Trouble in open throttle - The 1947 - Present Chevrolet & GMC Truck Message Board Network

mrladewig · 03-05-2003, 03:41 PM

I'm having trouble with my new to me 78 K-10 with 350 and new quadrajet and distributor. Also, I have a 4 speed Muncie, not that it is the cause, but...

When there is alot of load on the engine, (low RPM and 3/4 to fully open throttle), the engine is putting very hard, and producing very little power. It also does not sound like the quadrajet is opening. If I let off the gas (to near idle) the power remains the same, and I can press down on the accelerator again lightly and get a little more power. But if I mash it, it goes back to putting along. If I very slowly work the engine up, it will get up to speed and reved up, and eventually the quadrajet sounds like it is opening. It will also lighlty backfire when I let off the gas.

The distributor is set at 10degrees BTDC instead of the 8degrees in the book, because I live at altitude and it seems to suite the engine. The engine idles nicely and will rev up without trouble if it is not under load. All of the vacuum hoses on the front side are new and fit tightly, the carb base gasket is also new (Mr. Gasket paper). Ignition module is new Accel. ( the old distrib was eating ign modules) The only vacuum hose which is not new is the line going to the brake booster, and it looks a little corroded. It doesn't make it feel like I don't have power assist on the brakes though. Vacuum measured off the vacuum port on the carb going to the vac advance is 8 lbs. I thought it was supposed to be 15...

I am planning to replace the brake booster vacuum line asap, but am getting a little frustrated and poor replacing parts. Anybody have a good method for checking for other vacuum leaks, or thoughts on other places to look. Anything else I might not have thought of? Thanks for the help

PHOENIX · 03-05-2003, 03:46 PM

accellerator pump on the carb ok?
I have also heard bad things about the accel modules. I have heard it from atleast 3-4 people that the modules are junk.

Spray carb cleaner around gaskets to check for leaks, if the idle picks up then there is your leak. Also try propane out of a little torch type thing. I hear it works just as well without the mess.
Hope that helps some.

mrladewig · 03-05-2003, 04:17 PM

Accelerator pump is working good with a nice spray.

I have gone through 2 of the "cheaper" modules, and I still have the last one (it didn't get burned). It didn't get burned up, but I had read that the higher end modules more closely matched OE spec for heat and voltage protection but I don't know much about it. Before I rebuilt the distributor, it had some sort of voltage leak off the coil or pickup coil or something that was eating the ign modules. ALL of those parts are new, along with weight bushings, condensor/module plug, cap rotor. The ign module did not change performance. The distributor changes did help when revving up.

The previous owner installed the carb. I am really surprised that the new carb might have leaks in it's gaskets, but I had a friend tell me that he had heard of people getting factory rebuilt carbs with bad gaskets (These were Bing carbs for BMW motorcycles).

BTW the carb is a Carter Quadrajet(?) and says "built for GMC" (my truck is a chevy but no big deal). Anybody know anything about that. I thought Rochester made them all, and my father in law (who has worked on Chevys forever) had never heard of a Carter one, but it looks dead-on identical to the Rochester for 78.

82 Silverado 4X · 03-05-2003, 05:06 PM

"At idle", you should be getting a good solid steady 20hg of vacume at the port on the carb or even at the intake, that is unless you have a really wild cam.

PHOENIX · 03-05-2003, 06:36 PM

normal stock vacuum range I believe is 18-22 at idle. Just for reference.

TROYLV · 03-07-2003, 07:59 AM

check your exhaust system. we sometimes forget- if it can;t get out it can't have vacuum. brake booster diaphram leak?

c10scotty · 03-09-2003, 07:22 PM

it could be a little lower since you are at altitude, 8 is too low though...

RockHQ · 03-09-2003, 08:39 PM

Could the fuel pump be the culprit?

mrladewig · 03-10-2003, 12:03 PM

Found the leak with carb cleaner. The carb base gasket is leaking... bad... on three sides. I had just replaced it, but it is still leaking, so there must be something wrong with the surface on the intake. Last time I made the mistake of just lifting. This time, I'll pull it all the way off, and figure out why the gasket is not mating up. Thanks to everyone for the help.

85 K5 · 03-11-2003, 09:03 AM

Throttle shaft in the base plate of the carb could be leaking air. These can be re-bushed for $35. Go to a speed shop and ask them who they know of that can do this for you.

Your carb cleaner could be entering the fuel system there, making it look like a gasket leak.

mrladewig · 03-11-2003, 12:37 PM

Well, I got the new gasket installed and everything is much happier now. I can finally hear the quadrajet open up! I'm going to go for a shot at emmissions today. If I pass, it'll be the last time this truck has to go through emissions (25 yrs = collectors plates in CO, and no more emmissions unless I sell it).

03-05-2003, 03:41 PM	#1
mrladewig Registered User Join Date: Feb 2003 Location: Lakewood, CO Posts: 12	Trouble in open throttle I'm having trouble with my new to me 78 K-10 with 350 and new quadrajet and distributor. Also, I have a 4 speed Muncie, not that it is the cause, but... When there is alot of load on the engine, (low RPM and 3/4 to fully open throttle), the engine is putting very hard, and producing very little power. It also does not sound like the quadrajet is opening. If I let off the gas (to near idle) the power remains the same, and I can press down on the accelerator again lightly and get a little more power. But if I mash it, it goes back to putting along. If I very slowly work the engine up, it will get up to speed and reved up, and eventually the quadrajet sounds like it is opening. It will also lighlty backfire when I let off the gas. The distributor is set at 10degrees BTDC instead of the 8degrees in the book, because I live at altitude and it seems to suite the engine. The engine idles nicely and will rev up without trouble if it is not under load. All of the vacuum hoses on the front side are new and fit tightly, the carb base gasket is also new (Mr. Gasket paper). Ignition module is new Accel. ( the old distrib was eating ign modules) The only vacuum hose which is not new is the line going to the brake booster, and it looks a little corroded. It doesn't make it feel like I don't have power assist on the brakes though. Vacuum measured off the vacuum port on the carb going to the vac advance is 8 lbs. I thought it was supposed to be 15... I am planning to replace the brake booster vacuum line asap, but am getting a little frustrated and poor replacing parts. Anybody have a good method for checking for other vacuum leaks, or thoughts on other places to look. Anything else I might not have thought of? Thanks for the help

03-05-2003, 03:46 PM	#2
PHOENIX Registered User Join Date: Oct 2002 Location: Phoenix, Arizona Posts: 4,703	accellerator pump on the carb ok? I have also heard bad things about the accel modules. I have heard it from atleast 3-4 people that the modules are junk. Spray carb cleaner around gaskets to check for leaks, if the idle picks up then there is your leak. Also try propane out of a little torch type thing. I hear it works just as well without the mess. Hope that helps some. __________________ * AVOID: LOPER'S PERMORANCE / LOPERSPEED.COM OF PHOENIX, AZ & COTTMAN TRANSMISSION *

03-05-2003, 05:06 PM	#4
82 Silverado 4X Genuine Chevrolet Join Date: Feb 2003 Location: South West Pennsylvania. Posts: 582	"At idle", you should be getting a good solid steady 20hg of vacume at the port on the carb or even at the intake, that is unless you have a really wild cam. __________________ George 82 Silverado LWB 4x4 350 SM465 NP208 77 Chevrolet Blazer Chalet 400 TH350 NP203

03-05-2003, 06:36 PM	#5
PHOENIX Registered User Join Date: Oct 2002 Location: Phoenix, Arizona Posts: 4,703	normal stock vacuum range I believe is 18-22 at idle. Just for reference. __________________ * AVOID: LOPER'S PERMORANCE / LOPERSPEED.COM OF PHOENIX, AZ & COTTMAN TRANSMISSION *

03-07-2003, 07:59 AM	#6
TROYLV Registered User Join Date: Oct 2002 Location: LAS VEGAS, NV. Posts: 157	LOW POWER check your exhaust system. we sometimes forget- if it can;t get out it can't have vacuum. brake booster diaphram leak? __________________ troy wilson

03-05-2003, 04:17 PM	#3
mrladewig Registered User Join Date: Feb 2003 Location: Lakewood, CO Posts: 12	Accelerator pump is working good with a nice spray. I have gone through 2 of the "cheaper" modules, and I still have the last one (it didn't get burned). It didn't get burned up, but I had read that the higher end modules more closely matched OE spec for heat and voltage protection but I don't know much about it. Before I rebuilt the distributor, it had some sort of voltage leak off the coil or pickup coil or something that was eating the ign modules. ALL of those parts are new, along with weight bushings, condensor/module plug, cap rotor. The ign module did not change performance. The distributor changes did help when revving up. The previous owner installed the carb. I am really surprised that the new carb might have leaks in it's gaskets, but I had a friend tell me that he had heard of people getting factory rebuilt carbs with bad gaskets (These were Bing carbs for BMW motorcycles). BTW the carb is a Carter Quadrajet(?) and says "built for GMC" (my truck is a chevy but no big deal). Anybody know anything about that. I thought Rochester made them all, and my father in law (who has worked on Chevys forever) had never heard of a Carter one, but it looks dead-on identical to the Rochester for 78.

03-09-2003, 07:22 PM	#7
c10scotty ~THE SILVER BULLET~ Join Date: Oct 2002 Location: WYOMING Posts: 285	it could be a little lower since you are at altitude, 8 is too low though... __________________ ~1980 c10 chevy scottsdale~ ~2.5/4 drop 305/th350c~

03-09-2003, 08:39 PM	#8
RockHQ ROCKET POWER! Join Date: Feb 2003 Location: Finland, Europe Posts: 509	Could the fuel pump be the culprit? __________________ What did i do last nite? I had a wonderful time, That's what they tell me What did i do last nite? 1979 C-10 shortfleet: Olds 350, Th400, 12 bolt Current project: 1969 Olds 455 W-30, needing a rebuild

03-10-2003, 12:03 PM	#9
mrladewig Registered User Join Date: Feb 2003 Location: Lakewood, CO Posts: 12	Found the leak with carb cleaner. The carb base gasket is leaking... bad... on three sides. I had just replaced it, but it is still leaking, so there must be something wrong with the surface on the intake. Last time I made the mistake of just lifting. This time, I'll pull it all the way off, and figure out why the gasket is not mating up. Thanks to everyone for the help.

03-11-2003, 09:03 AM	#10
85 K5 Registered User Join Date: Nov 2002 Location: Spring TX Posts: 113	Throttle shaft in the base plate of the carb could be leaking air. These can be re-bushed for $35. Go to a speed shop and ask them who they know of that can do this for you. Your carb cleaner could be entering the fuel system there, making it look like a gasket leak. __________________ 350 Crate 10 bolt 3.73 2.5 lift 32x11.50 1904 Edelbrock http://www.oplnk.net/~spirit/K5front.jpg

03-11-2003, 12:37 PM	#11
mrladewig Registered User Join Date: Feb 2003 Location: Lakewood, CO Posts: 12	Well, I got the new gasket installed and everything is much happier now. I can finally hear the quadrajet open up! I'm going to go for a shot at emmissions today. If I pass, it'll be the last time this truck has to go through emissions (25 yrs = collectors plates in CO, and no more emmissions unless I sell it).



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