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Model Forum / Radio Controlled / Land Models / September 2005S-25 with annoying low-end bog.................
Just posted this to Savage-central.com and figured I'd post it here as well............. <PASTE> Howdy y'all, Long post, but please read if you have the time and make any suggestions/comments. I have tried to be thorough in my description. S-25 motor with Nova Race-Pro Head, OS A-5 Glow Plug, Uniflow Mod (18" pressure line and 8" fuel line), Ofna throttle mod, O'Donnels 20%, stock 15/49 gearing, three-speed and stock pipe. About 4 gallons thru her, never overheated (averages between 260*-290*), after-run oil after each use. Here's the deal, she runs like a raped ape 80% of the time, wheelies out the wazoo............until she gets to 1/2-1/3 tank, when she develops this really annoying low-end bog when going WOT from a standstill on high traction surfaces. It is a LEAN bog, not a rich bog. It is not consistent but intermittant. I have tried tuning it out going both ways (lean, rich) with LSN and HSN and cannot find a happy medium. I can get rid of the bog by richening up the HSN slightly, but this kills my top-end speed. I can get rid of the bog by richening up the LSN slightly but this kills my take-off speed and I get a rich bog. I can NOT find a happy medium. New carb and backplate (both silicone-sealed) replaced under warranty, new fuel and pressure lines x2, new glow plug x2, new clutch shoes and springs (stock). Compression/pinch is still awesome. Is this just acting like a typical S-25 or do I have me a "special" engine? Anyone else have similar expriences? Anyone have any suggestions for a fix? About to ditch this motor and grab a Novarossi or Collari............... Nutz4Nitro <my handle on Savage-Central> <END PASTE> Arrrrrgh! This is driving me nutso! Doc > Just posted this to Savage-central.com and figured I'd post it here as > well............. [quoted text clipped - 37 lines] > > Doc I personally dont think its a motor issue rather I believe its a fuel tank positioning problem. I dont have the problem (yet) but I've been trying to work out a solution so that if./when I get it, I can fix it. The only thing I have come up with is a new fuel tank design that sits higher and has the pickup higher (with respect to the carbs fuel line entry). Currently the theory behind it is the fact that the fuel tank sits quite low in the truck and when you get past half way, the carb has to suck the fuel up against gravity and hence your lean out. I have so far read about 30 odd pages of forum posts re this problem and there isn't a single thing you can do that will fix this problem outright. Some have good results with the uniflow mod (like you have tried) and others (like me) dont even have this problem. I wonder how difficult it would be to design/build a crank driven mechanical fuel pump. That way, doesn't matter what revs, allways the same fuel pressure. Sorry I cant be more help, just hope this gives you at least a small lead... Cheers Dre (actually I do have one final solution if I ever get it and cant fix it. Rip the engine out and drop a couple of brushless motors in there, not quite the same, but I'm sure it would hold its own :) Well I had the same thing with my quake. When the tank got half empty it bogged just like your dilemma. I caulked around the seem of tank thinking there might be an air leak; didnt help. The lid had a foam piece that I think was the culprit. I bought a different style tank and fit it in and problem solved. This tank (250cc)has a tube with a fitting at the end that lies at the bottom of tank I can run the tank empty now. So check your tank or if possible try/borrow/swap another. > Well I had the same thing with my quake. When the tank got half empty it > bogged just like your dilemma. I caulked around the seem of tank [quoted text clipped - 3 lines] > at the end that lies at the bottom of tank I can run the tank empty now. > So check your tank or if possible try/borrow/swap another. Problem solved. Was a LSN tuning issue believe it or not. I ended up setting the LSN 3/4 turn OUT from flush! Had to really crank up the idle to compensate, but she's power-wheelieing into 2nd again. I can't believe how rich I had to set that damn needle to get her screaming again. I think the fact I have a HS/HT servo on throttle/brake along with linkage/servo saver mod was compounding the problem. Unless the crankcase loads up with fuel at idle, when I nail it, the slide comes open so fast there has to be a decent fuel reservoir in the crank before the HSN takes over. Ran two tanks thru her from top to bottom with consistent performance throughout and temps around 270*. Doc |
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