Sudden change in engine behavior

[Flycast47]

1977 Bonanza 350 Auto -  Truck has been running fine until yesterday. I pulled into the driveway after a very short drive and put it into reverse and the engine immediately began to act as if was running on 2 cylinders.  Backed up and put in park.  Engine was idling a bit rough and when I pressed on the accelerator in seem to rev up very slowly.  Held the engine at about 3500 rpm is sounds rough and is popping through the exhaust.  It is drivable with difficulty.  Accelerates very slowly or not at all with part throttle.  Giving it full throttle will eventually get up to speed.  Checked the carb and the choke is opening.  Appear to have spark to all plugs (checked by clipping timing light to each plug wire).  I think it is a carburetor problem but not sure.  Carb is the original Q-jet and probably never rebuilt.  Any help would be appreciated.

[zieg85]

Has the timing chain ever been changed?

[Flycast47]

"Has the timing chain ever been changed?"

Unknown but not likely.  But this really does not feel like that type of problem because the reaction I get with the throttle open changes.  The engine revs fairly normally under no load except for the exhaust popping.

[Flycast47]

Any other thoughts on this problem?  I have ordered parts to replace the distributor cap, rotor, pickup and control module.  We'll see if that changes anything.

[bd]

Check the ignition timing and make sure the distributor isn't loose and spun around.

[BBM3]

Has the timing chain ever been changed?

I'm with zieg85.
Sounds like the timing chain has slipped.

First, what bd suggested. Grab the distributor and see if you can rotate it.
If the distributor is loose.... bd for the win.

Edit: Fixed spelling

[Stewart G Griffin]

Any other thoughts on this problem?  I have ordered parts to replace the distributor cap, rotor, pickup and control module.  We'll see if that changes anything.

Is this the original engine?

[Flycast47]

Is this the original engine?

Yes it is.  It has about 160,000 miles on it.  The truck was used on a ranch for a number of years and then parked for many more.

[Stewart G Griffin]

Is this the original engine?

Yes it is.  It has about 160,000 miles on it.  The truck was used on a ranch for a number of years and then parked for many more.

In that case i'd like to give my .02 cents;

Being that the engine is 38 yrs old, it may need a rebuild.  One or both heads may be warped more than .0003 to .0005 inch. 

My 305 just gave up the ghost last October.  Granted i did some stupid things, but i think the fact that it was 36 model years old had something to with it as well.

Also, you may want to remove the valve covers to see if any pushrods are bent or if the valves are ok.

[Flycast47]

Update:  Replaced distributor cap, rotor and ignition module and checked the timing.  Timing is OK but no improvement after the distributor repair.

 

Being that the engine is 38 yrs old, it may need a rebuild.

You could be right but it has ben running so well that I was hoping that is not the case.  I took it a reelable local shop that knows old trucks.  I had planned to do this anyway to get the rear main seal replaced and fluids changed.  I will provide an update when they figure out what's wrong.

[78BIG-TEN]

sounds loke carb flooded and fouled spark plugs,or possibly a stuck egr

[Flycast47]

The shop identified the carburetor as the problem.  They were not specific but implied that it was inadequate and erratic fuel delivery. I had planned to replace anyway and  I have ordered a replacement Q-jet from SMI (Sean Murphy Induction) in Huntington Beach, CA

[truckbabe]

was the carb getting enough fuel? have you changed the fuel filters? Fuel filters get junked up and restrict the flow to the carb. Just a thought if you still have the same symptoms after you put on your new carb.
Good luck

[Flycast47]

was the carb getting enough fuel? have you changed the fuel filters? Fuel filters get junked up and restrict the flow to the carb. Just a thought if you still have the same symptoms after you put on your new carb.

Thank you for the suggestion.  No I have not changed the fuel filters but will be sure to do that before I install the carburetor.