Backfire above 40mph climbing needing throttle

Joined
Feb 28, 2022
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Austin, TX
Hey all - I'm about a month into a new to me 1973 with the original F 155 and 3 speed transmission. It seems to run great under 40mph and also runs well on flat ground up to about 55. However, if I'm going uphill - even very slight hills - and need to add throttle it starts and then sputters and backfires. I get back down to 40 and I'm good. If I hit a steep hill around 40 I can shift down to 2nd and just hold speed or climb around 35mph.

I'm going to take it to the local cruiser specialist in 3 weeks for a more thorough review of the rig, but I'd like to do a bit of diagnosis so I'm a bit more informed and can hopefully fix these issues myself soon. Specifically, I'd like to run through a diagnosis and see if I can identify the issue. I'm interested in the order of actions I should take. I have the following in mind - but I don't really know what I'm talking about and would appreciate your thoughts on an ordered next steps to hopefully find the root cause.
  1. verify timing - maybe it's a tad ahead and hopefully relatively easy to check and confirm
  2. identify vacuum leaks - the threads with backfiring at throttle seem to point in this direction
  3. compression test - get a current baseline and take action if necessary
Is this the right order and/or are there some obvious things I should check before or along with these? Anyone have any good links / tips on the easiest way to identify vacuum leaks?

I've pasted a list of the work done by the PO over the past few months reviving the engine.
1649785175497.png



THANKS!
 

samatulich

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Valdez, AK
Get yourself a vacuum gauge. Tee into one of the vacuum lines that gets manifold vacuum and run the gauge into the cab. Take it for a drive and keep an eye on the gauge. Get online and look up vacuum gauge reading diagnostics.
 

Pighead

Stop calling it an FJ
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Could possibly be fuel restriction.
Yes, but. Fuel restrictions usually just sputter, not backfire.
I'm thinking more an over-rich mixture somehow leading to an occasional backfire, or wierd vacuum advance or retard thing
 
Joined
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Thanks all. I'll try to look further tonight (vacuum, fuel filter, timing) but it's likely a few days out. I did confirm that it's not just over 40. It's really anytime I push the pedal to the floor in any of the 3 gears.
 
Joined
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Is it backfiring through the carburetor(lean mixture) or out the tailpipe(rich mixture)?
through the tailpipe - so sounds like a rich mixture

Has it always done this or is it a new problem?
I've only had it a few weeks. I've been driving it around the neighborhood and just opened it up a tad more the other day for the first time and noticed this.
 

73FJ40

After another night of rust removal!
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Stock Aisin carb? Could be something to do with the secondary barrel, which opens up under load like when you floor it going up a hill.
 
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Do a valve adjustment also post a pic of your engine and if your running points check the dwell or point gap.
Thanks -- here is the engine pic. let me know if there is anything specific or another angle that would help. I'll add the point gap to the checklist. time is my enemy at the moment. it's going to continue to sit for a bit but I'll post again after a bit more analysis. Thanks everyone for the tips thus far!

1649852098661.png
 
Joined
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blurry pic but I did find one issue -- I have fuel slightly leaking from the fuel pump into the carb. I can see this while idling wiht the engine warm and assume it gets worse as the throttle increase. I'm going to get that sorted and check for vacuum leaks in the process. Do others have a hose clamped around that input? Wondering if I need some new connectors.

The timing is 7* TDC on the bubble and I'm also going to get a tach reading and get the idle RPMs set b/c it feels a tad high but I'm not sure ... anyways I'll get a new baseline and see where I stand. I appreciate the comments!

1650318616928.png
 
Joined
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Vacuum reading is 17.5 idling while warm. If I read the Haynes manual correctly it shows manifold vacuum for US markets at 16.5 and General at 18.1 so I figure I'm in the ballpark. I also blocked the air to the carb to see if it idled up before stalling (recent thread) and that didn't happen. So timing is good and vacuum I think is good.
1650393395243.png


I haven't adjusted valves or checked point/dwell gap (I am running points).

I definitely need a new connector from the fuel pump into the carb. Right now I have a clamped tube that is trying but fuel is definitely leaking out and it increases with throttle as more fuel is pushed so that's going to be my next step. Anyone know where I can source the connector (3rd pic). I'm going to call SOR and check there first.
1650393047181.png

1650393062564.png


this is what I'm looking for:
1650393137112.png



finally - here is peeking down into the carb barrels. I'm about to research how I can verify the secondary barrel is getting fuel and probably setup a tach to get the RPMs right. Double check the vacuum and timing with known RPMs and go through the idle/fuel mix adjustment process laid out in the manual.
1650393240386.png


Thanks -- Dave
 

Pighead

Stop calling it an FJ
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I've got that union between the hard fuel line and the carb. That might not fix your problem.
Why did PO put a nipple there?
As i said, the carb threads strip out easily. Did PO over torque and cause a leak? Did he patch it with that barbed nipple? I'll bet he didn't helicoil it to the correct size. Does that nipple hold torque?
Is that what's leaking?
I might put it back together, with new rubber fuel line and known good hose clamps and determine exactly where the leak is from.

20220419_115954.jpg
 
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yeah is it leaking at the threads where it goes into the carb body or just leaking out from the hose connection?

If it's just the hose try installing a new piece with a fresh clamp, will probably fix it
 
Joined
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Messages
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I've got that union between the hard fuel line and the carb. That might not fix your problem.
Why did PO put a nipple there?
As i said, the carb threads strip out easily. Did PO over torque and cause a leak? Did he patch it with that barbed nipple? I'll bet he didn't helicoil it to the correct size. Does that nipple hold torque?
Is that what's leaking?
I might put it back together, with new rubber fuel line and known good hose clamps and determine exactly where the leak is from.
No idea why it was done this way, but glad to have known items to fix ... thanks for that pic. I saw on another post on the size so I think I'll try to find one and more securely connect the line.
yeah is it leaking at the threads where it goes into the carb body or just leaking out from the hose connection?

If it's just the hose try installing a new piece with a fresh clamp, will probably fix it
... 99% sure it's just leaking from the hose because there is nothing remotely tying the 2 connections together. I'll get this sorted and also try to verify the secondary valve once I'm no longer leaking gas,.
 
Joined
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Messages
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Austin, TX
Fuel leak fixed. Runs a tad better but still losing power on full throttle.

pulled the plugs - they all looked the same -- dry black soot that I was able to scrape off. Not good.
1650998193744.png

so I cleaned them up best I could ... and made a note to order new plugs - not sure what these are - not used to 3 tips like this?
1650998221993.png


Compression test seems OK -- maybe a tad low, but the numbers were OK - 120-130 with most at 125:
#1: 120
#2: 125
#3: 125
#4: 125
#5: 125
#6: 130

Dwell was low - 29 - with RPMs around 700
And I was getting less pressure than the other day. I was around 15 versus 17.

So low pressure and low dwell. I tried adjusting the carb throttle / mixture screws and couldn't get the vacuum to change. The mixture screw seemed way out so I think I need to reset this and start over but first figure out if / where I have a vacuum leak.

In the next few days I'm going to:
  • check for vacuum leaks - not sure why I couldn't get the 17 I was getting last week
  • fix the dwell gap on the distributor
  • adjust the valves - I confirmed #1 intake was tight but didn't have time to go through the adjustment (too late > too loud > done for the night)
  • reset / validate timing
  • adjust the carb throttle and mixture -- note when I was doing this I couldn't get any change while adjusting the mixture screw. I'm going to go back and read threads on this b/c I couldn't get any vacuum changes bringing this all the way in or out
low vacuum:
1650998640793.png


valves ...
1650998687484.png


1650998687937.png
 
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