Rough idle, very loud backfires, and now won't start

amg5872

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Hi, all. I'm having issues with my 1973 Bavaria. A few weeks ago it started running intermittently rough at low idle (sub-2500) and occasionally backfiring when I let off the pedal. Whenever I came to a stoplight or stopsign and let off the gas, the tachometer would dive to zero, bounce around a little bit, and the exhaust would sound pretty choppy. But then about half the time it would drive absolutely perfectly for my entire drive. A couple of days ago the car was again running rough at low idle as I drove home from work. As I arrived at my house, I let off the gas and there was a very loud backfire. When I went out to start the car the next day it would not start. It just cranks but no sign that it is even close to starting. It's as if it had no gas. Any comments/suggestions? Thanks.

AG
 

jefftepper

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This is a general suggestion regarding your narrative. The three elements needed for an internal combustion engine to run are "Air, Fuel, and Spark".

I suggest starting the diagnosis by looking for a significant vacuum leak. My guess is that it is either a hose or an emissions component such as the Gulp valve that has given up the ghost. As for the possible impact of a backfire, old Holley carbs had a power valve that would blow out when a backfire occurred. Once blown out, the power valve would need to be replaced for the car to run well. More recent Holley carbs received a anti-blowout modification which protects the power valve. Note sure if there are similar sensitive components inside stock Bav carbs that might be damaged by a backfire.

Beyond that, ignition timing/distributor function and valve timing would be areas to investigate.

Good luck.
 

bavbob

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Assume you have original Zenith Carbs:

1) As above, make sure you are getting a spark and do a vacuum leak check.
2) Pull a plug and seen if you were running rich or lean.
3) If you have a mechanical fuel pump, pull the line off the exit and ensure you have fuel. You have a cup to catch gas in hand, someone else cranks the car. One would think these are all or none but mine died a slow death, crappy running and then nothing.
4) I have original Zeniths, backfire won't hurt them.
5) Make sure throttle plates are in appropriate position on starting.
6) I have never seen a post on the thermal start valve having an issue...that rectangular box on the driver side of the carb. This is equivalent to a cold start valve and fixed closed, car is hard to start, if fixed open, car will start but run too rich and run like crap (based on my e24 experience).

Keep us informed so we all can learn.
 

amg5872

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Thanks for the responses. I will try this over the weekend. Sorry, I should've mentioned that I have Webers.
 

Mike Goble

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The very first think I would do is assure that the ignition system is in good working order. It's quite common for a 'carburetor' problem to be in the ignition.
 

Stevehose

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Check all vacuum hoses/caps and manifold gaskets for leaks and report back. Espescially on the manifold side of the carburetor because it seems your problem occurs when throttles are closed.
 

bengal taiga

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Hi, all. I'm having issues with my 1973 Bavaria. A few weeks ago it started running intermittently rough at low idle (sub-2500) and occasionally backfiring when I let off the pedal. Whenever I came to a stoplight or stopsign and let off the gas, the tachometer would dive to zero, bounce around a little bit, and the exhaust would sound pretty choppy. But then about half the time it would drive absolutely perfectly for my entire drive. A couple of days ago the car was again running rough at low idle as I drove home from work. As I arrived at my house, I let off the gas and there was a very loud backfire. When I went out to start the car the next day it would not start. It just cranks but no sign that it is even close to starting. It's as if it had no gas. Any comments/suggestions? Thanks.

AG


As a supplement to the other good suggestions . . .

It might prove helpful if you described the engine’s overall condition, including maintenance and any upgrades. Without more specifics, long distance diagnosis is shooting in the proverbial dark. You could potentially have a compromised head gasket or skipped timing gear teeth - or something far easier to resolve, like a loose coil wire. If your engine will crank but not start, you need to tell us more, including whether you have decent compression and whether your spark plugs are receiving adequate firing voltage (blue spark) and whether fuel is being suppled to both the carbs. That includes clean, uncontaminated, fuel. From your symptoms I also have to question whether you might have a marginal fuel pump that barely supplies enough fuel to those thirsty carburetors? Ruling out something major, without addition information, I would consider performing a thorough tuneup with the hope that this will correct something obvious.

Assuming your engine will run, when you describe your problem as "intermittent," also resulting in the tach dropping to zero coupled with backfiring, my first inclination is to look for a poor ignition ground. Ignition cut out (intermittent rough running) has been known to occur after hard braking and sharp turns, especially with soft/worn motor mounts. Check for loose distributor/coil wiring or wiring shorting out against the bare metal bulkhead.

The secondary ignition leads “may” still hold a distributor in place despite a loose hold down assembly. Make sure things are secure and not susceptible to occasional “unwanted” movement.

Still using the original points-triggered ignition system? If so, have you examined all under-the-cap wiring, including the condenser? Maladjusted points and/or a slop in its movable mounting plate can lead to unwanted swings in ignition timing - and backfiring. Ditto, a nonfunctioning vacuum advance system.

Weber carburetors? Are they are equipped with idle shut-off solenoids that function properly? If one or both carbs have solenoid “issues,” including possible dirty idle jets, this can lead to poor idle and part throttle operation. Can also affect getting the engine to start. It is doubtful this would result in major backfiring, without additional contribution from something else that would affect either ignition timing or valve timing.

The search function can be helpful regarding common questions. Same with trouble shooting checklists found in most shop manuals ;-).

HTH
 
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amg5872

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Update: The car started up fine on Wednesday evening and this morning and ran flawlessly for the few minutes I drove it both times. I am still going to try some tinkering this weekend to see if I can track down the cause of the periodic roughness. I appreciate all of the ideas. To provide a little more detail, I bought the car a few months ago and am told that the only modification is the "upgrade" to Weber carbs. The first time the rough idle issue emerged (several weeks after I bought the car and after several hundred miles of driving) I took it to a local BMW specialist (www.bimmersrus.com). They tuned the carbs and replaced the plugs and wires. Since then, the car has run better than before when it is running well, but that work seems to have done nothing to improve the periodic roughness situation.

Someone below mentioned a potential connection to sharp turns and hard braking. You may be on to something there, as I noticed that the last time this issue started the car was running perfectly until I swerved to dodge a pothole.

Thanks again, I will keep the group posted on my progress.

AG
 

bavbob

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Toss a bottle of Techron into the tank. Also consider some Seafoam (I have had decent results with other cars, not tried on my Bavaria. Some think it helps others think it's worthless). There is 40 years of crap in that engine and those gas liines.
 

chope97

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Check Carb float

If the float in one of the carbs sticks it will allow extra fuel into the bowl which will leak out into the manifold and cause the car to run rich sporadically as the fuel drops into the intake manifold
 

bengal taiga

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If the float in one of the carbs sticks it will allow extra fuel into the bowl which will leak out into the manifold and cause the car to run rich sporadically as the fuel drops into the intake manifold


If the floats are adjusted too low, there may be inadequate fuel in the carburettor bowls to satisfy the engine's needs through sharp cornering or hard braking. At the other end of the spectrum, sticky choke plates or plates that are not fully open when the engine is warm could produce a no-start or rough idle situation. . . .
 

amg5872

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From bad to worse

Looking for some more help. The car was running great for a couple of weeks and then started running like crap again a couple of days ago. Typical scenario was i would start the car in thr morning and it would run rough at idles below about 2000 for a few minutes. After a few minutes it. Would idle nicely at 800 or so. Then i get on highway and car runs great on highway. But when i get off highway after a few mikes the odometer goes straight to zero when I let off the gas. Car wants to die at every stop. Loud backfires occasionally. Some smoke after the backfire. And tonight when the car died at a stoplight there was kind of a long slow steamy sound from the engine, kind of like a diesel choking but less coughing and more wheezing. About three seconds long. Then the car would start back up after some cranking and nursed it home with a lot of clutch and parking brake. I still need to do the diagnosing recommendee by the group below but I am wonderinf if these additional symptoms sound familiar for anyone. Do I even want to know? I fear the worst...
 

Ed G

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Is your ignition working properly? Do the simple stuff first.

Good advice. Reminds me of working for a rental car company in Boston years ago. We blew up a fair share of mufflers as yutes, shuttling cars through the Sumner tunnel by flipping the ignition key off/on while stomping on the gas pedal.

Yeah....good times.
 

dang

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Crap its the damn head gasket isnt it.

This would be way down my list of potential problems. I would start with ignition related things and then work toward fuel supply issues. The tach going to zero seems to be your biggest clue, pointing toward power to the ignition system getting cut off somewhere. Ignition switches are known to go bad and cause these kinds of things, points, coils, wires, grounds, etc.

Have you tried taking the cover off the steering column and moving the ignition switch wires while the engine is running? Swap the coil, check wire connections...

Dan
 

amg5872

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Thanks for the additional suggestions. I haven't yet had a chance to dig into the troubleshooting. I'm a newbie so for each suggestion I need to figure out exactly what is being suggested and how to do it. But I did buy the Haynes and Chiltons manuals so that's a start. I've been driving the car every few days for short drives to try to figure out with precision when the symptoms are kicking in. The car will drive and idle fine for the first 5-10 minutes. Then, once the temperature gauge hits the first mark (around the 1/4 position of the gauge), the tachometer will dive to zero whenever I let off the gas and will start to gurgle and occasionally backfire. If I pull over and let things cool down it will then run and idle okay for a while. So it seems to be temperature related though I suppose it could be related to something else that changes with drive time. I will update with any insights. Thanks again.

AG
 

bavbob

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Think you need to describe tach going to zero a little better. Electrical = instant nose dive, say 2500 rpm to zero.
 

amg5872

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The issue with the tachometer diving to zero happens only when I let off the gas, for example, if I am slowing down for a stop sign. If I don't nurse the gas, the rpms will want to dive straight down to zero. I'm starting to become a very good two-foot driver. It generally doesn't happen at all if I am in gear (although a few times while driving the tachometer has just blipped down and I've felt like I lost power for just a fraction of second). But generally the main problem is that if the car is in neutral or if the car is in gear and the clutch is pressed the gas needs to be nursed or else the tachometer will go straight down to zero and the car will gurgle and spit and try to die. I currently have a second shop taking a shot at diagnosing...
 
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