popping through the exhaust at 4500 rpm or so.. HELP!

[Zedyone_kenobi]

That is a great idea Steve! Thanks!

[Duffman]

Will be interested in what you find, Stephen, as I am cleaning up a set of carbs and finding my back carb piston is moving much slower than the front one, and it doesn't seem to be due to piston/dome/needle/nozzle friction. It seems that the damper oil and the brass check valve combo works differently in one versus the other, just like in Stephen's #209 post above. Weird!!

[Captain Obvious]

The drop test I was referring to is done with the pistons and covers off the car. Here's a link to the video that Blue posted in the SU section:

What you're talking about (pistons stopping 10% above full down with the carbs fully assembled with springs installed) shouldn't happen. I'm sure you already knew that... So, when this sticking thing happens, does it only happen with the damper valve installed, or does it still happen with the stalk completely removed

If you've got some misalignment with the piston against the carb bore, that could be made worse when the engine is running and manifold vacuum is pulling that piston in towards the engine. Can you see signs of rubbing anywhere on the suction piston? How about on the inside of the suction chamber dome? Any scratches?

[Zedyone_kenobi]

Correct captain, I only get an imbalance when the damper valve installed. When it is removed, they THWAK together...

It is such a fine adjustment that it seems crazy. They should not be that marginal. I and would wager that most of them are not. OR else nobody would ever get them to run right. Today I am going to reverify a lot of things and go for a drive with the wide band hooked up. Update today

[240260280z]

Some tests:

1. Try dropping with damper valve in and also with the damper valve out to check for interference there. (compare front to back as this is a relative test). Swap damper valves and repeat. This should give you a good idea on how all damper valve and pistons interact. (PO could have easily swapped parts so make no assumptions... but for ZT resto's assume everything was matched up correctly).

2. Try piston drops (with damper valves installed) but this time just have the 4 (or 3) dome screws backed off to "less than snug" so that you can just slide the dome forward/backward/side-to-side by hand . Continue dropping and testing the optimal position of he dome by sliding it around. Next continue the drop test and slowly tighten screws in a cross pattern by 1/4 turns to ensure correct dropping of piston without binding. (you can do this with jets out if you want to eliminate any potential needle bind from this test).

However I still think you have ignition/scatter problem.

Edited January 14, 2012 by Blue
improved sentence structure

[Zedyone_kenobi]

You could very well be spot on.

I am going to pull off the valve cover Monday if it is raining and have a look around. I will also do the measurements you described earlier

[Zedyone_kenobi]

Well I made a movie showing my disassembly of the carbs, cleaning them, performing the drop test, reassembling them, doing the clear fuel line trick to get your float levels right, and starting the car.

BUT it is 53 MB large and I cannot find a way to post it on the net. Everything tells me it is too big.

[mayitin]

some times if your gas tank has a dent where the it pick up pipe is will not suck the fuel as it was intended to do. I just mention it since I had that problem on a couple of Zs I owend.

[Travel'n Man]

Stephen -

I just stumbled on your thread -

I installed Mikuni's last year - you will not regret it at all if you opt to purchase - your engine will wake up and sing - it's a beautiful thing! When I was running my SU's I used only 3n1 oil for my pistons - never had any challenges at all. I believe I was setting my front and rear float levels at different settings though. I don't believe a float setting would cause popping - it would cause hesitation and sputtering - and sever lack of power (like going up a hill and running out of gas). I do have my SU's on a shelf in my garage - if you want to borrow them just pay for insurance and shipping (to and from) and feel free to use them to see if the challenge is your SU's. They are heavy and it may be expensive to ship but they were dead on with performance when I took them off and they are sitting on a shelf and have not been touched. Your call - feel free to PM me if I can assist any. My next purchase this spring is a O2 Sensor - I think it is a must for everybody.

[Bonzi Lon]

Zedyone, did you try the trick I mentioned in post 196? It should seal any air leaks around the throttle shafts. Then you can try a different pair of carbs.

Bonzi Lon

[Zedyone_kenobi]

No, not yet Bonzi. But it is on the white board in my garage under 'things I have not tried yet'. I write all this stuff down, and I try things when I can fit them into my schedule...Today I am off! Yeah federal holiday!

So I will do many things today before it rains.

[madkaw]

I bet that dry eraser board looks busy