gpar

My engine starts at high rpm .

Most of the times, keeping the throttle stick in the idle position the engine starts at about 5000 rpm and remains like that. Only after moving the stick to full throttle and back to idle you get the normal 2200 rpm idle and the engine works all right from then on.
I´m trying to figure out why this crazy startup and I think some excess fuel inside the engine is to blame, but I have not a good explanation and how to avoid such a scaring situation.
Any ideas ?

gboulton

My Feedback: (15)

First thing I'd check is not "excess fuel", since that would result in a high RPM situation only for a brief time...and you state the engine runs that way until you manipulate the throttle stick.

Rather, I'd suspect something in the throttle system. Throttle body itself perhaps sticking slightly open at shutdown, some slop in the linkage, a servo that doesn't return to idle, or something along those lines. In short, with the plane off before starting, look and see where the throttle butterfly itself is. I'd wager you'll probably find it to be about halfway open, and can then determine from there what's causing that condition.

HTH

Tired Old Man

My Feedback: (1)

I agree, the throttle servo/linkage is prolly sticking somewhere in the range of motion.

Craig 01

Along with gboultons' excellent advice,a couple of additional things to check and as a cautionary note,check throttle plate is centred and retaining screw is tight.If all checks good,make sure there are no air leaks that seal when engine temp increases,including the vacuum pulse for carb fuel pump, although probably not likely but possible.An engine needs increased airflow to combust the excess fuel you mention when engine-carb is set at idle wether it be a sticky linkage or vacuum leak.

exeter_acres

My Feedback: (2)

which engine?

3DDino

mine starts easier at 1/3 throttle instead of idle
how about everybodys'
thz

gpar

This is an spe 26 engine with a biger carburator bore ,which gives me about 600 rpm more and runs great.
After start at 5000 rpm the engine takes about five or more minutes to slow down to 2200 by itself.
The throttle linkage is all right.

Tired Old Man

My Feedback: (1)

Slow to idle down generally indicates lean.

shaunmichaeljohnson

yes but taking 5 minutes to slow down?

this is wierd i must say...hmm.
could you live with this problem untill you find the cause of it?

maby a little oil stuck in the low end needle so when it warms up or is exposed to high suction it leaks and returns the mixture to what it normally is.
i nkow that leaky needle's can cause funny situations,,, i had one fall out on me and man that made it go fast for a split second before cutting out[:@]
good luck.

shaun

RTK

My Feedback: (1)

Gotta be an air leak somewhere if the throttle plate is closed.

Truckracer

My Feedback: (19)

Yup, sounds like an air leak to me too. I wonder if the air leak goes away (seals itself) as the motor warms up and the case or cylinder expands. Air leak gets smaller = engine runs slower. Have all the engine bolts been checked for tightness? I would suspect the bolts that retain the cylinder to the case first.

gpar

I forgot to add:
This only happens at the start just after priming .Whitout priming start is ok.
Thats why I think it is due to an excess of fuel at the chamber.

Truckracer

My Feedback: (19)

But excess fuel would make it run slower.

Have you checked all of your case bolts ... carb retaining bolts, etc? The only thing that can make an engine run faster is both fuel AND air. Either through the carb or other sources.

Craig 01

Put some water in a spray bottle and adjust for a "jet" of water, the water wont hurt anything for what you are about to do.Start the engine and whilst COLD/COOL and in the revving situation you describe,preferably back at idle as you say it will go back with throttle actuation.Spray a jet of water at every inlet tract joint,carb gasket joints,case joints-barrell to case joint etc and crank seal.The water if a vacuum leak will slow the engine and possibly stop it at the point of possible vacuum leak.Respray with lubricant spray at crank seal area when done to prevent corrosion and restart engine if necessary if engine stopped in previous operation of squirting water to clear ingested water spray.If you have a vacuum leak this will certainly pin point the area of possible excess air ingestion while cool at first start.It wont be easy to get the water jet to hit the points of interest,you will need to have sprayer close to points of interest and BE CAREFUL OF THE PROP.

Craig 01

One other thing,Did this problem start with the fitment of the"bigger bore" carb?

gpar

Yes its related to the use of a 13 mm venturi carburator (insted of 9.6 mm).

I am sure there are no air leaks . All bolts are tight

I think the "extra fuel" we talk about got into the engine chamber ( during priming) thru the carburetor, so it is already a mix of air and gas. I tried a new spark plug with same results.

Sometimes start at about 3500 rpm and others around 5200 rpm. After pushing the gas stick all the way back and fore, you get a normal idle wiht some small trimming adjusments.

Despite this problem the engine runs fine with an extra 600 to 700 rpm more compared to the original carburator.

gpar

The problem was caused by a clogged screen (carburator grid).
Dirt in the screen produced a leaning idle, going to full throttle cleaned the screen.
Removing and cleaning the screen solved all problems

Craig 01

Well there you go,Happy to hear you have sorted your problem,would of expected a partial restriction in fuel inlet screen to have affected higher rpm operation at same time of idle problem reported.