Frustrating

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jstairwalt

jstairwalt

Joined
Jan 21, 2017
Threads
1
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Location
Gardnerville Nevada
my 1972 jf40 is completely stock I recently rebuilt the carburetor and replaced all ignition components except distributor new coil points condenser wires plugs cap rotor. I'm having issues with backfiring in the carb. When sitting its fine throttles up great but when I drive it when it's under a load it starts popping in the carb and lacks power. If I run the choke pulled out it is a lot better I have searched for vacuum leaks and have only discovered small leaks in the throttle shafts in the throttle body. Would that cause the problem I'm having
 
Check to see if your vacuum advance is working. I agree that sounds like a timing issue

JP
 
It is a vacuum retard. And when I first got it running it was great and slowly started running not so great. Doing what I have explained. I warmed it up good and warm after driving it set the timing with the retard vacuum line in hooked and plugged set it to the 7 degree bb. And set the idle to 650 it idles good except every 30 seconds or so the bb moves either direction. I adjusted the carb to set vacuum and it runs good vacuum not quite steady but between 16.5 and 17
 
I thought that as well. When I rebuilt the carb I did the carb dip and cleaned everything very well used carb cleaner to make sure nothing was plugged. I looked in the carb and when it's idle I can see fuel from the main jet streaming in and give it a little gas and you can see fuel from the accelerator pump streaming in. When it first started issues I checked timing and it was far advanced to where you couldn't see the bb in the window at all. I'm thinking I may need to pull the timing cover and make sure it's not off a tooth
 
Also it does have headers on it they where on it when I got it so there is no intake heating I read once that you can get hot exhaust into the intake when cold and crack it. Don't know if it's true. I have used carb cleaner to look for leaks and like I said the only place I get a change in idle is at the throttle shaft in the throttle body. Not sure if the vsv is working correctly either
 
Disconnect the vacuum retard for starters. That's a no brainer. Retest.

Check points with a dwell meter and reset until you reach specs. Then recheck point gap at specified dwell. If it's 80%or less of spec, the dizzy is toast.

Best

Mark

www.marksoffroad.net
 
jstairwalt said:
I will try that that. I set the points to 0.018 but did t check the dwell
Click to expand...

I will bet dollars to donuts your dwell is way below spec at that setting. Don't be surprised if your vacuum improves with the hotter spark as well.

Best

Mark

www.marksoffroad.net
 
Check your secondary. If it opens with the mechanical linkage, but not with vacuum it will act like a vacuum leak. I had a carb like that once and it drove me nut until i figured out what it was.
 
Monty231 said:
Check your secondary. If it opens with the mechanical linkage, but not with vacuum it will act like a vacuum leak. I had a carb like that once and it drove me nut until i figured out what it was.
Click to expand...

Well intentioned, but misses the mark. 1972 carbs has a full mechanical secondary.

Best

Mark

www.marksoffroad.net
 
I am about where you are at, trying to solve a puzzle.

Everything I've read says fuel shouldn't be visible a idle. Perhaps not your biggest concern but definately worth factoring into the equation. Can you hear your idle fuel solenoid click when you turn your key on?

How clean is you fuel tank, filter, ect?

I just installed Pertronix's in my dizzy, that was a night and day difference for my rig. Adjusting my valves also helped quite noticeably. I found the Go/No-Go feelers helpful.

Do you recall what your jet sizes are?

I've read that after you plug your vacuum retard dizzy, 10* is where you want to set your timing. I put a dab of gold paint on my bb. Easier for me to seeo_O.
 
thebigredrocker said:
I am about where you are at, trying to solve a puzzle.

Everything I've read says fuel shouldn't be visible a idle. Perhaps not your biggest concern but definately worth factoring into the equation. Can you hear your idle fuel solenoid click when you turn your key on?

How clean is you fuel tank, filter, ect?

I just installed Pertronix's in my dizzy, that was a night and day difference for my rig. Adjusting my valves also helped quite noticeably. I found the Go/No-Go feelers helpful.

Do you recall what your jet sizes are?

I've read that after you plug your vacuum retard dizzy, 10* is where you want to set your timing. I put a dab of gold paint on my bb. Easier for me to seeo_O.
Click to expand...
I don't recall the jet sizes but I do recall that they are correct the tank seems to be clean and the filter is new and clean. As far as the idle fuel cut off does click with power put to it but I did notice when I rebuilt the carb that when I ppulled the solenoid off that it appears the small shaft has been cut off it is only about a quarter inch long and there was nothing in the passage way on the carb. But would that really affect it driving like it's running lean? I have put serious thought into that electronic ignition. It really works that good huh. I'm about to just dump 800 into it and get the hei and Weber carb set from man a fre
 
I want to thank everyone that has replied I have some things to try now thanks to you guys. You all are awesome I will try to get more pics posted of my work in progress.
 
Well your definately getting idle fuel. I've read where guys snip of the end of the IS. But if you can see the booster nozzle participating in idle fuel delivery, it might be worth figuring out. If your idle cucuit's not clogged, is your idle speed screw holding your primary throttle plate too far open. Perhaps back off that screw and advance your timing some and see. Maybe that will pull idle fuel from the right location.

I don't think you would regret switching to Pertronix. Perhaps try that before you go Weber. As you know spark is very critical and I like the fact I don't have to tinker with my points. Or wonder if they are dialed in. But they will be in my glovebox as a spare.

Fuel level in your sight glass good?


Have you checked your valve lash?
 
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