carb help

[Happy Eeyore]

I am having trouble with the carbs. I have carefully adjusted the valves, reset the points, cleaned the advance plates, and timed the bike with a strobe. All seems to be right in that area. 

 I have replaced all the o rings, the gaskets,  made sure all the holes are free and put new diaphragms in.  The throttle cables are loose and the throttle plate is at max closure at idle but the throttle adjustment screws do not touch the stops.  The "chokes" are assembled right and pushed against their low stop. 

 I have the Bing manual and have studied it. .  I have looked at a dozen other sites for advice.  No air leaks.

If I adjust the idle  mixture screw to the Bing recommended start of .5 the bike runs like it is too lean.  If I lean the mixture screw any further the bike dies. If I open the idle mixture screws the bike runs better but it idles way too fast. . The throttle stop adjustment screws are not touching the stops. 

I have not replaced the floats but it seems they are working correctly. They look funky but seem to be working. I have not replaced the idle jets nor the main jets or needles.  The needles are the same length and size.  It is like the idle system is feeding too much fuel. 

It might be possible that the original idle jets could have been changed but their holes are very very small. I have not compared their numbers against the Bing table. The slides have been buffed and do not hang up.

 No matter how I adjust the idle mixture the right exhaust at the jug is about 100 degrees hotter than the left exhaust.

Any ideas at what I should do next.  I have ordered new idle mix screws.  I can make the bike run and can balance the carbs at higher speeds. But it is not right at idle. 

[Barry]

Possibly the butterflies are not fully closed due to improper installation.

If the butterflies have  been taken out at any time they have to be put back in the correct orientation and centred. They are not simple discs, they have bevelled edges top and bottom for a good fit in the venturi so they can be put back the wrong way around. The test is to hold the carb up to the light with throttles fully closed where you should see at most a feint even ring of light all the way around.

[Happy Eeyore]

Berry I will check again but I discovered that when I tried to reassemble them.  I did not look for a bevel. but I got a good fit  I think. I will look again.  I have checked the floats and they weight 12.4 and 12.7 grams and they sit parallel to the body.  The rubber tipped valve looks good,  the numbers of the jets are the same side to side.  I will go out today and buy a compression gauge just ot be sure nothing is amiss with compression.  The spark is strong.  At higher RPMS I can balance the pull but not at idle.  The slides at idle are all the way down. The slide needles extend 42 mm from the slide.

I have had a Morgan and an MGA and an MGB and learned a lot tinkering with those SU carbs .  So far with the Bings  I am stumped.  I can find nothing that signals why.  I may call Bing and pick their brains.

Thanks for the tips.

[Barry]

The difference between SU's and Bings is the SU's have no separate idle circuit so they are simpler. 
Your problem lies somewhere in the Bing idle circuit.

Another thought: are you sure the enrichers are correctly assembled and fully off.

[Happy Eeyore]

First thing I checked was enrichers. I had those assemble right and set up correctly. 

 But you get an atta boy thank you for the plate suggestion.  I ignored my punch marks and had one  in the wrong way round.  It was out enough to cause what would be an air leak  which made that side lean.  I never noticed before that they were beveled.

Much better balance now and I am going to call it good to go.  The good news is that I know the inside of those carbs and I can take them one off in about 4 minutes and take ione apart in 5 min. I hope that I can remember that process in a couple of years.  Thanks again.

[Justin B.]

Another thing that can cause high idle is stuck advance weights in the bean-can...

[Happy Eeyore]

Justin  Thanks for pondering my problem.  One of the first things I did was completely take apart and clean and very lightly lube the bean can. The advance seemed to work well.  I could push the weights out and they seemed to return with no problem. The timing strobe lite seemed to indicate the advance was functioning correctly.  Are there tests that can make sure they are functioning besides what I have done?.

[Justin B.]

One thing I remember when I had the sticky weight problem is that when I pulled up to a stop the idle was crazy high.  If I put my foot on the brake and slightly released the clutch, enough to bog the engine, the idle would normally drop to normal.

[Barry]

If it's not the advance mechanism then it can be the carb adjustment.  If the butterflies are too far open at idle perhaps because the idle mixture isn't right then it can bring the transfer ports into play which will result in an idle speed hang up.

The objective is always to end up with the  minimum possible throttle opening for a desired idle speed.  Use the mixture screws to maximise idle speed and then drop it down again if necessary with the throttle stops. Basically never reduce idle speed using the mixture screws.

[Bob_Roller]

On setting up the carbs one thing I learned was getting the butterfly plates open the same amount.
I back off the idle adjustment screws then place the thinnest feeler gauge on the stip and adjust the idle screw until it just grabs the gauge .
Then perform the carb synch .
I also put a line on the end of screwdriver so I know how much adjustment I did .
Getting the carbs adjusted correctly be challenging / frustrating at times .