Fluctuating rpms?

[Connor strom]

So heres my problem, iv got a 84 k5 that runs like a beast...when it runs right. My issue is with the idleing rpms. Today for example i reset the rpms and tuned the carb the best i know how bc it was idling low, took it for a test drive down to the gas station and drove like a champ, nice and solid. I get back into the truck to go home and it wants to stall out. I get it home some how keep it running long enough to hook up a timming light that can read rpms (dont have a tach in the dash) and the idle rpms are all of a sudden running at 420-430. I did notice my air cleaner came loose and when i took it off and turned the truck back on the rpms where actually running high? Could the air cleaner be my problem or is there something else i should consider?

 

[MEANSTREAK K30]

So heres my problem, iv got a 84 k5 that runs like a beast...when it runs right. My issue is with the idleing rpms. Today for example i reset the rpms and tuned the carb the best i know how bc it was idling low, took it for a test drive down to the gas station and drove like a champ, nice and solid. I get back into the truck to go home and it wants to stall out. I get it home some how keep it running long enough to hook up a timming light that can read rpms (dont have a tach in the dash) and the idle rpms are all of a sudden running at 420-430. I did notice my air cleaner came loose and when i took it off and turned the truck back on the rpms where actually running high? Could the air cleaner be my problem or is there something else i should consider?

Hello, new here. I too have an 84 pick up. Is your engine stock?My Quadrajet has had a problem with the choke sticking.Could this be your problem?

 

[Connor strom]

Hello, new here. I too have an 84 pick up. Is your engine stock?My Quadrajet has had a problem with the choke sticking.Could this be your problem?

Nah its not the stock carb on it i dont remember the specs off hand but its possible that the choke could be sticking i havnt had much time to play with it yesterday. I also know that the air cleaner question is kind of an odd one and dont see how that could cause change in the rpms but thought it was odd that it increased rpms after taking it off. Ill have to add checking the choke to my list for tomorrow as i honestly didnt even think of that haha. Thanks for the reply =]

 

[pblaze725]

Fuel delivery

Have you changed the fuel filter in a while?
~if no, that could be the problem
~if yes, there could have been a little piece of the hose peeled off on instillation making a restriction.

Are there screens in the carb?
In mine I have 2 places for a catcher screen, one on the inlet, and another down in the bowl area.
The rebuild instructions said to use one or the other but not both.
I used both and it restricted the fuel flow enough to lower the idle.

There could be a small piece of debris in the carb that blocks a jet, and then moves out of the way, and then comes back later and blocks it again.

I recommend taking the top off the carb and lookin down in the bowl where the floats hang out and see if the fuel looks clean.
Spray some cleaner through the jets and make sure they're clean too.

Vacuum

What's the condition of the vacuum lines?
~Could the lines be collapsing during idle?
~could there be a crack in a line, or vacuum cap?
~Are you running the manifold vacuum or the timed vacuum?
You don't really use the vacuum at WOT because when the throttle blades are open the manifold is at exposed to ambient air pressures.
If there is a vacuum line choking (pinching shut) it most likely would affect the idle.



Choke

How cold is it?
My carb hates the cold weather, it idles like crap in the cold, I really have to play with the choke to get the idle correct.
What kind of choke are you using?
~Automatic- it may need to be adjusted.
~Electric- is it working right? You may need to check the voltage.
~Manual- do you have full range, is is the cable operating properly?

I hope this helps.

 

[diesel4me]

If removing the air cleaner affects idle speed,and you say its not the stock air filter housing,it could be it is covering up an air bleed tube in the carb venturi...if the filter is really gunky it can richen up the mixture too..

Vacuum leaks can affect idle speed too,and some can come and go as the engine changes temperature--might only do it hot,cold,or in between during warm up...check every rubber hose for cracks,split ends,or soft collapsing sections,that they are all connected to the proper ports--and make sure the PCV valve is working properly..

You can spray carb cleaner around any suspected gasket leaks on the carb base or intake manifold with it idling to spot where any air might be sneaking in,it'll make the engine tend to stall out or rev up a bit if you hit a leaking spot..

Its important we know what brand and type of carb you have...it is possible someone put an Edelbrock or Holley carb on the original Q-jet intake,and the base of the carb requires an adapter to allow them to fit (spread bore Q-jet to square bore Holley or Edelbrock)--those often develop gasket leaks and vacuum leaks at that area..

 

[Connor strom]

If removing the air cleaner affects idle speed,and you say its not the stock air filter housing,it could be it is covering up an air bleed tube in the carb venturi...if the filter is really gunky it can richen up the mixture too..

Vacuum leaks can affect idle speed too,and some can come and go as the engine changes temperature--might only do it hot,cold,or in between during warm up...check every rubber hose for cracks,split ends,or soft collapsing sections,that they are all connected to the proper ports--and make sure the PCV valve is working properly..

You can spray carb cleaner around any suspected gasket leaks on the carb base or intake manifold with it idling to spot where any air might be sneaking in,it'll make the engine tend to stall out or rev up a bit if you hit a leaking spot..

Its important we know what brand and type of carb you have...it is possible someone put an Edelbrock or Holley carb on the original Q-jet intake,and the base of the carb requires an adapter to allow them to fit (spread bore Q-jet to square bore Holley or Edelbrock)--those often develop gasket leaks and vacuum leaks at that area..

As for the air cleaner it is aftermarket but i just replaced the filter so its for sure not all gunked up. The hoses for vacume lines at first glance seem to be okay i have yet to be able to do a real inspection of them. The carb is an edlebrock carb that i believe does have the adapter that you mention (trying to remember off hand) ill for sure check out those gaskets that you mentiond. Thanks for all the ideas guys i appreciate the help and hopefully will have it fixed up soon.

 

[MEANSTREAK K30]

As for the air cleaner it is aftermarket but i just replaced the filter so its for sure not all gunked up. The hoses for vacume lines at first glance seem to be okay i have yet to be able to do a real inspection of them. The carb is an edlebrock carb that i believe does have the adapter that you mention (trying to remember off hand) ill for sure check out those gaskets that you mentiond. Thanks for all the ideas guys i appreciate the help and hopefully will have it fixed up soon.

I noticed no one said to check bolts for tightness. You have probably already done this. The Edelbrock carburetor itself might need rebuilt. I, myself have seen several issues with these carburetors . If I were you , I would find a Qaudrajet to put back on it . My two cents.

 

[Connor strom]

I noticed no one said to check bolts for tightness. You have probably already done this. The Edelbrock carburetor itself might need rebuilt. I, myself have seen several issues with these carburetors . If I were you , I would find a Qaudrajet to put back on it . My two cents.

Yea i did check to make sure everything was nice and tight, and it just so happends that i went to try and find a qjet to rebuild today too haha to no avail though. Any who if any one cares for an update i think iv got it narrowd down to the pcv hose. So after i get out of work and replace the hose im hopping the problem will be solved, fingers crossed any ways. Thanks again for the imput guys

 

[Connor strom]

Update if any one is interested: so i ended up replacing all the vacume lines (theres only three so why the hell not) and that still didnt solve the issue. That brought me back to thinking fuel delivery problems. When i got the truck it had a mr gasket electric pump that the previous owner mounted in the completely wrong spot (under the hood higher then the gas tank) and that was causing problems so rather then cut the gas lines to re instal it in the right area i just put a mechanicle pump on there and put a filter inline running from pump to carb. And it was running great. Then all of a sudden i started getting the fluctuating rpms and thought there was a vacume leak, which brought us to this thread. Any who after replacing vacume lines and that not working i decided to test the fuel pressure and in doing so i noticed the gaskets in the filter i put on blew out and was causing it to 1, not keep the filter full of gas. And 2, seemed to be letter air into the line. I replaced the ****er and all seems well now. Let me know what you guys think, as far as if that could be the end all problem or if theres something bigger that im missing. Thanks =]

 

[JoshHefnerX]

I see you got it figured out, was going to suggest checking vacuum at the modulator valve - if you had one. had a vacuum leak down there that affected idle when hot.

 

[Connor strom]

I see you got it figured out, was going to suggest checking vacuum at the modulator valve - if you had one. had a vacuum leak down there that affected idle when hot.

Yea seems to be running good again. Ill keep that in mind for next time though if i have issues again, knock on wood,haha