Quadrajet, high idle with warm engine

[kikkegek]

Originally Posted by CaptRMW

JMAC 11 is exactly right and a good shot of Blaster or WD 40 on the SECONDARY shaft on "both sides" will likely cure your problem. You will need to give it a shot once in a while since Rochester didn't find a cure for this problem...

untill now that didnt help. Will try again 2night!

[kikkegek]

checked 2nite and the fast idle cam is clear as far as I can judge. I tested it and the fast idle cam is not keeping the linkage from going to full idle...

I did notice something else.

I made a bracket to change the springs into pulling opposit to the throttle cable, so the throttle shaft is relieved of stress...

here are some pics of the setup sofar:





the thing is....I drilled several holes to be able to adjust the spring tension...but even the biggest tension will not pull it back to the idle screw...al the way..

I did notice this evening that when the engine is warm and not running, the linkage runs free and very easy...when I start the engine...the last bit of the linkage seems to move less smooth...

man...what can this be....

[geezer#99]

Where's your vacuum advance hooked up to?

[kikkegek]

Originally Posted by geezer#99

Where's your vacuum advance hooked up to?

manifold vacuum, because I have all the pollution stuff taken of.

why? do you think there is a connection?

[geezer#99]

Hook it back up to a ported source. Idle will slow down.

[notyo_cheez]

Second. Distributor advance and the transmission modulator need to be fed from the carb, know this from experience.

[kikkegek]

okay, thanks, did just that and indeed idle slowed down...

but it did not solve the problem, that the last piece of my linkage feels heavy and it does not return all the way easily when the engine is warm and running.

The only thing it changes is that it wil not advance as much in idle, wich causes less efficient burning of fuel...better would be to have it hooked up the right way and reduce fuel consumption by dropping the idle down by adjusting the screw for it.

So I am gonna reconnect it to manifold and continu my search for the problem....solution...

heres some info on type of vacuum too choose:

from hotrodders.com...

This is a great article!!!!I had my vacuum advance hooked up to the ported vacuum port after reading this article I hooked it up to full manifold vacuum adjusted the idle rpm.The car has so much more power now.



As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

[ray_mcavoy]

Hi kikkegek,

I think someone might have mentioned this earlier, but have you checked the throttle shaft to see if it's loose?

It's a common problem for the throttle shaft holes in the baseplate to get worn ... especially on high mileage Q-Jets. That creates a vacuum leak and I believe it can also allow the throttle shaft to get out of alignment just enough for the throttle plates to catch a little bit.

I had this happen with the Q-Jet on my Nova. I had to tap the throttle to get it to idle down. I did some checking and ruled out a sticking cable, fast idle cam, and so on, but noticed that the throttle shaft was rather loose. Fixing the worn holes in the baseplate with some home-made teflon bushings solved the problem.

I picked up the home-made teflon bushing trick from Dave Ray (IgnitionMan) on another forum ... he describes it in post #9 ---> http://www.hotrodders.com/forum/weir...ems-78607.html

[kikkegek]

Originally Posted by ray_mcavoy

Hi kikkegek,

I think someone might have mentioned this earlier, but have you checked the throttle shaft to see if it's loose?

It's a common problem for the throttle shaft holes in the baseplate to get worn ... especially on high mileage Q-Jets. That creates a vacuum leak and I believe it can also allow the throttle shaft to get out of alignment just enough for the throttle plates to catch a little bit.

I had this happen with the Q-Jet on my Nova. I had to tap the throttle to get it to idle down. I did some checking and ruled out a sticking cable, fast idle cam, and so on, but noticed that the throttle shaft was rather loose. Fixing the worn holes in the baseplate with some home-made teflon bushings solved the problem.

I picked up the home-made teflon bushing trick from Dave Ray (IgnitionMan) on another forum ... he describes it in post #9 ---> http://www.hotrodders.com/forum/weir...ems-78607.html

hi thanks for the reply.

The shaft can move just a little from side to side...but I dont really notice any serious wear...I think...

problem is, if I take of the carb, the engine wont be running, and the linkage would run free, so how am I gonna find the the cause of it not returning to full idle...

I contacted ignitionman, maybe he can help me with the teflon mad...I dont really understand it.

Do you have any pics of how you did it?

[geezer#99]

So know we've found your problem. Due to throttle shaft bushing wear the side to side movement causes the primary throttle plates to hang up on the bore. It can be fixed but you'll need to find an expert that's done it before.

[kikkegek]

Originally Posted by geezer#99

So know we've found your problem. Due to throttle shaft bushing wear the side to side movement causes the primary throttle plates to hang up on the bore. It can be fixed but you'll need to find an expert that's done it before.

I am not 100% sure, because when the engine is off and cold I have no problem...but it could be...how can I check this?

[geezer#99]

Pull off the carb, flip it over and see how the throttle plates fit in the bore. Move the throttle shaft back and forth and see where it hangs up.

[kikkegek]

I'll probably do just that...just need to find some time for it, since we drive it daily...

I'll be back!

[PRattenbury]

Have you tried adjusting the choke thermostat one or two clicks to LEAN? Might want to give that a try, if it doesn't help you can return it to where it is now.

[kikkegek]

Hi PRattenbury,

I checked 2nite and it is already in the leanest position I think:

look at this pic. Is the silver stip on the black cover the indicator?



I turned even a little further, lets hope it helps. I need to take it for a drive. Because with the engine now cold and off...the linkage is just fine..

[kikkegek]

Originally Posted by PRattenbury

Have you tried adjusting the choke thermostat one or two clicks to LEAN? Might want to give that a try, if it doesn't help you can return it to where it is now.

tested it last night and this morning. a little more and a little less...no difference.

I guess there is nothing left, but removing the carb and doing some reasearch on it!

[geezer#99]

Or just enjoy it the way it is. Experimenting with your carb throttle plate could leave you with nothing to drive. When I worked on mine I had several spares. You probably don't or have access to any.
Or find a rebuilt and put that on.

[MadManAndrew]

Is your electric choke working?

[kikkegek]

Originally Posted by MadManAndrew

Is your electric choke working?

No. i think it is disabled. Since you need one when driving LPG.
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[tucsonjwt]

I am wondering about the loose throttle shaft/bad bushing theory. If jabbing the accelerator causes the engine to idle normally, how would a loose throttle shaft (and associated vacuum leak) cause the high idle? Isn't the throttle shaft still loose after you jab the accelerator? Wouldn't it still have a high idle after jabbing the accelerator? Makes me think some kind of mechanical linkage problem is the issue.

[MadManAndrew]

I personally gauruntee you that if you turn that choke on your problems will dissapear. The high idle of the electric choke will only go off when the choke has power and is functioming correctly.
Posted via Mobile Device

[geezer#99]

With a loose throttle shaft the throttle plate hangs up on the bore. If you stab it hard it slams shut. More closing force puts it back where it belongs. Gentle throttle movement allows it to hang up.

[tucsonjwt]

OK, thanks geeser- play in throttle shaft causes hangup - nothing to do with vacuum. I detect no play in the throttle shaft from up top inspection, and it did this since it was "professionally" rebuilt" about 2 years ago.
MadManAndrew - I have a manual choke on mine, and I can physically verify that it is wide open when the high idle occurs. So what do you think would be my problem? (I mean my truck's problem, not me personally.)

[geezer#99]

The hang up isn't consistent either. When I fixed mine I found where the plate was hanging up and used a dremel to provide a little clearance. Bushing kits and plate screws wheren't readily available then back in the 80's so I didn't go that route for a fix.

[CaptRMW]

Originally Posted by tucsonjwt

OK, thanks geeser- play in throttle shaft causes hangup - nothing to do with vacuum. I detect no play in the throttle shaft from up top inspection, and it did this since it was "professionally" rebuilt" about 2 years ago.
MadManAndrew - I have a manual choke on mine, and I can physically verify that it is wide open when the high idle occurs. So what do you think would be my problem? (I mean my truck's problem, not me personally.)

Tucson this is almost always a sticking SECONDARY butterfly but everyone wants to play with the choke and primary throttle shaft. You can close the sticking secondary by manually pulling the lever forward that is on drivers side of the secondary throttle shaft just to verify whether or not that is the problem..I am running a Quadrajet that has over 300,000 miles on it and it still hangs up once in a while if I don't spray it with WD-40 .