Guzzino Jumping out of gear.

Hi All,
Well an example of the old saying “pride comes before a fall”. !!..

I was feeling rather pleased with myself having completed the rebuild of my 1953 Guzzino after nearly 4 years of slow progress. Just took it for a test run and it keeps jumping out of first gear. damn damn damn.
No amount of adjusting and fiddling with the gear rod length seems to improve matters !!
Any ideas of a fix without dismantling the gearbox would be very welcome.

It also seems to be very low on power when pulling off in first. But is that just the small engine ?

Cheers Bryn