"Hot" plug puzzler

[Terry Caron]

Hi all -

I'm at a loss here. 
Bench running my '57 McCoy .35  on 8.3 nitro/28 oil with a 10-6 MAS, I've tried the hottest plugs I have, new McCoy MC-59, OS 6/A-3 and T-bolt R/C Long, getting 200, 400 & 200 rpm drop respectively when I remove the igniter.
In addition, with the MC-59 & OS 6 it peaks at ~ 9300 rpm, but with the T-bolt it won't turn more than ~8300.

Any thoughts on what may be going on?

Thanks.

Terry

[Balsa Butcher]

It's just a 58 year old engine. RPM drop while not ideal does not sound excessive. A 200 rpm drop can be needled out. In stunt, high RPM is not necessarily a valid indicator of an engine's effectiveness. Take it off the bench and put it on a model. Fly it and see how it performs. If you have no intention of flying it, it is running good enough to be considered satisfactory and put back on the shelf.

[Terry Caron]

I run the same fuel, plugs and appropriate prop sizes on even older Fox .35, Forster and O&R .29 engines without these negative results.
MOF, they all run much better than this particular RH, but age alone is perhaps an over-simplification.
It's the T-Bolt result that's the most curious.

Terry

[frank mccune]

     Hello Terry;

     Check out my post a bit further down in this site. Engine stops when plug is removed subject.  I am having the same problem with an early 50's Fox .29 stunt.

     I have tried different plugs props, nva fuels etc but the engine still sagged when the battery was removed. It is getting a bit better as I can run it with the battery removed. However, the needle valve is so critical that I can not enrichen or lean the engine without it coming to a stop.

     I do not know what to do with this engine except to put it back in the attic! Life is too short to put up with this garbage! I have other engines that are in great running condition so why beat my head against the wall with this one engine.

     I did clean p&c of varnish and that seemed to help a bit.

     It sounds like your plug may be bad.  I have had them look good, glow well when tested but under compression, they would sag when the battery was removed.  In every case, a new plug solved my problem.

     Keep us apprised as to what you determine was causing your problem.

                                                            Stay well my friend,

                                                            Frank McCune

   

[Terry Caron]

About all I can add is that with the T-Bolt, the needle isn't anything like linear, yet it performs perfectly in an OS .20FP and all 3 plugs are fine in 2 Fox .35s.

Terry

[Jim Kraft]

Terry, The only thing I can think of is that it might be a little tight yet. Sometimes they can leak down but still be to tight in one dimension. Like I said, my 57 will turn an APC 10-6 at 9800, but it is a really good one and has been flown for a while. Like was said above, I would put it in a plane and fly it awhile. Other than that it could have a slightly bent rod or crankshaft. With the plug out it should turn very freely. I cannot explain the difference in plugs you are having. I have been running T-Bolt plugs for years without trouble. Not to say you cannot get a bad one. I cannot remember if you are using an aluminum gasket in the head or paper. I have aluminum in all of mine if that makes a difference I do not know. I do believe it conducts heat a little better and lets the engine run a little cooler. One other thing, you said it had a Tigre spray bar and needle in it. I have not checked them with a mic but the Tigre could be enough larger that it is choking the engine enough to slow it down. Could easily explain a 5 or 600 rpm drop.

     My offer still stands though and if you want I will take a look at it.

[Terry Caron]

Thanks Jim - I'm keeping your offer in mind. 

It turns freely w/o a plug, the ST spraybar is .155" diam and it has fiber gaskets, didn't know Al was available.
Best I can tell with straight edges, the rod isn't bent and I can't see any runout spinning the crankshaft, no play in the bearing and it has nearly as much compression when hot as cold.
So I guess there was more I could add. 

Terry

[Jim Kraft]

Terry, I just checked a McCoy spray bar and they are .130". So that is at least part of your RPM problem. That cuts down the effective venture area by quite a bit as the 57 McCoys had a pretty small venture to start with. If I remember right the Tigre spray bars are .157". So it is .027" larger than the McCoy.

[Terry Caron]

Well, that's a significant reduction Jim.
I'll modify the ST to McCoy size and see if that's the culprit for the lowish top end.
Update tomorrow.
A big THANKS!  

BTW - the ST spraybar is a tight slip fit through the venturi holes - is the McCoy spraybar turned down in the center section?
I assume not just a sloppy fit.

Terry

[RandySmith]

Well, that's a significant reduction Jim.
I'll modify the ST to McCoy size and see if that's the culprit for the lowish top end.
Update tomorrow.
A big THANKS!  

BTW - the ST spraybar is a tight slip fit through the venturi holes - is the McCoy spraybar turned down in the center section?
I assume not just a sloppy fit.

Terry

Terry  if you turn down the ST spray bar that thin it will break, the only way to do that is to file or grind the sides down to the thinner sides , this way you will have enough brass to keep the bar strong, make sure you file even and keep the hole pointed down when you file the sides, and only file the part that will show inside the venturie.

Randy

[Terry Caron]

Sorry Randy - too late!!!  
I chucked it into my drill motor last night and filed it down - came out fine.
What's more:
I fired it up a few minutes ago w/a 10-6 APC:
4-2 break is at ~9000, tops out at 10,500!  

That's better than I ever expected and essentially matches my Fox .35s.

So kudos to Jim Kraft - I could kiss you! (metaphorically, of course  )
I'm a happy McCoy camper now.

Thanks to all for your interest and input.

regards,

Terry

[frank mccune]

     Hi Terry:

      I am confused about your problem! You first noticed a drop of rpm when you removed battery from the glow plug.  Later in the story, you were concerned that the engine was not turning the rpms that you thought that it should.  When you thinned the spraybar, the rpms increase to a level that you thought was appropriate.  Did this also cure the drop in rpms when the battery was didconnected?

      The Fox .29 Stunt that I have been having a problem with did not improve when I installed a thinner spraybar.  I still have a drop of pms and a very critical needle valve setting when I remove the battery.

                                                      Stay well my friend,

                                                      Frank McCune

[Terry Caron]

Hi Frank -

There's a bit of a back-story involved.
I'd posted previously about the low power output of this engine and had gotten advice from Jim and others about that, with no improvement.
Then I ran across the glow plug issue initially addressed here and that was quite perplexing, as the T-Bolt is perhaps the most highly recommended hot plug.
Jim's comments above have now led to a resolution of the power problem, and seemingly the plug issue also.
I just got through trying all 3 plugs again, and in fact, I still have a couple hundred rpm drop on the OS and McCoy plugs with the igniter removed, while the T-Bolt now shows no discernible drop.
200 rpm isn't a really big deal, as the engine varies more than that on 4-cycle, which at this point I'm chalking up to a need for more re-break-in.
With all 3 plugs it now turns a 10-6 APC to 10.7ish K, pinch-leaning.
I don't know how or if any of it relates to your difficulties but I do hope you find something as successful for you as this has been for me.
I'll read your posts and at least commiserate with you.  

regards,

Terry


P.S. - I'm still not a fan of the ST nva. I prefer a FOX!!!! 

[RandySmith]

Sorry Randy - too late!!!  
I chucked it into my drill motor last night and filed it down - came out fine.
What's more:
I fired it up a few minutes ago w/a 10-6 APC:
4-2 break is at ~9000, tops out at 10,500!  

That's better than I ever expected and essentially matches my Fox .35s.

So kudos to Jim Kraft - I could kiss you! (metaphorically, of course  )
I'm a happy McCoy camper now.

Thanks to all for your interest and input.

regards,

Terry

Hi Terry

It will still most likley break from the vibration and now being so extremely weak, if so just file the sides flat to 130  and  leave the meat on top and bottom, you will have an oval spray bar that will be much stronger than a turned one. I have seen this happens  many dozens of times over the years.

Hope it holds for you

Randy

[Terry Caron]

It will still most likley break from the vibration and now being so extremely weak, .... I have seen this happens  many dozens of times over the years.

Hope it holds for you

Randy

To update, I replaced the ST needle/collar with an OS remote and now needling is properly linear.
I have ~1 1/4 hrs bench time on it and it 4-2-4s nicely when rotated.
It fills the "I don't have a McCoy" hole in my engine accumulation just as I'd hoped.
But now I have an "I need a Zilch X" hole! 

Terry