not within spec but I was told you can’t test those things."

"Is it a four stroke? If it i

"Is it a four stroke? If it is then, depending on whether or not its cold outside or not then that also could affect it too. It would act like a car in a way, takes awhile to warm up. One thing is that the oil specific to your motor, if being cold (outside temp), at least seem like it to me, it would be thick(oil), once the oil became more viscous, it would make your motor run a little better. Hope this makes sense."

"Ya that makes sense, but I li

"Ya that makes sense, but I live on the North Island of New Zealand and it's about 80 F. There is the posibility that it has the wrond oil in it I got it used. It's a 4-stroke.

Adam"

"Adam - Certainly check the vi

"Adam - Certainly check the viscosity of the oil when it's cold. If suspicious, change it. But another possibility is that you may have a vacuum leak that is sealing once the engine gets warm enough. It would be very small and difficult to find - probably around a fitting. If that doesn't pan out, then hone in on the carb - it may have an air leak that is sealing once the engine warms up."

One of the locals mentioned th

One of the locals mentioned that and we have had the cab off twice but I should take another look. Is there anything that I can spray on the carb that would temporarily stop the leak so I could test it like white grease? I did a valve adjustment tonight and ran it with the cover off and it did the same old thing.

"Assuming your engine doesn&#3

"Assuming your engine doesn't already smoke, spray a small amount of machine oil (or carb cleaner) around each fitting, one at a time, and look for telltale white smoke. I would start at the gasket where the carb throat mates to the manifold - that would be the most likely leak area, since that is where the heat differential would show up first."

"The vacuum leak i thought of

"The vacuum leak i thought of too, but figured since its cold where i live it may be cold where he is."

It's not overpropped is it

It's not overpropped is it?(combined with cold engine not developing enough power to get up on the plane).

I sprayed some WD40 around the

I sprayed some WD40 around the gaskets and it bogged down when I hit the carb to manifold gasket and the head gasket. I fixed the carb but I need to do a compression test.

"re: Head gasket.

I think y

"re: Head gasket.

I think you will find that compression is low in one or more cylinders when cold, the result of not properly torquing the head in the past. Also check the head and make sure its not warped.

I had to replace the T'stat on my BF75 (basically the same engine but 7.5HP) engine this summer. I had a difficult time removing one of the bolts (bottom one). Usually, one just needs to break it free and then it spins out. This one needed a wrench all the way.

When I looked carefully, I discovered a "track" on the head that indicated that there was a leak between the water jacket and the exhaust in the area of that bolt. When I tried to reinstall the bolt, it would not run in freely...

So what I did was purchase a blind tap (special tap for non thru holes) for the bolt size and after spraying WD40 on the tap, CAREFULLY "chased" the bolt holes, removing the tap frequently (proper technique.. using a tap handle, carefully "start" the tap. Using finger pressure only turn CW one half turn, then CCW 1/4 turn... repeat.. after one or two full net turns, remove tap and clean it off, add more WD40.. repeat until the tap hits bottom). An amazing amount of junk came out of that hole on mine and several others.

My conclusion and the reason for this longist post... When the T'sat was last replaced the "mechanic" did not clean out the holes and just ran the bolts back in. He tightened the bolts to "tight enough", but unfortunately, that last bolt got "tight" due to junk in the hole before it clamped the intake manifold to the block tightly enough.
After I cleaned out all the bolt holes, all the bolts ran all the way in using just finger tip pressure and I was able to properly torque all the bolts. Using a torque wrench is VITAL on engines with aluminum heads as is chasing the threads."

"I don't know for sure if

"I don't know for sure if this is a completely safe procedure but I have seen a mechanic do this looking for a vacuum leak. With the motor idleing, take a handheld propane torch (never light it during this procedure) turn the knob releasing some propane and go over all the joints around the carb and intake or any other suspected vacuum leak area. When the leak sucks in some propane the engine speed will increase."