Cub Wont Run Properly

[old bones]

can any body point me in the right direction ?

I have just finished a cub ,it has had the engine rebuilt ,with PVL ingnition and a new concentric .I have fired it up and although it starts, it starts with dificulty. I started to adjust the carb ,you know screw in throttle stop adjust slow running and so forth then I noticed the choke was on as soon as I pull the choke off it dies and will not run. Has anybody got an idea? Have I missed something?

Thanks in advance

OLD BONES

[vinnied]

Take apart the carb and tell us what no's on the jets, the needle, and slide, there should also be a number on the side of carb, start there

Vinnie

[odgie]

If it is a brand new Concentric, it's also worth checking all the passageways are clear, I've had more than one new one come through with a bit of swarf in it...

[old bones]

Thanks Vinnied & Odgie for your replies

Have stripped the carb and all looks clean but I havent blown it out with air yet . as requested here are the numbers of the bits that are fitted,

The carb number is 622/L301.

Main jet is 100.

Needle jet 105.

Slide has two markings, Im guessing the 4 in a milled circle is the size and the other which is cast in looks like MB1.

The needle has the clip in the middle of three grooves there are no numbers but it does have two lines at the top as if another groove could be milled at this position.

Hope this helps all suggestions gratefully received.

OLD BONES

Edited May 19, 2007 by Old Bones

[odgie]

I'm not well up on Cubs, so you may need a better opinion, but 100 main jet seems too small to me. That shouldn't stop it running at low revs tho. All other bits seem okay (although I'd defer to anyone with more Cub experience). The two small slots on the needle designate whether it's for a two or four stroke (and without checking I ain't sure from memory which is which for certain, so best bet is check with someone like Surrey Cycles - they'd tell you correct jet size as well - to make sure you haven't got a two-stroke carb or needle, that'll be endless fun...).

Other things: Does the carb have a pilot jet? Some Concentrics do, others depend on a small drilling inside the body of the carb - notorious for getting blocked, tho usually only on carbs that have been stood a while. You really need access to an air-line. With the carb dismantled, and the mixture screw at a couple of turns out, blow air up the pilot jet hole in the body. It shoud escape from the back of the carb (the hole off to one side) and should also come up the very small hole down at the front of the venturi (you'll see there's two of them if you look down where the slide has come out). It may help to block the hole at the back with your finger, and squirt a bit of carb cleaner in first (so you can see it bubble up the hole - but watch for it ending up in your face...). If you can't get air out of that hole, the pilot jet or pilot airway is blocked (in which case get back to me... and you'll need some battery acid...). If you do get air to blow through, then put it all back together and go back to basics. Needle clip on middle groove, air screw in as far as it will go, then a turn and a half out for starters, set throttle screw so you can see a little daylight under slide, and try it again. Once you get it running you can set a fast tickover and tune the idle mixture and needle clip, I'll advise further if you need me to.

Good luck :-)

[totalshell]

there have been other questions in teh forum about the correct sizes for needles valves etc.

back to real basics though..

if the carb runs with the choke on but will not run without it the carb is too lean or more likely air is getting in somewhere between the carb and cylinder through either of the two joints or the manifold itself. so strip it off clean it up make sure that the faces that mate are flat on a piece of glass use new gaskets and bolt it all back up. if it still runs on the choke but not without it follow Odgies advice

[alan]

To check for an air leak you can try holding a smoking cigarette close to joints with the engine running. The smoke will be drawn in if there is a leak.

For non smokers, i have read that you can play an un-lit blow lamp around the joints with the engine running off choke. If the engine picks up it suggests that the gas is been drawn in to cure the weakness.

At you own risk though...............

[old bones]

Many thanks Totalshell and Alan for your advice

I will try and carry out your instructions asap,Ive looked at some previous postings and one suggestion was, that the needle with two rings on the top is for a twostroke so I will get this checked as well.

Alan Im not sure about the blowlamp trick sounds a bit iffey to me but perhaps if I get really desperate it might be worth a try ( I'll be checking the home insurance tonight or should I say The Wife Is!!).

Any way thanks ,If anybody feels like suggesting further possible cures , feel free , The more I have to check the greater the chance of solving the problem. Ta,

Old Bones

[vinnied]

110 Main 25 pilot 106 needle 3.5Slide

Vinnie

[old bones]

Thanks Vinied For your reply ,Im sorry for the delay in thanking you for the details of the jets to use in the carb ,Ive had the hard drive on the computer go faulty so Ive scrounged an old lap top from the wifes brother. Ive ordered the jets you suggested and along with the other suggestions I should have a good chance to sort it out and get it to run.

Regards OLD BONES

PS The needle with two grooves above the 3 circlip grooves is for a 4 stroke (thanks to surry cycles for info)