engine and drivetrain Hunting And Misfiring After Some Work

[jjbunn]

Getting the hang of the new site - looks snazzy!

 

I'm trying to fix a poor running condition, and would like some suggestions, please.

 

Engine starts from cold, goes up to ~2500 rpm, oil pressure is 60psi, stays around those revs until the temperatures rise, the water choke opens up the top butterflies, and then the revs fall to around 1000rpm but then go up and down every few seconds between 1000 and 2500. There's also some missing: the engine is shuddering a little in its mounts. Oil pressure is around 45psi at this point. Water temperature in the bottom third of the gauge.

 

Recent changes I've made:

 

1) new plugs gapped to owner's manual specs

2) valve clearances all set to 0.007 (cold engine)

3) static timing set using TDC mark on camchain sprocket with distributor rotor pointing to #1 plug

4) new valve cover gasket

5) new 20w50 engine oil and filter

6) removed and cleaned carb, replaced with idle screw bottomed out then backed out 1.5 turns

7) removed and cleaned mechanical fuel pump

8) replaced fuel filter between pump and carb

 

Once the engine is warm, I've tried loosening the distributor and slowly rotating it this way and that to try to improve the running, but no luck.

 

I feel like I may have missed something, or made some stupid error, so even the most basic advice is welcome 

 

 

[ahlem]

Sounds like you are running a bit lean.

Did you change the jets or shake something loose when you changed the idle screw?

What carb do you have? Did you try dynamic timing?

[jjbunn]

Sounds like you are running a bit lean.

Did you change the jets or shake something loose when you changed the idle screw?

What carb do you have? Did you try dynamic timing?

 

No, I haven't changed the jets - I just cleaned everything on the carb and replaced it all. The carb is a Weber 38 DGES (I think that's the model number).

 

I haven't tried dynamic tuning, as it seemed pointless when the static timing is so bad?

 

If I'm running lean, then wouldn't that be cured by screwing in the idle screw some? I did try that, but then the engine wanted to stall.

 

I'm wondering if I have a vacuum leak, but I see between 20 and 30 at idle on my vac gauge, which seems alright.

 

Thanks for the reply!

[path88]

There's also some missing: the engine is shuddering a little in its mounts.

 

I got the same thing going on.  Not sure what to assume, but I'm actively trying to diagnose.  Suspect ignition, but can't find any fault there yet.

Edited May 10, 2013 by path88

[Chris_B]

Here are a few questions/thoughts on your problem:

 

When did this problem start? Was your motor running acceptably at some point? If so, what was the first change you made before the problem started? Where did you get this carb (e.g., was it on the car when you acquired it, did it come from an outside source, etc). .

 

On the 38 DGES, you should have an idle mixture screw on each barrel. After warmup, you should adjust your idle speed screw so that the motor barely idles, then adjust each idle mixture screw so that you have the optimal idle, and then back it off slightly.

 

Does your carb have idle solenoids? If so, check that they have power and are working. clean/replace if necessary.

 

Dynamic timing adjustment is not a bad thing. Start your motor and turn your distributor in the direction of best idle. Then drive the car to make sure it doesn't knock under load.

[Hans]

Using the notch on the distributor and rotor is fine as long as they were both installed correctly in the first place. What you might do is use a 12V test light to set static timing - instructions are in Haynes I believe. That should get it running.