RTH — Fuji carb adjustment

[C6H12O6]

I'm stuck, and running out of time to leave for Rubithon. Before I start, I recognize I have made a bunch of errors in judgment, but here we are. The first and worst lapse in judgement was attempting this 48 hours before I pull out of Portland for a 4-day slow-roll down to the Rubithon. Kicking myself.

'74 FJ40 with stock motor, de-smogged. Have the CityRacer non-us points distributor (19100-61080) and was running it with the vacuum advance plugged. I had just done a tune-up (cap, rotor, plugs, timing at 7° BTDC), and it ran great with the stock '74 carb, albeit without vacuum advance. Poor decision to swap out the carb for the Fuji carb I had purchased a few weeks ago from CityRacer. There were lots of other, smaller lapses in judgment, like cutting the OE fuel lines to make one that fit the new carb, rather than find something that would bolt up, making it difficult/expensive/impossible to go back to the old carb. Anyway, new Fuji is on there and I'm having trouble getting it adjusted. I am not having the experience that many have had where it just bolts on and pretty much works.

I started with 1.5 turns out on the Idle mixture screw, as directed. I was following the OTRAMM video directions, but it won't idle even a little at 690-ish to start. It runs fine (great, actually) off the idle circuit to warm it up, but when it reaches operating temp, and I push in the choke, it dies. Attempting to set idle speed to where it will run on it's own, the lowest it will "idle" is around 800-850, and even then it isn't great. Not even sure if it's truly running on the idle circuit at that point. I tried running a rough idle in the 700s to go through richening up the mixture as in the video, and got it to where the vacuum was decent (15-16 inHg), but when I tried to lower the idle speed even to 700, it just got rough and wouldn't hold idle. No vacuum leaks that I can identify, vacuum is steady around 800+ RPM, but drops down below 15 if I try to adjust anything down.

Went back to all the way in on the idle mixture screw and started over, this time starting at 4 turns out and going through the lean-drop method. That got me a little closer, and I was able to drive it without stalling, but idle speed was still too high (850-900) by the time I got home. It would "idle" there, but was backfiring under compression downhill, and was burbling when pushing the clutch in. No identifiable holes in the exhaust, but not a lot of back pressure. Mark's headers, 2.5" exhaust, and a pretty unrestrictive muffler, FWIW.

Running out of time to get this right enough to head out in the morning. I'm stumped. I tried the cardboard over the carb at idle to make it die to see if it speeds up signifying a vacuum leak. Nothing, it just slowed down and died. Sight glass shows good fuel level. Fuel pump has been replaced recently enough that I'm not worried about fuel delivery. Everything was great with the old carb, and everything is not great at idle with the new Fuji. Off to Napa to find a new carb gasket, as I had to re-use the gasket between the spacer and carb. It looked fine, but I'm just trying to eliminate an internal vacuum leak between the primary and secondary.

Thoughts?

 

[rlong]

I would triple check for vacuum leaks; spray flammable carb cleaner etc around intake base etc; and or spare hose up to ear listen everywhere on carb, carb base, manifold etc. as symptoms do tend to point to that.

 

[rlong]

(Hose up to ear, probing around with other end of hose.

 

[bhsdriller]

are you sure timing is right

 

[C6H12O6]

 

[C6H12O6]

Well, the one spot I couldn't easily spray to find a leak...
I think this tab cracked when I installed it years ago. Don't remember what my fix was. JB Weld? Held for a while, and not sure why it just reared its head when I changed carbs, but maybe I just found the sweet spot with the old carb, and can't start from scratch with the new one. Definitely sped up with a shot of carb cleaner. Like, as much as spraying it down the air cleaner opening.

Off to find a gasket in town. Or, we wait until Saturday to leave and have to hustle. Good thing I was planning for more time down this year, and a couple days in Placerville for just in case.

 

[rlong]

Two truisms in the garage.
1) If it seems like a vacuum leak, but you can’t find it; It is a vacuum leak, you just haven’t found it.
2) If you just changed something that should obviously be the underlying issue; 50% chance it is a different only partially related, or not related at all issue.
Don’t listen to reasonable “logic” in your head too much!

 

[C6H12O6]

Yes, and I'd add at least one more:

1. If you leave maintenance and upgrades until the last minute before a trip, you have nobody to blame but yourself.

I didn't change the carb thinking it would be a silver bullet. I had ordered it weeks ago just wanting to add the vacuum advance before the trip, and not planning ahead for time for Jim/Mark to rebuild the old one and add the advance to my '74 carb. It was running fine, and I stupidly thought I could swap it out at the eleventh hour. I'm sure the manifold crack still existed, and I had probably just tuned the other carb into the sweet spot where it was working enough that I could live with it, or just blaming the symptoms on the lack of advance. Dumb.

Anyway, a healthy amount of JB weld and some TigerTape exhaust tape sort of worked well enough to adjust the carb close and get through the trip. I had to adjust the idle speed higher than I wanted to keep it from stalling, which negated some of the lower gears I have installed, but I made it down to and through the Rubicon. I picked up a gasket on my way out of town, and took my spare intake manifold with me just in case. Now, I can take my time to remove and inspect everything, get the spare manifold surfaced and milled down to the thickness of the header flanges so this won't happen again.