Steady decrease in power and mileage - The 1947 - Present Chevrolet & GMC Truck Message Board Network

crm318 · 12-07-2009, 12:04 AM

Recently, I have noticed that my truck is getting worse gas milage. The other day, I decided to give it a full throttle punch (fully up to temperature) and the truck fell on its face and then reved up, not really going anywhere in the process. I also noticed that the tach didnt really move either (it used to fly). I took all of the plugs out and they were all in good shape. I took the distributor cap off and it looked alright also. I did notice that the graphite piece in the center looked a little flat. The fuel filter is also clean. What else should I look for?

dan42 · 12-07-2009, 01:17 AM

Sounds like you have a conventional dizzy - you changed the points/condenser/rotor lately? Did you check the timing? You also might have carbuerator issues. You are narrowing it down, it's a matter of eliminating stuff.

crm318 · 12-07-2009, 02:09 AM

Oh, Im sorry. I didnt mention that I have an HEI.

68Stepbed · 12-07-2009, 09:07 AM

What kinda carb you got?

crm318 · 12-07-2009, 01:25 PM

I have a quadrajet.

dznucks · 12-07-2009, 02:19 PM

It may be the carb building up some kind of blockage in a jet or fuel passage.

How are you plug wires? are they routed close to any manifolds or headers?
How easily does you drivetrain spin by hand, ie your tranny and rear end?

andrewmp6 · 12-07-2009, 02:25 PM

Yeah start with a basic tune up if that don't work try rislone oil treatment its about 5 bucks it does work.

68Stepbed · 12-07-2009, 02:42 PM

It sounds like a carb issue. I don't know much about Q-jets, so I can't help much, but I'm sure someone here can.

crm318 · 12-07-2009, 02:53 PM

When it warms up, Ill give the carb a look over. The wires look to be in good shape. The boot of one of the wires comes a little close to the header on one of the plugs. What is rislone oil treatment. Does it go in the oil?

69GMCLonghorn · 12-07-2009, 06:15 PM

You have HEI, which is great. But a steady decrease in power might be pointing to some kind of a problem with it. You might be at that difficult point where you need a second, hands-on opinion. I've learned that a few bucks well spent having someone else look at it can save LOADS of time and possibly money in the long run.

68gmsee · 12-07-2009, 08:05 PM

What do you mean by steady decrease in power? Do you mean it has been decreasing over a long period like weeks, months? If so, how long have you been noticing it?

There's several things that can cause it and, as mentioned, you may need to just start eliminating possibilities one by one. The easiest first.

You've done the preliminaries so now you can continue with the other stuff.
The tune up is a good idea. New cap, rotor, plugs, filter change, and maybe even the plug wires. If they're old they can cross fire when wet -and the weather has been moist and wet the last few days.

Check your timing and then lastly remove the carb and clean it out real good.

Beyond that, you may be looking at a worn cam or other engine problems...

crm318 · 12-07-2009, 10:27 PM

Well. It has been going on for about 2 weeks (as the weather has gotten colder). I will go through the easiest stuff first and then tare into the carb. The timing is set at 14 degrees at idle. My engine specs are in my signature.

12-07-2009, 12:04 AM	#1
crm318 Alright Alright Alright Join Date: Mar 2004 Location: Midland, Texas Posts: 1,672	Steady decrease in power and mileage Recently, I have noticed that my truck is getting worse gas milage. The other day, I decided to give it a full throttle punch (fully up to temperature) and the truck fell on its face and then reved up, not really going anywhere in the process. I also noticed that the tach didnt really move either (it used to fly). I took all of the plugs out and they were all in good shape. I took the distributor cap off and it looked alright also. I did notice that the graphite piece in the center looked a little flat. The fuel filter is also clean. What else should I look for? __________________ 1972 Cheyenne Chevrolet red and white 357ci, Vortec heads, GM Performance high rise intake, Comp 268H .454/.454 218/218 110 LSA Quadrajet carb, Flowtech headers, 2.25 40 series HEI, Curve kit, TH 350, 3.07 gears 275/60/15s on 15x8 rallies

12-07-2009, 01:17 AM	#2
dan42 Registered User Join Date: Feb 2007 Location: Cali Central Coast Posts: 5,191	Re: Steady decrease in power and mileage Sounds like you have a conventional dizzy - you changed the points/condenser/rotor lately? Did you check the timing? You also might have carbuerator issues. You are narrowing it down, it's a matter of eliminating stuff. __________________ '67 SWB Fleetside ZZ4/350/Tremec 5-speed/4link/Scott's IFS - and fun as heck! SOLD Click here to support the board Philippians 4: 6-7

12-07-2009, 02:09 AM	#3
crm318 Alright Alright Alright Join Date: Mar 2004 Location: Midland, Texas Posts: 1,672	Re: Steady decrease in power and mileage Oh, Im sorry. I didnt mention that I have an HEI. __________________ 1972 Cheyenne Chevrolet red and white 357ci, Vortec heads, GM Performance high rise intake, Comp 268H .454/.454 218/218 110 LSA Quadrajet carb, Flowtech headers, 2.25 40 series HEI, Curve kit, TH 350, 3.07 gears 275/60/15s on 15x8 rallies

12-07-2009, 09:07 AM	#4
68Stepbed Registered User Join Date: Mar 2005 Location: 10-Uh-See Posts: 5,609	Re: Steady decrease in power and mileage What kinda carb you got? __________________ Matt 68 C10 stepside, LS1/700R4, TCI Engineering suspension system

12-07-2009, 01:25 PM	#5
crm318 Alright Alright Alright Join Date: Mar 2004 Location: Midland, Texas Posts: 1,672	Re: Steady decrease in power and mileage I have a quadrajet. __________________ 1972 Cheyenne Chevrolet red and white 357ci, Vortec heads, GM Performance high rise intake, Comp 268H .454/.454 218/218 110 LSA Quadrajet carb, Flowtech headers, 2.25 40 series HEI, Curve kit, TH 350, 3.07 gears 275/60/15s on 15x8 rallies

12-07-2009, 02:19 PM	#6
dznucks Registered User Join Date: Feb 2006 Location: Smyrna, Ga Posts: 1,647	Re: Steady decrease in power and mileage It may be the carb building up some kind of blockage in a jet or fuel passage. How are you plug wires? are they routed close to any manifolds or headers? How easily does you drivetrain spin by hand, ie your tranny and rear end?

12-07-2009, 02:25 PM	#7
andrewmp6 Registered User Join Date: Jan 2009 Location: Louisville,Ky Posts: 5,811	Re: Steady decrease in power and mileage Yeah start with a basic tune up if that don't work try rislone oil treatment its about 5 bucks it does work.

12-07-2009, 02:42 PM	#8
68Stepbed Registered User Join Date: Mar 2005 Location: 10-Uh-See Posts: 5,609	Re: Steady decrease in power and mileage It sounds like a carb issue. I don't know much about Q-jets, so I can't help much, but I'm sure someone here can. __________________ Matt 68 C10 stepside, LS1/700R4, TCI Engineering suspension system

12-07-2009, 02:53 PM	#9
crm318 Alright Alright Alright Join Date: Mar 2004 Location: Midland, Texas Posts: 1,672	Re: Steady decrease in power and mileage When it warms up, Ill give the carb a look over. The wires look to be in good shape. The boot of one of the wires comes a little close to the header on one of the plugs. What is rislone oil treatment. Does it go in the oil? __________________ 1972 Cheyenne Chevrolet red and white 357ci, Vortec heads, GM Performance high rise intake, Comp 268H .454/.454 218/218 110 LSA Quadrajet carb, Flowtech headers, 2.25 40 series HEI, Curve kit, TH 350, 3.07 gears 275/60/15s on 15x8 rallies

12-07-2009, 06:15 PM	#10
69GMCLonghorn Registered User Join Date: Dec 2006 Location: Porter Ranch, CA Posts: 978	Re: Steady decrease in power and mileage You have HEI, which is great. But a steady decrease in power might be pointing to some kind of a problem with it. You might be at that difficult point where you need a second, hands-on opinion. I've learned that a few bucks well spent having someone else look at it can save LOADS of time and possibly money in the long run.

12-07-2009, 08:05 PM	#11
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: Steady decrease in power and mileage What do you mean by steady decrease in power? Do you mean it has been decreasing over a long period like weeks, months? If so, how long have you been noticing it? There's several things that can cause it and, as mentioned, you may need to just start eliminating possibilities one by one. The easiest first. You've done the preliminaries so now you can continue with the other stuff. The tune up is a good idea. New cap, rotor, plugs, filter change, and maybe even the plug wires. If they're old they can cross fire when wet -and the weather has been moist and wet the last few days. Check your timing and then lastly remove the carb and clean it out real good. Beyond that, you may be looking at a worn cam or other engine problems...

12-07-2009, 10:27 PM	#12
crm318 Alright Alright Alright Join Date: Mar 2004 Location: Midland, Texas Posts: 1,672	Re: Steady decrease in power and mileage Well. It has been going on for about 2 weeks (as the weather has gotten colder). I will go through the easiest stuff first and then tare into the carb. The timing is set at 14 degrees at idle. My engine specs are in my signature. __________________ 1972 Cheyenne Chevrolet red and white 357ci, Vortec heads, GM Performance high rise intake, Comp 268H .454/.454 218/218 110 LSA Quadrajet carb, Flowtech headers, 2.25 40 series HEI, Curve kit, TH 350, 3.07 gears 275/60/15s on 15x8 rallies



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