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PostPosted: Sun Aug 28, 2005 9:29 pm
by lazflyn
I have been having some engine troubles that I cannot seem to correct despite having gone completely through the engine.

Would a worn cam lobe on a crankshaft affect the ignition?

I found that the side of the point that rides on the cam lobe has started to wear a groove into the lobe (due to my inexperience in knowing how to properly maintain the ignition system). This would probably account for why my engine slowly started to run rough at a certain throttle setting over time. Weird thing is it would run smooth most of the time. The problem was that it was just more unwilling to smooth out at full throttle. Does that make any sense? The first start of the day it would start right up. I would let it warm up for about two to three minutes. Then, I would slowly go to full throttle to check it out before flight. It would, as time went by, get worse and worse in cleaning out as it went to full throttle. It would run smooth as could be all the way to 3/4 throttle, then it would run rough and resist running smooth or advancing rpm. I could back off on the throttle and it would run smooth. Then I would try and advance the throttle and again at about 3/4 throttle, it would run rough again. If I played with the mixture needles, I could get it to work and it would run perfect the whole flight. But now it has got to the point that I can't ever get it to run smooth past 3/4 throttle. HELP!

George Curtis helped time my ignition and I found that it was set at 23 degrees BTD. That's just a little out of the window (17-13 degrees). :rolleyes: So we set it to 15 degress BTD. I thought it would definitely change and run better, as it was timed wrong. And I was confident because George pointed out WHY the ignition timing had probably changed. I took the engine back home and tried it the next day. It didn't do a darn thing, darnit! It would run smooth till it got to 3/4 throttle. Then it would fight the additional throttle input by running rough. I played with the needles and found a way to make it run REAL rough and backfire. :D I must be talented. :D Anyway, frustrated, I gave up till today. I tried today and I got it back to running great...except at 3/4 throttle on up. Actually, at first it ran right up to full throttle. I checked the high needle to make sure I wasn't too lean, then went back to half throttle for a minute. I had a quick thought: Maybe it's all good now. I advanced the throttle to full. As I got up to 3/4, it started running rough, as is now usual. I listened to it and felt the waves of frustration come over me. Then, for an unexplained reason and at 3/4 throttle, it just quit. So I am completely perplexed.

I rebuilt the carb twice. No luck.
I swapped the carb with the good running engine. No luck.
I rebuilt the entire ignition system. No luck.
I timed the ignition and put it at 15 degrees BTD.
I checked the point's gap and it's at .018.
I installed another brand new spark plug.
Still no luck.

I am at a complete loss as to what is wrong. The only thing that is left that I haven't replaced is the piston, crankshaft, and bearings. Everything else is either brand new (the ignition) or replaced (the carb).

If any of you have had experience with this or have any ideas, please help.


PostPosted: Mon Aug 29, 2005 5:29 am
by xgary

how are you adjusting the high speed needle ?

warm engine up and it should be responsive from idle on up.
Once at full throttle or near in your case turn high speed needle till it run rich
(counter clockwise)
then turn in clockwise till the rpms pick up ( assuming thet do)
keep turing till the rpm peaks an d starts to run lean and will actually cut out form fuel starvation.
If this helps them fine tune
-- find max rpm then 1/8 turn counter clockwie (RICHER)

try that.

If not ...........any help there.
Have you replace points?
a weak spring could casue points to float.
or abad condensor could be culprot.

Other engine runs ok ? If so swap ignition coils and spark plug lead.
if you have any current drain or bad coil it could contribute to this.

what oil and mixture are you using? ?
Is fuel fresh?
What did you do to engine since it started to do this?
are crank seals leaking?


PostPosted: Mon Aug 29, 2005 7:14 am
by flyalaz
Hi Mark,

I don't know if it makes a difference, but you say 13-17 degrees. Shouldn't it be measured in inches below TDC. George knows what he is doing, but I had to ask.
I also saw in your long list of things changed or reworked that you didn't change the piston rings. I think that it's the only thing you haven't changed, so maybe that is the culprit. How does the piston skirt appear- is there scorching past the level of the rings, or is it clean.


PostPosted: Mon Aug 29, 2005 2:25 pm
by lazflyn

Thanks for the advice. I'll play some more with the tuning, but from what I've experienced, it doesn't seem to be a little rich or lean. The needles have affected the problem, but I don't have any problems tuning the other engine. It runs perfect, even though I've messed with its needles and retuned it. This bad engine really has something wrong with it. I've taken the carb of the good engine and put it on the bad engine. I played with the needles and there wasn't any change to the way it behaved.

Also, I have replaced everything that has to do with the ignition (ie. new point, new condensor, new coil, new spark plug wire, new spark plug). The problem was happening before I replaced all these items and still does the exact same thing after all these items were replaced. The problem started late last August (I've had many flights with fresh full and oil since then) and at first I just put up with it because it would quickly go away. Now it has gotten to the point that it almost will never smooth out and go to full throttle.

I am not sure how to check for current drain, Shorty. Do you have any suggestions?


This spring I did a top end overhaul on both engines. I replaced all gaskets, had both cylinders honed (they were near perfect, but I had it done to have an even surface for the new rings to seat on), and installed new piston rings. The right engine works great. The left engine (the problem child) has the same problem it had before I did anything to it.

Oops, when I said degrees, I meant inches. We used a dial indicator when we did it. We set it to .15 in. BTC. The book states .138 to .158 inches. Sorry, I don't know why I got degrees in my head. When I first checked it, it was at .23 inches. Waaaay off!!! Thought for sure that would make it do what it was doing. But, unfortunately, it didn't change at all how it ran.

By the way, the piston looked perfect when I installed the new rings. The old ones were starting to stick a little. The piston looked brand new, as I could see the maching marks still on it from the base of the piston all the way up to the lower ring.


PostPosted: Mon Aug 29, 2005 3:10 pm
by xgary

try this guy

Bob Robertson Light Engine Services Ltd. Rotax Service Center St. Albert,
Ab. T8N 1M8 Ph: (Tech Support) 1-780-418-4164 Ph: (Order Line) 1-866-418-4164

Guy is excelent and a ROTAX TEACHER plus he runs his own shop

very good guy

PostPosted: Mon Aug 29, 2005 4:39 pm
by ozzie
do these engines have reed valves? worth checking

PostPosted: Mon Aug 29, 2005 4:49 pm
by lazairiii reed valves on 185's.

PostPosted: Mon Aug 29, 2005 11:41 pm
by lazflyn

I believe the seals are fine (no leaking oil). But I'm not sure as I am just a novice. What should I look for to tell if the crank seals are bad?

Does the guy you mentioned, Bob Robertson, have any experience with 185's? Will he know about their use on a Lazair? Thanks for you help.


PostPosted: Tue Aug 30, 2005 6:57 am
by flyalaz
Take a close look around the front and back shafts. and on both faces of the crankcase. You say it is dry and so it should be. If there was a leaky seal, there would be residue left.
Are the bolts that hold the crankcase together properly torqued? Another thing you could try would be to use soapy water along the joint of the two halves and turn the prop around to see if there is a leak there.

PostPosted: Tue Aug 30, 2005 7:07 am
by xgary
Give Bob a Call, He is thoroughly trained and experiened in a ll Rotax engines .
And guess what ? - he is A ROTAX engine instructor to Rotax mechanics..

Bob Robertson Light Engine Services Ltd. Rotax Service Center St. Albert,
Ab. T8N 1M8 Ph: (Tech Support) 1-780-418-4164 Ph: (Order Line) 1-866-418-4164

See what he says, I justa dufus :)
And he one of the nicest guys you could contact and he will give you advice right on phone. I order all parts from him for all planes that come in here and he ships out right away- I order by 4 pm EST from him and i get by 2 pm next day
BC or Alberta to Ontario to my shop.

I like good service and better yet I like a good running engine.