Tuning H6 100 M carbs

[Martinld123]

I just installed and started tuning new Moss H6 100M, 1 3/4" carbs with rebuilt distributor. My car is a 1956 BN2 with all 100M upgrades, 100M cam, larger M pistons, stronger valve springs, air box. I have had a fair amount for success tuning these style carbs in the past but I seem to be getting an on off surging at idle. I started at 12 flats out but had to go to 16 in front and 17 in back carb to make the surging less to none. I must be doing something wrong but I am using all of steps to setup carbs like balancing, lifting pistons to see if lean or not. My gray area is how to use flats to get engine to run smoothly. So 16 or 17 flats out seem too much but engine runs best in this area. At that setting my exhaust has a random burble every couple seconds or so with no surging. In short it seems to be running way too lean at 12 flats out and best at 16 to 17. Any ideas? My old carbs ran best at 17 flats out too. Thanks Marty

 

[EV2239]

Make sure the floats cut off fuel supply when they forks are 7/16" off the face of the lid. Use a drill shank. Make sure the throttle butterflies are perfectly syncronised with a 1.5 thou feeler under each when the dash pots are out and make sure that the jets are between 35-38 thou below the jet bridge. With all that right, it won't need adjustment.

However, of the timing isn't exactly right, you haven't a hope.

 

[Martinld123]

Make sure the floats cut off fuel supply when they forks are 7/16" off the face of the lid. Use a drill shank. Make sure the throttle butterflies are perfectly syncronised with a 1.5 thou feeler under each when the dash pots are out and make sure that the jets are between 35-38 thou below the jet bridge. With all that right, it won't need adjustment.

However, of the timing isn't exactly right, you haven't a hope.

Thanks for responding. I am not sure what you are saying. So you say to put 1.5 K feeler gauge under each idle adjustment screw instead on using paper with one turn in to give proper piston gap at idle? Everything I have read said this is a starting point? Thanks Marty

 

[dcarlg]

The suggestion was to check the needle valve settings on the fuel bowl lids. Also, check the floats while you have the lid off.

 

[EV2239]

Take the Lids off the float bowls, invert them and slide the end of a 7/16" drill under the forks and over the ridge on the lid. It should just just fit. If the float height for fuel cut off isn't right, the mixture will be wrong.

If you apply the settings as I describe, you won't need further adjustment, or to put it another way, adjusting will not make it better. This is all the works did.

If if the static timing is retarded it will appear weak and if advanced you'll have trouble slowing the tickover and it may hunt.

 

[Martinld123]

Take the Lids off the float bowls, invert them and slide the end of a 7/16" drill under the forks and over the ridge on the lid. It should just just fit. If the float height for fuel cut off isn't right, the mixture will be wrong.

If you apply the settings as I describe, you won't need further adjustment, or to put it another way, adjusting will not make it better. This is all the works did.

If if the static timing is retarded it will appear weak and if advanced you'll have trouble slowing the tickover and it may hunt.

Thanks this will be a big help.

 

[Martinld123]

Make sure the floats cut off fuel supply when they forks are 7/16" off the face of the lid. Use a drill shank. Make sure the throttle butterflies are perfectly syncronised with a 1.5 thou feeler under each when the dash pots are out and make sure that the jets are between 35-38 thou below the jet bridge. With all that right, it won't need adjustment.

However, of the timing isn't exactly right, you haven't a hope.

Thanks for your help. Speaking of timing my 100 BN2 would normally be 6 degrees BTDC but I have a DWR1 cam that is said to be the same as or a little hotter than a "M" cam which then I was told to try 12 to 15 degrees BTDC. Any thoughts on this? Thanks Marty

 

[johnea]

The stock distributor advances 30 crankshaft degrees. If you run 6 degrees (stock setting) BTDC, the max advance will be 36 degr. (30+6 degr.) If you'd try 12 degr. you would end up having a max advance of 42 degr. (30+12 degr.) which is too much. You would need to shorten the advance curve in your distributor from 30 to 24 degrees in order to go not beyond the max advance of 36 degr.

 

[EV2239]

You will need more static advance with a hot cam and you will also have tickover issues, so Johnea is spot on. Your distributor will need it's advance limited to suit.

Hotter cams always come at a price, power further up the Rev band and lumpy tickover. IMO they're essential for racing and they spoil a car for road use. The less revs you have to use in a Healey, the easier it is to drive.

 

[Martinld123]

The stock distributor advances 30 crankshaft degrees. If you run 6 degrees (stock setting) BTDC, the max advance will be 36 degr. (30+6 degr.) If you'd try 12 degr. you would end up having a max advance of 42 degr. (30+12 degr.) which is too much. You would need to shorten the advance curve in your distributor from 30 to 24 degrees in order to go not beyond the max advance of 36 degr.

Thanks for all of the great help from everyone. I should have stated that I had my distributor rebuilt and recurved at advanced distributors ( www.advanceddistributors.com ) and installed at the same time with my new carbs form Moss Motors. Jeff at Advance Distributors is the one who told me to try 12 to 14 degrees BTDC. I informed him about the DWR1 cam I was using so he recurved it as needed I am sure. He is known for quality work but the word try has me a little nervous, lol. I know he does great work.

So thanks to everyone's advice I think my next step will be to revisit my timing marks. My 100 BN2 never had any timing marks so I put TDC on using a spark plug stop to find it but I have yet to come up with a way to make an arm from the block extended out to front of the pulley. In the mail I have a timing arm used on MG cars hoping I can make it work. Anyone with other source or ideas??

I am using a digital timing light so my solid copper plug wire maybe giving me a problem. I think I will have to temporarily replace number one wire so it works? You have all been a great help. Marty

 

[busybrit]

Let your car idle and warm up. Now hunt down your vacuum leak, because I think you may have one.

My car is set up exactly like yours. DW1 cam, advanced distributor, carbs the lot.

If you set the timing at standard it runs ok but not great. Advance it a little at a time until it rattles on acceleration when under load, say up a slight hill. Then dial it back.

Set the jets like previously mentioned, about .038", you will likely be too lean with that. Lift the carb pistons up and let them fall and they should make noticeable clunk, if not your needles aren't centered. Both carbs need to mirror each other.

Your issue is the surging at idle, incorrect timing and the mixture settings shouldn't cause this condition. A varying issue is causing the surge, so its air or fuel, most likely air. Many cars I see are lean, rich, out of balance, non of which have a surge condition.

Hope this helps.
Johnb