Home » Public Forums » GMCnet » [GMCnet] Fuel/Carb question
| [GMCnet] Fuel/Carb question [message #338257] |
Wed, 31 October 2018 22:15  |
JerryW
Messages: 256
Registered: August 2018
Karma:
|
Senior Member |
|
|
Hi Michael,
Welcome. I will take a crack at your questions, but start with this info in mind. Our coaches were built for 1970 gasoline that was formulated to be very volatile so it would atomize in the very low pressure regime of venturi action inside the carburetor where light springs and vacuum are the only forces which attempt to create the proper mix of air and fuel for different load, altitude and heat conditions. Todays fuel is formulated very differently for a world in which alcohol laced fuel is pumped at around 50 psi through injectors which open and close with great precision to create the proper mix of air and fuel for different load, altitude and heat conditions. Trying to get yesterdays carburetor technology to properly burn todays fuel to push a 12,000 pound motorhome up a hill is more than a bit of a stretch. Everything in the system has to be just right for that to work very well at all, or consistantly.
Eventually most GMC owners will opt to ditch the old Q’jet carb and install a modern self learning EFI system specifically designed to retrofit to yesterdays cars, trucks and our motorhomes. Several manufacturers make suitable products to do this easily for $1000 to $1500.
Your fuel system is made up of two 25 gallon flat tanks connected by a common fill tube. The two tanks are interconnected by an electric solenoid operated tank selector valve that often fails from the alcohol in the fuel eating up the inside of that valve. If the valve works properly then the switch on the dash selects which tank fuel should be drawn from. The rear one is called main and the front one is called aux. The reality is fuel is drawn from both tanks simultaneously until the point where about 7 or 8 gallons is left in the tank not selected by the valve. As you drive gas will move from tank to tank depending on the slope encountered. As you go up hill gas moves to the rear (main) tank. As you go downhill gas moves to the front (aux) tank. Unless you drive only on flat land, you really have no idea how much fuel is in which tank. No matter, fill your coach, drive 250 miles with either tank selected, then stop and refill. You need a rest by then anyway. Never rely on that switch to really provide you with emergency fuel because if you need it, it may not be there for you!
To make the fuel system work the fill and run vents must be open. The run vent lines run off the top of the tanks. There are rubber isolator pads that are supposed to maintain about a half inch of clearance between the frame and the underside of the coach. If those rubber pads are squished, as they often are at this age, then the vent lines coming off the top of the tank can get pinched and no longer function properly. The vent system also includes a line that runs to a vapor/liquid separator in the DS rear wheel well. A ball float is supposed to keep raw fuel from passing any further through that system as the line goes from there around the back of the coach and forward on the PS until it reaches one or two (if a California coach originally) charcoal canisters that if working correctly (many re not) will trap the fumes. A vacuum line runs from the canister into the bottom of the carb where under high vacuum conditions the fumes are drawn out of the charcoal canister to be burned in the engine.
If any of these systems are not functioning properly, no telling what kinds of symptoms you might experience. Add to all this the fact that the passages inside the old Q’jet carbs are very small and easily clogged. That is the function of that final fine filter in the inlet to the carb. Unless your tanks are spotless inside as are all the lines and inline filters, then you need that last line of defense. For safety you want an all metal line going from the mechanical fuel pump to the carb. To make it easy to get at that filter, as others have said, loosen the four bolts that hold the carb in place after you break loose the fuel line on the front of the carb. Now raise the carb up and move it back a bit as you unscrew that line. Careful as the filter housing uses a very fine thread that is easily cross threaded when putting it back in. After changing the filter, start the fuel line and place the carb back on the intake manifold. Tighten the four bolts lightly. The torque spec is listed in the manuals and is much lighter than you might guess. Over tighten them and you run the risk of warping the carb castings. Once warped, the carb is scrap metal and cannot be successfully be rebuilt. Once the carb is torqued down correctly with the gas line already started it is easy to tighten it.
So, treat the fuel system all as parts of one very critical system that is not going to work all that well running todays gas. Go through it all and bring it all as close to what the factory did as you can. Many of us ditch the mechanical fuel pump for a couple of reasons. If the diaphragm leaks, it will leak raw gas directly into the oil pan. Even working properly it struggles to pull liquid fuel all the way from the tanks and up into the carb under different heat and load conditions. If the liquid fuel becomes fuel vapors on the way the coach will stall out from lack of fuel. Some add a small electric fuel pump to act as a booster to push fuel to the mechanical fuel pump and use it only when they detect stall. Many of us remove the mechanical fuel pump and install one or two Carter 4070 low pressure fuel pumps as close to the fuel tanks as possible and use those all the time to bring liquid fuel up and into the carb. You will find lots of references on how to do this with a bit of searching through these posts and the various GMC club web sites.
I hope this helps. We all faced these issues and solved them on our coaches so you are far from alone. Enjoy!
Jerry
Jerry Work
The Dovetail Joint
Fine furniture designed and hand crafted in the 1907 former Masonic Temple building in historic Kerby, OR
glwork@mac.com
http://jerrywork.com
———————
Message: 8
Date: Wed, 31 Oct 2018 18:45:15 -0600
From: Michael Stevens
To: gmclist@list.gmcnet.org
Subject: Re: [GMCnet] Fuel/Carb question
Message-ID:
Content-Type: text/plain; charset="utf-8"
Greetings all and thanks for the many replies. Great to have an amazing group of knowledgable and helpful people as I navigate these new waters.
So my coach has a aux fuel pump after between the tanks and the motor pump. The previous owner had put in a new motor pump and a stainless steel fuel
line to the carb. Unfortunately it is tough to get that SS line in and out to check the fuel filter near the carb. So on one little roadside challenge
I opted to eliminate it all together. In its place I put a fuel filter after the aux pump. So my coach now has two fuel filters, one before the aux
electric pump, and the other after it.
So I will plan to drop the tanks and replace the rubber fuel lines.
Here are a few questions (you all will have to be patient with me as I am no mechanic and certainly ignorant with these coaches):
1. Why do I need the Aux pump? I only use it when doing a cold start to get some gas into the carb, but shut it off immediately because, as mentioned
I thought I was dealing with flooding. If the motor runs without the aux pump then why bother having it?
2. How do I test the separator in the left wheel well or the charcoal canister to see if they need to be replaced?
3. Several people mentioned that it is likely not a carb issue, but a fuel supply issue, How can I check that? Do I need to buy a gizmo to check my
fuel pressure? How can I know if it is vapor lock, pressurized tanks, or flooding?
I realize these may be simple questions, but as I said, I'm new to this stuff.
Michael
PS. Don't know why I have a Canadian flag, I live in Boone NC with my wife and three kids ages 9, 15 and 17.
-----------------------
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
|
|
|
|
Current Time: Wed Sep 18 18:28:23 CDT 2024
Total time taken to generate the page: 0.00705 seconds
|
GMCforum
Search
Help
Register
Login
Home
