Another only runs when choke pulled thread (1 Viewer)

Guster

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I've tried to read as many of the "only running when choke pulled" threads as I could find but I'm still trying to figure this one out. The engine will run ok when choke is pulled and runs at around 1200 rpm with a low, steady vacuum reading around 12 (at sea level). At lower rpm it will die and pop back through the carb. I don't know of a way to get timing set correctly without being able to run at lower rpm. I'm trying to decide whether or not I should send my carb off for a pro rebuild, go with Sniper or if there is some other problem on the bottom end. If i have a manifold related leak or bottom end problem then I'm thinking going with Sniper won't help anything. Here's what I know and have done over the last several weeks:

  • 72 FJ40 F engine with unknown history
  • Replaced head gasket and valve seals, cleaned carbon buildup off of valves, cylinders looked good
  • Cold valve adjustment when putting back together and will be checking valves again today (cold check again since I have manifold removed)
  • Vacuum reading from fitting on the manifold in front of carb, brake booster vacuum line plugged
  • Manifolds machined for flat surface, gasket from an engine gasket set bought through good vendor, will likely try remflex gasket next
  • Large cap FJ60 distributor, I added a vacuum port to the carb and it has been capped during this process
  • Dry compression test 1-6: 150, 158, 140, 160, 160, 160
  • Idle solenoid confirmed working and clicks, since my vacuum reading at 1200 rpm is low I'm assuming that means the idle circuit is not the problem
  • Rotor is pointing at number 1 plug connection when timing mark is lined up with the line on the flywheel
  • PCV is connected to the insulator below the carb to the PCV valve and I did not plug this during testing
  • New exhaust system
  • I rebuilt the carb and certainly could have messed something up during that process

Based on all of that is this still most likely a vacuum leak?
Could it be a camshaft problem?
If compression is good does that mean valves are at least close enough until I can adjust with a hot engine? Cylinder 3 is a bit low compared to the rest so that one might be off.
What should my vacuum reading be at 1200 rpm?


head.jpg


rocker.jpg


manifolds.jpg
 

whitey45

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Compression numbers are great, My gut feeling is a vacuum leak. Have you sprayed carb clean around the intake runners and base gasket of the carb?
 

Guster

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Compression numbers are great, My gut feeling is a vacuum leak. Have you sprayed carb clean around the intake runners and base gasket of the carb?

I did and could not find an area with an obvious change but will do this again when running again.

Thanks @Skeet, the problem is I can't get it to idle low enough to start that process. I've tried various settings of the mixture screw with no major change to how it runs.
 

Guster

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Checked valves and most were a little tight and one was loose, I think #3. Not sure how I didn't notice before but the manifold studs are in pretty bad shape so going to order a set. Hopefully the tight valves were the problem.
 
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Check any diaphragms on your carb and distributor for air leaks. Sometimes during a carb rebuild junk can still be in there, taking it apart again is free and doesn't take very long. You can also double check that the idle solenoid functions properly visually. When you idle with the choke it pulls to much fuel out of the carb so it is not an indicator of the idle circuit functioning correctly.
 

Guster

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The valve adjustment did not change the vacuum number. I disconnected each plug wire and could notice a difference with each so all cylinders appear to be firing. Sprayed carb cleaner at the base of the carb and did notice a slight change in idle. I had used gaskets between the carb insulator and intake and also between the insulator and carb. I removed those gaskets and that might have helped a bit. Vacuum was closer to 15 but was running over 1500 rpm. Sprayed carb cleaner at base of carb again and there is definitely an rpm increase where the PCV fitting goes into the insulator so need to look into how to fix that leak.
 
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Arizona
Your distributor, is it points, electronic, or pertronix? Reason I ask is I had a similar issue with a points distributer , seemed to get better with more RPM. When I changed it to a non-usa with pertronix , I was able to idle properly and do the normal stuff needed. Old distributor had wear on the shaft and I believe it effected the points open / close the slower the shaft was turning. Just a thought.
 
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Jan 22, 2007
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Alva, Fl.
I've tried to read as many of the "only running when choke pulled" threads as I could find but I'm still trying to figure this one out. The engine will run ok when choke is pulled and runs at around 1200 rpm with a low, steady vacuum reading around 12 (at sea level). At lower rpm it will die and pop back through the carb. I don't know of a way to get timing set correctly without being able to run at lower rpm. I'm trying to decide whether or not I should send my carb off for a pro rebuild, go with Sniper or if there is some other problem on the bottom end. If i have a manifold related leak or bottom end problem then I'm thinking going with Sniper won't help anything. Here's what I know and have done over the last several weeks:

  • 72 FJ40 F engine with unknown history
  • Replaced head gasket and valve seals, cleaned carbon buildup off of valves, cylinders looked good
  • Cold valve adjustment when putting back together and will be checking valves again today (cold check again since I have manifold removed)
  • Vacuum reading from fitting on the manifold in front of carb, brake booster vacuum line plugged
  • Manifolds machined for flat surface, gasket from an engine gasket set bought through good vendor, will likely try remflex gasket next
  • Large cap FJ60 distributor, I added a vacuum port to the carb and it has been capped during this process
  • Dry compression test 1-6: 150, 158, 140, 160, 160, 160
  • Idle solenoid confirmed working and clicks, since my vacuum reading at 1200 rpm is low I'm assuming that means the idle circuit is not the problem
  • Rotor is pointing at number 1 plug connection when timing mark is lined up with the line on the flywheel
  • PCV is connected to the insulator below the carb to the PCV valve and I did not plug this during testing
  • New exhaust system
  • I rebuilt the carb and certainly could have messed something up during that process

Based on all of that is this still most likely a vacuum leak?
Could it be a camshaft problem?
If compression is good does that mean valves are at least close enough until I can adjust with a hot engine? Cylinder 3 is a bit low compared to the rest so that one might be off.
What should my vacuum reading be at 1200 rpm?


View attachment 2650156

View attachment 2650158

View attachment 2650157
Timing --rotor should be pointing at approx #4 cylinder when #1 is tdc(7 deg btdc)
Vacuum--12 in HG is quite low--recheck point gap--it may be too tight(you should be seeing 16-18 in HG at 700 rpm)--not bouncing around--see vac diagnostics
Carb rebuild--did you use Pinhead's carb rebuild 7 series videos for this?--if not, may want to review that-pretty much the gold standard(sounds like the emulsifier tubes in the idle circuit have some issues)

Vac diagnostics.jpg
 
Last edited:
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Sounds like a vacuum leak and/or idle circuit issue. Just because the solenoid is clicking doesn’t mean that the idle circuit isn’t obstructed.
 

Mtntopper

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Check any diaphragms on your carb and distributor for air leaks. Sometimes during a carb rebuild junk can still be in there, taking it apart again is free and doesn't take very long. You can also double check that the idle solenoid functions properly visually. When you idle with the choke it pulls to much fuel out of the carb so it is not an indicator of the idle circuit functioning correctly.
I had a similar problem a month after installing a professionally rebuilt carb. Needed full choke not to die. I ended up finding a cloud of particles in the float bowl. I soaked up the gas in there and wiped it out with papertowel. Been running normal since.
 

Guster

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Oregon Coast
Thanks everyone for the tips and suggestions. On the distributor question, I'm using a large cap electronic distributor from a FJ60. I'll take the carb apart today and go through it with carb cleaner. I did refer to Pinhead's videos when rebuilding the carb. For the rest of the fuel system everything is new except for the hard lines so crap could have reached the carb from those hard lines.

I pulled the fitting on the insulator and did this using Permatex Aviation form a gasket. Not sure if necessary or the right solution but figured if leaking this will help.

insulator.jpg
 

Guster

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That gasket maker was not a good option or I didn't let it dry long enough. The fuel bowl did have some crap in it. I sprayed out everything with carb cleaner and air. Also cleaned the hard line between the fuel pump and carb the best I could. It can now idle at lower rpm but idle is bouncing around vacuum is around 15. I still have a vacuum leak at the carb base insulator, get higher rpm and vacuum (around 18) when spraying carb cleaner at the PCV insert.

bowl.jpg
 

Guster

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So there is closure on this one. I sealed up the leak on the insulator and also changed out the PCV valve and grommet, which fit better than what I had. I was able to set timing and go through the lean drop carb adjustment process. Vacuum at idle is now steady at 19 and I was able to drive it down our street for the first time! Thanks everyone for the help.
 
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Cartersville, GA
I've experienced repeated obstruction in the idle circuit. On two occasions the circuit clogged soon after using fuel system cleaner. I run only 90-octane ethanol-free gas from a good station, so I don't think fuel is the issue. I've had the tank boiled and sealed with new lines to the fuel pump and original metal lines to the carb from there.

The fuel pump is a few years old - does anyone think the fuel pump might deteriorate and create this fine debris? Does fuel cleaner accelerate that deterioration? Does fuel tank cleaning/sealing sometimes not work? All I know is I'm getting tired of having to pull the carb off every few months! Would appreciate any insight from you all before I replace with a new tank.
 

Mtntopper

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I've experienced repeated obstruction in the idle circuit. On two occasions the circuit clogged soon after using fuel system cleaner. I run only 90-octane ethanol-free gas from a good station, so I don't think fuel is the issue. I've had the tank boiled and sealed with new lines to the fuel pump and original metal lines to the carb from there.

The fuel pump is a few years old - does anyone think the fuel pump might deteriorate and create this fine debris? Does fuel cleaner accelerate that deterioration? Does fuel tank cleaning/sealing sometimes not work? All I know is I'm getting tired of having to pull the carb off every few months! Would appreciate any insight from you all before I replace with a new tank.
Every time I've dug into this issue for my situation, I keep coming back around to the same answer: tank replacement.
 

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