power slippage

Hi all,

'66 2a 109", 2.25 petrol; engine recently rebuilt, all new clutch components, gearbox rebuilt. All has been well and good for about 1,500 miles.

But, recently, I seem to be losing power in gears 2/3/4 when the clutch is disengaged. When starting from 1st, all seems normal. Then, when upshifting, if I press the accelerator as normal the engine revs as normal but the power seems to not be being transferred to the drive wheels, at least not immediately. Instead, the engine revs, and the power seems to be lost for awhile...that is, the engine is revving (in response to the accelerator) but the truck is laboring to find the power. If I back off the accelerator, I can avoid this -- so, if I accelerate very slowly after a gear change, the power will ramp up slowly and do its job. But, if I accelerate too quickly (that is, in the manner I always used to accelerate, which is not quick at all), the engine revs but the truck doesn't speed up at first. If I maintain the power to the engine when this happens, eventually the truck will "catch up" to the revs and the engine power gets transferred to the wheels. It's as if I have the overdrive selected when I'm in gears 2 / 3 / 4 (I do not).

To test the clutch, if I start from a stop in 3rd or 4th gear, I feel no slippage and the truck stalls (as one would expect). Also, as mentioned, the problem does not seem to occur in 1st gear, when cold or warm.

Any ideas as to what is going on?

Thanks in advance.

that is among the better and detailed diagnosis's i've seen in a while!
My guess is the actuating rod for the clutch is at fault? this doesn't explain the explanation for 1st gear though, However, first gear is pretty low and may not slip if the clutch plates are just hardly touching.
either way, I'm figuring it's in the bell housing, there will be better than I come on here with ideas soon probably.
I wonder if there is something slowing fluid return to the resevour or perhaps some air pocket that heats up lessening the clutch pressure?

Did you replace the clutch master cylinder? A while back there was a huge batch of CV master cylinders that were just a little different than the original ones. The difference was that they needed more adjustment/slack? when releasing the pedal than the original ones. I think it might have been a difference in where the pressure return hole was? Not sure. But, when used on the brake system, and setup exactly like the one you were replacing, they would hold the pressure. I had one on an 88”. It worked fine, until about 5-10 pumps. Then the pressure would be so great, it would lock all the brakes. The fix was to let the master cylinder return a little farther than the old one. That allowed the pressure to “bleed off” back to the reservoir. If one of these were used on the clutch side, it would have the effect of keeping the clutch in, or mostly in. The good news is that there is an easy check for this. Pump the pedal up about 10-15 times. Then slightly crack/open one of the fittings/lines on the clutch line. If fluid comes out under pressure, with the pedal up, then that’s a problem.

Other issues could be linkage related as mentioned or something actually wrong with the clutch.

Thanks very much for the replies. The clutch master was replaced in the past and, tested as you note, seems to be fine.

I resolved the issue by merely adjusting the clutch slave cylinder actuating rod (about one full turn counterclockwise, as viewed from below, as reference for those with slippage problems in the future). In the past I've looked into the black arts of clutch slave adjustments and note, too, that the green bible claims that late type clutch mechanisms (like mine) require no adjustment. Well, what I did worked so I'm happy (for now) -- as it stands, what (limited) engine power there is is now going to the drive wheels as it should.