Starting then cutting out

[Jeff t]

Just done a couple of jobs so new kick start and clutch springs, I also took the carb off and reed valve as I was getting a bit of spit back Imola with phbh 30mm, anyway put everything back and now it will start with the choke but soon as I turn it off it just dies. I was careful putting back together and the inlet manifold looks seated properly only other thought is if I could have damaged the reeds but like I said I was careful, any thoughts before I take it off to check.

[johnnyXS]

well it sounds like an air leak somewhere on the carb or inlet doesn't it.
The air leak is making your mixture too weak which is being compensated for when you put the choke on .

[Warkton Tornado No.1]

by Jeff t » Tue Jun 28, 2016 10:23 pm
Just done a couple of jobs so new kick start and clutch springs, I also took the carb off and reed valve as I was getting a bit of spit back Imola with phbh 30mm, anyway put everything back and now it will start with the choke but soon as I turn it off it just dies. I was careful putting back together and the inlet manifold looks seated properly only other thought is if I could have damaged the reeds but like I said I was careful, any thoughts before I take it off to check.

You should check your pilot jet.

Your symptoms are as if that is blocked.

[Jeff t]

Just took both carb and manifold off and I'm pretty sure it's not an air leak , no damage on the rubber and the reeds look ok so I'll check the carb tomorrow. It does sound like it could be the pilot jet as if I Rev it then it's ok but let the throttle back and it cuts out.

[johnnyXS]

if putting the choke on stops the engine from cutting out on over run then the engine is getting a weak mix from the pilot circuit ie the slide the pilot jet and the air mix screw.

I assumed that you had checked that the pilot jet was not blocked and was the correct size.

If you have no air leak or fuel restriction then I would suggest that you reset the air screw with the engine warmed up. If you need to have the air screw screwed right in to get a stable idle then go one up on your pilot jet.

[Jeff t]

Ok well I've checked all the jets no blockage, I'll try the air screw and see if it's that, it was bloody fine before I started !!

[johnnyXS]

where exactly are you getting the spitback ? is it at low middle or high revs or right throughout the throttle range ?
I read on MB Devs site that spitback with these carbs is fairly common and often due to over jetting but that doesn't seem to be the problem with yours rather the reverse, too little fuel

[Jeff t]

It's after a run there's quit a bit of shite on the running boards round the carb and on the inside of the side panel, it was more of just checking why I was doing a couple of other jobs. The only other thing is I left the carb cables connected and moved out the way to remove the reed valve but as per the posts I've checked all the obvious.

[johnnyXS]

are you running without an air filter ?

[Warkton Tornado No.1]

Post by Jeff t » Thu Jun 30, 2016 10:14 pm
It's after a run there's quit a bit of shite on the running boards round the carb and on the inside of the side panel, it was more of just checking why I was doing a couple of other jobs. The only other thing is I left the carb cables connected and moved out the way to remove the reed valve but as per the posts I've checked all the obvious.

The problem sounds more like your reeds with your additional comment.

However, despite the risk of some more ‘dummy spitting’ antics, have you assembled the carburettor correctly?

It wouldn’t be the first time that a PHBH has been assembled with the needle retaining plate on the wrong side of the needle clip……

[Jeff t]

I hadn't touched the carb untl I had the problems after the rebuild and since then I have took out the jets made sure they are clear and put back together again I checked the reeds and they look ok no visible damage.
With the jobs i did, new clutch springs, kick start repair, removed the ramp on the end plate and fitted an Li version which I think would have nothing to do with the problem I have now unless I am really missing something.
With reagds to the carb end I removed the reed valve checked and reassembled including a new gasket took it apart again and checked the reed valve is seated correctly and I think it is. Yes I run with no air filter but always have since the kit has been on, the only other thing is I obvioulsy removed the exahust to get at the clutch.

One final recap it was running ok before I started just the spit back, this morning I have adjusted the mixture on the air screw and it made a bit of differnce as it ran a bit longer before dieing adjusted one full turn. I am sure theres somthing obvious I'm missing but buggerd if can fathom it out

[Jeff t]

Went back to basics what have I changed which was only the gasket took off the reed valve this morning and there is a fourth mounting hole at the front of the manifold that's not used the old gasket must have covered it but the new one doesn't. So there's about half a 5mm hole that is allowing air in past the gasket, hard to explain but thankfully sorted the issue.

Thanks for all the help it was a weak mixture in the end just no one could of known without seeing the actual scoot.