Vacuum lines & RPM flat spot

ahdoman · 06-17-2009, 02:02 PM

77 2.7CIS - I was having problems with my cold start idle (car would start but idle was really low and required some gas to keep it running until it warmed up after a minute or two). After reviewing the vacuum / emission diagrams I discovered things weren't hooked up correctly. I re-did all vacuum lines (new hoses). From cold it started and idled great around 1100 rpm! However, after warm up it hunts for idle if I set it below 1500 rpm. Also there is a flat spot around 3200 rpm that goes away after about 3600 rpm. If I disconnect the vacuum line to the distributor the flat spot goes away completely. I need to re-check timing but this is on a 914/6 and is really a pain. So, before I do that; would too much advance or retard cause the flat spot? What about idle hunt?

longhornchris04 · longhornchris04's Garage

A rich fuel mixture can/will cause the idle to hunt. Now, if your spark timing is off then it can mimic running rich (I think).

If you can, check your CIS pressures. Another option would be to lean out your mix just a little to see if that fixes the idle-hunt. Search for CIS in the 911 tech forum, there's instructions on how to adjust once the engine is warm.

From what you've posted, my first check would be timing (even if it is a pain). Your fuel could be spot on but if your engine is timed wrong it won't matter.

Also, the 77 WUR has acceleration enrichment via engine vacuum. Are the vac lines hooked up to it correctly. If not, then your engine isn't getting proper enrichment.

As to the flat spot... sounds like timing to me, but I'm certainly not an expert.

ahdoman · 06-17-2009, 03:20 PM

I guess I should have mentioned; CIS has been tested and mixture is correct. What is "77WUR"?

longhornchris04 · longhornchris04's Garage

Warm Up Regulator - its part of the CIS system that maintains the control pressure and adjusts it for warm up (hence the name) and vacuum (some years). Over the years they changed, the later models (those with lambda control) don't have the vacuum adjustment - yours from 77 should.

The vacuum enrichment is a misnomer, its actually vacuum lean. As the manifold pressure climbs the WUR decreases the control pressure, resulting in a richer mixture, tied to WOT.

Anyway, if this isn't working correctly then your mixture goes lean upon WOT until the airflow sensor catches up.

If your CIS is indeed functioning then its very likely that your timing has too much retard. This would make the engine behave like its running rich (the idle hunt).

Flat6pac · 06-18-2009, 06:41 PM

The vacuum connection to the dist. on a 2.7 is a retard and should only be at vacuum at idle to vacuum retard the dist. for emissions. Disconnect the hose and put it on your tongue and you should have vacuum at idle. If you dont have vacuum your hooked above the butterfly in the throttle body
When you lag in the low 3ks it could be the counterbalancers in the bottom of the dist. not free to advance. I prefer to soak the distributor with the pod removed in WD40 overnight and check that the dist. can advance freely. Blow it clean, oil it up and reinstall.
Bruce

ahdoman · 06-19-2009, 08:23 AM

Thanks Bruce. I have the vacuum lines hooked up per the pic below (minus the EGR valve). The weird thing is I don't get the flat spot with the vacuum line unhooked. Also, the flat spot seems to follow the timing. In other words if I retard the timing the flat spot drops to a lower RPM. If I advance it, it will go up to a higher RPM.

Once I get the engine at confirmed TDC I'll pull the distro and check it out.

T77911S · 06-19-2009, 08:33 AM

sounds like it was running rich to compensate for the air leaks. a control pressure check is always a good idea to know where you stand. otherwise, it is always a question as to if it is right.
check the advance, and retard in the dizzy, make sure both work. most remove the retard, i left mine connected. it lets me run a little richer and not surge at idle.
set timing to 0 TDC and check for full advance. once that is set, check the CIS stuff.
your car also had/has a themo time valve (TTV). it is a very nice little device to have. it richens the mixture up even more for cold starts by blocking the vacuum to the warm up reg (WUR). it is only active for about 20 seconds.

if you have gages, do your presure checks like this.
first, unplug the electrical connector to the WUR. unplug the connector on the rear of the air flow meter. turn key on and check system pressure, about 5 bar.
check cold CP with no vacuum- .8 bar
start car, monitor cold CP, should jump to 1.8 bar within 30 seconds.
connect power to WUR, CP should rise to 3.6 bar with vacuum to the WUR. remove vacumm and check for drop in CP to about 2.8 bar.
if all these check, the WUR is good.

set mixture and try again. set to the rich side, just to the point the RPM's want to dip when you let off the gas.

T77911S · 06-19-2009, 08:41 AM

that diagram is not correct for your car. the vacuum line from the WUR to the TTV connects to the TOP of the WUR, not the bottom.
the WUR connects to the rear near the bottom of the throttle body. it needs vacuum all the time, except for WOT. the dizzy connects to one just below the butterfly, it needs vac ONLY at idle. this is were a vac gage/pump is nice to have. you can meassure the vac and make sure it drops just as you give it gas.

with fuel gages connected, you can see the CP momentarily change if you blip the throttle real quick, that is the vac enrichment circuit working.

ahdoman · 06-19-2009, 10:19 AM

Thanks for the info T77911S. However, I am confused by your explanation. I do not have a TTV. Should I swap the two lines from the WUR on the throttle body? Switch bottom to top and top to bottom?

BTW - just pulled plugs and they look good and according to exhaust sniffer a/f seems to be correct.

psalt · 06-19-2009, 11:48 AM

If I disconnect the vacuum line to the distributor the flat spot goes away completely. I need to re-check timing but this is on a 914/6 and is really a pain.

Sounds like you found the problem. If you have a vacuum retard distributor, leave the vacuum disconnected and set the high spec timing to the factory spec. Vacuum retard was a crude emission device used to squeak by the then current curb idle H/C test. It has no effect on WOT performance and negative side effects, if you are not taking an idle H/C test, you are better off without it.

ahdoman · 06-19-2009, 04:47 PM

Hey all thanks for all the help. Flat spot gone, vacuum / emissions lines all hooked up correctly. Starts good cold and hot. Time to drive it!