Struggling with PDSIT synchronization

[Amskeptic]

hey guys! - here's an update
idles a little rough -16.5 MPG. seems to maintain power
sounds normal?

Sounds like a C/C+ midterm exam.
Colin

well that's not a great grade!

I'll finish the 'semester' up soon - but a question on the low idle balance to keep me focused- in your procedure you mention adjusting the throttle stop screws 1/4ish of a turn in opposite directions to compensate for imbalance, and leave it at that - so if with my "snail type" syncronizer, I see, for example, a "2" on the left side and a "5" on the right side, and the 1/4" adjustment helps nominally, shall I just leave it at that and move on with the tuneup?

thank you

Kris

Yes, synchronizing idle air flow with stop screws is sort of over the top, so you just try a little.
Man, I love the way you kids push back against grades . . .

[rustbus]

Hi!

So I did eventually complete the tuneup procedure.

After a new set of points and condenser that are NOT Bosch, the dwell and timing are solid at 48 and 5*ATDC. nice, finally staying put (my last bosch points pitted real quick...)

high speed balance was right on...I didnt have to tweek it. low speed still favours the one side, but alas, no more room for adjustment there. it idles pretty smooth now anyhow.

the procedure went actually kinda quick this time around, i did not have to adjust much, thats a good sign considering how many times i have been through it.

starting at 2.5 turns out on the mixture screws, i adjusted once 1/4" turn to the lean side and that was it. idle mixture, brought the screw way out until idle changed, then adjusted it for the 25rpm drop...finally, with some serious staring at the tach as i adjusted, i could perceive the small variations in RPM. before it was frustrating because no matter how much i move the idle mix screw, i couldnt get much change. anyhow, all good now, idles at or just under 1000.

only thing now is that the idle varies on occasion. will be fine for, say, 30 minutes of driving, then all of a sudden at the next set of lights it wants to idle low around 700. it doesnt do this as much after this last run through the procedure, but it still does. Colin didnt you mention something about hot idle degradation? i dont know, this symptom comes and goes, seemingly regardless of any external factors

overall, pretty happy, but my mileage is still low. last i checked still around 16mpg...i'll check it again in a tank or two now after this successful tune.

sadly, I am about 90% convinced I will be install L-Jet on this motor this winter season (if I still have access to our shop) I have everything, I mean everything, required for the swap - but none of the FI parts I have came off a running engine...both engines had compression issues on one cylinder. I really want to get it running on the FI so i can confirm the FI works, as next step in the next year or two is a rebuild or engine kit from you know who.

I will miss these duals. been through alot together, and a massive effort to return the whole deal to original 1972 factory setup. in particular i will miss the oil bath air cleaner, i think. they have never let me down, but everything i read about FI makes me want it. maybe the fuel economy will bump up too.

[Amskeptic]

Colin didnt you mention something about hot idle degradation?

mileage is still low.

I am about 90% convinced I will be install L-Jet on this motor

I will miss these duals. been through alot together, and a massive effort to return the whole deal to original 1972 factory setup. in particular i will miss the oil bath air cleaner, i think. they have never let me down, but everything i read about FI makes me want it. maybe the fuel economy will bump up too.

Yes, hot idle degradation due to fuel percolation. Do you have the oem pump?

I am about 90% convinced you are jumping the gun.

Everything you have read makes you want it? Covet it? Geeze, if I was out in the middle of the desert ... I am, by the way ... I would rather be on the side of the road with carburetors in 118* heat. You can fix carbs.

By the way, the best Type 4 fuel economy comes with the dual carbs. Just the *sound* of dual carbs barking makes me miss the Road Warrior, ask Stan in Cloverdale CA . . .
Colin

[satchmo]

Everything you have read makes you want it? Covet it? Geeze, if I was out in the middle of the desert ... I am, by the way ... I would rather be on the side of the road with carburetors in 118* heat. You can fix carbs.

By the way, the best Type 4 fuel economy comes with the dual carbs. Just the *sound* of dual carbs barking makes me miss the Road Warrior, ask Stan in Cloverdale CA . . .
Colin

I agree with Colin. The dual carbs, once they are dialed in, are the way to go. Don't give up just yet.

You may need to take a close look at your carbs for signs they have been mis-handled. Many uneducated mechanics/owners will screw the needle adjusters in so far they ruin the delicate aluminum seats and make it difficult to tune the carbs. This happens most frequently at the throttle plates. Sometimes folks have replaced the jets or drilled them out thinking this will make for more power. Instead they get excess fuel consumption and difficulty tuning. Finally, the throttle shafts can wear causing lean running. To compensate, one has to open up the needle adjuster a bit and that may cause lower mpg.

I've looked at and collected a LOT of used Dual PDSIT carbs. It is very rare to find one that doesn't have at least one of the problems above.

Tim

[rustbus]

Hey thanks Guys,

Colin, no not original fuel pump. its the "CB Performance" rotary, with 3.5psi output and I have the double headed Holley Fuel pressure regulator on there, system checked while running with a gauge tied in.

I'm now about 90% certain that I am uncertain about switching to FI. at least on this engine. maybe if i ever do get around to building one myself. I have a huge box of FI stuff with nearly 3 of everything, a fuel tank, cutout bits of aircleaner stand, etc. some gorgeous fuel pump and FI wiring harnesses...

it does sound nice sometimes doesn't it!

Satchmo, i know its impossible to be in someone elses shoes via the internet, but i assure you i inspected these carbs like nothing else i have ever done. i had 3 sets, and did everything to select the best and compatible parts from the lot.

my throttle plates do look like the mix screw was cranked in a bit ...in fact, i think one of my few other threads on itinerant air was about this specific thing with pictures. that thread showed me i may have been splitting hairs. census was they were OK.

m pretty confident the jets weren't tampered with - i had quite a few to select from, and they were stared at too!

throttle shafts were rebuilt by keifernet off the samba. minimal play, comparable to a nos one i have. (wrong style though, too old, cant use it)

Dont get me wrong guys, i have no flat spot, feels like pretty impressive power at speed given its the 1700 baby of the T4 engines, and oil temp never gets over 220 any more where as last year it would rock pretty high (pre-anal carb rebuild)

really, my only issue with the bus is a perceived, and sometimes quite noticeable chuffing style noise, which i believe could be a knock developing. I had my head in there with my ear on the fan shroud after a long drive (man it runs cool/smells good!) hugging the engine and listening to the sound deep inside. would have looked very strange to any passer-by. unrelated to fuel delivery! anyhow I'll keep an eye on it.

I have not ever verified my accelerator pump output volume.

so...does fuel percolation occur with electric or stock pumps?

[Amskeptic]

my only issue with the bus is a perceived, and sometimes quite noticeable chuffing style noise, which i believe could be a knock developing. I had my head in there with my ear on the fan shroud after a long drive (man it runs cool/smells good!) hugging the engine and listening to the sound deep inside. would have looked very strange to any passer-by. unrelated to fuel delivery! .

so...does fuel percolation occur with electric or stock pumps?

Your description leaves me sitting here puzzled. Do you have hot engine main bearing thumping? Verify oil pressure meets spec and set the endplay to .003-.0035" and then forget about it, these engines have more main bearing surface area than a Chevy 350.

Fuel percolation is far more pronounced with aftermarket electric pumps. You want 3-5 psi for factory dual carbs as mentioned in the Bentley. If your regulator allows you to hit 3/4/5 psi, that should give you an opportunity to see if any changes in driveability occur. Some regulators will not allow backflow during heat-soak. Then hot gasoline has only the carb throats to expand into. Hard hot starting *will* occur, but accelerator-to-floor will start it every time.
Colin

[rustbus]

Your description leaves me sitting here puzzled. Do you have hot engine main bearing thumping? Verify oil pressure meets spec and set the endplay to .003-.0035" and then forget about it, these engines have more main bearing surface area than a Chevy 350.

Fuel percolation is far more pronounced with aftermarket electric pumps. You want 3-5 psi for factory dual carbs as mentioned in the Bentley. If your regulator allows you to hit 3/4/5 psi, that should give you an opportunity to see if any changes in driveability occur. Some regulators will not allow backflow during heat-soak. Then hot gasoline has only the carb throats to expand into. Hard hot starting *will* occur, but accelerator-to-floor will start it every time.
Colin

hehe, i dont really know what i have when it comes to engine bearing noises... I have no OP gauge but the dummy light lights an goes out after a few cranks, start or not. never seen it come on, either with 20-50 or 10-40 oil. I set the endplay this past winter, but i do hear, sometimes, a change in the engine sound with the clutch pedal depressed. on startup i hear what i think is a knock, which diminishes quickly into a chuffing. but i swear i can hear and feel a knockish thing emanating from the engine. I could be making a big deal out of nothing! If you'll have a listen, I will try and record the noise and post it on photobucket or something, hopefully that works I've never uploaded video/sound myself.

Back to the carbs, I will check and adjust the fuel pressure this weekend. if my memory serves, I cut it very tight to the low side with the regulator. maybe too low. I'll bump it up to 4psi (or max pump output) and see.

About the heat soak, i dont know if this regulator backflows, but (and feel free to scold) I have installed a ball valve pre-regulator (actually threaded into the bottom of it) and once i arrive at a destination i will be at for more than say, 30min, I leave it running, jump out, close the valve, wait a bit, then kill the engine. cheap insurance? definitely allows fuel shut off if im servicing something downstream.
Hot startup has always been reliable, with me slowly pushing the pedal to the floor when cranking. real hot, yes, just as you say, all the way down to the floor.

anyhow, i'm all over the place with this thread!

[rustbus]

this may or may not work well...



thats a cold startup, after sitting for at least 7 days straight, no choke, no gas pumping until after start.

this is the loudest the knock has ever been, it surprised me, but not totally as i expected it to be louder after sitting so long and draining the oil down.

the knock is in the first 30 seconds or so, then after that i move the camera around to get more sounds...but my camera doesn't make a very good microphone!

took the bus out to my parents yesterday, ran good, but the idle wanted to drop out more. right to 0 rpm at a set of lights, then, say 10min later, idling at 900. can't figure why it want to drop out occasionally.

[rustbus]

oh boy.

so i believe the intermittent poor idle is solved....loose idle circuit solenoid....

while thats a super lame thing to overlook, at least i was able to use my brain to figure it out, rather than randomly feeling around the engine. I said to myself, "why does it change from super high idle (where i had adjusted it to in an attempt to compensate for the low) then back down to like 600, regardless of where it was set? why does it seem to 'switch' between idle speeds, with no regard for any other parameters? why always 6-700? why is that number familiar?....oh yeah, thats what Colin said the idle would be when you killed the central idling circuit, dumbass.

yeesh. tightened that on with more than just my slippery fingers and adjusted the idle back down.

i also checked the fuel pressure while in there, it was set to 2.5psi. again, what was i thinking. so, i slowly unscrewed the pressure regulator all the way up to 3.5psi, and thats where it stopped as that is what this electric pump is rated to put out. so regulator is now redundant, but i see not much point in removing it....although it would clean up the compartment a little.

anyhow, i'm off for a drive too see the results and verify idle at operating temp.

Thanks!

[Amskeptic]

oh boy.
at least i was able to use my brain to figure it out,

This is good. Let us know how it does.
Colin