TWO STROKE Question

[jerry-ahrens]

I went through a 40 hp Big Twin last summer, with all new ignition, carb kit, fuel pump, the works. That motor would only run on one cylinder [barely], and sometimes would not start and run at all. After pulling the reed block, I found 2 screws, one on each side, holding open a reed petal. Both cylinders were affected, since there was a screw lodged in each side. That one kicked my butt for a day or so before I figured it out. The screws came out of the recoil interlock on the front of the motor.
So yes it can happen..

[mightymite]

Well, I pulled the reed valve assembly and visually I do not see anything wrong. Is there a way to test the reeds? And if this is Ok what else could cause pressure back thru the carb ?

[frankr]

If they are lying flat against the plate, they are OK. Please describe exactly what is happening and why you are concerned. There is quite a storm going on in the carb venturi and some mist may blow back. But it should be a very miniscule amount and you shouldn’t be able to notice it unless really looking for it. The silencer should stop it and return it to the inflow. So, is that what you are seeing, or something more serious. Does the motor run OK?

[mightymite]

The motor is very hard starting. I have been using my i/2 electric drill in a tank w/the prop on. You start with full coke and then leave it full until the motor almost dies. Then you push the choke back in until it almost dies. Choke in and out, in and out until it warms up. Maybe a dozen times. Once it is warm it does not run to bad. However you can adjust the idle jet to where it idles real nice, but then when you advance the throttle the motor dies. Or you can adjust the idle jet so it rev’s up nicely with no hesitation, but then it won’t idle. I have used two different carbs, with the same performance results. Oh, and even when the motor is warm, you still have to use the choke to start it. I sure thought this was a dirty/clogged high speed jet, but no amount of cleaning has any benefit. Also, I put new pistons/rings in and my gauge is showing 75lbs per. Not super, but it should run with that.
I got the idea that the silencer might affect the tuning, so I put it on and that was when I noticed the blow back. There is a small hole in front of the silencer and you could put your finger on it and feel the air blowing out of this hole. Maybe about 5psi or there abouts.

[frankr]

Not making any accusations, but just an off the wall guess: Awhile back I bought a 3hp Lightwin that I spent much time on, and could not get it to start and run as it should. I finally discovered that the pistons were put in backwards!! Worth a looksee??

I sort doubt the air you are feeling at the fogging port really is blowback. I’m betting it is air pulses as air flows through the silencer.

OK, what are your compression readings?

[mightymite]

75 top/75bottom

[mightymite]

I just checked the pistons and they are in correctly.

[jerry-ahrens]

How about a crankcase leak? That would cause the motor to start hard. What type of sealer did you use on the halves?
The piston domes should slope toward the exaust cover.

[mightymite]

I don,t remember as it was several years ago I rebuilt this motor. I do remember using new gaskets. Is there a way to check for this leak ?

[fleetwin]

Did this motor ever run right after you rebuilt the powerhead? What did it run like before you did the work. 75PSI seems a bit low for that engine, but your gage might be off.

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