Idle woe '72 factory dual carb bus

[covelo]

So I spent some wonderful, but ultimately unsuccessful hours futzing with the bus this morning. Glorious morning here in SF and a great time to sit outside in the warm sun staring at your engine compartment. I checked the valves and most of the outside ones were tight (are those the exhaust valves?). Especially number 2 (intake and exhaust) were pretty tight after the long hot dead valley run.

Anyway, once the valves were properly adjusted I decided to tackle my idle problem. I unscrewed the big central idle plunger and indeed it was not operational. Sprayed some carb cleaner in there and it fired right up. Screwed it back in the carb, but no difference. So I took it out again and it works fine outside the carb, but there is no way for me to determine whether it works when it's screwed in. I did spray some carb cleaner in the hole as well, so any crud should have been gone, but I didn't remove the carb so it's hard to say what the overall condition of that hole was.

So I don't really know what to do next. The symptoms are not straightforward. The bus starts fine and idles fine, albeit a bit low and the big idle adjustment screw on the left carb top does nothing when I turn it, which is consistent with the idle circuit plunger not working. It also runs fine when I drive, but when I brake to make a turn or stop at a stop sign the bus stalls. Bumping or using the starter immediately and satisfactorily restarts it with a good (but low) idle afterwards. So I thought I might have a problem with the brake booster vacuum. However, when I brake without stopping, nothing happens, even if the car is in neutral and idling (like when I'm coming down a hill in neutral and brake). The only thing I haven't checked is what happens when I slow down hard without braking, but then that's difficult to do.

If it was the brake booster vacuum, where would I look to establish if there is a problem? Also, I assume that has no relationship to me not being able to get the idle circuit to do anything. Is that a correct assumption? Does anyone have any suggestions on what could cause the idle circuit plunger to work in the open air, but to fail when screwed into its little hole? Is there anyway to bypass that thing?

And lastly, I have an embarassing backfire on steep downhills in the city here. This is not much of a problem on the open road. The bus didn't backfire on even some of the steepest grades in Death Valley, but it always does it in my street. Could this be related to the idle air circuit problem as well?

My apologies if these are silly questions, but the whole dual carb fuel system is not yet intuitive to me. You learn something new with these buses every day. But at least I'd rather have a bus that runs well but stalls a lot than the reverse so I guess that helps.

Thanks!

[dingo]

So I don't really know what to do next. The symptoms are not straightforward. The bus starts fine and idles fine, albeit a bit low and the big idle adjustment screw on the left carb top does nothing when I turn it, which is consistent with the idle circuit plunger not working. It also runs fine when I drive, but when I brake to make a turn or stop at a stop sign the bus stalls. Bumping or using the starter immediately and satisfactorily restarts it with a good (but low) idle afterwards. So I thought I might have a problem with the brake booster vacuum. However, when I brake without stopping, nothing happens, even if the car is in neutral and idling (like when I'm coming down a hill in neutral and brake). The only thing I haven't checked is what happens when I slow down hard without braking, but then that's difficult to do.

If it was the brake booster vacuum, where would I look to establish if there is a problem? Also, I assume that has no relationship to me not being able to get the idle circuit to do anything. Is that a correct assumption? Does anyone have any suggestions on what could cause the idle circuit plunger to work in the open air, but to fail when screwed into its little hole? Is there anyway to bypass that thing?

And lastly, I have an embarassing backfire on steep downhills in the city here. This is not much of a problem on the open road. The bus didn't backfire on even some of the steepest grades in Death Valley, but it always does it in my street. Could this be related to the idle air circuit problem as well?

My apologies if these are silly questions, but the whole dual carb fuel system is not yet intuitive to me. You learn something new with these buses every day. But at least I'd rather have a bus that runs well but stalls a lot than the reverse so I guess that helps.

simplest thing first: disconnect your brake booster at the place where it receives manifold vacuum, and plug up both sides. then test run. I dont think thats your problem, but its an easy one to eliminate

Have you taken off the top of the left carb and checked to see that the emulsion tube that sucks fuel out of the float bowl is not clogged. Check by sucking/blowing...then rinse with Scotch. have you checked the tip of the thin metering needle ?

As for dying/stalling when you come to a stop..could be afew things, but badly worn throttle shafts and resultant vacuum will cause that symptom. Sounds like your idleing system is malfunctioning. Im sure Colin will soon respond with the full artillery.

*planning to have bonfire/beer tommorow at sunset with Djeep..usual spot..cruise by if you can

[Amskeptic]

I took it out again and it works fine outside the carb, but there is no way for me to determine whether it works when it's screwed in.
I have an embarassing backfire on steep downhills in the city here.

The big solenoid likes to have the plunger very slightly depressed inside the carb. If it does not contact the seat, the solenoid will not retract it. Make sure it is screwed in completely. If there is a rubber sealing ring between the solenoid and the carburetor, delete it. Your stalling problems will be history when you have the central idling circuit back.
Backfiring is usually a lean condition that you may not be able to get rid of if the engine is still cold. It should not backfire when engine is at operating temperature. If it does, richen up the dual carbs a bit. Remember when I set up the dual carbs a little lean and let the central idling circuit take up the mixture by being slightly rich?
You might fix thebackfiring and the stalling in one step!
Colin

[covelo]

The big solenoid likes to have the plunger very slightly depressed inside the carb. If it does not contact the seat, the solenoid will not retract it. Make sure it is screwed in completely. If there is a rubber sealing ring between the solenoid and the carburetor, delete it.

How do you know these things? I deleted the rubber ring and my idle is back! Thanks!

[Amskeptic]

How do you know these things?

Years of agony.
Colin
(vin#?)

[satchmo]

Covelo -

Are you running dual carbs on a 2 liter engine? What year bus are the carbs from? Did you have to change your jetting at all?

I am bumping up to a 2 liter and plan on using the stock dual carbs from a 73 bus. I know that carbs from 72 1700cc, 73 1700cc and 74 1800cc all had slightly different jets (at least that's what Bentley says).

Thanks, Tim

[Amskeptic]

Covelo -

Are you running dual carbs on a 2 liter engine? What year bus are the carbs from? Did you have to change your jetting at all?

I am bumping up to a 2 liter and plan on using the stock dual carbs from a 73 bus. I know that carbs from 72 1700cc, 73 1700cc and 74 1800cc all had slightly different jets (at least that's what Bentley says).

Thanks, Tim

Use the '74 jets and you'll do fine. Or, ask one of our European 2.0 carb engine people to tell you which jets they are using. The Type 4 dual carbs were used in South Africa and Australia also.
Colin

[satchmo]

Covelo -

Are you running dual carbs on a 2 liter engine? What year bus are the carbs from? Did you have to change your jetting at all?

I am bumping up to a 2 liter and plan on using the stock dual carbs from a 73 bus. I know that carbs from 72 1700cc, 73 1700cc and 74 1800cc all had slightly different jets (at least that's what Bentley says).

Thanks, Tim

Use the '74 jets and you'll do fine. Or, ask one of our European 2.0 carb engine people to tell you which jets they are using. The Type 4 dual carbs were used in South Africa and Australia also.
Colin

I don't have the Bentley in front of me (at this point in time, I have not memorized every page), but I seem to remember that not only did the main and air correction jet sizes change, but there are also changes in the pilot jet and pilot air bleed jet (or something like that). Unfortunately, the pilot jets are not identified in the diagram for PDSIT 32-34 carbs, and I wonder if they are non-removable anyway. Perhaps one needs to go with a 74 carb to get the whole enchilada?

Tim

[Amskeptic]

Unfortunately, the pilot jets are not identified in the diagram for PDSIT 32-34 carbs,

Your pilot jets are the electromagnetic cutoffs. There will be numbers on them, usually a 55.
Colin

[dingo]

I have a bucket full of PDSIT carbs that i wont be using...lemme know if you need some jets or solenoids or whatever

[satchmo]

I have a bucket full of PDSIT carbs that i wont be using...lemme know if you need some jets or solenoids or whatever

I have 'box o carbs' here too, with lots o jets. Still want to know what jets folks are using that have the stock solex PDSIT 32-34 running on a 2 liter engine. Covelo? European 2.0 carb people? Anyone? Thanks.

Tim

[DurocShark]

I've still got my box'o'carbs as well. So between us all we should be able to come up with whatever jets are needed...

[covelo]

I have a bucket full of PDSIT carbs that i wont be using...lemme know if you need some jets or solenoids or whatever

I have 'box o carbs' here too, with lots o jets. Still want to know what jets folks are using that have the stock solex PDSIT 32-34 running on a 2 liter engine. Covelo? European 2.0 carb people? Anyone? Thanks.

Tim

Sorry, I was out of the country. I am using the original 1700 carbs on a newly rebuilt 2l engine. Probably not a good idea.

[Amskeptic]

I am using the original 1700 carbs on a newly rebuilt 2l engine.
Probably not a good idea.

It is a fine idea under 4,000 rpm. How do your plug electrodes look?
If your Westy is able to maintain highway speeds, get decent fuel mileage (17+) and the plug insulators are not bleached white, yer in the ball park.
Colin
(still trying to get my brother corralled for our trip out west to visit Marina Chick. . .)

[satchmo]

I have a bucket full of PDSIT carbs that i wont be using...lemme know if you need some jets or solenoids or whatever

I have 'box o carbs' here too, with lots o jets. Still want to know what jets folks are using that have the stock solex PDSIT 32-34 running on a 2 liter engine. Covelo? European 2.0 carb people? Anyone? Thanks.

Tim

Sorry, I was out of the country. I am using the original 1700 carbs on a newly rebuilt 2l engine. Probably not a good idea.

Thanks for the info. Do you know if you have the stock cam or not? I assume you are not running hydraulic lifters.

I just about have my rebuild ready to go with a 2.0 crank, rebuilt rods, balanced pistons, 1700cc heads with larger openings and valves, and the old fashioned stock dual carbs.

Tim