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racer86

Brand New EVERYTHING - Misfire on 3&4 - HELP

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(edited)
Hi All,
 
Long time lurker and massive beneficiary of the wisdom of this forum so a big thank you for that...I hope to be able to give some back!  I've had my 2002 about 9 months now and have been working my through it over winter, taking it from a standard '73 2002 carb to a bit of a period correct(ish) cafe racer type feel.  Basically making it a bit more simple and fun on the road. 
I've done masses of work on it over winter: everything from full powerflex heritage rebush, AGK shifter and mounts, coilover conversion , a load of service stuff like thermostat, belt, earth leads, Cobra RSR seat, momo wheel...and more, and it's actually "finished" for now...apart from a small matter of a running issue....
 
Engine wise I've fitted a Fritz bits 4 branch manifold and super sprint exhaust and a new Weber 32/36 DGV with inlet manifold etc. 
The Issue I have is at idle/ low revs (up to 3000rpm part throttle) 3&4 misfire....it's ok and strong over 3000rpm but below you can pull the leads and it makes no difference...but I am getting strong spark out of them (all 4)
 
So far I've:
 
Replaced the entire ignition system (all top end Bosch/Been/NGK) :
Plugs (2nd new set BP6ES gapped to 0.025")
New leads (silicone posh jobs) even tried the old ones which made no difference.
Condenser
Points (gapped to 0.016")
Coil
Dizzy cap
Rotor arm
Checked timing (which is as I bought it and unchanged) and firing order correct. 
 
On the carb side:
Checked full throttle/ choke/ linkages all working and ok
Set float level to 34mm and confirmed bowl is full
New fuel
New fuel filter
Fuel pump is good and plenty of fuel getting through.
 
Jetting as follows:
 
Primary
Air corr 765
Emulsion tube F50
Idle 55 also tried: 60 65
Main 140 also tried 150
 
Secondary
Air corr 760
Emulsion tube F6
Idle 50 also tried: 55 60
Main 135 also tried 170
 
Compression test (cold, no oil in bores, WOT)
1 135
2 125
3 145
4 140
 
Valve clearances checked and set to 0.007"
 
It's seems to get better with more fuel but even jetted rich can taken the idle screw out over 6 turns! I've checked for vac leaks even disconnecting servo and plugging hole with no change. All gaskets new throughout.
 
I'm so at a loss I've removed intake and exhaust to check for physical blockage: there was none! Even ran it with no cam lid for a few seconds to check valve gear all functioning and not a broken cap or cam....there isn't.
 
I have 4 good sparks, fuel, compression, timing everything functioning.....I just cannot solve why 3&4 miss.....
 
I've attached some pics (you can see the primaries on 3&4 are not changing colour as much as 1&2) and a video of the miss. 
 
I should mention the engine had had a reputable rebuild and covered about 600 miles since.
 
Really got me beaten this one and the car is so close!  Thanks in advance for any help you can give me. 
 

IMG_0145.JPG

 

IMG_20200223_145855.jpg

 

IMG_20200224_200621.jpg

 

IMG_20200225_212257.jpg

 

IMG_20200315_210530 (2).jpg

 

Edited by racer86

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(edited)

Are you positive that you have the spark plug wires in the correct firing order?

 

Also... don’t let those plug wires rest against the fender. 
 

You might try swapping the plug wires and see if they are the culprit?

 

#2 compression number is not great against the others. 

Edited by zinz
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Brake booster hose vacuum leak.

 

You're welcome.

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3 hours ago, racer86 said:

Checked timing (which is as I bought it and unchanged) and firing order correct. 

 

Can you be more specific about your timing situation?

 

(a variable light will let you be much more specific)

 

Normally I would not think timing would be apt to affect the way 1&2 are running, compared to 3&4, but the difference in compression numbers between those pairs of cylinders may be affecting it.  Meaning, compression affects the amount of advance required.  I am not sure whether it is this 'fussy' though.

 

3 hours ago, racer86 said:
Compression test (cold, no oil in bores, WOT)
1 135
2 125
3 145
4 140

 

I see that there are two different coils in your photos, a black one and an aluminum one with blue stickers.  Which one are you using now and do you have a ballast resistor?

 

If  you can spring for an AFR gauge and a variable advance timing light, you will have some very specific information to draw upon when jetting your carb and setting your timing.


Tom

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4 minutes ago, ray_ said:

Brake booster hose vacuum leak.

 

You're welcome.

 

wouldn't that only affect number four?

 

 

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2 minutes ago, '76mintgrün'02 said:

 

wouldn't that only affect number four?

 

 

 

 

Let's see ?

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2 minutes ago, ray_ said:

Let's see ?

 

Hey Ray!

 

I don't see a check valve on that vacuum hose...

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A few thoughts...

  • Your "idle" jet circuit actually provides fuel up to about 2500 or so rpm, so if it's carb related, then the prime suspect would be the idle jets.  However, if they were plugged, you'd have the problem with all four cylinders.  
  • Have you checked your accelerator pump output?  Check with engine off:  work the accelerator linkage and look down the carb throat--you should see a healthy squirt from the pump nozzle.
  • If you're using a high energy ignition system, you might be getting some cross-firing with those spark plug wires all bundled up together tightly.  Kind of a black swan, but I've seen it happen, even with new wires.  
  • Look at #3 and #4 plugs...after running the engine:  are they squeaky clean, sooted, wet or ???  From your pictures, it appears that the exhaust manifold on cylinders 1 &2 show some heat discoloration, while 3 & 4 are not...It would almost make you think that those cylinders aren't firing at all, or not very much.  All four manifold pipes should be the same color...
  • spark plugs can be bad out of the box--although I've never had a problem with NGKs
  • +1 on the (apparently) missing check valve on the power brake booster hose.  Normally it's between the long hose and the manifold, connected to the manifold with a short length of hose.
  • From the video you posted, it sure sounds like the plugs simply aren't firing, and that diagnosis is strengthened by the lack of discoloring on the #3 & 4 exhaust manifold tubes.

Let us know whatcha find--we all like intriguing (and different) mysteries.

 

mike

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Hi All, thanks for the helpful and quick responses...typing all this through and sharing is a big help, never mind all the useful feedback! 

 

Sorry, I should have said before, the car ran and drove really well with the original Solex PDSI and standard exhaust for the few hundred miles I drove it before tearing it to bits.  I will say the carb was going off though, but it didn't bother me as I knew it was coming off. 

 

Ray: I wanted this to be the answer.....I blanked off the vacuum port on the inlet manifold completely (with a grommet/ some tape and my hand to make sure I had a good seal) and no difference.  I have now replaced all the vacuum hoses anyway. 

 

'76mintgrün'02: There is NO check valve....just a 3-way connector to the 2 boosters and then up to the manifold. Is this missing?  It did not have one fitted before? 

 

Timing I have not checked however I have not altered it since it was set up when I bought it and it was running/ driving well.  There are marks on the distributor which are still lined up.  I have tried rotating the dizzy to advance/ retard and no difference.  That said I don't know where the timing is at...I just know it ran well! 

 

Firing Order: Yes, I'm sure...i checked and checked and checked until I was going mad...I timed the engine up to TDC and matched the rotor arm to 1 (at about 10pm, bulkhead side as you stand at the front of the car), then 3-4-2 

 

Coil: Yes, the black one is the original Bosch one. I replaced it with another Bosch coil (the aluminum/ blue one shown) part 0221119027 which was listed for the 2002. There is NO ballasted resistor and wasn't previously. 

 

Valves:  I thought they sounded loose, but they are @ 0.007"....they were tight before...maybe I shoot for 0.006" this time.

 

 

Thanks again all! 

 

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I vote for vacuum leak from booster.  Pinch off the hose and see.  Get a proper timing light and a infrared thermometer.  It's pennies compared to what you spent.

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6 minutes ago, racer86 said:

'76mintgrün'02: There is NO check valve....just a 3-way connector to the 2 boosters and then up to the manifold. Is this missing?  It did not have one fitted before? 

 

Timing I have not checked however I have not altered it since it was set up when I bought it and it was running/ driving well.  There are marks on the distributor which are still lined up.  I have tried rotating the dizzy to advance/ retard and no difference.  That said I don't know where the timing is at...I just know it ran well! 

 

Firing Order: Yes, I'm sure...i checked and checked and checked until I was going mad...I timed the engine up to TDC and matched the rotor arm to 1 (at about 10pm, bulkhead side as you stand at the front of the car), then 3-4-2 

 

Coil: Yes, the black one is the original Bosch one. I replaced it with another Bosch coil (the aluminum/ blue one shown) part 0221119027 which was listed for the 2002. There is NO ballasted resistor and wasn't previously. 

 

I am not sure what you mean by two boosters, but there should be a check valve between the intake runner and the brake booster.

 

I get teased for recommending the Innova 5568 timing light so much, but it is a very useful tool (essential, IMO) and relatively affordable... get it before that changes.  Setting timing based on what it was really isn't very practical.  You've made a lot of changes since then.  The variable light will let you see what the timing curve is throughout the range.  I guess people who don't appreciate variable lights simply have never used one.

 

I don't want to be insulting, but you do have the firing order based on the clockwise rotation of the distributor, right?

 

The black coil needs a ballast resistor inline, as it came from the factory.  The early cars have a ceramic block style, with a coiled resistance wire on the back side and the later cars, with the coil mounted up forward have a resistance wire inside the wiring loom.  If you have a tester, you can see what the resistance is on your coil and add an appropriate resistor to bring that number up to 3 ohms.

 

This stuff really doesn't answer the 1&2 / 3&4 discrepancy, but is important.

 

Did you set your float height for the plastic ones?  I think that is around 35 mm, as opposed to the brass ones that get set around 40 mm.

 

Thanks for all the nice photos.  

 

Tom

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I can't see it being anything other than a vacuum leak. Go find it :)

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(edited)

Pursuing the vacuum leak theory I blocked off the take off from the manifold for the booster and started spring brake cleaner around (at this point had it burst into flames I'd have let it burn - frustration levels are HIGH!)  You can see spraying it at 3&4 lifted the idle and, not in the vid, but 1&2 didn't....

 

HURRAH, I'VE FOUND THE ISSUE......NOPE! 

 

I reassembled with sensibly copious amount of Loctite posh instant gasket, allowed to cure over night, fired it up and NO DIFFERENCE GIVEN THE SAME TEST.Maybe it was pulling it into the air filter? 

 

I've stripped it off and the image shows No.4 on a marble slab (kitchen shopping board, don't tell my wife) which is the straightest edge I have. Doesn't look look anything gaskets, bolts and sealer wouldn't solve. 

 

I'ave taken the manifold off again and have sent it to the machine shop to get it perfect.  I have new genuine BMW gaskets too.  

 

I fully expect this will make no difference. 

 

 

For reference, pug image is after 15-20 mins running to warm up and then drive up and down the road, some foot down, come cruise up to about 50mph.  Order in the image is 4-3-2-1. 

IMG_20200315_144627.jpg

 

IMG_20200317_204553.jpg

 

IMG_20200318_080335.jpg

 

IMG_20200312_121450.jpg

Edited by racer86

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