Bike idles but won't rev

[EddieDay]

Following my previous thread on "Low compression, head off" when I had the bike running well, I started it yesterday and had a lovely tick over but the engine would not rev. When I opened the throttle it began to miss and would have stalled if I had not closed the throttle, at which point a nice steady tick over resumed. The obvious issue is a blockage in the main jet of the carb. i.e. it is fine on the pilot jet but there is a blockage in the main jet passage. But...I have opened and cleaned the carb, including the float chamber, filter (both sludged up), the main jet and float needle.
Searching threads on the topic I realise that this is the reverse situation of most posts, where the engine won't idle but runs at speed.

Could this be an electrics issue, e.g. a faulty coil? (The bike is a 1958 coil ignition model). Or do I just need to check the carb again?

My fall back is to replace the coil, HT lead and plug. (The points and condenser are fine). But I have a feeling it is really a fueling problem.

(Spriddler, I started a new thread as I think it is sufficiently different from my original problem )

[SPRIDDLER]

When I opened the throttle it began to miss and would have stalled if I had not closed the throttle, ...The obvious issue is a blockage in the main jet of the carb. i.e. it is fine on the pilot jet but there is a blockage in the main jet passage.

Starting a new thread is helpful for others when using the 'Search' facility.

Its almost the opposite, Eddie. I think first you need to ensure that the pilot jet, which has tiny holes, is clear, particularly since you said the filter etc. was clogged with sludge.
The engine depends upon the pilot jet and cutaway up to about 1/4 throttle. The main jet really only comes into play after 3/4 open.
Also it's always worth/easy to try a new sparkling plug when suffering any sort of misfiring or poor running.

Following my previous thread on "Low compression, head off" when I had the bike running well,

It would be quite a coincidence for the coil to become faulty whilst you were sorting out the low compression issue.

Monobloc jet operation.JPG

Pilot drillings etc, items 7, 8, 9, 10, 31, 32, 33...........A good squirt of aerosol carb cleaner will usually penetrate and clear the drillings in the carb body.

Monobloc Pilot screw side.JPG

Monobloc key to diagrams.JPG

[Alexlib]

I feel your pain , I have been chasing a misfire which until today I was convinced was carburation , I have been changing jets but been unable to fully clean up the running, today the misfire got worse causing me to investigate the ht coil which I now believe is the problem. Like you the bike ticked over and started well for a couple of hundred miles but was getting worse each time I took it out.

[MatthewC]

Sometimes things aren't always what you expect. I rebuilt my Monobloc several years ago including new jets purchased direct from Burlen, but immediately had trouble. On the face of it the main symptom of running at low throttle openings and cutting out above 1/4 throttle suggested a blocked main jet, but it was actually completely the opposite! What turned out to be the problem was that the idle jet was either incorrectly calibrated, or not sealing correctly and allowing extra petrol past the threads, so in fact it was running super rich and fouling the plug within seconds, not enough for it to cut out at low throttle openings, but with the higher cylinder pressures from a wider open throttle the plug was just shorting instead of sparking. Taking a look at the spark plug will help.

[1608]

Timing too retarded or not advancing with throttle.

[g80csp11]

Reluctant to suggest as the obvious , make sure your you are opening the choke and not closing it . I have seen it before when choke cable came out the lever and id not noticed . assumed it was electrical until i noticed the cable . put it back in and everything was fine .

Also others running with choke on , and wondering why the engine struggles

[EddieDay]

Thanks for all your suggestions.
I took out the pilot jet and it was clean. The float chamber emptied itself when I took the end cap off so I take that as a sign that the passage to the pilot jet from the float chamber is clear. Same symptoms. (With choke open).
Checked that the weights on the advance mechanism are free and will open. Checked timing. OK. Checked points gap. No difference.
Changed the spark plug to an NGK B7ES after reading several threads on the topic. It was a B6ES. No difference.
Checked that the connections to the coil were tight and the HT lead in place. No difference.
Just ordered a new coil and HT lead. The old coil is stamped 2016. Not that old IMO but I am running out of ideas.

[Expat]

I too feel your pain Eddie and need to check similar things as you, on my bike. Just got it running as we’ve discussed, then on the second shakedown run at gentle revs after refusing to start for ages, there was a distinct lack of power. Other, unrelated issues followed, grrr!
Good luck with yours.

[jackstringer]

I chased a problem for weeks only to find it was the fuel pipe not allowing enough flow.

[Pharisee]

Thought everyone knew that carburettor problems are usually electrical and electrical problems are almost certainly due to the carburettor.