Mid-RPM, Medium-Load Missing (MADDENING)

[opus10583]

Hi,

It all started with a leaking distributor oil seal…

Had my engine rebuilt last fall, on one of Mark Allison’s 1800 short blocks, a recent rebuilt head; new starter, water pump, fuel pump; Al pulleys and a lightened flywheel from Millers.

Reused the original distributor which had been fitted with a Pertronix Ignitor, and the 32/36 DFEV I’d fitted earlier.

Over the, twice the estimated, time course of the refit, the distributor oil seal developed a leak. Heading into winter I bought a later electronic setup which I discovered needed a rebuild after I received it.

New pickup, module, new coil, oil seal, bearings, cap and rotor and good to go.

Tender break in miles revealed no problem, then vacuum capsule arrived, fit it in and car would not pull past 5,000rpm in any gear, really rough through 3-5k.

Reset timing, pickup gap; reset timing, pickup gap; reset timing, pickup gap…

Had original dist. rebuilt by Cardone, refit Ignitor, new cap and rotor.

Car stumbles through 3.5-4.5k…

Verified #1 TDC vs cam markings; tightened carb nuts, replaced rotor, plugs, wires.

Car stumbles through 3.5-4.5k…

Rejetted carb, 180main 165secondary air correcter, .47 idle jets. Found float hanging low, reset to spec. (these were a to-do list item that suddenly seemed pertinent)

Car stumbles through 3.5-4.5k…

Replaced fuel filter, inspected carb in-body filter, found it was crushed (see below, correct?) but intact and clean.

Car stumbles through 3.5-4.5k…

Sprayed carb-adapter-manifold interfaces with carb cleaner: Engine dies.

Replaced carb-adapter-manifold gaskets, applied high-temp RTV to sealing surfaces.

Car stumbles through 3.5-4.5k…

Sprayed carb-adapter-manifold interfaces with carb cleaner: Engine dies.

…What (TF) am I missing?

Thanks for any suggestions,
Mark

32/36 in-body fuel filter by opus10583, on Flickr

[RoyBatty]

What happens at wide open throttle?

If it passes thru this RPM range and clears up you may need to blow out the carbs fuel and air passages. Especially the idle circuit. You might have a plugged fuel circuit or air bleed.

[opus10583]

What happens at wide open throttle?

If it passes thru this RPM range and clears up you may need to blow out the carbs fuel and air passages. Especially the idle circuit. You might have a plugged fuel circuit or air bleed.

Hi,

It seems to clear up at WOT, but it could be masked as well.

Thanks,
Mark

[RoyBatty]

when the stumble occurs, is there smoke from the exhaust? Like a if you're to rich? Or does it act lime its not getting enough fuel? Does it misfire out the carb or exhaust?

[majicwrench]

HIgh temp RTV is not fuel resistant and will degrade into a worthless goo in the presence of fuel vapors.

That is not your problem however.

[opus10583]

when the stumble occurs, is there smoke from the exhaust? Like a if you're to rich? Or does it act lime its not getting enough fuel? Does it misfire out the carb or exhaust?

Hi,

I can't tell if it smokes, it only happens at speed.

Misfire in the exhaust.

Thanks,
Mark

[opus10583]

HIgh temp RTV is not fuel resistant and will degrade into a worthless goo in the presence of fuel vapors.

That is not your problem however.

Hi,

Hight temp and oil resistant Permatex.

Any idea what then?

Surfaces were all clean before hand, hadn't been an issue before a long winter of little activity.

Thanks,
Mark

[majicwrench]

When is the last time it ran well?? What has changed since then??

[opus10583]

When is the last time it ran well?? What has changed since then??

Hi,

Last June? Ran well but burned oil like a diesel. Started deteriorating rapidly.

Then, as above: NOS shortblock rebuild, then distributor started leaking oil, then the NE winter set in and then after two distributors, plugs, wires and carb attention it still has this stumble at 4k.

I'm out of ideas.

Thanks,
Mark

[RoyBatty]

Barking out the exhaust?
Too rich or late timing at that RPM. Or maybe a little of both.
Any way to check your distributor advance mechanism?
Also could still be a carb issue. Plugged air bleed or something similar.
Good luck.

[So Cal Mark]

I had a problem car like this last week. I found very worn advance weights/springs so the timing actually retarded as the rpm increased. A new Computronix ignition has the car running better than ever

[opus10583]

Barking out the exhaust?
Too rich or late timing at that RPM. Or maybe a little of both.
Any way to check your distributor advance mechanism?
Also could still be a carb issue. Plugged air bleed or something similar.
Good luck.

Hi,

I wouldn't say barking; missing, out the exhaust.

Advance seems fine, using an adjustable timing light. The distributor was rebuilt by Cardone, better than the vac advance one but problem persists.

What would "air bleed" look like on the parts explosion?

Thanks,
Mark

[opus10583]

I had a problem car like this last week. I found very worn advance weights/springs so the timing actually retarded as the rpm increased. A new Computronix ignition has the car running better than ever

Hi Mark,

This problem didn't manifest with the current distributor before, which was then rebuilt by Cardone this Spring, so I think that's ruled out.

I tried two sets of advance springs with the vac advance unit with no change.

Thanks,
Mark

[RoyBatty]

See if this helps.
If any of the air passages get blocked off with debris, it can be just as bad as a fuel passage being obstructed.
The draw of the fuel is aided by the air bleeds. And proper metering of the air thru these passages is a must.
Pull both the idle jets and use compressed air to blow out those passages as well as the main metering circuits.
Check the jets themselves for anything that aint supposed to be there. When you pull the jets, look for an air passage in the gasket surface above the jet. Typically you can look down on the gasket mating surface toward where the idle jets go into the carb body and see thru to idle jet bore.
The image is too large for proper display here. So here is the basic link.
http://2.bp.blogspot.com/_dlt8k_lxV9c/T ... ng+001.jpg

[rebar1111]

Separate problem: On your schematic, is the 50 on the pump discharge nozzle actually 0.050 inch diameter per orifice? or some other reference. I am trying to unblock my orifice to a reasonable size.