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Help please... this is a good one!


commplexone

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I have dual weber dgv5a's set up in my 72 240 and a problem has developed. Let me first note that i have rebuilt both carb's since discovering the problem and the same thing is happening so the rebuild did nothing for it. Anyways, here goes... when my car is cold and or off the carbs work great. When my car warms up my probelm starts, particularly between 3500 and 4000 rpm's. When my car is at high rpm's whether it is cruising down the freeway or when i really jump on it (80-90mph) my secondary carb just plain stops spitting fuel. Carbs work fine when i stop, shut it off, get out and then go check but when driving i have the problem. Keep in mind the primary carb (the one that gets fed first) has no problem but the secondary gets starved completely. When i rebuilt i checked all jets and everything was gutted to be completely cleaned. This is really confusing me because i have no idea what could be causing me to loose all fuel delivery in carb number two at higher rpm's. Please help, any and all advice is greatly appreciated.

P.S. I have had this car for a little over a month and this just started happening a few days ago.

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Are you running the stock fuel pump? you might find that it is not providing enough volume because it is getting tired. I am running a electric pump from a RX-7 that I bought at a bone yard for $15.00 . and it works great and is quiet. They pump 5 psi which what you are looking for . If you go this route , just be sure to get one from a carbed car not injected. '79 or '80 I know are carbed. The fuel filter could also be your problem as was mentioned . Your car is wired for a elect pump already . I just added a oil pressure switch to kill the pump in the event of a accident or the loss of oil pressure . The RX pump will supply a set of triples on a stroker , in case you are wondering . Gary:)

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There are any number of generic electric pumps out there that will do the job, although some may be noisy. (That's one reason people like the RX7 pumps, they are relatively quiet.) I picked up a generic pump at a local parts house for $30. No brand name, unfortunately. But tell them you want a small electric pump that delivers no more than 5 psi, and around 30 gallons/minute.

The one I got looks like the one in the following link. It was available in a 2.5-4.5 psi model, and also in a 5-7 psi model. You probably want the lower one.

http://mgaguru.com/mgtech/fuel/fp201.htm

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This could be more than just a simple fuel pump problem or filter problem. Are you sure it is not getting fuel at the higher RPMs Have you taken the hood off and let someone ride in the engine compartment to see if your second carb is getting fuel or not? No just kidding on that part :finger: Clean your fuel filter or get a new one and see if that helps. If not then run a fuel pressure test to see if you are getting the correct fuel pressure. What I would do is get some hoses from Home Depot that are long enough to run inside of the car. Then while driving it, check if the fuel pressure is reading in the correct range. If it is staying at 5 psi then you have another problem that would most likly be to electric. Could be distributor, points(if you still have them) or even the plug wires are not conducting right. It may run fine when cold but once heated up and spining at the RPMs you say it could be losing spark and not fuel. Just my 2 cents.

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NEVER run fuel lines into the cabin of your car...NEVER EVER. Can you say crematorium? If a leak developed you'd by shocked (and probably dead) by the volume of fuel a simple 5 psi pump can put out in short order. The idea of having someone riding in the engine compartment at speed is more sane.

Did you perform a tune-up on this engine (wires, plugs, etc.)? Had you performed any mods or service when the problem materialized? Did you clean out the 'last chance' filter where the fuel line attaches to the carbs? Did you replace the old fuel line from the fuel rail to the carbs? Are you using a fuel rail or just jumping fuel lines from pump to carb to carb?

I'm assuming that when you stop the car car that the rear carb is not visually providing fuel. It sounds like a starvation (possible fuel filter) or a vapor lock issue to me. Can you put some pictures up so we can see how everything is installed?

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Did you perform a tune-up on this engine (wires, plugs, etc.)? Had you performed any mods or service when the problem materialized? Did you clean out the 'last chance' filter where the fuel line attaches to the carbs? Did you replace the old fuel line from the fuel rail to the carbs? Are you using a fuel rail or just jumping fuel lines from pump to carb to carb?

I'm assuming that when you stop the car car that the rear carb is not visually providing fuel. It sounds like a starvation (possible fuel filter) or a vapor lock issue to me. Can you put some pictures up so we can see how everything is installed?

Only thing I did was change the oil, new thermostat, change coolant, new distributor, valve job, new battery, all new fuel lines except for return line, and add a tachometer prior to this happening. The car worked fine for a month or so while i performed all of the above and only started acting up randomly on the freeway this past friday night. When i noticed the problem i took both carb's off, completely rebuilt them (i mean completely) and put them back on with new gaskets. When i went to test drive the car after putting the carb's back on the problem was still happening. This morning i put in a brand new fuel pump and it is still happening. I am stumped because this car was driving great and then all of a sudden this happened. I have done nothing mechanical that would have caused it.

I am running a fuel rail and there is a brand new clear fuel filter so i can see whats going on. Plenty of gas is being given to the car cause the filter is always full. Like i said i replaced all fuel lines but the return line. The way it is set up (dont have any pictures) is from the tank it goes to the filter. After the filter to the pump and from the pump to the fuel rail. Carb number one gets fed first and has no problem. Carb number two gets fed second and is the trouble maker. After carb number two fuel is sent to the return line. I know that that carb isnt spitting gas at high rpms because when the problem happens i get out with the car running and manually pump the carbs. Number one works fine and number two shoots nothing. If i turn the car off for a few minutes, then restart it, both carbs will work fine until i get up to around 4000 rpms and it happens again.

I do have a few questions tho...

1. Can i get around the fuel rail and just go straight from pump to carb to carb?

2. What would happen if i was to pinch the return line off completely and just run straight to the carbs?

I really appreciate all of your suggestions by the way. If i didnt have a place to talk about this stuff i would be in a world of hurt. Thanks so much guys.

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Not being the mechanic that the other responders are, my suggestions may be moot, or not applicable. But with that out of the way....

Why not swap the fuel line to the Carbs, 1 goes to 2 and 2 to 1, then run your check again. If the problem now is in Carb 1, your problem is not the carburator but at a guess in the fuel rail between 1 and 2. If on the other hand, the problem is still with Carb 2, then your problem is in the inlet portion of that carburator.

Next, try feeding the fuel directly to the carburators from the fuel pump. Use hoses approved for fuel use as well as the "Y" splitter. Check again and see what your results are.

Unless you've managed to hook up the electronic choke on the carburators I wouldn't mess with anything electrical. All you'll do is compound the problem.

Eliminate the fuel supply as the problem or not, THEN go on to other things.

FWIW

E

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Sounds like the problem is with the #2 carb . I don't know a thing about Weber's so I cannot be of help there . Question do the Weber's have a filter or screen in the inlet to the float chamber ? The fuel rail is there so that the heated fuel gets flushed back to the tank rather than just sitting there over the exhaust manifold and boiling. There is a metering hole at the end of the fuel rail that keeps he pressure up in the system when the demand is greater . So it is not just a tubing that runs back to the tank . If at idle and the engine is running fine , can you shut it down and remove the float bowl cover and check the level , then do the same when the engine malfunctions . Just clutch it and turn it off and pop off the cover and see the fuel and compair . Just a thought. Gary :paranoid:

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I installed the oem mechanical replacement just to see if i was having a pump issue. I just got off the phone with top end performance and they said my lobe might be worn that drives the pump so i should definately just bite the bullet and go electric.

The lobes rarely ever get worn that bad, in my experiences. You can take your valve cover off and inspect it yourself, to rule that problem out.

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