Just rebuilt carb, gas leak help (1 Viewer)

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Jul 26, 2017
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Location
Castle Rock, CO
So after doing my first Carb rebuild and replacing some vacuum hoses and valves, I finally took the truck out for a long drive today to see if I solved the stumbling issue. The truck ran well, way better than before, and there might have been a tiny bit of part throttle stumbling while in 3rd or 4th gear, but it was pretty hard to tell. Overall I was pretty happy about the drive, and confident that I am headed in the right direction. However, I pulled the truck into my garage and stepped out about an hour later to grab a beer from the beer fridge and the gasoline smell was pretty overwhelming, so I popped the hood to see what was up. I discovered I have a small gas leak near the bottom of the carb, and I'm pretty sure it is coming from this doo-dad on the drivers side of the carb. I can't really see anywhere else where it would be leaking.
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Honestly, this is a part I unscrewed and set to the side and then when reassembling the carb, I just threw it back on, without much thought, just following the pictures I had taken when I took it apart. I'm ashamed to say I don't even know what this part does, though I'm planning on some research. (edit: apparently it is the secondary slow cut valve..still don't know what it does) Do i have it on right? It seems to be leaking around the rubber boot, so I am searching for a replacement, can I replace this or should I look for the whole valve. Thanks for the help!!!!!
 
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Slow cut valve something or other. I’ll find a link for it.
 
It doesn’t say as detailed as in another thread unless I skimmed to fast but you can tap and plug it. A bunch have. It’s a weird thing in my head but I suppose if it works then it works.
@mwebfj60 I can’t remember if you’ve gone over this system in all your carb dissecting?
 
Although that thread made my head hurt. Haha. What are people currently doing when their secondary slow cut valve goes bad since it is NLA?
 
Sounds like pulling the plunger and tapping it to JB weld in a bolt is the current option. Are there any other options I am missing?? I do have to keep it smogged and have not yet passed emissions here in CO.
 
The rubber dust boot on the SSCV is always hard as a rock after 10 years of underhood exposure. The slightest touch and it crumbles apart, if it was even still there (this applies to all carb, not OP specifically). There is no practical replacement of the boot. The bit of fuel that may ooze out around the plunger is only the fuel that is in the vertical passage above the SSCV. IOW, it can't siphon/leak all the fuel out of the bowl.

When the secondary is inop (normal condition for all 10+ year old carbs), fuel is never drawn up & over to the SSCV, so it never appears to leak. After the roached seconday diaphragm is replaced and the vacuum system is modified, the secondary goes from being a dead end, to a major air & fuel supplier. Fuel is seeping out now because the secondary barrel is actually working.

Plugging the valve so it stops leaking, but also stops flowing fuel, will cause a secondary tip-in hesitation.

HTH

@NeverGiveUpYota it seems @FJ40Jim explains it quite nicely
 
That’s a good description. I think the leak has more to do with the plunger shaft and it’s clearance with the bore and sometimes the 8 gasket. I’ve run for many years without the boot.
If you want to find out how it will run with it drilled and plugged, bend the linkage arm on the valve down to where you can open the throttle linkage fully and the arm doesn’t touch it. It’s pretty soft and bends easily. Try flooring it in different scenarios and see if it’s acceptable without having to compensate with float level, timing, or richening other circuits.

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That’s a good description. I think the leak has more to do with the plunger shaft and it’s clearance with the bore and sometimes the 8 gasket. I’ve run for many years without the boot.
If you want to find out how it will run with it drilled and plugged, bend the linkage arm on the valve down to where you can open the throttle linkage fully and the arm doesn’t touch it. It’s pretty soft and bends easily. Try flooring it in different scenarios and see if it’s acceptable without having to compensate with float level, timing, or richening other circuits.

View attachment 1655753

My understanding is when you plug it, you want to avoid blocking the two holes, effectively making the plunger permanently open. The only thing you are plugging is the plunger hole. Thus I’m not sure if bending the tab and keeping it closed the whole time simulates that, or maybe I am thinking of it wrong. Basically keeping this circuit I have highlighted in red open all the time to allow vacuum to pull fuel in.
 
View attachment 1655835

My understanding is when you plug it, you want to avoid blocking the two holes, effectively making the plunger permanently open. The only thing you are plugging is the plunger hole. Thus I’m not sure if bending the tab and keeping it closed the whole time simulates that, or maybe I am thinking of it wrong. Basically keeping this circuit I have highlighted in red open all the time to allow vacuum to pull fuel in.
Interesting, I had it in my head the other way. If that’s the case, what I said is wrong for sure. I guess you could use a zip strip and tape and pull the lever down and put the zip strip through the hole in the throttle linkage plate to hold it always open as a test. I kind of put it in the same category as grounding out the ICS.
 
yeah, you would want to bend the tab to keep the arm pushed down by the link if you wanted to see if it would affect things poorly before actually plugging it...
 
that valve opens the circuit to the secondary transition slow jet. if that valve is not actuated, fuel is not supposed to get to the secondary slow jet. pulling the plunger and sealing it off while keeping the ports to the carb body both clear allows vacuum to pull fuel to the circuit. if that valve is left on unactuated, or either of the 2 ports get blocked, there will be no way for fuel to reach the secondary slow circuit. as complicated as this seems, it is pretty simple if you take it circuit by circuit, and while those THC and CRLLC carbs seem very tempting, their lack of ability to self regulate over as wide an array of changing atmospheric conditions keeps me from taking the pill...these stock Aisans are pretty cool once you get them dialed in...all that said, carbs suck.
 
@Chad717, did you do the paperclip test to make sure the secondary diaghragm is opening. I don’t see that valve going bad from just removing it and putting it back on for a rebuild. Gas in middle of sight glass..linkages not hanging up..secondary butterfly valves closed in bore when not being pushed open..linkage adjusted properly to start pushing secondary linkage at the right time..cooling fan kicking on..
Seems there are more things to investigate before modifying the valve.

@LAMBCRUSHER I was nodding up and down with you till the last sentence, carbs were my #1 spot in the favorites list till recently, now #2 right behind the VB and A440
 
It seems like a lot of people have done this mod without to many negatives. I found a 8 X 1.25 mm tap at Lowes, just got to find some small bolts and i'll seal it up. Thanks for all the good replies. Love this site.
 

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