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Cursing issues at 40mph


PeterCS

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While there's already plenty of excellent advice provided above that will likely solve your problem, your description of the situation reminded me of a similar problem I had.  Turned out that I had set the ignition timing embarrassingly too far advanced.  Surprisingly, it seemed to run fine under load and higher rpm and didn't make any knock sounds. I made this mistake while using my brand new Innova timing light for the first time.  Took me quite a while to figure out how make the low speed rough running go away., but once I set the timing correctly, everything was fine.  

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Funny- I was having

Cursing issues at 40mph

yesterday trying to get home...

 

Sunday Drivers came out a day early...

 

t

 

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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I'm having the exact same issues with my car currently, including some sort of misfire/backfire/afterfire under the hood at steady state speed. I recently rebuilt my 32/36 with a sync link, and replaced jets with c.diesel's prescription, as well as intake manifold gasket. I suspect I'm getting a vacuum leak from somewhere but I haven't fiddled with anything other than the carb. Runs hard under full throttle, jerks like Steve Martin at cruise. Could it be the base of the carb?

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Do either of you have vacuum advance (or retard) distributors installed?

 

The vacuum feature cuts out when the throttle is opened up and pulls in more advance (or retards it) at light throttle cruise.  I guess it also depends on what vacuum source you're using.  (ported vs manifold)

 

40 minutes ago, 1dollar93 said:

I recently rebuilt my 32/36 with a sync link, and replaced jets with c.diesel's prescription

 

I think you might need to alter the jetting to make the sync-link work well.  There's a thread or two on that topic in the archives.  The c.d. prescription ran too rich for me (based on AFR readings) until I added a larger primary air corrector.  I'd imagine it is quite rich if both barrels are opening simultaneously.  

     DISCLAIMER -- I now disagree with much of the timing advice I have given in the past.  I misinterpreted the distributor curves in the Blue Book as timing maps for our engines.  I've also switched from using ported-vacuum to manifold, with better results.  I apologize for spreading misinformation. 

(3-28-2024)  

 

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9 hours ago, '76mintgrün'02 said:

Do either of you have vacuum advance (or retard) distributors installed?

 

The vacuum feature cuts out when the throttle is opened up and pulls in more advance (or retards it) at light throttle cruise.  I guess it also depends on what vacuum source you're using.  (ported vs manifold)

 

 

 

I had this issue on a non-bmw where the vacuum advance was advancing the timing too much at cruising speeds. Simple to check - unhook and plug the vacuum hose to the distributor and see if that makes any difference.

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11 hours ago, 1dollar93 said:

I recently rebuilt my 32/36 with a sync link

 

here's some reading about jetting 

 

 

AFR gauges are awesome tools.  (hint)  I thought I might use mine to get the jetting sorted and then remove it, but I really enjoy having that feedback while driving.  It is the second most important tool I bought for my car, after the variable advance timing light.  (hint)

 

Tom

     DISCLAIMER -- I now disagree with much of the timing advice I have given in the past.  I misinterpreted the distributor curves in the Blue Book as timing maps for our engines.  I've also switched from using ported-vacuum to manifold, with better results.  I apologize for spreading misinformation. 

(3-28-2024)  

 

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Did the car “jerk” after you had the engine rebuild last year or is it since the tune up last week?

Did you replace the motor mounts at the time of the rebuild?

Are you running the stock rod accelerator linkage?

I ask because I had a similar issue after my engine overhaul with larger pistons/cam which caused a larger than usual “bounce” in my engine between the mounts during various points of acceleration/deceleration that I was only able to cure by converting to a cable accelerator linkage system and getting new mounts.  On acceleration, the engine would lean over to one side and stay, but as I let off the pedal slightly at cruise, it would bounce back and forth causing a jerking surge.  If your car was running fine between last years rebuild and the last weeks tune up, then I would look to the suggestions above.

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On 8/20/2022 at 2:50 PM, PeterCS said:

The car has an electric pump because I put in a tii engine. 
I restored this car 15 yrs ago. It has never driven well. So I broke down and had the engine rebuilt during the pandemic. Was hoping that it would solve the problem. 
the carb was new 15 years ago an I doubt that it has more than 1500 miles on it. I think my intake manifold is standard. 
keep the ideas coming. 
thx, 

PeterCS

 

 
Has this carb been rebuilt or cleaned since it was new ?

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This thread needs more feedback.  

 

Photos add interest as well.

     DISCLAIMER -- I now disagree with much of the timing advice I have given in the past.  I misinterpreted the distributor curves in the Blue Book as timing maps for our engines.  I've also switched from using ported-vacuum to manifold, with better results.  I apologize for spreading misinformation. 

(3-28-2024)  

 

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