New rough idle at start, need advice (1 Viewer)

KingAir Driver

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Jan 7, 2016
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Spokane, WA
Looking for a little advice on my new rough starting/idle issue.

Backstory: So I decided to bring the 73 home to work on it more. It had been running fine, drove it up and down the street a few times. The morning I go to grab it, has a hard time starting, barely idles, backfires out the carb, accelerator pedal has almost no affect (no revs). I goose the clutch to get it up the the trailer ramp, as soon as the front wheels hit the ramp, Everything clears up! Idles great, back to normal. Get it home, drive off, go around the block. Good!

Let it sit overnight, back to be rough, won’t stay running. The carb was rebuilt, new tank, fuel pump and dual filters. I’m perplexed if it’s carb related or dissy related? The fact that the issue resolved when I went up the trailer ramp is odd.

Any help would be appreciated.
 
Joined
May 12, 2015
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Bakersfield, Ca
When's the last time it's been driven for a decent amount of time?

Might be too easy but maybe you just need to take it out and really drive it. Mine will do that if it's been sitting
 
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Feb 3, 2020
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Stanwood, WA
Quick, simple check is looking at the dipstick; overfull, smell like gas, etc?
does it run better with choke on or off?
 

KingAir Driver

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Spokane, WA
Oil level is ok, no gas smell. Cold starts have normally needed full choke and then after warm up no choke is necessary.

Just checked spark and coil = good
Power valve squirts gas, but struggles to stay running.
Carb Sight glass half full

My clear fuel filter prior to the carb is fuller than I’ve seen. I’m leaning towards something is plugging the carb, no gas going from the jet’s.
 

KingAir Driver

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Spokane, WA
Carb opened up, all ports are clear, needle valve and float operating. Blew everything out to make sure. Still doing the same thing. Struggling to stay running.

Switched to ignition side, point gap looked good at .015, rotor and distributor cap clean edges. I tried advancing the timing to 17-18, started running a little better but got smoked out of the garage. My timing light has the RPM readout, but it was very erratic going from 1000-200-600 in a 1 sec span. All the plugs show a black carbon glaze on them. Recheck of the coil next, wonder if it’s a weak spark or signal.
 
Joined
Jul 6, 2020
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Poulsbo, WA
Hey King, have you gone through the FSM tuneup procedures to try and isolate the issue to the carb or dizzy? I would test the idle solenoid and make sure it's clicking properly and its wire is not grounding out. I have seen them ground out through the insulation if it's degraded at all. Have you checked your timing yet or looked inside the dizzy? How about vacuum leaks, have you checked for those around the manifolds? After all of the replacements that you mentioned above, is your gas tank getting proper vent or is it drawing too much vacuum? Just a few thoughts from my experiences. I'm trying to plan a trip to Spokane with my boys, so I could stop by and lend a hand (or a :beer:) Good luck!
 
Joined
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Poulsbo, WA
Sorry, it sounds like you have already checked most of what I suggested. I would only suggest that 17-18* degrees seems way to advanced for idle timing. Both of my 40's run best right on the BB at 7 degrees. As long as the vacuum advancer is working correctly.
 

KingAir Driver

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Spokane, WA
@AntFarm thanks for the offer, welcome to stop by any time. Yea, I know it’s way too advanced. The 73 has a vacuum retard dissy which is capped off. The carb was set up per the FSM and running great prior to this incident. The idle solenoid is clicking when keyed on. Manifold gasket’s are on my list. Just going to keep checking everything.
 

KingAir Driver

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Jan 7, 2016
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Location
Spokane, WA
Welp got it running and drove her for about 10 mins. Here’s my course of events.

- New plugs ( spark tester was showing a weaker signal than my buddy’s 40)
- Pulled the idle solenoid to check that it bench tested good. It did.
- Shot carb cleaner through the idle circuit and then compressed air.
- Used starting fluid to get it initial running more the 5 secs.
-Saw gas coming out the center Venturi, which it shouldn’t at idle. Played with some carb screws.
- Had a buddy turning the distributor to get it to stay running, Venturi stopped giving gas.
- Adjusted the idle screw and mixture screw utilizing lean drop to get 650-700 rpm, choke all the off, 15 in vacuum.
- Tried to set timing, but the signal on the spark plug was erratic and wouldn’t give a constant strobe.
Of note, checking the gas cap, the tank was really pressurized. Left the cap off during these above events. Still smoking pretty good which I’ll attribute to valve seals for now or unburned gas due to weak ignition

Hopefully it starts again when it’s cold. I’m going to upgrade to a DUI distributor next. Also, get a vented gas cap until I can figure out the pressure.
 

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