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Wink Weber rebuild

I am quite certain that i need to rebuild my Webers on my 1970 911. After numerous attempts at tuning (ie air/fuel, idle adjust, synch.), i still have "spitting" back through the carbs and an occasional back fire out the exhaust. I think there is rubber piece in the carb that has deteriorated. This has to be eating HP and efficiency b/c when it spits, i can feel the engine stutter.

My theory is that the engine is getting too much fuel. There is wet fuel that sometimes comes out of the exhaust. I can't turn down the idle adjust b/c it doesn't idle as it is right now. I think the spitting/popping comes from excess fuel vapor exploding in the carb or exhaust when it gets hot enough.

This has been happening since: The engine was rebuilt w/ new solex cams. The carbs sat out for about a 1 and 1/2 years then put back in.

Well tuning hasn't seemed to work, but give me you thoughts and ideas.

Patrick O'Donnell
1970 2.2 Widebody Coupe
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Old 05-08-2002, 06:24 PM
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I think what usually happens is that the idle and fuel mixture gets clogged at the end of their tubes by carbon and unburnt fuel making it hard to adjust them. The only rubber part that could deteriorate would be the accelerator diaphragm. The only other rubber parts are washers.

Another possibility is the throttles. They can get sticky or wobbly (if they have excessive miles)

Rebuilding is easy. But you might try cleaning the throttles with a little B-12 Chemtool. Also you can use the B-12 to clean the tubes where the idle and fuel mixture screws screw in.

Bobby
Old 05-08-2002, 11:04 PM
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Are you sure one or more of the floats are not sticking? If the needle valve is not shutting off fuel supply when the float reaches the proper level, you will get the kind of rich condition you describe. Check all your float levels before you start tearing things apart.

TT
Old 05-09-2002, 07:50 AM
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what size are the idle jets. the number on the jet is not to be trusted, because too many have been drilled oversize. you need a jet gauge. you should have at least a .55, maybe even a .60 for larger and cammed engines. the small jets always cause popping and spitting. an incorrect mixture adjustment/balance can cause it too, among other things.
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Old 05-09-2002, 08:26 AM
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John's suggestion that (among other things) the idle jets may be poorly sized sounds right to me. In my experience, popping in the carburetor body has always betrayed a lean mixture. Explosions in the exhaust are nearly always caused by an exhaust leak.
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Old 05-09-2002, 08:33 AM
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By exhaust leaks your mean?: the exhaust valves leaking?
Also How easy is it to get at the throttles/jets w/ the carbs still on the engine in the car.

And yes i was thinking the accelerator diaphram was deterioted b/c of sitting out and dry hard. What would this do??

Patrick
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Old 05-09-2002, 02:03 PM
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Cerbin,
Those Webers on your engine are 32 years old. Typically, the clearance in the throttle shafts wear to a point they allow air to be drawn in and create a lean idle/part throttle mixture.

With the engine idling, press each throttle shaft (in turn) off axis and see if the idle changes; or squirt some carb cleaner (propane works too [unlit]) at the throttle shafts and see if it affects the idle speed. If so, you have leaking throttle bores and they will need to be rebuilt by someone like Eurometrix. BTW, don't discount the other suggestions provided for you.

Sherwood Lee
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Old 05-09-2002, 03:16 PM
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By exhaust leak I mean air getting past ann exhaust gasket, such as the six that are between the heat exchangers and the heads, or between various exhaust pieces. Every single time I have mentioned exhaust popping to a mechanic, the mechanic has assumed there is an exhaust leak. One caveat is that some of my fellow Board members are certain they have seen exhaust popping caused by other reasons.

Carburetors do wear at the bore that the throttle shaft goes through. Grab that throttle shaft and wiggle it.

Torn Accelerator pump diaphragms make a car hesitate when you open the throttle if there is only one carb. If there are multipe pumps, then one torn diaphragm would probably just make the car run poorly. I know nothing about triple-throated Webers, but I have been told they lack accelerator pumps.
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Old 05-09-2002, 03:47 PM
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If there is "wet fuel that sometimes comes out of the exhaust", as Patrick said in his first description, I don't see how this can be a "lean" condition. It sounds like a fuel over-run in the carbs, to me. This can be a very dangerous condition and could result in engine damage or, god forbid, a fire. I don't want to be an alarmist, and I am just a shade-tree, "hobbiest" mechanic, but BE CAREFUL! Raw fuel is a very volatile substance, and fuel vapor is explosive.

If liquid fuel is coming out the exhaust, it can also be washing the oil from the cylinder wall, bypassing the rings and contaminating the lube oil. This is not a good thing. Sample the oil from the sump. Does it smell like gas, or is there noticeable dilution? Are any of the plugs fouled or wet? This may give an indication of which cylinder (or cylinders) are problematic. The Webers have two float bowls for each bank, serving 3 barrels.

Checking float levels is not a difficult job, but you do need a special fixture to do it with the Webers. PMO improved the design by supplying a sight glass in each float bowl for checking this, but with the Webers you need to attach a float gauge to each chamber. Then use small copper washers under the needle valve to set the height of the fuel in the float bowl. If the level is set too high, or the float is damaged, leaking or sticking, the fuel pressure can push too much gas into the motor because the needle valve which the float actuates never closes.

Remember, free information you get over the internet is worth just as much as you paid for it. This applies to my message as well. All anyone has to go on is what you wrote in your brief, original message and their personal experience.

TT
Old 05-09-2002, 05:20 PM
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Cerbin
Are you sure that the liquid you see at the tailpipe isn't condensation?

To set up the Webers, you should have performed the following steps:
1)adjusted accelerator pump quantities
2)set float levels
3)equalized vacuum on each carb using idle air correction screws
4)balanced vacuum side-to-side @ idle and at 3000 RPM
5)adjusted idle mixture
6)set idle speed

Changing cams could result in the need to rejet. You'll never know unless you first try to set these carbs up properly. If that fails, then start looking at the things suggested by others in the responses above.

If you are unsure about how to set up your carbs, drop me an email. I have some step-by-step instructions that I developed that may help you.

Good luck.
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Old 05-09-2002, 06:49 PM
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Hey john I have a few tuning guides and such but please send your info my way. PattyOD911@aol.com

BTW the intake and exhaust popping usually only occurs on deceleration and cruising at constant speed.

Thnx for all the support
Patrick O
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Old 05-09-2002, 07:44 PM
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Cerbin
See below. I had to post in two replies due to the length. Hope it helps.
John

Weber Set-up
Part I

STEP 1- Accelerator Pump Volume
Measure the accelerator pump volume at each of the 6 nozzles. Before starting the car, turn the key “on” and energize the fuel pump. Let the pump run a minute or so to fill up the float bowls. The air cleaners are off. Now, lower the vial down the throat of barrel No. 1 and park it directly under the nozzle. Have the assistant in the car depress the gas pedal all the way, nice and easy. Withdraw the vial, record the results on a piece of paper, and dump the contents of the vial. Repeat at each nozzle. (You may have to remove the velocity stacks for cylinders 2, 3 and 6 to gain proper access to the nozzles for those barrels.) Adjust the nut on the accelerator pump rod as necessary. Turning the adjuster nut out will increase the amount injected, and vica versa. You want .6 to .8cc (ml) (.75cc works great). Get them all even.

This procedure can also be done on the bench. Just fill the float bowl that has the brass valve in the bottom that feeds fuel to the accelerator pump. Then activate the pump and measure. The advantage of measuring at this stage, as opposed to when the carburetors are attached to the intake manifolds, is that if there is a blockage, it is easy to get at the check valves, nozzles, pumps, and float bowl valves and clean them out.

This procedure also provides a good opportunity to check that the throttle plates are opening all the way (vertical) with the gas pedal floored. If they are not, turn the plastic adjuster under the gas pedal in to give more travel, or out (less travel) if the throttle plates rotate beyond the vertical position.

STEP 2- Float Level Adjustment
Remove one of the two float bowl plugs on one of the carbs. There are two plugs per carb since there are two float bowls and two floats per carb. Careful- all the fuel in the float bowl will dump out so catch it with a little jar. Now insert the tool and tighten it to prevent leaking, since any leaking at the washer will give false readings. You need to focus on the upper two lines scribed on the vial portion of the tool. The lower two lines are for the Zenith carbs that some 911T's used (70-71). Have the assistant turn on the key to energize the pump but don't try to start the car. Now the float bowl will fill and you'll see gas fill the vial. After the vial is through filling, carefully examine where the meniscus is. You want it right between the top two lines. Turn off the key. Remove the needle valve plug, back out the needle valve and seat and add or subtract washers as necessary to raise or lower the fuel level, respectively. (ADD SHIMS= INCREASE FUEL LEVEL; REMOVE SHIMS= DECREASE FUEL LEVEL). Replace the needle valve and seat and the plug and have the assistant turn on the key. Note the fuel level. Continue to adjust until the meniscus is spot-on. The washers that come with the tool differ in thickness. After you do one, you will develop a sense for how many washers you need to raise/lower the fuel level. Then remove the other float bowl plug on the same carb and repeat the process. Keep at it until you've got them all perfect.

A couple of things- some books say to measure the fuel level with the car running. I've tried it that way and believe me, it's pretty hard to do since the engine is vibrating when it’s running. This vibration makes the fuel is slosh around in the vial and thus it becomes difficult to determine if the level is correct. This is exacerbated by the fact that the two lines scribed on the vial are near the very top of the vial. Second point-- the washers under the float bowl plugs are crush washers that are good for one shot. After you replace the plugs, if you re-use the washers, they are likely to leak. If you have some extra ones, use them. If you don't, don't overtighten the little bastards. Just snug them and move on. You can always order more from PMO and then go back and replace them at your convenience.

STEP 3- Initial Start-up Adjustments
Make the initial adjustments to the screws as per PMO and fire it up.

Initial adjustments (PMO) are as follows:
Idle air correction: 1/8 turn out*
idle screws: ½ turn in after touching throttle arm on each carb
idle mixture screws: 2 turns out

(*I suggest turning all the idle air correction screws in (gently!) until they hit their seats, and lock them down for the initial start up. You want a nice rich mixture for starters to avoid excessive popping which is really distracting, and then work from there.)

Here is where it gets tricky- while your assistant in the car keeps the motor running by judicious applications of throttle so that the engine warms, you have to diagnose in a grossly rough way, what is going on and what you need to do to smooth out the idle so that you can get the car running tolerably well enough to begin the real work of balance, synchronization, mixture adjustment and idle speed adjustment.

If the engine idles (regardless of speed) with the initial settings, do the following: if any barrel is spitting, slowly open the mixture screw- like 1/8 of a turn at a time. This should reduce the incidence of popping/spitting. Once the popping is reduced, quickly measure vacuum on each barrel with your STE. Are the barrels of each carb pulling roughly the same vacuum? If not, and vacuum is all over the place, determine which carb is pulling, on average, more vacuum than the other. Then open up the idle stop screw on the carb that pulls less vacuum, or turn in the idle stop screw on the carb that pulls more vacuum, or both, so as to get both sides roughly equal. Keep engine speed close to idle speed (900 rpm).

If the engine won’t idle, have the assistant hold it at a faster speed and adjust the mixture screws to avoid excessive popping. Once the engine runs smoothly, release the gas pedal and see if it idles. Check vacuum and make any adjustments to the idle stop screws that are necessary to get the car to run at idle. Once the car is warm (140 degrees or better) and holding an idle, it is ready for accurately setting balance, sync, mix and idle speed.

END PART I
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Old 05-14-2002, 05:23 AM
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Cerbin
Part II below.
John

Weber Set-up
Part II

STEP 4- Vacuum Adjustment- (balance)
Disconnect the right and left carbs from the crossbar by undoing the short press rods on each carb. Use an 8-mm wrench to pop the ball cups free of the throttle arms. You don't want the linkage affecting this adjustment. Pick a carb. At 900 rpm, measure vacuum at each barrel and record results. Determine which barrel pulls the most vacuum (“reference barrel”). Lock down the jam nut on that barrel’s idle air correction screw. Now, carefully and slowly turn out the idle air correction screw on one of the two barrels that doesn't pull as much vacuum as the reference barrel. Get the vacuum exactly equal to the reference barrel. As you open the idle air screw to equalize vacuum, you are going to experience popping, since you are leaning out the mixture by introducing more air into that barrel's idle mixture. DON"T PANIC! Just open the mixture screw up a hair to compensate so that the popping doesn't distract you, or if it's just occasional popping, you can leave it alone for the moment. When vacuum is equal to the reference barrel, lock down the jam nut. Equalize vacuum in the remaining barrel the same way.

Now, you have all three barrels on the one carb pulling equal vacuum. Repeat the process for the other carb.

Step 5- Side-to-Side Balance @ Idle (synchronization)
Using the STE synchrometer, determine which side is pulling more vacuum. Turn the idle stop screws out or in a hair until both sides are pulling equal vacuum and you have 900 rpm idle. Turn the idle stop screws in to speed or out to slow, keeping each side balanced with the other. Now, the engine should be running at 900 rpm with each barrel pulling exactly equal vacuum. You are probably getting an occasional pop since your mixtures are all off.

Step 6- Side-to-Side Balance @ 3000 RPM (synchronization)
Reattach the press rods. Check the side-to-side balance at idle to make sure that the linkage isn't off at idle. If the press rods disturb the idle balance, adjust them so that they exert no preload on the carb throttles. Now you are ready for balance at 3000 rpm. Have the assistant hold the engine at 3000 rpm. Quickly run the STE over each barrel and record the results. Each barrel of each carb will match. But, your linkage could be off, causing the right carb to be out of sync with the left—that is, one carb may be pulling more vacuum than the other at 3000 rpm. If one carb is pulling more vacuum than the other @ 3000 rpm, lengthen or shorten the right/left press rods to compensate. When you get vacuum to balance at 3000 rpm, check that the linkage adjustment hasn’t screwed up idle balance. When vacuum balances at idle and at 3000 rpm, you are synchronized and ready for the "artful" part of this process, where it helps to have the ear of a musician.

Step 7- Mixture Adjustment
Speed the car to 1200 rpm using the hand throttle in the car to hold it there. (This is the BA method- put rock on gas pedal. PMO advises to disconnect the right press rod and speed the engine with the idle stop screws. I use the BA method.) Now start with barrel 1. Slowly turn the mixture screw in until you hear slight popping in the muffler- a little more and the idle speed drops. You may get an occasional BIG POP while doing this- this is normal. As soon as the idle speed drops, slowly turn out the screw until idle speed comes back up- this should take about 1/8 of a turn. You may hear that faint popping in the muffler at this point. If you do, turn the screw out a bit more until the popping disappears and the engine runs smoothly. (NOTE: Slow turns on the screw are warranted. Reaction to changes in the screw’s position is not immediate.) Now, note the position of the screw and slowly give it 1/2 a turn more. This is the initial setting. If it continues to pop or spit up before moving on to the next barrel, turn out the mixture screw an additional 1/8 turn. All should be well with that barrel for the moment. Proceed to barrel 2- same process- then 3, 4, 5 and 6. If it took ½ plus 1/8 of a turn to get barrel 1 to behave, then turn out the mixture adjustment screws for all of the other barrels the exact same amount.

After you run through them once, you will have some doubts as to whether you got the mixture on the first few barrels equal, since as you move through this process, you will begin to develop the "ear" and the "feel." Start over with barrel 1. Make sure that each screw is turned out the same amount from that point where the idle speed returns without faint exhaust popping after it has dropped as a result of turning it in. The point here is to get them all equal. Run through them as many times as it takes to smooth out all popping and spit ups and to get satisfied that you have them all open the same.

Step 8- Idle Speed Adjustment
Release the hand throttle and let the engine return to idle speed. If idle speed is off, disconnect the right and left press rods. Adjust the idle speed to 900 rpm with the idle stop screws. Make sure that side-to-side vacuum remains equal. The engine should be running smooth- no popping and holding a nice idle. Reconnect the press rods and check balance at idle and at 3000 rpm again just to be sure!

Step 9- Test Drive
Take the car for a test ride. Is there any surging between 2000 and 3000 rpm? Is it popping on deceleration? Is it transitioning seamlessly between idle and main circuits? That is, no flat spots, right?

Return to home base. Make any final adjustments that you feel are necessary to the mixture screws. In other words, if it popped occasionally or surged, turn all mixture screws out an extra 1/8. Repeat test ride. Make any further adjustments that are necessary.

If the mixture screws are turned out more than 3 ½ turns from full in, the idle jets are too small. If the engine stumbles on acceleration or is slow to return to idle, the idle jets are too small.

End Part II
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Old 05-14-2002, 05:26 AM
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Great write-up, John! Thanks for taking the time to post it. This should be archived as a tech article, I think.

TT
Old 05-14-2002, 08:08 AM
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John,
I concur with Tom (ttweed). Good job for a well-written (and accurate) description of the process. Everyone with Webers should save and archive this info.

I'd like to add that small adjustments of the idle mixture screws can make a big difference in the air/fuel mixture at idle. And since the idle mixture on these carbs continues into part-throttle operation, some fuel economies can be found in this area. Most cruising, even at highway speeds, is done at part throttle. I used one of the popular DIY exhaust gas analyzers to fine tune the idle mixture. My idle mixture went from from 8% CO down to 4% in just a few fractions of a turn of each adjuster. Mileage went up too which shows you how far off my adjustments were.

Sherwood Lee
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Old 05-14-2002, 09:39 AM
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