Changed fuel to lower Nitro, engine dies at mid-throttle? - RC Groups
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Jul 08, 2008, 03:05 PM
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Changed fuel to lower Nitro, engine dies at mid-throttle?

My local hobby shop has run out of the 30% Coolpower heli fuel I've been using for the past 2 years (they don't expect to get any more 30% this summer they said), so I bought 2 gallons of 15% Coolpower heli fuel instead. I knew I'd have to retune the carb on my OS 61 SXWC engine, but this has me stumped!!!

Everytime I get into the midrange (near half-throttle) the engine bogs way down and dies. I've tried resetting my needles (I have a 3 needle carb) to the OS recommended settings for initially starting an engine (2 turns out on high-speed needle, 2 turns out on idle mixture, and 1 turn out on mid-range needle) and it made no difference... can't get thru mid-range.

I also have 2 articles on setting up this engine/carb I printed off the internet which both suggest the same initial settings of 2 turns out on high-speed needle and 2 turns out on idle mixture needle BUT 0 turns out on mid-range needle (keeping it fully closed initially). That doesn't work either, engine still loads up and dies at mid-throttle.

So I'm about out of ideas here. The engine ran fine till I used up the last tankful of my Coolpower 30% heli fuel (for 2 years never had to do more than adjust the high-speed needle valve a click or two to adjust for weather conditions). I'm stumped on why this change to 15% nitro is kicking my butt.

Anybody got any good suggestions? I've spent a few hours already toying with the needle setting and nothing seems to make a difference. The plumbing in the tanks is good, I replaced the clunk lines just to be sure and the outside-tank lines are all in good condition. It has a header tank situated even with the carb level (carb needles are at about the midpoint of the header tank). Never had a problem before this!


- Michael
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Jul 08, 2008, 03:20 PM
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TheBum's Avatar
Lower nitro will require a leaner mixture. You may have been starting out too rich.

Also, have you tried changing out your glow plug? Lower nitro requires that the plug burn hotter, so if it has cooled down toward the end of its service life, it may not be getting hot enough.
Jul 08, 2008, 05:12 PM
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The plug was last changed around Christmas 2007. It's got about 3 gallons of 30% Coolpower thru it since. I'll try changing the plug however and see if I get any change (pun intended!).

Short of that, I also have another 1 of these engines with the same carb on it sitting on a shelf, so I may try swapping carbs completely and see if I can get a different (same model) carb to tune up correct. Right now I just don't know if it's something I'm doing wrong or what. Engine tuning isn't a strong point of mine. I'm learning though!


- Michael
Jul 09, 2008, 07:39 PM
Registered User
Is this a 60B 3-needle carb?

Try 1.5 on main
Low 1.25 to 1.5
Mid at 1

If it works, check that you're not too lean on the main
Jul 10, 2008, 11:39 AM
Registered User
Is this a 60B 3-needle carb?

I haven't had time to mess with it again since starting this thread... stormy weather here each day, and alot of work to do. I'll try some of these ideas out this weekend.

1 other thing I notice is that my Hatori tuned muffler seems to be leaking a little around the o-ring joint at the engine exhaust manifold... it's dripped some oil onto my floor directly under the engine (which it's not done before). The o-ring or whatever it is in there must be leaking... not sure if that could be causing a loss of tank pressure affecting the carb. Seems like it would lead to an over-lean condition though, not an over-rich condition. Other club members at the field last Sunday when I was working on the carb tuning said they also believed it was flooding out at mid-throttle, not leaning out and dying. Anyways I have a 1 piece style Revolution muffler I can put on it and try if necessary. However, I'll 1st try the advice ya'll are giving me 1 at a time and see what effect each change has.


- Michael
Jul 11, 2008, 08:41 PM
Registered User
No joy ya'll. I tried everything recommended to the letter... replaced the glow plug, set the needles as suggested, even changed the tuned muffler (the hatori pipe was leaking around the o-ring so thought it might be bypassing too much pressure and not pressurizing the fuel system correctly... put a 1 piece Revolution tuned muffler on instead).

So is it time to try a different carb yet? I have no better ideas at this point... nothing I do with the needle valves on the current carb seems to have much effect.

It'll barely get to mid-throttle before choking down and dying. There's no speed-up before it dies, in fact is bogs down, so I'm pretty sure this is an excess fuel issue.

Using my GV-1 engine govenor I was able to get it into the high-range 1 time this afternoon (with mid-range needle only 3 clicks open) and it ran with good power for about 60 seconds and then started bogging down with excess fuel and finally died. Auto'ed from a 5 foot hover.

I reset the needles to the OS recommended start settings again (2 out, 2 out, and 1 out on the mid-range needle) and it won't go over 1/3 thottle without bogging down and dying. I presume that something's wrong inside my carb and the 30% nitro was covering the problem up?


- Michael
Jul 12, 2008, 01:39 PM
Registered User
Can you borrow a tank of fuel (or try a tank from the other bottle you purchased)? It's possible that you bought a bad batch. If all was good, spot on, until you used the new fuel, 30 to 15% shouldn't cause that much of a tuning issue where it won't go over midrange. Set the needles back to where you had them with 30%, try a tank of known good 15% and lean the needles a bit.
Jul 12, 2008, 02:49 PM
Registered User
Well, I can try the 2nd jug of 15% Coolpower I bought the same day. I guess there's nothing to lose by trying it... what're the odds that 2 jugs would both be bad? (sigh)

- Michael
Jul 12, 2008, 02:52 PM
Registered User
Gaz-b's Avatar
This is very much like a problem i had turned out to be a tiny bit of tank plastic in the carb fuel nipple it started and ran fine untill the fuel was in demand by adding pitch engine would slow/stall/reverse the plastic would be forced into the opening cutting off the fuel supply.
Jul 20, 2008, 12:04 PM
Registered User
Ok here's the skinny... tried different fuel (same 15% nitro content) that another heli pilot at my club was using with his Raptor 50. Same result. We spent another 1 1/2 hours adjusting the needle valves on my engine. Finally with the mid-range shut completely off we got it to go above half-throttle and into the high-speed power band and hover ok. However it was a little weak going thru mid-thottle so I cracked the mid-range needle open 2 clicks... and it bogs down and dies at mid-throttle again.

So closed mid-range needle by 1 click (so it's only open 1 click now!) and it now powers thru mid-range and into high-range without hesitation and seems to run with good power. But geez, only 1 click open on mid-range needle does the trick? 2 clicks causes it to flood out... that's sensitive I swear!

I've run 3 tanks of fuel thru it since we got it going right and it seems to be just fine with only 1 click open on mid-range needle. Engine backplate is hot but not too hot to touch for a few seconds, power seems REALLY good at high-speed... better than it was with 30% nitro even! Engine dies in about 4 seconds when pinching off the fuel line at the carb while idling. Idle is nice and reliable.

Also nice to not have quite so much smoke.

So that's that. Just wanted to update this thread on what finally worked for me. It took almost 1/2 gallon of fuel and some 4 hours or more of messin with it to FINALLY get it adjusted correctly for the 15% nitro though. Ughhhh!

Thanks ya'll!

- Michael

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