Cold start issue

[Derek996]

My B32 has a 376 Monobloc and has always been a good starter hot or cold. From cold usually by the 2nd or 3rd kick but recently has become very difficult to cold start. I go through the usual procedures fuel on, 2 or 3 tickles, partly retard the igniton and full choke. It fires quite readily but won't pick up. I can eventually get it going after many attempts. Each time it will run a little bit longer but just won't accept any throttle at all. When I eventually get it to run it idles and runs perfectly. I've tried both richening and leaning the idle mixture to no effect. I'm a bit stumped with this, given that nothing was changed, and would appreciate any thoughts

[rhyatt]

My 1st thought is check the magneto points .012"

[Derek996]

Magneto points are spot on, it's a newly rebuilt mag.

[rhyatt]

Ok , so hopefully a good spark and no suppressed ht cap at the plug.
Has it previously run fine ( you've mentioned that it started fine.

My next thoughts - if it has been standing idle for a while - is muck in the pilot passage/jet.

This controls a lot of the low speed and transition the the throttle slide , which must be fine as you've said it goes well after you get it going.

[Derek996]

This afternoon, I have changed the spark plug, removed the pilot jet and check it was clear, checked the points gap and igntion timing. All OK.
Last year the bike started easily hot or cold. This year after the mag rebuild it fired 1st kick and started on the 2nd. Over the last 3 weeks I have done some work to replace the gearbox gaskets, it has the dreaded lightweight box and was leaking badly after replacing the kickstart return spring last year. I also adjusted the clutch and replaced the oil return pipe. None of these whould any affect on cold starting. Yet once I get it started it runs fine 

[rhyatt]

You've had it a while , so conversant with "up to tdc etc on the starting routine, so it's not that.
Stale petrol after a short lay up.
Carb sounds fine.
Has anything else happened that could interfere with starting.
Top of carb screwed on ok ?

[neil1964]

I had a similar issue last year.  I'd pulled my ZB34 engine to install the ABSAF 600 in my 'daily' bike a couple of years ago although running well with 30mm concentric (relatively new with modern float, viton needle, coated slide etc) with burgess silencer. Installed said engine/carb/magdyno into my 57competition rebuild. 
Bike would start first or second kick (usual ritual - tickle carb, half choke, half retardant, couple of priming strokes and long swing). But it would die after a few seconds or if I opened the throttle slowly. Whack the throttle open it would run until I backed off when it would stall and refuse to restart at all!
Only difference was the Armours mini GS upswept silencer.
I ended up stripping carb at least x3, flushing the lines, cleaning tank and filters (even swapping petrol tank for aluminium one I had). Drilled fixed jet out and went from 25pilot up to 35. Various manifold gaskets, compression tests, adjusting valve clearances and decompressor. Experimented with plugs (new/old, champion vs NGK) HT wires and plugs. Like you the mag is one I had previously rebuilt with rewound armature (gentleman in Bristol) and an easy cap. Points 12thou plug 18thou non resistor cap etc.
Got it running with 35 pilot and went for MOT (15 miles on trade plate) and had couple of misfires for few seconds and it cleared revving freely. Sailed through MOT  but then refused to start!
New plug and off. 
No problems since, although now have oil leak from head gasket around pushrod tube area!
Loathe to interfere now but I've had PP build me a solid crank with new Carillo rod so a future rebuild is planned.
In my case I think it was combination of pilot jet wrong size for new exhaust possibly marginal plug.  One thing I should have done is accurately check float height (in the way that it influences pilot jetting) as all I was concerned with was blockages, leaks etc. Also not sure how well modern plugs like old thumpers with flooding as method of cold starting.

[Derek996]

Well I've tried several plugs including a brand new unused one with no improvement. I'm fairly sure it's something to do with the carb. I've had the pilot jet out to make sure it's clear but it could be one of the air passageways that's blocked or partially blocked. I doubt if it's a float level issue as that wouldn't change all by itself over a few weeks. I think my next move is to strip the carb and make sure everything is clear. There is a tiny passageway at the engine side of the carb opening just behind the bottom of the slide. That will be my main suspect.

[Derek996]

Yesterday I stripped the carb and although I couldn't find any blockages I made sure that all the internal passageways were clear. I put it back together and refitted it to the bike. I got it to start but still rather reluctantly. Once it was warm I adjusted the idle mixture for optimum running and the idle speed as slow as it would run consistently. I'm think I should also have checked the float level, although I don't see how that would have changed. I will see what happens on the next cold start.

[Tigerfeet]

I had a lot of trouble last year with another sort of four stroke engine. Eventually after much carb stripping etc I found it was operator error. The standard cold start is prime then full choke - and in hot weather the engine nearly shook out of its mountings - whilst refusing to pick up /clear. Since that saga, I and my mates with similar engines, do not even look at the primer or choke if the ambient temperature is up into the 20C + range.  And the engines start perfectly on the button. Might be worth trying...