Tuning a Fox .35. Need advice.

You have the older spade type needle. These needles while they do work can be problematic. Do you have a small piece of fuel tubing currently on the needle to seal air leaks? If not, it will require it. If your not sure, I can send you a photo of what I'm mentioning. In addition, it's critical to have the holes aligned properly on that spray bar. There's two holes on that spraybar. When looking down the venturi, you should barely be able to see either hole. Rotate accordingly if not aligned. The best modification you can do to that needle is to clip the knob off directly at the round knob and bend the straight portion of the needle into a J shape. If not, your first doink into the ground or flip over will quickly result in a needle that will snap clean off with the spraybar. Prop choice and plug choice is critical as well as fuel considerations. While the engine is a iron piston engine, you need to keep the oil content high. I run a 50/50 blend in mine, but I highly recommend you using a all castor fuel of at least 23-25%. Prop choices are chosen according to the plane your trying to run it on. I like to suggest big plane, big prop and vice versa. That being said, 10x6 for the larger stuff and 9x6, 9.5x6 for the medium sport stuff. Keep your tank height slightly above centerline approx 1/4" which will alleviate some of the transitional burping when going inverted. A hot plug and not the Fox plug. I sound like a broken record here but the Thunderbolt R/C long or Sig R/C long, K&B 1L are far superior choices to the Fox plug. Ken

If your looking for a far better run for your Fox, the Super Tigre needle will work far superior. Although, the engine will slightly be down on power. Here's the Tigre needle http://www.ebay.com/itm/ST-NVA-ST-G-20-51-FOX-35-/221358154140?hash=item3389f9619c The spraybar diam is .155". The stock fox is turned down in the center to .135". While this sounds miniscule it will make a difference and measuring one Fox spraybar to another you would be very surprised of the wonderful tolerances they held. I 've measured them from .127" -.137". If your running the stock muffler, it's hurting the run more than helping. If you can run without the muffler, the engine will make more power and not retain heat as bad. The stock down swept blimp like muffer needs to have it's stinger cut off and the exit hole enlarged. Don't bear down on the muffler screws. This will squeeze the case and bind the piston in the liner. How much time is on the engine? This is a major determining factor to achieving a good run with these. I expedite much of the break in process by helping it along or I should state doing what Fox should've done. If there's a leak in the backplate, all bets are off and you will absolutely not achieve a good run. It may just show signs of stuttering or just shutting off. Remove the backplate and lap the case on a piece of glass using 600 and oil until the entire surface doesn't show any imperfections and is completely flat. At this point the cork gasket can be substituted for anaerobic gasket maker which will also combined with the lapping of the case bring your backplate slightly deeper into the case similar to the stuffer plate that Fox offers. While the stock unit isn't deep like the stuffer, the stuffer plate is a machined and hard anodized part which will retain the conrod far superior than the stock backplate does.Ken

Thanks. I added that spraybar to my watch list. I'll install it when I am ready to fly my Stunt 35.

I think the RSM needle is turned down or used to be, you can also carefully open up the venturi to compensate for the larger dia. needle, but would advise not.
http://www.rsmdistribution.com/index-2.htm

Thanks Ken.

...the RSM needle is the same, only $5 less and you're buying from a control-line company.

I didn't see any mention of the Randy Smith PA needle valve available at Aeroproducts, Another good choice that works out of the box.

Also the new Fox NV design works really well, but is a tad pricey.

Ron

Yep, good call Ron...I wonder if those are the ones I was thinking that the spray bar was turned down on?

Mark Boesen wrote:Yep, good call Ron...I wonder if those are the ones I was thinking that the spray bar was turned down on?

Yes they are turned down. Supposedly they are the best Fox .35 replacement needle out there. Funny Ken didn't mention it as he was the one that turned me on to them.

The ST NV is what Lew Woolard installs on his modded .35 stunts.

In short, anything besides the stock NV is an improvement.   

I'm looking into it now.

Yep, legend has it Duke claimed the flat needle reduced needle vibration, when it was really more likely a cost savings?

I always liked the precision feel of a Fox needle: click, click, clunk, click!

FYI, you can always mark a ST spray bar with a sharpie, and using a drill as a lath and a fine small file carefully turn down the diameter of the spray bar.

Just ordered this:
http://www.foxmanufacturing.com/index.php?main_page=product_info&cPath=166&products_id=366

I was going to mention the PA needle, but until you get a few broken ones under your belt, I figured the stock ones or the Super Tigre would work sufficiently. Today we had a blast flying. It was Fox .35 day. Nothing like it when it's 20 deg out and snowing. One puked the crank out the front. Here's a shot of what we were up to. https://www.facebook.com/photo.php?fbid=611504878919650&set=pcb.611505045586300&type=1&theater We have a video but not yet on the site

I see Ken,

Yeah I flew my first full size stunter with a bog standard wide bypass Fox sporting a 60yr old NV so it can be done. Love the Ringmaster on ski's Ken!

Matt,

How was the Fox mounted when you were running it?

Ron

Brrrrr! I need hot chocolate just looking at that picture.

...or a cold beer!

Cribbs74 wrote:
Matt,

How was the Fox mounted when you were running it?

Ron

Well. I ordered the wrong thing. Hopefully I can call Fox tomorrow and clear things up.

The engine was in my test stand. Cylinder up.
P.S. I don't have a muffler.

Matt,

That would be the best thing to do. Sharon is very helpful and if you tell her what you want she will find it for you.

Don't be surprised if they don't have it though as they have halted engine production to stay afloat in these lean times.

I grabbed a couple NV's last year from the factory and I am glad I did. If I knew then what I know now I would have bought several engines too.

Ron

A lot of things can affect a Fox run. If that test stand has any give the engine will vibrate the crap out of itself and result in a poor run, also if the fuel was being shaken that could do it too.

Couple more questions:

What fuel were you using?
Was the prop balanced?
What glow plug is installed?

Maybe if you described the run we can help out a little more.

Ron

After re-reading the posts again, I opted for the Super Tigre needle. I am also going to try to get the RSM needle (the correct one) as well since I have a pair of Fox .35s. I'll post my results when the parts come in.

Thanks again for the wealth of knowledge. I'm still learning.

Cribbs74 wrote:A lot of things can affect a Fox run. If that test stand has any give the engine will vibrate the crap out of itself and result in a poor run, also if the fuel was being shaken that could do it too.

Couple more questions:

What fuel were you using?
Was the prop balanced?
What glow plug is installed?

Maybe if you described the run we can help out a little more.

Ron

Tatone stand hard mounted to a bench.
10% fuel. Can't remember brand off hand.
10x6 balanced prop
Fox short plug. Med temp.

I can make it run, but only if it is tuned extremely rich.

Short plug? Should be a long.

Not sure how that would affect your run though as I have always used long plugs. It probably doesn't help compression though. Did you check the spray bar orientation like Ken suggested?

Stunt .35's are pretty tolerant assuming it's basic needs are met. It could be as simple as installing a new plug.

Ron

Long 'hot' glowplug and lots of oil, if you're using a short plug and R/C fuel, I'd say that was 75% of your problem.