I ahve a 78 CB 400T that when I bought it the previous owner told me he had replaced the clutch. However I don't think he was very mechanically inclined at the look of the rest of the bike. Here is the problem: when the bike warms up and I crank on some pwer the clutch locks and won't disengage. No matter how much pressure I put on the lever it won't budge. I just have to cruis around and ride it out for about a minute then its OK. I have changed the oil with proper M/C oil as I thought the PO may have used car oil and that was causing the problem, it still does it but not nearly as much. Any ideas of what I should do?