Vacuum leak or carb, or both?

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I resurrected a 1979 fj40 after sitting for 20 years. All emissions equipment intact. Fully rebuilt carb, new fuel tank and fuel filters.

Its first tank of gas everything worked/ran flawless. I set the timing yesterday and went for a drive afterword. Ran great until I ran out of gas, had a buddy bring me gas, no big deal. Driving back to the shop I noticed it would stall at stop signs. Had to keep my foot in the gas to keep from stalling, also slight hesitation when accelerating from low RPM's.

Back at the shop and it will barely idles, eventually stalls out. Unhook the idle cut off solenoid, and it clearly won't run at low RPM's. I determine idle cut off solenoid is not the issue. Hook up vacuum guage which shows poor vacuum, pull choke knob and vacuum is better but still weak. Next I remove air cleaner and spray starter fluid near intake manifold. There is clearly an RPM increases when sprayed near #2 and #4 cylinder on base of intake manifold. Also very slight increase when sprayed a base of carb.

These vacuum leaks are surprising to me because I had both manifolds off, new hardware, gaskets and proper torque on everything. Not to mention the truck ran flawless the prior week.

Here is where I get really puzzled. Since I had the air cleaner off I squirt a liberal amount of carb cleaner down the carb. Start it up and it seams to run better, do it again and it runs even better. It's almost back to the point before I started having an issue. Still stalls at idle but much better than before the carb cleaner trick. Drive it home tonight with the choke ever so slightly out to keep it from stalling and performed great.

Any thoughts as to what could be going on here? The more I think about it I am starting lean more toward a carb issue, the carb cleaner made a big difference. Also, I am second guessing the vacuum leak test using starting fluid. The starting fluid could be getting sucked up somewhere along the air rail causing the increase in idle. I am pretty sure those air rail tubes are not totally air tight.

Eventually my plan is to do a full desmog and go with a sniper. At this point I'd just like to establish a good baseline of everything working/running as it should prior to any mods.

Any help or advice is appreciated.
 
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I’d lean towards fuel system clog. Ran to the bottom of the tank and sucked something into the lines. Could have made it to the carb.

I’d start simple and change the fuel filter.

As far as the manifold gasket…. I switched mine out without machining the surface. Used an oem replacement. Still leaked. Shop changed it out to an aftermarket that has a way larger tolerance for flatness and it took care of the issue.
 

ToyotaMatt

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Maybe its time for a more robust approach ?

rebuilding 30-50 years age and time wear parts don't work after a while


install a KYRSTAL KLEAR fuel filter pre oem filter , then you can confirm the bad fuel theroy or NOT ?

its TIME for a NEW OEM carburetor perhaps .......



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Pighead

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Air rail tubes go just to the exhaust, not intake. Have you re-torqued your manifold studs? Should do routinely after a change of gaskets...
 
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Air rail tubes go just to the exhaust, not intake. Have you re-torqued your manifold studs? Should do routinely after a change of gaskets...
I don't know why I was thinking the air rail fed the intake, not much sense in that.

Looks like next step is to pull the carb and re-torque the manifold studs.
 
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I’d lean towards fuel system clog. Ran to the bottom of the tank and sucked something into the lines. Could have made it to the carb.

I’d start simple and change the fuel filter.

As far as the manifold gasket…. I switched mine out without machining the surface. Used an oem replacement. Still leaked. Shop changed it out to an aftermarket that has a way larger tolerance for flatness and it took care of the issue.

Contaminated fuel system is unlikely but I will check after I go through the manifold. Currently the paper fuel filter shows only clean fuel, sight glass in carb is always full and it run great at full throttle.
 

65swb45

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If the sight glass is full, that would be an indicator of a problem. Should be around half. And you didn’t mention your vacuum numbers OR the last time the valves were adjusted.
 
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If the sight glass is full, that would be an indicator of a problem. Should be around half. And you didn’t mention your vacuum numbers OR the last time the valves were adjusted.
@65swb45

My mistake, by full I meant half sight glass. Valve clearances were done within the last hundred miles.

Vacuum numbers were at about 16 - 17 inches off brake booster line. Very low RPS's, barely running at this point.

When I sent my timing earlier that day the vacuum was right around 20 inches. No tachometer hooked up but sounded like around 650 RPMs. This was before I ran out of fuel and started getting the erratic idle problem.

In addition to poor idle, there is a noticeable flat spot when driving. Acceleration is great, but about mid RPM's when cruising there seems to be a flat spot.
 
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mine was doing same and it was because fuel tank had crud in it, partially clogging and then fully clogging the fuel line. if you ran out of gas, you may have sucked something down the fuel line. as said, check/replace your fuel filter. while its unhooked, make sure your getting proper fuel flow from the tank.
 

Pighead

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Both a failing fuel pump and a line blockage could be ruled out by watching the porthole while it idles and dies
 
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Good information everybody, I appreciate all the help. Looks like I got a little in front of me.
 
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Typically a vacuum leak would cause a fast idle. At least that has been my experience. But if you noticed an accelerated idle after spraying starter fluid around the intake then you definitely need to address that as others have suggested. Trash in the fuel line would be my guess. Start simple, replace the fuel filter with a cheap one from auto zone. If that's the culprit then buy a good oem one. You say your buddy brought gas, was it new gas, old gas that's been sitting forever, new gas in a dirty can?. I had someone bring me "gas" once. Turned out to be varsol. Drained the tank, put fresh gas in and problem solved. If it was me I would probably drain the tank too. That way you know you have fresh gas. Drain tank, new gas, new filter see what happens. Fix your vacuum leak, then set your idle on your carb. Hopefully no trash got to your carb. If it was old gas with ethanol sitting in garage for a year then you may need to rebuild the carb too.
 

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