4 bbl carb on wrong? - Page 4 - JeepForum.com
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post #46 of 71 Old 06-16-2021, 06:55 AM
Matt1981CJ7
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Originally Posted by 85Tuxedocj7 View Post
I know very little about vacuum on a carb, I did hook a vac gauge up to it and at idle it pulled 20" and when i throttled up it dropped to around 17"
That's very good vacuum readings.

I haven't read the entire thread, so I don't know where you are at on the carb positioning, but you are in good hands with Gutthans and Bagus.

Matt



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post #47 of 71 Old 06-16-2021, 04:27 PM
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post #48 of 71 Old 06-17-2021, 09:47 AM Thread Starter
85Tuxedocj7
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Originally Posted by gutthans View Post
The one pictured is a 1987 258 block. Head is a 7120 casting from a 1995; used a SD530 head gasket. Headers are Clifford 3 into 1 dual exhaust.

I used to go with Clearwater conversion heads, but they stopped supplying 7120's exclusively. So I just scrounge and pick-up used ones. Do a clean-up plane of the surface .003 - .005, 3-angle valve job, cc the chambers, install full silicone bronze guides, stop up the water jacket steam pockets (pic) because up to '89 the pockets hang off of the block edge (NOTE: HESCO sells them now, but instead of welding, they use DEVCON...so I just do it at home with JB Weld and save $300)
. Use the CJ to early YJ oil pan with the middle sump.

Pretty much all stock parts except for performance upgrades (cam and etc.)
I bought a used 7120 head (don’t know what year it came off,) but started reading about having to change push rods, headers, fabricate a PS pump bracket, I never have been able to find an answer on if my stock 258 cam will work, and so on and got a little intimidated and backed off the idea.

Oh and I couldn’t find long tube 3 into 1 headers for the 4.0 head to keep from having to cut into my existing exhaust. my setup has long tube 3 into 1 Hedman headers.

Damn you have a clean undercarriage! Is that a TDK Frame?
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post #49 of 71 Old 06-17-2021, 11:25 AM
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Yep TDK. I used a 1990 PS setup with late CJ brackets and pulleys ('86-'87 because I run V-belts). Did have to dremel the bracket a bit, but that's mating up to an OFFY and not a stock manifold.

You DON'T need to change the cam, but you will probably drop $30 on a set of pushrods...mostly because the rocker arm height MAY be too close after planing the head. I don't recall if the 3/8th 258 p-rods are too fat on the newer head holes, but if you don't swap the running valve train over entirely, new p-rods are only about $30 a set. I used custom Crane p-rods here, but I put $19 Olsmobile Tornado ones on the last stroker 258. It's more about the right length to get the proper pre-load. The stock setup allows for quite a tolerance in length.

You WILL get 30 - 40 rear-wheel hp from just changing the head and the exhaust. Unfortunately, the exhaust must match the head and the 4.2 heads have a center bolt that is not on the newer heads, so you have to upgrade the header to match.

Clean? Yea, I don't mud run and there's no road salt!...it's only got 900 miles since put together and I keep oil wiping or WD-40ing everything. I hate working on a vehicle and having dirt and crap drop on my head.
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post #50 of 71 Old 06-17-2021, 02:00 PM
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Clean? Yea, I don't mud run and there's no road salt!...it's only got 900 miles since put together and I keep oil wiping or WD-40ing everything. I hate working on a vehicle and having dirt and crap drop on my head.
Looks very clean, nice Jeep. You might want to look into Gibb’s Brand aerosol oil preservative, vice WD-40. It’s the Bomb!

I sprayed a whole can of it under my JLU to help preserve it. It will keep bare metal from corroding, and anything that is coated with paint or anodized hardware, or other corrosion prone areas will have an extra layer of protection.

You can get it online if you Google it.
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post #51 of 71 Old 06-17-2021, 02:37 PM Thread Starter
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Originally Posted by gutthans View Post
Yep TDK. I used a 1990 PS setup with late CJ brackets and pulleys ('86-'87 because I run V-belts). Did have to dremel the bracket a bit, but that's mating up to an OFFY and not a stock manifold.

You DON'T need to change the cam, but you will probably drop $30 on a set of pushrods...mostly because the rocker arm height MAY be too close after planing the head. I don't recall if the 3/8th 258 p-rods are too fat on the newer head holes, but if you don't swap the running valve train over entirely, new p-rods are only about $30 a set. I used custom Crane p-rods here, but I put $19 Olsmobile Tornado ones on the last stroker 258. It's more about the right length to get the proper pre-load. The stock setup allows for quite a tolerance in length.

You WILL get 30 - 40 rear-wheel hp from just changing the head and the exhaust. Unfortunately, the exhaust must match the head and the 4.2 heads have a center bolt that is not on the newer heads, so you have to upgrade the header to match.

Clean? Yea, I don't mud run and there's no road salt!...it's only got 900 miles since put together and I keep oil wiping or WD-40ing everything. I hate working on a vehicle and having dirt and crap drop on my head.
I'd love to have a TDK frame. my dad and I built this when i was in middle and high school. my frame is sprayed with something like bedliner (guaranteed to hold moisture) but I can't afford a new frame at the moment... Ideally id have a black or gray CC finish on everything below the tub but that's not happening with this jeep oh yeah its a fiberglass tub supposedly came from Canada.

Mine is more for driving on the beach in NC but i have a farm i don't mind ridding around the fields on but it gets washed before its parked.
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post #52 of 71 Old 06-28-2021, 09:47 AM Thread Starter
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EUREKA!

I rebuilt the carb, and got it put back on (the correct direction) this weekend. It's so much more responsive.

In the attached photos you see how the two holes closest to the engine are sooted up, while the two away from the engine are clean? The way the plenum is divided, the two holes closest to the engine are smaller runners intended to be used with the primaries of the carb, while the outboard ports are intended for the secondary's, When my father and I built this jeep in the early 2000's the carb was installed 180° from what it was supposed to be, the engine was always slow, the carb would load up and take forever to accelerate, it felt like I was dragging an anchor. He/ I had several well respected mechanics try to tune the carb to no avail. I pulled the carb off to rebuild it and saw the carbon in the intake and realized something was wrong... Thanks to you guys i learned it was a divided plenum and have switched my carb and it runs like a champ. I can't wait to get it completed and put it on the road. I am currently waiting on a fuel sending unit, new throttle bracket and longer throttle cable then it's going on the road.
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post #53 of 71 Old 06-29-2021, 05:32 AM
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Share this information with ALL of the 'well-respected' mechanics and anyone else that missed this obvious goof! Wouldn't be the first time nobody checked to see if it was plugged in to the wall...
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post #54 of 71 Old 06-30-2021, 12:19 PM Thread Starter
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Originally Posted by gutthans View Post
Share this information with ALL of the 'well-respected' mechanics and anyone else that missed this obvious goof! Wouldn't be the first time nobody checked to see if it was plugged in to the wall...
I guess they assumed we had the carb on correct or didn't consider the plenum was divided. I never heard of a split plenum until I started researching this... I was 13-14 when it was put together originally.
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post #55 of 71 Old 07-02-2021, 06:28 AM
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Ah yes, the misspent youth..............

Glad you got to the bottom of it!!!!

The vaccumm readings are important as you are trying to suck open the air valves. That should be enough but you need to find out when they are opening, which is the sudden rush, and lighten it if it is happening too late by drilling the weights. Given the engine is limited in revs i would expect it to come in by 2200 to 2400 rpm, does anyone think differently?

I had a Lancia once with twin Dellorto side drafts on a 1300cc V4, they were sequential secondaries and the car was a Jekkyl and Hyde, drove smoothly until you hit 4000rpm and then went wild to 6500rpm, gave great economy but could be thrashed, it was a class winning rally car. It weighed 960 Kg / 2100 lbs so did not need a large engine. 1972 was a good year for Italian cars and about the peak for carbs.

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1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
1995 Cherokee 4.0 - CHEROKEE
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post #56 of 71 Old 07-02-2021, 07:00 AM
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Originally Posted by BagusJeep View Post
The vaccumm readings are important as you are trying to suck open the air valves. That should be enough but you need to find out when they are opening, which is the sudden rush, and lighten it if it is happening too late by drilling the weights. Given the engine is limited in revs i would expect it to come in by 2200 to 2400 rpm, does anyone think differently?
Not being intimate with this particular carb, I can only suggest finding a looong straight road and gradually easing up in 4th to where the secondaries tip-in. It might also help to 'T' a (hand-held) vacuum gauge inline (if you don't have one plumbed in the cab) so you can watch for the fluctuation.

A good place is around 2200 as Bagus noted. Most driving will be under that, so that helps with mileage. My motor is set to have full advance around 2000-2100 and that's also where the secondaries come in. Around town 1800-2000 is pretty typical operating range for me.
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post #57 of 71 Old 07-02-2021, 07:15 PM
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Quote:
Originally Posted by gutthans View Post
Not being intimate with this particular carb, I can only suggest finding a looong straight road and gradually easing up in 4th to where the secondaries tip-in. It might also help to 'T' a (hand-held) vacuum gauge inline (if you don't have one plumbed in the cab) so you can watch for the fluctuation.
.
Excellent idea on using the vacuum gauge.

I am not familiar with this carb, is there a port you can connect to ABOVE the secondaries? If so it will show very little vacuum at that port until they open and then you will see it spike on the gauge.

Failing that you will see a drop in vaccum in the manifold when the secondaries open.

BagusJeep lives in Bali with far too many 4x4s:
1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
1995 Cherokee 4.0 - CHEROKEE
1980 Land Rover Series III 109" troop carrier - ROVER
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post #58 of 71 Old 07-03-2021, 06:36 AM
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Here's another lovely caveat to that damn split-plane intake; if you run the PCV from the back of the carb, you're likely to end up with the same issue because that secondary plane doesn't flow anything until you get deep enough in the throttle to open the secondaries so anything that does make it past the barely-cracked throttle plates will just bounce back and forth in the lower plane with wave reflection.

Then if you're really "lucky", there'll be a standing wave right where you spend most of your driving time and it'll actually push the gasses from your PCV right up out of the carb and turn the whole thing sooty on the outside; secondary side only.


Shawn

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post #59 of 71 Old 07-03-2021, 07:26 AM
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Here's another lovely caveat to that damn split-plane intake; if you run the PCV from the back of the carb,
Shawn
I note in the first few pictures that there is a closed port on the secondary side. A second picture of the carb shows a nipple also on the primary side of the carb. Since the manifold itself has a port for the vacuum brake take-off, below the carb pad, it seems like that nipple can be plumbed for the PVC on the primary side. Yes???
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post #60 of 71 Old 07-03-2021, 08:05 AM
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Originally Posted by gutthans View Post
I note in the first few pictures that there is a closed port on the secondary side. A second picture of the carb shows a nipple also on the primary side of the carb. Since the manifold itself has a port for the vacuum brake take-off, below the carb pad, it seems like that nipple can be plumbed for the PVC on the primary side. Yes???
The port on the front side of the carb is the way to go. It's easy to want to plumb the rear of the carb for PCV where it's away from header heat and easy to access but now you're in trouble.

I wouldn't use the port in the intake for PCV. I'm sure it's fine with its non-directional suck for the brakes. Funky intake design, for sure. I'm wondering if a Holly-style carb that uses a vacuum pod for secondary activation rather than a counter-weighted air flap would have worked well mounted forwards. Not that it matters here; just wondering.


Shawn

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