Rough Running Engine

[JFB]

Hi folks - I am new to the forum.  My son and I have a 69 Bronco that we've been fixing up over the last year.  It has a 302 that until recently has been running great.  We've replaced : distributor, plug wires, plugs, and coil. It is bored .030 over and we honed cylinders and installed rings and rod bearings - no issues with smoking or plug fouling.

So - suddenly it has developed a severe rough running condition - hard to start and will only run very rough at 1/2 throttle, will not idle.

Here is the weird part - yesterday I started it ( had to use starting fluid ) and it was doing its usual rough running  - but then suddenly cleared up and ran perfectly for 2 minutes - then just as suddenly back to the rough running.

I has an almost new Holley 2 bbl that is getting fuel, pumps squirt and the fuel pump is pumping ( at least it squirts with the line off and engine cranked ). 

At first I thought it might have 'jumped time' - but I checked the rotor position at TDC # 1 and it looks really close + if it has jumped time, how to explain the sudden smooth running for a brief period ?

Anyway - I am at a loss for what to try next.           Any similar experiences or advice would be greatly appreciated !

Thanks,  John

 

[Rons beast]

Hey John, welcome.

Look for a vacuum line that may have a small crack...but most likely is a carb issue, with something foreign floating around and plugging jets and /or passages.

Good Luck

 

[JFB]

Hey Ron - thanks for the reply.  The carb is a Holley 500 2 bbl that has the float bowl on the front with the metering block right behind it.  I took the float bowl off - it looked clean - and then took off the metering block, blew it out as best I could.  I don't see any way to get into it to check the emulsion tubes ?  There are a couple of brass plugs in the top of the block that don't appear to be removable.    Do you know of any other cleaning operations I can do to this thing ?

Also checked the float level and fuel is right at the bottom of the inspection port - so I guess that is right.

Still searching !!

John

 

[Bully Bob]

Hi John,

Ditto Rons suggestions and, I would add;

Timing chain/gears do what you're describing. Key off.., turn the crank nut while watching the

rotor in the dizzy..!  They should move at the same time, or very nearly so. (do this back-n-forth)

If there's a lot of slop.., that could be your problem.

Auto or stick..? Check the rubber hose on the modulator (if auto)

Keep us posted.......... B)

 

[JFB]

Bob,

It is a 3 speed ( dreaded column shifter, but that is another saga ) -  I'm digging back into the carb - don't think vacuum leak is the problem ....... but here's the question :

if it is cam / distributor timing related to worn timing chain or jumped tooth - can that appear as an intermittent problem ?  I'm still puzzled by the short episode of 'perfect running' that  I experienced yesterday.

Thanks,

John

 

[Bully Bob]

I drove my '73 to the bank (some time back).., it ran good & smooth.

Came out, it wouldn't start. Towed it home, did the test, replaced the chain/gear set,

It started right up & ran perfect ever since. ^_^ /emoticons/[email protected] 2x" width="20" height="20" />

It acted up a couple times here-n-there prior to the bank trip.

Yours still could be something else. (all we have is gas, air, & electrical..., not likely air problem)

A vacuum gage &/or a timing light could detect radical timing/firing.

 

[genthree]

Did you check the fuel filter?  My 73 didn't even have one....

 

[JFB]

Hey Genthree - yes has a new fuel filter

Here is an update:

New timing chain/gear set ( old one had marks lined up and was not worn that badly )

New plugs and wires

Changed carb to a rebuild Autolite 2100  - no change

Compression on all 8 cyliners

Hot wired directly from battery to coil - has spark from coil to distributor and tested each plug for fire

Fuel pump 'squirts' fuel when disconnected from the carb

Checked distributor timing - rotor points to # 1 on the compression stroke

Is it possible for the fuel pump to seem like it is pumping but actually not be delivering enough fuel to keep it running ?

The accelerator pump squirts fuel when looking down the carb and working the throttle.

This thing about has me whipped !

John

 

[Bully Bob]

John..,

You may be here for a while gett'n this solved. It would benefit you to fill in your signature/info pages complete.

That way we, & others know at what point your at & whats been tried.

You see ours below & left of ea. post.

Trust that more folks with info./help will "jump-in" knowing the history, vehicle., eng., etc.

"We've replaced : distributor, plug wires, plugs, and coil."

This should be entered but be more specific.., like what dizzy.., what coil brands.., new--used.?

At this point., I'm more suspicious of the ignition.

 

[genthree]

Could it be a broken ground wire?  

 

[Bully Bob]

Yep.., wiring is always a suspect.

This reminds me of another test. 

Run a jumper wite from the POS.  on the battery to the + side of the coil.

Start engine.

Should it run normal..., the wire in the "run" circuit is likely the issue.., or maybe the key switch.

YOU WILL need to yank the jumper wire to get eng. to stop running after key is turned off.

CAREFUL, that wire is "hot"  (installing & removing)