More Weber tuning help needed

si2t3m · 05-27-2002, 10:04 AM

Hi,

I'm trying to sync both right & left carbs to finish the ajusting procedure as posted by John a while back. Problem is that when i start playing with the throttle stops bolts, i either get a very low idle or the idle will start creeping up and go to 2000rpm (probably getting to the point where advance starts to kick from the advance mecanism in the dizzy).

I have new plug wires, new cap & rotor, MSD ignition, new BP6ES spark plugs gapped at .42, timming is set to 5BTDC a 950rpm. Took the distributor out, check & cleaned & lubed the internals with Bosch dizzy grease.

This is on a 3.0L that i just installed in my 914-6 conversion. I've rebuilt the carbs, adjusted the pumps, checked and ajusted float level. Jetted as per PMO's instructions, started with the basic settings for idle and air screws, same thing for throttle stops screws.

Got the engine to run at 950 rpm by holding my foot on the pedal, for timming purpose, then ajusted it around 1200 rpm to sync the individual barrels on each banks as per instructions. got it to idle at 1000-1200 but with uneven sync between right & left and checked individual sync on each bank again, and again...

Just trying to figure out what is happening to stop running from side to side of the car for hours!!!

TIA

Marc-Andre
914-6 conversion 3.0L, Webers

Raynald · 05-27-2002, 10:55 AM

Hello Marc-André,
what is the size of your idles jets ?
and how is your mixture screws ?

si2t3m · 05-27-2002, 11:26 AM

I'd have to go back home to check the idle jet size.

My idle mixture screws are all new from PMO. Carbs are IDT and as one of the idle mixture screws got ... lost... well i had to order a set of 6 but for IDA's. IDA's are more like > while the IDT's are like }. Mr Parr told me that this wouldn't make much a difference and to at least get 6 of a kind. As for now, idle mixture screws are 2 1/2 (for the air bypass that i didn't open) and up to 3 turns out for the ones that i had to open up the air bypass.

Marc-Andre

jabb · 05-27-2002, 04:26 PM

Hi Marc

One important factor is to have the Linkage adjusted properly.
If it is not that can give you problems with adjusting the idle.

I believe the Haynes Manual might have the procedure for adjusting the linkage. Sometimes you just have to go back and start from the begining. I would start with the linkage adjusments.

T Bird · 05-28-2002, 07:33 AM

Marc
When you balance vacuum side-to-side, you must have the linkage disconnected.

I presume that you have balanced vacuum at each carb--that is, with the linkage disconnected, you have each barrel of the right carb pulling equal vacuum at idle speed, and each barrel of the left carb pulling equal vacuum at idle speed. The vacuum reading for the right carb probably does not match the vacuum reading for the left carb. In order to get both sides to pull equal vacuum, you must keep the linkage disconnected (disconnect each press rod connecting the crossbar to the throttle arm of each carb). Now, slowly turn in the idle stop screw on the carb that pulls less vacuum until the vacuum reading increases to match the other carb. Get them equal. Where is your idle speed now? If it's too fast, slowly turn out each idle stop screw. Keep at it with the idle stop screws until you get the proper idle speed with vacuum balanced.

Now reconnect the press rods. If they are either too long or too short, they will exert preload on the throttle arm and screw up the idle balance. If that's the case, with the press rods connected, loosen the lock nuts on the press rods and twirl the press rods back and forth between your thumb and forefinger. You will see the rod get longer or shorter as you twirl. You will also be able to feel the middle spot between extremes, where the rod is just the right length. At either extreme (i.e., too long or too short), the rod will be difficult to twirl, because you are fighting the resistance from the throttle arm on the carb (i.e., either trying to open the throttles, or pull them shut). When you find the sweet spot between extremes, lock down the nuts on the press rod and repeat for the other carb. Measure vacuum again to make sure that you are now balanced.

Check side-to-side at 3000 rpm next, obviously with the linkage connected.

What are you using to measure engine speed? The tach on the dash or an external tach?

yeuporsch · 05-28-2002, 08:53 AM

John,

I followed your thread on tuning the weber. I believe it's about 90% tuned, very slight exhaust backfire, but a whole lot better than before. I can't believe how much better the throttle response and acceleration on my 72 911S. I will run over the adjustment again this weekend. Thanks for the excellent outline on the weber adjustment.

John
72 911s

P.S. When you guys talk about adjusting idle jets, to get them larger, does that mean going to a machine shop and having them drilled out?

T Bird · 05-28-2002, 09:49 AM

John
Thanks. Glad to hear that the car runs better now than before. Do you mean changing idle jets? You can buy new idle jets in bigger sizes for the Weber 40 IDA 3C from PMO. You can also buy a set of jet drills and drill out the jets to bigger sizes. I would just buy new jets if needed, unless you are going to do this a lot. Then the drills make better sense.

Keep at it until you get it perfect. This is really an exercise in patience. Remember that you must nail down each step in the process before moving on to the next step.

Good luck.

si2t3m · 05-28-2002, 11:46 AM

Finally got them synched after taking a step back. 1-2-3 didn't react much when i played with the idle stops as compared to 4-5-6. So instead of trying to bring down 1-2-3, I increased 4-5-6 up and then got them both to come down slowly.

Now all barrels are even and synched.

Next step Idle Mixtures... working slowly i got 6-5-4-1 to react when turning in the idle mixture screw in. 2&3 even with the idle mixture screws completely closed doesn't do a thing... I checked the idle jets and they are clean. Checked to see if i've got spark. Yes on #2 erratic on #3. I checked this with the strob light.

Repeated test on all of them. #3 gets the stob light to flash but there is long moments where nothing is going on. I'm trying to rule some stuff out now concerning #3 wire. I have another cap so i'll try it. Have spare wires too so i'll try them also.

I'll be pulling that bank off and inspecting it again carfully again tonight.

I think that my idle circuit might be clogged. Did a little test run (after all this work, had to take it for a spind). It's jerky at low rpm but at 3000rpm+ then fun starts.

I'll get there... Slowly but I'll get there!

Marc-Andre
70 914-6 conversion 3.0L, Webers