Help, engine misses under load - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**MJN** · 02-23-2003, 02:42 PM

'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!

mikep · 02-23-2003, 02:58 PM

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?

JimKshortstep4x4 · 02-23-2003, 03:03 PM

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.

Jim

**MJN** · 02-23-2003, 03:17 PM

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.

JimKshortstep4x4 · 02-23-2003, 03:34 PM

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.

Jim

**MJN** · 02-23-2003, 06:43 PM

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.

Chevy Wrench · 02-24-2003, 03:50 AM

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.

Huck · 02-24-2003, 09:08 AM

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~~~

**MJN** · 02-24-2003, 07:53 PM

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!!

mardenmaine · 02-25-2003, 01:57 PM

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

Project1970 · 02-25-2003, 02:05 PM

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.

jku72 · 02-25-2003, 06:11 PM

Fuel filter will do it too. When my truck does that, it almost always turns out to be a restricted feul filter.

02-23-2003, 02:42 PM	#1
MJN Registered User Join Date: Feb 2000 Location: Maquoketa, Ia/ Orr, Mn Posts: 2,999	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! __________________ '51 South Bend Model "9A" '56 Chevy 3600 NAPCO '74 Chevy Cheyenne Super K20-restored on 37's '72 GMC Sierra Custom K2500- in progress '08 GMC Envoy Denali '12 GMC Sierra 2500HD Denali '17 GMC Yukon XL VCCA member #58596 http://www.nohrco.com

02-23-2003, 02:58 PM	#2
mikep Used to have a truck Join Date: May 2002 Location: port orchard WA Posts: 1,552	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? __________________ No truck :-(

02-24-2003, 03:50 AM	#7
Chevy Wrench former desert rat Join Date: Feb 2003 Location: salt lake city, utah Home of the 3 M's Mormons, methlabs and minivans Posts: 320	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. __________________ 94 Chevy Silverado Ext cab 4/6 drop w/ 18's and 2500watt system (work truck). 67 Chevy swb under const. frame off custom, 20's, bagged and a large rat under the hood,and a 1940 Chevy truck under const.

02-25-2003, 01:57 PM	#10
mardenmaine Registered User Join Date: Apr 2001 Location: Guilford, ME, USA Posts: 3	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

02-25-2003, 06:11 PM	#12
jku72 Registered User Join Date: Jan 2002 Location: cornville, maine Posts: 659	Fuel filter will do it too. When my truck does that, it almost always turns out to be a restricted feul filter. __________________ jku Cornville, Maine 68 C10 burb 42 Willys MB 46 Dodge WF-32 1.5 ton 53 & 56 Olds' 60 Cadillac 22 Dodge Brothers

02-23-2003, 03:03 PM	#3
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	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. Jim

02-23-2003, 03:17 PM	#4
MJN Registered User Join Date: Feb 2000 Location: Maquoketa, Ia/ Orr, Mn Posts: 2,999	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.

02-23-2003, 03:34 PM	#5
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	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. Jim

02-23-2003, 06:43 PM	#6
MJN Registered User Join Date: Feb 2000 Location: Maquoketa, Ia/ Orr, Mn Posts: 2,999	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.

02-24-2003, 09:08 AM	#8
Huck Senior Member Join Date: Jun 2001 Location: Shelbyville, KY Posts: 3,269	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~~~

02-24-2003, 07:53 PM	#9
MJN Registered User Join Date: Feb 2000 Location: Maquoketa, Ia/ Orr, Mn Posts: 2,999	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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