The Stovebolt Site Forums The Shop Area Engine Shop 261....mild rumble....?

I am having a problem fine tuning a 261 and describing the "hesitation' ...puff....rumble... by email is difficult but here goes....

Totally rebuilt 261 (truck eng. with solid lifters)with less than 50 hours of running.

Oil at 40 psi...
vacuum at 22 in. and steady
....idles well around 425-450 rpm
Pertronix ignition....
not running hot...
revs up good.....
Std Rochester B carb totally rebuilt and adjusted on a flow bench
Using clean gas from a removable outboard motor plastic tank and a filter in line...
10W30 DIESEL MOTOR OIL with two remote
PH 8 oil filters

Symptoms...

slight ticking sound from valve train probaly one of the front cylinders on the loose side..

...the part that annoys me no end is a slight rumble hesitation in the exhaust pipe.....not a steady hesitation but a slight puffing sound that comes and goes without a specific repeat pattern... almost as if a cylinder was not firing completely...

Could I have a valve adjustment problem.... if one is clicking could I have one not closing properly but then it has to be interminently...?

I have tried removing one plug wire at a time but did not noticed any significant changes... now to be honest I may have to repeat that process with a pair of appropriate rubber gloves as the extra voltage from the Pertronix coil gives you repeated wake up calls....

Am I missing something else...overlooking the obvious...?

Comments suggestions...?

Bob C.

RAC, Does the engine have a cam intended for solid lifters or have you just used solids on the stock hyd lifter cam?
Cams ground for solid lifters have a clearance ramp ground into the profile to take up the specified clearance gradually before the valve opening starts of course hyd lifters are not supposed to have clearance so their cam has no such ramps.
Solids used on a hyd lifter cam should have the lash set very tight usually .002-.004 (hot)to try and duplicate the no lash condition the cam was designed for.The valve timing will be drastically changed if solid lifter cam clearances are used and the engine is in danger of valve breakage and seat/face damage from the valve not being set back on the seat gently.Excessive noise goes without saying.

Another thought: timing point at idle.I have been able to help people with a sound engine seeking a smooth regular idle and hearing these irregular misfire/sput/valve leakage type sounds at idle.Many times they have timed the engine by "ear" to get more pep or acceleration at low speeds and have the timing advanced too far.For a stock cam designed for smooth low speed and idle characteristics if spark is advanced more than 4 deg from specified the intermittant misfire is likely.The tricky ones which had you setting the timing with vacuum line unhooked because they had manifold vacuum to the advance chamber will probably jump to over 20deg when the line is hooked up and the questionable idle will be there.
Test my theory.Use a timing light to get an idle setting of 4 deg BTC after verifying that the mark is still very close to where it belongs if it is on the balancer and see if you hear a difference.Maybe even retard it another little bit to see what it sounds like.
If you want smooth regular idle and snappy response too you may find that getting the service of a mechanic with a distributor machine to build a custom advance curve is the way to go.
Compromises abound when we start "tweaking" these engines for our entertainment--but ain't it great?

Did the truck 261 in Canada come with hydraulic lifers/cam?

The automobile 261 in Pontiacs in Canada came with hydraulic cam/lifters.

Thanks for the feedback guys.

Thid is a genuine honest to goodness trcuk engine....
Engine came from an old 58 chev fuel delivery truck ( 5 ton) and had standard solid lifters.....not a Pontiac ...and it has the big 1/2 inch oil lines to the filters. Lifters have been replaced and so was the cam..... paid for a stock truck cam..... I really have no way of knowing if they installed the proper one.... now I am worried..... but everything else they did was spot on.....

Timing..... well yes ...even after using the timing light on the ball/pointer of the flywheel I did "tweak" the dizzy by hand... am I bad ..??

According to my manuals for the late 261 they were tuned TDC with no advance.... or again am I wrong on this...?

So I better try to get the timing back on spec.... then go from there....may also satisfy myself and pull the valve cover and recheck the valve lash....

Could the change over to Pertronix pointless system have any impact on the timing....? I know from experience that, as suggested in a web posting, I opened up the sparkplug gap to .040 once the Pertronix was installed and the engine really settled down to a smoother idle.

Thanks for your comments/suggestion...will keep you posted of my findings.

Bob