Flying a new ST 75 in the 4*ARF. Since day one I have been having engine hesitation problems, (only on take-off). I noticed today that the pickup fuel line, where it comes thru the firewall is close to an inch lower than the centerline of the carb. Supertigre, as well as other engine manufacturers recommends the fuel pickup line being no lower than 3/8 ths of an inch.
My question is #1, could this be causing my engine hesitation problems. Where the engine is struggling to pull the fuel up.
2nd, has any other 4*60 owners experienced this problem.
3rd, do I need to pull the tank out and raise it? That would be a pain.

Wow...I just looked

(suppose I should have noticed this in the build!!![][:@])

and my carb is about 3/4in higher than the tank...
it is a 4 stroke but that shouldn't matter...BUT...mine runs like a champ, never a single hiccup...

Lets wait and see what some others say.....

pelham,

The tank position is a possiblity but you may need to a little more diagnosis.

If it is only on takeoff then I would think the low speed needle is a little rich.

Try this. Make sure you are holding the plane really good

Start the engine and let it warm up. Now throttle up quickly to only 1/4 to 1/2 throttle. Does the engine hestitate, and spit fuel out of the muffler?

If so the low speed needle is too rich

If doing the above and the engine hesitates, does not spit fuel and dies or almost dies then it is too lean.

In a too rich setup turn the low speed needle 1/8 turn clockwise and try this test again

If it is lean turn the low speed screw counter clockwise 1/8 turn.

Keep doing the above until is goes to half throttle smoothly and quickly.

Once that is done then go from idle to full throttle, again if it spits fuel and hesitates the high speed needle is rich, it it tries to die it's lean

Based on your description, it sounds as though the low speed is rich...

pelham,

I've always had some random problems with my 4*60 engine quitting when in flight. I've had the same problem with two different engines so I know it's not the engine. I have replaced the fuel lines and tank and still the same problem.

I read an article on Uniflow fuel system setup and I think it is worth a try. Here is the link to the article: http://www.fraserker.com/heli/uniflo...flow_works.htm

My experience indicates the relationship between the carb CL and the tank CL has always been very important for proper engine operation. And yes I would move the tank. Some can get the engine to run but the settings are always more difficult when the carb tank relationship is incorrect. Rather than trying to adjust around a known bad circumstance I find adjustment to be much easier when starting with the tank at the correct height.

Four stroke engines do not seem to be as critical as two strokes.

Bill

Interesting thoughts.....

But I think the tank in this model is as high up as possible.....

So (I'm honestly asking) is this a design flaw with the 4*60, arguably one of the more popular 2nd (and much further) planes out there???

Let me apologize right off for the way this post is going to sound. It's not meant to be harse, I think more diagnosis is really needed here.

Remember he mentioned that this is only on takeoff. He did not mention that it was at any other time.

Normally a tank problem where the tank is too low would create a lean condition so in the air it would only be worse all the way through the operatin range. but since it appears that the problem goes away in the air 2 things could exist here

1. Low speed needle is slightly rich, giving the hesitation
2. Tank position.

I would like to hear the results of this test. Fly it, get it up good and high to prepare for a possible dead stick. Cut throttle to idle, wait a few seconds then quickly go to full throttle. If it hesistates then I agree it could be the tank or even still be richon the low end. If it goes right to full throttle then I stand by the low speed needle being to slightly rich by an 1/8th or even a 1/16th turn.

I have had 2 of these planes, one with a 2 stroke and the current one with a 4 stroke and have had no problems at all. I did look at mine last night before I made the post and mine is also low in relation to the carb on my OS91 FS and it just purrs right along without a care in the world.

bubbagates,

You didn’t say anything harsh. It is fun and educational to work out the problem both as a design issue and as an ‘adjust around issue.’ Generally speaking I find it best to remove the design issue if at all possible. Enough bad things happen without encouragement from me.

Since the beginning of time the tank/carb relationship for a 60 has been 3/8�. Generally speaking less engine tweaking is required. Also the tank relationship turns itself upside down when the airplane is inverted. Just an opinion.

Bill

Thanks Bill,

I was thinking about the inverted thing but did not want to push my luck. What's weird here is my OS 61 2 stroke was about 2/3 inch below and I had absolutely no problems with it. The 91 4 stroke purrs nicely from idle on up to full throttle.

That is why I am pretty convinced on the low speed thing. If all of the testing and tweaking was already done then I have just stuck my foot in my mouth (not the first time, tastes funny too [:'(]) and I apologize.

it is odd that this mount is so much higher than the fuel tank.......

but as bubba, my engine runs great too, and nice transitions... did play a bit with both low and high needles, but it flies great

I will try leaning the low speed needle and see if that corrects the problem. Thanks for all yor input, very helpful to us beginners.

Let us know how you make out

Flew today, leaned the low speed needle. It helped a bunch. Thanks again!

Glad to here it.