Stopping the popping

[manoa matt]

Trying to diagnose a problem with a friend’s 1978 spider. She bought the car new from the dealer in 78 and has a 2 inch thick folder full of maintenance records. She wanted me to do some routine maintenance and replace a few parts. Throughout its life it’s had some strange mods by previous mechanics.

It had an electronic ignition installed, but with no vacuum advance. The distributor shaft was also pretty stuck and would only advance a few deg. It also would not snap back via the springs. The cooling fan was wired on constantly with the engine running. External thermostat was removed with a flex hose from the lower rad to the pump. The car was actually running without a thermostat. The heater core was removed, but the heater tube and the port on the head were just plugged, and not connected. Several of the emissions ports on the carb were hooked up wrong/crossed. Despite all this the car ran OK without any issues.

I replaced: the plugs, cap, rotor, pickup, control module, vac advance, and removed, cleaned and lubed the dizzy shaft. Now it had more movement/advance and would snap back via the springs. I hooked up the fan to the rad switch, installed a new external thermostat, new water pump, and new hoses. I connected the heater tube to the head port and installed a “T” fitting. I replaced the mechanical fuel pump and replaced almost all of the rubber hoses and made sure they were properly routed to the respective carb/emissions ports. I also replaced the timing belt. All timing marks lined up prior to, and after new belt installation. I turned it over twice by hand afterwards to make sure.

The car ran pretty good after all this. The odd thing was the water temp gauge would be almost in the red before the fan would come on. I verified the head temp and the temp at the fan switch via a non contact laser thermometer. Both readings were in line with standard operating temps of around 190 deg. I figured the gauge must be wrong and tried two other gauges with the same high readings. I then removed cleaned and replaced the temperature gauge resistor that’s up under the dash and the temperature gauge started to read more accurately. By that logic, with the fan constantly on and running without a thermostat the car was showing a “normal 190 deg.” reading but running MUCH cooler.

Upon rapid decelerations and going down hills the car would pop in the exhaust. I had an extra fast idle diaphragm, told her that would fix it, and I could install it the next weekend. Jump to the next weekend. In the process of replacing the fast idle diaphragm I ended up cleaning and rebuilding the entire carb. Checked all the adjustments in the 32 ADFA work book and everything was still within specs. The carb was missing the actuating rod for the float bowl valve, so the float bowl cam was just zip tied to the adjacent fast idle cam to prevent it from swinging down and jamming the accelerator pump cam in the open position. I did not have time to remove my carb to make an extra actuating rod so I just left the zip tie, but unhooked the float bowl cam return spring so that it would not have a negative effect on the fast idle cam.

I tested the fast idle system via the button. The RPM’s would only pick up to around 1200, but the spec is for 1600rpms. If regular idle is 800 and it picks up 400 to get to 1200, then it still has another 400 to go to get to 1600. Basically the fast idle system is only pulling half of what it should. I also checked the clutch pedal switch and the transmission switches. It only has the large switch on the side of the trans and not the small switch on the top in front of the shift housing. I figured the fast idle system would not activate on its own without the second trans switch.

I removed the test button and ran some wires into the cabin so I could use the test button to activate the fast idle diaphragm while driving. While going down a hill and the popping is occurring I pressed the button. The idle picked up, but did not stop the popping, and by the results, I figure that even with the fast idle system fully operational it still will not be enough to overcome the popping in the exhaust.

The popping is worse now than before the carb was rebuilt. Also the car did not pop at all before she brought me the car. Of course she is in her 80’s and drives very slow, so her driving style is less likely to produce the popping.

The first thing I might try would be to install my carb from my 78 which runs perfect and does not pop on decal. I’m going to make a float bowl actuator rod and install that this weekend. The only other things I think it could be are incorrect idle speed and mix adjustment, float level set too high, or adjustment of the fast idle system. However all those were adjusted according to specs.

Sorry for the long description, but I felt the details were relevant. Any and all suggestions will be welcomed.

Thanks,

Matt

[manoa matt]

She has the stock 4-1 exhaust manifold, straight pipe, resonator, and rear muffler so there should be enough back pressure. Also, no emissions regulations here, so solutions that include capping/eliminating will also work. How do you guys with DFEV carbs deal with the popping if there is no high idle system?

[mbouse]

i was going to ask about the exhaust, and you beat me to it. you are correct, that with the resonator, a common muffler and the 4-1 you should have plenty-0 back pressure.

i drive my '80 like it was stolen...all the time. i've had the DFEV on an 1800 intake with a 4-2-1, cat eliminator, resonator and ANSA for nearly three years.... not one single pop. it is my #1 means of transportation 6 months a year.... so i see all kinds of driving conditions, hills, twisties, long straight stretches at highway speed, and loads of city driving. not one single pop, ever.

last year i swapped out the two litre head for a reworked 1800 head; neither head produced your symptoms.

my emissions equipment was shipped to Allison's Automotive, so that equipment is not in the picture either.

i've not experienced your issue with this carburetor.

[racydave]

If it has a AIR pump it could be related to the control valve or vaccum to it. Also, if the distributor is like the stock 79 was, any attemt to remove the vac advance and hold the plate still will most likely have ruined the bushings for the plate.This will result in a very eratic pick-up signal and funky timing. Id try a good dizzy. Popping on de-cell sounds like advanced timing.

[manoa matt]

Emissions stuff was removed many years ago. No air pump, no EGR, The air injection ports in the head don't have plugs. They were welded shut!

The only emissions stuff still hooked up is the fast idle diaphram and electrovalve. The charcoal cansister connected to vacuum on carb throttle plate with gas tank and float bowl vents (float bowl vent valve inopperable at the moment due to missing actuator rod) Stock 78 air cleaner with crankcase vapor and vacuum lines.

The electrical connections to the EGR electrovalve are connected to the valve to retain continuity in the wiring circuit, although I don't think this is necessary.

Air fuel mix screw is exaclty 2 full turns out.

[So Cal Mark]

popping in the exhaust is normally caused by unburnt fuel in the exhaust from a rich mixture. If the air pump is still intact, the diverter valve may be bad or misconnected. Erratic timing should be more of an issue on acceleration.

The fast idle system was to prevent the throttle snapping shut on decel, causing a very rich mixture. This was primarily an emissions feature as the feds were learning about high emissions under certain operating conditions

[majicwrench]

Mark is right, fuel is burning in your exhaust, hence the "pop". But in order for that unburnt fuel to ignite, (pop) it needs to have a supply of fresh air. Somewhere, air is being sucked into the exhaust system. Even a tiny leak that you can barely hear, on decell, will suck enough air to cause problems. There is no way to keep some fuel out of exhaust on decell ( some jap cars shut off the idle mixture soleniod on decell, which worked slick) so you gotta keep the air out,or "pop"!
It's just me, but I dislike seeing the emisions stuff eliminated, it is all there for a purpose, the car was designed to work well with it in place, and the car didn't backfire when new, but if stuff already gone, not much you can do about it.
The "idle-up" system work as Mark said, by keeping the idle high it burns the mixture completely, thus minimizing the fuel in exhaust on decell.
Can you screw in idle mixture screw a bit, till idle just falters?? Minimizing the amount of fuel at idle should, in theory, help a bit. And take a darn good look at the exhaust, and the plugged air injection. Gotta be a leak in system somewhere.
Keith