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Rpm still not better

1968 volvo

Well-known member
Joined
Apr 29, 2021
Location
Santa Rosa California
Hi Volvo Nerds :)
I have a 1968 Volvo 142s with b20 engine and the suhs6 carbs. It idols at about 1300-1500rpm.
Here is the way that it acts.
-It idols high only when it warms up
-It only idols high only after you rev it
-It goes down to a reasonable rpm when in gear. (It has a BW35 transmission in it)
-It seems to be running rich but that could be the carburetors not being very well tuned

Here is what Ive tried to fix the problem
-I had the carburetors rebuilt by a gentleman by the name of Thomas Bryant (They needed to be rebuilt anyways)
-I replaced the carburetor to intake manifold gaskets
-I replaced to intake manifold to head gasket
-I removed the second set of butterfly valves in the intake manifold and plugged the holes
-I replaced all the vacuum hoses
-I replaced the one way valves for both the pcv and the brake booster
-I replaced the brake booster( I did this prior to trying to fix the rpm when I built the brakes back up)
-I made sure that the fuel pressure was in line with what the Haynes shop manual recommends by installing a fuel pressure regulator

I'm at my whits end and was wondering if anyone had other suggestions. I tried everything people recommended last time and I'm back for more.
 
Hi Volvo Nerds :)
I have a 1968 Volvo 142s with b20 engine and the suhs6 carbs. It idols at about 1300-1500rpm.
Here is the way that it acts.
-It idols high only when it warms up
-It only idols high only after you rev it
-It goes down to a reasonable rpm when in gear. (It has a BW35 transmission in it)
-It seems to be running rich but that could be the carburetors not being very well tuned

Here is what Ive tried to fix the problem
-I had the carburetors rebuilt by a gentleman by the name of Thomas Bryant (They needed to be rebuilt anyways)
-I replaced the carburetor to intake manifold gaskets
-I replaced to intake manifold to head gasket
-I removed the second set of butterfly valves in the intake manifold and plugged the holes
-I replaced all the vacuum hoses
-I replaced the one way valves for both the pcv and the brake booster
-I replaced the brake booster( I did this prior to trying to fix the rpm when I built the brakes back up)
-I made sure that the fuel pressure was in line with what the Haynes shop manual recommends by installing a fuel pressure regulator

I'm at my whits end and was wondering if anyone had other suggestions. I tried everything people recommended last time and I'm back for more.

Try resetting all fuel and air settings. Warm it up first.

loosen linkage. Back off choke and idle set screws until there is absolutely no contact. Back the chokes off an extra full turn.

Get the throttle screws to just touch the stop, then give them 1.5 turns.

Raise the jets to the bridge. make sure they are all the way flush with the bridge in the carb throat. Pull the dashpot dampers and check that the piston falls with a clink. if not, recenter the jets.

Lower the jets 12 flats (2 full turns). check that the piston falls freely to the bridge. Put the dampers back in.

Start the engine. It should idle high, so get it back in line by adjusting each carb. Use a sync tool and get the air flow correct, and idle where it should be.

Lift the front carb piston 1/8" and see what changes. If it stumbles, richen by 1 flat, if it raises the idle lean it by 1 flat, and blip the throttle after each adjustment. Do the same to the rear. Go back and fourth until lifting the piston doesnt change the idle speed.

THE MOST IMPORTANT PART:
Make sure that they are even and balanced. Make sure that all other things are correct. So valve lash, and ignition timing. Vacuum leaks are a huge problem, so check for vacuum leaks again.

If you dont have a sync tool. Go to the hardware store, and get some clear tubing (about 12 feet) a yard stick, and a couple packs of zip ties and make a manometer. Pretty easy.
 
Last edited:
Good afternoon Redblock Redliners,
Thanks to cwdodson88 for the advice. This morning I woke up in a terrible mood. So instead of being a pain to my family I went out to my car, the one thing that doesn't complain when I'm a nightmare.

I did what was recommended of cwdodson88 and tried to tune my carburetors. Right off the bat I noticed that one of the carburetors was making a hissing noise. When I looked closer I saw that the rear piston thingy didn't drop nearly as low as the other after reving it. I think that this is maybe the brass piece on the butterfly valves making that noise..... Let me know what anyone thinks.
 
When decelerating with your foot off the pedal (like going downhill in gear) a lot of vacuum is generated which draws in a lot of fuel that isn't needed. The decel valve opens under high vacuum conditions to reduce the amount of fuel entering the engine. Slightly opening the throttle will do the same thing if you remove the valves.

I don't know of anyone selling replacement parts for the decel valves. If anyone has them it would be Joe Curto. He also has the plates without the valves.
 
Does this Joe have a website or where can I reach him? If I do solder the valves should I do both or just the one that is being problematic? Also am I at risk of having a chunk of solder fly into the engine if something goes wrong?
 
Decel. valves could be an issue too, but my opinion is that your intake manifold is slightly warped based on the symptom that it is fine when cold but the idle speed increases when warmed up.

Even the new gasket is not sealing properly when warm.

Basing this on my experiences with a few B20s in my 140 series cars.
 
I removed the valves and soldered up the holes on a few but don't remember what kind of solder I used.
 
Success. It was the decel valve. The idle is perfect even after a long drive. I am still going to do a half tune on the carburetors. Feels good to have that complete. Now what next?
 
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