New SUs- stumble off of idle

[Zaspen]

I measured the floats using the clear tube method and the fuel level is 23mm below the bottom of the float lid (where it meets the gasket) for the rear carb and the front is at 28mm for the same measurement.  I've read a lot about 23mm being the correct level but am I measuring it from the correct point?  I'm assuming they both should be 23mm?  The front carb had a lean backfire a few times.  I suppose this could be the reason.

[Terrapin Z]

The three screw carbs had different length float valves than the earlier 4 screw carbs. 

Here is some info about fuel level settings from a few years back.

 

[Zaspen]

Thanks for all the float info.  I'm going to try the 10 turn method over the weekend.

[Zaspen]

I set the floats this weekend using the 10 turn down method and it runs pretty well at lower RPMs.  It idles good and pulls well up to about 4K and then starts to break up.  It will rev up to 7K fine but just breaks up along the way.  I thought it might be lean but it behaves the same way if I have the choke fully on.  I also noticed that the timing was advancing from about 10 BTDC to around 50 BTDC at higher RPMs.   I thought it could be detonation so I pulled the vacuum advance so it would only advance to around 35 BTDC with the mechanical advance but no change in the break up.  I suppose it could be ignition so perhaps I should try a coil, cap/rotor.  It has a Pertronix ignitor in place of the points.  Interesting note is that the plugs are brick red when I kill the motor under load at around 2K and a little lighter if I kill the motor at 4500.  I'm running 87 octane with a lead additive for the valve seats.  

[madkaw]

Yep- disconnect that vacuum advance . Does it miss at lower rpm pulling a slight hill? 

How did you know it was 50 degrees ? Was this just revving in the driveway?

Ignition issues like to show up with heavy loading at lower rpms -especially if it’s lean. 

Have you checked your mechanical advance operates smoothly?

 

[Mark Maras]

 How old are the spark plugs? The only time my Z misfired above 4000 RPMs was a track day. (Murphy's Law) The old plugs looked pretty good, but a new set cleared up the problem.

[Zaspen]

I will have to try lower RPMs up a hill with the vacuum advance disconnected. I havent checked the mechanical advance besides seeing the timing advance. I will do that. I saw it advance to 50 while revving it with no load- is this invalid?  I added extra timing marks to get to 45.  Plugs only have 500 miles on them but they did get pretty carboned up with the Holley on there. I will try new ones. Thanks 

Edited May 28, 2019 by Zaspen

[240260280]

20mm down from bowl edge is fine (this is 23mm down from inside ceiling of fuel bowl as referenced in the service manual).

28mm is too low.  Both levels should be the same as a starting point.

Edited May 28, 2019 by 240260280

[madkaw]

10 hours ago, Zaspen said:

I will have to try lower RPMs up a hill with the vacuum advance disconnected. I havent checked the mechanical advance besides seeing the timing advance. I will do that. I saw it advance to 50 while revving it with no load- is this invalid?  I added extra timing marks to get to 45.  Plugs only have 500 miles on them but they did get pretty carboned up with the Holley on there. I will try new ones. Thanks 

Some dizzies have 17 degrees of vacuum advance so I guess you could see that much with the advance plugged in. There’s a chart out there that tells how much advance based on dizzie number .  

 

[siteunseen]

13 minutes ago, madkaw said:

Some dizzies have 17 degrees of vacuum advance so I guess you could see that much with the advance plugged in. There’s a chart out there that tells how much advance based on dizzie number .  

 

Here's an Excel chart I think your talking about?

Distributor advance curves S30-S130 (1).xls

 

Edited May 28, 2019 by siteunseen

[Zaspen]

Mine would be a 72 with US emissions so the numbers match what I'm seeing. Thanks for the chart! 

[siteunseen]

I cannot recall the member that did that but I've passed it on many times for others.

I wish I was able to take credit for it but no. I'll remember as soon as I disengage.

@Walter Moore thank you!

Edited May 28, 2019 by siteunseen