Carburetor Rebuilt - Bogging and back-firing

Joined
Jan 7, 2011
Messages
55
Location
Peoria, AZ
 
Hi all, I just had the Carburetor rebuilt and has an issue when under load - low and middle mostly. Idles fine. Any suggestions?

Factory Carb - 1970 FJ40

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Joined
Feb 14, 2013
Messages
736
Location
Wake Forest, NC
 
‘70s are a special year. You have an extra port on the carb base by your idle mixture screw near the valve cover. Yours is open in your first pic. It needs to be capped to prevent loss of vacuum. Instead of capping, the port can be connected to a vacuum advance dizzy. However, your stock dizzy is vacuum retard.
 
Joined
Aug 4, 2006
Messages
3
Location
Cincinnati, OH
 
 
I assume it had some issue before you had the carb rebuilt. Might be an issue the carb rebuild would not fix. I think "stoshzack is on the right track poor vacuum will do weird things. Does it do better when you pull the choke under load? Check your vacuum, and timing. Fix any issues with the vacuum. If still not running well in low-mid range check compression. Good luck and nice engine/carb (clean), and engine bay.
 
Joined
Jan 7, 2011
Messages
55
Location
Peoria, AZ
 
ok so I think I corrected the vacuum lines as seen in pix. The one next to the idle is now going to the dizzy. I also checked the timing and it was off. So I advanced it a bit and it does run better - issue is it sputters at low but cleans out mid to high load.

Going to advance the timing to see if this helps.

Other suggestions are appreciated.

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Last edited:
Joined
Feb 14, 2013
Messages
736
Location
Wake Forest, NC
 
ok so I think I corrected the vacuum lines as seen in pix. The one next to the idle is now going to the dizzy. I also checked the timing and it was off. So I advanced it a bit and it does run better - issue is it sputters at low but cleans out mid to high load.

Going to advance the timing to see if this helps.

Other suggestions are appreciated.

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Timing needs to be set with your now connected vacuum line to the dizzy plugged (i.e. disconnected and plugged up - I use a golf tee). Also, if you have the stock dizzy, it is vacuum retard, so connecting that vacuum line to your dizzy will actually further retard the timing as your speed increases. You do not want this. If you have a stock dizzy, you need to just cap that extra port coming off the carb. I ended up buying a earlier dizzy/non USA (I think '68) which is vacuum advance. Once timing is set correctly, you can dial in using you idle mixture screw.
 
Joined
Feb 14, 2013
Messages
736
Location
Wake Forest, NC
 
And yes, that dizzy is vacuum retard. Cap the extra port on the carb and set the timing a little higher than the normal 7 degree mark to improve timing.
 
Joined
Feb 14, 2013
Messages
736
Location
Wake Forest, NC
 
Good info here:
 
Joined
Jan 7, 2011
Messages
55
Location
Peoria, AZ
 
And yes, that dizzy is vacuum retard. Cap the extra port on the carb and set the timing a little higher than the normal 7 degree mark to improve timing.
ok - just wanted to confirm - I have an electronic ignition if this changes the timing.
 
Joined
Feb 14, 2013
Messages
736
Location
Wake Forest, NC
 
ok - just wanted to confirm - I have an electronic ignition if this changes the timing.
If by "electron ignition", you mean a PerTronix dizzy, you would still want to cap the extra port on the carb and not run the line to your dizzy. You would also set timing with this port capped. Restated, the diaphragm in your dizzy might not hold vacuum. So it is just easier to cap it.
 
Joined
Jan 7, 2011
Messages
55
Location
Peoria, AZ
 
If by "electron ignition", you mean a PerTronix dizzy, you would still want to cap the extra port on the carb and not run the line to your dizzy. You would also set timing with this port capped. Restated, the diaphragm in your dizzy might not hold vacuum. So it is just easier to cap it.
ok - capped the port under the fuel/air and tee'd off to the dizzy. Tomorrow I'm going to set the RPMs with the air/fuel and fast idle screw to specs. thanks for the help and stay tuned.

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Joined
Jan 7, 2011
Messages
55
Location
Peoria, AZ
 
Hi all, I verified timing is set to 650 and 7 degrees BTDC. Mixture and fast idle speed also properly adjusted. Vacuum also confirmed to be good.

Starting to wonder if something has went wrong with the carb rebuild as none of these symptoms were present - backfires at the carb, hesitation and bogging until RPMs are up.

And something I also discovered is when the choke is full closed, nothing changes.

The only thing I changed was adding a new Insulator as the prior one was leaking.
 
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Joined
Oct 4, 2018
Messages
269
Location
Marietta, GA
I learned the hard way you cant assume the carb is plug and play after a rebuild. I had mine rebuilt recently by one of the carb guys everyone knows and found 3 things that had to be fixed before it would run right.
 
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