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Petronics Issue

Discussion in 'Huskies, Imps, Rapiers, Fastbacks, etc.' started by absunbeam, Sep 20, 2019.

  1. absunbeam

    absunbeam Platinum Level Sponsor

    Here's a head scratcher. The Super Minx 1725 cast iron head is having a rough idle when cold. Checked for vacuum leak via spraying gas/starter fluid around the intake connections. Decided to check the timing which was correct. Checked the plug wires with timing light induction harness and found 1 & 3 to be dead(only running on 2 cyl) has a petronics ignition and coil which has less than 400 miles on it. Any ideas of whats going on? -Al
  2. sunalp

    sunalp Platinum Level Sponsor

    Did you try swapping plugs, the working plugs for the non working ones? Did this pertronics
    unit work previously, or is it a new unit? Could be defective if new. I've never had this type of issue with pertonics.
    I had a miss on a 1725 with a weber and aftermarket manifold. Turned out that someone had put the
    brake booster on the front fitting and it was pulling the fuel away from that cylinder. Removed that and it ran perfectly.

    Check those items and let us know what you find out. If you have points, put those in to see if the issue goes away.
  3. absunbeam

    absunbeam Platinum Level Sponsor

    Thanks Steve- Changed plugs to the same, had 3 different brands in it when i did a compression check. Had Petronics in it and ran ok. There is no brake booster, the PCV hose needs replacing. I do have points and that will be the next task.
  4. husky drvr

    husky drvr Gold Level Sponsor

    Hi Al,

    Just curious about your compression test results, but I'm fairly sure your problem does not reside there.

    Cylinders 1 and 3 are adjacent in the firing order. The wires for those two cylinders are side by side on the distributor cap. It will probably be a good idea to test the coil wire output with your inductive light and see the if the coil is being given a signal to fire for those two cylinders. If the coil IS tasked to fire on those two cylinders, then you likely have some type of damage or wear in the distributor and/or cap not allowing the spark to travel to the properly timed cylinder. If the coil IS NOT signaled to fire on those two cylinders, your Pertrnonix unit probably is at fault.

    Just a thought and HTH,
    Last edited: Sep 22, 2019
  5. absunbeam

    absunbeam Platinum Level Sponsor

    Thanks for the input, started with Steve's suggestion on the plugs. All were heavily sooted with black carbon, sand blasted the carbon off and restarted. Timing light then worked on all wires after that, guessing what I saw was the fouling of the plugs. Got the timing rechecked and advanced slightly to factory spec 6-7 deg. Zenith 1 bbl carb was next so took the top off and blew out the jets and idle air passage. Got it all back together fired up and just purred. Next is to figure out why it is putting so much gas down the carb. Reading the manual thought maybe the o-ring wasn't sealing and sucking gas out of the bowl. Carb was rebuilt about 100 miles ago. Will have to take it out for a test drive to see if that cures it. It has always put out black smoke at idle and carbons up after a short warmup. So will give an update to see if that improves it.
  6. husky drvr

    husky drvr Gold Level Sponsor


    I think your carb is a Zenith 34 IV by my service manual -WSM 145. Was your SMinx originally equipped with the 1725? Or was it a 1600? If it was a 1600, then is your carb original to the 1600? I am only speculating after looking at the diagrams and a quick perusal of the operations descriptions because I don't think I've ever had a 34 IV needing work. Do you have any idea what size venturi is in your carb - 24 or 27?

    You should not be able to see black smoke at idle, that's way too rich.

    Just a couple more things to check:

    > manual choke should be completely opened after start and slight warm up.

    > fuel pressure should be 1.5 to 2 PSI. Fuel pump should have a thick spacer type gasket between pump and engine block. The pressure is too high without one.

    > correct float level setting?

    > are all settings set to OE specs - idle speed screw, idle mixture screw, air volume screw? I think most carbs with an air volume screw work in reverse for lean - rich settings.

    > are all jets in the proper locations?

    > are all check valve balls in the correct locations and functioning?

    > is the economizer valve functioning properly? There should be an air bleed in the economizer passages which also acts as an anti-siphon protection. If the economizer is not functioning correctly, then unneeded extra fuel will be supplied at all high vacuum conditions.

    Just a few thoughts that might not be applicable.

    Have fun,

  7. absunbeam

    absunbeam Platinum Level Sponsor

  8. George Coleman

    George Coleman Gold Level Sponsor

    @ cents worth if the distributor has not been rebuilt and you and a Pertronix unit you are just masking the under lying problem. The dwell on a old distributor is constantly changing and the Pertroinx just makes it seem better.
  9. absunbeam

    absunbeam Platinum Level Sponsor

    Does a distributor with a worn shaft (side to side) have any effect on a Petronix set?
  10. Barry

    Barry Platinum Level Sponsor

    The combination of Pertronix Ignitor and a worn distributor shaft can cause timing variation, but it does NOT affect dwell; that is determined by the electronics.
  11. husky drvr

    husky drvr Gold Level Sponsor


    Couple more quick thoughts. Oil smoke is usually bluish-gray. If you are noticing signs of oil out of the exhaust, it is probably the result of excess raw fuel washing oil off the cylinder walls.

    The WSM145 section on your carb say the starting point for idle mixture screw is 1.5 turns out and is sensitive. Yours at 1/4 turn indicates excess fuel is dumping down the carb without being controlled by the mixture screw. Guess that's obvious, right? If you are willing and curious to explore the WSM145. It is available as PDF on the home page under the tech manuals listing. There is some info under general data and an extensive section on the 34 IV carb itself.

    It sounds like you have the car back together and driving. If so, maybe a test to see if you can see any obvious internal fuel leakage.

    Try this, when the car is cold:

    > remove air cleaner,

    > disconnect power to coil so NO CHANCE of spark,



    > crank engine about five seconds longer than it normally takes to start engine when cold,

    > go look down carb throat to check if you see any wet fuel and maybe where it is leaking - no leaks and no fuel is what you should see.

    If you don't see anything doesn't mean that there is not an internal leak between passages. Most likely source is probably a missing plug where a passage is drilled and was then plugged to control flow direction.

    Good luck,
  12. absunbeam

    absunbeam Platinum Level Sponsor

    Don-Did the cold crank test, no fuel going down the carb. Which was promising. Started it after that and looked like a lot of fuel was going in the center at idle. I would expect to see less than that.
  13. husky drvr

    husky drvr Gold Level Sponsor


    At idle, there should be no fuel exiting the center of the venturi. That is the discharge for the main circuit. The main discharge should not deliver fuel at all without an airflow induced vacuum in the venturi. There is not enough airflow at idle to create that vacuum. The only things that come to mind for an idle discharge in the main circuit at idle are > fuel bowl not properly vented leading to higher pressure in the fuel bowl > static fuel level way to high in fuel bowl which could be caused by several possibilities. Too high a fuel pressure - leaking needle and seat assembly (either in or around the assembly) - plastic floats that have become fuel soaked and lack buoyancy to close and seal needle valve - might be an issue with the vacuum controlled power valve.
    I'm sure there must be other possibilities.

    I will ponder some more,

  14. absunbeam

    absunbeam Platinum Level Sponsor

    Don- checked the fuel pressure and it's @ 2lbs. Will have to look @ the power valve next and check float buoyancy. If the o-ring is the wrong size could it create an issue?-Al
  15. husky drvr

    husky drvr Gold Level Sponsor

    Hi Al,

    Sorry not to have been back to this sooner. I've been studying the drawings in WSM 145. Since I don't a Z 34 IV in hand to check or past experience, I've been reading, studying the drawings, and pondering. I kept trying to decide about your description of the leakage "in the center". So one pic to eliminate the thousand words coming up. After examining the description of the "power valve" operation, I'm not sure that it functions in the way I was thinking. It seems to be an AIR valve and not a fuel valve. So it is probably NOT an issue.

    In this picture, are you seeing the leakage at the red arrow or the blue arrow?

    If the leakage is at the blue arrow showing the outside edge where the emulsion block joins the carb body, Then I think you have identified your most likely problem - the O-ring. In the drawings, it does seem that the O-ring is below the fuel level in the bowl.

    If you are seeing the leakage at the red arrow, then that is probably caused by different issue(s).

  16. absunbeam

    absunbeam Platinum Level Sponsor

    Thanks Dan- Will look into that, may need a thicker dia o-ring if that's the case.
  17. absunbeam

    absunbeam Platinum Level Sponsor

    Coming out of the red area, the center Venturi. Any way to adjust that flow?
  18. absunbeam

    absunbeam Platinum Level Sponsor

    Dan- Found the issue, the economy device (3 screws)was missing the 2 gaskets that sandwich the diaphragm. Had them in the kit but they did not fit the post, opened them up got them to fit. Put back together and now can get it to idle without missing. Thanks for your help in getting this resolved.
    Al IMG_3030.JPG IMG_3032.JPG IMG_3033.JPG
  19. husky drvr

    husky drvr Gold Level Sponsor


    No, no adjustment. If you look at the picture, the horizontal light dotted line is, I think, the reference line for design fuel level. Notice it runs through the float pivot point. The passage that travels to the auxiliary (booster) venturi extends to below fuel level. It also travels a distance above the fuel level so fuel will not be pulled through the main system until enough airflow at the booster venturi(BV) causes enough vacuum in the BV. The depression in the BV will not be sufficient to start drawing fuel over the high point until the upper ranges of the idle system is reached. You can sort of see where the throttle blade should be for that transition point when the throttle blade just passes the idle progression holes just above where the idle mixture screw passage is located near the base. There is nothing to adjust between the base and top of that passage (air emulsification passages don't function without airflow through the carb, either).

    Your fuel bowl is filling up to a level that allows excess fuel to drain out the BV like an overflow, instead of air at normal atmospheric pressure in the fuel bowl, you might have air at a different pressure than normal that causes the fuel bowl to overfill and overflow. The vents might not be working properly, but that seems to unlikely. There are two vents, one in the top cover and one that is a passage into the top of the carb throat.

    Too high an internal air pressure would tend to push fuel up and out the BV. I just don't know what would cause a consistently high internal pressure at idle.

    Too low an internal air pressure would cause the pump pressure to be more effective and possibly overpower the needle and seat. If the floats are possibly in a condition where they are already lacking the authority to control fuel flow, then this condition would only be worse. The only method I envision for this is if engine vacuum is leaking past or through the economizer diaphragm and venting that is not operating properly.

    Another possibility seems there might be a pressurized leak path before the fuel even gets to the needle valve and fuel bowl. The drawings seem to show a potential across the top of the emulsion block straight to an opening in the passage to the BV. Two drawings seem to disagree about this so I'm not sure. Ensure the gasket is in good condition an properly placed between cover and emulsion block and neither surface is warped.

    Float level setting can't be set by checking the fuel level in the bowl. When the floats are removed the level drops making the new level unsuitable for measuring. The proper way to check is with the needle and float assembled to the top cover assembly, hold upside down and measure the distance from the highest point( bottom edge as normally installed) on the float to the gasket surface. The setting is 32-33 mm at that point. Check both sides. I guess it might be possible to install the floats so they can't close the needle valve completely, but don't think the carb could be assembled in that manner.

    I'm out of time for this evening. Hope this helps to some degree. Still just some random thoughts.

    Good luck,
  20. husky drvr

    husky drvr Gold Level Sponsor


    That's great. Glad you found the issue.

    Keep us updated on how it all finally sets up and works.

    Have a good evening.

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