Mid-RPM, Medium-Load Missing (MADDENING)

[bradartigue]

Sprayed carb-adapter-manifold interfaces with carb cleaner: Engine dies.

As in you spray carb cleaner near the base of the carb and it kills the engine? That's an air leak and could cause the symptoms you describe. If that isn't it...

The range you describe is the transition off of the idle jet, where the carburetor will stop using fuel from the idle plumbing altogether, and start using just fuel from the mains. It is the only time you're using the secondary idle jet. If the secondary jet is too small or - depending on what car your DFEV was originally made for - nonexistant - it may cause a stumble. Normally when your primary main jet is too small and your idle jet is just barely big enough you get this kind of crappy mid-range "flat" and you tune it out by increasing the mixture or the idle jet sizes slightly.

[georgeramos]

I just "stumbled" upon this post because I am now experiencing the exact same thing. it was not happening before I did the following...

-Marked location and loosened the dizzy nut
-Fiddled with single point setup as i had noticed the car was running a little bit stumbly but not under load per-se
-loosened point setting screw to slightly widen gap (NOTE: when I tightened this screw I noticed it keeps turning, it does not get tighter until it no longer turns, not sure if this was the case before)
- As i was putting the rotor back on it crumbled so i replaced it with an older one I had around. this seems to fit and the car runs but it is black, not brown like the cap and the other rotor. it does say Marelli on it however...I will order a replacement

Now the car has a slight mis/stumble at 3.5 - 4.5k RPM's. Also think it backfired once loudly thru the exhaust.

Also noticed oil leaking from dizzy location after I loosened the nut which I had never seen before.

I guess what I'm saying in part is I think this is an ignition issue, not necessarily a carb issue.

I havent checked timing yet. Could my point gap cause an issue like this?

I just read another similar post and there was mention of a thin piece of metal that holds the rotor in place, is the absence of this something that might cause an issue? I did not see one.

[georgeramos]

...since i kinda hijacked the thread with my last posy (sorry) i will close my story by saying it was plug #1 who's ceramic insulator had vanished...

[opus10583]

See if this helps.
[...]
The image is too large for proper display here. So here is the basic link.

Thanks! That was helpful, removing and blowing everything through did not solve the issue, however.

Thanks again,
Mark

[opus10583]

The range you describe is the transition off of the idle jet, where the carburetor will stop using fuel from the idle plumbing altogether, and start using just fuel from the mains. It is the only time you're using the secondary idle jet. If the secondary jet is too small or - depending on what car your DFEV was originally made for - nonexistant - it may cause a stumble. Normally when your primary main jet is too small and your idle jet is just barely big enough you get this kind of crappy mid-range "flat" and you tune it out by increasing the mixture or the idle jet sizes slightly.

Hi Brad,

Thanks. Increasing the idle jets from 47/50 to 50/60, primary/secondary, didn't resolve, restored to baseline...

The only thing I haven't cleaned or blown out is the power valve diaphragm, which I will tonight/tomorrow.

Current jetting is

Idle 47/50

Fuel 137/140

Air 175/165

Thanks again,
Mark

[DanD]

I am trying to read this right. Are you saying you replaced the distributor with a different one, and now it doesn't run right ? Do you have the old distributor, and can you put it back on ? Did your old distributor have vacuum advance ? does the new one ? Is it hooked up ? Plug gap ?

Are you saying you spray carb cleaner at the base of the carb, and it stalled out ? Does it still do that ? Are you saying that you tightened all the screws holding the top of the carb on ? Did you warp the top of the carb by over tightening ? Does the carb have an accelerator pump diaphragm ? Is it split or leaking ? Is there a secondary idle circuit for the second barrel ?

A lot of times we assume the problem is in the carb, but often it is electrical or inadequate fuel supply. Most likely, you only have one problem, so if changing something doesn't help, go back to what worked before things got messed up. Confirm engine timing. Confirm ignition timing. Check plug wires, plugs, tune up parts, filters, etc...

If you have to rebuild the carb, put it on a bench, and clean it all out. Keep the jets organized. Only take apart what you need to to clean out the fuel passages.

[opus10583]

Hi,

Two distributors (original was rebuilt and is in it now), new cap rotor plugs and wires, fixed the carb seal, confirmed the timings, cleaned or replaced everything in the carb, as of last night, verified all vacuum plugs and connections.

Replacing the power valve diaphragm only made it worse

Thanks,
Mark

I am trying to read this right. Are you saying you replaced the distributor with a different one, and now it doesn't run right ? Do you have the old distributor, and can you put it back on ? Did your old distributor have vacuum advance ? does the new one ? Is it hooked up ? Plug gap ?

Are you saying you spray carb cleaner at the base of the carb, and it stalled out ? Does it still do that ? Are you saying that you tightened all the screws holding the top of the carb on ? Did you warp the top of the carb by over tightening ? Does the carb have an accelerator pump diaphragm ? Is it split or leaking ? Is there a secondary idle circuit for the second barrel ?

A lot of times we assume the problem is in the carb, but often it is electrical or inadequate fuel supply. Most likely, you only have one problem, so if changing something doesn't help, go back to what worked before things got messed up. Confirm engine timing. Confirm ignition timing. Check plug wires, plugs, tune up parts, filters, etc...

If you have to rebuild the carb, put it on a bench, and clean it all out. Keep the jets organized. Only take apart what you need to to clean out the fuel passages.

[opus10583]

Hi,

After two days of re-re-verifying everything from coil connections to cam timing to intake leaks, and resetting carb jetting back to (almost*) zero; the stumble's all but gone(*). Don't know what it was, and admittedly I've been changing multiple variables...

Now I've revealed another weak link: http://www.fiatspider.com/f08/viewtopic ... 63#p214001

Thanks to everyone for all the suggestions and guidance, I'm sure the problem was one of them!

Thanks,
Mark

* the original secondary air jet won't go back where it belongs (?), the one I replaced it with is too large and could be the reason for the slight hitch that still exists.

[sptcoupe]

Back firing in the exhaust indicates a rich mixture. This sounds like a fuel delivery problem. Have you checked to see what happens top your fuel pressure when it is missing? The best way is to get a shop fuel pressure gauge and hook it up, and tape the gauge to the windshield so you can watch the fuel pressure when the miss is happening. That said, I would go with an idle circuit issue as the culprit.