302 Engine Idles up and down, and Engine bogs at higher speed - Vintage Mustang Forums

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post #1 of 6 (permalink) Old 09-30-2013, 12:39 PM Thread Starter
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Question 302 Engine Idles up and down, and Engine bogs at higher speed

I have a 68 with a 302 engine, 4bbl demon carb. I cant get the engine to maintain a steady idle. It races up, then goes down to almost a stall, then goes back up..then down..etc. (this is in park)

When driving, it bogs heavy at higher speeds. Ive also noticed a dramatic loss of power

Any ideas??
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post #2 of 6 (permalink) Old 09-30-2013, 03:16 PM
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is this a recent occurrence or has it always done this with this carb?

sounds rich like a high float level possibly if prob is new.
post #3 of 6 (permalink) Old 09-30-2013, 05:23 PM Thread Starter
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The engine ran great at all speeds and idled around 800-900. Steady idle. never went up and down like its been doing and never bogged over 40mph...so this is a recent problem. Whats a high float level and how can I check? Most things I can figure out on my own, but not carbs and fuel problems. That's when I ask for help

Last edited by scott68; 09-30-2013 at 05:32 PM.
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post #4 of 6 (permalink) Old 09-30-2013, 08:08 PM
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Sounds like you have a vacuum leak. That could cause some surging.

Check all of your connections before tearing apart the carb.

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post #5 of 6 (permalink) Old 09-30-2013, 08:31 PM
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check ign timing, should be around 10 at 800 rpm with distributor vacuum connected to ported vac not manifold vac.

check for air leaks as mentioned by plugging all fittings to intake and carb including the pcv and power brake hose.

spray carb base and intake at head with flammable brake fluid using narrow long nozzle. spray gently.

if still bad adjust fuel mix valves until highest rpm is achieved.

reset idle to 800.

if still bad remove carb top and check fuel level.

see youtube.com video for instructions for your carb type.

Last edited by barnett468; 09-30-2013 at 08:34 PM.
post #6 of 6 (permalink) Old 09-30-2013, 10:31 PM
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If you don't smell gassy exhaust or black smoke/soot from the tailpipe(s) then I'd look at a plugged fuel filter, weak fuel pump, partially plugged idle circuit or float level too LOW. If you see black smoke/soot then float level too HIGH, blown power valve or leaking float needle/seat. Also to be considered if it starts hard and is down on power is a slack timing chain that has jumped a tooth or two, a restricted exhaust or weak spark. Check your transmission fluid...you could have a ruptured vacuum modulator diaphragm sucking fluid into the engine but should have corresponding white smoke. Throw a vacuum gauge on a manifold vacuum source and analyze the readings.


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