Engine just quits when HOT (200 degrees or so)

[pigman]

I have a 400 CID stroked to 470 CID with .450 lift, rollers, 3 inch pipes, etc ... The problem is that when she gets hot she quits.

I can immediately pour some gas down the big Holley and she will fire up and burn, but then dies.

I think that means my MSD 6AL is doing the job. (I am thinking vapor lock)

I wonder if it's as simple as a bad gas cap? What does a 1972 Roadrunner use for it's gas cap? I.E. vented or unventilated? The one that is on there is about 12 years old so I don't mind replacing it.

I put 3 inch pipes on the headers and ran em to the back .. wonder if they are too close to the metal fuel line?

Could the mechanical fuel pump work ok cold and then quit when it gets hot?

I have considered insulating the fuel lines and may look into a heat blocker of some kind where the metal fuel lines run across the engine block.

Any help would be appreciated. http://www.tpigman.com/plymouth/rr.html

 

[Jonnyuma]

Sounds like classic vapor lock. Whether it's happening under the hood or further back is gonna be a tough diagnosis.

 

[shag766]

it fires pouring direct to carb. so, humph, pump can fluctuate if old. short of throwing parts at it, if possible i would insert 2 temporary pressure guauges in the line. 1 after the pump and 1 well back before pump. pour gas in carb slowly and controlled, eye ball gauges.its a 2 person op but it will narrow down the location. or throw on a pump and go from there. .02 cents

 

[383man]

I would go with an electric fuel pump and see if that fixes it as if it is vapor lock I bet an electric pump will fix it. You will have fuel under pressure coming from the tank instead of fuel under vacum being sucked up to the pump. Fuel under pressure has a higher boiling point as the fuel in a vacum will vapor lock much easier then the fuel under pressure. Its worth trying if you think its vapor lock. Ron

 

[pigman]

Thanks guys ... not that it matters that much but the valve lift is .550 not .450 (LOL got to keep these specs right) .. I think we will start by putting a fuel pump gauge just before the dual feed Holley and probably replace the mechanical fuel pump first.

Then look at the fuel lines to see if they run anywhere close to the pipes. Thanks for the replies ... does anyone know the type of fuel cap to purchase? When the engine stops and I go back to check on the cap .. I unscrew it and hear a nice "whoosh" sound. I can't tell if it is air going IN or air going OUT. Cap is old .. don't mind replacing it.

 

[pigman]

More .. I noticed when they rebuilt my engine .. they dropped the line that went to the (useless) charcoal canister ... I need to make sure my gas tank is vented as another possibility is after about 20 mins a vacuum could build up in the tank and refuse to let fuel flow to the pump and on to the carb.

Will check that out too.

 

[pigman]

Note the single hose going to that canister in this shot. http://www.tpigman.com/plymouth/engine2/176.JPG

 

[pigman]

I would go with an electric fuel pump and see if that fixes it as if it is vapor lock I bet an electric pump will fix it. You will have fuel under pressure coming from the tank instead of fuel under vacum being sucked up to the pump. Fuel under pressure has a higher boiling point as the fuel in a vacum will vapor lock much easier then the fuel under pressure. Its worth trying if you think its vapor lock. Ron

Thanks Ron .. your description of vacuum and sucking makes sense .. first thing I am going to try is a fuel pressure gauge on the other side of the fuel pump to see if that thing is not happy. Then make sure I am getting my fuel to that Holley under pressure ...

Will let you know.

 

[pigman]

it fires pouring direct to carb. so, humph, pump can fluctuate if old. short of throwing parts at it, if possible i would insert 2 temporary pressure guauges in the line. 1 after the pump and 1 well back before pump. pour gas in carb slowly and controlled, eye ball gauges.its a 2 person op but it will narrow down the location. or throw on a pump and go from there. .02 cents

good advise and I plan to take it to see how it plays out~

 

[pigman]

 

[pigman]

 

[Jonnyuma]

If the old canister line has been plugged then you'll need a vented cap. Or, if you want a non-vented cap (for originality OR local emissions compliance), what I did was make a filter out of an old sending unit sock and put it right at the tank, where the line from the charcoal can used to go. It doesn't have to be fancy, its just to keep the crud out of your tank.
As gas is removed from the tank, it has to be replaced by air. If air can't get in...vacuum, and the hissing or whooshing sound when you release the cap.
By the way, you don't need an expensive, racy electric pump. You can get a cylindrical in-line pusher from the parts store and plumb it into the fuel line back by the tank. About 20bux. You can rig it to a switch to only come on when you want it to, too. As a backup or, to impress your friends.
And if your lawn mower runs out of gas you can just connect a longer hose, flip the switch, and transfer the gas you need.
Notice that the cheapo electric pumps aren't regulated, so you'll probably want to installed a pressure regulator, also pretty easy.
That said....the rebuildable, more expensive pumps would be better and more durable for heavy race use.

 

[pigman]

If the old canister line has been plugged then you'll need a vented cap. Or, if you want a non-vented cap (for originality OR local emissions compliance), what I did was make a filter out of an old sending unit sock and put it right at the tank, where the line from the charcoal can used to go. It doesn't have to be fancy, its just to keep the crud out of your tank.
As gas is removed from the tank, it has to be replaced by air. If air can't get in...vacuum, and the hissing or whooshing sound when you release the cap.
By the way, you don't need an expensive, racy electric pump. You can get a cylindrical in-line pusher from the parts store and plumb it into the fuel line back by the tank. About 20bux. You can rig it to a switch to only come on when you want it to, too. As a backup or, to impress your friends.
And if your lawn mower runs out of gas you can just connect a longer hose, flip the switch, and transfer the gas you need.
Notice that the cheapo electric pumps aren't regulated, so you'll probably want to installed a pressure regulator, also pretty easy.
That said....the rebuildable, more expensive pumps would be better and more durable for heavy race use.

Thanks ... :hello2: I do notice that when the engine stops .. I can go back to the licence plate (LOL) lift it up, undo the gas cap and hear a pretty loud hiss ... I don't know of course if air is coming out or going in.

I like the idea of a pusher pump as I could use it when needed. I am sure a pressure regulator would be mandatory if I push more fuel than the mechanical pump can push.

I suspect vapor lock cause it is def fuel starvation .. but it could just as easily be NO FUEL to the pump.

Just for the hell of it .. I purchased a new gas cap and noticed that I could suck air through it .. but not blow air THROUGH it .. so I would guess that proves it is a ventilated cap.

So far .. I have not gone with blowing or sucking my old gas cap .. so many gallons (I get about 6 MPG) of gas it don't seem appealing.

 

[pigman]

Update: Okay .. so my actual problem is after running about 20 mins, the engine stops with a typical out of fuel experience. I dump raw gas down the carb, she fires up and runs till that gas is exhausted. I suspected vapor lock as it happens when the engine is hot.

So .. put a fuel pressure meter on the line between the fuel pump and carb and fired her up cold and after about 20 mins .. she died. Checked the gauge and no pressure. Checked the pump and no gas.

So ... now I suspect either a clogged gas tank, bad cap (won't let air in) or clogged fuel lines from tank to pump.

Dropped the tank and found no problems .. the filter sock was almost clean and as far as I know is the same one put their in April of 1972.

Blew air through the fuel line from the back of the car and sometimes it was fine, and sometimes not so much.

So ... I suspect I will have to run new fuel lines to fix this issue. thanks for all the pointers ...

Pigman

 

[pigman]

Well .. found at least a serious root problem. The new fuel pump didn't perform any better than the old one. Took out the rod that rides the cam and drives the fuel pump and found it to be about 1 inch shorter than one from another 400 CID block.

With my expensive Comp cam and valve train we are going to put in a rod with a brass tip to avoid cam lobe damage in the future.

 

[pigman]

Specs

Ross flattop 99495 forged racing pistons.

400 CID to 470 CID Long rod

Stroke 3.915"

Rod Length 6.760"

Piston Height 1.238"

Rod Ratio 1.726

 

[pigman]

I think the vapor lock was brought on by the fuel pump not working right and then allowed a smaller amount of gas to flow and vaporize. I thank all of you for thinking this through for me.

 

[splicer]

...Had precisely the same issue with my engine and it was definitely an overheated carb, which in turn percolates the fuel out of the bowls and engine quits.
505 stroker
870 Holley Avenger
Carter 120 GPH mechanical fuel pump.

The following solved the problem:
1) Mr Gasket aluminum heat shield
2) 1" phenolic carb spacer
3) Insulate fuel line and filter between pump and carb

The above steps were taken in the order listed and 1 and 3 made the biggest difference.
Engine can now get up to 200 deg in traffic and not burp, stumble or quit.

Hope this helps.

Splicer

 

[pigman]

...Had precisely the same issue with my engine and it was definitely an overheated carb, which in turn percolates the fuel out of the bowls and engine quits.
505 stroker
870 Holley Avenger
Carter 120 GPH mechanical fuel pump.

The following solved the problem:
1) Mr Gasket aluminum heat shield
2) 1" phenolic carb spacer
3) Insulate fuel line and filter between pump and carb

The above steps were taken in the order listed and 1 and 3 made the biggest difference.
Engine can now get up to 200 deg in traffic and not burp, stumble or quit.



Hope this helps.

Splicer

Thanks for taking the time to reply .. I had the stock TQ with the phenolic space which I kept and put on top of an edelbrock performer intake. I also took your advice and insulated the fuel line. But the real problem was a worn fuel pump rod .. see photo .. it is a good 1/4 inch shorter that it should have been. I put in a new one with a brass tip and even though it is between 90 and 100 degrees out on the road here .. I have had no further trouble!

 

[pigman]