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73-87 Chevy _ GMC Trucks => Performance => Topic started by: Ben_Greenberg on April 05, 2015, 09:29:06 am

Title: 78 c15 heavy half acceleration
Post by: Ben_Greenberg on April 05, 2015, 09:29:06 am

Hey guys and gals,
My 78 c15 heavy half automatic has had lots of problems in the past on cold starts. All remedied by realizing how the choke pull off spring was supposed to be installed so I fixed that.
Now, in the first minute or two of driving the truck accelerates without me pushing the pedal. After a few minutes the Carb appears to respond fine, but initially it just keeps accelerating. I am sorry this isn't a great description, not sure how else to describe the issue. I slow the idle speed down by pushing on the throttle pedal after a few seconds of idle, but once the truck is in drive, and I don't touch anything it will accelerate up to about 40-45 mph.

To me it sounds like something is sticking open, but before I just start ripping everything apart I was looking for some ideas. Any help is appreciated. Thanks everybody!

Title: Re: 78 c15 heavy half acceleration
Post by: LTZ C20 on April 05, 2015, 11:52:52 am

Welcome from California. Take a look at your throttle return springs, if it's warn out and not holding tention then it won't hold the throttle plate fully forward in the idle position, so it would be like having your foot on the gas, it's pulling the throttle open at idle just enough to increase speed instead of putting it to idle speed.

You should be able to get new springs at any auto parts store if needed. They usually come in a pack of 2. Use them both.

Title: Re: 78 c15 heavy half acceleration
Post by: Ben_Greenberg on April 05, 2015, 11:56:01 am

Thanks LTZ I will take a look at that while I am poking around on some other things. I wouldn't doubt that some of the springs are super old. The previous owner did some work to this thing, but the parts in terms of age are all over the place.

Title: Re: 78 c15 heavy half acceleration
Post by: LTZ C20 on April 05, 2015, 12:02:29 pm

Your welcome. You have the possibility of having 1 worn out spring or 2 worn out springs. You should always use 2 springs, if 1 breaks and your only using 1 then your screwed.

Title: Re: 78 c15 heavy half acceleration
Post by: LTZ C20 on April 05, 2015, 12:07:01 pm

This is an example, I entered what info you gave about your truck and guessed you had a 350.

http://m.oreillyauto.com/mt/www.oreillyauto.com/site/c/search.oap?year=1978&make=GMC&model=C15%2BPickup%2BRWD&vi=1142907&keyword=spring+&un_jtt_redirect

Title: Re: 78 c15 heavy half acceleration
Post by: blazer74 on April 05, 2015, 01:01:22 pm

Is it accelerating due to being on the fast idle cam? What's is the idle speed, high and low?   Accelerates to  what speed?
Once the choke coil heats up after a few moments fast idle cam drops down to and car acts normal.
At first thought sounds normal choke operation since this started when you set the choke for the cold start issue.
Make sure you use the dual return spring setup.


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Title: Re: 78 c15 heavy half acceleration
Post by: Ben_Greenberg on April 05, 2015, 01:33:42 pm

Thanks for all the info guys! The idle is slow when in park, but when the truck is in drive it accelerates as if I had my foot on the throttle.