'72 bus carb popping back

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Dr. Scott
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'72 bus carb popping back

Post by Dr. Scott » Sun Apr 27, 2008 6:51 pm

Status – 1972 bus / with 1973 factory replacement motor – dual solex carbs (PDSIT 2/3) backfiring a little out of the left carb and out of the exhaust - on acceleration only. None on de-acceleration. Plugs clean and gapped. Timing set. Smog system removed, but vacuum and hose lines that connected the two carb / throttle bodies are in place. Idle is low. Runs great at high rpm’s. Loves going 45 mph in third and climbing hills. Does not like low rpm’s especially in second. Carbs have been rebuilt after recent engine pull. Brake booster line is hooked up. Electromagnetic jets in place. Carbs synced. Seems like fuel is not getting burnt? Exhaust is new and may not have tightened down enough under the heater boxes when putting everything back together - ran out of daylight tonight to really check that out. The rest of the accessible exhaust is tight.
Any ideas, suggestions or directions?

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Mark
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Post by Mark » Sun Apr 27, 2008 8:07 pm

Lean (or rich...hard to gauge from your findings?) mixture, correct dwell, valve clearances verified, your aforementioned possible exhaust leak? Check the easy & free things first. Get a good trouble-light maybe? :pirate:
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Amskeptic
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Re: carb farting

Post by Amskeptic » Sun Apr 27, 2008 8:31 pm

Dr. Scott wrote:Status – 1972 bus / with 1973 factory replacement motor – dual solex carbs (PDSIT 2/3) backfiring a little out of the left carb and out of the exhaust - on acceleration only. None on de-acceleration. Idle is low. Runs great at high rpm’s. Carbs synced.
Seems like fuel is not getting burnt? Exhaust is new and may not have tightened down enough under the heater boxes when putting everything back together - ran out of daylight tonight to really check that out.
Carbs not synchronized. "But I just said. . . " Anyways, backfiring upon acceleration is a classic symptom of one carb working harder than the other, and in your case, it is the left. If you have a 1973 set of carbs, they come on tall intake manifolds with EGR valves and a plastic box air filter, if you have just a 1973 longblock but the original manifolds and carbs with the low profile oil bath air cleaner, ignore the red.

You will eventually need to compare float levels, jet sizes, accelerator pump discharge quantities, nozzle positions, and you must extra-double check that the EGR valves located on the intake manifolds are properly CLOSED and that the diaphragms inside of them are not breached. Actually, they probably are. What then happens is that the little tiny holes that you can spot on the periphery of the cans of the EGR valves will be drawing a secret vacuum. Plug them.The central idling circuit is what connects the intake manifolds to each other through those little pipes with hose connectors that lead to the left carb. A robust high velocity central idle circuit improves smoothness and power down low right off idle. It needs your vacuum retard unit on the distributor to be functional so your idle speed will be something less than 1,300 rpm with the central air screw opened up but good. The 5* ATDC/10*ATDC timing is to allow lots of air flow through the central idling circuit AND an idle speed of 1,000 rpm. What happens is that the vacuum unit fails, the idle "sticks" at a high rpm, and people turn the central idling circuit speed screw all the way down to lower the idle, and driveability? Buh bye. The 5* ATDC/10*ATDC reading is actually just there at idle, and disappears the minute you crack the throttle. IF you time to 5* ATDC/10*ATDC with a leaking vacuum retard, or even a missing vacuum retard distributor, then your backfiring at low RPMS may very well be because the whole timing map will actually be way too retarded. With a leaky or non-existent retard unit, you best set your idle timing to 6-7* BTDC at below 1,000 rpm which you can't get without shutting off the airflow through the central idling circuit. See how inter-related/inter-dependent it all is? There is a lot to digest here, but check it out and get back to us.
Colin
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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Post by Dr. Scott » Tue Apr 29, 2008 6:37 pm

thanks for the feedback - I will get to work and let the site know of the results - appreciate you for taking the time to reply. I will honor it with the results....(it may be awhile - I am limited to weekend work and got son's birthday and Mother's Day the next two weekends...priorities, priorities!)

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Post by Dr. Scott » Sun May 18, 2008 5:18 pm

Ok Colin - and others - the update on the 72 bus with a 73 motor (put in by VW dealer in 74)
finally got to the bus today -
Plugged up the EGR valves (they had duct tape around them when I got the bus - when I pulled motor and rebuilt carbs I thought "why the heck is that duct tape on here?" and cleaned them up real good. Now they have duct tape on them again - seems to work).
Turned down fuel mixture screw and idle screw all the way and backed fuel out 1 1/2, then another half later on...
Idle screw backed out about 2 1/2 also.
Timing - played around with it - went everywhere suggested - now at 7* BTDC...
Seems I must have the leaking / non existant vacuum retard unit...
Adjusted the idle screw on side of left carb out about 1 1/2 turns.
Electromagentic jet on right carb is stripped - Locktighted it in there...
Oh - and yeah, Colin, I did have to re-sync the carbs...they were off...and yes, I have the tall intake manifolds...I liked your "But I just said..." - three words I don't mine saying here "You were right".
Obviouslly I was way too rich - lots of black smoke before -
Now - smoke is gone - mostly mostly -
Backfiring from carb and exhaust are both gone ---
Performance is pretty darn good - 45-50 mph up the mtns/hills in Dahlonega - Put some Marvel Mystery Oil in her (Walmart did not have Sea Foam)
Drove her around for 20 miles or so...I am not a hot rodder but she still likes/performs better at high RPMs and 40+ mph better than low speeds.
I am gonna change the fuel filters.
wanna take her camping up in Bryson City this weekend - but gotta feel safe (got the family on board...) Will have another bus along just in case -
I do think the idle is a little high, but motor sits pretty smooth...
seems to be running the best she has since she joined the family in August...
but what should I be looking for now...

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Post by Dr. Scott » Mon May 19, 2008 6:25 pm

Admin - I noticed the re-title on the post. I will be a little more descriptive next time and a little less juvenile. I work with adolescents...and tend to stay in that mindset a little too often. But got the idea of how to behave here - thanks!

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Amskeptic
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Post by Amskeptic » Mon May 19, 2008 7:30 pm

Dr. Scott wrote:Admin - I noticed the re-title on the post. I will be a little more descriptive next time and a little less juvenile. I work with adolescents...and tend to stay in that mindset a little too often. But got the idea of how to behave here - thanks!
I only did it for the search function. It is actually suggested in the FAQ for the technical forums. Nottaproblemo. . .
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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Post by Dr. Scott » Tue May 20, 2008 4:48 am

now a little backfire on acceleration and a little surge when backfiring?

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Amskeptic
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Post by Amskeptic » Thu May 29, 2008 4:07 pm

Dr. Scott wrote:now a little backfire on acceleration and a little surge when backfiring?
Backfire = loud report from exhaust.
Pop = muffled report through air intake.
Which under what circumstance?
Colin
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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