Power Slipping

Soft_Click

Member
1966 IH 1200A Pickup, SV304 engine with 4-speed T98 transmission.

I finally got my timing dialed in and carb tuned nicely so I’m idling at 18 vacuum. Starts and idles great, accelerates immediately and revs pretty high. Haven’t really pushed it to the limit.

I went for my first (somewhat) successful drive since I bought the truck, which was a rush. Unfortunately I couldn’t really accelerate beyond 10 mph. My engine would speed up, but the wheels wouldn’t.

I’ve already pulled the transmission and inspected the clutch which seems to have plenty of life (see attached). I also got the flywheel resurfaced and reassembled everything.

I couldn’t loosen the bleeder nipple on the slave cylinder and stripped it even with my wrap around wrench, so I replaced it. I adjusted the clutch linkage so there’s no play at all.

I think it’s both not releasing the clutch all the way and not engaging all the way somehow.

Some potentially relevant symptoms:

Engine idling, everything warmed up. Clutch in, wait a few seconds for transmission to stop spinning, grinds into first gear. If I got second then first, no grind. It’s still spinning I think.

Rolling downhill in clutch pedal depressed in neutral and there’s a distinctive whine I haven’t seen before. Maybe the engine being driven by the wheels?

Clutch only engages right at the top. I’d really like it engaging closer to the bottom of the pedal stroke.

The big on of course is I go to accelerate and it grabs initially, then seems to slip above 10mph.

Any ideas?
 

Attachments

Last edited:

Soft_Click

Member
I had this problem before checking the clutch and didn’t adjust the pressure plate at all, just left it and torqued it to spec. I don’t know much about the pressure plate, but I know with a new clutch disk it needs to be reset? Just guessing.
 

Soft_Click

Member
I gave myself a little slack at the clutch fork and that eliminated the grind into first. Getting a little better. Still not getting the power down. And reverse makes the whole truck hop around... lots to do haha
 

Soft_Click

Member
I was just inside the transmission a week ago and found some small metal shavings that I cleaned out. My reverse gear is pretty chewed up already, so I was thinking that’s where it’s from.

Should I try to drive in second gear to rule out 1st slipping?
 

FDChappie

Active member
Gears, as a rule, don't slip, they strip. Or with missing teeth they skip. Did you try the transmission in the different gears while it was out?
 

Soft_Click

Member
I only ran it in reverse and first.

reverse was super jumpy, mostly because I don’t really have room to engage the clutch partially, it’s like all or nothing at the top of the pedal. Of course I didn’t go over 10mph in reverse.

First engages all the way at the top and seems like 0-10 could be normal. Above 10 all the power slips away.

I can try it in second tomorrow if you think that could make a difference.
 

Zaedster

Future Owner of IH Parts America
Post a video of the clutch throw-out bearing engagement, with someone filming and another pressing the pedal. If the gears were stripping, you would clearly hear it. Sounds like a clutch engagement issue, don't continue to operate the truck as you are probably causing more damage to what's already happening.
 

1975IH200

Member
10 mph is about the limit of speed in 1st gear and Reverse gear due to the gear ratios of the T-98 manual transmission.
Try driving it normally by shifting into 2nd, 3rd and 4th gears as vehicle speed increases.
 

Soft_Click

Member
Here’s the view inside the bell housing:

confirmed the clutch is installed the way I said it was, which I believe is correct.

Maybe 10mph is just the limit in first! That would be wild, but I’ve never driven a 1966 before haha
 

Zaedster

Future Owner of IH Parts America
Here’s the view inside the bell housing:

confirmed the clutch is installed the way I said it was, which I believe is correct.

Maybe 10mph is just the limit in first! That would be wild, but I’ve never driven a 1966 before haha
Can you adjust the throw-out bearing further out? Looks like it's still applying load when released.
 

Zaedster

Future Owner of IH Parts America
I could spin it by hand while I was down there. I can also push the clutch fork back by hand a little bit.
Since it all checks out, I would recommend operating the gear shifter device located on the transmission tunnel. 1st gear is very low, 2nd is good for starting out, see if the speed increases.
 

Soft_Click

Member
Can you adjust the throw-out bearing further out? Looks like it's still applying load when released.
This may be the solution!
Initially, it was too close, which is why it whined going down hill in neutral and maybe why it slipped. I backed it off a bit before taking that last video, then today I backed it off a lot. I realized the farther away I adjusted the throw out bearing, the lower down on the clutch pedal and as long as the transmission is completely disconnected from engine power by the time my foot hit the floor, I think it’s okay.

I couldn’t test drive it today because it’s raining and I don’t totally trust this thing in dry conditions yet, but I did drive 20 feet forward and back into my garage. The clutch is grabbing near the bottom, which makes it much easier to drive. I accelerated quickly and chirped the wheels which could be good news.

I’ll know for sure when I really drive it once it stops raining. Thank you!
 

Soft_Click

Member
I just got back from a nice test drive, things are improving, but not quite road-worthy yet.

I took Zaedster's suggestion and threw it into second gear once the power started slipping. I had power once again and got up to 15 miles an hour when I felt it slipping again, I threw it into third and the engine stalled!

I would say I have a usable power band of 800-1800 RPM, just guessing. I haven't installed a tach yet. I think it has to be somewhere between the engine and transmission. maybe my pressure plate isn't applying enough pressure?
 

Zaedster

Future Owner of IH Parts America
Are you sure the carburetor is operating correctly? These engines don't have enough power to slip clutches that easily, especially an original one.
 
Top