Stumbling and backfiring carb question (1 Viewer)

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I recently purchased a 1972 Land cruiser with the original F engine. The engine idles rough and backfires and stumbles unless I set the timing way advanced ~30btdc. Everything appears stock. I’ve gone through the timing/dwell, full tune up. The motor has compression 130-150 on all cylinders. I believe my issues are related to the carburetor. I just rebuilt it with no change apparent. I noticed during the rebuild that I am missing the “pump discharge weight”. Someone had put two small balls instead of one weight and one ball. #22 in the diagram. I also have about 3/4 full in the sight glass. Would either of these things cause my issues?

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I recently purchased a 1972 Land cruiser with the original F engine. The engine idles rough and backfires and stumbles unless I set the timing way advanced ~30btdc. Everything appears stock. I’ve gone through the timing/dwell, full tune up. The motor has compression 130-150 on all cylinders. I believe my issues are related to the carburetor. I just rebuilt it with no change apparent. I noticed during the rebuild that I am missing the “pump discharge weight”. Someone had put two small balls instead of one weight and one ball. #22 in the diagram. I also have about 3/4 full in the sight glass. Would either of these things cause my issues?

View attachment 2565781

Never seen two balls used. Aisan used a ball and weight or a ball and spring (in later models). I’d also check for vacuum leaks. Spray carb cleaner around the carb while it is running. If it idles up, you have a leak in that area. Also plug your vacuum line to your brake booster and see if that helps. Your brake booster diaphragm might be toast. If you have original drum brakes, you would never be able to tell.
 

1911

chupacabra
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In very general terms, backfiring through the carb is usually indicative of a lean condition. I'd start by looking for intake leaks at the base of the carb, the intake manifold, then vacuum leaks elsewhere.
 
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A 1972 should have a brake booster. It may have a vacuum leak. Listen for a hissing noise way up under the dashboard where the brake pedal rod goes through the bulkhead into the back of the booster. A short piece of hose might help hear it. Could also have a leak on the front, where the master cylinder bolts to the booster.

I would start by capping the vacuu line at the manifold and just isolate the booster, and see if it runs better.
 
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Thank you all. I checked with the brake booster line plugged, didn't seem to help. I put a new hose on the booster anyway because the old one was cracking. I did notice an RPM increase when I sprayed WD-40 directly where the intake manifold meets the head. The manifold nuts and bolts are tight but I am going to pull the manifold and put a new gasket on with a little copper sealant on both surfaces and torque. If I still get a leak there I will have the manifold machined. I have been through absolutely everything else so fingers crossed that this works!
 
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Thank you all. I checked with the brake booster line plugged, didn't seem to help. I put a new hose on the booster anyway because the old one was cracking. I did notice an RPM increase when I sprayed WD-40 directly where the intake manifold meets the head. The manifold nuts and bolts are tight but I am going to pull the manifold and put a new gasket on with a little copper sealant on both surfaces and torque. If I still get a leak there I will have the manifold machined. I have been through absolutely everything else so fingers crossed that this works!
A lot of Mudders use a double gasket for warpage issues. I was told by a machinist that the intake and exhaust manifold cannot be machined. Not sure if he was telling the truth or lazy. I might have the carb weight you need. Also, my carb shows 3/4 full during operation. Pretty sure that is not an issue.

@Pin_Head might be able to weigh in
 
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I have had good experiences with Remflex gaskets. They actually expand when exposed to hot exhaust gas, so they can fill quite a gap. I torque them to Toyota specs and not the tightness that Remflex suggests.
 
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I have had good experiences with Remflex gaskets. They actually expand when exposed to hot exhaust gas, so they can fill quite a gap. I torque them to Toyota specs and not the tightness that Remflex suggests.
Pin_Head, did you see my first post about the missing weight? Have you seen that before. I suspect someone lost the weight and just threw a ball in there. I wonder how that is affecting function.
 
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A lot of Mudders use a double gasket for warpage issues. I was told by a machinist that the intake and exhaust manifold cannot be machined. Not sure if he was telling the truth or lazy. I might have the carb weight you need. Also, my carb shows 3/4 full during operation. Pretty sure that is not an issue.

@Pin_Head might be able to weigh in
If you have that weight please let me know and I will pay pal or Venmo the cost. Thank you!
 
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Personally, I have never seen a carb with a missing weight, but I have not had that may to observe compared to people here that make their living working on Toyota carbs. Luckily, a weight only has to fit and weigh so much, so making a substitute should be easy if necessary.
 

thebigredrocker

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I'd have a machine shop plane the manifolds. Do it with the exhaust manifold bolted to the intake. They will true both together on a big belt sander. Seen FJ40Jim and MarkW recommend the FelPro intake/exhaust gasket w/ no sealant on flat manifolds. The above program worked like charm for me.

Looks like stoshzack has you covered on the weight. In case not, I'll post measurements.

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weight replacement idea.
Rest in Peace Tools R Us
Brass circuit board standoff nut may work?

should be a big ball in the discharge hole and the smaller ball in the pump plunger hole under the e-clip. Small ball big hole, big ball small hole :cheers:
 
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I did attempt to use the Fel pro gasket but I didn’t like the fit. The gasket they are selling is like a combo gasket for both F and 2F. Notice the bolt holes on each end.


Fel-Pro Intake And Exhaust Manifold Gasket Set  Primary Image

The original one looks like this


MAHLE Original Intake and Exhaust Manifolds Combination Gasket  Primary Image


I used a different gasket and I think it’s pretty tight now. I left the intake loose from the exhaust when I bolted them to the head so they could adjust as flat as possible when torqued down. I’m now running pretty well at about 20 BTDC. If I set the timing any lower I get backfiring and sputtering under load.

I’d like to see exactly what the original weight looked like. The FSM says it has to be oriented in a certain position when installed so I’m not sure any old weight would work.
 
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Backfiring through the carb and bucking during high engine load, like long uphill climbs with wide open throttle are more likely signs of insufficient fuel delivery. There is something wrong if you need 20 degrees of advance to “run” well. It could be that your centrifugal advance is stuck, but the problems would be RPM dependent and not load.
 
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There is something wrong if you need 20 degrees of advance to “run” well.
Couldn't agree more! The idle is ok at 7 BTDC but it stumbles and backfires as soon as you take off. Fuel pump looks relatively new, I am going to replace the fuel filter next. I'll check the advance and see if the timing changes at higher RPMs.
 
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I also took some vacuum measurements.

vacuum at the brake booster is 21in
vacuum from VSV to the bottom of the carb is 14in
vacuum from the manifold to VSV is 14in
vacuum to distributor from VSV is 0 - revving has no effect
vacuum to top of carburetor (actuator) from VSV is 0 - (only opens at speed?)

Looks like my filter off the intake is plugged giving me a low reading from that port.
 
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thebigredrocker

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Not sure what filter you'd have off the intake. Is the PCV valve stuck or gummed up? Should be able to disconnect it from side cover and hear a ball rattle.

Sounds like you still have a 72 retarding dissy. Might try to run with it disconnected/capped from vacuum source. At the same time disconnect/cap the Vacuum Switching Valve.
 
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62E32C94-BFAA-401F-B0D8-317B538D7CCD.jpeg

Filter in that fitting on top of the intake to keep backfires from damaging the VSV I think.
 

thebigredrocker

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81465186-6DBC-451B-9FB8-3A3C019C9B02.png
Are you referring to the circled part. I think that is just a vacuum fitting w/no filter. You should have close to 18hg off that fitting.

Have you done a valve adjustment yet? When you re-gasketed your intake did you inspect for cracks?
 

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