Aisin carburetor idle circuit routing (1 Viewer)

[Scotty2]

There is the vac leak.

I never even thought about this area until now. I’m more familiar with PCVs that stick into the valve cover, like V8 Chevys have. Just to make sure I’m not tilting at windmills here, I’m gonna include a photo of metal tubing that extends out of the crankcase right under the #1 spark plug. It has a funny shaped end on it where the rubber tubing attaches. This isn’t some kind of PCV that’s connected to the hard tubing is it? I mean, my thought would be to install an inline PCV in the rubber tubing in an accessible place.

The other end connects right to the front of the carburetor base plate. I’ll include a photo of that end too for your viewing pleasure, and an aerial view of the whole engine bay.

 

[Grayscale]

Why is there a T-fitting in the vac line going to the brake booster?

 

[pb4ugo]

In the 1st pic, The metal thing the hose is clamped to is supposed to be the pcv. It pulls out of the grommet that's attached to the side cover.

 

[Pighead]

This isn’t some kind of PCV that’s connected to the hard tubing is it?

yup, looks just kike a PCV valve. Might as well take it out and have a good look at it, squirt some carb cleaner in, make sure the ball in it rattles and put it back in,

 

[Pighead]

Why is there a T-fitting in the vac line going to the brake booster?

That's a good question.

 

[Scotty2]

Why is there a T-fitting in the vac line going to the brake booster?

That was the best place for me to connect my vacuum gauge.

 

[Scotty2]

Ok, I pulled my PCV valve and the check valve “slug” did seem to be moving. I sprayed a bunch of carb cleaner in it and ilwhen I blow through it, it passes flow in one direction and not the other, just like it should. And boy does that taste good in the morning!!

It has a solid grommet seal it seems. So I got excited about nothing.

So today I’m gonna roll it out into the fresh air and try out my propane vacuum leak tester. Half of me doesn’t want to find an intake manifold leak, and the other half wants to find it just to solve this idle problem that’s driving me bonkers.

Thanks again, guys! I’ll report my findings when I’m done.

Scotty2

 

[Scotty2]

Ok, I pulled my PCV valve and the check valve “slug” did seem to be moving. I sprayed a bunch of carb cleaner in it and ilwhen I blow through it, it passes flow in one direction and not the other, just like it should. And boy does that taste good in the morning!!

It has a solid grommet seal it seems. So I got excited about nothing.

So today I’m gonna roll it out into the fresh air and try out my propane vacuum leak tester. Half of me doesn’t want to find an intake manifold leak, and the other half wants to find it just to solve this idle problem that’s driving me bonkers.

Thanks again, guys! I’ll report my findings when I’m done.

Scotty2

 

[Scotty2]

View attachment 3800617

I’m happy you folks haven’t gotten sick and tired of my nube questions, so here’s another. I’m scoping out the intake manifold on this F155 engine, and I’ll attach the best drawing I could find of it in the TSM. How many bolts/nuts secure the intake manifold?

I don’t think the drawing shows that little detail with very much clarity.

PS the first odd thing I see is that there’s a missing bolt on the very leading edge of the intake manifold, but it really looks like placing a bolt here doesn’t secure the manifold, only the gasket. I’ll add a pic of this too. It may be nothing, but the propane test will tell me!

 

[Grayscale]

The hole in the gasket is intended for the 2F exhaust manifold, or header. The 2F has couple of extra holes in the block for the exhaust manifold.

 

[Grayscale]

View attachment 3800617

Is there only one screw holding the alternator mounting bracket to the motor's head?

 

[Scotty2]

The hole in the gasket is intended for the 2F exhaust manifold, or header. The 2F has couple of extra holes in the block for the exhaust manifold.

Thank you, sir!!

 

[Grayscale]

So today I’m gonna roll it out into the fresh air and try out my propane vacuum leak tester. Half of me doesn’t want to find an intake manifold leak, and the other half wants to find it just to solve this idle problem that’s driving me bonkers.

Scotty2

You reported 15-inches of vac at 1000 rpm. If memory serves me right, vacuum goes down as rpm goes up? If that is correct, than you should be close to vac-spec if you can make the motor spin at 600 rpm.

The carburetor was chemically 'dipped' during the rebuild for varnish removal? Or was the carb always in service and no fuel dried up in the internals? Corrosion looks harsh on your carb's exterior?

 

[Scotty2]

You reported 15-inches of vac at 1000 rpm. If memory serves me right, vacuum goes down as rpm goes up? If that is correct, than you should be close to vac-spec if you can make the motor spin at 600 rpm.

The carburetor was chemically 'dipped' during the rebuild for varnish removal? Or was the carb always in service and no fuel dried up in the internals? Corrosion looks harsh on your carb's exterior?

Yes, I dipped the three sections and then sprayed the passageways with carb cleaner. Twice. Put a kit in it. Didn’t find anything plugged, clogged, or severely coated with varnish, etc.

 

[Scotty2]

I’m gonna try this vacuum leak test with propane to put my mind at rest that there’s no leak. If I don’t make an ash of myself!!

 

[Pighead]

I’m gonna try this vacuum leak test with propane to put my mind at rest that there’s no leak. If I don’t make an ash of myself!!

You might find it slightly easier if you loosen the fan belt enough that your cooling fan isn't spinning...

 

[Scotty2]

Like I mentioned in today’s earlier post, I eliminated the PCV valve as the vacuum leak, since it opens and closes.

I administered propane all around the intake manifold and the carburetor, but detected no leaks.

I got a little nervous administering propane to an area when the hose was melting on the exhaust manifold, so I quit for a while and let it cool off.

The best I can do with timing at the BB, is 850 rpm, producing intermittent vacuum from 12-15 in Hg. And THAT is only possible if I leave the choke on slightly.

Next, I’m gonna pull the carb one more time.
- gonna check mating surfaces for flatness, AND dress them lightly with 240 grit paper on glass
- Gonna flush all idle circuit passages, AND run monofilament thru each of them


It’s time for a story about my 1982 Chevy Silverado, with a 250 ci, 6 cyl, 3 on the tree, 2bbl. It started idling really rough (after it overheated due to a bad radiator cap). The Chevy garage explained that this engine had an “integral head-intake manifold”. When they got hot, the intake half of the integral would crack, intermittent vacuum leaks, idle issues, etc. I got rid of it.

This reminds me of that intermittent idle. Uggh.

 

[Pighead]

Can’t get the engine to idle with the timing at the BB. Have to use choke to keep it running.

Have we ruled out your fuel cut-off solenoid yet?

 

[Scotty2]

Have we ruled out your fuel cut-off solenoid yet?

I tested it on the workbench and it opens and closes. The engine doesn’t continue to try to run when I turn the key off.

Is there more?

PS Here’s the view of the intake manifold innards and the carburetor spacer and gasket set. I re-used these; no new ones in my rebuild kit.

 

[Grayscale]

When you depress the throttle, does the engine stall, particularly when it is not gradual but all-at-once?

The float adjustment is hard to read thru the window. In all the LC carbs I've opened up, the needle and seats were slightly different. This might affect float level? I've missed the float spec before, and it flooded the bowl. I pulled over, and the light was finally just correct, and I could see the bottom of the float in the window! I believe that I limped home with more choke, as I know that I didn't open the carb in the parking lot.

How does Scotty2's carb get vacuum for the power valve?