I'm trying to sync the carbs (Weber IDFs) on my new engine. I warmed it up and then let it idle at about 800 rpms. I disconnected the linkage and used the synchometer. On one carb, both barrels were very close to each other (5 and almost 5), while the other carb read a 5 and a 3. Is this indicative of a bent shaft or some other problem, or can this be adjusted?

I do recall removing the throttle plates from one of the carbs while I was cleaning them. I was going to remove the shaft but decided against it and put the plates back on. Might I have changed the position of one or both of the throttle plates in relation to the bore? Would this cause the problem?

Also, fuel seems to be almost weeping in the area of the brass bolt and washer (#5 and 6 in the picture) directly under where the feed line connects to the carb. I used a new washer and tightened the bolt real good. Any ideas?

Click to view attachment

Chris, unfortunately there could be a few other factors that are causing one cylinder to pull less, valve adjustment, lower compression in that cylinder, intake leak, right off the top of my head. You also need to make sure there isn't an air leak on that velocity stack.

Do a search for carb tuning, and you'll probably find my method. The key for the air bypass is to start with all of them screwed all the way in on thier seats, find the barrel that's pulling the most, and match the others to that one.

I'm not 100% sure, and it's been a while since I've had a set of those carbs apart, but I think there's only one way to put the throttle plates.

The aluminum sealing washers can be a pain to get to seal. You will probably have to just tighten them a bit more.

When you could get new Weber 3 barrels, they torqued the heck out of covers for the float valves, to the point that you could strip the hex head on the cover plugs. PMO used to and probably still sells fiber sealing washers that didn't require that much torque.

1. There should be an adjustment screw for each venturi that you turn to get the #'s equal, looks like #41 is it. My carbs are Dellortos so I am not sure on the Webbers...

2. Get a new copper sealing washer and heat it with a torch, let it air cool and then put it on.

Perry, I found your writeup. I will go through that procedure once I get the engine in the car. Kinda anxious to see how it pulls.

The washer that came in my rebuild kit was a fiber washer. It's red in color.

Regarding one cylinder pulling less, is there supposed to be a gasket between the velocity stack and the base of the air cleaner?

Hi,

Fully seat air bypass screws. Use bypass screws to balance throats one carb. No worries about the number as long as they are the same. Do the same to the other carb. Now use idles screw to make left and right even. Spend the next 3 days noodling with linkage. You're done.

Except for the fuel leak.

Carlos

You can also flip the carbs from side to side to see if the problem follows the carb or stays at the cylinder in question.

If it follows the carb, you need to reset the carb using the air bypass screws first to get both barrels to match.

Rich

Yes, there should be a gasket at the velocity stack base, #2 in the Weber diagram.

Looks to me that screw #48 is the air bypass screw.......#41 is the idle mixture screw.....

Click to view attachment

#48 is air bypass. Loosens the lock nut and slowly open the screw. This changes the ratio of fuel to air, opening will take you leaner as you are letting in more air past the butterfly.

#41 is the mix screw (it has the spring). It does not effect the ratio of fuel or air, just the volume of it. As you know, this only needs to be open enough to keep the cylinder running happy.

Before fiddling with these, though. look for leaks. Run a propane torch so that only propane is being put out (no fire.) while the motor is running. If the RPM picks up while you have the torch near a joint, you have found a leak.

Zach
Zach

Sounds like a vaccum leak or a plugged idle jet. I'd blow the jet clean and check for vaccum leaks.

Double post.

......AFTER removing the throttle plates.."on one carburator"...DID YOU REINSTALL them , with the proper orentation?...I.E. do they look like the ones you did not mess with?.

I recall putting the plates back on the way they came off. I just barely tightened the screws and then opened and closed the plates a few times to make sure they were seated correctly, and then tightened the screws.

.................OK, just wanted to know if you might have reinstalled the butterflys backwards....they HAVE , a specific orentation......