2F cuts out when braking, nose down? (1 Viewer)

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Strange but more and more I am noticing that when coming to a quick stop while going downhill in my 1980 FJ40, the engine to cut out almost like it has run out of gas.

Almost everything under the hood is stock. I did replace the fuel filter about 6 months back so I wonder if I have a carb problem or perhaps a fuel pump issue? Neither have given me any trouble for as long as I can remember but things do happen.

I wonder if anyone else has seen this and found the cause?
 

John Smith

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Is this only with the engine hot? What is your idle speed? What is the vacuum at idle? Possible causes are anything from vacuum leak, to carb problem, to ignition issue...
 
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I would think carb problem, when you take you foot off the gas and put on the breaks the fuel pump does not need to pump much fuel. Maybe your float bowl does not have enough fuel in it, so that when down hill and slowing down the gas get pulled to the front of the bowl and no gas is getting down carb. That is my guess. Check your vacuum hoses, maybe they are shifting and leaking only during down hill braking, but seems like remote possibility to me.
 
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No snorkel or any other mods. Just stock carb and air cleaner.

This generally happens when hot, but that's just my driving pattern.

I did have the idle adjusted about six months ago. It idles faster than before now that it is adjusted.

Vacuum leak is an interesting idea. I will have to hook up a vacuum gauge will driving and monitor it. Right now though, I don't hear any vacuum leak.

The carb is also a possibility. Is there a sight glass anywhere on the carb to show the level of fuel in the float bowl? That would make it easy to see if it's a float problem.
 

Cdaniel

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Do you have power brakes?
If so, what shape is the booster in?

A possible vac leak appearing only under hard braking=big vac leak and rough running and or stall.

I had a truck stop dead in it's tracks one time. Big bump knocked the hose off of the booster. Instant stop.
 
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I was poking around under the hood tonight looking for vacuum leaks and could not see or hear any.

I did notice one strange thing. There was no sign of fuel through the sight glass in the front of the carb. It float bowl appeared to be dry.

Is there an easy way to check the pressure the fuel pump puts out?. AFAIK, the fuel pump is original and with many years and miles on it may be the problem.
 
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Search this forum, you can disconnect the fuel line from the carb and direct it into a container. There are posts on how much fuel it should pump while cranking for an amount of time. Check your fuel filters often along with what degnol said. All can cause this problem. My bet is on the float not adjusted otherwise I think you would see other problems other then only while braking down hills.
 
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I'll check the fuel filter tonight -- this is very easy. Not sure about the float, needle/seat and fuel screen.

The 1980 smog laden carb is a little intimidating to look at. What would I need to remove to get inside the float bowl?

Would I need to replace the gaskets while putting it back together?
 

John Smith

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I was poking around under the hood tonight looking for vacuum leaks and could not see or hear any.

I did notice one strange thing. There was no sign of fuel through the sight glass in the front of the carb. It float bowl appeared to be dry.

Is there an easy way to check the pressure the fuel pump puts out?. AFAIK, the fuel pump is original and with many years and miles on it may be the problem.
Remove the fuel supply hard line from the front of the carb and disconnect the coil wire to the dizzy. Put a catch can under the fuel line while someone turns over the engine for 30 seconds. If your fuel pump is any good, and the line from the tank not clogged, you should have a cup of gas in your catch can. Report back.
 
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It's either 1) carb float adjustment or 2) massive break booster vac leak.

Rule out the vac leak in the booster by hitting the brakes while idling in the driveway. If your idle doesn't change for the worse, it's probably not the brake booster.

You won't necessarily "hear" a vacuum leak
 
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Minor update today. I tried the brakes on hard in the driveway and no affect on the idle so vacuum leak is not the issue. Looks like a fuel supply problem.

I also checked the fuel filter next to the fuel pump. It was full of fuel so I suspect the problem is between the pump and the carb.

I'll try disconnecting the hard line this weekend and see what I find.
 
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My gues is you could have a weak fuel pump or. I had this happen to me on my 79. It would idle fine but once you got on the highway it would start bucking....not every time but only did in highway speeds. You may have a faulty fuel pump but it also would not hurt if you put some sea foam in the tank if you have not done so recently.

If you have A bad egr valve it would not idle. However, I have seen them work fine but once you come from highway speeds and slow down to a stop you engine tends to die. If run out of options the egr could be a culprit as well. But I rule the fuel pump first or a vaccum leak before considering the egr valve.
 
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A carb can slosh out the float bowl vent on hard breaking and flood out the engine. I am not familiar with the carb on the 2F but it could be your float level is too high or you are missing the vent tube riser, if there was one - again not familiar with your carb.

I have a sbc with holley 650 in my 40, when i put the front disc brakes on, i had a similar problem with the engine cutting off on hard breaking. The drums never stopped the truck fast enough for this to happen. I removed the stock vent tubes and installed longer ones made of some ss fuel line i had laying around - that solved the problem.
 

Southeast Overland

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I had a very similar problem last year in my 55. Ended up being a pretty big leak at the intake/exhaust manifold to exhaust. Replaced with a header and Remflex gasket and problem disappeared.
 
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I ran the fuel line into a bottle while cranking the engine for 30 seconds. I was surprised to catch 2.5 cups of fuel -- is my pump on enhancing drugs or is this normal?

I also did not find any screen to clean in the fuel inlet area so I'm beginning to think it is a float adjustment problem?

One other observation I made was after having the line disconnected from the carb, I saw the fuel level at midway in the site glass. It made me wonder if I was mistaken about the fuel level in the float bowl being too low. Maybe it was so high all along that I couldn't see the fuel level through the site glass.

Not sure how the diagnosis changes if the fuel level is really too high in the float bowl? Should the level always be visible at half way in the site glass?
carb.jpg
catch_bottle.jpg
 

John Smith

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Well your fuel pump appears to be working well. I think you can rule that out. The fuel level in the sight glass should be in the middle. If the fuel level is too high or low you need to adjust the float. Test for vacuum leaks by starting the truck and spraying carb cleaner around the intake / exhaust manifold gasket area and the carb base. If the engine idle speed goes up while you spray you found a vacuum leak.
 
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I had a friend that had this problem with his 75, turned out to be a bad wire on the coil. When he hit the brakes hard on the down hill the wire would move just enough to seperate and cause the engine to die.
 

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