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Compression Test Results


Columbo

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I've been troubleshooting a miss at idle.  So far I have set the valves at .007, set the timing, swapped the dizzy cap and roter, points and plugs and wires, and new coil.  Ive checked for vacuum leaks.  I even checked the ground strap!  The car runs well through the gears with a good pull.

 

I just did a compression check with a rental unit from my local parts store and it came out: 120, 120, 115, 115.  The engine was warm (not hot) and with open throttle.

 

It is a 1975 2002 base with a Weber 32/36 (water choke) and everything else pretty much stock.  Are those acceptable numbers and where to go from here?

 

This is just a thought but if the idle jet was clogged, would it miss fire at idle but run ok while driving?

 

And one other thing I just thought of:  On the bottom of the intake manifold there is a device screwed in that has a push on wire connector...but nothing connected to it and no wire hanging anywhere.  

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Compression results are fine. Low but fine. I have similar compression results and have been running a bottle of engine restore with each oil change and it did seem to help my numbers a little. 

 

Bottom line is compression is not your problem. 

 

The device underneath the intake manifold is for emissions which had obviously been removed from your car. Leave it out replace it with a suitable plug.

1976 BMW 2002 Chamonix. My first love.

1972 BMW 2002tii Polaris. My new side piece.

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Specifically, the device under the intake manifold is the T2 Temperature Switch.  It is closed below 113F open above 113F and grounds the EGR and the Choke Relays.  The fact that it is not connected begs the question, how much of the emission system for your 75 has been removed and is everything buttoned up properly?  What new coil are you running and is it compatible with the internal resistor wire found on the 75 model?  You didn't mention replacing your capacitor, which could cause an ignition miss.  Blow out your idle jet, which may improve how smoothly your car will idle.

 

Like Stevenc22 mentioned, your compression numbers, although low, are at least relatively even, so I would chase ignition and fuel issues.

 

Good luck,

 

Mark92131

1970 BMW 1600 (Nevada)

 

 

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On a stock engine with stock cam you should see 140psi as a minimum for a compression test.  Your engine is getting tired but at least all the cylinders are similar.  Add some moly disulfide (MOS2) to the oil.  It will help a little.  Dirty idle jet could be the issue.  Put your timing light on each plug wire.  When it misses a beat you'll be able to see it.

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How do your plugs look after the car has been idling for a bit and misfiring? Which plugs are you using? What plug gap are you running? Yes, it is possible that one or both of your idle jets could be partially blocked and the motor would otherwise seem to run ok in the transition.

Chris B.

'73 ex-Malaga

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Yeah, I re-adjusted the carb today.  I was running a bit rich by the looks of the plugs.  I took it for a drive and re-inspected them and they looked grayish brown so I think the mixture is where it should be now.  This afternoon I removed both jets and blew them out.  I carefully used a sewing needle to make sure all the holes were free of gunk but didn't take it for a drive due to a snow flurry passing through.

 

I'm using NGK BP****, can't remember the number but it's what are commonly recommended on this forum.

 

By miss, I mean the engine is smooth and then "hiccups" and it jerks in the mount.  It's random in frequency.  Gap of the plugs set to .024.

 

I should be able to take it out tomorrow and will report back if cleaning the jets worked.

 

Thanks for all the tips fellas!

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So I still have the miss.

 

I removed the distributor (Bosch JFED 4).  There is a lot of up/down play (axial??), about .045 of an inch worth.  I've got awn IE distributor with electronic ignition on the way because I read somewhere the max play should be .005 of an inch.  Can someone confirm this?

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4 hours ago, Columbo said:

So I still have the miss.

 

I removed the distributor (Bosch JFED 4).  There is a lot of up/down play (axial??), about .045 of an inch worth.  I've got awn IE distributor with electronic ignition on the way because I read somewhere the max play should be .005 of an inch.  Can someone confirm this?

 

Getting a new distributor definitely won't hurt. I have been slowly chasing down a similar miss myself. 

 

Take me the air cleaner off, start the car and run till warm. Get a flashlight and peer in to the carb chokes from above. Do you see any dripping from the auxiliary venturis? 

 

I had had this issue with my 32/36 which would see a periodic blob of fuel drip onto the back of the throttle plate. This would cause a little bit of a stumble. I needed to rebuild the carb and shim the venturis to stop that. 

 

 

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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Simeon, that is really interesting and I will check for a drip as soon as I get the new distributor in.  The drip theory sounds VERY plausible as its smooth for a few second and them, like you say, hiccups one or twice and then repeats.  Again, I cannot tell if it is happening while normal driving but seems smooth with ok power.

 

 

 

 

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I determined it was down to a poor fit for the venturis that allowed air in the side which then allowed fuel to drip out in an uncontrolled manner. 

 

See this blog entry for what I did:

 

 

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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