wingstrut

Gentlemen, I have new Zenoah GT-80 that starts and runs great, but as I am flying at half or full throttle I hear a skip in the engine about every thirty seconds, I replaced the Champion RCJ7Y's and they gave me Champion 5843, they claimed these were the new easier start plugs.
Well, It was miss-firing every two seconds, it sounded like the ignition system was shorted.
I took those out and replaced with NGK BPMR7A which is the replacement for the Champions.
It ran better but still had the skip, but now it is about a two minutes apart.
I replumbed the fuel tank three times, I have soldered on barbs and ty-wraps on all the fuel lines inside and out of the tank, when running the engine the fuel flows through the lines without and signs of bubbles foam or anything else.
I took several temperature reading of the cylinders, the cyl on the left as you are sitting in the cockpit runs about 235 deg., the other cylinder runs 260 deg.
The cooler running cyl. is further back from the cowl, maybe it is getting air more efficiently, just guessing.
Is this temperature too high and maybe causing the plug to over heat causing the miss-fire?
I haven't tried using a colder plug in this cylinder yet.
I am using Exxon regular mixed with Amsoil Dominator at 40 to1.
I am also using a 42 percent ignition cut off module.


Thank you for any ideas, it has me stumped,...........Ron

"the temperatures were taken on the ground without the cowl, I have air ducts around the cylinders.

arobatx

I'd try a heavy duty switch in place of the ignition cut-off for a few test flights, in an attempt to further narrow down the source. It is a GT80 or ZP80? (Magneto or EI?)

Pete Bergstrom

Ron,

When you say 'new' Zenoah GT80, are you referring to a 'new' magneto GT80 or one of the ZP80T engines with the electronic ignition? I suspect it is the EI version as you mentioned using the 42% products ignition cutoff module.

Your description certainly sounds like an ignition event of some kind - either the ignition is mis-firingintermittently or you have some pre-ignition going on.

1. Have you tried bypassing the cutoff module to isolate that as a possibility?

2. Have you tried to richen the low speed needle to avoid an overlean mixture which could be causing occasional pre-ignition?

3. Assuming it is a ZP80T version, what batteries are you using to operate the ignition module?

Pete

pe reivers

How are your plugs gapped? try increasing to 0,025"
There may be too low voltage to your EI. OTOH, some ignitions "skip" when using too high voltage.
Often this "skipping" is associated with early ignition timing as well.

Truckracer

My Feedback: (19)

And the standard question, are the needles set properly?

wingstrut

Sorry guys, I failed to mention that it is a magneto Gt-80 swinging a 26 x 8 prop.
I have correctly set the needles at least six times going from just right to slightly rich.
set low end to maximum rpm then backing off about 300 rpm, high end same way.
I have set the high end rich and the low end lean.
low end rich and high end rich and vise versa.
I may try to disconnect the engine shut off module to eliminate that part, I bought it used.
It seemed to change the interval when I changed plugs, that is why I suspected the one cylinder running hotter may be the culprit.
Is 260 degrees too hot.?

Thank you fellas for the help..........Ron

pe reivers

260F is quite cool, depending on how you measure it. Topmost cooling rib, against cylinder wall, sensor isolated from cooling air.
The best way is to measure plug washer temperatures. In that case, 400F to 450F is quite normal.
Wingstrut did not provide any information on temperature sensor location/reading time and operating conditions at all. Maybe he can correct this, and provide some more useful information.

K-Bob

My Feedback: (1)

Definitely ditch the 42% ignition cutoff. If yours works at all you have been very fortunate.

wingstrut

K-Bob, do you have a recommendation for a transmitter operated engine cut off?

Thank you for your time..............Ron

wingstrut

The engine is a Zenoah GT-80 swinging a Xoar 26x8 sabre WW1 style prop, Exxon regular gasoline mixed with Amsoil dominator 40 to 1.
I ran the engine without the cowl so I could adjust the carb, there is ducting around the cylinders.
I measured the temperature after running the engine for four minutes at half throttle on the ground, I did not fly.
I used a Fluke 561 IR Thermometer at approximately 18 inches from the cylinders I pointed the IR beam at the center of the cylinder half way between the crankcase and the spark plug. I did not point it at the fins.
I could not get a cyl. temp at the rear of the engine because of obstructions.
Viewing from the cockpit the left cyl. temp was 235 degrees Fahrenheit the right cyl was 260 degrees Fahrenheit.
I ran the engine at various rpm's for another four minutes on the ground and the temperature remained the same.
The temperature outside was a steamy 89 degrees here in florida.




Thank you for time,.....Ron

Tony Hallo

Several years ago there was a threadover at the giantsthat discuss adjusting the cylinder temperatures with additional gaskets under the base. The hot side was raised to achieve balance.

karolh

sub.

do335a

On mine, I ran a micro switch for the ignition cut off.

There was a separate servo on a separate channel, 2 position switch activated, so that I could cut the ignition by grounding the magneto at any throttle setting at any time.

In the pic, you can see the servo and micro switch on the right rear of the engine box. The other servo, on the left, is for throttle.

Additionally, the magneto ground was set up in parallel so that it could be activated by a separate fuselage mounted switch on the ground. That way, much as with a full size plane, it was not hot until deliberately made so.

While parked, I activated both cut offs, just to be certain. No bystanders could inadvertently get the engine to fire.

wingstrut

That is very interesting, lower the compression and change the timing, could work.
I think I found my misfire, I hate to admit this but my engine was loose, I have never had an engine come loose before.
I believe it could have been creating RF frequencies that could possibly have been interfering with my electronic engine cut off module.
I came to this conclusion after tightening the engine back down and flying the plane there were no misfires actually I received comments on how smooth the engine ran.

Thanks tony for that info

wingstrut

The engine loosened back up, I don't understand this, but I'm breaking out the locktite and safety wire, I will fix that sucker.

I really appreciate how you guys jumped right in to help, fantastic information.......Thank you again................Ron

wingstrut

Hey folks! I just noticed that my GT-80 doesn't have a gasket between the carb and the phenolic / plastic spacer, there is one between the spacer and the intake manifold.
Does anyone else have this on their GT-80?
I have tried to find a decent parts breakdown but couldn't find one good enough to verify this.
At one site they listed two gaskets at another they listed one gasket but no views.

Ron

karolh

All of the Zen manuals with an exploded engine diagram I have seen show gaskets on both sides of the carb spacer block so it's fair to assume that the GT-80 should be the same. Have seen expl. diagrams of all the Zen engine but never one of the GT- 80.

wingstrut

That could be part of my problem.
I will have to check further to be sure.
Thank you for your information.....Ron

Pete Bergstrom

Ron

PM me with your shipping info and I'll send out gaskets for you. Yes. There should be two gaskets there.

Pete