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Old 11-21-2017, 08:14 AM   #16
Scoutboy74
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Default Re: always surgeing

That's pretty low unless you're at a high elevation. I wouldn't expect that cam to be lumpy enough to effect it that much. Fuel at the vac port, low vacuum... something's up with that carb
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Old 11-21-2017, 09:43 AM   #17
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Default Re: always surgeing

I think I will pull carb and rebuild. I already have kits. Should I at this time go back to 6.5 PV that came with carb? should I change main jets from 66 to 64 or 62 or at all?

Question: Does someone move these posts or copy them to more relevant threads? I'm only asking since this is in the ignition thread. Or should user (me) be aware and copy or move as appropriate?
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Old 11-21-2017, 11:04 AM   #18
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Default Re: always surgeing

The records I have show that IH used 2.5 PV's in the carb on the 392. In a heavy vehicle/truck a lot of time could be spent with the PV open. Instead they jetted a bit on the rich side and opened the PV under really heavy loads.

Really the answer to your question depends on what do you plan to use this vehicle for. Then the carb can be tuned for that purpose. In my carb tuning adventure I found lots of info for hot rod tuning, but a dearth of info on tuning for trucks. In the carb thread referenced, my goal was to tune for MPG by having the primary's toward the lean side and a 2.5 PV and when the power was called for have the secondaries run rich. Unfortunately with a 600 cfm carb this type of tuning is not as viable as the secondaries will rarely open.

Running the vacuum gauge for a while will give you an indication of the vacuum level in your actual driving conditions. Decide where you want the extra power to come on and install the PV to match the vacuum level in those conditions.
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Old 11-21-2017, 11:21 AM   #19
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Default Re: always surgeing

Quote:
Originally Posted by Oil Scout View Post
Surging is most prevalent when trying to maintain constant speed or rpm, but is strongest when slight acceleration to maintain rpm. When letting off just the slightest bit or coasting in gear there is no surging.

Vacuum rarely drops below 5, so I'm not sure if PV (3.5) opens very often. I noticed when rolling quickly on accelerator only twice that vac reached less than 5. I was able to find a sweet spot of the surging at 2200 rpm in third gear and observe. It really seems like it is starving. Not understanding how this carb could be starving the engine.?

I will try to post video and pics when phone finishes updating.
In the low end of the cruise range, you're on the pilot jets, then mid cruise range you're primary main and power valve. The 3.5 will delay the enrichment staging. This can leave you lean in cruise. Go back to the numerically larger PV. Test,

Leave the primary mains asis for the time being.

Leave the following alone for now. I'm just elaborating on tuning mistakes.
I have to assume that pilot jets are as supplied by the factory (Holley) and probably OK. A lot of would be tuners like to drill out the main air jets. This helps with generating a smoother transition where you would normally have a hole or hesitation, while going from the pilots to the mains under light acceleration on largely cammed engines. In that case the main jets are increased. On a small cammed engine with good manifold vacuum, it can end up lean in pilots light cruise.
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Old 11-21-2017, 12:54 PM   #20
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Default Re: Surging 392

Okay, Carb was pretty clean. Not unexpected since I rebuilt it six months ago. However, there was one plugged hole on metering block see picture attached with picks pointing out plugged path.

Also, the jets were 50% plugged, picture attached to see comparison of dirty and clean.

Correction on previous post: Jets are 65(s) not 66. and PV is 4.5 not 3.5.

Question: see pic of carb/manifold gaskets, which one should be used, thick pressed paper or thin paper?

Square one on right was what was on this when I purchased vehicle. Ive seen a couple maybe three different styles of manifolds, is there any detail about this one that someone could add color for me.
Attached Files
File Type: txt Metering Block.txt (23 Bytes, 31 views)
File Type: txt jets clean and not clean.txt (23 Bytes, 20 views)
File Type: txt Carb to Manifold Gasket.txt (6 Bytes, 19 views)
File Type: txt 392 Manifold.txt (23 Bytes, 20 views)
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Old 11-21-2017, 04:09 PM   #21
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Default Re: Surging 392

Your photo's came through as text files, so we can't see them. Half plugged main jets would definitely do what you describe. 4.5 is reasonable on the PV.
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Old 11-21-2017, 05:29 PM   #22
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Default Re: Surging 392

Carb rebuilt, installed, new filter, and will not run. I have spark at each plug, timing is in the ball park (if not right where I left it), fuel in the bowls, and checked vacuum lines. It will start, but very rough and dies without holding down accelerator. My hunch is a big vacuum leak, but from where? Can you get a vacuum leak through the vacuum secondary's diaphragm? I'm grasping...
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Old 11-21-2017, 08:30 PM   #23
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Default Re: Surging 392

I can't answer your specific question, but your timing and ignition haven't changed. Only your carb. I still believe most if not all your issues were and continue to be housed in that device. I believe when you do get it squared away, you will see your vacuum at idle in the high teens and your engine being tolerant of a more reasonable amount of static timing advance.
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Coal Trickle - '99 Dodge Ram 2500 Q-Cab SWB 4x4 - 5.9L 24V CTD/NV4500
Hooty - '74 SII - 392/TF727/D20/3.73 D44's/Spartan FA/Krac-lok RA/RC 4" SUA/34x10.5 TSLs/33g Fuel/HFT 8k winch
Mongo - '71 1210 Std Cab 2WD - 345/TF727/RA17 D60 4.10 Trac-lok - "Mongo love candy! Duh, huh, huh!"
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Old 11-21-2017, 08:38 PM   #24
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Default Re: Surging 392

I agree. I have been working on everything else for so long and it has needed attention from the beginning. With the issues found today I expect it to work. Just canít think of what is wrong now.
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Old 11-21-2017, 08:54 PM   #25
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Default Re: Surging 392

Trying to post pics again
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Old 11-23-2017, 07:18 AM   #26
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Thumbs up Re: Surging 392

Okay, metering block was still not flowing properly. Needs to be rebuilt. Couldnít find parts readily available, but Summit Racing in Arlington had a new block in stock for $43.00. Installed the block and carb. Fired right up with 5 inches more vacuum than I ever had before. Still not as steady as I like, but 16 to 17 in Hg is great. Also, I can know get timing below 5* without dying. Set timing to 8* and drove like a new engine should. Still need some tuning, but very happy with results.

I will post additional results and pics later to hopefully make this useful for others.

Right now Iím taken by a couple of days of with family. Happy Thanksgiving everyone!
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Old 11-23-2017, 08:25 AM   #27
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Default Re: Surging 392

Hey, that's excellent progress! Use your vacuum gauge along with your ears to adjust your air idle mixture screws for an optimum ratio. And be sure the screws are the same number of turns out from seated on both sides. Happy Thanksgiving!
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Favorite hobby...Driving Salma Hayek in my Scout

Coal Trickle - '99 Dodge Ram 2500 Q-Cab SWB 4x4 - 5.9L 24V CTD/NV4500
Hooty - '74 SII - 392/TF727/D20/3.73 D44's/Spartan FA/Krac-lok RA/RC 4" SUA/34x10.5 TSLs/33g Fuel/HFT 8k winch
Mongo - '71 1210 Std Cab 2WD - 345/TF727/RA17 D60 4.10 Trac-lok - "Mongo love candy! Duh, huh, huh!"
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Old 11-23-2017, 10:08 AM   #28
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Default Re: Surging 392

Glad to here it's fixed. There are just passages in a metering block, so they may have plugged with the same debris as the jets. Enjoy the family and your running Scout. Happy Thanksgiving.
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