engine and drivetrain Drivability/hesitation Issue

[slowguy]

Folks,

 

I’m trying to diagnose and fix a drivability problem with my 1974 2002 – a hesitation that occurs in very specific circumstances.  The car has over 150,000 miles without a motor rebuild.  The current carb is a 38/38 Weber, installed by a prior owner.

 

This is the issue.  I autocross the car, which means that it sees a series of 30 second to one minute runs in first and second gear at relatively high RPMs.  For the first two or three runs, all is well.  Once the motor has warmed up (the temp gauge is reading above the blue zone but not above half-way) I get an intermittent lack of power for the first 15 seconds of the run.  The car launches well, and I short shift to second before hitting red line in first.  Then I get about 10 to 15 seconds where throttle response is erratic and the car seems to have much less power when I ask for it.  After that, the problem goes away for the rest of the run.  An autocross course is by definition twisty, but I can’t association the problem with specific lateral accelerations or hard braking.  Nor do I hear any obviously missing ot backfiring from the motor.  For the next run (and the next and the next, assuming that the car had not cooled down), the problem persists in exactly the same way.  The issue only occurs in this context – never in driving on the street in any gear at any speed, and never in the later parts of an autocross run.

 

When I first got the car (a couple of years ago), it had an occasional rough running issue in street driving, particularly under acceleration.  Since then:

-- I had a compression test performed, which showed no issues,

-- I replaced the spark plugs,

-- I replaced the distributor with an Ireland Engineering unit with an electronic ignitor,

-- I replaced the ignition coil,

-- I had the carb checked for blocked jets, etc., which showed no issues,

-- I replaced the ignition wires with an Ireland Engineering 8mm set.

 

The prior owner had had at least some of the fuel lines replaced after they clogged.  I don’t know if the fuel pump was replaced at that time.

 

The new distributor and coil cured the minor drivability problems on the street, but didn’t solve the issue on autocross runs.  The two most puzzling things (to me) about the problem on autocross runs is that it only occurs after the motor is warmed up and it only persists for the first part of the run.  I would be very grateful for any ideas as to what could be causing this issue.

['76mintgrün'02]

I replaced the distributor with an Ireland Engineering unit with an electronic ignitor

 

I would bet money on the electronic ignitor.

I would wager the cost of a set of points and a condenser.

I would also install a vacuum advance distributor (curved to suit your engine).

 

You could simply try another ignitor, (if you are fond of that feature).

 

edit:

There was a thread recently where a faulty Pertronix unit was misbehaving once warm.

http://www.bmw2002faq.com/topic/159339-once-warm-stumbles-tach-bounces-dies/

Edited November 9, 2014 by '76Mintgrun'02

[kbmb02]

I would also install a vacuum advance distributor (curved to suit your engine).

 

There are benefits and applications where a vacuum advance distributor can be helpful; however, in my opinion, auto-x is not one of them.

 

As I've been taught, the vacuum advance on a distributor functions as a light throttle mileage maker, by increasing the ignition timing under light throttle cruise conditions (high vacuum). The increased timing improves engine performance/reliability.

 

As for the second part "(curved to suit your engine)", I'd agree that curving a distributor to match the engine build and usage can be quite helpful. -KB

['76mintgrün'02]

I appreciate the clarification.

In my 'daily driver mindset'    Carburetor + Vacuum = Happy    

 

My addition of the sentence you quoted was self indulgent.

It's not like the lack of vacuum advance has anything to do with the problems at hand.

 

Limited Knowledge + Soap Box = Embarrassment.

Edited November 10, 2014 by '76Mintgrun'02

[jireland2002]

Possibly a float level issue, but I suggest you check the needle and seat.  You should be using the 250 size not the 200.

 

[Beaner7102]

Possibly a float level issue, but I suggest you check the needle and seat.  You should be using the 250 size not the 200.

+1 for carb issue, not ignition. But I would still check both. Swap in some points or a new Pertronix and replace the needle & seat and check float levels.

 

Beaner7102

[slowguy]

Thanks for the ideas, folks.  I'll definitely get the carb looked at as suggested.  One other thought -- could this have anything to do with the rev limiter?  I gather that '74s had them, but that's the extent of my knowledge.

I'll also think about swapping in another distributor -- but the fact that the problem has persisted through two distributors inclines me to look at other things first.

['76mintgrün'02]

Have you been using the same Ignitor module through all your trouble shooting efforts?

[peterschop]

The rev limiter is built into the rotor which should be replaced with a non-rev limit rotor. I'm thinking that you are starving for fuel. When was the last time the fuel filter was replaced? Or maybe you have a sticking float in the carburetor. Also make sure you have the 250 needle and seat in the carb like jireland2002 said.

 

My experience with a bad ignition ignitor is for it to fail when hot and not work again till it has cooled off.

[Hans]

I'm with Peterschop: fuel filter, fuel pick up screen in tank. Do you have electric pump? If manual pump, check for grunge. 

[slowguy]

'76Mintgrun'02, I'm not sure what you mean by "ignition module?"  I did replace the distributor (with a tii electronic version from Ireland Engineering) and the coil.  The specific problem I'm describing did persist after the replacements.

 

It sounds like the weight of opinion is tilting towards fuel delivery, with ignition in second place.  I'll check the carb and jets over the winter, but will have to wait for next year to retest on an autocross course.

 

Thanks to all who commented.

['76mintgrün'02]

I replaced the distributor with an Ireland Engineering unit with an electronic ignitor

 

I was referring to the "Electronic Ignitor" 

Sorry if my hunch was misleading.

[peterschop]

Before you open the carburetor, check the fuel pressure and flow after the pump and before the carb.