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Utah02

Car Wont Accelerate past ~4000 RPM Under Full Throttle

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(edited)

Hey Everybody,

 

I have written in on this before so I apologize if you're seeing it again but I am truly stumped and this has turned into an issue far beyond what I could have imagined. Last time I wrote I wasn't extremely detailed which left a lot of overlapping troubleshooting and confusion so I am also going to be very extensive on my detail with this so we all know what I own, what the problem is, and what I've tried to fix it.

 

I have a 72 2002 base that has an Carbureted e21 M10 with a Weber 32/36. I purchased the car with this issue as well. The car runs pretty well with the occasional misfire and will accelerate well. Though under WOT I can accelerate hard up to roughly 4000 rpms then it just cuts all power. I can feel an immediate hesitation and a sort of popping rumbling sound and the car will just sit right around 3500-4000 rpms and wont go above. This is in every gear as well.  It only happens in gear too, If i am in neutral it will rev higher than 4000. If I back off the accelerator just a smidge it can accelerate passed 4000 rpms so this only happens when I am at full throttle. I have done a long list of replacements with the help of  parts car to fix it but i am now on about 3 or 4 months on and off of trying to fix it and the problem still remains. I have a Carter Low Pressure Fuel Pump Located above the gas tank bolted to my rear shock tower well. with a filter just before it and a filter right before the carburetor.  It also has Pertonix ignition with a normal distributor rotor and cap. Here is a long list of things I have tried and nothing has worked. This is in order as well. All parts came off a running 2002 and I am positive work

 

Originally had a Weber 38/38 carb switched to a Weber 32/36

Timed

Cleaned Distributor Cap and points

Changed distributor rotor

Changed distributor cap

Changed Spark plugs 

Changed Spark Plug Wires

Change Entire Distributor system with cap and switched from Pertronix to Points

Timed again of course

Changed Ignition coil

Bypassed ceramic ballast resistor

Changed to mechanical fuel pump at different location

Checked Fuel Pickup and replaced Screen

Changed Battery

Changed Alternator

 

Some of these things I know were far fetched but I was at a point of desperation and was just hoping they might work

There are even things I have probably forgotten that I did. I've spent multiple entire days to fixing it and been in the garage for 12 hours at a time. Now I am not the smartest when it comes to everything but I know enough to do what I did so I might have questions when you suggest things. 

 

If you have any further questions I can do my best to answer. Help is really needed here and I thank everyone in advance for taking the time to help me. And again I'm sorry for writing in again just need the help. Here is a link to a video I filmed of what exactly the car does when it bogs.

 

Thanks again,

 

Colby

 

Edited by Utah02

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I would have said that your fuel pickup screen is clogged. That's the first symptom...starving for fuel at high rpm.

But obviously you've checked that. Still seems like a fuel issue.

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Fuel issue seems most likely to me too. I haven't checked my fuel lines thoroughly but when I had the carpet rupees out I inspected them quickly and they looked fine. I was told a super simple test would be to buy 5/16 hose and run it from the tank out the trunk and around the side of the car as awake shift fuel line just to see if that changed anything.

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I agree, fuel.

$20 fuel pressure gauge, tee it @ the carb, run it so you can see it

when you're driving (zip- tie to hood louvers)

 

Then you know .  Could be electrical, too, but eliminate fuel first.

 

t

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(edited)

agree.  check fuel first.  volume and pressure at the carb.  I would also delete one of the fuel filters.

 

then...the distributor.  looks like that is still original.  (unless "changed entire distributor system" included the dizzy itself....)  when you say you timed the engine.  how?  was that just the base timing at idle or did you check it also at full advance?

 

since you say it happens at same rpm, regardless of gear, I am leaning to more an electrical/ignition/timing problem.

Edited by mlytle

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I know t is simple / silly thing, but it sounds like other than fuel it is ignition and possibly the dizzy rotor you installed had a 4k rev-limiter style? I know you can buy these rotors in various rev limiting types... Lastly, your points could be off and a simple/cheap thing to replace.

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One other thing to check...the linkage on your carb.  If the second barrel on your 32/36 isn't opening, you've got not nearly enough carb throat for a 2 liter engine.  Or just the opposite--the second barrel is opening too quickly (admittedly this is unlikely unless you've fiddled with the linkage or there's a Sync-link not properly installed) you'll have a huge flat spot at about those revs.  

 

And finally...if you had the carb apart, it's possible to install the spray bar backwards.  That's the drilled piece that takes fuel from the main jet and sprays it into the airflow through the carb's venturi.  If you did that on the primary barrel, the car would barely run but if you did it on the secondary barrel, you'd get a flat spot where the secondary barrel starts to open--because it won't be getting any fuel.  

 

cheers

mike

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Sure feels electrical or airflow to me... since it happens with just revving the engine and under load at the same rpm. 

 

Blockage in the exhaust system?

Is this an electric choke?  My car wouldn't get over 4k rpm when I forgot to get power to the choke and it wasn't opening. 

 

Timing: I'm with mlytle on this one.  Check the advance.  A timing light with an advance function if great for mapping out the advance curve.  

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agree.  check fuel first.  volume and pressure at the carb.  I would also delete one of the fuel filters.

 

then...the distributor.  looks like that is still original.  (unless "changed entire distributor system" included the dizzy itself....)  when you say you timed the engine.  how?  was that just the base timing at idle or did you check it also at full advance?

 

since you say it happens at same rpm, regardless of gear, I am leaning to more an electrical/ignition/timing problem.

I will check fuel. I replaced the entire distributor. I timedwith a timing light at idle. Full Advance? I don't believe I've done that

 

 

One other thing to check...the linkage on your carb.  If the second barrel on your 32/36 isn't opening, you've got not nearly enough carb throat for a 2 liter engine.  Or just the opposite--the second barrel is opening too quickly (admittedly this is unlikely unless you've fiddled with the linkage or there's a Sync-link not properly installed) you'll have a huge flat spot at about those revs.  

 

And finally...if you had the carb apart, it's possible to install the spray bar backwards.  That's the drilled piece that takes fuel from the main jet and sprays it into the airflow through the carb's venturi.  If you did that on the primary barrel, the car would barely run but if you did it on the secondary barrel, you'd get a flat spot where the secondary barrel starts to open--because it won't be getting any fuel.  

 

cheers

mike

This seems likely but I originally had a 38/38 and it had the problem. Switched to the 32/36 and nothing changed. Both carbs worked fine on another 2002.

 

Sure feels electrical or airflow to me... since it happens with just revving the engine and under load at the same rpm. 

 

Blockage in the exhaust system?

Is this an electric choke?  My car wouldn't get over 4k rpm when I forgot to get power to the choke and it wasn't opening. 

 

Timing: I'm with mlytle on this one.  Check the advance.  A timing light with an advance function if great for mapping out the advance curve.  

Ok, exhaust blockage is new, not sure how to check that out but I can look that up and look into it. The 38/38 was an automatic choke and the 32/36 is a manual choke. I will also do an advanced timing and see if that works.

 

Thanks everybody!

 

Colby

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I know t is simple / silly thing, but it sounds like other than fuel it is ignition and possibly the dizzy rotor you installed had a 4k rev-limiter style? I know you can buy these rotors in various rev limiting types... Lastly, your points could be off and a simple/cheap thing to replace.

I've used 3 different rotors. Two that were the normal without a rev limiter and on that was a rev limiting rotor and all 3 didn't change anything.

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Oh I also forgot to mention that when I am in neutral and not in gear it doesn't happen. I can rev up to a higher rev limit passed 4000 rpms. I'll edit the original to mention that

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Here are the first two threads on this problem

 

http://www.bmw2002faq.com/topic/165901-car-chokes/

 

http://www.bmw2002faq.com/topic/166190-car-bogging-troubleshooting-stumped/

 

I have reread them and sort of like the "fly stuck in the fuel line" theory... seriously.

I would pull the line at the tank and the firewall, then let it drain back into a jar, or bottle.

Then I would use compressed air to blow out the line (very carefully, of course) and see if any insects come out.

 

In one of your earlier posts, you mention the carpet limiting pedal travel.  I am wondering whether you have inspected/adjusted the pedal travel yet?  There is an arm that clamps onto the throttle pedal shaft, on the outside of the pedal box.  On my car, the clamping portion of the arm had distorted and the two sides were touching, before sufficient clamping could occur.  I removed a little metal between the tabs and it has stayed put since.  This is not likely to be the source of your problem, but it is an important step in setting up the carb; to make sure full pedal equals full throttle... with no slipping.

 

It is a little weird that you can achieve higher rpm as long as you do not give it full throttle.

 

You have replaced the fuel pump in the trunk and one of your early photos showed nice new fuel lines, so old cracked lines are not likely the problem.  Is the plastic sleeve intact (not cracked) on the outlet nipple of the fuel sender (at the tank)?

 

 

 

 

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can you adjust your valves first. then set your ignition timing correctly based on the distributor model for a base line setting. generally 1600rpm vacuum advance disconnected and plugged to the ball or 2400rpm with a centrifugal advance only distributor to the ball. If you do not know how to do this to achieve a base line for your timing can you please use the search function and learn. timing the ignition at idle will not give you the correct advance at higher RPM. then double check your best lean idle carb settings. put in new spark plugs(which ones are you currently using?) gapped correctly and report back. when doing so, show us a picture of the spark plugs

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