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HELP, no fuel

Mud Rat 12

Mud Rat 12

1/2 ton status
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Jul 31, 2000
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Location
Citrus Heights, Ca
I was letting it idle so that it could warm up and it slowed from 800rpm to 400rmp. I tryed to give it gas to speed it up and it died. Now i can't get gas to the carb, i changed to filter and it does not get gas that far up. It was sitting for a year and i had it running fine last week. I hevent had time to pull the pump but how can I see if that's what it is:confused:
 
If you've got gas coming out of the feed line when you take it off of the pump , it's the pump. You could remove the feed line and replace it with a pipe and funnel and see if the pump sucks it through. They do go out and often after sitting around for a spell.
 
If your motor is stock or there abouts, just use a mechanical one. They're cheap and will last years. Bolt it on and fugetaboutit!
 
the motor is not stock. it is 9.5to 1, full msd, 600cfm edelbrock carb.. i know it sucks, and a rv stage 3 cam. it is rated at 350hp and 400ftlbs. I am looking into efi swap later but my budget is stoping me. I just want it to run at this piont:( .
 
Good mech pump will be fine. Save your pennies for the EFI swap, that is worth it's weight in gold!
 
check the rubber fuel line down by the frame and fuel pump , maybe a collapsed line
 
I recently had a similar situation and replaced the performance Holley pump with a $16 Napa special.

I did some searching online and found that your necessary Gallons Per Hour on your pump at Wide Open Throttle is ROUGHLY HorsePower/12. My Napa pump says that it does 23 GPH which means it should support around 276 HP at WOT.

**EDIT** <not necessarily, see posts below>

Desktop Dyno claims I'm just over 300, so I may be returning to a High Performance pump if it starts getting too lean when I lean on the stupid pedal.

Side Note:

I had to get my Blazer "running" with starting fluid for a few seconds in order to get enough suction with the mechanical pump to get it primed. Lesson Learned for me :)

-Ben
 
Last edited:
I actually just learned a little more.....
That rated GPH is for "zero load"... not what a pump sees when it's pushing against a load like a Carb.

I just found this site this morning:
http://www.centuryperformance.com/fuel.asp

He claims that the divide by 12 rule is only for zero load. (oops)
(multiply by 1/2 and divide by 6 = divide by 12)

For realistic numbers, he says to multiply horsepower by .23 for "dead head" style regulators. (which is what a carb essentiall is, I think).

While this site is based on Electric pumps, it probably carries over to our mechanical world fairly well.

So.... for your 375 Hp engine and his math, you would need a roughly 86 GPH pump.... 3.75 times more than what my Napa pump is pushing.

I don't want to lead anyone wrong here, but I'm goign to be upgrading my fuel pump!!!!

-Ben
 
Another thought on CFM.
I always hear that people don't need nearly what they think they do.

Any internet caclulator is just a guess.... but I checked this one for mine:

http://www.corvetteactioncenter.com/tech/tools/carb.html

By iterating I found that with my 383, a 750cfm Quadrajet, Dual Plane Performer Intake and .85 VE (see their recommendation) I would max out at about 5300 RPM... hotter than I'll ever need to run. I think my torque curve maxes around 3000 RPM :)

Throwing in your engine at 350 ci, .85 VE and a Dual plane intake (guess) your 600 CFM carb should limit out around 4700 RPM.... probably plenty!

Like I said, only rough calculators, but I like to use them to see if I am 'close'. I'm no engine builder, so I need all the help I can get!
 
so if the math is correct i need a lot bigger pump then i have to fix my lean issue. I am going to look at new pumps tomarrow. What do I do with the factory return line... plug it?
 
That, my friend, is an excellent question. You could plug it.

(I'll be honest here.... I'm just now learning about a lot of this while I'm troubleshooting my system. Take everything with a grain of salt because I don't "Know" yet. I'll give you links to what I've learned so you can decide what you want to do.... :) )

http://www.chevyhiperformance.com/techarticles/0606ch_carbureted_motor_fuel_system/

http://www.circletrack.com/techarticles/58838/

http://www.holley.com/data/Products/Technical/Fuel%20Pump%20Tech%20Info.pdf

My understanding is that the return line greatly helps to avoid Vapor Lock because the fuel is always flowing, refreshing itself, and staying cooler.

You can run one line to the carb and have it dead-end into the carb if the pump is low pressure (they manufacturer websites list whether you can do this or not). I am running mine like this currently. I don't have a return line and I think I am having some vapor lock issues.

I suppose one way to go might be to get a better pump and run it into a 'return flow' regulator. From there one could be the carb line and one could plumb into your return line. A lot of parts, plumbing, and money, but it would get you where you want to be without making the full jump to an electric pump.... which would probably work best with the return regulator anyway.

I'm at about the limit of my knowledge here. Hopefully someone else will chime in and contue to lead us to perfect fuel delivery!
 
All I've got to say is that the whole calculator for CFM carb requirements *I think* is being proven bunk.

GM used 800CFM carbs on 5L engines at least as far back as the early 80's, and I've got an article sitting right here in front of me (Engine Masters Fall '06) that dyno tested a 440 with 4 carbs from 650-1050CFM. I'd suspect that based on the carb caluclators, the 1050CFM unit is overcarbed. Well, it would be wrong. The 1050CFM unit made more torque AND horsepower at every RPM, than 650, 750, and 850CFM. These carbs were all individually tuned, so it wasn't just a slap it on and hope deal.

The RPM range started kind of high, but the difference between the 650 and 1050 unit was 28HP at 3500RPM, and it made 49lb ft of torque at the same RPM. That shows that even at lower RPM's there would be a gain.

These dyno tests with so called "over carbed" engines have become increasingly popular, and so far I haven't seen one that showed less power (torque particularly) the bigger you went with the carb.

I checked the engine masters website, the article isn't up (yet) that I see.
 
Dorian,

I think I agree with you.

I was more trying the address the concern earlier that he could be forcing the engine 'lean' by only having 600 cfm. The quick calculator was showing me that the carb wasn't that far 'out of line' for the engine. I was just looking for a ballpark guess.

I also used it to reassure myself that the 750 Q-jet wasn't going to choke my 383.

I was using it to answer the question: "Is my carb too small?", rather than "is my carb too big"..... :)

From what you're saying, I would think that there is every possibility that a bigger carb could be better.
 
I was just trying to figure out why I could not get it to run right at freeway speed, was 3500-4000rpm on my 36's. Now that I have 42's it will be a lot lower. I am going to put a 110gph pump on it and see if that help's. I also am going to put a filter midline to stop that crap in the bottom of the tank out of the pump and carb.
 
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