Car limited to ~20mph... Fuel? (no longer Clutch slip)

[DaveR]

EDIT: This is reference, but I don't believe it's the clutch. More likely a fuel pressure or delivery issue. Maybe in the carbs as well. Everything replaced and carbs rebuilt, so its pretty surprising to have this kind of issue. Need to get a fuel pressure tester...

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Something strange happened today that was new.  After driving to work in the morning and things feeling pretty normal.  It wasn't an aggressive drive at all. On the way home there was a lack of power transfer at certain RPMs/torque range.  I first noticed it going up a hill, the car seemed to be struggling to have the power to push up it. It originally felt like it was in too high a gear and didn't have any torque available, but even in lower gears the same thing happened. Just push the throttle, engine sound might change a bit but RPM's and speed cap out very slow. ~20-25mph tops.

I've never felt a clutch slip, I suppose I would have expected the engine to continue to rev higher but the speed not increase accordingly.  The engine really didn't rev higher, even though I could hear it trying to do so.  If i disengage the clutch the engine rev's happily.

I ended up having to limp home in higher gears but super low RPM.  I couldn't really get the car over 30 mph or so, the power just wasn't being transferred. Also whichever gear I selected I really couldn't go beyond 2k RPM or so without getting no power transfer.

Very recently I did a clutch slave adjustment. I was trying to back it off at the slave so there was a little play with the clutch pedal fully up. (so I wasn't putting undue pressure on the diaphragm spring)  Maybe I went too far? I would assume this failure mode would be that I couldn't disengage the clutch enough to change gears, not that the clutch would then slip.  Am I all backwards?

I suppose I could have backed off the adjusting nut too far and there is now more clearance than there should be between the withdrawal lever and the release bearing. But again, I wouldn't expect the clutch to slip in this case, just that I couldn't properly change gears.

Edited July 1, 2017 by DaveR

[Zed Head]

Good idea to check what you last worked on, but your symptoms don't really sound like a slipping clutch.  A slipping clutch will make more engine nose as  RPM increase but the car won't go any faster.

Lack of power seems more engine related.  This might sound too simple and obvious but check your throttle linkage, even the carpet under the throttle pedal.  Could be that your foot action wasn't translating to throttle valve opening.  If you open the hood and contort you can see the end of the linkage at the throttle body, to see if foot movement = blade movement.

The engine doesn't require much air when it's out of gear to rev freely so that could be misleading.

Check your air filter and inlet also.  A plastic bag or other debris could have been sucked in.

Edited July 1, 2017 by Zed Head

[DaveR]

Okay I'll scope it all out. Was very strange indeed that this morning it was fine and this afternoon was no good. I've driven maybe 200 miles on it since rebuilding basically everything so it really must be either something silly like that, or my poor clutch adjustment.

[siteunseen]

What's the clutch pedal to the floor measurement, 8.25"?  0ff the top of my head maybe 7.25"?

280 right?

[DaveR]

240. And I either checked or adjusted the clutch pedal inside the cabin to the factory pedal distance. 

 

[Mark Maras]

 This sounds like lack of fuel to me. I had a similar problem years ago when my Z wouldn't go over 45 MPH, in any gear. Engine revved freely to redline, in neutral. I limped it to my Dad's house. After explaining the symptoms, he explained that regardless of what gear you're in, it takes virtually the same amount of gas to go a specified speed. We went out, opened the hood, blew out the filter on the fire wall, started it up and the problem was gone. I drove it for a couple of days before replacing the filter. No problems. Easy thing to check before you get into things too deep. If the filter check doesn't solve the problem I'd check the other filters, and fuel pressure and volume flow.

[DaveR]

Few more facts to add to the mix.

1) Air filter and intake are clear. Don't think it was starved for air. Carb pistons move freely as expected.
2) Clutch slave had been replaced with aftermarket. There is no return spring (not sure if it is required with this slave).  The piston extends outward automatically to meet the withdrawal lever, so any of my adjustments to the nut aren't going to mean anything... unless of course I need the return spring.  Either way I adjusted the nut out so the piston on the clutch slave is basically at the start of its travel, like it would be with a return spring. The auto adjusting outward force is not strong at all so I don't think its significantly preloading the clutch.
3) I have a secondary inline fuel filter directly by the tank for before I knew I was going to refurb the entire tank.  A Fram G3 inline. It was a little smushed over, possibly could have been restricting flow some although it's been there all along.  Since I didn't need it any more I removed it, maybe this is my fix.
4) I will check the OEM fuel filter as well, but given the G3's presence before it I really doubt anything made it that far.
5) All the other fuel lines in the whole car are new, as is the mechanical fuel pump. 

I'll try to drive it tomorrow and see if anything has changed.

Probably unrelated, but my Clutch Master Cylinder's fluid is like dark brown/green.  Uncool since I replaced the clutch hose last year and bleed that system. That's a really short window for it to turn murky.  Hmm

Edited July 1, 2017 by DaveR

[DaveR]

Well, blew out the other fuel filter by the fender and still no luck.  Fuel pump is new (Carter), but I suppose maybe something is happened there.

My next course of action is to check fuel pressure and flow post pump and see what it looks like there.  Then maybe I'll double check the carbs (newly rebuilt by me) are okay. I know the linkage travel is good, and the pistons seem to move freely, but I'll take off the domes and peak around.

There's not much to go wrong in this car so hopefully I figure it out soon.  I don't believe the clutch is slipping anymore at least, and in the process I think I got rid of some leaks in the clutch hydraulic line.

[Zed Head]

If I read the operating description in the FSM right, the pistons control air flow and the pistons are controlled by intake manifold vacuum.  So, basically, if the pistons don't rise to let more air in then the engine is effectively throttled.  I'd like at where the pistons vacuum source is, and if it's possible that it's blocked or lost.  Maybe the carbs are loose, causing a big vacuum leak, or there's a fitting or plate that's loose.  Seems like the pistons must not be responding to intake vacuum is what I'm implying.  They're probably open a certain amount for idle and low RPM, so you do get some response.

It's a neat concept, but the carb's air opening is controlled by a secondary mechanism, the piston, not a primary one, like a throttle blade.  If your throttle linkage is working, seems like it has to be the pistons.  Assuming that the other engine parts are correct, like ignition and cam timing.

I could be way off, I'm just using your problem to learn SU stuff.

[DaveR]

Assuming something from the recently refreshed fuel tank didn't break free and make it past 2 fuel filters to clog up something in the fuel delivery lines, or the brand new fuel pump failing within 200 miles...

I would point towards the carbs as well as the likely spot for a potential issue.  I'll take them apart and make sure my pistons are moving smoothly, my jets are unclogged, my last chance filters are clear and my floats are okay.

First up is definitely the pistons. The throttle opening creates the vacuum that drives up the pistons, but if they hit a snag and don't go up any higher then your speed will certainly be capped.  This is probably one area that I wouldn't be too surprised to have an issue as getting the originally to travel smoothly was a bit tricky.

Thanks for the help all.

Edited July 1, 2017 by DaveR

[DaveR]

Did some work on it and drove it again. It definitely pegs around 20-25 mph.  I can floor the throttle and the rev's and speed increase but just BARELY.  Up until that point it sounds and responds normally, its like it hits a limiter or something right at that speed.

Crossing things off the list still. There's fuel everywhere it should be and all the filters are clear. 

Carbs look really good, everything slides smoothly, all very clean and tidy. Jets are clean.  The fuel levels in the bowls might be low, and slightly mismatched, but never gave me a problem when tuning it.  If the consensus is that low bowl fuel levels here could cause a sudden onset issue with fuel delivery I can adjust them, but I don't imagine this is the case. 

I have not YET tested fuel flow and pressure, as I do not yet have the means to do so. I suppose that's next up on the list.  Still seems crazy the fuel pump could have failed so quickly.

One thing I'll do is check the fuel lines for any potential sources of air, especially on the suction side of the fuel pump. The fuel hoses are all new and with hose clamps the first tightening seemingly always needs to be re-tightened later after the rubber relaxes.

Edited July 2, 2017 by DaveR

[siteunseen]

My floats were bad off and the car was starving for fuel. Couldn't climb a hill and eventually got so bad it wouldn't go over 30mph. Had to gradually get to that speed.

If you could put a glass filter before the oe fuel filter you can see trash from the tank and if it's not full, has air in it, your bowls aren't either. When the floats are right the filter will be full of fuel, at least on my car with a mechanical pump.

Good luck.