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Quadrajet Question
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Supporting Member of Barthmobile.com 12/20
Picture of Paul And Jaime
posted
Hello all. Been having a problem with a backfire in the manifold while under load ( foot to the floor, climbing a hill ect.) Consensuses was a starving fuel issue. So yesterday I finally got around to changing the fuel filter. Surprise!! The PO had removed the filter and left it out and put a inline filter in. My question is do you think I should rebuild the carb and put a filter in ? Does it hurt the performance of the Carb to run with an inline filter instead of the little filter in the Carb? I also have the quadrajet bog. The secondaries seem to be working properly and the choke is also in good working order. This is my first quadrajet by the way. Youtube is not the best alterative to experience.
 
Posts: 101 | Location: Ky | Member Since: 10-20-2020Reply With QuoteReport This Post
Glassnose Aficionado
Supporting Member of Barthmobile.com 2/09
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I had a backfire problem when I first got the 78. It turned out to be the distributor wasn't advancing properly. Make sure your vacuum tubes are all good and any unused ports are capped off.


79 Barth Classic
 
Posts: 3426 | Location: Venice Fl. | Member Since: 07-12-2004Reply With QuoteReport This Post
Supporting Member of Barthmobile.com 1/21
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Another item to check, are the advance weights free.
Good luck


It's amazing what you find when you go look.
 
Posts: 1018 | Location: St. Charles, MO, USA | Member Since: 10-09-2003Reply With QuoteReport This Post
Official Barth Junkie
Supporting Member of Barthmobile.com 2/22
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Ah yes, the (in)famous Quadrajet carburetor....

One of the last of the dinosaur fuel systems. It was a fairly complex carburetor. When it was set right, the smaller primaries give reasonable fuel economy while the big secondaries allowed for good airflow when wide open. It has primary, secondary and main jet systems. Also accelerator pump, choke plate for starting and power valve for wide open flow.

Net result is a complicated expensive carburetor that works well when set right. I believe the high cost to produce carbs and looming emissions restrications convinced the companies to go the fuel injection route.

Meanwhile, the carb tuners found that many of the changes they made to one system (ie low speed vs high speed) affected other systems and made it difficult to get everything right.

OTOH, if the thing ran well at one point, it will probably get by with some minor maintenance.

Once the exterior is cleaned up, remove the top. Replace the float valve and set the float height. Replace the accelerator pump piston. Replace the top.

Set choke linkage to specs, be sure it is moving freely. Check to be sure the choke heater is working.

There are two plugs in the bottom of the carb that are pressed in and sometimes get loose and leak fuel from the bowl. Be sure they are staked tightly.

The internal fuel filter is a joke. Even a small amount of water or crud will plug it and starve the engine. Worse yet, it is spring loaded so if it gets plugged, the fuel will bypass it, crud and all. Inline is much better.

Speaking of starving the engine, the 454 is a thirsty beast so be sure your fuel pump and lines are clean. Old motor homes are notorious for rust and crud in the tanks, lines and filters. Be sure your fuel system is flowing well.

As others mentioned, there are other problems besides carb leaning that cause backfire.

Be sure your distributor cap is clean inside and plug wires are good. At higher rpm, the old coil systems had less voltage to the plugs.

Finally, be sure the timing is right and the advance is working.

In the twilight years of carburetors, many attempts were made to reduce exhaust emissions and improve mileage/performance. These were contrary goals. In the interest of emissions and economy, the engines were set up to run lean, with lower compression ratios. Good for reduction of hydrocarbon emission but not for performance.

In addition, the leaner mixtures burned hotter and produced more NOX emissions. To reduce peak burn temps, they introduced oxygen-poor exhaust gas into the fuel/air mix. (So-called exhaust gas recirculation: EGR). They also retarded ignition timing. Both of these reduced engine performance further.

So, given that most of the changes reduced performance, the net result was increased fuel consumption and minimal gains.

So, considering that most of the OEM changes were futile attempts and the smog equipment is no longer working now, it makes sense to tune for best performance (if allowed by your state.)

Once the carb is set, check the ignition timing. In the good old days, base timing was usually set about 8-10 degrees advanced. (BTDC: before top dead center). The later ones were retarded, 0-4 degrees after TDC.

I would set timing at around 8 BTDC (with vacuum advance temporarily disconnected and plugged), then if you have no knock on full accel, bump it up a degree at a time until you have knock, then back off about 2 degrees. I ran my 454HO motor at 10 degrees advanced. The more advance, the better they run until they knock.

In addition to base timing, be sure your advance systems are funtional. The centrifugal advance is done by weights inside the distributor. Depending on rpm, they will deliver 0-10 degrees advance as the rpm increases. As long as the springs and weights are not stuck, they should work ok. If you watch the marks with a timing light, you should be able to see the timing change when you blip the throttle.

There is also a vacuum advance. This senses manifold vacuum from the base of the carb, often ported internally to regulate the vacuum signal. At high vacuum, the advance will be around 8 degrees. On many emissions models, there were valves to regulate/delay the vacuum advance so it could only work in top gear, engine warm. These usually do not work and they prevent full advance. Be sure the vacuum line from the carb base goes directly to the vacuum advance unit.

With all systems working, you will get about 25-30 degrees total advance at high rpm and medium throttle.

Finally, disconnect the vacuum lines to the EGR. It is likely plugged and not working anyway, but disconnect to be sure. Also, check the heat riser valve in the exhaust mainfold is not stuck.

With these changes you should have an engine that runs well and likes throttle. Good luck! Mechanic


9708-M0037-37MM-01
"98" Monarch 37
Spartan MM, 6 spd Allison
Cummins 8.3 300 hp
 
Posts: 4831 | Location: Kalkaska, MI | Member Since: 02-04-2011Reply With QuoteReport This Post
Supporting Member of Barthmobile.com 1/21
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"Golly Sarge, now that there is a post!"


'92 Breakaway
Cummins 190hp, No Modifications
Allison AT542
Floor Plan 30-BS-11B
9205-3798-30BS11B
 
Posts: 355 | Location: USA | Member Since: 11-07-2014Reply With QuoteReport This Post
Official Barth Junkie
Supporting Member of Barthmobile.com 2/22
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Yeah, sorry, I do get long winded at times.
Just my 2 cents (or maybe 200 cents worth.) ROTFLMAO


9708-M0037-37MM-01
"98" Monarch 37
Spartan MM, 6 spd Allison
Cummins 8.3 300 hp
 
Posts: 4831 | Location: Kalkaska, MI | Member Since: 02-04-2011Reply With QuoteReport This Post
Supporting Member of Barthmobile.com 1/21
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Thumbs Up Valuable information! cheers





#1 29' 1977parted out and still alive in Barths all over the USA




 
Posts: 1024 | Location: Floral City FL | Member Since: 04-25-2008Reply With QuoteReport This Post
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Hi All, Both my classic Chris Crafts had quadrajets. Loved the sound of both boats when you opened the throttle. However, I did my share of tinkering with them. Always dreamed of buying a "Holley Sniper EFI for quadrajets". I thought an electronic system such as this may make tuning a little less stressful. Although this system will lighten your wallet considerably.

Good Luck,

John
 
Posts: 33 | Location: Central Pa. | Member Since: 03-24-2019Reply With QuoteReport This Post
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quote:
Paul And Jaime


You are on the right path. Exact issues will be yours to diagnose, but you are certainly asking the questions that will get you to a solution.

Steve VW: Excellent review. The Quadrajet in the last incarnation, on the 454, was a wonderful and majestic mechanical computer, solving the equation of gasoline proportion to air mass. The supporting gear around it certainly needs regular maintenance. The carb itself does as well, as throttle shafts loosen, and other parts wear and age.

Hills were a problem for me at one point, due to the 2" section of hose between a pipe and a pump loosening up. This was located in the fuel tank. Running with a full tank helped to deal with this.

Vacuum hoses might look OK, but when you handle them, you will see some turn to powder, especially when on attachment points, like the ported vacuum switch. There is also a vacuum delay valve.

It's been a while since I thought about the EGR valve, perhaps that could cause issues. I replaced mine at one time.

The "results oriented approach", especially when dealing with mechanical items that are decades old, sometimes is to remove and replace all the low dollar value items, like the vacuum hoses, the PVS switch, EGR valve.

Learning to observe the timing marks on the harmonic balancer, from below, is something you'll never forget!

Happy Motoring,
Matt


1987 Barth 27' P32 Chassis
Former State Police Command Post
Chevrolet 454
Weiand Manifold, Crane Cam, Gibson Exhaust
 
Posts: 454 | Location: Massachusetts | Member Since: 07-28-2006Reply With QuoteReport This Post
Official Barth Junkie
Supporting Member of Barthmobile.com 2/22
Picture of Steve VW
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I replaced the Quadrajet on my Regal with a throttle body injection system. Never looked back. The engine ran better at all temps and loads, better mileage too. There are more choices now than ever for fuel injection upgrades.

Here is my thread: http://www.barthmobile.com/eve...1087061/m/3173973877


9708-M0037-37MM-01
"98" Monarch 37
Spartan MM, 6 spd Allison
Cummins 8.3 300 hp
 
Posts: 4831 | Location: Kalkaska, MI | Member Since: 02-04-2011Reply With QuoteReport This Post
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Steve,

Great old thread. Six years before my time on this forum. In most cases, it's hard to beat modern technology. CAD-CAM has completely changed the graphics & the sign business.

Absolutely love your Dad's boat. He is a very lucky man to be able to enjoy it for so long. Been to many antique & wooden boat shows(Clayton, NY). I love the lines & bright work of the classic wooden boats. Heck, I even enjoy the unique smell.

Good Luck,

John
 
Posts: 33 | Location: Central Pa. | Member Since: 03-24-2019Reply With QuoteReport This Post
Supporting Member of Barthmobile.com 1/21
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So $1500 will buy a brand new Holley Stealth TBI kit, fuel pump is included, throw the EGR system and A.I.R. pumps in the bin put in a new cartridge inline fuel filter, clean the fuel tank(s) and fuel lines. Replace the fuel tank pick-up with a straight pipe, enjoy life! Total cost about $2000. Check the distributor and add as much advance timing as possible. The Holley will optimize itself after the initial set-up.

OR

Clean the fuel tank(s), and lines, put in a high-quality low pressure electric fuel pump and cartridge fuel filter, a straight pipe fuel pick-up. Buy a wide range fuel mixture gauge kit, install the oxygen sensor. Toss the EGR and A.I.R. pumps in the bin. Keep or remove the mechanical fuel pump, kept mine because it has a fuel return circuit. Clean and rebuild the Rochester, kits and instructions are easy to find. Check the distributor, set the timing with as much advance as possible. Check the fuel ratio at cruise speed, re-jet only the primary rod jets to get the correct ratio (I could not find new rods only jets). Enjoy life! Total cost about $600.

Either job is at least a weekend, longer if you have to have the fuel tank(s) professionally cleaned and you want to totally remove the EGR system and weld up the exhaust pipe. Maybe another $100 if you have it done.

The electric fuel pumps should be tied into the oil pressure, so they shut off when the engine is not running for safety's sake.

Must mention either case requires the manifolds to plugged where the AIR pump injection was or install new headers to replace the manifolds.

These changes may not be ok in some states.


25 Ft Glassnose, 2792, 1982
454 Engine
Plain Jane Interior
Original Paint
 
Posts: 1827 | Location: Clinton Iowa | Member Since: 04-02-2017Reply With QuoteReport This Post
Supporting Member of Barthmobile.com 12/20
Picture of Paul And Jaime
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Thanks to all for the great advice. I have a rebuild kit on order from quadrajet power, in the mean time I ran a seafoam kit through it ( after the seafoam I can slowly get to WOT with out the backfire or stumble, but still has a miss randomly at lower RPMS) and it turns out I have an HEI distributor. Will update after I try to rebuild my first quadrajet and have to order one all ready rebuilt.

I have also printed out Steve VWs suggestions and I am going to work my way through them one at a time. Thanks Steve!!
 
Posts: 101 | Location: Ky | Member Since: 10-20-2020Reply With QuoteReport This Post
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If you are getting in to it to do your own rebuild, consider reading the book by Cliff Ruggles, "How to Rebuild and Modify the Rochester Quadrajet".

No telling what you'll find when you dig in to the carb. Rebuild kits do nothing for wear between the throttle shafts and the base plates. Cliff explains how to use "chucking reamer" and install brass bushings. Having that area of the carb being loose, allows more air in, and allows for a lean condition.

Inspect that fuel line from the carburetor as well. Make sure there are no signs of fuel seepage, due to the steel rusting through.

The mechanical fuel pump on the engine: some companies say that there pumps can be used, but not all are the same internally, to provide the extra suction force needed to move fuel from far back in the RV chassis.

Matt
 
Posts: 454 | Location: Massachusetts | Member Since: 07-28-2006Reply With QuoteReport This Post
Supporting Member of Barthmobile.com 12/20
Picture of Paul And Jaime
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Cool Book. Just bought it on kindle. I was also looking at this
https://nationalcarburetors.co...ter-M4MC_p_1484.html

Have no clue to the quality of the the carb or the company but seemed like a good price. This was my backup plan if the rebuild failed.
 
Posts: 101 | Location: Ky | Member Since: 10-20-2020Reply With QuoteReport This Post
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