Air leak?

GoldenMotor.com

el_yopo

New Member
Mar 15, 2013
8
0
0
Argentina
Hi all,
after a couple of weeks of assembly, today was the first engine start!

Everyhing went smooth, it barely required any efforts to make it run. Fuel on, primer on, a couple of wheel turns and voila!
But after warming up the engine for a couple of minutes, I disengaged the primer and the revs went up wildly... Checked the gas cable and it was fine... it also tried to rev it up even further.
After putting back the primer revs went down again.

So I guess it may be an air leak somewhere (when the mix is rich due tomthe primer it is "fine" but in normal mode it it just too much and it revs up)...
That and checking the carb and cable....

Any other hints?
I'll be digging deep in the forum for further tips and take a look as soon as I have the time...

Thanks and cheers from Argentina,

Alex
 

crassius

Well-Known Member
Sep 30, 2012
4,032
158
63
USA
remove the idle screw and work the throttle a few times listening for the slide to hit bottom - if it is going all the way down every time without binding, then put back the idle screw and try adjusting idle again with the choke (primer) off

if that doesn't work, try spraying some water around the intake area to see where the leak is
 

el_yopo

New Member
Mar 15, 2013
8
0
0
Argentina
Thanks for your prompt reply.
Will try that to make sure the gas cable is long enough and that the slide is free to reach the bottom of the carb. Also, covering the different potential sources of leak, one at a time (the intake manifold, the carb cap)
 

2door

Moderator
Staff member
Sep 15, 2008
16,302
175
63
Littleton, Colorado
Alex,
Concentrate on the intake manifold, especially the gasket where it attaches to the cylinder. We also recommend that the carburetor be sealed to the intake. The metal to metal connection point is often inadequate and requires a sealant to make it air tight.

There are other areas we can examine if the intake proves not to be the issue.
A fast idle speed is typically a symptom of excess air entering downstream of the carburetor.
Just to offer a little clarification so others aren't confused; the device you're referring to as a "primer" is in fact the 'choke'. Not talking down to you, sir. Just offering some information that you might find helpful. The choke is used to enrichen the fuel to air mixture during start up. After a few moments of engine warming the choke lever should be moved to the full open position and left there. Hope that helps a little.

Let us know what you find.

Tom
 

UVsaturated

New Member
May 15, 2008
140
4
0
Dayton, Ohio
Let us know about this issue. I reinstalled my engine once and had something incorrect in the assembly of the throttle barrel and it was stuck wide open. The throttle action is the first thing to check to ensure that it isn't just stuck.

Like other members have noted, vacuum leaks can definitely cause a high idle. I've not gotten to sealing my carb to my intake manifold and it idles extremely high because of this. It only runs fast at idle when the engine is warmed up though. Certainly that is my problem and could be yours.

Another source of leakage not mentioned is through the crankshaft seals. Not sure if the motor you have is new or used, but they are sometimes shipped missing this seal or the bearing seal has been known to disintegrate causing a vacuum leak right out of the side of the case, beneath the magneto. This would cause a huge leak and would affect total performance remarkably which does not seem to be the issue with your motor.

Like others have said, the intake manifold is more than likely the source of your problems.
 

el_yopo

New Member
Mar 15, 2013
8
0
0
Argentina
Hi guys,

well, first pass of debugging is done. Looks like it was the gas cable. Apparently it was just too short and the slider was not reaching the bottom of the carb. Just found out by peeping from the idle adjust screw hole (thanks crassius!)... As a tryout, instead of cutting the sleeve I just removed the counter-nut on the input to the carb to give the sleeve half a centimeter more room and that made the trick.

Anyway, I had also selaed the intake manifold with plumbers teflon even before giving teh bike a try, after reading a bunch of pasts here - and also made an RTV-sealant o'ring on the top of the carb: that thread seems kind of flaky from day one.

So, armed with my properly idling engine I took the china girl for a ride. cvlt1

What I found was:
- The clutch was a bit too loose - worked but made some chirring sounds. After I removed the cover I found greasy clutch powder at the bottom. So I tightened it a bit and now seems fine (less noise, no dust).
- The engine works well, though it seems to starve if I "press the pedal". Looks like either the mix is too lean already or that there is not enough fuel going to maintain very high rpms (not that I care much for the time being since its just running it up, so no high revs for now)... maybe air leaks are there after all.
What I'll do is run it at those rev levels where it starts coughing, shut it down, close the fuel and check the float level right away to see if it's empty.
(checked that the tank had the tiny openenings to avoid vacuum, but maybe they are not big enough??).

Will see... I'll keep you guys posted.

Alex
 

el_yopo

New Member
Mar 15, 2013
8
0
0
Argentina
By the way, yes, I don't know where I took that "primer" thing from!!

It is a choke!! Yes, it is a choke!!!! :-D

A primer is... well, I am not sure what a primer is (except for a textbook).


Seems the half of my brain taking care of English is short of one half...
Thanks for the clarification!
 

crassius

Well-Known Member
Sep 30, 2012
4,032
158
63
USA
after taking that nut off, you might have to check that it can still go all the way up without hitting the bottom of the adjusting screw - if that is a problem, put the nut back on & use a razor blade to cut a small amount off the throttle grip where it hits the stop inside the mount (it is very soft, so be careful not to cut too much)
 

maniac57

Old, Fat, and still faster than you
Oct 8, 2011
4,484
22
0
memphis Tn
I'd wait until you have run at least three or four tanks of fuel before you worry about tuning the carb needle. These motors take a while to break-in and will not tune consistently until they have a few miles on them.
Try loosening the gas cap next time to see if your tank is vacuum locking and restricting fuel flow.
 

el_yopo

New Member
Mar 15, 2013
8
0
0
Argentina
Well, a few weeks have gone by and the little china girl was not run much (maybe once a week for a dozen kilometers each time).
Anyway, what I found now is that it won't idle - at least not very stable- unless I keep the choke half-on.
Also, it will not develop any high speed or power unless the choke is half open.

So I am guessing all this point really to an air leakage, right? So too lean a mixture (unless half choked) and then no power and high(er) temps.

What do you guys think?

I will put in under surgery this weekend to try and fixit...

I'll keep you posted.

Alex
 

maniac57

Old, Fat, and still faster than you
Oct 8, 2011
4,484
22
0
memphis Tn
Yes, it sounds like an air leak somewhere. I'd check the crankcase seals if you find nothing elsewhere.
 

2door

Moderator
Staff member
Sep 15, 2008
16,302
175
63
Littleton, Colorado
Yes, it sounds like an air leak somewhere. I'd check the crankcase seals if you find nothing elsewhere.
I agree. The seal behind the magneto rotor (magnet) is highly suspect. If the factory doesn't drive the seal in corrrectly the rotor will rub it destroying it. I've seen a couple of those but members here have also found the right side seal behind the primary gear defective too.
If you're confident that you've sealed the intake and carburetor and you still have the symptoms you're describing then take a look at your crankshaft seals.

Tom