Problem after valve adjustment

expatriot98 · 06-07-2001, 04:52 AM

After doing my first valve adjustment, I've noticed an occasional backfire through the carbs. It seems to happen soon after I go into throttle as in after a gear change or even starting out in first. The car is a '70E w/webers. Any thoughts?
Thanks in advance,
Mark

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Mark B. Wilson
1970 911E
mbwilson@home.com
http://www.marksobsessions.20m.com/

Superman · 06-07-2001, 05:05 AM

Not directly the valves' fault, I suspect. Just sounds lean.

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'83 SC

Jdub · 06-07-2001, 05:30 AM

Congrats! That's great. Now I would have the CO2 adjusted and the dwell, then timing, checked. Kind of like ironing out the wrinkles on the bed; you start at one end then proceed to the other end. Valves, dwell, timing, then the carbs, last of all!

Jw

Rot 911 · 06-07-2001, 05:43 AM

Just a thought: Could also be that one or more of your intake valves are not properly adjusted so that they are not closing completely and on the firing stroke you get the backfire thru the intakes.

Kurt V
72 911E

expatriot98 · 06-07-2001, 06:52 AM

I adjusted the valves as the Haynes instructs to a "stiff sliding fit" of the feeler gauge.

If one of the intakes is adjusted too tight, is that what Kurt is saying?

I also failed to mention that the car has the Pertronix thingie. I replaced plugs and cap while I was there, but haven't checked timing yet.
Mark

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Mark B. Wilson
1970 911E
mbwilson@home.com
http://www.marksobsessions.20m.com/

addictionMS · addictionMS's Garage

a piece of advice I was given that has served me well, was to go to someplace like Sears and buy a Micromiter that you can set to .004". Use the micromiter set to .004" to get the feel of how much drag there is on a feeler gauge when you have a gap of the correct size. I ofter re-calibrate myself prior to every valve adjustment session.

Jdub · 06-07-2001, 07:27 AM

Yes, I think Kurt is saying that a valve may not be loose enough, thus not shutting and causing blowback as the piston rises.

How were the valves as you adjusted them? All tight? Was any particular valve way out of spec either way compared to the others? It is possible that, IF you pulled your plugs just before adjusting, you may have lodged some carbon between a seat and valve face. This would have been a "my that valve is very loose" observation. Screwing down the adjuster on a valve in this condition causes it to be very, very tight.

Could also be that you are on the correct road though, and simply need to bring the other specifications into line. CO2, etc.

Jw

BK911 · BK911's Garage

Excellent idea with the micrometer! Popping could be due to too lean or improper timing. Set/verify your timing first, then adjust your carbs. Did you adjust your valves on a cold engine?

Rot 911 · 06-07-2001, 08:12 AM

Mark, yeah what I was saying, not too clearly, is that one or more of your intakes may be adjusted too tight. I know it doesn't seem possible, but it can happen. This winter I adjusted my valves, with the engine out of the car, and ended up having both the intake and exhaust too tight on number one. Discovered this when the cylinder was not firing and I realized I had no compression on that cylinder. I have no idea how I managed that feat of carelessness. But readjusted and all works fine.

Kurt V
72 911E

expatriot98 · 06-07-2001, 08:47 AM

The micrometer idea is perfect, Addictions. That was one of the areas I was confused on. A "stiff sliding fit" is a pretty nebulous thing.

I think the first thing to try is to recheck that side of intakes. Then as you guys suggested, timing and then carbs. All of the settings have been as the PO had the car set up for Seattle. The carbs probably do need a little tweak for Dallas.

Another Weber question. This set of carbs have probably 100K miles on them. Is the mileage an indicator for the need of a rebuild. The car runs good through the rpm range, but there are times when she "loads up" and needs her pipes cleaned out. Does a rebuild have to be done by a pro, or can I kit them just like my old muscle car Holleys?

Thanks for all the help guys
Mark

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Mark B. Wilson
1970 911E
mbwilson@home.com
http://www.marksobsessions.20m.com/

Early_S_Man · 06-07-2001, 10:11 AM

Mark,

Yes, you can do your own Webers, just like the old Holleys ... might take a bit longer, though!

There is a fairly common phenomenon that explains how some valves on the VW air-cooled and 911 engines can end up too tight after adjustment. On certain rockers, the drag between rocker threads and adjuster thread is minimal, and the adjuster screw has much more drag between it and the jam nut, even though the amount of engagement is only about 1/3. In cases like this it is very common to have the clearance tighten-up when you tighten the jam-nut against the rocker. If you look closely at the slot of the adjuster as you tighten the nut with a box-end wrench, you can see whether it is 'following' the jam-nut as you tighten. If this is one of the 'troublesome' adjusters, then it will take a few tries to judge the amount of arc angle to back-up before tightening. But, the clearance should be checked on all valves after tightening, whether you notice movement, or not!

So, recheck your clearances, and keep the above in mind on future valve adjustments! In a typical VW or 911 engine, there will only be a couple of 'troublesome' adjusters, but it doesn't hurt or take long to recheck each valve clearance after tightening the jam-nut. Good luck!

expatriot98 · 06-07-2001, 12:34 PM

Thanks Warren and all for the help.....
Mark

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Mark B. Wilson
1970 911E
mbwilson@home.com
http://www.marksobsessions.20m.com/

zotman72 · 06-07-2001, 02:16 PM

Mark where are you at with the MFI
system to replace those carbs? You
were donated a T system right?
I just got my S motor running today
with Denver's MFI wizard John Eisenbud.
My car really skoots. MFI rocks when
it works!
Dump those carbs and get with what
the Factory put on that engine. You
will never look back.

Bill
2.4S zoomer

expatriot98 · 06-07-2001, 02:52 PM

Hi Bill,
Yes that was me. RarlyL8 (Brian) very kindly donated his 2.4 mfi to me a while back. I thought about doing a 3.6 transplant but have decided to leave the 2.2 in place and go back to mfi. I want to rebuild the engine at the same time I install the mfi and that may still be a year out. In the mean time, I want get the carbs up to spec. When I do the change over, I can then sell the webers with a good conscience.

Another variable is that during a previous rebuild, 2.2S P&C's were installed. The decision there is whether to get the heads reworked and cams reprofiled as an S and have the mfi pump rebuilt for S specs. That seems appealling.

Oh well, it's all academic now. Several drastic changes occuring in my personal life right now are clamping down on the Porsche spending. Hills and valleys, man.

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Mark B. Wilson
1970 911E
mbwilson@home.com
http://www.marksobsessions.20m.com/