Backfire on load - idles nice

[jp16v]

This is my first STF post. I've tried everything I've read about folks issues similar to mine, to no avail. PLEASE HELP!! Forgive the long post, I wanted to provide as much detail as possible.

If you frequent volkswebbin, forgive the x-post. I put this out over there as well...

I have a VW Beetle based dune buggy with a '72 1600 DP motor that I've never really had running great. I pulled the motor off of a wrecked '72 Super. Compression looks good across the board.

Here are the symptoms: it has a beautiful idle and starts up with no problem. When accelerating under load, it backfires (not violently, just a pop) through the carb. This condition can also occur if I stab the throttle with no load. It falls on its face during acceleration off idle. Once it's past the idle circuit, it seems to move along just fine through the rev range. After a while of tuning, it fouls the plugs (black sooty foul) so I know it tends to idle rich. The pop when accelerating make me suspect it's getting a shot of air w/out enough fuel to match (vacuum leak).

The facts (parts):
Rebuilt German 34Pict-3 Carb with nice, tight throttle shaft (I've tried two others: Bocar and another Solex)
Newly rebuilt DVDA (retard disconnected and plugged) dizzy set at 7.5 degrees BTDC w/vacuum off and plugged (I've tried an 009 also). Advance (both mechanical and vacuum) is working well. New condensor.
Silicone 8mm JC Whitney plug wires (I have some doubts here) tested for continuity and resistance when flexed. Tested fine (I think there are no resistors in these as I understand stock ones to have)
Fresh (as in today) Bosch W8AC plugs set at .028
Valves set at .006
Points gapped at .016
I've tried a known good coil (off a bug runnin great)
Main jet 130
Idle jet 55
Idle solenoid works (distinct click every time)
Choke is working well and stands up when warm
Good gas (I burn most of my gas in the driveway tuning this beast)
Restored tank (glazed internally)
Good fuel pump and verified pressure with Fuel Pressure Gauge
Less than ideal dual cannon exhaust (not a big fan)
New urethane intake boots

Things I've done to try to troubleshoot (other than setting it up like above):
I have checked for vacuum leaks thoroughly by disconnecting the generator belt and spraying the different intake connections (including DP Castings to head) with starter fluid (w/fire extinguisher in other hand). Also sprayed around the carb, especially the throttle shaft.

I have pointed the accelerator pump nozzle down the throat when cracking open the throttle butterfly. It gives a nice steady stream of gas. I've tried all (well most positions) of the accelerator pump bell crank (same symptoms). Symptoms remain the same regardless of the carb I put on.

I've tried multiple dizzys, carbs, and coils.

Points of interest:
The volume control screw doesn't have a huge impact on the idle, but the air bypass screw does (also makes me think vacuum leak).
The strobe timing light picks up a very erratic signal from the spark plug wires (hit, hit, hit, miss, miss...). Are the silicone jackets too thick to get a consistent signal?

Next steps:
Bosch spark plug wires...

I want to drive this thing. Please help!!

[Piledriver]

What pressure were you actually getting from the fuel pump??

Uncap the retard side of the vac can. Leave vented to atmosphere..

You actually have ~12V going into the coil on the hot (+) side when running? Points connected to (-)?

Brake cleaner can work better to find vac leaks and is non flammable, it will slow/kill the engine if it gets sucked in, puts the fire out.

The resulting exhaust can put you out too, only try this in a WELL ventilated area.

[Scott Novak]

I'm assuming that you are using an inductive pickup timing light. It doesn't matter how thick the insulation is as long as you can completely close the inductive pickup core around the wire. That being said, it's possible that the core of your inductive pickup could be damaged, or something is preventing the core from completely closing around the ignition wire. Test your timing light on another car to be sure that it's working correctly. Also make sure that the inductive pickup is connected properly. There is usually an arrow on the inductive pickup that indicates the direction of the spark plugs.

I would recommend that you remove both the vacuum retard hose and the vacuum advance hose from the carb, and cap the vacuum and the retard ports on the carb, just for troubleshooting purposes. Then set the distributor at 30 degrees at maximum advance. Check it at 3,500 RPM or so.

Idle is often where intake manifold leaks will show up the worst. If the volume screw is not having much effect, as you mentioned, and throttle shaft is as tight as you say, an intake manifold leak is likely.

Scott Novak

[Marc]

have you been using the same main jet size (130) in all of the carburetors? Do they all have the same air correction/emulsion tube?
Have you checked that the intake manifold preheat passage is clear? You don't get much heatriser flow from that type of exhaust system to begin with, but if the intake manifold pipe is flowing freely and the holes at the #2 & 4 exhaust flanges are drilled out as large as possible without hitting daylight you should be getting enough manifold heat from convection to make the flat spot bearable.
A cold manifold makes the fuel condense when the throttle's opened and manifold vacuum decreases. The fuel eventually makes it to the cylinders but it's in the form of droplets that are too large to burn completely. This can make the plugs look as though they're burning rich even though the flat spot is caused by a momentary lean condition. Dual-ports are much more susceptible to this problem because the intake manifold is larger than the singleports', which reduces the airflow rate at low RPM and puts conditions inside closer to the "dew point".
What aircleaner are you running? A functional thermostatically-controlled stock unit that'll feed warmed air to the carb can help. Over the years there have been several designs; first they were weight-controlled (only drew warm air at idle/low speed), then they tried controlling them with a cable linked to the thermostat flaps. The latest models were vacuum-operated, and they're not bad - but personally I prefer this type, that has its own self-contained wax-pill thermostat and requires no external linkage or hoses:
http://www.thesamba.com/vw/classifieds/ ... ?id=537470
If you have one of the other types and it isn't functioning properly, you might try jury-rigging it so the flap is always in the "warm-air" position to see if that helps enough to justify buying another one.

Most of the time the best place to aim the accelerator pump nozzle is so that the stream barely misses the venturi wall.

An early spark has a better chance of igniting a lean mixture, which is why a vacuum-advance distributor can mask the root problem (they kick in a little extra advance off-idle, a trick that a centrifugal-advance distributor is incapable of).

[jp16v]

I'm assuming that you are using an inductive pickup timing light. It doesn't matter how thick the insulation is as long as you can completely close the inductive pickup core around the wire. That being said, it's possible that the core of your inductive pickup could be damaged, or something is preventing the core from completely closing around the ignition wire. Test your timing light on another car to be sure that it's working correctly. Also make sure that the inductive pickup is connected properly. There is usually an arrow on the inductive pickup that indicates the direction of the spark plugs.

Yep, it's an inductive timing light (just a FLAPS cheapy). I've never noticed or looked for an arrow on the inductive pickup, but I'll take a look tonight. I'll test the light on my truck (other cars have coil packs so can't test on them)

I would recommend that you remove both the vacuum retard hose and the vacuum advance hose from the carb, and cap the vacuum and the retard ports on the carb, just for troubleshooting purposes. Then set the distributor at 30 degrees at maximum advance. Check it at 3,500 RPM or so.

I'll try this timing setup tonight and report back the results.

Idle is often where intake manifold leaks will show up the worst. If the volume screw is not having much effect, as you mentioned, and throttle shaft is as tight as you say, an intake manifold leak is likely.

That's what's strange, the idle is rock solid. The acceleration feels like a vacuum leak induced lean condition. I'll verify straightness of carb flange on intake manifold, too for good measure.

[jp16v]

have you been using the same main jet size (130) in all of the carburetors? Do they all have the same air correction/emulsion tube?

I've been using the 130 in all the carbs. The "other" Solex I have has a doctored-with air correction/emulsion tube. I'm staying clear of that carb and its parts altogether (also determined that the throttle plate was installed upside down )

Have you checked that the intake manifold preheat passage is clear? You don't get much heatriser flow from that type of exhaust system to begin with, but if the intake manifold pipe is flowing freely and the holes at the #2 & 4 exhaust flanges are drilled out as large as possible without hitting daylight you should be getting enough manifold heat from convection to make the flat spot bearable.

When I built the buggy, I took time cleaning out the preheat tubes with a section of frayed clutch cable. Also, the exhaust had the flange for the preheat, but no hole drilled at all. I drilled and opened this up, though it could likely use some more. I'll look into doing this. How hot should the preheat tubes be at idle? 3rd degree burn hot?

What aircleaner are you running? A functional thermostatically-controlled stock unit that'll feed warmed air to the carb can help. Over the years there have been several designs; first they were weight-controlled (only drew warm air at idle/low speed), then they tried controlling them with a cable linked to the thermostat flaps. The latest models were vacuum-operated, and they're not bad - but personally I prefer this type, that has its own self-contained wax-pill thermostat and requires no external linkage or hoses:
http://www.thesamba.com/vw/classifieds/ ... ?id=537470
If you have one of the other types and it isn't functioning properly, you might try jury-rigging it so the flap is always in the "warm-air" position to see if that helps enough to justify buying another one.

I'm running a spun aluminum offroad air cleaner (looks like a tophat). There is no thermo controls contained therein. I don't have any thermostat flaps on the buggy (no heat). I'll shoot some pics of the motor, maybe you guys can see something that jumps out at you.

Most of the time the best place to aim the accelerator pump nozzle is so that the stream barely misses the venturi wall.

This is where I've pointed the pump nozzle... right down the side of the wall where the throttle plate cracks open.

Keep the ideas coming guys. I'm dedicated to seeing this thing through...

[Marc]

...How hot should the preheat tubes be at idle? 3rd degree burn hot?...

With a stock muffler one end will be "2nd-degree" hot and the other somewhat cooler; with your setup both ends will be about the same temperature since there's no real flow, just convection.

...I'm running a spun aluminum offroad air cleaner (looks like a tophat). There is no thermo controls contained therein...

The combination of no warm air to the carb and inadequate manifold heat is infamous for the symptoms you've described, after you've ruled out the ignition and an induction leak I think you'll need to look there. You may be able to cobble up something like a coffee can with a pipe welded to it for the warmair hose to attach to which can be placed over/around the aircleaner.
The 130 main is already the largest stock offering for a 34PICT-3 but you may need to step it up slightly. It'd be worth trying a 60 pilot jet too.

[LeeVW]

I would be very interested in seeing if the Bosch plug wires help. Popping or backfiring during acceleration sounds like a timing issue to me.

Lee

[jp16v]

I'm going to swap in a set of known good Bosch plug wires tonight from my friendly fellow buggy owner/neighbor.

I'll report back my findings... in the meantime keep the suggestions coming.

[Marc]

...Popping or backfiring during acceleration sounds like a timing issue to me...

It's certainly possible to get a backfire in the intake (I refer to this as a "sneeze" to differentiate it from afterfiring in the exhaust) because of early spark timing/cross-firing but the symptom is classic for a lean mixture, although I can't explain the physics that cause it. One theory is that the too-lean mixture is still burning late into the exhaust stroke and causes ignition in the intake during the valve overlap period, but if that's all there is to it it seems to me that sneezing would take place more often than not.

[Scott Novak]

If you say that the volume screw doesn't have much effect, but the engine idles well, it could mean that you have richened up the mixture enough at idle that the intake leak dilutes the over-rich fuel mixture to the proper air fuel ratio to idle well. It may be that using a larger main jet and idle jet may help compensate for this.

But as Mark was saying, an insufficiently warm intake manifold can create instantaneous lean conditions when you suddenly open the throttle. That along with an intake leak could cause the backfiring.

I haven't tried this before, but I was thinking that you might try smearing grease around the outside of the throttle shaft. I think that it would, for a short time, seal up any leakage through the throttle shaft bore to help eliminate that as a possible source of intake leak.

I've been running a German 34-PICT-3 with a 127.5 main jet and g55 idle jet. I'd have to look at the carb, but I think the air corrector/emulsion tube is a 75Z. The choke is connected. Fuel pressure is set at 4 lbs. I had been running with an EMPI single tip exhaust which has the preheater tube running from Cylinder #2 to Cylinder #3. I used the stock air cleaner, but it the air duct was not connected to the preheat duct to suck hot air from the cylinders.

I have a Mallory vacuum/centrifugal advance distributor with the grey/grey springs. I use a Jacobs Pro Street ignition with a Jacobs Ultra Torquer ignition coil, 8.5 mm Jacobs Energy Core magnetic suppression wire with NGK BP7HS-10 spark plugs which are a colder heat range than stock with 0.065" spark plug gaps. The engine is 1600 cc with mildly ported and polished heads that I did myself with a semi-hemi cut, 1:1.25 rocker arms, stock cam, and a 6.8:1 compression ratio.

I have not had any problems with backfiring or spark plug fouling. I don't have any hesitation problems when the engine is warmed up. But the vacuum advance and high energy ignition helps to overcome any hesitation problems that I might have with the less than ideal intake manifold heat setup. I also took extra care to open the intake manifold heater tube ports, on the exhaust system, as far as possible, and the intake manifold heater tube was cleaned out before using it.

Scott Novak

[jp16v]

Don't take my "volume screw doesn't have much effect" as gospel. It may be doing its thing, but my lack of experience with a nicely running 34Pict3 means that I don't know exactly how it's supposed to react with the screwdriver in hand. There's a world of difference between me reading that its supposed to react a certain way, and actually experiencing the correct change by a turn of the screw.

I understand the concept of instantly leaner conditions by not using warmed air from two standpoints. Cooler air is denser than warm, thus packing the cylinder with more air vs fuel per cycle. Also, warmer air is less likely to cause condensing of the gasoline in the long intake runners.

I'm going to do another look over tonight and test for vacuum leaks... again. It's funny how when your behind your car working on it, you're sure of what you see... but the next day at work, you start to question your observations...

[LeeVW]

""backfire in the intake (I refer to this as a "sneeze" to differentiate it from afterfiring in the exhaust)"

Good point. I did in fact mean "sneeze", as my theory was that a cylinder was firing (or partially firing) while its intake valve was open.

"I've been running a German 34-PICT-3 with a 127.5 main jet and g55 idle jet."

Same here on my 1600DP, and it runs great! The engine stays cool even on the freeway in the desert when it's over 100 degrees F out. The engine does get a bit rich at higher altitudes (>3,000' ASL), but I'm not going to switch jets because I drive it all the way there and back. I have also never had any popping, backfiring, or plug fouling with this setup. I also have a Mallory HyFire 6A. High energy ignition is a wonderful thing!

I tired a 130 main jet on two different occasions, and it was WAY too rich both times. The stock 127.5 seems to be the ideal main jet size for a stock or nearly stock engine.

This is a very interesting thread. I am learning a lot from all the theories. Keeping fingers crossed that this gets solved soon!

Lee

[Scott Novak]

Regarding your air cleaner. The stock oil bath air cleaner basically has it's own built in 4.5" high velocity stack that contains the fuel fog that can rise above the carburetor.

Granted, the newer OEM VW air cleaners, that use pleated filters, don't have this velocity stack, however they are still designed to contain the fuel fog.

If your air cleaner doesn't have at least 3" between the carb and the base of the air cleaner, you might want to make a 3"-5" long extension.

Scott Novak

[jp16v]

There's a good 6" section of wire-reinforced hose that the air cleaner is connected to. I had aspirations of being in deep mud, so it's sort of like a snorkel.