RPM drop in 1st under load (FJ60) (1 Viewer)

[Slow Left]

I like your idea about going back and rechecking valve clearances and running the leak down test first.

I'd be slow to come to the conclusion that the head needs to be removed and rebuilt until there is proof worthy of such an indictment.

Problem is, I can't give you the exact advice you need to prove or disprove.

 

[vszUnclear]

Thanks Slowhand. I've checked the valve clearances on two separate occasions. I guess I could check them again or have a pro do it.

Like a poker hand, I feel like I'm in this 2F pot pretty deep. I've rebuilt the carb, manifold gasket, Jim recurved dissy, galley plug, etc. I think pulling the head will be tough but it's probably something I'd do at one point here anyway since it's unlikely that I'd swap for a Chevy. Learning experience and all.

 

[Slow Left]

Understood.

Guess I feel the same way about my rig. One step at a time.

 

[Slow Left]

...but since you seem to be comfortable with handling uncertainty (and a glutton for punishment), I ran across this thread earlier (trying to resolve my own conundrum...):

https://forum.ih8mud.com/threads/hesitation-on-acceleration-with-trollhole-carb.601437/#post-7708327

The thread talks about setting the accelerator pump and secondary valve on the carb. I have purposely avoided making any radical adjustments to such linkages on my carb (after having rebuilt it) simply because I'm working through and making sure that everything is working correctly. But after having read this thread, I might go and check the travel on the accelerator pump. Should be a straightforward measurement.

Am wondering if, on your carb, the accelerator pump is withing travel tolerance (9.5 mm travel), since fuel starvation under load could explain the symptoms you are seeing, too. Just another idea.

 

[vszUnclear]

Slow left, I'll definitely check the AP. I meant to do it when rebuilding the carb but rushed a bit there at the end. Far easier than tearing down the head.

It's the oscillating vacuum though that troubles me. I'd welcome any advice to the contrary of course.

 

[Slow Left]

How did you hook up the vac gauge?

In other words, where did you port the vac from on the intake manifold?

 

[vszUnclear]

It's a good question. I took Mark's advice in this thread, except that I T-ed into the large brake booster hose for vacuum (he used the smaller hose).
https://forum.ih8mud.com/threads/aftermarket-guages.55911/#post-630320

Since they both go into the same hole in the manifold I figured it's fine.

 

[FJ40Jim]

Just read this thread, here's random notes:

Dunno what's going on w/ a BVSV, but the ported vacuum connects directly from carb to dissy vac advancer. The BVSV is only used for the canister purge system.

Pull vac gauge reading off the manifold vac port at the top center of carb. The PB fitting is not good because it sees cylinders 5-6 much more than 1-2, which causes the jittery needle.

The symptom (carb bogs on tip-in) is probably due to a lean mixture at that small throttle opening.
A few possible causes for this...
1. Lame AP, which doesn't give correct fuel for the rapidly revving engine in 1st gear. In slow revving gears i.e. 4th on the highway, the AP is much less important.
2. lean stumble because idle mix is too lean, carb is running out of fuel at the top of the slow circuit.

 

[Slow Left]

That smaller hose goes to the A/C idle up. I have my vac gauge t'd into there, rather than the larger, brake booster hose.

The thing is, your vac reading at idle seems low to me. I can easily get 21 inHg vac at idle.

So what is making that 5 inHg in difference? Could be a vac leak, but could mal-adjusted valves also cause it? Leaky valve guides?

By the way, that increase in vac you see when going down hill in gear is normal. You noted a 22 inHg reading. I get up to 25 inHg (with my 21 inHg at idle).

When you accelerate uphill, you should see the vac drop to around 2-5 inHg, and then, as the load decreases at the top of the hill, the vac should go up to around 15 inHg while holding the throttle steady.

Have you put vac on the brake booster diaphragm using a hand held vac to see if it holds vac? That would be a possible source of vac leak, too.

 

[vszUnclear]

Thank you Jim! You just saved me a lot of work that wouldn't have paid off (apparently)

Just read this thread, here's random notes:

Pull vac gauge reading off the manifold vac port at the top center of carb. The PB fitting is not good because it sees cylinders 5-6 much more than 1-2, which causes the jittery needle.

Doh, I thought I was on pretty firm footing with the brake booster tap, but all assumptions need questioning.
To be clear do you mean the port pictured here?

The symptom (carb bogs on tip-in) is probably due to a lean mixture at that small throttle opening.
A few possible causes for this...
1. Lame AP, which doesn't give correct fuel for the rapidly revving engine in 1st gear. In slow revving gears i.e. 4th on the highway, the AP is much less important.
2. lean stumble because idle mix is too lean, carb is running out of fuel at the top of the slow circuit.

OK, I will get on those two. Jim, I just rebuilt my own carb and replaced the AP in the process. I noticed though that both the old and new AP were tough to push down into the hole. I realize there is a spring under it, but it was more than that - a friction resistance along the leather perimeter of the AP as it gets deeper into the hole. Is that normal?

 

[FJ40Jim]

Yes, that's the correct port.

You didn't replace the AP. You replaced the plunger, which is one part of the AP circuit.

There can be to much friction between plunger and bore, which will cause AP to not move at all or move slow and not get a full stroke. OR the AP overtravel spring can be too weak, or the starting position can be too high, or the stroke can be too small or the nozzle can be restricted, or the check ball seat can be corroded, or....

 

[vszUnclear]

That smaller hose goes to the A/C idle up. I have my vac gauge t'd into there, rather than the larger, brake booster hose.

Interesting, I'm guessing you see steady needle? Jim's previous post indicates that this fitting may cause oscillation in general.

The thing is, your vac reading at idle seems low to me. I can easily get 21 inHg vac at idle.
So what is making that 5 inHg in difference? Could be a vac leak, but could mal-adjusted valves also cause it? Leaky valve guides?

Right, so I probably still have a vac leak on top of a crappy AP or idle mixture.

Have you put vac on the brake booster diaphragm using a hand held vac to see if it holds vac? That would be a possible source of vac leak, too.

I'll have to check that. I need to find where the diaphragm is located first...

There can be to much friction between plunger and bore, which will cause AP to not move at all or move slow and not get a full stroke. OR the AP overtravel spring can be too weak, or the starting position can be too high, or the stroke can be too small or the nozzle can be restricted, or the check ball seat can be corroded, or....

Jim, I will be in touch to get you my carb if I hit my limits without finding the solution. I'm very excited to have the drag net narrowed down though.

 

[Slow Left]

I do see some oscillation in the needle on my vac gauge, but only about 1 inHg. But yes, according to what Jim notes, having my vac gauge t'd into that union at the back of the intake manifold will bias the accuracy of the reading in favor of what is going on in the 5 adn 6 cylinder...

You'll know soon enough (after you switch the vac line to port off the carb) whether or not your vac measurement is instilling unwarranted concern.

I would still assume that you do have a vac leak or untuned component somewhere, accounting for vac readings of 16 inHg at sea level.

The brake booster diaphragm is inside that black, oval shaped metal thing on the firewall right behind the brake master cylinder. In fact, to measure whether it holds vac or leaks, just unattach the brake booster hose (the one you have your vac gauge t'd into) at the union on the intake manifold, and attach the handheld vac there. Then pump like hell (large volume, so it will take a bit of effort to pump all the air out...)

 

[Slow Left]

I ran a search for past threads on brake booster troubleshooting:

https://forum.ih8mud.com/threads/brake-booster-check-valve-s.568755/#post-7415991

https://forum.ih8mud.com/threads/brake-booster-s.490030/#post-6694496

https://forum.ih8mud.com/threads/should-brake-booster-hold-vac-w-mightyvac.459999/#post-6375307

https://forum.ih8mud.com/threads/brake-booster-question.88639/#post-1066808

sounds like a quick test on the brake pedal will tell you alot about how the brake boost system is working (or not).
Also seems that the check valve on the brake booster is more often than not the culprit for vac leaks.

You probably already saw this thread on Vac in general, but here it is:

https://forum.ih8mud.com/threads/more-vacuum-stuff-search-already-done.153108/#post-2121839

 

[vszUnclear]

OK I T'd into the center top carb port Jim suggested and while the vac needle is more steady, it still has some movement. Now instead of the crazy back-and-forth oscillation, the needle drops from 16 in by about 2" a couple times per second (at idle). Would love to know what that means. The guide Slow Hand posted seems to indicate a valve issue still, but I'm not confident in that diagnosis.

I think my brake booster is fine based on the pedal tests I did. Thanks for pulling together all those links.

I'm going to check on the AP and the idle mix shortly.

 

[vszUnclear]

The plot thickens, and I feel stupid for not checking this earlier...

Some people suggested my problem could be misfire related. While idling, I pulled each spark plug wire one by one.
Cylinders 1,2,5, and 6 all degraded my idle when the wire was pulled. Cylinders 3 and 4 made no impact to idle.

I had a couple of extra denso plugs in the rig so I swapped them in and did my test again: cylinders 3 and 4 made no impact.
The old spark plugs were dry (not damp).

The wires are new/good, I could see them sparking when I pulled them out and held to ground. My plugs are only 3 months old and I swapped them with spares so that can't be it either. Dissy has a new cap, the dissy was tested/rebuilt by Jim recently. Last time I checked, compression was 140, 140, 138, 139, 137, 145 PSI

First - can anyone confirm that cylinders 3/4 affect idle (I can't imagine they wouldn't but who knows...)

Second - what do you guys recommend I do now? Leak-down tests?

 

[FJ40Jim]

Pull all 6 plugs, see what they look like, run compression test.
Post results.

 

[vszUnclear]

Jim, thanks for helping with this. The compression results seem grim:
120, 110, 5, 15, 100, 117

I've also attached pics of the spark plugs.

I am not an expert but this can't be good

 

[vszUnclear]

the last plug

 

[Slow Left]

The first set of compression readings you posted looked pretty good? When did you take the readings for the first set?

The second set of compression readings (the ones you just posted - I'm assuming you just took those readings within the last day or so) look bad?

How did you take those compression measurements?

Was there any difference in how you took the measurements the first time versus how you took them most recently?