<div style="margin: 0in 0in 0pt">Ok I have a glow plug question I have an OS 91 FSIIp in a Balsa USA &frac14; cub I&rsquo;m running Powermaster 20/20 and flying from a field with an elevation of 2589 ft and have been for 2 years.</div><div style="margin: 0in 0in 0pt"></div><div style="margin: 0in 0in 0pt">The idle has always been impressively low and consistent. Last month I replaced my glow plug (OS # F) with a new (OS #F) witch only lasted 3 flights and 2 idle screw adjustments.</div><div style="margin: 0in 0in 0pt"></div><div style="margin: 0in 0in 0pt">I only had an OS#8 in my flight box when the second plug went 10-7. I know have 2 more flights and have had to adjust the idle screw twice and I&rsquo;m still not happy with the idle.</div><div style="margin: 0in 0in 0pt"></div><div style="margin: 0in 0in 0pt">Is there a plug that would work better for my purposes? I get confused on what plug works best in a particular application.</div><div style="margin: 0in 0in 0pt"></div><div style="margin: 0in 0in 0pt">An example last weekend we had a young man show up at the field with a trainer with a new OS 46LA and a #3 plug. The motor would not idle somebody made the comment that it needed a # 8. They swapped out the plug with a #8 and it purred like a kitten. The motor came with the #3 so that is what the guy had for a replacements. They tried three of the #3 plugs (new) before the #8. It was like magic and the guy with the # 8 was a hero of the hour.</div>

hangar 9 4-stroke plug.

YS about had a cow when I sent them in an engine with one of the H-9 plugs?? Seems they just never tested them in there engines, I have had good luck with them, the YS plugs and the OS type F have all been great. A lot of guys here use McCoys. A four stroke plug is a hotter plug then the 8 so just go out and buy a few different makes of four stroke plugs and try them.
Have you bothered to adjust your valves in a while?? Sometimes you start having little problems as the diaphram in the regulator starts to get old or if you get some crud in the system.
The glow plug is just one place that can cause a problem.

Merlin plugs Rock.

If the new plug only gave you three flights, I would suspect somewhere you are running lean.  Either you adjusted too lean when you installed the new plug, or you may be having pump problems?

I agree with Gray. A valve adjustmet will do wonders. mostley when there tight. If they are tight when the engine heats up it will lose compression.

<p style="MARGIN: 0in 0in 0pt" class="MsoNormal"><font size="3" face="Times New Roman">How do you know when to adjust the valves? The motor came with the airplane. I picked it up at an auction three years ago. </font></p><p style="MARGIN: 0in 0in 0pt" class="MsoNormal"><font size="3"><font face="Times New Roman"> 

</p></font></font></p><p style="MARGIN: 0in 0in 0pt" class="MsoNormal"><font size="3"><font face="Times New Roman"><span style="mso-spacerun: yes"> </span>I would think valve lash would increase with wear and I would be having float at high RPM before I would notes an idle problem. </font></font></p><p style="MARGIN: 0in 0in 0pt" class="MsoNormal"><font size="3"><font face="Times New Roman"> 

</p></font></font></p><p style="MARGIN: 0in 0in 0pt" class="MsoNormal"><font size="3" face="Times New Roman">Is there a test that I can do to diagnose any pump problems? </font></p><p style="MARGIN: 0in 0in 0pt" class="MsoNormal"><font size="3"><font face="Times New Roman"> 

</p></font></font></p><p style="MARGIN: 0in 0in 0pt" class="MsoNormal"><font size="3" face="Times New Roman">Who sells <span style="mso-bidi-font-size: 8.5pt">Merlin plugs</span>?</font></p><p style="MARGIN: 0in 0in 0pt" class="MsoNormal"><font size="3"><font face="Times New Roman"> 

</p></font></font></p><p style="MARGIN: 0in 0in 0pt" class="MsoNormal"><font size="3" face="Times New Roman">What is the difference in the OS#8 and the type F? </font></p><p style="MARGIN: 0in 0in 0pt" class="MsoNormal"><font size="3"><font face="Times New Roman"> 

</p></font></font></p><p style="MARGIN: 0in 0in 0pt" class="MsoNormal"><font size="3" face="Times New Roman">Will a hot plug give better idle than a gold plug? </font></p><p style="MARGIN: 0in 0in 0pt" class="MsoNormal"><font size="3"><font face="Times New Roman"> 

</p></font></font></p><p style="MARGIN: 0in 0in 0pt" class="MsoNormal"><font size="3" face="Times New Roman">I am think that an on board glow would make this a non-issue but it has always done so well with out one. The motor is mounted sideways not inverted. I have a Saito 125 mounted inverted with no on board glow and it has never misses a beat as long as I feed it powermaster 20/20 but use 15% cool fuel and it will dead stick every time I make a long approach or idle back for a long dive. </font></p><p style="MARGIN: 0in 0in 0pt" class="MsoNormal"><font size="3"><font face="Times New Roman"> 

</p></font></font></p><p style="MARGIN: 0in 0in 0pt" class="MsoNormal"><font size="3"><font face="Times New Roman">I don’t think my fuel has gone bad as my Saito is still purring like a kitten and it has always been finicky about its diet. <span style="mso-spacerun: yes">      </span></font></font></p><p style="MARGIN: 0in 0in 0pt" class="MsoNormal"><font size="3"><font face="Times New Roman"> 

</p></font></font></p><p style="MARGIN: 0in 0in 0pt" class="MsoNormal"><font size="3"><font face="Times New Roman">Are there any advantages to using a McCoy #8 a friend was using them in his Ram 500 and no longer has a use for them?<span style="mso-spacerun: yes">  </span></font></font></p>

When the engine gets hot the valves expand. if they are tight now when cold it will run worse arter it's gets hot.I see this alot at my local field. Guys buy old engines at sales ect. Pull off your rocker boxes to see what the clearence is it cant hurt then you will know for sure if that is what it is. As far as plug differences the f plug is longer one the tip. And it is a little hoter..

According to this page http://www.rcplanet.com/Glow_Plugs_s/870.htm the MC-8 would be a terrible choice for any four stroke engine.

Mc Coy Glow Plugs Chart

MC-4C : 4-cycle plug and used in all Saito engines.

MC-55: The MC-55 is considered a hot plug and is designed to run on fuel with a low nitro content. Applications for this plug would be planes, tether cars, etc.

MC-59: Like the MC-55, the MC-59 is considered a hot plug. The difference between the 55 and 59 is that the MC-59 is designed to run on fuel with a higher nitro content (low to medium nitro content) than the MC-55. Applications: Boats, planes, helicopters, and cars.

MC-8: The MC-8 is a medium to cold plug, designed to run on fuels with a medium to high nitro content. Applications: Boats, cars, and planes.

MC-9: The McCoy MC-9 is considered a cold plug and is designed to run on fuels with a medium to high nitro content. Applications: Cars, boats, ducted fan and Giant Scale planes.

MC-14: The MC-14 is a hot plug, designed for helicopters and 4-stroke engines. This plug has a 24K gold stem.

=======================

First I would get a 'four stroke' glow plug. I like the OS-F but others probably also work well.

Next I would check the valve lash and adjust if needed.

Gray Beard mentioned making sure the fuel pump is working correctly. I do not know anything about pumps but it makes sense that it could wear out or get debris in it. You know it is at least three years old. I have no idea how long the pump diaphragm and other parts should last.

I agree with you that the lash probably did not tighten up in 3 years of running, but it is something to check since you are having problems.

http://www.merlinglowplugs.com
The 2010 appears to be their recommendation for four strokes

<span style="FONT-SIZE: 12pt; FONT-FAMILY: "Times New Roman"; mso-fareast-font-family: "Times New Roman"; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA">Thanks for the links</span>

If you have never worked on a four stroke engine you should get one of the books about them. Istill have my trusty OLD A Guide To O.S.Model 4/Strokes written and illustrated by Graham C. Rice. Some of this little book is out dated but only because of some of the newer minor changes that have been made over the years. The last time it was updated was in 1999. It's from RCM Anthology Library Series, it was only a couple of bucks and well worth it.
Ihave found much better books at one of my LHS book racks and should up grade my reading. This little book is great to help understand what is inside one of these engines but doesn't go into trouble shooting, has nothing on pumpers or pumps.
Your fuel pump either pumps or it doesn't pump, not a lot of gray area here and it is a replacement part, can't be rebuilt. If it's a regulator problem it's a send into O.S., this is one of my sore spots with O.S. they will sell you a new regulator cover but will not sell you a diaphram, you have to send in the complete carb to them over a 50 cent part.
Go to the glow engine forum and you can ask Bill Baxter of O.S. about engine problems, the man knows his stuff and will answer you back pretty fast.