'74 tii stumbles at part throttle

[Aaron R]

Hoping you, the 2002 wisdom sphere, can help me out a little. 

 

I just got my '74 back on the road (again), and it starts and idles pretty well, but has an annoying issue. When it's dead-dead cold, it accelerates smoothly and all seems well. 

 

But once it gets any heat into the engine whatsoever, it will miss and stutter, especially under part throttle. If I blip it from idle to 2,000 rpm or so at a stoplight, it'll backfire a bit. 

 

The pump linkages are all adjusted correctly according to the book, it has a 123 ignition Bluetooth distributor in it, a new Bosch AL41X alternator that is charging, the grounds have all been renewed. It started doing this before the new alternator came along, though it has gotten worse. I put clean NGK BRP6ES plugs in it, which made no difference. I also swapped coils, which did nothing. 

 

The other day the warmup regulator locknut and top hat were gone when I got back from a drive, so I guess the vibrated off. I have a washer and nuts on there now, and the WUR extends to full distance and moves the lever. The miss was there before this happened, though, so I'm not sure it's that. 

 

The fuel filter has less than one tank of gas through it. The oil looks decent. The electric pump is a Bosch from an E28. But since the car runs fine when cold and clears up if you mat it, I don't think it's fuel delivery. 

 

Lastly, I had the head and the upper timing cover skimmed last summer, reseated the valves, and put it new valve seals. 

 

So to recap -- My tii has a consistent, significant miss/stutter/stumble after warmup at part to medium throttle, especially between 2,200 and 3,500 rpm. It runs and revs perfectly smoothly before the temp gauge moves. It has a bunch of new parts. 

 

Any ideas would be appreciated. 

[e31fan]

Just to rule it out, have you checked and R&Red  fuel pressure regulator valve on back of Kpump? Checked front screen at banjo fitting on front of Kpump? Hooked a fuel pressure gauge up and seen what pressure you are getting at idle and when it starts happening? Checked fuel pickup screen in tank? Plug wires?

[Aaron R]

1 minute ago, e31fan said:

Just to rule it out, have you checked and R&Red  fuel pressure regulator valve on back of Kpump? Checked front screen at banjo fitting on front of Kpump? Hooked a fuel pressure gauge up and seen what pressure you are getting at idle and when it starts happening? Checked fuel pickup screen in tank? Plug wires?

Have checked the banjo, but not the fuel pressure valve recently. Plug wires are new, will check pickup screen. 

[Stevenc22]

What sort of timing do you have programmed into the 123?

[Aaron R]

14 minutes ago, Stevenc22 said:

What sort of timing do you have programmed into the 123?

Max of 29 degrees after like 3,500. Set at the factory 25 at 2,400. Checked the timing against an adjustable light, and it's consistent between the 123 and the light. 

[Son of Marty]

Sound like it's running lean when warm, when cold the wur is covering up the leanness.

[Aaron R]

Thank you all. The check valve seems good, the front screen is clear. I'll get into the tank later and check that screen after making sure something isn't blocked, I'll check the WUR adjustment. Again. 

[Stevenc22]

I have a AF gauge and when I set the linkage per the manual, I was lean 14-16. I reset it, keeping tension on the linkage while pinned, which biases it to open more, and the AF went to 12:1

[NMDerek]

9 hours ago, Aaron R said:

Hoping you, the 2002 wisdom sphere, can help me out a little. 

 

I just got my '74 back on the road (again), and it starts and idles pretty well, but has an annoying issue. When it's dead-dead cold, it accelerates smoothly and all seems well. 

 

But once it gets any heat into the engine whatsoever, it will miss and stutter, especially under part throttle. If I blip it from idle to 2,000 rpm or so at a stoplight, it'll backfire a bit. 

 

The pump linkages are all adjusted correctly according to the book, it has a 123 ignition Bluetooth distributor in it, a new Bosch AL41X alternator that is charging, the grounds have all been renewed. It started doing this before the new alternator came along, though it has gotten worse. I put clean NGK BRP6ES plugs in it, which made no difference. I also swapped coils, which did nothing. 

 

The other day the warmup regulator locknut and top hat were gone when I got back from a drive, so I guess the vibrated off. I have a washer and nuts on there now, and the WUR extends to full distance and moves the lever. The miss was there before this happened, though, so I'm not sure it's that. 

 

The fuel filter has less than one tank of gas through it. The oil looks decent. The electric pump is a Bosch from an E28. But since the car runs fine when cold and clears up if you mat it, I don't think it's fuel delivery. 

 

Lastly, I had the head and the upper timing cover skimmed last summer, reseated the valves, and put it new valve seals. 

 

So to recap -- My tii has a consistent, significant miss/stutter/stumble after warmup at part to medium throttle, especially between 2,200 and 3,500 rpm. It runs and revs perfectly smoothly before the temp gauge moves. It has a bunch of new parts. 

 

Any ideas would be appreciated. 

 I google what does mat mean and got car mats, manifold air temperature? Thanks for killing this 🐈‍⬛ 

[Aaron R]

3 minutes ago, NMDerek said:

 I google what does mat mean and got car mats, manifold air temperature? Thanks for killing this 🐈‍⬛ 

Mat it as in “put the accelerator pedal on the floor mat.” Floor it. Open the throttle all the way. 

[dlacey]

as @Son of Marty said, sounds weak when warm. Since you did the head skim last summer, the Throttle body or manifolds must have been removed and re-installed, that likely has disturbed the mechanical alignment of the linkage. You say the linkage is adjusted correctly, all good... now you need to do a warm running adjustment of the mixture with the small screw inside the tuna can.

 

1) warm-up the engine fully

2) open tuna can and mark the small screw with a marker pen so you dont lose track of turns.

3) use a small screwdriver to turn the small screw~1/8 turn anti-clockwise ..we might expect revs to rise.

4) continue 1/8 turn increments and monitor revs each increment.. there may be a few increments where revs stabilise, and then revs start to fall again...once that happens, turn the screw one increment back into that zone of maximum revs.

 

Replace Tuna can lid & test drive... if it still doesnt pull smoothly, try an extra 1/8 turn anticlockwise, but dont go too rich as it will kill your economy.

 

Once setup, these KF synchronisations stay good for years.

[Aaron R]

I have had the tuna can moved enough that the car bogs and dies at idle on one end, and so that the idle rises and falls on the other. Found a happy medium in between where it will idle, but it still has a hard, stuttering miss at part throttle that goes away when you open it up.

 

The air hose that goes from the warmup reg to the intake gets sucked flat when the car is running and warm. Both ends of it are tight, but I haven't had it off to check if it split. It's a new hose from W-N, so the collapsing is a little baffling. The car has blow-by, but the PCV is working, so I wouldn't expect that to cause it.

 

The plugs were fouled with black soot before, when I made the original post, so I don't think lean was my issue.

On 4/8/2024 at 7:49 PM, dlacey said:

as @Son of Marty said, sounds weak when warm. Since you did the head skim last summer, the Throttle body or manifolds must have been removed and re-installed, that likely has disturbed the mechanical alignment of the linkage. You say the linkage is adjusted correctly, all good... now you need to do a warm running adjustment of the mixture with the small screw inside the tuna can.

 

1) warm-up the engine fully

2) open tuna can and mark the small screw with a marker pen so you dont lose track of turns.

3) use a small screwdriver to turn the small screw~1/8 turn anti-clockwise ..we might expect revs to rise.

4) continue 1/8 turn increments and monitor revs each increment.. there may be a few increments where revs stabilise, and then revs start to fall again...once that happens, turn the screw one increment back into that zone of maximum revs.

 

Replace Tuna can lid & test drive... if it still doesnt pull smoothly, try an extra 1/8 turn anticlockwise, but dont go too rich as it will kill your economy.

 

Once setup, these KF synchronisations stay good for years.

 

[dlacey]

The fact that the pipe is sucked flat suggests no leaks... but agree this replacement hose seems too soft for the task, it should be a thick-wall vacuum hose.

4 hours ago, Aaron R said:

goes away when you open it up

That would indicate the top-end is not so bad, so i encourage you to continue experimenting with Tuna Can adjustments, try going leaner maybe? to see if that addresses your part-throttle miss.

 

(all this assumes ignition is properly set) 

[Son of Marty]

Check to see if your cold start injector is shutting off and not dripping.

[Aaron R]

Pulled warm compression numbers yesterday. Not encouraging. Haven’t done a leakdown yet, don’t have a leakdown tester. I guess that’s next, then … head gasket, maybe. 
 

1: 138

2: 120

3: 150

4: 160