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Old 02-23-2003, 02:42 PM   #1
MJN
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Help, engine misses under load

'72 Blazer with 350, HEI, misses under load and heavy acceleration, like going up a hill the engine will start to miss on one cylinder with a lot of hesitation and gets better when you let off gas. Seems to happen more around 2200 to 2800 RPM. I replaced cap, rotor, wires all new. Switched ingnition module with a good one, and adjusted carb and timing. Timing is set at 12 degrees before and vaccum at idle is around 16psi with idle set at 800 RPM in park. Last thing I can think of is that maybe one plug is bad or is going bad. Just replaced those in early August with AC R44T. Any ideas of what might be happening would be appreciated. Thanks!
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Old 02-23-2003, 02:58 PM   #2
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if you have full voltage to the coil the firt thing I would strat with is the selection of plugs. The r44t is a fairly cold range. you might want to go with the R45T. What carburetor are you running and is the jetting right?
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Old 02-23-2003, 03:03 PM   #3
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I would change the plugs. The newer plugs seem to gas foul a lot easier than they use to. I understand that it is because they don't coat the inner insulating cones any more. I have changed my plugs on my plow (3 times since winter started) due to gas fouling. I just changed out the choke pull which had gone bad and caused the engine to run rich as it warmed up. The explanation that I was given was that with the fuel injection on cars now days gas fouling is not as common.

It makes sense to me.

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Old 02-23-2003, 03:17 PM   #4
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The carb is a quadra jet off a '72 vette. Bought it at a swap meet from a guy that rebuilds them. I think I will try plugs, nothing else left. Thanks guys.
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Old 02-23-2003, 03:34 PM   #5
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If you don't make long trips, I would go with Mike's suggestion of the R45's. I run R45TSX with the gaps at .045 to .050.

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Old 02-23-2003, 06:43 PM   #6
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I drive it everyday, the farthest it goes is maybe 40 miles one way, thats only maybe once a month. I'll pick up a set tomorrow after school. Thanks.
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Old 02-24-2003, 03:50 AM   #7
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You also might want to check the float level, and it wouldnt hurt to check out the accel. pump for leaks. Those old q jets can be tricky. One more thing, try retarding the timing by 2 degrees. Sounds like you're having detnation problems. Trust me, I work on this stuff every day, its what I do.
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Old 02-24-2003, 09:08 AM   #8
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I would start with a compression check to make sure the basic is solid. Then I would reset the timing to 8 degrees advanced. Make sure your ignition wire from the fuse box to the dist is a heavier gauge then the original stock wire. A weak coil or marginal condenser will also give you fits so consider those as well. R44 or 45 plugs should be OK gapped to .045. The other consideration is which vac advanced is feeding your distributor --is it connected to timed vacuum or full manifold vacuum? I have best luck (when using HEI) with connection to the timed vacuum port (no vac at idle). Good luck and let us all know what you find~~~
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Old 02-24-2003, 07:53 PM   #9
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Just put in new R45TS plugs and runs like a champ. Found the number six cylinder plug was full of carbon deposits. I tried different timing and carb adjustments but it idles smooth right where it is now. The vaccum advance is on the timed vaccum port. Thanks a lot guys!!
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Old 02-25-2003, 01:57 PM   #10
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seems simple

have you checked your plug wires... this is not uncommon to find broken plug wires which cause this failure under high load (high voltage) situations
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Old 02-25-2003, 02:05 PM   #11
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Re: seems simple

Quote:
Originally posted by mardenmaine
have you checked your plug wires... this is not uncommon to find broken plug wires which cause this failure under high load (high voltage) situations
Quote:
Originally posted by MJN
I replaced cap, rotor, wires all new.
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Old 02-25-2003, 06:11 PM   #12
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Fuel filter will do it too. When my truck does that, it almost always turns out to be a restricted feul filter.
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