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Old May 19, 2011, 02:11 AM
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Joined May 2011
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ASP 91 Four stroke tuning

Hi all,
I'm new to this forum but have been gas flying for years. I have always flown 2 strokes but have just purchased an ASP 4 stroke for teh first time.
I am, however, having trouble tuning it.
I am running it inverted in a BT tempest scratch build.
I have been trying to run it with 16% nitro fuel as I was told you can use the same fuel as 2 strokes (being an old mechanic, this made NO sence!)
firstly, does anyone have, or can I be directe to a manual for this engine? couldn't find one on this or other sites.
secondly, is the tuning procedure the same for 2 and 4 stroke motors? and if not, can anyone give me a procedure to follow?
any help GREATLY appreciated

cheers
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Old May 19, 2011, 04:05 AM
most exalted one
Canada, BC, Blind Bay
Joined Aug 2002
3,594 Posts
You can use 2 stroke fuel. It doesn't have oil in the crankcase so needs the same oil a 2 stroke needs. I understand it's best to angle an inverted 4 stroke a bit, so the oild doesn't collect right at the glow plug if there's any excess. Tune it so it smokes a bit. Should initially be on the rich side as I understand it. I got good results out of .61FS ASp, etc.
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Old May 19, 2011, 05:03 AM
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Joined May 2011
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Thanks 4*60
It does seem to be a problem with the engine inverted, because when I leave the stick on the glow plug, it idles nicely.
However, my two stroke engines are directly inverted and run no problem...is it different for a four stroke?
anyone have a directly inverted 4 stroke that runs nicely?

cheers
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Old May 19, 2011, 11:21 AM
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Northwest Indiana
Joined Feb 2011
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Quote:
Originally Posted by gprs12 View Post
Thanks 4*60
It does seem to be a problem with the engine inverted, because when I leave the stick on the glow plug, it idles nicely.
However, my two stroke engines are directly inverted and run no problem...is it different for a four stroke?
anyone have a directly inverted 4 stroke that runs nicely?

cheers
2 things:

1) make sure the gas tank output is just below the carb needle if possible....this seems to be the biggest issue with being inverted.

2) If it won't run without the glow ignitor on, it is probably way to rich on the low end. This is typical for break in but after that first tank, I start leaning things out with each successive tank. this includes the low end and the high end. By the time you get about 6-7 tanks though it, you can dial it in for just slightly rich from peak rpm and make the low end lean enough to have a good idle and transition.

I have an ASP 91 (just broke it in) and it runs like a top! Oh, and I use cool power 15% in my 2 and 4 strokes with great success.
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Last edited by CRFan1; May 19, 2011 at 11:31 AM.
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Old May 19, 2011, 11:35 AM
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United States, TX, Weatherford
Joined Dec 2006
1,681 Posts
grps12, tuning, ala needle valves, is about the same for two and four stroke engines. And fuel is essentially the same. Yes, I use the same mix in both two and four cycle engines, always with some Castor in the blend. There is good advice given by all.

Tank location is critical for inverted engines. That has already been mentioned. So is tuning the low speed needle valve for idle and acceleration from idle.

As far as a manual for the ASP-91 simply down load the Magnun 91 manual, they are the same engine. Magnum is a US brand name, while ASP is generally used in the rest of the world. Valve clearances are the same.

I run my Maggies on 10% nitro but they will tolerate much more, just not necessary in my way of thinking.

I think Maggies are good engines and cost a whole lot less than their brothers, OS and Saito.

Chip
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Old May 19, 2011, 05:10 PM
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Joined May 2011
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Thanks for all your help and advice, you've been great.
as for the location of the tank , I've gone off the "BT Tempest 5" design, so the tank height was already set...maybe he was designing for a 2 stroke engine? this shouldn't make a difference, should it? his plans don't specify which engine was used in the prototype.
the tank centre line is centred to the engine. if I disconnect the fuel hose to the carby, the fuel doesnt drip out untill I lower it PAST the line of the carby.
can you repostion the spay bar to make any difference? ( like Supre Tigre 2 strokes?)
I may disconnect the air pressure line to the tank and give that a go.
I have put a piece of fuel tube around the mix needle as I have read they leak a bit.
I will post my results and appreciate any more replies

cheers
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Old May 19, 2011, 08:15 PM
TigreJohn
United States, CA, Corona
Joined Aug 2008
1,002 Posts
The manual for the Magnum 91FS is available for download at: www.hobbypeople.net. For what ever reason the Magnum manuals are better than the ASP, even though both are made in the same factory.
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Old May 20, 2011, 01:38 AM
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I've put almost two tanks through the engine and I can get it running without the stick at about 1/3 speed. I should really order a tachometer ( the thing that reads the prop speed...don't know the correct term) as they say for four strokes it's hard to tune by ear.
is this normal for it to take so long to get it to idle at low revs?????
it IS starting to drive me a little crazy.........
jjkupinski, I can't find teh manual for the magnums on that site...do you have a direct link?
cheers
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Old May 20, 2011, 09:35 AM
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Northwest Indiana
Joined Feb 2011
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Quote:
Originally Posted by gprs12 View Post
I've put almost two tanks through the engine and I can get it running without the stick at about 1/3 speed. I should really order a tachometer ( the thing that reads the prop speed...don't know the correct term) as they say for four strokes it's hard to tune by ear.
is this normal for it to take so long to get it to idle at low revs?????
it IS starting to drive me a little crazy.........
jjkupinski, I can't find teh manual for the magnums on that site...do you have a direct link?
cheers
Have you leaned out the low speed mixture at all? Again, from the factory they are WAY to rich, mostly for initial break in. Screw it in (clockwise) to lean it out. Go in a 1/4 trun at a time until it idles good than do smaller increments until you get a good idle and a good transition to WOT. Keep in mind that you need to dial in the High end first, than the low end. If you make a change to the low end, you need to adjust the high end, etc. They both affect each other.
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Old May 20, 2011, 01:44 PM
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United States, MD, Bowie
Joined Sep 2009
137 Posts
What prop are you using? This would also affect idle tuning. And yes, I have (3) 4 strokes that are inverted in my warbirds without any problems running. (OS91FS, SAito 125, and and OS200FS)
Try leaning the low end when running (around 1/4 throttle) until it dies then back out about 1/2 turn. This would give you a ball park figure. Like CRfan1 said adjust the high end first.
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Old May 20, 2011, 10:51 PM
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Joined May 2011
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FINALLY GOT IT!!!!!!!!!!
I'm using a 13 x 6 mater airscrew, as I am putting the third tank through, i've got it to idle nicely and transition well....must need a few tanks to bed the moving parts in

still running it a little rich on the top end, though, as this is apparently good for it.

thank you once again for all your help, you've been great

cheers
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Old May 20, 2011, 10:57 PM
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sorry,
it's a 13 x 8 Master Airscrew

cheers
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Old May 20, 2011, 10:59 PM
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United States, MD, Bowie
Joined Sep 2009
137 Posts
THe 13x6 might be a little too small for the 91FS. (the 80FS uses that prop) Go with at least 14x6-7. Yo don't want too much rpm for the 4 cycle.



13x8 is much better...
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Old May 21, 2011, 12:13 AM
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o.k.
will do....that means re-tuning, hey?
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Old May 21, 2011, 12:24 AM
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do you think I could use a 14 x 7 three bladed prop?
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