Guy

Need the collective wisdom. Put a refreshed engine in my 79. Everything is new including fuel distributer, rebuilt WUR etc. Fuel pressure readings all tested and retested and are all dead on.

Car starts fine each and every time. Fast idle is 1500. Drops to around 950 after a short time. Runs fantastic at first, I mean simply amazing. Pulls wicked strong but then once it warms up I start experiencing a floating idle with poor throttle response. Idle hangs at 1500 to 2000 and is slow to return. This seems to happen only after I wind it out a bit. Mixture seems to be a bit rich so I leaned it out slightly with no noticeable difference.

I think I am chasing a vacuum leak but have been unsuccessful in locating it so far. Have not used a smoke machine but i have used the ether test.

Thoughts and suggestions appreciated!

Daniel5691

I am certainly no expert, but this thread goes through a lot of hot idle tests/issues....
https://rennlist.com/forums/928-foru...e-problem.html

GlenL

The decel valve can stick open. That'll hold the idle up but it shouldn't affect driveability.

The control pressure is good when the engine is really hot? Is the system pressure good when really hot?

karl ruiter

I'm still working on getting my '79 perfect.

One of these:
http://www.ebay.com/itm/AEM-30-4110-...ZT8RqB&vxp=mtr
really helped a lot. $150 well spent.

A leaking decel valve can cause the idle to move around. You can cap it off with two 1/2 copper pipe ends. Also, there may be an adjustment on it. I have mine removed and I think that causes a small problem in that the idle tends to drop too much when you let up. Might be some other issue though.

I recently bought a cheap smoke setup for mine. Just like this:
http://www.ebay.com/itm/EVAP-Vacuum-...DPL.CASSINI%26
works well, but have not had success with it yet due to the difficulty in blocking off the main air intake. Thinking about making a plate with rubber covering that could be tightened down.

For air leaks that effect the mixture but not the idle, have a look at the idle mixture adjustment screw. There is an O ring that is supposed to seal it, but kinda micky mouse. Another source of air leak can be the plastic boss that holds the cold start injector. The boss can crack. Also have a look at the boot that goes between the throttle body and the spider body. These can get hard and crack also.

Guy

Thanks! I had read that thread. Hard to follow but sounds like he had a mixture problem. Will re read.

Control pressure when hot is 44. I really think the deceleration valve could be an issue. The symptoms seem to fit. I had it cad plated and may have caused an issue.
I may be responsible for the driveability issues as I keep tweaking A/f and timing. Will work on this.
Thanks! I hunted around the mixture screw and found a vacuum leak at the base gasket.

I made a new gasket as the replacement is NLA. I also replaced the o ring. This eliminated that vacuum leak but hasn't seemed to help the original issue.

I have a Gunson Colortune on its way to me to fine tune the mixture.

Going to work on the decel valve next. Keep the ideas coming. Such a big help! Thanks to all!

Guy

I had an extra decel valve. I tried testing it with a hand held vacuum pump on the small nipple on the back side. Pumped it up to 25 and was able to watch the metal plate on the large center opening move down and open up. Released it and it went back to closed.

I am going to remove the unit on the car and see if the idle is changed. Perhaps I can swap this extra unit in to retain the valve.

I kept one spare of every vacuum valve from being plated. I purposely didn't have this one plated just in case the process damaged the valve. It was capped for plating but the process is pretty intense.

GlenL

The easiest test is to remove the vacuum line.

vze2jshn

I have a Gunson Colortune on its way to me to fine tune the mixture.

What model and/or adaptor works?

Guy

G4074 14mm is what I ordered

Guy

Thanks Glen! Removed the vac line and capped the vacuum source. Drove a couple miles, got the revs up past 3500 and returned to idle. No repeat of the floating idle. Car felt great even though it still needs a final tune.

Am I ready to say its fixed........90 percent. This car has taught me to be very cautiously optimistic

Just curious, what vacuum should I expect at idle? I am getting a steady 22 at 900 rpm

Thanks

karl ruiter

I have a colortune but have not been able to achieve much with it. For example, here was my latest problem:
-I had a stumble coming off idle.
-With my A/F meter I saw the mixture falling when accelerating off idle.
-Maybe a fuel filter issue?
-But under hard driving the mixture was fine, so how could it be filter?
-Now what?
-I put a pressure meter coming out of the pump.
-Fuel pressure out of the pump was dropping at idle.
-My alternator does not charge at idle, and I was seeing a 2V drop from system voltage to pump voltage (old wiring).
-I realized my pump was not seeing enough voltage to keep the accumulater charged at full pressure at idle so when I punched it the fuel was just not there, until the motor started spinning and the alternator came on.
-Put a relay to give the pump direct battery voltage, and problem solved.

Point is, I would not have figured this out without being able to see the mixture under driving conditions.

Guy

Great example! Yes an A/F gauge would definitely be helpful. If I had a o2 sensor, I think that would have been my first course of action.

I did use the colortune last night. Easy, easy easy. Once warmed up, I installed it and ran the car at idle, Color was pure orange. I leaned it out and watch the color go to blue. Took it a little further and the car stumbled, then turned it rich about 1/4 turn. All in all, it was a little rich by about 3/4 turn. I then tweaked the timing a little to get the vacuum a little higher. Got about 19 on the vac gauge.

Test drive revealed that the car runs fantastic and smooth from 1800 rpm on. Just a little rougher than I would like up to 1800rpm. Don't get me wrong, it starts smoothly under any condition and idles nicely but has a hint of movement at idle as if it has a light miss or vacuum leak. Can even hear is very slightly out the tail pipe. As soon as the revs hit 1800, it's a silky smooth symphony

​I think the hunt is still on but we are making significant improvements

Guy

So I think I got this fixed. So I decided to check the tune again now that I found and fixed a few vacuum leaks. Long story short, the timing was way off and the mixture needed some tweaking. I actually adjusted the mix, timing and idle speed in concert with each other. My timing was off because I kept adjusting it for max vacuum, which wound up being way too advanced. So I re timed it with the light, then adjusted the mixture and idle to smooth it out a little (very slight) all the while keeping the colortune in place. Then I bumped just a little more advance (around 31 degrees at 3k). Idle is nice and smooth and car no longer stumbles at low rpm.

Amazing how such small changes can make a difference.

kiwiokie

Glad you have it beat. Did you dismantle your throttle body and grease the bearings when you had it out? Trying to decide if I want to mess with removing the screws on the throttle body shaft and risk buggering them up. Cheers, John

kiwiokie

Did you have any trouble removing the butterfly screws?