engine and drivetrain Sudden power loss, rough idle and shudder through 2,500 RPM

[Smitty]

Hope you guys can help diagnose this problem. For background, the car is set up as follows: 1976 2002 with 320i 5-speed, rebuilt M10, 292 cam and Weber DCOE carbs. It was 60 degrees outside yesterday and the car had about a quarter tank of gas and fresh oil.

I was driving on the freeway at approximately 60 mph in fifth when the engine began to lose power. It would not hold speed in fifth but would maintain 50 mph in fourth (just). It could build revs slowly but was very rough through the area of 2,250 rpm, after which normal smoothness returned (albeit with a third of normal power).

I exited the freeway and pulled to a stop with the intention of limping to a nearby parking lot. The engine would not hold idle and was extremely rough until around 2,250 rpm. I shut the car off and had it towed.

Ideas? Thanks!

[jdm1602]

Sounds ignition related, still running points? Fuel pump O.K.? Let me know if you want some help I'm in Markham and have lots of spares.

[Mike Self]

carb problem. However, if the ign checks out, remember that the idle circuit on your carbs provides most if not all the mixture up to about 2200-2500 rpm; thus I'd suspect a problem in the idle circuit. You may be getting too much fuel (a sticking float in one carb) causing an over rich condition and flooding one or more cylinders.

Lots of black smoke at low rpms? Tailpipe and plugs coated with soft black deposits? That's usually a sign of an overly rich mixture.

Conversely you may have experienced a sudden vacuum leak. Vac advance dizzy? check the vacuum advance diaphram for leaks. Remove hose from manifold, pop the dizzy cap and suck on the hose. You should feel resistance and see the point plate move.

Also check any unused vacuum ports in the manifold or on the carbs to see if a cap has fallen off. A very lean condition would be accompanied by excessive pinging/detonation.

If it happened suddenly as you stated, it's probably gonna be something relatively easy to track down.

Happy troubleshooting

mike

[pikeperkins]

This (sorta) happened to me after I put pertronix in my car. The little(!) bolt that holds down the disk to the dizzy had backed out and consequently the timing was all over the place. Good luck.

[Smitty]

Thanks all for the replies! I've been very busy and didn't have the opportunity to work on the car last week, but hopefully I'll get some time soon to diagnose this properly. Would be very relieved to discover that it's just the jets!

I'll post up the resolution. Thanks again.