I just started having a problem with my engine that I have not experienced before and was wondering if anybody had any suggestions for the 1st place I should look for the solution. I have a thunder tiger 46 pro running 10% nitro 18% synthetic/caster blend (same fuel whole time). I started the day (as all other days) and the engine ran fine, good idle great transition and good top end. I had it set just audibly below peak rpm, the pinch test increased rpm substantially. I have the three line setup in the tank, (intake with clunk, exhaust pressure with clunk and plugged vent for fueling). The tank is visible and not foaming. The transition was almost instantaneously from low to high rpm (fast reving). After about 4 - 9 minute flights, I thought the engine started to run a little lean during flight so I brought it down to richen it up but found out that the transition was very slow. From low throttle to high throttle the engine does not rev very fast. It takes about 5 – 10 seconds to reach full rpm but is smooth and not burbling. The idle is still good and the top end is still there, just takes a while to get there. It is like the throttle is being pushed slowly but it is not, the servo moves the barrel wide open very fast. At the field, I took out the high speed needle and blew air through the barrel and air came out the needle hole.

What do you all have for the diagnosis?
Thanks for any replies.

Rich idle mixture and lean main needle.  Warming up makes it worse.

Thats what I was going to do next. I was going to lean the idle mixture and then retune the high speed. It was a fairly cool morning and it got warmer as the problem presented itself, so this may be the culprit.
Thank you.

Did you try a new plug?

I didn't try a new plug yet because it seems to run just fine at all throttle positions, it just takes a bit of time to get from idle to full thottle. It never dies though until I cut the throttle all the way. You can let it idle for about 3 minutes and then slam the throttle to full and it just spools up smoothly and slowly until it gets to full throttle, almost like you raised the throttle slowly.

Air temp changes always require mixture changes to compensate. This is less critical on the low needle, much more on the high.

To me it sounds like the egine is in an overloaded state, as if the prop is too large (which it is not). Maybe one of the bearings is in a semi-seazed state? Is the engine turning over smoothly?

The engine turns over very smooth. The engine reved just fine with the same prop before the problem started. The engine peeks out well and idles well, it just take a good 5 seconds to get from idle to full rpm even when the barrel is opened quickly. it is the same from half throttle to full throttle, slow increase in rpm until it reaches full rpm. At any throttle setting, it holds a good stable rpm when it gets there. Its running a good vapor (oil ) trail. I will try and lean the idle and retune the high end tonight as well as change the glow plug just to be sure and let you all know if it solves the problem. Originaly I thought it was a partial blockage that would allow the engine rpm to increase as the pressure in the tank increased slowly that's why I blew out the needle valve at the field but did not have any change. If the idle mixuture change does not solve the problem then I will just disassemble and reassemble, that always seems to fix any problems and everything gets cleaned up.

You might check the pressure nipple on the muffler - they have been known to plug up with crud. Try a pin or paper clip.

To me, it is a restriction to the free flow of fuel.
If you use no filter, I would insist on the needle valve assembly.

A faulty needle seal can allow air intake, which compete with the flow of fuel drawn by the vacuum of the Venturi.

You have not said how you tune the two needles, but to me it seems like like you have set it too lean on the low speed needle (the midrange needle would actually be a better name as it effects the whole middle range and can even restrict the full throttle setting). A video would help with the diagnosis though.

Set the main needle on the rich side, so that it drops a little in the peak rpm. Then set the low end on the rich side too. If the low speed needle is set to what may seem "just right" at constant rpm then you will get a slow throttle response. It needs to be set on the rich side of that seemingly correct setting, so that you get crisp throttle response. I.e. you set the low end needle to get a crisp throttle response rather than doing a pinch test or something else at a constant rpm.

I didn’t get a chance to work on it last night, was getting my son ready for his first Boy Scout Jamboree. I usually tune my engines by setting the high speed needle to max rpm then turning rich about 300 rpm, then turn barrel to idle position as low as it will idle and let set for a minute or two then increase throttle fast and listen for an abundance of fuel ( engine stumbles) or lack of fuel (engine just dies quickly), then I adjust the idle mixture screw the appropriate way then readjust the high speed needle again then back to idle and keep going this way until it runs smooth and good.

I am starting to wonder if it would be the needle at all because the engine was running great and normal for 3 flights and then right in the middle of the 4th flight it started this behavior, I have never had a needle just turn itself out of tune all at once. I am thinking there might be a blockage because when the barrel is quickly fully opened from idle, the engine doesn’t stumble or anything, it just slowly in a matter of 5 seconds increases rpm to normal wide open rpms just like if you slowly opened the barrel, then at full rpms, if you pinch the fuel intake line the engine increased in rpm, and then dies just like normal.

It is almost like there is a restriction in the inlet and when the barrel is opened, the pressure builds in the tank from the exhaust and starts to push more fuel into the carb and that creates more pressure and pushes more fuel and so on until the engine cannot build any more because the air through the carb has reached the full flow rate.

Sorry for the long winded reply, but I will try the idle mixure screw tonight.

Sounds like a fuel delivery problem.  Ditto on checking the pressure vent on the exhaust/muffler.  Maybe a piece of stuff inthe clunk partially blocking the fuel flow.

Chip

Just thought I would follow up on what solved the problem. I checked the fuel and exhaust lines for blockage and found none. So I leaned the idle mixture and re-tuned and that ended up solving the problem. I ended up turning the idle mixture screw in about 3/4 to 1 full turn, which surprised me that it was that far out of tune. I am getting 11,900 max rpm and it idles at 2,600 rpm with a 12x3.75 apc prop on 10% nitro. It will idle endlessly and then rev up without hesitation now. Thanks to all for the help I got to fix this.

One full turn off in about 4 - 9 minute flights indicates a problem with the retention of that screw.

Thanks for the update.