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Engine Miss And Lean Condition


doug73cs
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Winter finally gave up and the car is back on the road but running poorly - was fine last fall when took it off the road.  Stock engine with 15,000mi since rebuild with 38/38 webber that starts instantly and idles well at 800rpm - slight stumble.  Timing set to factory numbers. 

 

Symptoms are missing/rough running as rpm increases and air fuel ratio becoming very lean under throttle.  Plugs 1 and 4 black and sooty, plug 2 very lean and 3 light tan with sooty base.

 

Checked valve at .007 throughout, plug leads from new dist cap to correct plugs, Petronix wired correctly and everything involved is new within the last 3 years and very low mileage.  No pitting or carbon on the rotor or distrib cap.  Carb settings unchanged since last fall and set using wideband A/F meter.  Checked vacuum advance by sucking and blowing on the hose with the engine running which may be crude (and rude) but should an effect on the engine operation.  

 

With the engine running I pulled each plug lead off the plug and the engine stumbles on cylinders 1,2 and 4.  Pulling lead 3 had no effect on how the engine ran so thought I had found the problem.  Replaced plug lead on 3 but that had no effect so it seems there is no spark to cylinder 3 and it is upstream from the plug lead. 

 

So before I start replacing parts - culprits would be coil? dizzy wobble?  

 

Comments?

 

Doug 

Edited by doug73cs
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Thanks, I have checked for vacuum leaks at the carb.

 

No spark on cyl 3 points to the distributor as most likely problem.  Petronix?  Probably not coil since it sparks 1,2 and 4 just fine.  Hope it's no the distributor itself but agin works on 1,2 and 4 so....

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Thanks, I have checked for vacuum leaks at the carb.

 

No spark on cyl 3 points to the distributor as most likely problem.  Petronix?  Probably not coil since it sparks 1,2 and 4 just fine.  Hope it's no the distributor itself but agin works on 1,2 and 4 so....

I would start by pulling #3 plug and put wire back on. Hold plug to head to make ground and have someone turn the engine and look for spark. If no spark, try it with the plug from #2 on the #3 wire. If no spark, pull #2 wire from cap and put in #3 spot and check for spark. If still no spark using #2 wire and plug in place of #3 on cap, I would suspect the new cap maybe bad. 

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How do you check if the Petronix pickup is not the problem? If the Petronix works on cylinders 1, 2 and 4 can it fail on cylinder 3?

I would not think so. I had my Petronix fail last year. I worked fine when cold but after about ten miles of driving it would heat up and start cutting out. It would give an erratic spark on all cylinders. Once it cooled down, it ran fine. An easy way to see if it is firing to all cylinders, hook an induction timing light to each individual wire and look at the light. Low RPM is easier to see if the spark is consistent. 

 

But first, make sure you don't have a bad plug in #3.

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  • Solution

Through switching plug leads and plugs around I confirmed that plug  #3 is pooched, bought a new set of NKG (since they didn't have Bosch WR7DC) and did the pull the lead on each cylinder with the engine running to test.  The engine runs much better (still a little rough) and I can continue setting up the carb when my main fuel jets arrive via post.

 

Thanks all and Peterschop in particular. 

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Still have to figure out why the 1+4 are rich and 2+3 are lean.  With only one carb into a common manifold it isn't the carb/fuel system.  My worst nightmare would be the timing chain skipped a tooth left or right which might explain this issue.  Can't remember how many teeth are on the cam sprocket but one tooth could be 10 - 15 degrees which is huge.

 

But thanks again.

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Still have to figure out why the 1+4 are rich and 2+3 are lean.  With only one carb into a common manifold it isn't the carb/fuel system.  My worst nightmare would be the timing chain skipped a tooth left or right which might explain this issue.  Can't remember how many teeth are on the cam sprocket but one tooth could be 10 - 15 degrees which is huge.

 

But thanks again.

I don't know enough about this since I have have only been into 02's for about a year but maybe it is the manifold design letting more fuel going into 1 and 4 cylinders... Or then again maybe it is your valve adjustment. Now that you have new plugs, check again that all the valves are at the right settings (.007 cold). You can then check your plugs in another couple of months.

 

I doubt that you jumped time. That would not explain why 1+4 are rich and 2+3 are lean. I would concentrate on  timing and vacuum lines are not leaking or your vacuum advance - retard on the distributor are not leaking. 

 

My engine in neutral had quit a shaking vibration. I want to replace the engine mounts,  thinking that will solve the problem.

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