Flat Spot when adding Power

D

dtreid

New member
Finally got the Cruisair up tonight after a few months of hibernation. I've noticed this since I got it, but just thought I'd ask.

During final approach when I'm carrying little throttle, if I add throttle, the engine goes through a bit of a flat spot between 1200 and 1500rpm. If I slowly apply throttle the engine catches up after a few seconds, but it makes me think if I really firewalled it, the engine might sputter and quit.

Has anyone else experienced this? I thought I heard something about making sure to carry power on final with the 165 Franklin but thought I'd ask for other's experiences.
 
Red Flag Warning !
Get some professional help before you fly the thing again.

This is lawn mower 101.. but you aren't flying a lawn mower !
Undershoot your final approach, the engine quits, and then where are you ?


just from general engine experience here is a list of possibilities:

1. Carburetor Ice - check systems involved.

2. Mixture problem. Carb has adjustable Idle Mixture, and smooth transition depends on this not being too lean or too rich.

If main jet is wrong... for what ever reason, ditto.

3. Carb has an accelerator pump, it's function is to dump some extra gas in the mix to well.. help the engine accelerate !
The linkage is subject to an AD.. but that doesn't guarantee it works right.

--- This sounds like an accident waiting to happen - and it pretty much demands you check the whole fuel system
from tanks, valves, strainers, lines, to carburetor for contamination.. and if it were mine I would be getting a good
A&P to check your carb, timing, and plugs before I flew it again.

All it takes is a little crap to partially block a jet in the carb to ruin your whole day.

Spark plugs load up. I had one cylinder "suddenly" start oiling the lower plug.. no apparent reason than just wear.
Go figure. Stuff happens between annuals.

How is your idle, Cold Starting, and Run Up ?

Have you gotten used to "imperfections" in these areas ?

That one almost bit me in the ass big time.
 
yup.
I was going to mention all those things plus intake leaks but heck, I'm an A&P/IA with no Crusiar expereince so I'm not to be trusted..
I guess 16 years with a Frank powered Stinson doesnt count :D
I also would have suggested that you start with jamming it off-idle on take off and see if it stumbles, then if it does you can abort.
If I was trustworthy, that is.
 
I finally found a AME who knows Franklins. As soon as I told him my problem he said that the forward intake balance tube needs to be insulated. He said what happens is that on extended finals with low power setting, the intake crossover cools off too fast and you end up with condensation forming in it, that is sucked into the engine when power is applied. Seems like a logical explanation.
 
I'm certainly not questioning your AME but rather am simply curious. Did they have insulation on the forward intake balance tube when they rolled out of the factory?
Gary
 
They did not..nor did any other airframe manufacturer. This is a warm air area inside the cowl at all times during engine operation. This is not an answer to the problem.
Dan
 
I know there can be a number of things to make an engine act this way, but when I was a student and flying a Cessna 150, I was complaining of the same problem and then while practicing pattern work in cross-wind situations, I lost a cylinder and got a real lesson that day.
 
Blimpy, I think the accel pump linkage AD is for the MA3SPA on the 150. The accel pump linkage is different on the MA4.5 I think, and there is no AD on it...though the problem does sound kind of like an accel pump problem.
Dave, have you tried to go up to altitude and see what happens when you try to duplicate the problem?
 
Sorry, I got wrapped up in generic engine faults, and for got he has the 165. And the intake leak sure bears looking at
 
The real aviation mechanics in the group should probably speak to this.


You can test the tightness of the intake manifold bolts at the cyl head. See torque table in Franklin Overhaul Manual.
You may remember 4 of them Fell Out of my engine, in January !
I was having low speed running problems on the ground, and when it finally crapped would only make 2000 rpm
at full throttle with carb heat on and mixture full rich.. otherwise it just didn't run.

There is a rubber tube which joins the "balance" tube at the front of the engine,
which joins the right and left intake manifolds together. Check that for condition,
and the clamps for tightness.

There are flange gaskets at each end of the cast intake manifold. These can be installed improperly.
The flange bolts can be checked for tightness. Unless these flanges got loose and moved, I doubt that the rubber
ring seals will go bad. You probably would have to take these junctions apart to see if the seal was bad, and then you get to put in a new one , no matter what !

At the aft end of the intake manifold, there are also seals where the cast manifold joins the tube that bend back and joins the carburetor.. these can be put in wrong also ( read franklin overhaul manual for description of how this is to be done,
I have never done it ).

Somebody with more experienced than me can comment on all that.

If you find everything nice and snug, and nothing moving around that shouldn't.. then I cant shed any light on the subject.

There are old automotive tricks, like listening for leaks with tubes, and stethoscopes, and also
squirting oil on suspected intake leaks... either of which could be life threatening with a prop involved !

Time to hire a real mechanic !

--

I would inventory any running quirks and make a list of anything out of the ordinary.
Presumably this problem either just started, or has been getting worse - right ?
So list everything out of the ordinary.

Does it start easily.
Does it idle smoothly, and will it idle slow even when cold. It Should.. and if not that is a red flag.

Read your spark plugs. Top and Bottom. What do they look like. Brown, white, oily, black, clean or clogged up with deposits ?

Will it run smoothly at all speeds on the ground... idle, taxi, fast taxi, tail up FAST down the runway, and abort ?
Will it do this both Cold and Hot ? It should. Can you duplicate the problem with the wheels on the runway ?
Try it. This is no different than wheel landing, or aborting a take off.

Try mag drop at idle, ~ 1300 rpm, 1500, 1800, 2000, etc.
Make notes.

Is the carb heat working right ? It will be worthwhile to pull the prop and cowl,
so you can fully inspect all these things.

You tested the fuel system like I suggested right ?

Mag timing, and point gap/ condition ok ? Not guess so, but tested and inspected ?

In general it is very important to have EVERYTHING to do with Ignition Spot ON.. before adjusting or delving into
you Carburetor.

Except checking for clean fuel, which would be First on my list.

Do you have a Manifold Pressure Guage ?

This can be a useful diagnostic tool.

At idle and low rpm.. you should NOT see any ticking on the Manifold Pressure gauge.
 
Figured out a few things with this problem.....some remain.

I removed and cleaned all of the fuel screens, drained the carb and cleaned the intake screen in it (they were all spotless when removed)
I replaced all of the spark plugs. I had the old ones tested and only 5 of them were much good.
I replaced all of balance tube packing seals as well as the short piece of hose that connects the halves of the front one.

Then I found the intake manifold bolts on cylinders 4 and 6 loose. I tightened these up and now I have a very smooth start up with no missing. A definite improvement there.

The problem that I am still dealing with is the transition from the idle to main jet about 1200-1500 rpm. When the engine is warm there is a lag as it goes through this zone. I've played around with the idle mixture and it doesn't seem to make too much difference.

Are there any carb experts that can shed some light on the subject? It seems to me that I've pretty much eliminated everything but some sort of contamination in the carb idle circuit.
 
Well, the Marvel MA4-5 carb IS field serviceable if there is talent of the right sort in your neighborhood.
A misadjusted float level (too low) is consistent with your description of the problem.

Since the carb is not particularly unique, it can be serviced like any Cessna or Piper carb. It might be worth having an experienced local mechanic have a look at the float level and eyeball the jets and throttle shaft.

ron
 
It costs nothing to pull the carb, and blow Berrymans Chemtool through all the jets, and ports.

Is your accelerator pump working ?

You can set the float level while it is out.

Somebody with a flow bench can pin point the problem easily.

Timing is spot on ?

You did a LOT of work... and all the right things before tearing into the carb !

Those sneaky intake bolts ... same thing :oops: scared the living crap out of me !
 
The accelerator pump is working. If I pump the throttle a few times gas runs out on the floor.

Another little issue that popped up on me last night. I richened the idle mixture a few clicks before I went flying. I went up for about 1/2 an hour. When I taxied back and took off again, just after liftoff, RPM went from 2600 to 2400 for 10 sec or so. I landed and took off again and the same thing. I taxied back and leaned the idle mixture 4 clicks, went for a high speed taxi and things were back to normal. Now I'm just not sure if having the idle mixture too rich (if it in fact was) caused this momentary rpm drop on takeoff.....this old girl just keeps me guessing.
 
Hmm, got me a little concerned. The idle mixture should have little[nothing] to do with anything at higher power. 200 rpm is a bunch of power loss. I once had a june bug stuck in the secondary venturi and the engine quit cold. The 7AC and I glided back to the airport.
No air filter, right? Or even via the heat muff. Could be some debris in front of the main jet venturi. Time to ground the a/c and get to the bottom of it. No option.
ron
 
adjust the mixture so you have a nice stable idle. then after idling for 30 secs or so, briskly pull the mixture to cutoff and be hyperaware of what the engine does.. it *should* slightly increase in RPM then shut down.
if it doesn't increase, your idle is too lean.
if it takes a moment, THEN increases before it shuts down, it's too rich. - start leaning it 1/2 turn and try it again. when you get to the point that it doesn't speed up, back it out until it does and call it good.

if you cant get a stable idle, you've most likely got an intake leak.

of course, if your carb is junk, none of this may work.
 
I yanked the carb off and sent it to Ted VanLaningham in OK today for an overhaul if needed. If nothing else, he should be able to let me know if the carb is the cause of my grief.
So much for flying for awhile, but I really do want to get to the bottom of this.
 
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