Kitfox 4 + HKS, the saga continues

[foxfever]

Thanks for the replies....

Just curious - What accuracy can be achieved with a conventional (non electronic) protractor? I thought 1/2 a degree was quite good.

Roberto.

I have owned and flown 3 blades of several manufacturers.

The 3 I had that adjusted similar/same as yours are warp drive, gsc, and powerfin props.

I can say that within 1/2 degree is acceptable and likely not contributing to your problem.

When I was in a hurry and re-pitching, within 1 degree was good enough for me.

I would then try to get it within 1/4th later, and found no significant difference in smoothness.

Also, of the 3, I felt the PF was the smoothest.....

[CDE2FLY]

Roberto - I suggest trying to isolate the problem. In general, the source of the vibration is either due to engine operation or pitch/tracking/balance of the propeller.

With the prop off, it sounds like the shaking is not nearly as bad. Does the substantial drop in EGT occure when the prop is off? If so, sounds like a cylinder is not firing properly. I assume you have two carbs, 4 cylinders, and you're monitoring EGTs on two cylinders not fed by the same carb. Would it be possible to monitor EGT on all 4 sylinders to determine if both cylinders fed from a particular carb were dropping? This would suggest something is going on with the carb supplying fuel to the cylinders with the low EGTs.

My best guess is that the prop balance is the problem and you can only know for sure by using a dynamic prop balance tester like the Dynavibe. The broom handle approach will not provide nearly enough "resolution" to be useful. I was amazed how much smoother my engine was after dynamc balancing. A very noticible difference was achieved with several very small flat washers installed on the spinner back plate approximately 6 inches from the center of the prop. A very small imbalance can make a very big difference when "multiplied" by the length of the prop blade(s).

Again...try to isolate potential sources of vibration and rule things out one variable at a time. One additional thought - have you contacted the engine manufacturer? Chances are they've had customers with similar issues and might have some valuable insight.

Good luck and keep us posted.

[Timberwolf]

HKS is a 2 cyl engine

[CDE2FLY]

Yes - I should have looked at the pics more closely...clearly a 2 cylinder engine!

Couple of additional thoughts....with the prop off the engine ran smoothly up to 3000 rpm but with the prop on lots of vibration above 3500 rpm. Can you run the engine at 3500 with the prop off to see if the vibration returns?

You mentioned with the prop on there is a bad shake at 2400 rpm but up to 3000 rpm with prop off the engine with smooth. That would suggest the prop is causing at least some of the vibration.

Also, the drop in EGT has me thinking there is something else going on as well...a significant drop in EGT would suggest you're not firing on that cylinder which would cause a significant vibration in a two cylinder engine.

Again....best advise would be isolate the variables. Dynamically balance the prop if all all posible. Once the prop is balanced, if there continues to be a vibration, it's likely due to the cylinders not generating equal power...carb, fuel delivery, ignition, and compression are all possibilities.

[rwaltman]

There are two separate problems,

(a) Vibration
(b) Engine not developing full power.

Taking (b) here, (a) in a separate post. [ It is going to be long, even for my standards... ]
In previous episodes I did not want to run the engine above 3000 RPM without the propeller, so I don't know if the problem shows up or no.
Went to the airport today planning to do just that, and to redo the carb balancing more carefully and at higher RPMs. (more below)
Couldn't do either. The oil temperature gauge stopped working. Without a reliable oil temp reading I didn't want to do anything with the engine beyond starting it and running at low RPM for a couple minutes.
Spend a lot of time trying to figure out how to get to the wiring and maybe remove the oil temp gauge itself, without much luck.

[ The most productive thing I did today was to assemble a tool cart that has been waiting in its box for 1 week.
When my wife saw it ( < http://www.homedepot.com/buy/storage...art-99980.html > ) she asked if I'm going to serve tea in the hangar. ]

My suspects are:

(1) A fuel flow restriction in one of the carbs. Did open the top and cleaned the needle in the carb that, (from memory,) was feeding the cylinder that quits at > 3K RPM. Did not change anything (pushed it briefly to 3300 RPM)
(2) A bad carb sync that is noticeable only at higher RPM.
I'm not sure I did the procedure correctly with regards to the vacuum balance tube between both sides. I remember pinching it close when I did the idle point, not sure I did it when I balanced again at 3000 RPM.
(3) Like (1), but not a restriction but an "abnormality" in the fuel flow due to engine (and carbs) vibration.
(4) Bad compression in one cylinder. The compression was checked when the plane went for its annual and it was OK. (First day, I was there) Of course something may have happened between then and now. A stuck valve?
I will ignore the last two for the time being.

So, the steps now are (a) fix the oil temp reading problem, then (b) balance again the carbs and, if the problem is still there, (c) send the carbs to somebody for a full disassemble and clean-up.
There aren't many people familiar with the HKS engines, but the carbs are the same Bing 64 used in the Rotax 900s, so this should not be a problem.
Hopefully there will not be a (d), scratch head vigorously and go back to empty drawing board...

[ I did ask the mechanic to check the carbs when he had the plane, and he told me he did,but I'm not sure what that means and I wouldn't trust it anyway. ]

This is beginning to be frustrating. Due to work, travel time to the airport, weather, etc. I've been able to work a little on weekends only. Anything I can not solve on the spot (such as ordering a new oil temp sender or gauge) means instantly an additional 1-week delay ...

Roberto.

[pedro10012000]

Roberto - it seems the issue with one cylinder must be the first thing to resolve. Have you discounted that the rough running over 3000rpm could be something as basic as a bad or poorly gapped spark-plug. An easy way to check is to switch the plugs from one cylinder to the other.

Likewise you can may be able to switch over the leads from each CDI to check nothing is amiss with them (it could even be a grounding problem with one of them)

Otherwise strip and clean the offending carb, plus make sure the throttle and choke cables are running freely.

Did you see this discussion re the issue of the Bing carb vent tube length and locations - possibly similar symptoms to yours so worth checking ..... http://www.teamkitfox.com/Forums/showthread.php?t=3869

Does the aircraft have a genuine HKS rev counter? It seems a lot of non-genuine ones are prone to incorrect readings, meaning the pilot may pitch the prop incorrectly, which allows the engine to overrev by about 1000rpm. This can cause bending to the pushrods, which if slight, may only become evident (thru rough running) at higher revs.

I think from your pictures that you have a 2.58 ratio gear box - if so it is only specced to a MOI of 3000kg/cm2. That should be ok with the 3 blade Powerfin, provided you solve the engine running issue.

PS the HKS gauges often seem to have grounding issues - it may be worth checking the ground of your Oil Temp guage

[rwaltman]

pedro10012000 wrote:
-- the rough running over 3000rpm could be something as basic as a bad or poorly gapped spark-plug. An easy way to check is to switch the plugs from one cylinder to the other.
Plugs have 0-time - Changed when I bought the plane, even though they had only 80 hours. Still, valid point, will check. I kept the previous set.

-- strip and clean the offending carb, plus make sure the throttle and choke cables are running freely.
By "strip" you mean remove from the plane? Yes. Cables feel OK.

-- Did you see this discussion re the issue of the Bing carb vent tube length
Yes, and you mentioning it just brought something else to mind: I have been doing these experiments without the cowlings, that is, with an altered air flow. Should put then back.

-- Does the aircraft have a genuine HKS rev counter?
Yes, I am looking for a cheap optical tach to double-check it.

-- I think from your pictures that you have a 2.58 ratio gear box"
Thought I had the 3.47, but seems you are right. Found this in Yahoo's HKS group:
"look at the side of the gear housing. The 2.38 has 3 reinforcements in the casting on each side. The 3.47 has 4."
Next time I remove the plugs for any reason I'll check it turning the prop and counting piston cycles.

-- the HKS gauges often seem to have grounding issues - it may be worth checking the ground of your Oil Temp
Will do. By the way, this (grounding, wires shaking loose) is the kind of problems I'm concerned the excessive vibration may cause.

Roberto.

[Dave F]

How does this engine run at wide open static?
or at 5000 rpm. ?


possibly nature of the beast?

you would not be the first to think that is is very bad when it is just normal.


I would round up another HKS owner to evaluate.

[rwaltman]

Loaded with the propeller does not go past ~3300 RPM.
But now I know, or think I know, why. Did some progress this weekend.

[rwaltman]

Finally, a light at the end of the tunnel.

First - Fine tuning the prop.
My first attempt was more of a sanity check than a careful measurement.
I measured the pitch at the blades tip, that is, with the angle meter on top of the leading edge protection tape, something that Powerfin specifically says not to do. Also the plane was outside, on a windy day, with the wings rocking all the time.
I did it again, twice, inside the hangar, more carefully, not on top of the tape, and found a larger error than before. Two blades within 0.2 degrees, the third 0.8 degrees from the lowest of the first two.
Also, measuring from the hub's edge to the blade's tip, one blade was 1/32" longer than the other two.
The assembly instructions say to pull out the blades (as centrifugal force would) when assembling the hub, apparently this was not done consistently.
Prop was removed, disassembled, cleaned, reassembled and re-pitched correctly.

Second - My carb balancing was worthless.
There is a "vacuum balance tube" linking both air/fuel inlets, that needs to be closed when doing the carb balance. I overlooked the fact that in my setup it is not a tube, but a "T", with the center section feeding the fuel vacuum pump. So, one cylinder drove the carburetor on its side, while the other drove its carburetor and the fuel pump. Not really balanced.
Process repeated with the balance line removed from both ends, holes plugged, electric fuel pump on.
As before, 1st balance without prop, idle at ~1900 , high power at around 4000 RPM.
Then the prop went back on. Idles at 900! So the HKS recommended 1400 RPM is perfectly reachable.
At 900 - 1000 RPM is shaky but stable, doesn't try to quit. There is still some vibration until going above 2000 RPM, but much less intense than before.
So far so good. Further away, not so good.
As before, trying to go above ~3300 loaded with the prop causes shaking, vacuum unbalance, 1 EGT diving down.
This time I had reinforcements. "Rotax guy that knows" was by my side. He is convinced there is a carburation problem. The suspect carb is removed and disassembled, he says something "doesn't look right" but we had to leave it like that, was getting late in the day (yesterday)
We meet again today, the other carb is removed to compare and we find out that:

Third - Bing carburetors do not work correctly when not all the parts are there.
One of the bronze thingies is missing. (Will call it by its proper name when I get the manuals and learn it.) Not a clue how this did happen, I can only suspect the A&P from hell.
So, tomorrow I'll order the Bing manuals, the missing part, a rebuild kit if available, (gaskets, O-rings etc.) By next weekend should (one more should) have everything back and running, and bring back my attention to the CHT and fuel gauges.

Hey, at this rate I may greet 2013 with an airworthy Kitfox ...

Roberto.


Find the differences: