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Jett 76L Probs

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Old 09-08-2008, 10:40 PM
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dwcrow
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Default Jett 76L Probs

Hello Guys,

I am having trouble getting my .76L to transition smoothly thru the throttle range. The tank set up is a normal non bladder type but is isolated w foam for vibration purposes. Opening the throttle slowly, I cannot get the flat spot out with the throttle hole opened about .080". Blipping the throttle will get it thru. Cannot get it to run at 1/3 throttle to pinch the line to check. Makes it impossible to just cruise. If I continue to open the throttle slowly pretty much dies every time. Idles ok though. The tank is centerline with engine with the carb sideways. Have not tried the K&B 4520 plug. Tried starting over on the tune by following the instructions to no avail. I do not know if you can expand any more than the instructions say but I am giving this a shot.

Bummed
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Old 09-09-2008, 01:23 AM
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rmenke
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Default RE: Jett 76L Probs


dwcrow:

I have 6 of these things from a 35 to 90's and have never had a transition problem with the Jetts. After some 60 years of playing with these things, maby we can get it sorted out. Would like to know a couple of things about the engine please. Age, estmated gallons of use, front or remote needle. Ok, lets review basics first. Remove the needle and with a small copper or brass wire, run it through in all directions. If you have a needle in carb type, take the carb off and blow out the fuel exit from the bottom. Use a high pressure hose, first blow out the line from needle, then center, then fuel fule ends. If you have a nut and nylon insert on the needle side, order and replace it. While at it, order the carb O ring and replace it also. Blow out the system from the fuel side. You should be able to see through the needle system or at least light reflection with th remote needle system. Plastic tanks shed small pieces that can and will stop up the works, or worse yet, partially block a system. Ok, go to the fuel tank clean it out and replace all the lines inside and out including the bronz metal lines. If you are using aluminum metal, chuck them. Use good grade of fuel line, like DuBro, almost as good as the old and gone Prather. Do not omit this pain in the tail step. Re-set the idle needle per the Jett web site instructions and start her up with the main needle 5 turns out. Keep the glow igniter on the NEW OS8 plug and needle in until the engine is running a little rich but cleanly. Now throttle down and get the engine to idle correctly. Play with the idle screw until you have a reasonable idle speed. Open her up and re-adjust the high needle to normal rpm. Should not be far from when she gets on the pipe. Again check idle. It should idle without loading up or going lean for at least 45 seconds. Assume your piston/sleeve fit is still tight on top and you good compression? Just for grins, let le know how things work out. ENJOY
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Old 09-09-2008, 11:20 AM
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Default RE: Jett 76L Probs

Hi,

Thanks for writing.......

A few questions to help diagnose the problem:

Did you complete the engine break-in process on the test stand?
Was it running and transitioning properly before it was installed in the aircraft?

Did you adjust the low/mid range needle valve at all ?
(the low needle is factory pre-set when Dub test runs the engine. It is not a typical low end mixture adjustment. Turning it in or out more than 1/8 turn from that postion really can mess up transition, and can also affect top end)
If need be, I can provide a proceedure to re-set the idle/mid mixture.

What prop are you using?

Which muffler? Lite-Jett or other?

Is the needle valve assembly airframe mounted or is it still on the engine? (airframe is prefered)



The first step in correctly diganosing any issues.... install the engine back in the test stand. Easier to work on, eliminates any variables, and provides a baseline. (The first thing Dub will ask you on any diagnosis is if you have the engine in a test stand).

Assuming for a moment that everything is by the book and break-in went just fine to start with ....

Set the high-end mixture first - full throttle - find peak rpm, back off 300-400 rpm.
Check the transition again.
Note factory position of the low end mixture screw. (stock position is somewhere close to the screw being flush with the nut)
Adjust the low end needle open 1/8 of a turn. See what that does.
following that, adjust the low end needle closed 1/8 of a turn from the original setting. See what that does.
One or the other will improve the condition.
From your description, it sounds like the low speed mixture is a little lean.

I recommend trying an OS type F plug. Yes, the 4c plug.

Let me know on some of the variables..... I will try to help.

Bob Brassell
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Old 09-09-2008, 10:27 PM
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Default RE: Jett 76L Probs

Thanks a Lot for the replies,

Answers to your ?'s

I bought the eng. new.
Has been broke in per instructions on test stand with proper fuel, specific Powermaster.
I have totally jacked with the adjustments. I will admit that.
12x8 prop for now, want to go to a 14x4 eventually.
Turbo Jett Muff..
Needle is attached to frame.
Sullivan tank and inline filter with brass lines installed.
Cannot say that I really got into the throttle transitions while on the stand.
Mid/Idle screw is out past flush about .025".

Should I go ahead and try to tune with a bigger prop? I think she's definitly broke in by now.

I will get a OS Type F plug for grins.
Oh Yeah, I do not know what this means ->(Yes, the 4c plug.)

Thanks Again! I will take any advice thrown at me!


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Old 09-10-2008, 04:59 PM
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Default RE: Jett 76L Probs


dwcrow:

Follow Bob's directions to the letter, he is "the man" and still teaching me things missed through the years. Did not intend to inject into the Jett site, a no-no. Blow out the needle system regardless. Am interested in Bob's reaction to the 14 X 4 (I assume for 3D ) prop on this engine. I have a 90L going to pot in a Alley Cat that that is just not fast enough for my needs/wants. A Jett 60LX fills all my speed needs and then some. Thinking of stuffing the 90L in a 69" QQ Yak or similar. Yes, love to fly 3D things, improves my flying skills and reactions that need improving. ENJOY
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Old 09-10-2008, 08:08 PM
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Default RE: Jett 76L Probs

Thanks rmenke and gang,

Just tried the 4C plug. Seemed to improve transition. I think I am pretty close. Will go from idle to full rpm in an instant. If I open the throttle very slowly it seems to lean out when it gets about .125" open. If I leave it there it will eventually die. Will idle for a minute or so with no problem though.

still corn-fused
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Old 09-10-2008, 08:20 PM
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Default RE: Jett 76L Probs


dwcrow:

Follow Bob's instructions, open idle screw 1/8 turn and see results. If no improvement, go back 1/8 to original position and close 1/8th. You should have to beat that thing with a baseball bat to quit maintaining a idle. ENJOY
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Old 09-10-2008, 09:54 PM
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Default RE: Jett 76L Probs

Yep, that is exactly what I have been doing only in smaller increments than 1/8's. I have only made it worse so far. Never had a sweet spot so hard to find. I will keep trying.

Thanks Again
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Old 09-12-2008, 01:15 PM
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Default RE: Jett 76L Probs

If you have the 14x4, try the transition with that.

Give me a sense of baseline performance since that is what we test it with.

Keep track of where the midrange needle is now.

Run it at full throttle - set the mixture at about 300 rpm off peak. The 14x4 and turbo should get to about 11,000 rpm.
Pull the throttle down to that 1/4 to 1/8 open position. (somewhere in there). This is low throttle to a very high idle (3500 rpm?)
Adjust the low end mixuture so it runs smoothly there and transitions fairly well out from that throttle setting.

Let me know how that goes.

Bob
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Old 09-27-2008, 11:28 PM
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Default RE: Jett 76L Probs

Hello Again,

Well, I finally got the big prop on this puppy. An APC prop. Sure took the zippy rev out of it, acts like it has a heavy flywheel on it now. Definitely closer on transition though. Out of curiousity I held the plane verticle and played with throttle like I was in a hover. It did not like it at all, quite running every time. Almost like the tank was not pressurized enough. I checked the fuel system for leaks, cannot seem to find anything.

Thanks for the help but...

still cornfused
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Old 09-28-2008, 11:10 AM
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Default RE: Jett 76L Probs


Do you have a photo of the aircraft and installation. That might help.

Tyically fuel draw is very good. I set the 76L about 500 rpm off of peak ...... does not seem to mind midrange much at all.

What is it doing in midrange? lean? loading?
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Old 09-28-2008, 08:07 PM
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Default RE: Jett 76L Probs

Here Ya Go,

Hope the pics reveal something obvious. FYI tank is sandwiched between two pieces of foam front and back in case you cannot tell.
About the lean or loading up question. I can only presume lean just knowing gravity would make it harder to pull fuel uphill. Also, when you say "back it off peak", you mean lean it down at full throttle to achieve max rpm then fatten it back up to lower it back down right. Just making sure.

Thanks again for the help
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Old 09-28-2008, 10:56 PM
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Default RE: Jett 76L Probs


Just 2 cents worth. Picture one shows a in line alm. filter adjacent to a motor mount bolt, isolate it in foam just to be sure. Notice you have some blue foam in contact with the tank. That stuff has been the source of friends tanks foaming in my past experience. To be safe, use nothing for your tank and receiver but white foam RUBBER you can get from Hobbico. 1/4" and 1/2" stock works best. Dont be afraid to glue pieces of the stuff togeather, hold up just fine. Saw a QQ receiver install. 1/2" white foam rubber glued to the bottom, receiver taped with black tape and glued to the foam. Using it for about 6 mos. with no problem in a Mojo. Try it once, you will like it. Can see everything shaking, the receiver remains vibration free. ENJOY
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Old 09-29-2008, 10:20 AM
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Default RE: Jett 76L Probs

Installation looks ok... looks pretty much the same as on my Edge.

The tank padded front and back helps, but it has to be padded top and bottom too. Can't touch the wood directly.

Only difference I have is I installed an 8oz bubble-jett fuel tank. But that should not make a big difference.

Yes, at full throttle, briefly find peak ground rpm back off from there 500 rpm.

With the turbo-jett muffler, you will get a little bit of a pipe step effect... its not as noticable as on the tuned mufflers or a pipe, but it is there. Maybe that is part of what you are feeling with the throttle response.

For a while I was flying mine with a new type Zinger 14x4 - seemed to be lighter and spooled up a little better. Had just a little less static pull than the APC at the same rpm, but that actually allowed me to run a click or two more throttle in midrange, and that kept the engine on the "far side" of the muffler step.
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