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Y Block Idle RPM

Posted By Tedster Last Month
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Tedster
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This is more of a general question, or maybe an observation on the factory Tune-Up specs. I've noticed that many if not most engines seem to "like" a bit higher RPM? The Y-Block factory spec was 475-500. Does that seem a little bit low, do you tend to set the idle RPM higher on your Y Block?

A well tuned factory cammed engine in good shape "should" idle smoothly down low without complaint. Doesn't mean it needs to remain there, but it is actually a kind of diagnostic I suppose. In that sense it is pretty common to see people try and compensate for different engine derangements by ramping up the idle speed.

The reason I ask is I've been going through the 2100 carburetor on my Y and discovered that the throttle rod and throttle arm itself wasn't staked or secured properly, at some point in its life bushings were installed to clean up a vacuum leak. But whoever reassembled it, the result was quite a bit of slop in the movement to open and close the throttle blades. The butterflies seem to fit the bores very well, but in the past it was difficult to achieve a consistent factory idle down low, even for test purposes and I kind of gave up. At 600 or so it idles absolutely smooth. Having made what I think is a lasting repair to the throttle assembly I'm interested how it will turn out when things warm up a bit.
Dobie
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My '55 Merc likes about 600 RPM. I don't know if it has a stock cam. If it isn't stock it's very mild.
Ted
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Engines in tip top shape will have a better manifold vacuum and those will idle better at low rpm versus those engines that are worn thus not having as good a manifold vacuum and/or a loose or worn timing chain that retards the camshaft also affecting the manifold vacuum.  A higher idle speed produces a better carb signal as a result of ‘improved’ manifold vacuum so that would be a Yes in that a higher engine idle rpm would produce a smoother idle.


Lorena, Texas (South of Waco)


57RancheroJim
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My stock cam 292 idles great between 500-525. My other 292 with Isky E4 does better at 600, both being stick trans.. Auto trans cars usually need a little more than sticks
bergmanj
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Folks,
I'm definitely dating myself here; but, will go ahead anyway: In 1963 I was a High-School senior and my dad's 1954 Ford 239 OHV engine was getting "tired"; so, I proposed to him to let me "rebuild" it. Surprisingly, he agreed - what the H___ did a 16 year-old know???  Well, I took my time and did it "right": That engine would start in the very coldest weather (-45 F here in N MN in winter) at one fan-blade part of a turn on a three blade fan!

To the point, though: In experiment, I was able to get it to idle smoothly down to around 300 RPM (maybe less), still pulling a high  intake manifold vacuum, and you could still stand a nickel on edge on that manifold - a very proud "kid" at that time. Put it back up to factory idle RPM's after experimenting though, to preserve cam lobes on their oil cushion.  I've since built a few more "nice" Y-blocks, also with really good results.

A really good Y-Block can run smoothly at very low RPM's.

Regards, JLB

55 Ford Crown Victoria Steel Top
Tedster
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bergmanj (2/21/2021)
.

A really good Y-Block can run smoothly at very low RPM's.

Regards, JLB




Sounds like the same conclusion I arrived at. Since I had trouble achieving a smooth idle at low RPM early on eventually I learned that could only mean defects in ignition quality or ignition timing, manifold vacuum and valve timing etc, as Ted mentioned.

Also a consistent valve lash adjustment and proper valve spring pressure improves idle smoothness I have found. That's when "balancing nickles" started to be doable! Still, was starting to run out of possibilities.

Have been chasing this one for a while, it otherwise runs excellent but was one of those annoyances that by definition indicates "sumthin' ain't right". It wasn't so much the idle wasn't smooth at 450 or 500, but It sure didn't seem to like it very much - the butterflies would seem to "stick" in their bores maybe, the idle simply wouldn't return back to the same RPM twice in a row after revving down low.I think the huge amount of slop found in the throttle lever would also interfere with keeping the transfer slot covered properly.

As an aside I've always admired the idle note of a straight 8 when tuned correctly ... thump ... pause ... thump ... pause ... thump ... That's just great!!
FORD DEARBORN
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I can understand having an unpredictable idle speed if the carb throttle assembly is not properly staked on to the throttle shaft. Many years ago I used a bit of JB weld to fix that on a 2100 carb.  My personal idle rpm for my Y-block is 600 manual trans.. Not that the engine can"t idle smoothly at a lower rpm but at 600, more oil should be slung of the crank delivering more oil to the cam lobes and also tends to bring the charging system into the positive range. Tedster, , were there any issues with a slight stumble on acceleration? If I can remember, the loose throttle arm caused the accelerator pump to lag slightly, in my case.


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DryLakesRacer
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Been chasing mine since I installed the PCV system. Still have 1.5” on the crankcase at in gear idle. After a get together with like minded folks this morning I’ve got it where I can live with it at 600 in drive. Manifold vacuum is 17 inches and the carbs are in sync. Found one tiny vacuum leak on the rear choke housing.

56 Vic, B'Ville 200 MPH Club Member, So Cal.
Tedster
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FORD DEARBORN (2/21/2021)
ITedster, , were there any issues with a slight stumble on acceleration? If I can remember, the loose throttle arm caused the accelerator pump to lag slightly, in my case.




Yes, though intermittent. Or, maybe I'll be surprised how much better it runs? That's usually how these things go. I think the loose throttle arm was getting worse over time of course too. I did use JB-Weld for the repair as well, I think it should hold up in this application. Should be almost 40F later this week, I will probably set the curb idle and idle mixture and such then, if not actually drive it.
Tedster
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Turned the engine over by hand a few times to fill up the carb bowl, made sure it had several good accelerator pump shot squirts and it fired right up.

It does idle down low now pretty nice. One thing though I'd kind of forgotten about is there is a fair amount of piston slap when engine is cold at low RPM. It is completely absent when engine is warm, so I assume that's what it is.


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