Engine hesitation at around 3500 RPM

[Fraidknots]

Hi all,

 

Would appreciate any input.  I have a 74 2002 I recently purchased and overall, the car is in amazing shape.  I have sorted out many of the gremlins over the past few months but there is one thing I can't figure out.  At around 3,500RPM, the car will hesitate, almost like it is misfiring. SO far, I have done the following to try to fix the issue;

 

• Replaced the spark plugs
• Replaces cap and rotor
• New Bosch ignition coil
• New 8MM high performance spark plug wires

 

The car fires up instantly and once past that hum, will run all day.  Any ideas as to what may be causing this?

 

Thank you.

[Schnellvintage]

When was the last time the points were replaced and dwell was set? Or what type of ignition do you have in the distributor? Also at what load is the engine under at 3500 rpm when the miss occurs. Full throttle, cruising or deceleration? It could be carb related.. also. What did your old parts look like when you replaced them?
Matt


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[Fraidknots]

HI,

 

The old parts all looked fine, actually none of it really needed to be replaced but since the car had been sitting for a while, I figured it was money well spent.  

 

• The distributor has been upgraded to electronic ignition and looks fairly new
• The car is in acceleration mode when the misfire happens.  I have to back off the throttle, downshift to get the RPMs high and then work through that range'
• If I accelerate very very gently, the misfire seems to be better

Could this be a timing issue or a fuel starvation issue?  Would it make sense to replace the fuel pump?

 

Thanks

['76mintgrün'02]

Which carburetor do you have?

 

Which distributor model?

 

Is the electronic ignition a Pertronix unit?

Does it have the proper resistance (3 ohms)?

 

This statement you made in another thread is a little confusing...

"The ignition system is all new.  Switched to electronic ignition with new points and coil."

https://www.bmw2002faq.com/forums/topic/231196-speedometer-reading-to-high-and-tach-is-bouncing/?do=findComment&comment=1295837

 

You are saying that the problem is only at 3500 rpm while accelerating?  Fine at rpms above that?  Fine if cruising at 3500?

 

I'd say stop buying parts (for now) and buy tools.

A variable advance timing light will tell you how  your distributor is behaving.

An AFR gauge will tell you where you have fueling issues.

 

[Fraidknots]

a webber 30/38 i think

 

[adawil2002]

If you are using the original distributor, that's where the advance stops. Send it to be rebuilt and recurved to your carb. Can go with a 123, the internal seal will fail at between 16-30K miles and will be fixed under warranty, they'll say the engine has too much crankcase pressure. 

[joebarthlow]

check timing, dwell.

 

How do your points look, assuming you are running points with stock distributor?

 

Mechanical fuel pump or electric?

 

Identify the problem before throwing money at expensive bandaids.

[Fraidknots]

Updated electronic distributor and points look good.  Mechanical fuel pump.

[Simeon]

13 minutes ago, Fraidknots said:

Updated electronic distributor and points look good.  Mechanical fuel pump.

 

Again, this is the point that needs clarification. Typically if you have an ‘electronic’ distributor then you will not have points. You can trigger an external electronic ignition using a set of points but then the distributor isn’t electronic. 

 

We we just need to be clear on what you have here as your symptoms sound a lot like a dwell problem, points problem or even a failing condenser. 

 

Another question: what are your ignition leads, cap and rotor like?

[FunElan]

My 32/36 has stumbled for years. We recently took out the Pertronix electronic ignition in favor of points. Runs much better now - set the gap to .016".

[worzella]

Might be way off but isn’t 3500 about when the second barrel kicks in... at least on a 32/36?

 

But like I said I just recall previous discussions on fuel starvation up around thar RPM when people had bad jets/issues on second barrel

[Fraidknots]

Ok, let me try cleaning that all up and see if there is any improvement

[Mike Self]

If you have a Weber 32/36 carb, that's about the time the second barrel starts to open, so make sure the linkage is allowing that barrel to open.  You didn't indicate that this occurs while you're accelerating smartly through the gears, and that at 3500 rpm, regardless of gear, it stumbles.  If that's the case, with the engine off, peer down the carb throat and work the accelerator linkage quickly.  You should see a spurt of gas emerge from a small nozzle in the primary barrel.  If there's no squirt, or just a dribble, the accelerator pump or its passages are at fault--either the diaphragm is toast, or a passage is clogged.  Replacing the diaphragm is pretty easy, and you can buy just the diaphragm (without buying the whole rebuild kit) from O'Reillys.  

 

If you have a 38/38, do the same test, but look for a simultaneous squirt into both barrels.  

 

Hope that's the problem, as it's an easy fix.

 

mike

[zellet]

I suspect carburettor jetting (Weber?).
Also, weird gasoline changes this time of year (summer gas vs. winter gas) could be an issue.
I’ve had “dead spots” in my throttle over the years.
Usually carb-related.


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