1969 Bus - Ending the never ending popfire

skin daddio · Post by **skin daddio** » Tue Jul 28, 2009 12:34 pm

69 single port
035 dist
30/31 solex, auto choke set for summer
tach reading 825-850, but it doesn't spike, it hunts a little.
1 irritating backfire

i have had a poprocks sound for so long but i can't ignore it any more. doesn't sound like backfire bangs, although i'm not a hundred percent sure. during valve adjustment all four intakes were okay but all four exausts were tight. so, i feel like i need to address it and solve things once and for all.

brand new points, dwell at 52*, static timed at 0 tdc, carb adjusted every damn way possible on earth; basically as described in bentley. with that i'm expecting intake leak. so far it starts up sharp, idles nicely. upon five minutes of warm up street driving the popping starts. usually in second gear after letting up on the gas anticipating a stop sign. more dramatic popping on a mild down hill with foot off accelerator in second. greater downhill=greater popping.

popping happens with dwell in the 44-52 degree range and all carb adjustments. my best recollection is that it happened with different carbs too.

regarding potential exhaust leaks, there are no carbon deposits and everything seems tight. regarding bad valve cover gaskets, i've addressed that. i changed over to an all-cork one in a case where the rubbery one failed though no change in the popping.

last year, anticipating an intake leak i removed the engine and slid a sweet nos gasket ring between the head and manifold on the left side. evidence of oil spray around the left firewall led me to think the problem was right there. i synched it down as best i could and thought the problem would be corrected.

this is causing me to use my brakes more since i have to take it out of gear a lot when letting up on the accelerator. bummer.

including your thoughts, a question i have for the group: is there a spray [which one] that i can feed this and all potential areas for intake and exhaust leakage?

hambone · Post by **hambone** » Tue Jul 28, 2009 12:39 pm

Richen up the mixture, classic symptoms of too lean. Also check your exhaust for leaks.

Manfred · Post by **Manfred** » Tue Jul 28, 2009 1:07 pm

Your dwell maybe set too high. 50 is the max for new points. At least, for my make and model. The spark may be jumping the points when they are open. I've heard this may cause the back fire, causing your popping sound.

vdubyah73 · Post by **vdubyah73** » Tue Jul 28, 2009 3:53 pm

With your foot off the throttle and decelerating, the mixture into the cylinders is too lean to burn. It gets into the hot exhaust and pops off. Pretty much all cars did this back in the day. Just a bumba da bum bum bumm while decelerating. Decel solenoids were invented for carbs to eliminate it sometime in the 70's, one of the early emission control devices. All it did was prevent the throttle from snapping shut when the gas pedal was released, it kinda eased the throttle shut slowly. A minor exhaust leak makes it worse as it lets fresh air into the exhaust to mix with the unburned vapor from all the cylinders. When the unburned vapor from all four cylinders gets together in the hot muffler off it goes.

skin daddio · Post by **skin daddio** » Tue Jul 28, 2009 5:26 pm

vdubyah73 wrote:[snip] A minor exhaust leak makes it worse as it lets fresh air into the exhaust to mix with the unburned vapor from all the cylinders. When the unburned vapor from all four cylinders gets together in the hot muffler off it goes.

aside from visually noticing leaks is there a liquid method you'd suggest to trace tiny leaks. i seriously can't see like i used to.

initially i was looking for carbon deposits thinking exhaust leak meant burnt stuff not getting out through the tail pipe. like carbon showing inbetween a bad fitting muffler/damper or damper/tailpipe. now i will change gears to look for clean spots that could allow fresh air in. pretty sure before its done i'll have to remove the muffler and deal with snapping the dreaded rusty bolts that never give a break. but if that is the case, its worth it to reduce or remove this danged popcorn maker.

vdubyah73 · Post by **vdubyah73** » Wed Jul 29, 2009 3:47 pm

Have a helper block the tailpipe with a wet rag, I mean really block it, try to stop the exhaust from exiting the tailpipe. While the helper is blocking the tailpipe You are underneath trying to pinpoint where any hissing sounds are coming from.
If the exhaust is tight you can actually stall the engine this way.

skin daddio · Post by **skin daddio** » Wed Jul 29, 2009 3:51 pm

roger that. THANKS!

Post by **Amskeptic** » Thu Jul 30, 2009 10:11 am

skin daddio wrote:roger that. THANKS!

My bus is popping now too. It is the rotted-out baffles in my "lifetime stainless steel muffler" from Bus Depot. Looks great! on the outside. Made of crap on the inside.
Colin

skin daddio · Post by **skin daddio** » Mon Dec 21, 2009 7:43 am

when i said neverending pops i meant it. frusterating.

i removed the engine a few weeks ago and really, i mean overcarefully got into vac leaks. meticulous, i spent a long time buttoning it back up. i used four nylocks on the intakes at the suggestion of beloved flaps. driving was fantastic for nearly 450 miles until, sure enough, backfires and oil splatters on, around the firewall, etc.. what a nemesis. i'm weird about putting any more stress on the head intake studs while away from my garage. really getting to be too much. the gaskets are the good copper ones, i can't see why the intakes keep slipping up. damn!

Post by **Amskeptic** » Tue Dec 22, 2009 11:08 am

skin daddio wrote:when i said neverending pops i meant it. frusterating.

i removed the engine a few weeks ago and really, i mean overcarefully got into vac leaks. meticulous, i spent a long time buttoning it back up. i used four nylocks on the intakes at the suggestion of beloved flaps. driving was fantastic for nearly 450 miles until, sure enough, backfires and oil splatters on, around the firewall, etc.. what a nemesis. i'm weird about putting any more stress on the head intake studs while away from my garage. really getting to be too much. the gaskets are the good copper ones, i can't see why the intakes keep slipping up. damn!

Once you have taken care of vacuum leaks, you must go elsewhere to track them down. Are you fully informed of the jet sizes in the carb? Do you know that the baffles are still good inside of the muffler?

Play. For example, try this experiment:
If you have an electromagnetic cutoff jet, make sure that it has the rubber sealing ring with the collar. Tighten it and then go back loose 1/8 to 1/4 turn. Adjust idle mixture. Go drive. Notice how the engine has better response and doesn't backfire. Retighten cutoff jet. Electromagnetic cutoff jet is a jet , it is not a simple stopper like the later air bypass big solenoids. Make sure it is clean.

Your float adjustment is critical. Do you know that your fuel level in the bowl is good? Find out, it you don't know. You have a little pilot jet on the right side? Clean it too.
Colin

skin daddio · Post by **skin daddio** » Tue Dec 22, 2009 7:16 pm

what about the oil spray patter on the firewall, both sides, and oil around the intakes, i.e. rubber engine seal, rubber ignition wire boots, etc.. related to your theory?

Post by **Amskeptic** » Wed Dec 23, 2009 7:47 am

skin daddio wrote:what about the oil spray patter on the firewall, both sides, and oil around the intakes, i.e. rubber engine seal, rubber ignition wire boots, etc.. related to your theory?

Oil spatter is oil spatter. Fix it. Air-cooled engines appreciate cleanliness.
Cooler seals? Cracked case at a gallery?
Colin

skin daddio · Post by **skin daddio** » Sat Jan 09, 2010 5:24 pm

a twelve hundred mile run gave me data.

mile 000: engine out, tighten everything down pertaining to a possible vacuum leak. remove cylinder head, correct push rod tube and re-seal, tighten bail wire at the cover.
mile 450: 1 north. looping west marin the backfire reappeared in bolinas. restless night sleep near ocean beach.
mile 700: bus doesn't want to leave goldworthy's spire in the presidio. hard start, return to cousin-in-laws in cole valley. time taken to think things through at the academy of sciences and de young. three test drives to 1] presidio to noe; 2]house call for commission and separate dinner in the mission; and 3] exploratoruium. all tests were in different hilly directions lasting fifteen minutes.
mile 900: 199 north. the straightshot from grants pass causes me to think i was mildly loosing compression. bus wanted to return downhill to trinidad. dungeness crabs at rock booty prices. bus knows, and it saw that lucy at beachcomber cafe.
mile 1200: from the safety of my driveway i took out the bus for a one mile postal run. bus was postal. when i returned i found the dwell at 70, and if that wasn't enough two one inch size bites were missing from the top of my cork valve cover gasket. needless to say i'm draining the oil while stacking my multi-tasking. kinda creamy and thick from what i'd seen so far. plan to swap a better matching damper for best fit at the muffler before i start banging the danish hole three ways.

last dangity, this ain't the first gasket this cylinder head tried to gargle neither. why does my engine think one of the gaskets is a piece of bread?

Post by **Amskeptic** » Sat Jan 09, 2010 5:41 pm

skin daddio wrote: dwell at 70,
two one inch bites were missing from the top of my cork valve cover gasket. why does my engine think one of the gaskets is a piece of bread?

Dwell can be squared away with correct grease on the breaker cam lobes and a check of the points where the hold-down screw contacts. Some people over-tighten this screw which leaves an impression on the plate. Subsequent adjustments creep back to the impression. Replacing the points and not over-tightening is the cure for that.
Bites are usually a rocker arm interfering with a poorly positioned gasket.
Colin

skin daddio · Post by **skin daddio** » Sat Jan 09, 2010 6:22 pm

yes, and i think the bus is sucking my cork.

Itinerant Air-Cooled

Itinerant Air-Cooled

1969 Bus - Ending the never ending popfire

1969 Bus - Ending the never ending popfire