Help, new 3.5 swap doesn't run right

[echappe]

Gentlrmen,
I have been beating my head against the wall with this for a few weeks now.
3.5l from 89 735i was recently used to replace 3.0. the head was gone over with new valve guides and seals. new 008 distributor Bosch blue coil and 2 new weber 38/38 DGAS's.
It starts with just a flick of the key and idles OK but above 3000 RPM's it starts popping and backfiring through the tailpipe and carburetors , blows black smoke and quickly fouls the plugs. I have double checked cam timing,ignition timingwith oscilloscope. The 38's came from Jam engineering and i have been told they are jetted correctly for this application.
Does anyone please have a clue or suggestion ?
Thanks in advance,
Ernest

 

[sfdon]

What is your voltage at the coil at 3200 rpm?

Classic question- are you running rich or do you have a weak spark....

What does your spark look like at 3200?

What is your battery voltage at 3200?

Have you tried a home run wire from battery to coil?

Which wiring harness?

Yes- there is a chance that you are running rich ... and a chance that your spark is petering out out. Which is it? Tell us your voltages. Tell us what coil voltages you have and what ohm readings you are getting at the coil. Ohms at the rotor... ohms from cap to plug..

 

[echappe]

thanks SFdon, I will do those checks monday and get back to you

 

[MMercury]

Gentlrmen,
I have been beating my head against the wall with this for a few weeks now.
3.5l from 89 735i was recently used to replace 3.0. the head was gone over with new valve guides and seals. new 008 distributor Bosch blue coil and 2 new weber 38/38 DGAS's.
It starts with just a flick of the key and idles OK but above 3000 RPM's it starts popping and backfiring through the tailpipe and carburetors , blows black smoke and quickly fouls the plugs. I have double checked cam timing,ignition timingwith oscilloscope. The 38's came from Jam engineering and i have been told they are jetted correctly for this application.
Does anyone please have a clue or suggestion ?
Thanks in advance,
Ernest

Given your description of the symptoms, the problems could be simple or complex, ignition and/or fuel related. My initial reaction to your carb coughing/high speed misfire, is incorrect ignition timing and/or an extremely lean mixture. Both could contribute to misfiring and yes, sooted up plugs. (While it may sound counter-intuitive, black soot is not always the result of an overly-rich mixture. A perfect 14:1 mixture that is unburnt, still results in soot.)

You mentioned double checking the cam timing, does this include checking the valve lash too?

While everything sounds as though it should work well, given all of the new parts, begs the question: what settings are you using for normal adjustments?

For example, you mention an "008" distributor (which sounds like a four cyl. tii distributor found without vacuum advance). Despite the name, it may have a far different advance curve than the stock distributor and thus require entirely different timing settings. If my guess is correct, using the stock recommendations for timing may leave you with too much total advance in the upper rev range leading to incomplete combustion.

This is only part of the equation, because maybe your engine is being starved for fuel at the upper rev range? Without proper jetting, even the best carburetors in the world are worthless. Your retailer can offer you "ball park" settings, but this is just an approximation. Altitude, ambient temperatures, engine displacement, compression, camshaft selection, and anticipated engine use are all factors to be considered in initial jetting selection. And even then, there are many variables that may still require tweaking idle, main, and air correction jets, not to mention emulsion tubes. Nevertheless, even assuming your carb jets were perfectly suited to your engine, are your floats properly adjusted? If the float levels are adjusted too low, it may be that when your engine is its thirstiest, the carb bowls are not full enough to keep those cylinders quenched.

If you are the doing your own work here, I would suggest trying the easiest things first. As a starting point, reexamine your ignition system. Make sure your plugs are the proper heat range and properly gapped. Try timing the distributor by ear, see if you can affect the running by retarding (or even advancing the timing). Using a timing light (stroboscope?) isn't particularly effective unless you have a spec known to work for your engine. (If you truly meant an oscilloscope, it may tell you that your ignition system is functioning well, but may not directly diagnose an advance curve that is not tailored well for your engine. If you did not obtain a spec sheet with your distributor, and if you are so inclined, with a decent tachometer and an adjustable timing light, you can roughly plot out the distributor's advance curve and at least compare it with the curve you would have had with your stock distributor. For that matter, you might want to try reusing your old distributor.

While on the subject of ignition, you mentioned running a blue coil. Although there are exceptions, most of the blue coils are internally ballasted. Thus, if you are not bypassing your original ballast resistor, you are likely restricting full voltage to the coil. This could also account for your poor high speed engine operation. Likewise, if you have a defect in the cap, or if you are running a rev limiting rotor with a weak spring.

If the foregoing were not enough, it might be helpful if you could list your carbs' jetting. Even knowing the idle jet numbers might prove helpful if only to determine a baseline.
http://www.e9coupe.com/forum/showthread.php?t=1589
http://www.e9coupe.com/forum/showthread.php?t=81

hth

 

[RonP]

Have had similar problems and after replacing all fuel lines, electrics, etc., it turned out to be a bad ground strap. I would also check the AFM as I have had the same symptoms with a bad flap.

 

[dp]

Have had similar problems and after replacing all fuel lines, electrics, etc., it turned out to be a bad ground strap. I would also check the AFM as I have had the same symptoms with a bad flap.

Grounding is a good tip. He's running carbs though so no AFM

 

[execmalibu]

Clogged Exhaust?

Had a similar experience with a vintage Motorcycle! Bike ran OK until I installed a new top end...

Would run great for a short time (1 mile )then putter out and die... Took the Head off 4 times, replaced Carb, Ignition, Plug, Fuel Lines... I tried everything and Was seriously considering using a very large hammer on the motor :neutral:but then I realized the baffle was partially clogged with oily sludge.. Too much back pressure choking the new motor... Cleaned the baffle

The Bike ran amazing after that!!!

 

[mpgandco]

I had a similar problem with a 1973 Porsche 911 "S" I recently bought.It would run ok for the first few minutes and then it would start to backfire and would barely run.

In my case it was the voltage regulator.I put a volt meter on it and it was overcharging.It was putting out 17 volts.I replaced the voltage regulator and that fixed the problem.

 

[execmalibu]

mpgandco... A 1973 911S Porsche??? Who with an E9 would ever want a Porsche?

mpgandco

I hate early 1970s cars especially vintage Porsches!! Thats why I only have (7) Porsches ( five 911s and two 356s) but only one Dino Spyder, one 280SL and only one chrome bumper Corniche Convertible... And now one E9!:-D

 

[mpgandco]

mpgandco... A 1973 911S Porsche??? Who with an E9 would ever want a Porsche?

Are you kidding me?I have to say I can't quit driving my Porsche 73 S!!!I find myself driving it more than the 993TT.The sound that flat six makes at 7300 redline is priceless.

My 73 S Targa looks to be almost the same color.What color is that?Gemini Blue Metallic?

I have to admit you own some of my all time favorite Vintage cars!!!You could buy a real nice modified CS for the price of those Rudge wheels you have!!

I sent you an Email,if you need and help with the Coupe

 

[execmalibu]

Impgandco... In 1973 Porsche had 2 Metallic Blues! Gemini Blue and Metallic Blue

In 1973 There were 2 Metallic Blue 911 colors...

Gemini Metallic #334 (light metallic blue) and another slightly darker Blue color called just Blue Metallic #335...

My car is the darker Blue #335

The numbers are from my fading memory so I might be off...

A few more toys...

 

[mpgandco]

Ernest
Sorry I hi jacked your thread!!!I hope you get your 3.5 motor transplant problem figured out.

Exec:
Thanks for the info on the Porsche colors.I will send you a PM about the color of my 73 S and maybe you can educate me a bit.