Starting and tuning rebuilt motor...timing travails, vacuum leak vexations and carb conundrums (1 Viewer)

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Well I put her back together and she started right up...adjusted timing a but (it was a retarted, starts and idles ~12 degrees.) But I still have a lean idle unless I choke it back, and screwing in the bypass still gets the AFRs down but it dies. I am going to keep playing with it now, but while I am doing that perhaps someone can chime in with advice about the best way to experiment with richening up idle some more...raise the float level or find a bigger main jet?

Thanks!
 
Joined
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Toronto
So I had another go yesterday with more dramatic action and outcomes...

1. I pulled the carb and checked the settings from TLC. Overall I was frustrated by the TLC experience (putting in and charging me for a port vacuum which I didnt ask for and dont need, non-responsive to emails, 4 months wait, $725...) and found notes that there were jets missing etc but those werent replaced and I think it was set up generically rather than according to the requests and specs I provided....I found the float had been replaced but the fuel level was low in the bowl, so adjusted that up. I found a 120 main jet which I see the owner of TLC has said in this forum is not right for my carb and setup but there it was returned in my carb. Neither spare jet was replaced. My stamped ID tin was removed and not returned.

2. I had my own spare 114 jet so I drilled that out to 135,

3. I had a spare 55 slow jet and installed that in place of the 50.

That richened up idle significantly, and brought the throttle and bypass back into range for making adjustments.

4. I still couldnt get her quite right so I wanted to rule out a vacuum leak. I made my own smoke machine out of a crusty old soldering iron and a coffee jar. I used the bicycle pump to push in `2 psi and found only 1 small leak (from that un-asked-for port!) No vacuum leaks aside from that tiny one, so thats a relief AND confusing...

So now that I have a reasonable rich idle, ruled out vacuum leaks and have timing working properly hopefully I am in the home stretch!

PS. the smoke machine worked so well I tried it out on the outlaw....no leaks there either!

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Dizzy

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In the image above, your fuel level in the bowl looks low. In the Hayne's, it says something like a float height of 7.5mm. When I set mine there, using Pinhead's suggested set of feeler gauges, the fuel was up to the top of the window, thankfully Toyota gave the carb a window. So, it was set out of spec by a full millimeter, and the fuel is dead center on the dot. It was important to make the float drop height (like a mm gap between the needle and the float) correct after that, as I had fuel beyond the top of the window.

Is your engine bored out beyond factory diameter? I have a hard time believing that a 120 is correct as my '75 came with a 144, and I'm running a 156.

Also, between my two carbs, both of them have a leaky choke breaker diaphragm, I don't know if that is a common problem, but I wish I could get a new one.
 
Joined
Nov 21, 2017
Messages
380
Location
Toronto
In the image above, your fuel level in the bowl looks low. In the Hayne's, it says something like a float height of 7.5mm. When I set mine there, using Pinhead's suggested set of feeler gauges, the fuel was up to the top of the window, thankfully Toyota gave the carb a window. So, it was set out of spec by a full millimeter, and the fuel is dead center on the dot. It was important to make the float drop height (like a mm gap between the needle and the float) correct after that, as I had fuel beyond the top of the window.

Is your engine bored out beyond factory diameter? I have a hard time believing that a 120 is correct as my '75 came with a 144, and I'm running a 156.

Also, between my two carbs, both of them have a leaky choke breaker diaphragm, I don't know if that is a common problem, but I wish I could get a new one.
Yes, those pics were after the truck had been sitting for a few hours and the idle solnoid had been clicked on and off a few times testing so some fuel trickled out. On normal running the fuel level is just below the middle line. I followed Pinhead's whole video series when I rebuilt the thing the first time (Thank you! Pinhead!) and revisited it again yesterday when making those changes I referenced above. I didnt bend the tab enough though and it still sits a 0.5mm low...will fix that...

The motor is bored over slightly, still 3.9L with 2F heads.

Thanks!
 
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Joined
Jan 22, 2007
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Alva, Fl.
 
 
The really low vac readings seem to me to be the culprit--The carb could be perfectly fine, but without good vac(>15 in Hg), fuel will have a difficult time getting through the idle ckt.
There was a vac t'shooting chart on MUD several yrs ago. (I think you said the vac gauge had been proven accurate)

Vac diagnostics.jpg
 

Dizzy

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Santa Fe, New Mexico
I agree with ssgoat. However, if you carb is off, you don't have a source for vacuum because the throttle plate angle is too high. This idea goes along with the Air/Fuel exhaust readout, and the good compression test. The smoke test suggested that the seals were good. It is my opinion that the Air/Fuel exhaust sniffer is far more relevant than reading vacuum, especially on a carb where the idle speed screw isn't specified by the factory (unlike on a Weber).

So, did your vacuum go up when you drilled your jet?
 

FJ40Jim

The Cruiser Whisperer
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Sorry I'm late to this. Here's some random info that may help.

There is no air bypass screw, that's a VW thing?
There is an idle mixture needle where screwing it in restricts idle fuel, backing it out increases idle fuel.
Next to the idle mix screw is a new ported vacuum fitting that is needed for a vacuum advance distributor.
120 is a good main jet for that carb.
On a tight test engine it pulls 13.0 AFR at idle at 800'MSL.
All symptoms point to a vacuum leak.
The difference in AFR between fuel level at the bottom edge of the sight glass and at the top edge of the sight glass is immeasurable.
Putting the 2 marks together on the timing gears is the correct assembly method. The rubber damped timing gear is known to fail in old age, allowing the outer ring of the cam gear to walk around, effectively retarding cam timing. If it is bad enough, the symptoms will be low compression, low power, weak idle.

HTH!
 

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