Eratic engine run, preignition?

[frank mccune]

      Hello all:

      I need some help with an engine that appears to be experiencing preignition.  I can not get a smooth engine run in either a 4 cycle or a 2 cycle mode.  The engine sounds like it has a crackling sound and it runs very erratic.  Maneuvers are very risky as the engine loses power as the engines sags in power.  I have never had an engine do this before.  No matter how I adjusted the needle valve, it still sound that it had a pronounced exhaust crackle and an unsteady run.

    Some specifics:

    Engine OS .40S from the 60's.  New ring fitted.  Perhaps it needs more break in time?  I did break it in on the bench.

    Prop 10-6 Top Flite

    Fuel 5 nitro-14-14 oil

    Plug New Fireball Med RC

    Airplane Green Box Nobler

    Tank Uniflow The tank appears to perform as a Uniflow tank should by going lean for about 2 laps before quitting.

    The only thing that I can think of causing this I perhaps a burr or something in the combustion chamber that may be hot enough to cause ignition when fuel is introduced.  I will remove the head and examine the engine for some problem in the combustion chamber.  I will also change fuel and another type of plug.  This engine has been spot on in the past, thus making me think that it is the fuel.


      Suggestions and/or suggestions?
 
                                                                                                      Tia,

                                                                                                      Frank McCune

     

[frank mccune]

    Correction:

    The prop is an 11-6.

[Phil Krankowski]

Pre-ignition is typically an over compression situation.  It can also be a hotter than desired glow plug. 

Did you use the same head gasket?  Reusing head gaskets can sometimes mean they are "squished" especially if they are fiber gaskets.  If the gasket was replaced was the replacement the same thickness?

You are using a new plug, so try running a plug that ran well before if you have one.

Phil

[frank mccune]

     Hi Phil:

     Thanks for the reply;

      I will add a head gasket and then try a short reach plug in an attempt to lower the compression as I have no more head gaskets.  One would think that this engine would have been designed as a low compression engine with one gasket.  Will keep you posted.

                                                                                                                Cheers,

                                                                                                               Frank Mccune

[C.T. Schaefer]

Sleeve in backwards?

[frank mccune]

     Hello All:

     The cylinder is installed correctly.  The ring was made and fit by Mr. Bowman.  It used to be very easy to hand start even while being inverted.  Now electric starter only. ?

     I changed plugs by different mfg. but no improvement.

     Any other suggestions?

                                                                                                                   Cheers,

                                                                                                                   Frank McCune

             

[TDM]

Maybe you are looking at the wrong cause of the problem.
Many times a bad motor run is attributed to bad weak nose construction. If the nose is weak there is nothing you can do to get a good motor run other than converting to electric.
Vibrations cause the fuel to foam up so there goes the needle setting then on top of that you get a lot of energy instead in going in to spinning the prop it goes in to twisting the nose back and forth.
You can have a good rigid nose construction and a crap motor and the motor will run pretty decent but a bad nose construction with a good motor and you will never get the motor to behave no matter how good is it.

[frank mccune]

      Hi All:

      Thanks for the replies.

       I have ordered 12 K&B plugs.  I will mix up a batch of 5-28- all castor and see if that helps.  Perhaps a run on the test stand would help diagnose the problem(s)? I checked for air leaks but found none.  There is always the line to the engine that seems to go bad.

                                                                                                                    Tia,

                                                                                                                    Frank McCune

[Lauri Malila]

Was the sleeve re-honed before fitting new ring? If not, it can be quite hopeless to get it well seated.
Every time I let Bowman do the fitting of new ring, the result was ok, never good results with buying a stock of his rings and assembling by myself. It's logical because cylinders have their small variations, it's like 1 in million chance to have one that fits perfectly. In my opinion a ring has to be tailor made to work well. L