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Old 10-17-2008, 06:42 AM   #1
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Default Engine tuning questions

I have two engines that I am attempting to get dialed in for my mugen MBX5r's. They are both 3 port ofna hyper 21's that I robbed out of other models and I am running a dynamite 007 tuned pipe. I am running O Donnel 30% fuel.

On the first engine I am having a problem getting the tuning to hold consistent. I get it dialed in where it really runs good on the top end and the temp runs anywhere from 215 - 235. It wil run really well like this for about 5 minutes, then the temps start to creep up. Do I need to richen it up and put just take the lower performance? Would shimming the motor to lower the compression help? Or would a colder plug help? I really like the performance I am getting on the first five minutes of the run and would like to get the motor to the point where it will consistently stay at on temp.

My other engine tuning question is a little more straight forward. I have another ofna hyper 21 3 port that is starting to lose some of it's compression. Can I take out a shim to try to bump the lost compression, or is this a bad thing to do? The motor runs really well so I don't want to do anything that will change it's consistency.

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Old 10-17-2008, 12:15 PM   #2
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the first thing i want to say is you dont have to listen to me at all but.........
i think on your first engine you say your temp is creeping higher after you tune it to run how u like it, well all i can say on that is that it sounds like you are tuning it cold. second of all if you are tuning it at running temp and still after 5 mins of running on a track like situation (continuous running around a track slowing for corners etc) then it sounds like you are too lean if your temps are too high, if you want your temps to be in a consistant temp range then u must tune it a little richer than it currently is and then run it for 5 mins and see what its performance is (even if it feels too rich for the first 4-5 mins).it might slot into line once its warm enough to effect performance ( remember some engines sometimes take upto 5 mins to fully warm up to max running temp.).
also sounds like your top end might be a tad too lean.OR your top end is fine(at peak) like you said and your bottom end is a lil lean there fore making it creep
, also try leaving your top end how u like it (maybe just slightly on the rich side of degrading its peak performace)and run the bottom needle alot richer but not so much that it rich bogs and loads up ,then check your temp. .
you can only try and see what happens i guess.
if you are not a newbie then u probably know alot of this and it may be just a tiny air leak im not sure. check your fuel line for smal pin holes.or your tank lid
u may have to settle for slightly less performance until it gets hot,that is if you want it to be in your temp range but if its not too too hot (with in reason) then let it go, if your performace is peak.
some engines like to run a bit hotter than average just richen it the slightest little bit only just taking off a little bit of performance and let it go.
btw i think that tuning your bottem end a tad rich like i said cant hurt too much as it prevents hardcore wheel spin upon take off hehe.
anyway try all these things i have said seperately one way or another is bound to work and see how ya go bro.

have you tried to just do what you are doing and let it go for 5 mins then bring it in check temps and if too hot just richen top end a fraction still making sure bottem isnt too lean till temp drops also try this seperately

as far as your second engine goes if its running well then leave it.
some engines run well loose (no pinch) just depends on the engine and how its tuned. you may just have to run it with the idle up a little higher than if it had pinch (compression)
i really dont know much about shimming but i do know this, like you said removing a shim will gain you a little compression all you could do is try it and see for your self of what it does. it shouldnt hurt it as far as engine life goes but if its lost compression then its on its way out already so you may aswell experiment with it anyway.

hope this helps happy racing. peace.
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Old 10-17-2008, 02:18 PM   #3
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You should tune after the engine is warmed up as downunder23 mentioned. Tune for performance first, and then check temps to see if they are within range. Trying to tune by temperature can cause you to chase your tail all day. A properly tuned RC motor should run at the same temp from about 3 minutes to 20 plus minutes. A low fuel tank or low pressure from the pressure line can cause temps to rise.

The way I prefer to tune is: Tune your top end first down the longest straight so that the needle is about one hour (1/12 turn) in from running clean (not rich). You can fine tune from there. The low end should be adjusted so that it just starts to run at the same rpm after pulling the throttle trigger and letting it idle, provided the idle screw is set properly and there is some clearance on the carb link shaft. Fine-tuning can be made from this adjustment. Idling fast then slowing down is a sign of richness. Personally, I prefer a slightly rich idle. One rule of thumb is to let your RC idle for 6 seconds and punch the throttle. Of course, if you get a lean bog, then richen the needle.

For head shims, an old rule of thumb is 8 thousandths per 10% of nitro. A 30% mixture should have around 24 thousandths of squish area. One-way to measure this is to use solder bent into a 90 degree curve. Remove the glow plug and insert the piece of solder into the combustion area with the end touching the side of the sleeve. Rotate the piston to top dead center or past if possible. Remove the solder and measure the end. Slide calipers work nicely.

Removing shims can increase compression and increase timing. A colder glow plug can retard the timing some without losing much performance. The danger of removing shims is too advanced timing that causes detonation. Detonation is when the air/fuel mixture ignites too soon and can cause pitting in a piston and head button. Severe detonation can cause even more damage.
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Old 10-17-2008, 07:40 PM   #4
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Without playing with the motor the above posts are most probably correct sounds like a lean bottom needle to me.Also the that particular motor just doesnt have any bottom end compared to average race motors & if you have good bottom your too lean.Many with that motor also run heavy clutch springs to make it work like other motors.I ran that motor for a year or so & can tell you it runs hot 80 deg in SOCAL = 260deg + to keep up & power drops off a bunch after 3-4 gallons no matter how you tune.Carb on that motor is also very un reliable but I forgot what carb I used in its place,sorry. Check out the motor section on the OFNA site forum & you'll learn more than you ever wanted to know about that motor & then some.
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Old 10-17-2008, 08:43 PM   #5
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Not to jump to a diagnosis but I too had that engine and it ran great then all of a sudden it wouldnt hold a tune. Took the carb apart and found the orings were flatspotted and some were even starting to come apart. I rebuilt the carb and it fixed the problem for awhile then started acting up again. I am gonna put a nova rossi carb on it this time. Moral of the story... If after following the above posts you still are having a erratic tune disassemble the carb and check out the orings. Nitrohouse has a oring kit for it very cheap.
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