View Full Version : Rough Running Rotax

Irish Falcon - n/a
7-Sep-07, 05:50 PM
I'v got a 503 in an Osprey hull, a few weeks back I removed the engine to fit a new fan hub, replacing the blades at the same angle as I removed them. Now it will rev to 4500 rpm ok above that it starts to run rough.It used to rev to 5800 smoothly. I'v cleaned the carbs,filled with fresh fuel,checked plugs and leads and no improvement. Any ideas whats up? Philip

Ian Brooks - n/a
7-Sep-07, 06:02 PM

Sorry to hear that you're stil having trouble.

Did you try putting the old exhaust back on just to see if thats upsetting things?

Did you try switching out each ignition in turn to see if that affects things?

Or disconnect the kill lines completely to see if they are leaking to earth slightly?

Fuel pump OK? (Pump on the primer bulb a few times to "assist" it, if that clears it up then could be the fuel pump not up to scratch or the pulse line blocked/kinked/perished)

Air filter? If it's paper based, I have known them go bad after getting wet, foam based are best for the Rotax if they can get wet.

Ignition high tension lines all clean and in good order?



Irish Falcon - n/a
7-Sep-07, 06:05 PM




Changed filter


and it still wont run smoothly. What am I missing? Philip

Ian Brooks - n/a
7-Sep-07, 06:16 PM
Excuse the back to basics approach, might jog some idea:



Strainer in tank

Kinked pipe

Filter blocked/clogged

Pulse pump not working properly:

Pulse line kinked

Connected backwards (OK OK....!!!! had to say it)

Dirt in carb:

Float valve blocked

Main jet blocked

Float damaged or not assembled properly

Carbs not opening properly


Filter blocked (K&N type bad after getting wet)

Something getting sucked onto filter


Spark plugs

Coils & leads clean

Kill line leaking to earth (disconnect near ig unit)

Advance retard not working (strobe)

Ignition unit not good (would have to be both gone bad, unlikely)


Are the blades really in the same angle? Could there have been some mistake?

Belt too tight (absorbing excess power)



Irish Falcon - n/a
7-Sep-07, 07:42 PM
Thanks again Ian,

The engine stars good, runs ok till approx 4-4500 Rpm, it will make 5700 but will be runnning very rough,fault is very consitant. Before I removed the engine it was running perfectly.While it was out it would have been on its end a few times.

All fuel should be ok, I have clean lines and filters, I'v cleaned the carbs,bowls jets etc, all seem ok,I'v tried pumping with the primer bulb-no change.

Air, clean & dry foam filters.

Ignition, I'v disconnected kill wire-no difference, both systems working.Plugs ok. <u>ADVANCE RETARD</u>? is this a possibility? I'm not a petrol head, disel in my viens. How do I check it?

Blades were fitted in same sequence and angle as previous owner had them set. Belt tension is also the same. Philip

Mart366 - n/a
7-Sep-07, 09:26 PM
put the old blades back on, and check the rpm again.

have you got the same type blades? 4z/ 5z


Irish Falcon - n/a
7-Sep-07, 10:00 PM
Same blades, same pitch 5z. Philip

hovmart - n/a
7-Sep-07, 10:40 PM
try yet another set of plugs

Irish Falcon - n/a
8-Sep-07, 07:42 AM
Will do so today but I have a dual ignition rotax so unlikly that two plugs went bad at the same time. I'm rechecking EVERYTHING again today. Philip

andycollins - n/a
8-Sep-07, 04:56 PM
A while ago I heard tell of a duff batch of plugs doing the rounds. I can't remember which ones they were. I'm sure Ewan or The Boss will recall though. It could be you've got a set of those.

Otherwise it sounds a bit like coil breakdown or something EHTish.

If it's points rather than CDI they could be dodgy. If it's points there should be a condenser (sorry not really into Rotax so I don't know) If there is it could be breaking down.



Irish Falcon - n/a
9-Sep-07, 07:28 AM
Update. Checked most things again yesterday,started up PERFECT. Max revs smoothly. Only one thing to do head for a beach with easy access to trailer in case of problems. All good for 20 mins then back again,smooth to 4500 rpm them rough to 5500, close throttle, at 4500 all ok above 4500 not good. Seems electrical? Could not get a set of plugs yesterday, when I get some I'll post results. Thanks for the replies. Philip

hovmart - n/a
9-Sep-07, 09:09 AM
will not be plugs then, sounds like cdi box problem

peterd51 - n/a
9-Sep-07, 06:01 PM

does it have an ignition coil?

If so, does it get hot?

Maybe it's getting hot and breaking down internally?



Mart366 - n/a
9-Sep-07, 08:08 PM
have you checked your compression?

could be worn barrells, once warm they open up slightly and give rough running, when there cool they run ok


Irish Falcon - n/a
10-Sep-07, 03:29 PM
Latest update. New plugs - no difference. Rotax is a dual CDI engine,both systems working. Hot or cold makes no difference. Ran it for a few hours yesterday, did about 30 miles, got full power sometimes but always smooth below 4500 RPM.Will check electrics in the dark later, maybe something will show up. Philip

chrismdaly - n/a
10-Sep-07, 05:00 PM
My 447 had the sme symptoms. The cure we used was to dimantle the carb and use a carb cleaner and a good air line on every part.

Three races later we had to re build the engine. Try checking the compression when it is warmed up.We had a worn out engine and the bores were severely scored.

I am still not sure if the first cure merely delayed finding the second and more serious problem.

Good luck.

Irish Falcon - n/a
10-Sep-07, 05:18 PM
Thx Chris, I cleaned the carbs as best I could with carb cleaner in situ but may try again with carbs off and with an air line. Talked to some jet ski friends with a similar problem. They had to use ultasound cleaners to get carbs really clean. Whats really bugging me is how consistant the problem is,4500 rpm ok, 5000 not ok. Philip

Ian Brooks - n/a
10-Sep-07, 08:54 PM
I'm mystified right now... I would agree that the carbs should come right out, be dismantled and cleaned using an air-line... but be really careful to note how the jets, etc go in, it is easy to put something back in upside down.


jon_curtis - n/a
11-Sep-07, 08:08 PM
ITS FUEL STARVATION http://hovercraft.org.uk/images/icons/smiley_icons/icon_razz.gif

Irish Falcon - n/a
11-Sep-07, 10:08 PM
Hope your right Dangerman, at the moment working far from home every day prevents any checks on the engine but I will blow out carbs again and try gravity feed before our planned cruise on sat & sun. Philip

Iceman - n/a
12-Sep-07, 07:17 AM

when you took the engine out did you remove the throttle cables from the slides?

If you did then you probably refitted them incorrectly! Check that the circlip on the needle is BELOW the white plastic cup.

Do you have a rubber coupling in the transmission system?

Try reducing the pitch on your blades, 5800 is too low a max rpm (assuming full carb slide travel) and you will be better running it at 6500 max rpm.

I doubt it is electrical or fuel related.

Irish Falcon - n/a
12-Sep-07, 06:28 PM
Hi Iceman,

I did not remove the cables, slides are fitted correctly but will check anyway. No rubber coupling is fitted. 5800 worked ok all year but I will check for max slide travel. Philip

Paul Fitz - n/a
13-Sep-07, 04:57 PM
Hello Philip,

I think most of the obvious things have been suggested. The symptoms you described were not detailed enough to determine if this is a single cylinder problem or something affecting both cylinders together. As you have removed and refitted the engine, I would look at 3 areas in particular.

The exhaust. - Remove it and check that it is not partially blocked.

Carbs - Check that the needles are not damaged and that both are fitted at the same height. Also check that the float heights are identical in each carb (if two fitted) and the needle valves close properly, Check that the throttle slides open together to maximum height and that any choke/cold start mechanisms fitted are working correctly.

Air leaks. - Check for air leaks around the inlet manifolds when hot. Check also the cylinder-head gaskets/rings, by doing a compression test when hot. (not easy I know with 4 plugs). – remember to pull the engine over at least 3 times for each test

to give a maximum reading on each.

I have known of similar problems occurring when engines have been stored 'on end' for prolonged periods causing damage to head gaskets and oil seals, but I doubt that this is the problem with yours if it was stored upright and out of the craft for a short period.

Added - If the above does not solve it run the engine using the old plugs until the problem occurs and do a plug chop. i.e run at full throttle for 10-15 seconds and kill the engine, keeping the throttles fully open until it comes to rest. Check the plug colours to determine if either of the main jets require changing.

Good luck.

Irish Falcon - n/a
13-Sep-07, 07:46 PM
Thanks Paul,

I'v just gone through the carbs again,checked slide travel etc,started up easily as usual and went to 6000 rpm smoothly so I replaced the engine cover and am hoping for the best. We hope to have three craft together here in the south west for the weekend and I intend to show the others some good cruising areas on the edge of the local pond ( also known as the north atlantic). Anybody that wants to join us is very welcome. Philip