*edit* Note, cylinder bank 3/4 is not firing. Plugs are oil fouled. All four cylinders have decent compression, considering that the rings probably are not set yet. Carbs are fresh rebuilt, I have yet to check spark.*/edit*

I have set my idle at 1K rpm.
Idle adjustment screws for the 3/4 bank is almost all the way out.
Idle adjustment screw for 1/2 bank several turns in.

Both carbs are idling at 5 kg/h according to my flow meter.

Mix screws for all 4 carbs are 2.5 turns open.

When I open the throttle to ~3K rpm, the 3/4 bank only sucks slightly more air. The 1/2 bank jumps up to about 15 kg/h.

So, where am I off.

I have my weber tech manual too, but thought I would ask here to see if I got a faster "ah-ha" type answer (like usual).

Freshly rebuilt carbs, by the way. All the passages should be clear. They have only been run for about 25 minutes.

Zach

Well, all things considered, for an engine running on 2 cylinders, it is running really smooth.

*sigh*

Started plying with the mix screw on 3&4, and neither did anything.
I pulled the no3 plug wire - no effect. I pulled the no 4 plug wire - no effect.

My motor is running (really smoothly, at around a 650 rpm idle) on only cylinder 1 and 2.

I am going to go do a compression test...

Zach

_something_ in the linkage is out of whack.

Compression numbers - keep in mind, this is a fresh rebuild that has not seen any road time (Bench test only), so I doubt my rings are set yet. Engine was warm for the comp test.

Cyl3 115
Cyl4 105
Cyl1 115
Cyl2 122

So, the engine is making compression.

The spark plugs for 3/4 are oil fouled (duh) and the plugs for 1/2 look perfect.

I did leak down (engine cold) the other day, and everything was between 8% and 12% leak. Again, for a cold engine, with rings not set yet, thats not too bad.

What do I want to be looking for? Spark next I guess.

spark will have nothing to do with how much air the engine is sucking in.

your fouled plugs match the side with bad airflow.

find out why that side isn't open as much or why the other side opens too much. you do not need the engine running for this - you can eyeball the bores and check it out. if there are connector links from the individual bellcranks those must be exactly the same length.

at half throttle it should be easy to see whether or not the butterflies match exactly.

You didn't say (in this thread) how big the engine is nor the venturiis. 44's pretty big. BTW - if the venturiis are not the same on both sides (you checked, right?) you will never match flow....

Engine is a 2056.
Don't know the venturi size, or jet size. I bought the 2056 engine and carbs together, and then rebuilt the engine and carbs. The engine due to poor leakdown (collapsing registers) and the carbs just because I wanted the experience.

I should have made note of the venturi and jetting when I had it apart, but I did not. Looks like another rain band coming in, so I have to get the engine cart back in the garage.

By the way, I am getting spark to all 4. I cleaned the oil fouling off the plugs with a wire wheel, and the bank fired again. Engine idles great as low as 500 rpm.

Where should I set my engine idle?

The 3/4 bank is still sucking less then the 1/2 bank. Bring up to about 3K the 3/4 bank rises to about 7.5 kg/h, while the 1/2 bank is 2x that.

I'll eyeball the butterflys at 1/2 throttle, and see what I can see.

Zach

switch the carbs left/right and see if the problem follows the carb ...
Andy

Its raining now, so I can't run the engine w/o gassing myself. Will try swapping carbs tomorrow.

I did pull the jets, and all the main jets are .125
The Aux Venturi are all 4.5
I can't figure out how to get to the main venturi. it looks like that Aux Ven. are staked in place, so I can't remove them. I don't know how to pull them out without damage. I know to remove the lock nut and remove the gas jet to pull the Ven, but can't do that with the AUX ven staked in.

How do I do this?

Zach

you're seeing a huge difference in airflow, and even allowing for nonlinearities it'd take a pretty big (i.e. easily visible) difference in venturii size to be the cause.

-probably- it's not a venturii difference unless someone went out of the way to mismatch something (that is - no one sells venturiis in pairs...). I think it's probably a linkage geometry difference you'll figure out once you start measuring stuff.

good luck with it.

Ya needs a UNISYNC.....

Disconnect the linkage and check the flow on all four throats....

Clean the plugs and re-install. If the rings need to seat, let it run and watch the oil temps....

You did measure the airflow with the linkage removed at least on one side yeah? And start from scratch?

Why are they oil fouled? Carbs won't cause that.

Do a,leakdown and compression check. It sounds mechanical to me.

Swapping carbs will tell if there is an issue with a carb, if the problem follows the carb, thats what it is.. If it maintains the issue on the 3-4 bank its something else.

Process of elimination.

I am using a syncrometer, but it is not Unisync brand. is there a difference? I am using the plastic and rubber one that pelican sells.

Engine is not in the car, and car is not ready to drive, to getting the load to seat the rings is problematic.



I set idle without the bar linked up. Tomorrow I might start fro scratch again.



Leak down, done with cold engine and probably non-seated rings was between 8 and 12 percent for each cylinder.

Compression test done today on a warm engine netted results:
Cyl3 115
Cyl4 105
Cyl1 115
Cyl2 122

My guess is that the oil on the plugs was from 2 sources, me putting oil down the spark plug hole the first time I started the engine for the 20 minute break in, and since the 1/2 bank was doing all the work, the 3/4 bank probably did not get hot enough to become self cleaning. When the rings seat, I would expect all the compression numbers to go into the 120s. It makes sence to me, but what to I know?

Tomorrow I will swap the carbs, and see what happens.

Zach

Nowadays I prefer the synchrometer to the UniSyn.

I still have a vintage (35+years) UniSyn in my tool chest and I always will, but the Synchrometer is a better product these days, at least for the 6's I work on.

Coupled with the fact that the new UniSyn's are cr&p. I posted comparison pictures a while back in some thread or another. The new ones have untrimmed sprue on the float, huge parting lines on the castings and other indications of very crude manufacture. It almost looks like someone bought the 40-year-old tooling and just kept using it. There are places in that tool where tolerances are critical.

So the "old guys" who got great results with a UniSyn are correct - it used to be a great tool. But what you can buy today is not that tool. A Synchrometer is the way to go.

BTW - as long as you are checking everything - badly-adjusted intake valves is another reason airflow gets out of synch. Especially since you have some time on the motor, maybe it's time to recheck cylinder head nut tightness and valve lash.

Do they have the old cold start thingy on the sides? If so you need to check and make sure the rods are fully shut. Bill Grimstead had a problem with #3 being dead it had spark and fuel but still no go and we rebuilt his carbs before we put them on.
When I took off the carb on that side to check it one of those rods was open allowing
a lot of gas to get by and creating a overly rich condition.

while there may indeed be other issues, here's a easy linkage confirmation. on the side that's running high, back the idle speed screw COMPLETELY off the stop. using the other idle speed screw, increase the idle to calm the pulsing down you might note with the synchrometer. i like the pointer to remain calm. now measure the air flow. if there's a dif adj the 'free' side threaded rod to match the flow up. i try to maintain a idle @ 900-950 rpm. double check all of the fasteners on the intakes and carbs to ensure tightness.

One thing I have found...and it really pisses me off...is that age has robbed me of the ability to see in low light. I was changing the oil on the wife's Volvo and I assumed that the gasket was seated right on the oil canister. It wasn't....farge it....not the Exxon Valdez but a solid drip at idle....

So, to sum it up, check the baseline. Use good light, take your time.... Gaskets, cylinder compression, proper plugs and gaps, float levels, clean jets, proper sized jets, jets that haven't reamed by someone else to a larger size (yes that happens!)

Intake leaks at the base can be a problem and are hard to see...even with good eyes. Take carb cleaner or quick start and LIGHTLY spray at the base of the carbs. Any idle change and you have an intake leak.

I forgot to mention that. Valve lash was reset to .006 in and out as to the specs on the cam card. I did that just before going to sync the carbs. The valves probably have less then 10 minutes on them running at idle or just above.

All the rest of the stuff I will do, but it is pouring outside, so probably not today.

Zach

I had a lot of trouble with #3 oiling up and not having any adjustability at idle. It was the "choke " circuit. I have never seen this on a weber before so I never noticed that on this set of 40MMs there were sliding brass valves under the plate on the side, usually just a plate over a blank space. This set of carbs ( made in Spain) had sliding valves with springs on top to hold them down. The problem was that the springs are supposed to have "keepers" to keep the springs down and therefore close pressure on the valves. These keepers were missing and the springs were only held down by the paper casket that , after it soffened a little allowed one of the springs to work up therefore allowing one cylinder to run rich at idle.

2) when the engine is idiling put your hand over one carb then the other. If the engine speeds up when you close the top then the shaft seals are bad and you are sucking air through the shafts and not through the top as intended.

Over the years i found that for some reason i found the best tune i could get out of a carb set up was by ear.
With the unisyn even when i had them all matched, the engine didn't run as well as when i used a length of rubber hose and matched the hiss of each carb intake to all sound the same. If you can tune a guitar you can do this.

No but I CAN Tuna Fish....

Al, go ahead and bolt on the Chinese carbs and that problem will go away :-)

Damn thats a hard thing to say!

Okay, so I got a break in the weather and ran out to do some tuning.

I took some pictures for Jake. His guess is that my vents are 34mm.


I took the carb off the manifold and inspected the venturi closely, but could find no marking, so it must be engraved on the under side (which at the moment I can not get to).

I put everything back together and started really examining the linkage. I noticed that the geometry from the pushrod that worked the throttle was steeper on the 1/2 bank then the 3/4 bank - seemingly by design. I am guessing this is why the 1/2 bank is doing most of the work.

So, I mounted the 3/4 pushrod farther back on the linkage arm. I also turned the turn buckle to lengthen the arm a little. Then I powered up the system, and let the fuel pump run for a few moment to refill the float bowl on the carb that I had pulled off.

Suddenly I smelled gas. I must have bent the float arm a little, as the 1/2 carb was overflowing with fuel. *DAMMIT* I sopped up as much as I could, and took the carb apart. Reset the float to 10mm to closed.

Not knowing what to do about the gas sitting in the intake manifold, and not having a baster to suck it out, I ran the starter without fuel or spark and pumped it out. I suppose I need to remove my exhaust and drain that fuel out before I blow mu muffler off. I'll have to do that tomorrow.

I'll test to make sure that my carb float shuts off as it is supposed to first though.

I do not think that these carbs have choke circuits, but thats something that I will look for tomorrow.

Zach

Your carbs have the choke circuit as I can see the jet and the brass valve for it at the top of your pic. Just remove the cover that has a lever on it or some have a block off plate thats in the middle of the carb facing the engine. Look inside and you can see the 2 brass valves and make sure they are all the way down and seated and that they can move freely in the bores.

looks like accellerator pump parts to me ...