engine and drivetrain 72 high RPM running issues

[Chris_B]

If you have only the stock coil with no other "black" boxes, then you probably have a basically stock ignition system. How do you have a pertronix system? Did you install it? Do you have an adjustable timing light?

[snowy]

Petronix came with the car. No timing light.

[72_02]

My issue has basically gone but I will admit I paid a garage.. I was sick of it. Let's see how long it lasts!

 

New plugs and coil. Tuned carb. Smooth now, very responsive, buuuut it does still feel like the odd misfire or something, cruising isn't ultra smooth.

 

Maybe it's just old. Not sure.

[Chris_B]

Snowy, you are probably going to need an adjustible timing light to work on this problem. Are you up for that? $30 or so on ebay or amazon. Are you up for that/
 

[rey949]

I had the same problem. It ended up being my rotor. After we installed pertronix the rotor was a little too tall and as I hit 3800-4K Rpm it would just lose power.


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[snowy]

Ok I'm picking one up from autobahn tomorrow, adjustable timing light that is.

so Rey949 did you just swap it out?

thanks 72 02 I may end up doing that but haven run out of patience yet, I know the feeling well.

[PaulTWinterton]

High rpm stumble is quite often due to fuel starvation.  Have you checked all fuel filters? Especially in the fuel tank.  This one is especially suspect.

[71bmr02]

RE: rotor button,  I was suffering much the same problems.  I checked out everything, carb, timing, valve adjustment changed distributors. spent hours on this and finally decided to check rotor button.  Using a multimeter i disovered there was no continuity from end to end on the button and it was just the garden variety button not the rev limiter style.  replaced the button and all is well.  I had never seen that type of button fail in that manner, there must be a resistor under all that epoxy.

 

Gale H

[Buckeye]

2 hours ago, 71bmr02 said:

there must be a resistor under all that epoxy.

 

yes there is one.

 

 

['76mintgrün'02]

Rotors also come with 1,000 Ohms of resistance.  It often says R1, or R5 on them, referring to their resistance.  Either should be fine.  (edit, R5 is stock and I'd stick with that.  Resistance here affects the spark duration, so I have read).

 

Resistance is a cumulative thing though, so if you also have spark plugs with built in resistance, (there is typically an R in the plug model number with them) and add to that spark plug wire connectors which have more resistance built in, you could start seeing problems.  I learned this from Ray_'s former running mate, so I consider this the Truth...

 

rey949 brings up a good point as well.  Sometimes the magnetic ring is not slid down all the way on the shaft and does not leave enough height for the rotor to properly seat.  Is your rotor sitting down hard on that ring?  If so, measure the amount of shaft above the ring, as well as the depth of the inside of the rotor, to make sure it is all the way down.

Tom

 

P.S.

If you find that the ring is fully pushed down on the cam portion of the shaft and the rotor is too tall to fit above it, you can shorten the rotor fairly easily, assuming it does not need more than about an eighth of an inch removed.  There is a metal spring inside, which holds it tight to the distributor shaft.  It may be easier to pull that out, while shortening the rotor body.  

 

I shortened one when I added a dwell adjusting mechanism to my points, which raised them an eighth of an inch.  I made a fixture which the rotor slides down into, leaving the part to be removed sticking out, so it can be sanded off.  The 'fixture' was just a piece of 3/4" plywood with an 11/16" (17mm) hole (iirc) drilled in it and a slit added, through the hole, to allow it to open up and accept the rotor.  That also allows it to be clamped tight on the rotor for sanding/shortening.  If this is what you end up choosing to do, I will post photos... if needed.

 

Edited July 16, 2020 by '76mintgrün'02

[Chris_B]

The clogged fuel line theory doesn't make a lot of sense. If you start your motor and let the car idle for a few minutes, the bowls should fill up with fuel in each carb. The fact that you can drive the car away and it runs ok if you keep it below 2500 RPM proves that you have fuel in the carbs. On the other hand, if you start the motor and let it idle a bit, then rev it up to the 4K point and if it immediately cuts out, that can't be due to a fuel delivery problem because there should be enough fuel in the bowls to allow an unloaded motor to rev up to any RPM level, relying on the fuel in the bowls. I hope that makes sense.

 

You should check your fuel pressure at the carbs- you want to see 3-4PSI. If you don't have a fuel pressure gauge (buy one, they are cheap), then just do a flow test by disconnecting the hose and running the fuel into a can. I am sure you know that gasoline vapors are explosive...

 

BTW, there is no filter in the fuel tank- it is a screen at the bottom of the fuel gauge float chamber, and it can indeed become clogged (often with rust flakes). You may also want to check the screens at the fuel intake on the carbs. Unscrew the 19mm brass bolt on top of the carb and check the plastic screen.

 

[snowy]

Thanks for all the rotor info and will get into that today. I've emptied the fuel tank and cleaned the filter. The fuel lines are new but I've blown out the fuel line with the compressor and the breather, so all good there.

 

[72_02]

This is a theory but the garage says it's running well, which is right, but as soon as I hit an incline I have the issue.

 

It wouldn't be something as simple as the angle of the fuel tank would it? For example mine is pretty solid on flats and great downhills, even at high rpm, but with a hill it nearly always dies unless I really nurse it.

 

Itll pull strong up to 80mph in 3rd and 4th but the hills, doesn't matter what gear; it'll have trouble.

 

 

Edited November 27, 2016 by 72_02

['76mintgrün'02]

The float in your carb pivots in such a way that the incline could affect it, although it shouldn't matter.

I'd suggest that you take a close look at your float.

Weber floats come in either brass or plastic.

The plastic ones can become saturated with fuel over time/heavy and sink.

The brass ones can leak and fill up with fuel.

The two styles also have different height settings, which could have been confused.

That is where I would start.

The float comes off with the top of the carburetor, so you do not need to remove the whole thing to check it out.

Tom

[72_02]

21 hours ago, snowy said:

Thanks for all the rotor info and will get into that today. I've emptied the fuel tank and cleaned the filter. The fuel lines are new but I've blown out the fuel line with the compressor and the breather, so all good there.

 

Snowy (sorry to hijack thread) did you have a single plastic inner/rubber outer hose from tank to inline filter in the engine bay on your car? Do you mind sharing the genuine part number if so?

 

My car has a single line the entire way through which I will replace to try eliminate any air leaks.. BMW site says 32 dollars - this the correct line?

 

Mine says BMW 8x13 on it.