2.0 dual weber 40's, you guys know. trying to get a perfect balance on the carbs, pass side (using a spectrometer) reads 5 on both barrels, driver side reads 3 and 5 , if i adjust the throttle screw 5 and 7. have the idle at 900, timing and dwell are on. no more backfiring or popping, it actually sounds good. can't find a way over the last hump. is it a valve porblem?

Are all the jets EXACTLY the same size?
Valves are adjusted the same
Idle screws optimized for best running on all cylinders?
Spark plug wires fresh
Spark plugs fresh gapped the same

If so then I would adjust the air by-pass screw

However I suspect a partially clogged idle jet. or some crud in the idle circuit

this was my first thought... you sync each throat, and each carb...

Click to view attachment

each throat 1 1/2 turns out, wires, plugs, valves, points, plugs all correct. whats the procedure for individual throat syncing
joe disasembled both carbs, blew out all the circuits and jets prior to mounting the carbs, the jets are the same.

A partially clogged idle jet would affect the flow of air through the carbs? How? It seems to me that throttle plate position and any air bypasses in the carb would be the contributors to the flow of air through the synchrometer (assuming the engine is running at a constant speed). Interesting . . .

use the adjustment screws... 1 1/2 turns is the starting point. one or the other may need to go a little more or less... plus it's impossible to unscrew them the exact same.. so you may have to turn one an extra 1/8th - 1/4th to get all 4 throats equal on the sync.

With the butterflies closed air enters through the idle jet. There is some leakage around the butterfly and small holes in the plate. But it's minimal. What Joe said above is dead on.

If you adjust the mixture screws for flow you'll screw up the best idle mixture. You can't change the air fuel mix with the mixture screws. You need to open the air bypass on the low barrel and again adjust the mixture screw. Usually it's a plugged idle circuit. Make sure it's clear before you go this route.

Interesting. Really seems like the bleed of air around the butterfly with it X degrees before closed to set the idle speed and any additional air bypass bleeds would far outweigh any air bubbling through the idle jet, but if thats what you see then thats what you see.

The throttle shaft could be tweaked too.

A tweaked shaft will give you one high flowing barrel and 3 low. One low is most likely a plugged up jet. I'd peal it apart and blow it all clear first.

listen to jmill and joe, they know more than me. mine was on, but I thought I'd read somewhere the ind. throat might need a slight adjustment...

o/.k i am heading over to the car and am going to blow out the air jets and circuits again (compressed air) when that is done should they be put back completely in (closed)?

You need to have an equal combustion event to have equal air flow.

Air passing over the 28mm venturi creates enough air speed to suck fuel out of the idle jet.

At idle turning the screw in and out does affect idle mixture. only at idle up to 1200 or so RPM.

after that the air passing over the opening throtle plate sucks fuel through the progression ports via the idle jet.

after that 2500 to 3000 RPM the main jet will start flowing.

My carbs are nearly dead nuts the same on air flow. However they are not stock out of the box anymore.

I have reamed the idle air correction jets EXACTLY the same. I reamed the acceleration pump nozzles which were WAY off.
All my idle jets were not the exact same size as measured with a ream. Now they are and it made a big difference is smoothness.
Main jets and main jet air correction also got that treatment. I did some porting in the carbs too.

Jets all the way in with good O-rings.

By-pass screws all the way in

Idle mixture screws 1 1/2 turns out to start

Then adjusted to leanest possible before engine speed drops then back out 1/4 turn. (go slow and listen for the engine to react.

If you can turn in the screw past past 1 full turn from closed then you still have crap in the idle circuit passages for that cylinder.

Good to know, thanks

just tried all that and blew out the idle jets and circuit,removed the linkage, set the idle @900, no change. allow me to put forth a theory

when i had the problem with the valve adjustment screw stripping off on the passenger side, the balance on the drivers side was fine , since i replaced the screw and readjusted the valves on the pass side, the driver side carbs are off. could it be that i need to readjust the driver side valves, they were done before the engine went into the car. the engine hasn't been run for more than 2 hours since.

I replied on the other site just for sport.

LMAO

Actually the idle jet doesn't care about the venturi. The butterfly is closed. Air and fuel are pulled into the carb due to differential pressure (read suction). The idle jet has a set air/fuel ratio. By adjusting the mixture screw you can only increase or decrease the volume of that ratio at idle. Thats what I tried to say poorly.

i guess i have to start from scratch, readjust the valves and see whats what. am ordering a new cable and cluster bushings.will i damage anything if i drive it unbalanced? i want to have a little fun amist all this crap.
what does LMAO mean?

lmao = laughing my ass off

no, you will not hurt it with 1/4 of your cylinders a little out of sync if you want to drive it a little... it will provide motivation too...

Having your valves out of adjustment will affect your flow. If the gap is set bigger than the rest your intake will open late and shut early = lower flow. Check the valves. If thats not it pull out the mixture screw and the idle jet. Blow out the idle port with them removed. Blow out and inspect your idle jet. Check the holes in the butterfly. Make sure they're clear and that they look the same as the others. Some backyard tuners drill out the holes to lean up the progression ports. If that doesn't work I'm out of ideas. Maybe Joe can come up with something. He usually catches what I miss.

A copy of my carb tuning method post for Trekkor, a few years ago.

Assumptions: the carbs have the optimum jet and venturi package (good luck on this one), the float level in the carbs is correct, the cams are correctly timed, the valves are properly adjusted, the ignition timing is dead on, you have the proper spark plugs for your engine, the linkage is good, the fuel is good, the engine is good.

Remember that the mixture and air bypass adjustment screws are precision needle valves, not head gaskets. Use your fingers to tighten them, not your fist.
Start and warm up the engine.
Make sure the two drop links for the throttle linkage are exactly the same length, and disconnected. You can use a 8mm thin igntion wrench to snap them off.
Turn the mixture screws all the way in and then 5 half turns out.
Turn the air bypass screws all the way in.
Turn the idle speed screws out til it just touches, and then in 5 half turns.
Put on your hearing protection and start the car.
Use your STE and find the barrel that pulls the most. We'll call this one baseline.
Balance the barrel in the other carb that pulls the most with the idle speed screw. (if you have a Uni-syn, give it to someone you don't like and purchase a STE airflow meter)
Go back to the other carb, with the baseline barrel. You will have one all the way in, then use the air bypass screws and balance the other two barrels.
Go to the other carb and do the same thing.
Snug the jamb nuts on the air bypass screws.
All six (or four)barrels should pull the same amount of air at this point, if not repeat air adjustment proceedure.
Snap the throttle linkage drop links back on the carbs. If the idle changes then you need to barely adjust the linkage mounts so snapping the drop links on, doesn't change the side to side idle balance.
Use the hand throttle or a vice grip and rag to lock the linkage between 1400 and 1800 rpm.
Start back at the baseline barrel and adjust the mixture screw in or out, to get the smoothest running and highest rpm, then turn it in 1/4 turn.
Do the same with the five other mixture screws.
If you have to turn the mixture screws more than two turns either way, you've got the wrong jets.
Recheck side to side and individual air balance, adjust as needed.
Road test the car.
If you get snapping and poping out the intake, it's generally a lean condition.
If you get heavy exhaust fumes, or pboofing out the exhaust it's probably too rich.
If you get a flat spot or popping out the intake at between 2800 and 3200 rpm, you probably need larger idle jets.

That's a rough, five minute draft of my carb tuning proceedure, hope it helps!

If it goes good it should take about 45 minutes, if not about three years.

PK

Another good tip: If you think you have a lean cylinder (idle jet plugged) you can back out the jet carrier a half to one full turn and the engine should stumble, if it doesn't change theres a good chance the jet is clogged. This primarily applies to the 3 barrel carbs, as the 2 barrel IDF's have a different type idle jet set up.

Don't diminish the value of correct ignition timing, and valve adjustment, it makes a difference. Also make sure your linkage is dead nuts the same length.

1) welll yea!!!Please do get ALL the valves adjusted FIRST.2) may I suggest that the ...butterflys are not aligned , equally, with the IDLE ports(fuel ports,) .....I think you originally said ...everything was balanced and running well ..EXCEPT...one cylinder was off....that (cylinder may have a bent or distorted butterfly, which needs to be realigned....equally opening with it,s partner on the other bore,side.(both butterflys open equally.)....3) you have gotten some really good advise from those who made comments....you just should remove the carbs and overhaul them properly......and use a hose clamp on the fuel lines.....checking ALL the jet sizes ,and correction jet sizes is real good advise......postpone driving until the carbs are repaired,fixed..torching a valve,or burning a piston WILL really P.Y.OFF.

Man I really hope you get to drive this weekend. Good luck Rich

He is really just an RCH from perfect.

Something else to do that has not been mentioned yet. you have been setting the balance of the carbs at idle. so this means you should have one side of the linkage disconnected. and setting things with idle stop screws.

Once you get that sorted. you have to connect the linkage and artificially pull tension on the throtle cable till you reach about 2500 RPM now you need to synch the carbs using the hiem jointed reach rod on one side.

That is the final step when the carbs are pulling the same at "x" rpm everything will seem right with the universe.

[
o.k running better. took a nice 20 min drive. still high pitch popping on acceleration out of the carbs hesitation on acceleration. dobn't think this is pedal cluster related. redid all the valves, balanced the linkage, set the air bypass screws all in, mixture screws a .2.5 turns out, idle about 1000 rpm's, 3 barrels @5 on the syncrometer 1 @ 3.5., cannot find the trick to this one, if i raise the idle or tweak the mixture, both sides go up exponentially on that carb, if i back out the idle jet screw on the low barrel nothing seems to happens, if i back it out on the good barrel it stutters, i.m gonna buy new jets, venturi's and so forth but i have 2 questions
could this be a timing issue. i timed it @ 27btdc, this is a stock cam converted to carbs by the po.
given that. what size venturis , jets etc should i get.
i am posting this on 2 sites for maximum info

For kicks you could swap the carbs over and see if the probelm moves wth them - you have to move the linkages from one end to the other on teh throttle shafts, but it's not difficult - I did this on mine to try and isolate a bad barrel on one carb ( #2 cylinder) and it had no effect, the same cylinder was screwed up - found a nick in gasket to manifold on that barrel that i did not see before.
Just a thought - you are expert at removing them anyway, by now they should jump right off into your hands when you snap your fingers

[[/quote]
For kicks you could swap the carbs over and see if the probelm moves wth them - you have to move the linkages from one end to the other on teh throttle shafts, but it's not difficult - I did this on mine to try and isolate a bad barrel on one carb ( #2 cylinder) and it had no effect, the same cylinder was screwed up - found a nick in gasket to manifold on that barrel that i did not see before.
Just a thought - you are expert at removing them anyway, by now they should jump right off into your hands when you snap your fingers
[/quote]
are you suggesting switching just the carbs or carbs and manifolds?. would'nt that put my linkage in the back? neil while i have your attention. i see on your linkage a large spacer on the pass side. i didn't get one in my kit. i put an extra nut on to keep the angle symetric.

its a bit of a project but he is suggesting switching the carbs...that means you take off the linkage plates, swap carbs, then put the linkage back on and re-sync.

It is a valid test, but all it will do is show you one of two things.
You have one cylinder that has the valves adjusted wrong...or its got a leak...
OR
You have not adjusted the carbs correctly.

Each cylinder will perform differently, especially on an old motor.
Each venturi will behave differently also.

That is why there are individual adjustments for the air/fuel mixture on each venturi right from the factory...cause they all are different...just a fact of life.

I would suggest that you check the valve adjustment again on that one low cylinder, then, and only then, if everything else is ok...adjust the lower screw, not the one with the spring, not the idle just...
The lower one with the lock nut on it.
Adjust it while you have the sync tool in place.

Rich

[quote name='nycchef' date='Aug 6 2009, 04:35 PM' post='1200215']
[[/quote]
For kicks you could swap the carbs over and see if the probelm moves wth them - you have to move the linkages from one end to the other on teh throttle shafts, but it's not difficult - I did this on mine to try and isolate a bad barrel on one carb ( #2 cylinder) and it had no effect, the same cylinder was screwed up - found a nick in gasket to manifold on that barrel that i did not see before.
Just a thought - you are expert at removing them anyway, by now they should jump right off into your hands when you snap your fingers
[/quote]
are you suggesting switching just the carbs or carbs and manifolds?. would'nt that put my linkage in the back? neil while i have your attention. i see on your linkage a large spacer on the pass side. i didn't get one in my kit. i put an extra nut on to keep the angle symetric.
[/quote]
Switch the carbs only - leave the top plate of the linkage where it is and the manifold to head. Do not adjust anything, see what the sync says.

The CB kit for a 914 should come with that spacer, its black about 1.5 inches long - with the ball end for the drop link. Without it the two linkages cannot be parallel, they need to be in both in the front to back and side to side planes, or the carbs will be out of sync at full throttle - the drivers side carb will lead the other - not good, at full throttle the drives is wide open, the passengers still has a way to go.

Rich makes a good point also if the lower screw is not perfectly seated, or the O ring is messed up, you will see a difference on just one cylinder. Did you remove these at any time, maybe accidently swapped them over. Remove the one on that cylinder only and examine it, if the point is not perfect and not seating that could be your probelm.

you guys are taling about the idle jet correct, i backed the offending barrel just about all the way out and had no effect. the other side started to shudder wheb i did that. i removed the idle jets and blew out the circuit , i did not see an o ring. does it stay in the carb?

Idle jet is at the top.
It has a rubber o-ring.
It might stay in the carb...get a mirror and CHECK all four of them...make sure you have an o-ring on each one.

Idle air bypass is the screw with the spring...that is the one that gets screwed in tight, then backed off 1.5 turns.

Idle mixture screw is typically something that is set at the factory and should not be messed with..
Its setup on a flow bench to match each port to the other port...that is a simple function..

so, if that screw has been moved (it has a lock nut on it) then you may have setup that port wrong...it wont match the other port (thermo dynamics thing...air flow etc)

So, we are talking about three screws here.

What have you changed?
Did you adjust the screw with the nut on it? Its at the base of the carb, it does not have a spring.

Top screw, Idle Jet
Bottom middle screws, Idle mixture screws
Bottom screws with Jam nuts...Air bypass screws (used to balance carb on flow bench)

So...which ones are set where?

Rich

Agree, i'm also talking idle mixture screw, which on my carbs is seated firmly and has not been adjusted after i did carb rebuild - most sources seem to say close it completely and don't touch it as it is very sensitive - which i did.

No, sorry...that is why I posted the picture.
Idle mixture screw is the one with a spring. Its typically seated, then backed out about 1.5 turns...then you adjust each one in until the car starts to die...then back it out about a 1.4 of a turn. Done.

I posted the pic so we would use the same terms.

The screw with the jam nut...that is not so easy to mess with...
That screw.....the "AIR bypass screw" is exactly what its called.
Its set at the factory.
If you remove it when rebuilding or cleaning...that could easily be his issue at idle.
It could explain why they do not sync up at idle from one side of the carb to the other.

Rich

Rich is correct, I replied without seeing his diagram first- we are both talking about the same screw - the one with the jamb nut - the AIR bypass screw - now - Richy - did you adjust it or not ?
I'm going back to my 928 and getting the a/c reinstalled and working - r_towle has the floor

air by pass screw yes i have been screwing with it all day. have never touched the idle jet, am goin back to the car to make sure they are all the way in and then try to adjust it using the right screw.

With your mixture screws all the way in I have no doubt you had some popping going on.

The air bypass is for equalizing air flow between barrels at idle. You only open it to adjust air flow. It will be set closed from the factory. Sometimes you open one, none or 3. You use it to bring the flow up to the highest flowing barrel.

I set the idle mixture at 2.5 to start with. At 1.5 you might not be out far enough to adjust in to best mixture. If your best mix is at 2 you'll miss it.

Um....
First...remove each idle jet and check with a mirror that each one has an o-ring...
You need to know.

then screw them in all the way...they are not adjustable.

rich

I am watching this with high hopes. For some reason I really want your car running this weekend.

its within an hour...he just needs to be methodical.

Rich

every thing seems to be in place. no amount of adjusting seems to have any effect on the #2 barrlel. hell i even pulled the spark plug wire off to move it out of the way and nothing happened, i cannot figure...wait, hold on, why would'nt the engine stumble when i remove the #2 cyl spark plug wire, it stumbles for 1,3 4. am i that dumb that all this is is as bad wire?

firing order correct? 1432 clockwise.

Dont be mad...we have all done it.

Rich

i not mad ,i'm a moron, firing order is correct, pulled the wires off 10 times. 1, 3, 4, all miss 2 just sits there. i assume this would give me a low reading in that barrel, if the gas wasn't exploding?

swap one and two.

Pull the wire off and unscrew the end of the wire that goes to the plug.
re-screw it into the wire the right way...

The plastic piece that fits on the plug actually screws into the wire so the middle screw touches the metal wire in the middle...be gentle and you can fix it.

Rich

thanks rich, do it in the morning. for now it's a cold beer , lay in the pool, watch the yankees

Ouch....I changed the channel.

Rich

The cylinder not firing wouldn't mess up the flow. The carb flows on the intake stroke and it fires on the compression stroke. Again, it sounds like a plugged idle jet. If you have no fuel it won't fire. If its plugged no matter how much you tweak the mixture needle nothing will change. If it's plugged your flow will be low on that barrel. If it walks like a duck....

Check that you have spark on that plug. If it does it's a plugged jet.

Not to be rude, but have you ever heard of overlap on a camshaft and its purpose in the process. No spark would absolutely change the flow of a port on one cylinder.

Backfiring...well there is raw fuel in the exhaust...that might contribute to it.

Rich

Has anyone mentioned..(again)...that the butterfly,(throtle plate)s, are probably NOT EQUAL.....causing an uneven air flow reading at Idle....and all this confusing multiplicity of adjustments with idle air, idle fuel,etc.etc.etc.etc.....and if there is an ignition problem....it WILL affect airflow readings.