Rough idle after valve adjustment

[digger]

When you set the valves tight like 0.15 mm (0.06") you end up with rough idle for the following reasons:

-Seat to seat duration increase 16 degrees on stock cam.
-25% bigger overlap triangle area.

This is why idle degrades.

The actual profile at higher lifts barely changes e.g. the 1 mm net lift duration only gets 2-3* larger with the smaller clearance.

Peak Lift is 0.1 mm higher so the actual area under the valve lift curve doesn't change much in the areas where the piston is moving fast.

if you set them loose like 0.35mm then the take up velocity is 4x as high (1600mm/s vs 350mm/s at 7000rpm) so the impact energy is 16X larger which is not good for rocker and valve train life plus they will get very noisy. if you set the clearances a little on the small side you are being more sympathetic to the rockers

Note that some catcams cams are specced to use 0.15mm clearance

[digger]

after 5 years i hope he got his idle sorted...haha

[estoguy]

Something else that might help with your idle... what's your plug gap? When I first got my E30, it had a rough idle and a DIY article on the old E30World site recommended setting the sparkplug gap to the upper end the spec to help even out the idle. Did that, and things were much better. Maybe it'll help you.

I use NGK ZGR5As, and I leave the gap where it is when they come out of the box, which is the upper end of the spec... never had problems with my idle since.

[richardb]

Old topic here but thought I'd drop this video for those who are searching details on adjustment:

[bastianshaw]

no. the eccentric being too close to the valve. you can actually slightly open the valve if you pull too hard on the eccentric while you tighten it down. that makes it run like crap.

I used the special tool last time - works perfectly, no guesswork.

where can i get this tool

[nando]

no. the eccentric being too close to the valve. you can actually slightly open the valve if you pull too hard on the eccentric while you tighten it down. that makes it run like crap.

I used the special tool last time - works perfectly, no guesswork.

[bastianshaw]

a slightly tighter valve won't cause issues, but an overtightened valve definitely will. Especially if it runs like ass when cold, and then smooths out a little when warmed up.

what waffleswaffleswaffles this mean? like if the eccentric nut is too tight?

[nando]

I do it under the eccentric by my head isn't old, I also do it hot.

[TimR]

So, do you set them with the Bentley Book method or back of the cam?? I prefer hot on an older engine with 150K + miles

[nando]

I wear leather gloves. It's hot but I try not to touch anything but my feeler gauge, 10mm wrench, and spring loaded eccentric tool..

[Cabriolet]

to do the timing hot the job has to be done very fast. and chances are 99% you will be burned. but if you don't do it fast enough when hot the valves will be too loose and making the taping noise. I also found that if I did a valve job when the weather was near <40F they came out a bit too tight.

[Brandon12V]

It's also likely you'll burn your hands. lol

Main reason I do mine cold.

[nando]

wait wait... Nando you just blew my fuckin mind... I thought the purpose behind a larger gap when hot was due to accuracy? Adjustment on a cold motor will always be more accurate than on a hot motor. Isn't this why Porsche only reccomends to adjust valves cold? I'm so confused now.. I really want to know the reasoning behind this.

nope! The gap definitely gets larger.

Either way is just as accurate. I like setting mine hot, because that's actually what they are going to be at with a running engine. It's not usually recommended because it's harder and you have to be fast. It's also likely you'll burn your hands. lol

[SpecM]

yep, that was it!

#2 and 3 exhaust valves were too tight (like .005-006in), I guess I goofed yesterday

I set them all to .008in cold, that seems to work for me. No lumpy idle and still runs like a scalded dog (as much as a stock m20 can)

[Brandon12V]

wait wait... Nando you just blew my fuckin mind... I thought the purpose behind a larger gap when hot was due to accuracy? Adjustment on a cold motor will always be more accurate than on a hot motor. Isn't this why Porsche only reccomends to adjust valves cold? I'm so confused now.. I really want to know the reasoning behind this.