Start Then Stutter at Open Throttle

DCMotoring · Post by **DCMotoring** » Wed May 23, 2012 1:41 pm

I have done a bit of searching but haven't found any answers directly related to my problem. The engine starts and idles fine but when I open the throttle the engine stutters and nearly stalls. Once it warms up, it operates as normal.

I recently replaced the fuel pump and all the fuel lines. I was having some cold start issues during the winter but didn't get to diagnosing and fixing it and now that it's warm out, it isn't a problem. Is this engine stutter during warm up related to a component of the cold start system or do the symptoms indicate an issue with the MAF Sensor or Throttle?

Mike W. · Post by **Mike W.** » Wed May 23, 2012 9:06 pm

It's not related to the cold start system, it is cold start, not cold run, and only works while cranking. It could be an AFM problem, though not a MAF since it doesn't have one, it could be fuel pressure, coolant temp or most likely a vacuum leak. Check the timing too.

bizz · Post by **bizz** » Mon May 28, 2012 11:50 pm

I have the same issue with my 520i. Let me know if you fix the problem.

When i get a chance i'll be following Mike's advice.

bizz · Post by **bizz** » Tue Jun 26, 2012 12:28 am

Hey guys, i bought a 76' 528 recently to replace my 520i - curiously it has the same issue on full throttle, however only when opening the throttle fully from idle.

Once the revs are > ~ 2k the engine revs freely.

I noticed with the air filters / shroud removed the rear carb backfires & spits fuel out the top when the throttle is at WOT from idle.

Further to this the car doesn't seem to want to idle once warm & will stall when coasting in gear (auto)

Hope this makes sense, any ideas are welcome.
(35 y/o carbureted sixes are a bit foreign to me!)

Thanks
Ben

Falkenberg · Post by **Falkenberg** » Tue Jun 26, 2012 5:53 am

Carbureted? The 2 Zenith?
Check if the accelerator pump is working. With engine stopped, look in to the smaller venturi, pull the accelerator (or the small lever that operates the pump) and see if the small tube of the accelerator pump spits out fuel.

I'd also say that you better might open a new topic for carburetor problems. It might get long.

Mike W. · Post by **Mike W.** » Tue Jun 26, 2012 9:51 pm

The Evil Zeniths??? I had those on a Bav (E3) and hated them. Ours may have been a bit worse with emissions controls, but I had a terrible time keeping them in tune. And if I adjusted them for a good idle, I had a flat spot everywhere in the RPM range when I gave it gas. If I screwed the idle jets way in I could pretty much eliminate that flat spot, but then after a matter of days I lost my idle. Good power, good economy, but terrible driveability, especially at low speeds in parking lots. I finally fixed it by taking the whole L jet FI system off a 530i E12 and installing it.

bizz · Post by **bizz** » Wed Jun 27, 2012 12:00 am

^^ Yup, evil Zenith's!

Damn, didn't want to hear that. The car's only got a 129k kms on the clock so it hasn't had a lot of driving in the past.

It's not registered so i'm hoping i can solve this issue myself before i get the car inspected - if not i'm hunting down a good bmw mechanic as we speak.

Maybe i should consider the L-Jet upgrade too ... hmmm
Any other advice out there on the Zenith's, or a suitable upgrade (cost effective)?

Thanks

Falkenberg · Post by **Falkenberg** » Wed Jun 27, 2012 3:58 am

You can get the Zeniths to work, I've got them on my car, they are not brilliant but neither are they evil. It just takes some time to set them up. I can't say that I've needed very frequent tune-ups. Maybe it also depends on how much of a perfectionist you are. I've been running mine without adjusting anything since July last summer, until last month I decided to tried a colortune spark plug to check the mixture (which was a bit lean, so I've adjusted it a bit).

If your's are in a bad shape, you can still find repair kits for them. Just be sure that they have no vacuum leaks in the lower stages. If nothing leaks, I strongly recommend to NOT remove them from the intake manifolds, because that might give air leaks when you put them back and tighten the 4 13mm bolts uneven etc. A removal is not needed anyway when you just want to clean the stage with the jets. Be careful not no rip the gaskets when you open them up. I found that they open nicely when you, after removing all the screws (mind the one under the air box bolt!), tap the unscrewed part with a small hammer or a crew driver.

Check all diaphragm, all moving bits, etc. Don't give up too quickly in them.

Usefull links:
http://www.e9coupe.com/tech/zeniths/bmw2/bmw2.html
http://www.e9coupe.com/tech/zeniths/bmw1/bmw1.html
http://www.ruddies-berlin.de/35INATbmw.htm

bizz · Post by **bizz** » Wed Jun 27, 2012 5:32 am

You sir, have made my week!
Hopefully I can make some headway this weekend.

I'm still feeling my way around all the bmw resources out there, so I appreciate your assistance with the links

Falkenberg · Post by **Falkenberg** » Wed Jun 27, 2012 3:04 pm

Info specific for E12 zenith is hard to find, but fortunately there is some info on E9 and E3 fora. It's pretty much the same engine in those cars.

Mike W. · Post by **Mike W.** » Wed Jun 27, 2012 10:37 pm

Falkenberg wrote:
If nothing leaks, I strongly recommend to NOT remove them from the intake manifolds, because that might give air leaks when you put them back and tighten the 4 13mm bolts uneven etc. A removal is not needed anyway when you just want to clean the stage with the jets. Be careful not no rip the gaskets when you open them up. I found that they open nicely when you, after removing all the screws (mind the one under the air box bolt!), tap the unscrewed part with a small hammer or a crew driver.

Agreed, rarely is it necessary to remove them from the manifold. I could make them run better with a larger main jet, but mileage went to hell. Maybe memory is playing tricks on me, but it seemed like I had them open weekly. I'd just take the top off while they were on the car, pull the jets out, spray carb cleaner down them and thru the jets, then reassemble. Then I'd be good for another few weeks. On US cars at least, the "jet" in the accelerator pump sprayer was just too small. I never did figure out a fix for that. Yours might not be so cursed, but they are still not friendly carbs.

It was interesting what happened when I went to L jet though, I lost torque, I lost some economy, I maybe gained a little top end, but it would rev forever, and it just drove a thousand times better. Like a new car, step on the pedal and it went. Part throttle and it would cruise nicely. Perhaps less sporting in a sense, but a far nicer car.

Falkenberg · Post by **Falkenberg** » Thu Jun 28, 2012 9:21 am

Mike W. wrote: Yours might not be so cursed, but they are still not friendly carbs.

Possible, I'm just sharing my experience, just as you are.

Mike W. wrote: It was interesting what happened when I went to L jet though, I lost torque, I lost some economy, I maybe gained a little top end, but it would rev forever, and it just drove a thousand times better.

I've noticed some time ago that max torque number for the zenith 528 is bit higher and at lower RPM than for the fuel injected 528i. That would be conform your experience.

I'm not disputing the better driveability of L-jet, I'm just saying that a good Zenith setup is not a nightmare. A good L-jet (or motronic) will most likely perform better, else evolution would become involution.

In the end I think it just sums up to what would be the easier way to get a usable / satisfactory car. Since no time is spend yet on the Zenith, my argument is to at least give them a try. Pennies.

bizz · Post by **bizz** » Sat Jun 30, 2012 5:08 am

Had a bit of a fiddle today & the rear carbs fuel flow is audibly louder when applying throttle from idle. I assume it's a syncronisation issue going off section 7.3 on the following doc:
http://www.e9coupe.com/tech/zeniths/bmw2/bmw2.html
In fact, any of those issues sound possible.

I'll be getting a pro to look over them in the near future, will advise of the outcome.
Thanks.

bizz · Post by **bizz** » Fri Aug 03, 2012 7:41 pm

Mike W. wrote:And if I adjusted them for a good idle, I had a flat spot everywhere in the RPM range when I gave it gas. If I screwed the idle jets way in I could pretty much eliminate that flat spot, but then after a matter of days I lost my idle.

I had the carbs adjusted & overall the car runs fantastic.
However i have the same issue with idle - identical to what you said Mike, just a pain to drive around parking lots etc..

Anybody got a miracle solution or should i ditch the carbs?

bizz · Post by **bizz** » Tue Sep 04, 2012 10:11 pm

Just for future reference (if in fact there's anyone else out there that still has the zeniths) the solution to the problem was to remove the blocked accelerator pump from the rear carb, blow some carb cleaner through it & reassemble.

10 minute job, car runs perfect - no stutter or stalling at idle.
Thanks to Falkenberg for his spot on diagnosis