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Old 04-30-2012, 04:54 AM
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Idle stumble

This is a Chevy 6 with a mild cam,more compression and headers.I modified the stock intake to accept a large base Rochester 2G carburetor.I enlarged the carb idle restriction "jets" and a main jets a few sizes for best performance. The intake is water heated.The timing is set at 10 initial with the total mechanical timing of 36 degrees.Has a vacuum advance.The engine runs very nicely,no bogs,no flat spots,excellent throttle response at any rpm,starts instantly hot or cold,never any carb flooding,slight idle lope with 18 inches of vacuum and makes good power for what it is...except for one small issue.
I'll explain it this way.You're sitting in the truck holding the engine at let's say 1600 rpm.Take your foot off the gas,the engine speed drops down to around 400 rpm,stumbles a bit,then returns to the normal 700 rpm idle.If you ease off the gas pedal slowly it returns to idle just fine.On the road this means when coming to a stop unless you fiddle with the gas pedal,the engine does it's stumble trick. It never actually stalls but is a bit annoying in traffic.Once the thing is idling at 700 rpm it's very steady..No apparent vacuum leaks
The brass carb float is set to 3/4 inch at the float seam as shown in Rochester repair manuals.I have moved the float up and down several times with no difference in the idle stumble.The carburetor never gets hot and doesn't ice up......Throttle shafts seems to be ok,no more or less play than many other carbs I'messed with.
Any ideas???

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Old 04-30-2012, 12:19 PM
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Originally Posted by F-BIRD'88
Turn the idle speed up a bit more.

What is the spark timing when vacuum advance is plugged and the idle speed is slowed down to 400-500rpm? Is it now less than 10deg?
I actually checked the total mechanical advance.....The initial is about 10-15 degrees at 600 rpm from what I saw with the timing light.The engine runs so nice I didn't fuss over the initial timing.Vacuum advance is ported....
This is a 261 Chevy 6 with the same vacuum advance that turns the whole distributor like the 216 and 235.Maybe you're on to something about the advance.Might be some slop there allowing the distributor to retard momentarily???
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