Surging...

[dutch]

Hope you guys can lead me some here so I don't spend a lot of time wasted...
My '68 suddenly developed a surging or jerkiness when driving and I would like to have some input to where anyone (everyone) thinks I should look for a cure. The car suddenly lately seems to require a lot more cranking over to get it to catch than previously after it sits for a few days and seems a bit rougher until it warms up some. It does still drive normally until I either let it cruise under 2000 rpms at a steady throttle or take my foot off the pedal slightly to decellerate some - then it starts to buck in any gear. It gets progressively worse to the point where I either have to stick in the clutch when approaching a light (much sooner than I would normally) or drop it down a gear or two to get the revs up but it is only a cure until it slows and starts it again.
I'm not lugging it as this occurs between 1200 and 2000 rpm and it never did this previously - just started the other day and seems to be getting a bit worse and somewhat harder to start after it sits for a while (hour or more) where it started great previously in that timeframe. It still idles great and has good pull through the gears.
My thoughts were in some order of importance and effort (a) float level issues which I haven't checked yet (b) carb issues with needle and seats being the prime area to look at after a general clean out (c) I've got a new set of plugs ready to go in (delco R45's) but these characteristics don't really make me believe that they would fix a surging problem (d) changing the inline fuel filters to try and get any type of fuel flow improvements that might bring - but the ones I have in now don't appear to have any crap in them and my tank was lined a few years back and never has shown any junk in the fuel (e) I am runing 100LL aviation fuel and it seems to work great - this is the only thing new to the set up and operation of the car in the past year or two prior to the surging issues lately which appear to be getting slowly worse as I mentioned.
Any ideas as to where I should best aim my efforts so I don't head off on a tangent here. Specs - 350  / 9:75 cr / Holley double pumper 650 / '68 Z28 intake & heads / mild cam 232 - 238 degree @.050" / headers / timing 14 degees initial - 34 total @ 2600 rpm...
Thanks for any comments or suggestions so I don't spend a lot of time endlessly wandering through this...  Randy
   

[Sauron327]

 I can only tell you what I did to alleviate a similar problem. I did not have a hard starting problem however:
   
  Changed the points, Set dwell and timing. Reset float levels( holley 670 Street Avenger), Adjusted mixture w/ vacuum guage. Surge/Jerkiness now gone.

   How old are your ignition system components and what is the condition of the disributor? If it is sloppy and worn this will affect your performance.
   I look forward to other's comments on this subject as well. Others are more qualified for diagnostics than I.
   Footnote: I switched to full manifold vacuum( for vacuum advance) as per John Z's advice.

[dutch]

  
  Changed the points, Set dwell and timing. Reset float levels( holley 670 Street Avenger), Adjusted mixture w/ vacuum guage. Surge/Jerkiness now gone.

   How old are your ignition system components and what is the condition of the disributor?
   I look forward to other's comments on this subject as well. Others are more qualified for diagnostics than I.
   Footnote: I switched to full manifold vacuum( for vacuum advance) as per John Z's advice.

I am using the 266 distributor from my 302 and it is in perfect condition and has the limiting sleeve for the shortened advance  - threw in a Pertronix Ignitor II a couple of years or so back replacing the points so dwell and adjustments are not applicable here. Aside from points I wondered if the weights might be sticking since the car sits 10 moths of the year - but assumed it wouldnt idle down well if they were doing that.
What's with the full manifold vacuum? - I had heard a bit about it in the past but I must admit I am not quite sure where to obtain it from - right now I am pulling from a port on the front passengers side of the carb approximately 1/2 - 2/3's of the way up the metering block and I suspect that is what is considered ported vacuum. How do I get full - right off the manifiold somehow? And what does it do better than the ported type?
I just went out and looked at the carb and I see that the port that is on the front of the base plate that I had capped (which might be a full vacuum port?) has a very small tear in the end of it it so I will try another one to replace it - could it be that is maybe all it could take - hard to imagine but I will try it anyway...
I haven't yet as I said, tried anything to cure this surging problem and was just hopefull the majority of any and all suggestions posted back would help me cut down on the things I would need to try before I could get it solved.
The full vacuum is interesting but I seem to recall putting the dist canister line on the lower post that I mentioned and I couldn't get the timing to settle down at idle sufficiently to warrant using it. - Thanks for you input - Randy

[Tinkerr]

have you checked the choke, making sure its fully open, secondarly the power valve in the carb will sometimes cause these characteristics. If the engine has backfired through the carb it will sometimes ruin the power valve.

[jdv69z]

Will it idle? If it seems to run fine above 2000 RPM or so, (when the main jets are functioning) but runs really rough at idle or almost stalls, could be piece of dirt or varnish in idle circuit. I've had this with my Holley where something clogs a passage in the idle circuit (like dried out gas turned to varnish or dirt) and it runs like crap. I've disassembled the carb and cleaned the metering block passages, and this has solved my problem. It is amazing how a blocked idle passage in a carb probably only .032" in diameter can bring performance to a dead halt.

[dutch]

I removed the choke altogether years ago and plugged the holes in the body - it was a pian always ending up partially closed at some point and I just removed it rather than fighting with it or trying to wire it open. I have thought about the metering blocks as I had trouble with varnish or a small particle of something stuck in there a few years back and after fighting with it and finally dislodging whatever it was the difference was amazing in the smoothness and operation of the carb. The gas I stored it with this winter is the avaition fuel I mentioned which according to the guy who distributes it to our local airports and private operators, contains a certain staliizer compound so it won't varnish up in the carbs of planes on floats that sit unused over the winters here - so I'm assuming it shouldn't be varnish. The one thing I'm not sureabout is if it is hard on the rubber or viton needle and seat parts like racing fuels are.

[dutch]

I zapped myself offline before I finished the last part - The car hasn't backfired for years and doesn't run rich at all even with this new surging issue - so I doubt that the power valve is bad. I thought that surging is ususally a symptom of a lean condition if I recall correctly.
If my little vacuum port plug was in fact leaking due to the small crack in it, would or could that small amount cause it to suck enough air if it was jetted close to possibly get it into the lean spectrum of operation? Opinions? - Thanks - Randy

[rich69rs]

I would concur with your first thought - float level adjustment.  Might want to look down in the carb and see if you can see anything leaking in from the main metering circuit at idle - if so, float level is definitely off spec.

[tom]

A small vacum leak would be more prominent at idle, but would be insignificant at higher rpm's. I would look at the hoses. I was able to locate a leak on one of the cars by spraying carb cleaner around while the car was idling rough. When I sprayed around the leak the idle smoothed out temporarily.

Good hunting.

Tom

[rich69rs]

Referring to Page D of the Diagnosis, Testing, Troubleshooting Section of the Glenn Mitchell, Inc. National Service Data TUNE-UP MANUAL for all U.S. manufactured cars between 1961-1969:  for a surging enigne that has reached operating temperatures; i.e. a "hot" engine,  the following (in the order show) are listed as the items to check:

A.  Carburetor is adjusted too lean
B.  Fuel pump issue - low flow or pressure
C.  Carburetor float level adjustment is incorrect
D.  Dirty or Obstructed fuel passages in the main metering circuit of the carb.

[dutch]

Referring to Page D of the Diagnosis, Testing, Troubleshooting Section of the Glenn Mitchell, Inc. National Service Data TUNE-UP MANUAL for all U.S. manufactured cars between 1961-1969:  for a surging enigne that has reached operating temperatures; i.e. a "hot" engine,  the following (in the order show) are listed as the items to check:

A.  Carburetor is adjusted too lean
B.  Fuel pump issue - low flow or pressure
C.  Carburetor float level adjustment is incorrect
D.  Dirty or Obstructed fuel passages in the main metering circuit of the carb.

 Rich:

  When it is stated 'carb is adjusted too lean' does that just mean that the metering block adjustments are so - or is it referring to the main jets themselves? Obviously I haven't played with the jetting both main or idle circuit systems since I rebuilt the carb a few years back and it has always run pretty good and actually gets decent mileage if you keep the foot out of it and stay on the primaries.
  I did consider the fuel pump - but it's only got a couple thou on it since new and I was more inclinded to worry about the filters upstream initially and thinking that they might be starving the rest and therefore be the problem.

                                                                                                                                   Randy

[Sauron327]

Randy,
   Begin process as stated in Rick's excerpt from the manual.  A. and C. apparently were contributing factors in my situation. B. and D. may be yours as well, or a combination of the  items listed. If it was running good once before there should be no need to change the jets unless it will run better with new jets.
   You have to check your timing prior to carb adjustments. Just because the fuel pump is new does not mean it is good. Check the pressure.
   Eliminate A, B, and C then perform D if neccessary and arrive at initial diagnosis. Keep in touch.
 

[dutch]

I appreciate all of the input - I will try and get some time on the weekend to see what I can discover using the suggestions given... somewhat in the order of importance and ease. Thanks to you all again - Randy

[rich69rs]

Randy,

Nothing additional was given in the troubleshooting section.  I do believe that they listed the items to check from A-D in order of ease to check, whith disassembly of the carb (in order to check C and D) as being performed last in a logical problem solving approach; i.e. eliminate the easy to check possibilities first.

Also, I would make sure that nothing has happened that is causing the carb see more heat than normal (stuck heat riser valve as an example), which might cause sporadic vapor lock and perhaps the potential to surge at normal engine temps.  If you are using a heat riser valve, my experience with the ones currently available from any parts source that I've seen are junk and will bind up after fairly limited use.  The aftermarket does offer an open spacer to replace the heat riser valve.

Good liuck this weekend.

Richard

[dutch]

Thanks Rich
  - No heat riser in the works and the order in my mind originally was to do the easy and less complicated stuff first to try and weed out the issues and your list and the advice from everyone pretty much correspond to my thinking. Actually I originally posted to see if there were any less obvious but yet easy things that I might be overlooking before I charged into it.

    By the way, one thing I meant to ask originally was do you or anyone else here know where I can get a Sun Super Tach II (a sort of day 2 or somehwhat period correct tach for the car) checked over and rebuilt if necessary? The needle on mine jumps around lots for some reason now and it is first on my list to disconnect before I chase fuel problems on the slim chance it may have some bearing on the surging problem as well. I figure that's a real slim chance but regardless I would like to get it reconditioned to settle it down. It doesn't have the box that I have heard some have attached to them as far as I can tell - appears to just be a straight wire hook-up to the coil and ground. Thanks again...
                                                                                Randy