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Post  Ivanhoe on Sun Apr 15, 2012 1:22 pm

I finally got around to seriously running the Fox .049 FAI Special I saved from obscurity a while ago.

Fox .049 FAI Special Fox_fa10

I finally got a cheap tacho, so I can see what it's doing, but I have a problem, the engine won't hold max revs for some reason.
I was testing it using an old Topflight 6" x 3" and 25% Nitro, it starts easily and screams straight up to a measured 18,200 on the 6" x 3", but then, after a few seconds it drops to around 15,500, then back up to max again, and continues to do this. The piston/cylinder are not new, having had what seems a reasonable amount of use, so I think I can rule out the engine not being run in, all gaskets and bolts are new replacements, as is the glowhead, so I also rule out air leaks or a dodgy head, so I am a little bemused at the moment.
If I can get the engine to smooth out, I am most impressed by it's performance, if it will hold 18,000 + on a 6 x 3 I will be well satisfied!

Only other problem is, if I run this much I'll need ear defenders, by 'eck it's loud!
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Post  Cribbs74 on Sun Apr 15, 2012 3:53 pm

Sounds like fuel starvation issue. When the revs drop do you mess with the needle till it picks back up?
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Post  Ivanhoe on Sun Apr 15, 2012 4:43 pm

cribbs74 wrote:Sounds like fuel starvation issue. When the revs drop do you mess with the needle till it picks back up?

No, I tried that and it just causes the engine to cut out, the one thing I did consider was that, because the fuel line connector end of the spraybar is very small, I'm using Cox-size small fuel tube, and I'm wondering if, because of the high revs, the thin fuel tube isn't delivering enough fuel. Trouble is I don't know what I can do about it, bigger tubing just slips off the engine.
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Post  sdjjadk on Mon Apr 16, 2012 9:02 pm

Try running lower nitro. I had an old Johnson .29 C/L engine that had bad rpm fluctuations and I switched from 15% to 5% and it ran great.

Hope this helps.

Shawn
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Post  nitroairplane on Mon Apr 16, 2012 9:12 pm

Sounds like a leak around the NV so it's going rich an lean (more air then less air
) so try putting tubing around the NV.
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Post  Ivanhoe on Tue Apr 17, 2012 3:01 am

Thanks for the input folks. I solved the problem, as I thought, it was using too narrow a fuel tube. I put a short bit of the Cox narrow tubing over the spraybar end, then managed to get the larger tube over it, which stopped the larger stuff pulling off. Problem solved!
Engine now runs steadily, weird thing is the max revs have now dropped from 18,000 on the 6 x 3 to 17,800! I've lost 200 revs somewhere! (Yes, it's leaned out as far as possible, any more and it cuts out)
If I can ever get hold of some 30% + Nitro I'd love to try it on that!

My ears are still ringing, it's probably the loudest .049 I've ever run!
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Post  Kim on Tue Apr 17, 2012 3:22 am

Ivanhoe wrote:

My ears are still ringing, it's probably the loudest .049 I've ever run!

EAR PROTECTION !!!! When hearing goes away...it STAYS away !!!!!


But besides that...Really Cool Engine !!!!!
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Post  andrew on Tue Apr 17, 2012 6:54 am

Kim wrote:
EAR PROTECTION !!!! When hearing goes away...it STAYS away !!!!!



What did you say, young fella'???
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Post  Cribbs74 on Tue Apr 17, 2012 7:49 am

Ivanhoe wrote:Thanks for the input folks. I solved the problem, as I thought, it was using too narrow a fuel tube. I put a short bit of the Cox narrow tubing over the spraybar end, then managed to get the larger tube over it, which stopped the larger stuff pulling off. Problem solved!
Engine now runs steadily, weird thing is the max revs have now dropped from 18,000 on the 6 x 3 to 17,800! I've lost 200 revs somewhere! (Yes, it's leaned out as far as possible, any more and it cuts out)
If I can ever get hold of some 30% + Nitro I'd love to try it on that!

My ears are still ringing, it's probably the loudest .049 I've ever run!

Lol,

That's what I thought. Same symptoms as my Babe Bee. I had Installed some .010/.020 fuel tubing on the pickup just to get it running. It would come up to peak then drop off. I had to keep richening the needle until it would pick up again and so on to keep it running.

That's why I thought it was fuel starvation.
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Post  stevej on Wed Apr 18, 2012 2:18 pm

You might clean the spraybar out. I use these-
http://img.walgreens.com/dbimagecache/07094230306_450x450_a.jpg
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Post  Ivanhoe on Wed Apr 18, 2012 2:34 pm

stevej wrote:You might clean the spraybar out. I use these-
http://img.walgreens.com/dbimagecache/07094230306_450x450_a.jpg

Yes, thanks Steve, but I'm pretty sure those aren't available over here.

Spraybar, and everything else, was cleaned thoroughly when I rebuilt the engine, this is how I got it........

Fox .049 FAI Special Fox_110

So it was stripped completely and rebuilt from the ground up, luckily most old Fox parts are still available.
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