Acceleration problem Jabi 3300

[peter0]

Morning.
Has anybody by any chance experienced this problem on a 3300 engine? My engine is running perfect except for when you add sudden power from idle. In this condition she struggles for about 1 second before gaining revs as per usual. Normal 3 second slow application of throttle results in normal gaining of engine revs to normal static revs. I know people will say that you should never put balls to the wall and I agree but the point is that the engine should handle this and it always has. It seems to me like it could be a rich cut because the EGT's are also running lower than usual. If one adds full power from 2000 revs then she reacts normally so it's only from idle that theres a problem. Like when does too quick a go around.
We have been through absolutely everything in the aircraft and don't see any issues. I've checked the following so far.
1. Spark and ignition system etc..
2. Fuel filter and fuel flowing properly into carb.
3. Carb has been gone through top to bottom and all free of dirt. Choke plates also clear.
4. Float level correct
5. Scat hoses not folding or blocked.
6. Flywheel bolts fine.
7. Valve clearance set.
8. No mag drop.
9. Carb atmospheric vent clear.

My thoughts are that it's possibly carb jets? That's about the only thing that we haven't done. Not something one can check to easily without actually replacing them. I thought that it could be a blocked idle jet but this has been found to not be the case either.

Has anybody please got any thoughts on this issue?
Peter.

[scsirob]

This is just a wild-a** guess, but you do mention SCAT hose. Slamming the throttle open will cause a quick change in vacuum in the intake system. If you have such a hose in your intake system, the sudden vacuum might make it collapse briefly. Especially if it's longer than need be, the hose may collapse 'diagonally' (not sure how to explain it better). My first cause of action would be to replace this hose. Make sure it's the single-walled stuff, the hose with inner lining are more prone to collapse and block the airflow.

Another factor could be the air filter. Have you replaced it lately? Have you flown in moist conditions from dirt strips? A clogged air filter will cause more vacuum and a bigger chance of something collapsing.

The jets are an unlikely cause if this didn't happen before. These are just solid metal with a hole in it. Unless something blocks the hole, not much can happen with these jets.

[peter0]

Thanks and your explanation is perfect .
Definitely not scat hoses. I've checked and even had someone stand by the engine to look as I rev her. The air filter is good and new. So that is also highly unlikely. Jets is not the perfect explanation because as you also mentioned, they don't wear out suddenly but then in the same breath this might have been going on for a while and I only noticed when someone else was flying and pushed throttle in quickly. Not something that I would usually do.

The one thing that puzzles me is the reduction in EGT temps. That would have slowly changed as the jets wore out and that I would definitely have noticed!

[Tasmag]

Sounds a little like it is running rich, might explain EGT's as well.

Possibly float level or restriction at air intake.

What about trying the old filter to see if the new one is faulty.

[Rotor kop]

Jabi accelerating? I believe that Jabi's are the only aircraft that have bird strikes from behind...................

Just repeating myself from the Peaceful thread

[scsirob]

I believe that Jabi's are the only aircraft that have bird strikes from behind..............

Interesting comment from someone who wants to just hover

[Christo]

Jabi accelerating? I believe that Jabi's are the only aircraft that have bird strikes from behind...................

Just repeating myself from the Peaceful thread

Yup, they should put those Harley handlebar stringers on the wing struts by default so they can see which way they are going

[happyskipper]

Car carbs normally have a pump that spurts an extra bit of fuel when the throttle is taken from idle to full power very quickly. Accelerator pump, I think it is called.
I don't know if Jabs have these - but the solution is easy - don't accelerate too quickly, and all is fine...

[Marius Schrenk]

Car carbs normally have a pump that spurts an extra bit of fuel when the throttle is taken from idle to full power very quickly. Accelerator pump, I think it is called.
I don't know if Jabs have these - but the solution is easy - don't accelerate too quickly, and all is fine...

I do not know much about the Bing carb....but the thought also crossed my mind. If it has a acceleration pump the diaphragm might be shot.

[Chalkie]

The one thing that puzzles me is the reduction in EGT temps. That would have slowly changed as the jets wore out and that I would definitely have noticed!

I don't know the Jabi engine, but I do understand carburettors. In any carb with a slide and a needle extending into the fuel emulsifier / atomiser; the only jet that can "wear out" is the needle jet. The needle touching the sides of this jet will make the jet bigger, and wear the needle smaller; both of which will cause a rich mixture. (High in the RPM range)

The idle jet will control mixture until you have lots of throttle AND lots of RPM. (About half way through the rev range.) If the engine is slow to accelerate, the idle mixture could be too lean. (Accelerator pumps allow a lean idle mixture, without one the idle mixture needs to be a bit richer to aid acceleration.)

If the carb has a diaphragm, then check this for any perforation. Hope this helps.

[peter0]

Sounds a little like it is running rich, might explain EGT's as well.

Possibly float level or restriction at air intake.

What about trying the old filter to see if the new one is faulty.

I also suspect/suspected a rich cut. Have replaced jets and still have the same problem. I've also bypassed the air filter to see if that makes a difference and that has no effect. Float level is correct.

[peter0]

The one thing that puzzles me is the reduction in EGT temps. That would have slowly changed as the jets wore out and that I would definitely have noticed!

I don't know the Jabi engine, but I do understand carburettors. In any carb with a slide and a needle extending into the fuel emulsifier / atomiser; the only jet that can "wear out" is the needle jet. The needle touching the sides of this jet will make the jet bigger, and wear the needle smaller; both of which will cause a rich mixture. (High in the RPM range)

The idle jet will control mixture until you have lots of throttle AND lots of RPM. (About half way through the rev range.) If the engine is slow to accelerate, the idle mixture could be too lean. (Accelerator pumps allow a lean idle mixture, without one the idle mixture needs to be a bit richer to aid acceleration.)

If the carb has a diaphragm, then check this for any perforation. Hope this helps.

Needle and needle jet have been replaced with no positive effect. Carb does have a diaphragm. I've checked this and there's no perforation.

The way I see it. The carb has a idle jet for up to a certain revs as you mention. Then the fuel flow/mixture moves over to a tapered needle and the idle jet becomes inoperative basically because the fuel it supplies is negligible compared to the main jet.

I don't think that there is anyway that one can adjust the idle mixture? If I'm not mistaken the only way to adjust mixture is to replace main needle and jets with a different size. This would then adjust mid to high revs mixture. There's surely no reason an idle jet should wear?

[Chalkie]

What, no tapered spring loaded idle adjustment screw?

[heisan]

What, no tapered spring loaded idle adjustment screw?

There is an idle air screw.

The problem with an issue like this, is that just about anything can trigger it.

Various possibilities:

There is no accelerator pump, so simply banging the throttle open will always cause a stumble. Perhaps you misremember how fast you could open the throttle without causing a stumble?

A kinked sense pipe (from carb to air box inlet) will prevent the diaphragm from moving quickly, causing an over-lean mixture in rapid acceleration.

Too lean idle mixture will make the motor reluctant to accelerate until the main jet starts opening wider.

Too rich a mixture (as noted by others) will also hamper acceleration.

Poor spark will first be noticed under less than ideal mixture conditions (i.e. during engine acceleration). (Should see things get substantially worse if tested on one mag.)

Basically, you just need to test everything until you find the culprit.

[peter0]

Sorry Chalkie. My mistake you right about the idle mixture screw. I'm only a pilot, not a carb expert Am I correct in saying that all that will do is allow for smooth idle and not affect the increasing revs from low to high?

Im starting to think that I might have a bad fuel pump. That's pretty much the only thing that hasn't been gone through.