Unresponsive idle mixture screw...

[Jim Ratto]

I have to say it. Change that spark plug. So many times I chased my tail swapping jets and fixing what I thought were vacuum leaks and cleaning carbs and setting floats, only to have the same symptom persist.
Now yes, sometimes a driveability issue was carburetor related.
But just as many times it was an bad plug or some other ignition related fault.

You are creating more work for yourself by NOT swapping a plug.

I once fought my 48's for an entire day off from work (because I had to drive the car to work the next day). I had the problem carb off the car 3 times, thinking eventually I would find the issue. Nope. I finally realized I had swapped plug type for something I thought would work better, but they clouded over easily (no MSD in my car back in those days). I had a spare set of Denso resistor plugs in the cabinet for some Vanagon motor and since my heads then were 14mm long-reach... I decided I might as well try them.
Viola- I had 4 hard hitting cylinders.

Also, when I first got my Super Flow head/2276cc motor going in 1992, I had been playing with 48IDA for only about 18mo, so I was real green at making them work (at all). My boss offered to help me after work one night. We spent 90 minutes swapping jets and driving around, to no avail. Either it was too lean or it was pig rich and falling on its face. He suggested we remove the Bosch blue and 009 and drop in a locked Vertex. Instantly the motor woke up. The carbs were 1,000,000 easier to adjust mixture on. I, still to this day, have eyt to see anything done to my engines/tunes that had such a profound effect as his magneto had.

So don't overlook plugs/ignition. You may think they're OK.... but...

[Bill Schwimmer]

99% of carb problems are ignition. Say this to yourself over & over.

[type149doug]

   Thanks to both of you....   I know this is truly a carb issue.  I found a twisted throttle shaft.  YAY!!!  I'm finally a 1%er. I had a complete Mallory ignition system on the engine and there are some issues with it.  Therefore It was suggested that I remove the whole system and go with a basic blue coil and points distributor.  I installed a known good functioning Blue coil and a fresh 010.  It fires and runs great on this ignition set-up...  I could try every trick in the book to try to sync the carbs but the barrel over #1 was consistently showing more flow on my SK sync tool and I couldn't get the idle mixture screw to change the speed of the engine at idle.  My hypothesis is this:  The throttle plate over #1 was open just enough to lessen the vacuum under the throttle plate. Therefore less vacuum means the fuel cannot be drawn from the idle discharge orifice, hence no result from adjusting the idle mixture screw.  I will hopefully make more headway tomorrow after I do a compression check and see if I have any weak cylinders. Thanks again and keep your ideas coming while I dial this thing in.  Doug.

[j-dub]

   I did some redneck engineering and did a simple flow check on the offending carb.  I used some duct tape, shop vac and my SK flow tool.  I found that the barrel over #1 flowed 5.5 and the barrel over #2 flowed 3.5. Twisted throttle shaft... 

I actually do something similar as well. I have manifold to mount a single IDF or DRLA that has been modified so the runners are no longer individual, they are common plenum. On one side of the manifold I put a shop vacuum and block off the other side. With a flow meter I measure if the throttle shafts are bent and with some careful finesse I bend them back when needed. Works great! I have it on good authority that this is actually somewhat common practice but fancy shops don't use a shop vac, they use a flow bench.

[type149doug]

    I did a compression check and found that I have a low cylinder.  #1 is at 95psi and the other three are consistent at 115psi.   Now what to do... I really don't want to pull the engine to have the cylinders honed and install new rings.  But I'll do what I have to in order to get this thing running tip top again.....   

[Sam K]

You may want to do a leakdown test as well.

[Martin S.]

To test for leakdown, rotate the engine by hand until each cylinder approaches TDC. With the plugs installed you can hear the leakage past the rings into the case, or past the valves, or past the head to cylinder seal or a combo. Bug engines have plenty of leakdown normally, but if you have a leak you have a leak.

[type149doug]

     After finding the compression to be low on one cylinder, I have chosen to remove and tear down the engine.  I will have the cylinders honed and install new rings. Hopefully this will also mend some of the high crankcase pressure problems I've also been experiencing.  I look to have it back on the road within a month and we'll go from there.  Thanks for all the advise   Doug S.

[Martin S.]

Check your heads for warping too. A leak-down test before tear down will help you find exactly where the compression is leaking from. Could be valves as well. A couple of my (high mileage) engines developed loose head stud nuts and warped heads were the problem.