engine and drivetrain Engine bogs down

[2double02]

I just acquired a 74tii that was professionally converted to a Weber 38 back in 2012. It has sat for at least the last 18 months. It has an e12 head with mechanical fuel pump. Fuel pump was bad when I got it. I replaced the pump and filter and cleaned the screen on the gas tank tii fuel pickup. The car idles beautifully and pulls hard in first gear. For subsequent shifts it wants to bog down but will recover when the pedal is aggressively depressed at which time it pulls strong again. I have tried two good carbs with the same result. Tried an electric fuel pump also with no change. Feels as if it is starving for fuel. If you press the pedal very quickly from idle in neutral, you can create a similar effect. Is there anything I could be missing in the fuel system short of blowing out the lines?

Car has new plugs and wire along with pertronix electronic ignition.

Thanks in advance for your advice.

[peterschop]

Pull the fuel line after the pump and check the output with a pressure gauge. 

[TobyB]

Check the bowl for crud- if it's got a lot in it, that can do it.  Especially varnish build- up.

 

An air leak in the intake line can cause slow fuel delivery, too, as can a plugged pickup

screen in the tank.

 

I too would start by looking at fuel pressure- if it stays up, you go after the carb,

but if it drops to zero, go after fuel delivery.

 

t

[2double02]

Just an update and request for more advice. I have confirmed excellent fuel pressure. I also tried two different tii distributors. Playing around with the timing, I can minimize the hesitation that occurs only between gears when leisurely shifting or slowly pressing the pedal from a steady speed. With a little more aggressive driving, the car has exceptional power and pulls strong. It starts and idles perfectly. The previous owner was setting this car up for the track prior to his death. A friend mentioned he may have installed a cam that is best suited for more aggressive driving. Could that be the case? The car is an absolute blast to drive when powering through. It is as powerful as any tii or carbed 2002 I have ever owned.

[zinz]

What ignition timing have you tried and what are you running, now?

Edz

[2double02]

Began with 15 degree advance as I recall.

[zinz]

Sorry, if I don't understand...

 

Are you just static-timing? Or using an advancing timing light and the TDC mark on the flywheel (the OT line)? Or a simple timing light against the steel ball on the flywheel? 

 

My specific question would be, how many degrees advance and at what rpm?  What have you tried and where are you now?

 

Ed Z

[kbmb02]

Just an update and request for more advice. I have confirmed excellent fuel pressure.

 

How much fuel pressure? If it's a Tii fuel pump and no fuel pressure regulator has been installed, it's possible you've got more pressure than a Weber 38/38 wants or can handle (let's say 3.5 PSI). Then again, if the conversion was 'professionally converted' we'd think they addressed that... -KB

[dang]

Any signs from out of the tailpipe?  Smoke?  It sounds like you've eliminated the carb and fuel pressure as the problem, that leaves ignition timing and air flow.  Is there a chance that its not getting enough air when the throttle opens quickly?  I guess I'm asking if you've run it without an air cleaner?

Edited March 3, 2015 by dang

[2double02]

I don't have any more specifics on timing. I had a shop check it with a light and I gave them factory specs. I just bought a light and will be learning how to use it soon. Would timing differ for a tii versus carbureted car?

Distributor is the correct 002 tii unit.

It has a new stock 2002 mechanical fuel pump installed on the e12 head. The fuel filter is is the standard tii vertical location although it is just a typical plastic filter. I may move it to a horizontal location to see if there is any difference.

Air flow is sufficient.

Thanks for the continued advice.

[zinz]

I doubt you have enough ignition advance dialed-in.  Because you have a modified engine, the factory timing specs will only get you so far...yes, it will run, but it won't run to it's full potential.

 

Here's a nice write-up that will give you some insights  http://www.bmw2002faq.com/topic/67587-ignition-timing-methodology/

 

As for using your new timing light... read Hack's comments near the bottom of the page here: http://www.bmw2002faq.com/topic/124816-using-an-advance-timing-light/

 

Basic timing specs off the steel ball:

 

Look for the OT line in the strobe of the timing light (it can be a devil to see, if it hasn't been painted)

 

There's no telling where your shop set the timing (it's a Tii engine converted to a carb) You can hunt for the steel ball in the strobe by scrolling thru the advance buttons on the timing light, but as you will read in the attached thread, you could probably add 5 degrees to the stock setting and start experimenting from there.  Just be mindful of detonation (pinging)...any pinging at all and back it off a couple of degrees ( or buy higher-octane fuel). 

 

I've advanced my ignition by about 10 degrees to find the sweet spot for my set up.. but your engine is certainly different. 

 

Good luck!

 

Ed Z

 

 

[dang]

I agree that its probably timing.  You can bypass the fuel filter to rule that out, but I think you'd have enough fuel in the bowl.  I think a restricted fuel filter would cause problems with throttle under long loads, like going up an incline.

[Roland]

I just acquired a 74tii that was professionally converted to a Weber 38 back in 2012.  . . .   I have tried two good carbs with the same result.

 

 

I tend to agree with others regarding ignition timing,.  However, re-reading your original post gives me pause.

 

Knowns versus unknowns?

 

A "professional" conversion is usually better than an amateur conversion, but it does not guarantee against simple mistakes.  The factory is filled with "professionals"  - and dealerships seem to be filled with other professionals redoing the work of the factory professionals.

 

You mention nothing about your carb or the two "good" carbs you have tried, e.g., how they are jetted.  Although you describe a smooth running engine - even a slight jetting change can make a big difference in engine operation and that includes idle, full throttle under load - at different engine speeds, and hesitation.  Filters and low pressure can affect fuel availability to the carburetor, but so can float levels and its hard for anyone to make long distance guesses concerning the level settings in the two or three carbs you have tried.  

 

Fuel and ignition have to be harmonious for an engine to sing well.  Often, over-advanced ignition timing can mask or overcome carburetor deficiencies.  You mention new plugs but fail to mention type, heat range, and gap  -  any of which can contribute to engine operation "peculiarities."  (I didn't say they do cause problems in your case, only that they are possible causes.)

 

Finally, there is the camshaft that you mentioned may or may not be original.  Different profiles understandably affect torque and power production in different ways and at different engine speeds.  I'd recommend exploring under the valve cover to confirm what you have or do not have.  A different cam may or may not require valve lash settings different from recommendations for the stock cam.  Those settings may or may not impact engine operation at different engine speeds.

Edited March 4, 2015 by Roland

[Russell74Fjord]

I have been struggling with this too, with a 40/40 Weber, DGAV on an older MM 2200 Sport engine.

 

Mine is more random, which is really hard to get a diagnosis on. Feels like fuel starvation at times, and I did move the fuel filter to a vertical position from horizontal, that may have helped, since I was getting some stumbling and hesitation on uphill grades. It still seems the fuel filter has a lot or air in it. Folks tell me that is normal, and at times I do see the fuel filter is about half fuel, in it vertical position.  

 

Petronix and MSD, mechanical advance distributor. It has gone 30k miles without a hitch, now being weird. Cleaned mechanical advance mechanism, counter weights, timed, check fuel pressure was 4lbs. Checked fuel pump push rod against another one, very slight wear, so installed the longer one. Seems despite pressure could have been a volume issue?

 

Still baffled though, since it is not running consistently good.

[2double02]

This is all great advice.  I will remove the valve cover this weekend to take a peek.  Over lunch I tried to time it with a vacuum gauge which actually improved it a little.  My old school mechanic friend called it power timing.  Still a hesitation but barely there.  I will keep plugging away at it.