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High speed idle CAM adjustment

Tiny
88
  • MEMBER
  • 1988 CHEVROLET CAPRICE
  • 5.0L
  • V8
  • 2WD
  • AUTOMATIC
  • 120,000 MILES
After cold start idle stays high too long. Sometimes choke sticks closed after a short run and is difficult to start. Car diesels when turned off after a short run.

It looks like the tension on the high speed idle cam is too tight, preventing the cam to rotate back to position in a timely fashion. This, in turn, prevents the choke from returning to the open position.

I have found, with my fingers, a small hex with what feels like a spring on the right side (passenger's side) of the carburetor just under the cam assembly which seems to be the tension adjustment I am looking for. However, I see no way to get any tool, even with a universal joint fitting, in place to make the adjustment.

Please give me, if you can, a procedure for clearing the area enough to make this adjustment. My Haynes manual seems to be useless in that regard and I do not want to start taking things apart that I do not have to.

Thank you, 88
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Sunday, November 13th, 2016 AT 1:14 PM

14 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Most of the time this is no adjustment as they are not necessary. It is either not getting heated correctly or more like the choke linkage is full of dirt/oil take choke cleaner and clean all around the area and hold throttle open a little and work choke up and down and linkage while cleaning it. That may take care of it.
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Sunday, November 13th, 2016 AT 2:21 PM
Tiny
88
  • MEMBER
  • 59 POSTS
Thanks. I will try cleaning the linkages, but they seem to be free of gunk. I saw a YouTube presentation which shows the cam adjustment. It looks similar but not exactly like what I am feeling (it seems to have a small hex head instead of a Phillips unless the Phillips is within the hex head). The three position cam looks just like what was shown. It is a Rochester carburetor, if that is any help.
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Sunday, November 13th, 2016 AT 4:52 PM
Tiny
88
  • MEMBER
  • 59 POSTS
Just another thought. What is the proper operating temperature for that engine at normal operating speed?
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Sunday, November 13th, 2016 AT 4:54 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
195 is good it will show 210 or so on gauge, but it is under pressure temperature sender is in head so it will be hotter than it actually is.
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Sunday, November 13th, 2016 AT 5:02 PM
Tiny
88
  • MEMBER
  • 59 POSTS
Thanks. I will check that in the morning.

Still would like to see a procedure to clear that area so I can have a look for myself to see if the tension on that high speed idle cam is adjustable. It is a 4bbl carburetor, if that is any help (no specific model number listed on the spec sticker under the hood).
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Sunday, November 13th, 2016 AT 5:44 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
There is a screw that will slow or speed up that will ride on the high speed cam. We have no pictures of this, but screw comes in from back. I am sure that Is not your problem however and it is dirt/oil which should be cleaned with choke cleaner all the linkage on that side. You need to pull air cleaner off to see things. Car should idle at 2200 rpm's when set or if you want lower to 1500 rpm's at the minimum.
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Monday, November 14th, 2016 AT 7:00 AM
Tiny
88
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  • 59 POSTS
This morning, car seems to be running at recommended temperature when it is fully warmed up. Idle came down after full warm up, but still seems to be hanging up on high idle longer than it should. Dieseling is still a problem, especially before it is fully warmed up.
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Monday, November 14th, 2016 AT 1:19 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
If you don't put a tach on it you will never know what it's doing. I don't understand what you mean idling longer than it should when it's starter there are a couple of high idle stepson the cam one is very fast then it slows a little then slower before hitting regular idle which you need to know what normal warm idle is. The cam is controlled by the choke heater which is non adjustable. Idle should be 700 rpm with solenoid disengaged and 800 with solenoid engaged there are a nut like for solenoid adj and a small screw for other unplug wire to solenoid and make that adjustment if solenoid is holding it higher then back off screw to solenoid until it goes to 700 then plug back in and it should go to 800 at least those are the only adjustments.
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Monday, November 14th, 2016 AT 2:59 PM
Tiny
88
  • MEMBER
  • 59 POSTS
What I mean is that it comes down one step but stays on that step longer than it should. Then it will comedown another step but again stays there too long. The last two steps only come down after the car is stopped after it has come up to temperature and started without touching the gas. Sometimes it forces one to touch the gas a little and catches the second time BUT if you step the gas down too far it may activate the choke and then it won't start unless you go under the hood and retract the choke. I'm not even sure if the idle comes all the way down even under the most ideal situation. That's why I wanted to see if I can reduce the tension on that cam on my carburetor, but you say there is no adjustment so I'll try your suggestion above.

When it was newer it went through all the stages like clockwork, idled nice and low and quiet and would start after it was warm the minute you turned the key with no throttling necessary. It also never stayed on the higher idle cycle for more than a couple of minutes. Now sometimes I have to gear down in traffic just to drive without braking.
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Monday, November 14th, 2016 AT 8:49 PM
Tiny
HMAC300
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30 year old cars don't act like new ones
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Tuesday, November 15th, 2016 AT 5:55 AM
Tiny
88
  • MEMBER
  • 59 POSTS
Thanks! (LOL)
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Tuesday, November 15th, 2016 AT 9:55 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 42,895 POSTS
Hey 88,

It sounds like you have a primary choke pull of that has failed, we used to replace those all the time in the shop for this reason. I marked the part in the image below.

Please let us know what you find so it will help others.

Best, Ken
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Wednesday, November 16th, 2016 AT 3:21 PM
Tiny
88
  • MEMBER
  • 59 POSTS
Sorry for taking so long to reply. I wanted to post a picture of the carburetor on my car to see if we are on the same page or if the carburetors are similar enough to carry on with the same advice.

I have not, as yet, done so. For now, let me report that the car seems to be idling at much more acceptable speeds with no other adjustments having been made except to lubricate all the linkages and moving parts on the carburetor once again. The cold start is still good but cranks a bit more than it used to before ignition and stepping gently on the accelerator pedal to hasten ignition causes a very slight pop which I assume is a misfire.

Thanks again. 88
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Saturday, December 17th, 2016 AT 2:18 PM
Tiny
HMAC300
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No pic is posted
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Saturday, December 17th, 2016 AT 6:17 PM

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