Backfires through carb, won't idle smooth...

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E9Paradise

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I recently bought a '74 3.0CS. It won't idle smoothly and it backfires through the rear carb. The carbs are stock Zeniths. The timing has been set and seems to be steady. It has new plugs, wires, cap rotor. Points are in good shape.

Observing a vacuum gauge on the rear manifold shows a very erratic and inadequate vacuum. I haven't compared to the front manifold yet. There is a lot of carbon around the rear carb intake and evidence of heat. It can run smoothly while driving but will occasionally backfire through the carb then as well.

I'm wondering what might be the problem. I suspect a vacuum leak on the rear manifold... Anybody have any other thoughts?

Thanks! John
 
Update

Going on the vacuum leak clue, I found that the manifold to head bolts are loose. This could be a problem... I will pull the intake manifolds off and put new gaskets.

That will probably fix it.
 
The vacuum leak needs to be dealt with but generally backfiring through the carbs is sign of ignition timing that is way off (advanced). It's easy to make this mistake when lining up the silver balls / marks on the fly; there are usually two and on is TDC and the other is the timing mark. You also need to have the engine speed set correctly and usually have the vacuum lines off the distributor and plugged. What is the timing procedure you are using?
 
JJ, I was setting the timing by lining up the ball in the flywheel window at 1700 rpm. The thing I didn't do was disconnect the vacuum line. I'll do that once I get the vacuum leaks fixed.

Thanks for the advice! John
 
Another unpleasant possibility is that the cam and/or crank timing gear have slipped a tooth (didn't explain that too well but you know what I mean).
 
Philip, Yikes, I hope it didn't slip a cog... That would be the poptop on a can of worms. How can I pull the head and fix the timing without pulling the engine, rebuilding it, repairing all the rust, repainting, reupholstering, renewing the suspension, etc... etc...

Good thought though. Thanks.

JJ, I'll double check to make sure the timing mark is correct. Thanks!
 
Don't dispair!

When I got my coupe the chain was off by one cog and it was backfiring through the carbs. To top it off I had an intermittently shorting condensor (capacitor) on the distributor that would cause it to randomly stall for 2 seconds to 2 days. Anyway, I can assure you that while, as Malc might say, reseting the chain one link is "fiddly but doable" without pulling the head. The valve cover and upper timing chain cover (with distributor) must come off and you'll probably want to pop the tensioner to get some slack. Heck, I didn't even buy gaskets when I did this job - I cut them by hand and used some Pematex RTV in the corner areas. It is simple and worthwhile to check the cam timing. :?
 
It is not completely obvious but Corsachili helped me get it right with this advice:
I've rebuilt three M30 engines and the bolt in the timing gear should line up with the milled/raised section on the head.
Click to expand...
The milled/raised area (notch) is on the front top of the forward cam bearing - it's just a bit of flat smooth aluminum near the cam gear. The #1 cam lobes should be up or down. One of the bolts on the cam gear should line up closely with the notch. I actually had to stare at mine for hours and drink at least 5 beers to figure it out. Fixing it involved cursing and considerably more beer. However, it was easy to see that it was wrong; figuring out what it should look like was harder. Bottom line, if your cam is off by one tooth it will be obvious; nothing will line up with anything at TDC. If this turns out to be the case post again and I'll see what I can do to dig up or take a photo of proper alignment.

As a final note, even with my cam off a tooth I was able to get my car to about 120mph on 101N well north of San Fran. :roll:
 
Rough idle and backfiring could also be incorrect valve adjustment, although they typically don't change without someone messing with them. This would be consistant with your erratic vacuum reading. A vacuum leak can also show signs of rpm hunting while cruising at a constant speed (not accelerating, not deccelerating).
 
Thanks for the advice JJ and Dang. I hope to dig into it this weekend. The first thing I will do is fix the manifold vacuum leaks and see if that fixes it. If not I will be pulling the cover and looking for the things you guys suggested.

News at 11:00
 
Backfire

I had the exact same condition on a 533i and it turned out to be a head gasket had failed between two cylinders. This would be your worst case scenario. Try all the timing adjustments and carb adjustments first. A severly lean mixture will also cause a backfire condition. It also might not be the carb causing it, a failing fuel pump can produce severe backfires through the intake and even fire in the air filter. Very rare but possible.

Best of luck with your troubleshooting,

Flywulf
 
FlyWulf,

Thanks for the additional ideas.. I was talking with a friend and he was also suggesting a very lean mixture, possibly due to restricted jets. The car was not driven much when I got it.

I wonder if a compression check would reveal a bad head gasket?? Also, the fuel pump looks like the original, I'll check it out.

Thanks again!
 
Unless a head gasket has a serious failure a compression test may not reveal a problem; however, it will get worse. Often when they fail between cylinders it causes performance problems (misfiring under load) but the compression will be nothing more than suspect. Even so, a compression test tool is pretty cheap and it worth having in your tool box. I also recommend running the test several times (average results) on each cylinder and recording all the results in your permanent records for future reference - running a test each time you change plugs isn't unreasonable to spot trends. Most tools aren't well calibrated but you are normally looking for relative differences anyway. I seem to recall 10% consistency is not too bad but I am sure there are some more expert opinions out there.

Thinking further of the general topic there are some other simple things that can cause backfiring through the carbs. If you have old spark plug cables they can cross couple and fire adjacent cylinders at the same time (when intake valve is slightly open). Similarly, I've seen more than a few distributor caps with carbon arc filaments (hair line - use a magnifying glass) between electrodes. This will do exactly the same thing.
 
Update on bad running issue

Not having time to get the coupe running correctly, I took it to the local guru shop to have them diag and fix. They said they could probably rebuild the carbs, do a tune up, etc to get it running nice but wouldn't I rather swap in a FI system? They happened to have a Bavaria CSI that some customer got tired of paying for and it is kind of ratty anyway so they offered me the FI system for $800.

The end of the story is they are going to install it, install some new headers I had and adjust the valves for about $1700. I couldn't pass it up. I will hang on to all the old stuff to go with the car if I ever sell it.

Should be done in about 3 weeks. Can't wait.
 
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