rem0.061

My Feedback: (30)

What is the difference between hot and cold glow plugs? Do hot plugs stay lit longer than cold plugs? I have a super tigre 90 with a #8 plug(I think it is hot) and it idles fine after warming up, but when it idles for a short while it stutters and dies. Should I use a type f plug to help it idle?

earlwb

It sounds like your engine idle is a little on the rich side. You need to tweak the low speed needle a little bit.
A Number 8 OS plug is sort of a medium heat range plug.
A cold plug sort of retards the ignition timing some whereas a hot plug sort of advances the ignition timing. You normally use the colder plugs in racing engines with higher nitro percentage fuel at higher RPMs. Sport engines normally use the medium heat range plugs and four stroke engines tend to use the hot glow plugs more. There are exceptions to everything of course.

If the idle mixture is set good and it still seems to stumble then you could try a Fox Miracle glow plug and see if it helps or not as it is a more hot glow plug and it sometimes helps with issues like this.

Reference http://www.rcuniverse.com/forum/m_88.../tm.htm#880566
and
http://www.scootworks.com/rdrc/gloplugs.html

rem0.061

My Feedback: (30)

Thanks for the help.

blw

My Feedback: (3)

The F is hotter than the 8.

jaka

Hi!
If your engine dies after a long idle period (20sek or minutes) then you might need to turn in the idle needle some. Have you tried that?

j.duncker

My Feedback: (2)

The older ST 90 is known to have carb problems esp on throttle up after a period on idle.

There is a mod for this and a search on the net should find it.

One of mine ran well with an unmodded carb on a ENYA No 3 as for the other well ??

1QwkSport2.5r

I have a Super Tigre S90K from roughly the mid 80's or so with the original ST mag style carb. If the needles are adjusted properly, and the spraybar is adjusted properly, it does transition pretty well. In some cases, the fuel delivery isnt linear and drilling out the inlet nipple and part of the spraybar to a slightly larger size makes the fuel delivery more linear. Unfortunately with some carbs, making the transition better will make the idle quality worse and require a higher idle speed. An unseated ring can cause similar issues as well.

I use McCoy MC59 hot/long glow plugs in every engine I own, both Aero and Car. Not all engines transition perfectly, and the carb has about (IMO) 80% to do with that. The other 20% is the fuel and glow plug. I only own 1 engine not designed for long plugs and thats an Enya. To remedy the plug reach I use two copper washers. With shipping costs included, its not economical for me to use Enya plugs, and I will never use another OS plug as long as I'm a modeler. I used them in the past and I was never happy with their performance. The LHS owner gave them for free even.

downunder

Essentially the difference between hot and cold plugs is the ease with which they can ignite the mixture, not so much the actual temperature of the plug. A hot plug ignites more easily than a cold plug so this advances the ignition point which gives a means of adjusting the timing (by selecting the right heat range plug) much like adjusting the distributor in a car. That's for full throttle though. Like a car, the ignition timing has to be retarded at idle and glow plugs do this by cooling down a little (the actual temperature of the wire) during the comparatively much longer period between cycles and this retards the ignition. And this is where I'd disagree with point number 2 in that Rule of Thumb above. Plug heat range is selected for highest revs at full throttle, you live with what happens at idle.

It's possible your plug's on the way out but first I'd go by all the recommendations to check the idle mix with the pinch test.

rem0.061

My Feedback: (30)

The #8 plug is new, I'm going to try a fox miracle.

Sport_Pilot

Some ST's need low nitro or cold plugs. I would not recommend using a Fox Miracle plug in this engine. You may have used higher nitro fuel than this engine is designed for. You compensated for the tendency to detonate by adjusting the mixture richer. Now the idle is also rich and there is little adjustment left on the idle mixture screw. Though it may seem a hot plug should fix it, the opposite may be true. A colder plug will mean more RPM at full throttle and you will need to lean out the top end, this will also lean out the low end somewhat and leave more adjustment on the low speed needle. Or alternatey you can use lower nitro fuel which will also retard the ignition and allow a leaner setting.

rem0.061

My Feedback: (30)

I was told to use Super T fuel, I don't know how much nitro it has, but it does seem to be firing a little early just under full throttle. Someone I know told me that they use 15% nitro with no problems in the .90. I may adjust the carb later today.

Sport_Pilot

Don't use Super T fuel on a .90 sized ST.  Super T fuel is for the larger 1.4 and up.  The larger engines don't need as much oil.  Super T fuel has 9 to 10% oil instead of the required 17 to 20% oil.  Hope you did not run this long.

rem0.061

My Feedback: (30)

Ive seen two kinds of super T, super T and large super T. I have the regular super T.

Sport_Pilot

Here is the exerpt from the current ST .90 on fuel.

Quote:

High Quality Fuel: The recommended fuel for your SuperTigre engine should contain a minimum of 18% - 20% oil (either Castor oil, Synthetic oil, or a blend of the two), 0%-15% nitromethane and the balance Methanol. Almost all of the fuels on the hobby market with between 0% and 15% Nitro content will be fine, but you may get better results from a Premium fuel as opposed to a Sport fuel. Higher levels of nitromethane may require adjustment of the compression ratio to prevent detonation. Check with your hobby dealer to see what fuels are popular in your area.

Morgans Super T is 12% oil, Large Super T is 9%. I think the Super T is for the ST 2300 and the Large Super T is for larger ST's.

rem0.061

My Feedback: (30)

Oh, I will mix some oil in it... Darn I am already almost to the field.

turbo.gst

My Feedback: (8)

I never knew of a problem with the ST 90 carb. I have and run every type of 90 Super Tigre / Como ever made. I have used from FAI to 10% nitro with good results. I use 20% to 22% oil in my fuel. Those Big Tiger are for the S-2000 and above( exception for the ST 2300). I use 5% nitro & 20% oil in my 2300. The 2300 did have a run of carbs with a 'dog leg' groove in the barrel that had some mid-range issues.

If you loose rpm when you disconnect your glow driver, you need to try a different plug. It sounds like the low speed needle might be rich. Make sure you have a little time on the engine before you expect it to be super reliable. Once the engine is fully broken-in and set, the ST carb is one of the best made. I actually like the older series engine better than the G-90 ( and I have 4 of them). The 90 Tigre is one of my favorite sizes.

rem0.061

My Feedback: (30)

It flew great on the super T, I have added some more oil to it. I also got the low speed tuned right so it doesn't die. Now I need to figure out how to loosen the throttle, it is a little too stiff for the servo to close.