Flattop to Roundtop carb conversion. Help with hoses, intake manifold, etc please!

[yoshi_w]

Hi, so I am in the process of converting my car from flattop to roundtop carbs. I have the intake manifold, balance tube, and carbs for both setups. The engine is an L28 that came with flattop carbs on it and a mess of hoses and wires. I need help identifying hoses and figuring out what I don't need, etc. I have removed the flattops and their balance tube, then bolted on the balance tube for the roundtops to notice that there's no spots for some of the hoses that are there. I have pictures attached. The next thing I noticed is the roundtops won't fit on the manifold and need the larger bolts of the manifold that came with them, so does this mean they won't work on the n33 manifold? Next, there's no spot for the coolant hose, does this mean the roundtops don't need coolant, and if so, how would I remove this hose and block it? As you can tell I am pretty clueless but thanks for all the help!

[Bruce Palmer]

Dangit!! There was a drop dead killer thread on here within the last week showing step by step photos with discussion on this very subject. Hateit when that happens. Bueller? Anyone?

Why isn't this ibn the carb section?

[yoshi_w]

I searched the forums but couldn't find anything, as for being in the carb section, I didn't realize there was one sorry. Any help is appreciated, the flattops were poorly installed so I am almost positive there are extra hoses than on a normal flattop to roundtop conversion. It is a 240z that has an l28 with flattops which I am now converting to roundtops so its technically gone through two intake systems before this (fuel injected and flattops).

Edited February 3, 2012 by yoshi_w

[Captain Obvious]

Here's the thread that Bruce was talking about. It's not that detailed as far as exactly what to do with the hoses, but it does give a good mechanical overview.

http://www.classiczcars.com/forums/showthread.php?44969-Flat-top-carbs-replaced-with-ZTherapy-round-tops

I'll get to the details of your hoses in a second. I have to open up your pictures first...

[Captain Obvious]

Do you have copies of the service manual(s)?

You need two copies... One from 74 and one from 72. The 74 will help identifying the hoses from the flat-tops, and the 72 will help identify what you need once you're done. The manuals aren't perfect and there will still be questions, but they will help a lot.

OK, Here's your first pic. The two hoses that come off the original flat top balance tube... The larger one should be for your brake power booster, and the smaller one should be the vacuum source for your heater system. If you want power brakes and a heater that works, you're going to need to figure out a way to get vacuum to both of those hoses.

Your second pic... A little blurry and tough to figure out what you're identifying, but I'll give it a shot. Going clockwise starting in the upper right.

a) Brake booster - You need this.

Coolant out of rear flat top returning to water pump - Pull the hose and cap it off near the firewall.

c) Coolant supplied from thermostat housing into front flat top - Pull the hose and cap it off at the thermostat housing.

d) Bundle of snakes identified with star - Can't tell. Need a better pic.

e) Two fittings on front intake manifold - F = Throttle opener system, R = anti-backfire valve system.

f) The smaller starred circle - Can't tell. Again, need a better pic.

The red and yellow fittings you have capped off in this pic are for the throttle opener system. It's function is to reduce emissions by opening the throttle a little under very high manifold vacuum. There are some good pictures of that system in the FSM. Look there first and after that, if you still have questions, come back and ask.

On your last pic, the tube in question is coolant supply to the balance tube. Coolant is supplied to the tube you have identified, flows through the balance tube under the EGR valve, and then returns back to the engine through the cut off tube sticking out the bottom in your pic.

Study your service manuals.

[Captain Obvious]

Pictures worth 1000 words?

[Bruce Palmer]

Is this a great forum or what?

Final analysis. Our Euro balance tube has provision for PCV and brake booster and one cartb has a tap for vacuum to the distributor. All the rest of that is just supefluous..... Flat top manifold, early balance tube, long studs, thick insulators, round tops, heatshield of your choice and you should be good to go. Oh, and the early air cleaner. Been done thousands of times.

I am still surprised about how many cars are just now getting around to making the swap as NISSAN did the swap for new owners when the flat top cars first hit the streets, to keep the new owners from mutinying.

[yoshi_w]

You guys are both amazing help, especially captain obvious hahah. You seriously are not only helping me work on my car, but its also motivation at the same time, cause when you don't know what to do you just lose the spark to work on the car. Its also amazing how you labelled the function of each hose, very helpful and its nice to look in the engine bay and know what everything is there for. Thank you soo much I cannot express in words how helpful this is! Ill try to nab a shot of the unidentified hoses by tomorrow. Thanks again!

[yoshi_w]

I managed to work on the car and bolted up the intake manifold that came with the roundtops since it has the longer studs. I also managed to get some pictures of the hoses which I am attaching to this, the numbers correspond to show what they are attached to. Also, after everything is hooked up, is there any specific starting procedure? or should I crank it over until it starts? Thanks for all the help again!

[Captain Obvious]

I think those two tubes you show in the last two pics connect to your flow guide valve. It's an early crude evaporative emissions control system who's purpose is to capturing and store the fuel vapors that evaporate out of the gas tank when the car is not running. It's the predecessor to the carbon canister containment systems that are on newer vehicles. Plenty of details in the fatory service manuals.

By the way, I know you've got an L28, and I know you are pulling off flat tops and replacing them with round tops, but what year is the car?? With flat tops and a flow guide valve... I'm guessing it's a 73?

If it's a 73, then you can find info on the flow guide valve on page EF-20 in the Engine Fuel section.

As for starting it... When you get to that point, you could just crank it and (hopefully) go, but sometimes it can be a pain to get enough fuel to the carbs to get it to catch in the first place. I would recommend filling the carb float bowls with a funnel and short piece of tubing first, before you connect up the real fuel lines to the carbs. That way you shouldn't have to count on the fuel pump until after the car is running.

But before you bother even putting the carbs on the car... Have you checked your round top float levels?

[Bruce Palmer]

It would have been easier to screw the longer studs out and put them in the other manifold than it was to swap manifold. A small pair of vice grips should start'em and finger 'em out from there. I know, too late now. Hope you install seals uo alright and you have no leaks.

Oh, and I'm one of those who surmise the flat top manifold "might" flow a little better than the early ones. That's a totally undocumented "nothing behind it" surmisings based on just looking in the intakes side by side......

[LeonV]

It would have been easier to screw the longer studs out and put them in the other manifold than it was to swap manifold. A small pair of vice grips should start'em and finger 'em out from there. I know, too late now. Hope you install seals uo alright and you have no leaks.

Oh, and I'm one of those who surmise the flat top manifold "might" flow a little better than the early ones. That's a totally undocumented "nothing behind it" surmisings based on just looking in the intakes side by side......

FWIW, never use vise grips on an exposed stud. Instead, lock together two nuts by spinning them into each other and then proceed to loosen. I've learned this lesson more than once...

Good luck Yoshi, hope you get it started without trouble! Filling the floats as Captain suggests is a good idea if you want to get it started without cranking forever.