1978 Winnebago Sportman

[Catbones]

Hi,

Pretty new here and recently purchased my RV. I've notice that the cluster interrupts itself while driving, like a bump and I loose speed. I was told about this before purchase but apparently it's just tightening the bolt on the back of the gauge? In other words, at some points the arrow falls and I have no clue how fast I'm going nor do the miles spin.

Anyone know how to access the back of the gauge or cluster to tighten the bolt? Do I remove or partially take apart the dashboard or is it accessible from beneath?

Any help would be appreciate it and thank you in advance.