Take off RPM

[Al Downs]

Not sure what is going on but something has changed. We are at an elevation of 700'. On the ground I get 5200 rpm at wide open throttle. In take off it is 4800 and the plane feels very slow. Really haven't had anything done to it but it just feels slow.

[FlyingMonkey]

Are those the RPM values the airplane has always had, or have they changed?  Those numbers seem too low, you should see 5600rpm or more in cruise WOT and over 5000rpm on the takeoff roll.  

My airplane had takeoff rpm numbers like yours when I first got it.  Re-pitching the prop flatter (I think by 1.25 degrees), gained me over ten knots in cruise and over 200fpm in climb.  It sounds like the prop needs a flatter pitch, unless something has changed with the engine.

[Tom Baker]

There has has been a couple times mine has done that. What I found was the carb heat was stuck on. If you cycle the carb heat and don't see a drop I would look there.

 

[Roger Lee]

A 400 rpm isn't carb heat. That drop usually only gets you 0-50 rpm depending on when it's done and which model CT you have. Is the mag drop fairly equal at the same time?

 I would pop the cowl and start with the carbs looking at the throttle opening and closing and then moving on from there. If throttle linkage openings are equal then check things like the choke closing and then throw a set of sync gauges on the carbs (not an electronic one).

[Tip]

We have also had the carb heat stick in the on position on a couple LS models. The return is helped by a very tiny spring. 

[ctfarmer]

Just one occurrence of this or repeatedly? 

Material in fuel bowl blocking jet ?

[Roger Lee]

Part of the problem is the weak screw and nut FD used. They are easy to break and or slip.  You can also bend the cable during a hose change which will keep the butterfly from opening and closing because of a bent cable flex. I prefer to toss the crudy screw and nut that breaks too easy if you try to snug it down and use the one off the carb throttle arm that holds the main cable. That one will never break or slip.

[Al Downs]

These rpm.s are lower than before. The only thing done was carb rebuiilds. I had it rechecked and all seems ok. I think I will need to have the carbs checked again.

 

[Roger Lee]

Make sure the carbs are fully opening. There should be nothing after a carb rebuild that should cause a 400 rpm drop unless you see something on a carb sync that's way out of whack.

[Al Downs]

Found carb heat stuck on

[FlyingMonkey]

My carb heat does not change my RPM noticeably.  Are you sure that is the problem?

[Tom Baker]

4 minutes ago, FlyingMonkey said:

My carb heat does not change my RPM noticeably.  Are you sure that is the problem?

Andy, the CTLS is different.

[FlyingMonkey]

19 minutes ago, Tom Baker said:

Andy, the CTLS is different.

Really?  I didn't know that...  What's the carb heat system like in the LS?

[Al Downs]

That definitely took care of the problem. Now we are looking at  the wot rpm. I  am getting 5300 max and researching setting up to do the 5600-5650 wot.

 

[Tom Baker]

2 hours ago, FlyingMonkey said:

Really?  I didn't know that...  What's the carb heat system like in the LS?

The LS has a standard Rotax air box, Rotax part number 667355.

[coppercity]

I had the Carb heat return spring break on my CTLS last year, its located on the back of the airbox.  My wife, while flying with a student noticed a drop in climb and cruise performance during the event as well as a 150-200 rpm loss at max throttle.  A similar symptom can occur if the outer carb heat cable housing comes loose from the barrel connecting it to the airbox.

[Al Downs]

The end of the cable came apart on mine. A pretty easy fix with safety wire but about three hours of labor. It is really buried.

[Roger Lee]

Hi Eric,

On the CTLS carb heat screw it is too small and the hole that's drilled through it seriously compromises the screw integrity as far as breaking and doesn't hole well. I have tossed them in the past and used the same one that holds the throttle cable on the throttle arm. It neve slips and it never breaks.

[WmInce]

2 hours ago, Roger Lee said:

. . . "On the CTLS carb heat screw it is too small and the hole that's drilled through it seriously compromises the screw integrity as far as breaking and doesn't hold well. I have tossed them in the past and used the same one that holds the throttle cable on the throttle arm. It never slips and it never breaks."

It's posts like this that make this forum great. Thank goodness we have guys like Roger (and a few others) who make worthy contributions.