Air leak?

[Pip Holmes]

Hello karting fraternity!

I have what i think is possibly a crank seal air leak. I am no expert though, and have come to this rather shaky conclusion using my own extensive knowledge of engines and engineering, and many many forum searches!

The symptoms are basically:
1. at high revs the throttle sticks open and runs really lean until braking forces the revs back down.
2. i get a bog when pulling away. I think its called a lean bog?

I have just cleaned the carb completely and changed the gasket. The jet i’m running is a 158, which i believe is too low, and i’ve ordered a 168. I live in thailand at roughly sea level, the temp is usually around 32-36 C… (And the RM owners manual points to that as correct, although all the Thai mechanics think i’m crazy!) But it has made no difference… Still waiting on the bigger jet though. (i know it won’t help this problem though)

I just overhauled my power valve as well which turned out to be jammed closed!

I bought the kart second hand recently, and it’s my first experience of the Rotax Max engine – which is good…apart from this!

Sorry to go on!

So i think i need to check/change the crank seals and have no idea how to do it and wondered if anyone had a guide or link they would share?

Thanks in advance!
Pip

[Steen Carstensen]

This should get you on the right track. http://www.superkart.org.au/Documents/rotaxmanual.pdf

[tony zambos]

Pip,
You might want to listen to the Thai mechanics. That Rotax jetting chart will give you a jet that is very safe to use, but not very accurate for jetting a competitive engine. If you believe that your 158 jet is too lean, take the kart out to a track and after running a couple of laps, pull the choke lever up. If the engine runs better, you had too small of a jet in. If it gets worse, you were too rich to begin with and should go to smaller jet. You can also run the kart on the track and come into the pits running as close to full throttle as possible and shut the engine off. Check how wet the spark is and how dark the exhaust pipe is.

Your post seems to indicate your throttle is sticking open. Is slid not coming down when you take your foot of the accelerator? If the carb is not closing, that’s another problem that is a safety issue.

You mentioned that you clean the carb and that it has a 158 jet in it. Did you remove all the jets and atomizer tubes and inspect them for blockage? Besides the 158 main jet, what is the size of the other jets and atomizers and what are you float heights?

John Savage has an article on the Rotax carb and if you haven’t read it, do so. http://www.karting1.co.uk/new-ThePreparation&JettingoftheRotaxCarb.pdf

LAD Specialties customer / tony kart / rotax / kt100

[tony zambos]

Pip,
Forgot to ask what weight floats you’re using as float weight makes a difference in jetting.

LAD Specialties customer / tony kart / rotax / kt100

[Pip Holmes]

Thank’s guys – awesome info. As i said i am quite new to the Rotax Max, so it is a gradually unraveling mystery!

My floats are 5.2 grams.

I have thoroughly read that awesome article/guide by John Savage, really helpful.

I actually think my issue is not actually with jetting though, but rather some sort of air leak. I have noticed a bit of play in the throttle cable where it joins the carb cap, and it seems that the previous owner has put a short piece of fuel pipe over the connection and cable tied it in place, maybe to prevent air entering? I don’t know how likely that is, or if an air leak there would affect the mix? I assume it would.

I have checked the rubber between the carb and the reed valves – all good. I have checked every tiny bit of the carb itself, surgically clean!

If the throttle cable was unwound to it’s limit – should the engine still be able to idle? Because right now even with the idle screw turned right in, it doesn’t idle unless i wind the throttle cable up a bit… Is that normal for a well used engine? I don’t think it’s ever been rebuilt – next on my list i think! Especially if i’m going to race it – although they are really lax on stuff like that here!

I’m leaning towards the crankshaft oil seals still….. Is it likely to be these – or am i barking up the wrong tree completely?

Thank you again!

[tony zambos]

Pip,
That piece of fuel line at the inlet of the throttle cable is nothing. If you want to replace it with the rubber boot that comes with a motor, go right ahead. It is not a source of an air leak. If your throttle cable is uncoiling or worn, replace it. But it too is not a source for an air leak or the cause of a bog, There should be some slack in the throttle cable.

You have a racing engine. It’s not going to idle. Never going to beat any one on the track by idling.

If you have an unsealed motor, no log book and know idea of how many hours its ran, send it to a shop for a full rebuild whether you’re just going to just practiceor race it. If it blows up, it’s going to be more expenssive than a rebuild. Keep track of run time when it gets back.

LAD Specialties customer / tony kart / rotax / kt100

[Pip Holmes]

Hi guys!
OK just an update – so i had the engine rebuilt, all new gaskets, oil seals etc, and the problem totally vanished! Happy karter!
BUT now i have a different problem for which i will start a new thread! lol
Thank you for your help!
Pip

[tony zambos]

Good news. Thanks for the update.

LAD Specialties customer / tony kart / rotax / kt100

[Author]

Posts