3.0CS car bogging down at around 4k rpm

[CTD]

Hi everyone,

Since a few weeks I am having an issue with my 3.0CS. Engine was completely rebuilt, new coil, distributor, cap, wires, sparks,...
Whenever I drive it and the revs get up to 4k rpm the engine starts bogging and won't stop untill I let go of the throttle and keep it below 4k rpms where it runs fine.

Checked the timing, spark advance, carbs were tuned-in.

Does anyone have an idea?

 

[thehackmechanic]

Are you running points/condenser or an electronic triggering system like a Pertronix? I'm a big believer in Pertronix on 2002s, but when I installed one in my Bavaria, it caused the car to do exactly what you're describing. Put the points and condenser back in and the problem went away. Never figured out why.

 

[CTD]

Are you running points/condenser or an electronic triggering system like a Pertronix? I'm a big believer in Pertronix on 2002s, but when I installed one in my Bavaria, it caused the car to do exactly what you're describing. Put the points and condenser back in and the problem went away. Never figured out why.

I am using this:

BMW-6-R-V - 123ignition

The 123BMW-6-R-V is designed for 6-cyl. engines: - 2.5 CS, 2.8 CS/L/A - 2500/A, 2800 /CS/A - 3.0 L/S/CS/A - 3.0 SI/CSI/CSL (not for D-Jetronic injection) - 3.3 L/A - 320/6, 520/6 (early types)

123ignition.com

 

[Mike Goble]

Do you have a rev limiter set in your ignition?

 

[Stevehose]

Clogged fuel filter, bad fuel pump, leak in fuel hose at tank can do this. What model coil are you using?

 

[Luis A.]

It could be a number of things as mentioned above, but as you are running a 123 check that the coil's 1ary resistance is over 1.0 Ohms.

 

[dang]

Probably worth swapping the original distributor back in to test.

 

[DWMBMW]

Make sure you have bypassed/ disconnected the original resistor when using the 123.

123 distributor conversion, '73 CS

Would I be correct believing if I replace my stock distributor with a 123 that I no longer need the ceramic resistor in the circuit?

e9coupe.com

 

[CTD]

Make sure you have bypassed/ disconnected the original resistor when using the 123.

123 distributor conversion, '73 CS

Would I be correct believing if I replace my stock distributor with a 123 that I no longer need the ceramic resistor in the circuit?

e9coupe.com

Which resistor is that?!

 

[Stevehose]

Which resistor is that?!

Ballast resistor next to coil. Being a 73, you shouldn’t have the resistor wire which replaced that in 74.

 

[Dick Steinkamp]

Has it done this ever since " Engine was completely rebuilt, new coil, distributor, cap, wires, sparks,..." or did it just start doing this?

 

[CTD]

Has it done this ever since " Engine was completely rebuilt, new coil, distributor, cap, wires, sparks,..." or did it just start doing this?

I have done about 3.5k miles since everything was rebuilt. It started doing that all of a sudden.

 

[Arde]

I have done about 3.5k miles since everything was rebuilt. It started doing that all of a sudden.

That helps. I would be curious to know if it bogs down at 4k RPM in all gears. If it is fuel delivery, fuel filter, air filter, AFR, or carb related, the effects would differ in different gears as it takes less fuel to rev up in lower gears than higher gears I think. I assume with no load, in neutral, the engine revs fine to redline, right?

 

[deQuincey]

always alert, today i had an stupid problem,

well you have to be very attentive

Today I started, and so I didn't want to give it importance, but I heard a whistle, and it seemed strange to me.

In the end I started to check and the tube that goes from the carburetors to the air filter was loose.

Looking closely it was cracked, I cut 10mm off the tip and reconnected it, no problems.

 

[CTD]

Here's what I did so far:

-Removed the resistor as it was still attached;
-Replaced coil to a new one recommended by 123ignition.

Made no difference.

When I rev at idle it revs high fine.
When I drive in 1st it revs untill about 5k and then starts bogging/stuttering
In second gear about untill 4500rpm and in third untill 3800-4000rpm. When I let go of the throttle it runs fine again.

As you all figured, the car is an automatic.

Fuel system is clean.
Mechanical pump maybe?

 

[ScottAndrews]

Sounds to me like a fuel issue. You addressed the obvious ignition issues above. The issue seems to be load related too. If it was simply an engine rpm ignition issue, then you would expect it to occur under no load as well. Check your fuel pump and filters. If it is not that, then perhaps the rebuild of the carbs was not perfect, although that seems unlikely, unless the radically re-jetted them or something. I suspect you are simply going lean at high revs under load. You might also check to see if perhaps you have a latent power brake booster leak. As the revs get higher under lead the vacuum in the intake manifold increases, if that is somehow initiating a vacuum leak, presumably in the booster, then that would also lean out the engine without the carbs knowing anything about it, thereby causing the bogging/misfire.

 

[Dick Steinkamp]

As the revs get higher under lead the vacuum in the intake manifold increases

Actually the opposite. Full throttle pulling hard there is near zero manifold vacuum. Highest vacuum is with a trailing throttle down hill. Idle and steady state cruise also prodices good vacuum. That's not to say there isn't a vacuum leak somewhere that is contributing to the problem.

 

[ScottAndrews]

Actually the opposite. Full throttle pulling hard there is near zero manifold vacuum. Highest vacuum is with a trailing throttle down hill. Idle and steady state cruise also prodices good vacuum. That's not to say there isn't a vacuum leak somewhere that is contributing to the problem.

Interesting. I questioned myself when I wrote that, but didn't think it through. If that is the case, then a secretive vacuum leak would not then present itself under open throttle conditions, so that points to fuel starvation. Either a fuel pump/filter issue, or a carb issue. I'm suspecting a carb issue if the fuel pump and filters have not changed since the rebuild...

 

[Stevehose]

Clogged fuel filter, bad fuel pump, leak in fuel hose at tank can do this.

Next on the list of things to check!

 

[Dick Steinkamp]

Next on the list of things to check!

Check the fuel flow out of the fuel pump. If you have good flow, on to the next potential solution. It beats replacing a lot of good parts if you don't need to. (ALL cars run on good used parts )