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Engine Misfiring, Can't Find the Problem

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I'm a very novice mechanic, and I'm trying to shake out a new-to-me 1974 2002. I want the car to run reliably before doing much else to it. 


The engine is intermittently misfiring once warm, perhaps 40 misses per minute at its worst. There are no backfires. The car starts fine when cold and idles high but smoothly at 1,000-1,300 RPM, but when warm it usually starts misfiring, but not always. It wasn't an urgent problem until, during a misfiring fit on a 100°F+ day, the car died entirely in an intersection (!) and wouldn't restart for more than twenty minutes. Hopefully this isn't too comprehensive, but after searching the forum extensively, here are the conditions that might be relevant:


- I shipped the car to my home in Utah, which has an elevation of 4,500 ft., from southern California, where the elevation was certainly much lower. I haven't messed with carburetor jets at all. Carburetor is a Weber 38/38.


- There is noticeable but not extreme soot on the floor behind the tailpipe in my garage where I test the engine. There is also soot on the underside of the bumper near the tailpipe.



- I have replaced the fuel filter, which had become rust colored, with a new one. I don't know how long it had been in place. 


- The distributor is a Pertronix unit and looks clean to me. The ignition coil is blue.


- I tested compression, and it does seem low but pretty consistent. Results after six cranks per cylinder with a warm engine and wide open throttle (if I keep cranking it will go a little higher):
Cylinder 1: 43, 68, 91, 105, 115, 123 :: 123
Cylinder 2: 42, 69, 90, 105, 115, 122 :: 122
Cylinder 3: 39, 65, 85, 100, 110, 119 :: 119
Cylinder 4: 40, 65, 85, 100, 111, 118 :: 118
The tip of the compression tester came out quite dirty.



- Spark plugs are a little wet in the threads, especially #1 and #2. (See photo)



- The underside of the oil filler cap is oily but not brown or milky. 



Any help with this engine would be greatly appreciated. Thanks!





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Your plugs look good, and your compression's ok for your altitude.


If it was me, I'd start with the ignition- first check the timing, and see how much the spark wanders around

while you rev it and let it idle down.


Then cap, rotor, etc, and look to make sure you don't have resistor plugs AND wires AND rotor- one of the three's enough, with the blue coil.

Also wiggle the distributor shaft side- to- side and up and down to see how much free play it has.  And check the magnet ring

for the pertronix- they have been known to slip, crack, ride up, etc, and do this.  USUALLY the pertronix is not the fault- but it's not foolproof,

especially on the hot fail side of things.  A weaker spark can let the engine flood, and then not be able to fire hot enough to clear it, too.


Then, just because of your rust- colored filter, I'd pull the top off the carb (don't lose the little choke clip- you'll see what I mean) and clean out the float bowl.


...for starters.








"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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I'm with Toby on his ignition trouble shooting. Two plugs look a little lean to me, far right one is perfect, though. 


What at kinda plugs are those ? They have a needle-like electrode tip?


There was a similar problem noted here last week and it was discovered that the electric choke wasn't even hooked up. It ran great when cold but, because the choke wouldn't disengage, it would choke the engine out. 


Perhaps your our choke needs some adjustment? (Even though your plugs don't look rich at all)


Here's a quick write up on Jetting adjustment for altitude. 

http://www.lcengineering.com/LCTechPages/pdf/Tuning Weber Carbs for High Altitude.pdf


...but your plugs look pretty good. 


Maybe be you have a plugged idle jet?





'69 Granada... long, long ago  

'71 Manila..such a great car

'67 Granada 2000CS...way cool

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4 hours ago, TobyB said:

Your plugs look good, and your compression's ok for your altitude.


If it was me, I'd start with the ignition- first check the timing, and see how much the spark wanders around

while you rev it and let it idle down.


Oh wow, I didn't consider the effect of thinner air on compression numbers. I feel a little better now. I'll start checking on ignition items like you recommended. Like everything else on this car, I'll be doing it for the first time. I'm eager to learn though; it's one of the reasons I bought the car. I'll search the forum to learn how to check timing etc. Thanks for your help.


3 hours ago, zinz said:

What at kinda plugs are those ? They have a needle-like electrode tip?


Yeah, they're NGK Platinum plugs, BPR6EGP. They're the ones AutoZone handed me to replace my old Bosch plugs when I was first trying to fix the engine problem. I was such a n00b that I didn't even know how to remove the female plug ends to reveal the threaded male end underneath. It's been a process. :)


2 hours ago, zinz said:

Get some NGK BP6ES plugs gapped to about 0.034 and don't change anything else..see what happens. 


I'll do that as well. Thanks for all the help, Zinz. 

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I order mine from Amazon, make sure it is the non-resistor plug. BPR6EGP The "R" means resistor


I would start by checking the fuel pickup screen filter in your tank. If the fuel filter was rust colored it definitely means there may be some rust in the tank, and after a bit of driving, it can concentrate on the pickup screen and starve the engine. When it sits for 20 minutes, the rust falls to the bottom of the tank and the cycle starts over again.


The correct plugs may help to run a bit hotter and burn off any oil that is leaking past the valve guides, leading to less misfires. I also found solace in non-resistor performance plug wires, but I have a 2002Tii so you may not have to go this route.

1973 2002Tii Agave "Gerta"-----1972 2002Tii Verona project-----------2003 Porsche 911 X51-------2016 FIAT Abarth--------2003 Porsche Boxster----------2005 Honda Element

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So I've heeded everyone's advice and started learning all the infuriating minutiae that encompasses ignition. And for me, "learning" = screwing up every possible thing as much as possible and learning to fix it. It's my own little course of Ignition 101, which is probably like everyone else's, but with more fuming, swearing, and staring into space while I contemplate what generally the hell is going on this time.


Toby and Ed listed off some basic things to start looking at. I've decided to do this by throwing parts at the problem, which will likely have two benefits. One, I'll have to "learn" every step along the way (see above), and two, I'll probably stumble into a solution at some point. I therefore bought a new coil, new Pertronix, new distributor cap, new distributor rotor, new spark plugs, and new ignition wires.


The parts didn't show up all at once — they trickled in via the UPS man — so I did things in the order they came to my door. First up, the coil. Replacement was easy and amazingly the car started fine. But it didn't fix the problem. Next up was the plugs. I ordered the wrong ones, NGK BP6ES, because my plug wires need the tiny little threaded ends, not the big silver ones and the tips don't unscrew on those plugs. I'll get the right ones later. Moving on.


It was the distributor cap where my work finally went to hell. Oh, that silly little cap. I removed it once to check the innards, then put it back, and suddenly the car wouldn't fire. I checked the firing order, nothing, and then I removed it again and found that I'd broken the little spring-loaded conductor inside. I was screwed. But suddenly a came a knock on the door and in rode UPS-Claus, who brought me a brand new cap, which I promptly installed. A twist of the ignition, and still nothing. [swearing] After a long while I realized that while loosening the distributor for a timing adjustment, I'd pulled the distributor gear out of its seat and the gear had seated again, but in a random direction.


Now, for a lot of you all this is sososo basic, but for me it's all new so it's hard. Anyway, I learned to remove the valve cover to find TDC, found the TDC mark on the timing chain gear and centered it, then seated the distributor properly. I placed the #1 plug wire at 4:00 on the distributor, which it never was, and found that the distributor had been rotated all wonky all along so I corrected that too. Double check, double check, double check. Turned the ignition, VROOOOOOOM!


Now for timing. Plugged in my advanced timing light, connected the lead to plug #1, and it wouldn't flash. Just a flicker now and again, but no flashing. Hmm. Connected the lead to plug #2, flashing. Same for #3 and #4. But not #1. With the engine running I went around to check the plug wire seating on plug #1 and zap! I got shocked by the plug wire. I tried again to wiggle the connection, and the wire itself pulled clean out of the connector housing and ended up between my fingers, zap zap zap zap zap zap zap! Painful, yes. Anyway, that wire is old and now it's broken completely, but thankfully I have plug wires coming tomorrow. When I get them installed, I'll report back. 

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That was a really good read.


It would be a good thing to test the leads for resistance and compare them to the new leads, both attached and detached to/from the distributor cap. I believe that there are specs for this in the blue book. What are the specs required for petronix?

02tii 2751928 (2582)

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If the plug leads clip onto the threaded post on the plug then they are likely to be quite old. That is not necessarily a problem as long as they pass the resistance test but given the way that no. 1 fell apart I think you are definitely due a new set on your path to misfire freedom. 

rtheriaque wrote:

Carbs: They're necessary and barely controlled fuel leaks that sometimes match the air passing through them.

My build blog:http://www.bmw2002faq.com/blog/163-simeons-blog/

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With the new plug wires installed, I'm no longer getting shocked! :P Another upside is that the car runs rather well now. In fact, after installing the new parts I haven't had a single misfire. Now, because the problem was intermittent, I can't definitely say that I've slain this dragon yet, but it looks very, very promising. The problem seems to have stopped after replacing the Pertronix module with a new one, which is what I expected because I was having the same symptoms as some other hot fail cases. After a few days I'll report back.


The other other problem is timing. The car will only run well without pinging/knocking at around 20° of advance; I can't get anywhere near 30° without a lot of noise in third gear. But as Toby mentioned, "look to make sure you don't have resistor plugs AND wires AND rotor", I do in fact have all three. My guess is that I'm getting such a weak spark that I can't burn fuel fast enough so pinging is occurring toward the edges of the chamber, away from the insufficient main explosion. I'm planning to retain the Bosch resistor rotor, but change out my wires and plugs to non-resistor versions to help solve the problem. 

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cool, that sounds like progress!  My misfire, a while ago, was caused by crappy new wires.  I suspected the Pertronix as well and took it out.  It is still in the box.  Anyway, Kingsborne makes nice wire sets.  If you like the look of the plastic tubular loom, they will build them in one of those for you.  I prefer the little clips on the valve cover, myself.  The rubber lined 'conduit' clips from the hardware store fit well and are padded to protect the wires.  A one inch and three quarter inch fit well with my 8mm wires.


How are you setting the timing?

BB in the hole at 1400 rpm?


thanks for the update.  I am eager for the next one. :)


EDIT:  duh, you already swapped the wires, nevermind.

Edited by '76Mintgrun'02



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Sounds like your self-training is going very well. Your posts are clear and fun to read, too.

Did you happen to change the coil wire, too? It should have been in the wire set, but you didn't mention it. 

Finish off that nice tune up with another positive learning experience - a valve adjustment. It's easy, after you've done the first couple of valves for the first time. I believe there's a how-to article in the technical portion of the faq web site. After that, all your remaining problems should be carb related.

I had an experience with an electric 38/38 similar to what zinz mentioned. The electric choke was hooked up in this case, but the butterfly wouldn't open. Since I didn't understand how the system worked (pre-internet, it was harder to look up solutions back then), I changed it to a manual choke. The soot went away, the car ran much better and mpg improved significantly after that, but I recommend you get the automatic choke working right instead of doing what I did. Forgetting to shut off the choke can be bad for your engine.


no bimmer, for now

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