Rough Idle, Carb Synch Way Off

[admin]

Getting the 356 going after some months in storage. Had distributor rebuilt, new igntion parts, fuel pump, 40IDF's rebuilt, etc. etc.
Car starts fine but idles rough. No smoke from exhaust.Dynamic timing is spot on. Checked carb synch and cylinders 1 and 2 are balanced at 4 kg. When I put the synch meter on 3 and 4 the needle bounces wildly from zero to 30 and back. The meter is actually making a loud tapping noise when the red indicator hits the top of the gauge. Never experienced this before.
Any help with this would be really appreciated.

[S.J.Szabo]

Mr.Xenon:

Allow me to fill in some gaps so that the guys who know what they're talking about will have more info to help you faster.

Please review these assumptions:

1. The engine showed no such symptoms prior to being parked.

2. Nothing was done to the engine itself (valve train, heads, manifolds, etc.).

3. The engine was running the same carbs and dist. prior to their rebuild.

4. You have verified the correct operation of all the components of "spark" (rotor, cap, wires, connections, plugs)

5. You have verified the correct operation of the carb in question.

6. You have pulled the valve cover to verify correct operation of pushrods, rockers, springs, and valves (while turning by hand and by running).

I'm not smart, but I'd start with no. 6 and work up.

Someone knowledgeable should be with you soon.

[bbspdstr]

Mr. Xenon,

Yes, I, too, am waiting for someone who REALLY knows about 356 tuning to jump in.......

In the meantime, I'd assume that if the engine ran OK before and the valves were not touched.....then I'd still set them or at least check the lash.

Then, if you are absolutely sure timing is both statically and dynamically OK, I'd look at the carbs.

I was trained to initially set the idle air mixture screws two turns out from a gentle fully closed (in, snug) beginning. Then, once the motor starts and is warmed, those thumbscrews can be changed, in sequence, to make the engine idle as high as it can from adjusting in or out at each screw.

(I don't know squat about Webers anymore....we used to replace worn Solex and Zenith carbs with those Webers decades ago but now were are replacing Webers with Solexes and Zeniths now that they can be rebuilt well and the Webers became a less quality product than they were originally.)

Anyway, you then move to a final set with the air intake metering devise to a side-to-side balance of 1-2 to 3-4. Actually, check side-to-side to a rough balance, first...but all final adjustments need to be done warm and around 1000upm/rpm.

It doesn't hurt to fiddle with all this as you're killing time waiting for the warming...or, if it sounds well enough, go for a short drive, then get back to final adjustments without the linkage attached until last. You'd see or hear a change in idle if the linkage makes any effect/affect. Address throttle linkage issues separately but be sure the down-rods do NOT influence the carb arms at all. Actually, a small amount of 'slop' is better than perfect "full throttle".

Then, there are always 'the books' telling all this better than I can..

Good luck,
-Bruce
PS- don't discount that there could be dirt in a jet(s) on the obstinate side carb.....

[admin]

Thanks for the detailed replies.
Cylinder compression on cold engine with slow cranking is 113 psi on Number 1, 120 on number 2, 101 on number 3, and 122 on number 4.
Pulled rocker cover on 3-4 and all valves appear to be functioning normally. Valve lash is unchanged at 6 thou. Held a piece of paper in front of exhaust pipes and getting only positive flow.
Snail gauge on RH Weber is 5 kg and steady. Left carb is at 18. Getting an occasional spit from both carbs. Idle adjustment screw on number 3 will not turn either way with fingers.
Number 1 and 2 plugs are tan. Number 4 very light coloring and number 3 pure white.
Rough idle but seems to smooth out at higher rpm's.
Bad carb? What causes the high vacuum at idle?

[S.J.Szabo]

Mr.Xenon:

In addition to Mr.Baker's Always Excellent advice, I might offer a couple of thoughts from the cheap seats.

Let's start with a disclaimer: I remain a Uni-Syn kinda guy, so I can't really address the snail's readings.

That said, the fluctuations you described originally made me think of a sticking valve(s), but if things are steady again, that's probably not the issue.

On the other hand, I would note that while .006" may be correct for the exhaust valve, the intakes need a .004" set. If you're running larger here, the valve is not opening all the way, for one thing.

For another, I would surmise from your info that the "stuck" adjuster on no. 3 is screwed in very tight (verified by the plug color that is screaming "lean", I should think)and may be the root of part of the larger problem.

It looks like the next best step would be to re-set all four throats on the carbs. There is one little thingy tho; when you get no.3 adj. screw out (and maybe others as well), you will probably find a clear dent in the taper (from being screwed hard into the orifice) that will complicate a good setting during adjustment. For that matter, you should check them all for such damage. If you can replace damaged ones, do.

Now, I run IDFs too, and for whatever it's worth, I offer this: when (gently) bottoming the screws (to back out 1 1/2 turns to start tune), you will also be compressing the screw springs pretty tight, which can make it hard to tell if you've really hit bottom or not. I take the time to remove the springs and seal and count the turns in to bottom. Re-assemble, and count the turns going in, minus a turn and a half. It's kind of a pain, but eliminates a couple of potential problems.

Also, you might want to take advantage of the Webers' nifty air-correction screw; very handy to solve small balance issues.

At this point, I would suggest this:

Get all the plugs nice and clean, swap 'em around, double-check the sync of the carb drop links (since the carbs have been off and on again), re-set all the idle screws to 1 1/2 out and do the tune over again.

[admin]

Many thanks for the advice and suggestions. Very gracious to take the time to write thoughtful and detailed responses.
Turned out to be a vacuum leak in the carb. Installing new gaskets and/or adding some additional torque to the carb to manifold nuts solved it.
I should not have assumed that the problem was storage related.

[Craig Richter]

S.J.,
Good and well-presented carb tuning advice, especially referencing Weber's usually ignored idle air balance system. Many carb tuning authors suggest closing these screws and never to mess with them. I find their use can allow 700rpm idling with still immediate throttle response. If you like to tinker to get the set-up right, old Webers provide great performance.
C.R.

[bbspdstr]

..... If you like to tinker to get the set-up right, old Webers provide great performance.

Ahhh, Craig, the key words are "tinker" and "old." We 'in the biz-ness' used to joke that "you can't get rid of the Weber flat spot, but you sure can move it around."

Sure, over time, those who "tinkered" long enough found ways around that...but still professed a preference to Dellotos.

The OLD Webers were pretty good, then production changed to another country and we'd open fresh boxes and carbs to find chips and shavings and other debris. The quality and QG fell wa-a-a-a-y off. We welcomed those who, like Euro Metrics and Harry Bieker, found the interest in getting the carbs designed into Porsche engines rebuilt to be 'as new,' thus feeding to the "originality" mandate the change in ownership demographics demanded.

Still, as for carb attractiveness, for me nothing beats a Weber 48 IDA.

[Craig Richter]

Ahhh, Bruce, of course they are key words. Why do you think I used them?

[S.J.Szabo]

Mr.Xenon:

Glad you got it straightened out. I'm not surprised that my well-intentioned counsel missed the mark completely, but that's the hazards of listening to amateurs.

I do hope, however, you re-visited the no.3 idle screw.

In any case, I have long been enamored with the old-country 40IDFs that I installed in the middle '80s, replacing the Solexes I built the engine with. They are the older models with the cam-and-roller enrichment link and between this and the lovely throttle-shaft bearings, they have worked flawlessly for very many miles.

Perhaps its where luck figures in, but I have never had any flat-spot issues with these carbs, and they respond very well at low revs, allowing smooth idle-speed launches and are happy to tick over at 6 or 700 rpm.

Another case of being odd-man-out, I suppose.

[Don C]

S.J.
My car, a '65 356C, came with old Italian Weber 40 IDF's installed sometime in the early 80's. This car always had the hesitation off idle even after an entire engine and carb rebuild. Drove me nuts tinkering with them. I finally got the guts to attack the roller cam accelerator pump. To say it squirts a LOT of fuel per stroke is an understatement, something over .5cc if I remember. The reading I did for a 356 said it should be down around .1 to .15cc per stroke. After a lot of trial and error, disassembly and assembly, I got it down to .15cc per stroke. Upon reinstallation on the car it ran wonderful, pulls clean from idle on up. I don't know how the previous owners lived with it all those years. If you have the stock roller set-up and are working good I'm amazed!
The photo shows the slightly modified stock cam on the right and the final modified cam on the left.