Puzzle under the Hood

[Captain Obvious]

You probably know this already, but when you put the round tops on, your original center linkage between the two carbs won't work anymore. You'll need to swap over to the round top center. Also, you'll need to fashion some kind of return spring and figure out means to hook it to the heat shield because the round tops don't have integral return springs like the flat tops do. You can use your original flat top heat insulator blocks between the carbs and the intake manifolds, but most people switch over to the thinner square style that came with the early round tops. If you switch to the thinner ones, you'll need to pull all the studs and replace with shorter because the longer ones that work with the thicker insulator will now be too long and will hit the round top carb bodies.

What else? You mentioned above that you wanted to keep your EGR... I guess you could hook your EGR control solenoid valve to the port vacuum source on the front carb (the same port that supplies the vacuum advance signal to the distributor). The rear round tops don't have any vacuum nipples on them at all so you'll have to route to the front carb to retain some sort of EGR control. The port signal from the front round top is a little different in design and location than the port signal from the rear flat top, but it's the closest thing you'll have available once you swap round for flat.

Good luck and if there's questions about what hoses do what, let me know. I (unfortunately) know what each and every hose in that bundle of snakes does.

PS - Your round tops are probably not real "Skinners"... They're probably Hitachi.

[ryanonthevedder]

Yeh, you re right. Hitachis they are.

I was considering where to route the vacuum line for the egr, thanks for the suggestion. The insulators and center linkage came with the pkg that I bought and it came off of a 73 as well. The carbs have adaptor plates so I can use the stock 73 air filter and hang on to a somewhat stock look.

Plus: I was able to get the egr temp switch working again. It is a bit of a process, and its cheaper than buying one (if you can find it $80).

You have to chip out the epoxy filler until you get down to the switch. A bit of heat REALLY helps.

There is a steel retainer ring that holds the switch into the brass body, it has to go. You will probably destroy it getting it out (I did)

Remove the switch by opening up the lip around the top of the brass body. Once the switch is out de-solder the leads and put them aside.

Use a pick to bend back the aluminum rim on the top of the switch and pull out the plastic top. There are three parts inside: the switch (attached to the plastic top), the plastic insulator, and the thermo-activated disc.

It is probably the disc that is causing the problem. The disc has a dimple in it that will pop up and down with heat. I found that mine no longer "popped". I used an 1/8" flat faced punch and a small hammer to adjust the dimple. The shallower the dimple the easier it is to pop off.

I tested it with a soldering gun and with about 50 degrees C of heat it would jump 2" into the air. When it cooled it would pop back. Success!

Reassemble and test several times; cold the switch is closed and hot the switch opens with a click. Seal with epoxy of your choice.

Its not related to the topic, but its important to us egr guys.

Cheers

[Captain Obvious]

Or... If you're really really sure that you will never be putting the flat tops back on the car, you could do what I did with mine. Here's round tops redressed with flat top throttle shafts. Took some machining work, but now they're a direct plug-n-play drop in to where flat tops normally live. Integral return springs and all:

Thanks for the tips on the thermo switch. Surprising, but I hadn't considered trying to take it apart. I never imagined there could possibly be anything salvageable in there. I must be slipping.

I did looked (briefly) into a replacement, but didn't turn up anything good. The newer ones for the 280 are cheaper and plentiful, but they are backwards acting compared to the earlier years. I considered using one to actuate a relay to reverse the logic, but all of that is low priority. Once my engine is up to temp, the switch works just fine.

[ryanonthevedder]

Ooo, nice throttle shafts. I do have a spare set of flat tops for parts... but I am getting enough grief as it is working long hours in the garage :-)

I got the zx distributor installed today. It worked quite well and I just had to make up a little jumper to get it all working. I was able to use all the stock wires, and I was able to pull 12v off of my thermo switch. Great success!

I sprayed some carb cleaner into the manifolds and it fired right up. I retarded the distributor all the way and I was still able to get fire out of it.

The bonus is that it throws spark faster and stronger than a cop at the Vancouver airport! Horray for electronic ignition!

[Captain Obvious]

Wait..... So does that mean that the problems you were having have been fixed? It was an ignition problem?

The bonus is that it throws spark faster and stronger than a cop at the Vancouver airport!

Haha! Nice.

Yeah, my modified round tops turned out pretty well. I bought a spare set of flat tops and they were damaged in shipping so by the time they got to me they were parts only. I didn't feel too bad about them giving their internals in the name of experimentation.

[ryanonthevedder]

Yes I think it might be fixed. It was somewhere in the ignition system; either the coil or the distributor. I am leaning toward the distributor, but as was always the case I could not find anything wrong with it. Once I get the new carbs on I will be able to make the final determination.

And those zx distributors, how about that spark. What an improvment!

[Captain Obvious]

Yes I think it might be fixed. It was somewhere in the ignition system; either the coil or the distributor. I am leaning toward the distributor, but as was always the case I could not find anything wrong with it. Once I get the new carbs on I will be able to make the final determination.

And those zx distributors, how about that spark. What an improvment!

Excellent! So it's working well with the flat tops and as soon as the round tops are done, you'll toss those on and see how they work?

Here's a starting point for you... The flat tops are the same thing as round tops with the nozzles three turns down and then an air bleed around them to lean the mixture back out some. Keep that in mind when you're tuning the round tops. If you find you need three turns or more down to get it to run right, you've most likely got a vacuum leak somewhere.

First time I tried this swap, I reused the same insulating spacers and associated gaskets. Problem was, however, that the bulges in the gaskets from the water passageways in the flat top carb bodies didn't seal perfectly against the round top bodies. It ran OK for a proof of concept test, but I needed more turns than I should have to account for the extra unfueled air leaking past that insulator gasket.

So, the next time I had the round tops off, I replaced those gaskets and everything got better.

Then the next time I had them off, I pulled all the studs and replaced with longer and went with the thicker insulators and all new gaskets.

Then, I pulled it all back off and went back to the flat tops.

I've not yet messed with the ZX ignition module, but I've heard good things about them. Mine (fingers crossed) seem to be working fine and I'm inclined to leave sleeping dogs lie.

So... Are your timing numbers back to reality?

[ryanonthevedder]

, not so quick there friend. I have not put the flat tops back on yet. I am inclined to think I won't ever put them back on. I fired the motor up on carb cleaner by spraying it into the open manifolds (carbs off). I then spent my energy blocking off hoses and getting my emissions stuff working better. As you know well, those flatties are a pain to re & re.

With all that said, I got the round tops on this afternoon and got the car running OK. I still have to go through with a fine tooth comb to iron out the wrinkles, but it ran not bad; and more importantly it ran with 8 degrees of advance.

All is well in the universe. I must apologize for not doing the job one item at a time, and it means that unless I probe deeper into the components I may never know the real cause of the problem. However the car runs again and that was my ultimate goal.

Why might I ask did you go back to the flat tops in the end?

[Captain Obvious]

Gotcha. was just getting a little ahead of myself. So now the round tops are on and it's running OK. And your timing numbers are back to reality. Sweet!

I fully believe that the carbs weren't the original root of the problems as all evidence points to ignition, but if you're happy with the round tops, it would be a completely academic exercise to find out for sure. Sounds like you're not that curious, and I don't blame you. I agree with your assessment about resale value and the round tops... Whether the flat tops reputation is deserved or not, it's just so prevalent and has existed for so long that there's just no way most people will ever believe otherwise.

Without getting into the design details, I went back to the flat tops because I believe that the flat tops are inherently a better carb design than the round tops. IMHO, the single biggest advantage of the round tops is the ability to make large adjustments to the mixture by cranking the nozzle knob while the engine is running, and not only that, but you can do front and rear independently. The air bleed style mixture adjust on the flat tops works too, but it's range of adjustment is much smaller than the round tops which means that you must have everything else working just right, or you won't be able to get the flat tops mixture right no matter where you crank that screw. And you only have one adjustment for both carbs.

Other reasons?

I've made a bunch of improvements to my flat tops to improve some of what I consider shortcomings, and I have so much time invested that I refused to lose that battle.

Also, there's some sick satisfaction in being able to make work what everyone else says is impossible. I assume it's the same thing for the people liking Gremlins or Azteks, dome houses, Brussels sprouts... An "underdog" kind of thing.

All that and the simple "stockness" of the flat tops on my 74. Anything else in there just looks wrong to me.

You didn't ask, but it makes sense to explain the other side of the question... "Why did I put round tops on then?" Because before all of my carb work, I bought into the "flat top boat anchor" hype and picked up a set of round tops. My flat tops clearly weren't working right, and I used the round tops to keep my engine running while my flat tops were under the knife, and also to see how my engine "should" perform with the "good" carbs on it. Kind of a baseline for proper performance. Now that I'm done, I think my performance is about the same between the two designs, plus I get all the improvements of the flat tops.

It's probably off the direction of your original topic, but if you want details about design improvements, etc, LMK.

Edited March 9, 2012 by Captain Obvious
why did I try round tops then?

[ryanonthevedder]

Well she's all good again. Starts well, idles smooth, and gets up and goes pretty well. I havn't taken it out for a good rip yet, but things seem promising. I was able to lean things out a bit too, and according to the exhaust gas analyzer on my face it is running a lot cleaner. I am a bit more confident that it might pass smog. I appreciate the flat top sentiment, but those round tops look pretty good all cleaned up in there.

Thanks for all the comments, I will PM you if I ever figure out the actual culprit.

[Captain Obvious]

Glad you've got it licked and I wish you good luck with the smog.

So... What's your plan with the original flat tops? I'm going to be putting my 74 up for sale and I'm not sure I want to saddle any new owner with my modified flat tops. I mean I think they're better than original, but it's not like you can open up the manual anymore and have everything look right.

It's a labor of love, and I'm just not sure it would be a good idea to pass those mods on to someone else unless they know what they are getting into. I don't want a future owner taking his car to someone for work and having the mechanic go "What the heck is that? That's not right!!" Haha! Does that make any sense at all?

Anyway, it sounds like your flat tops are in pretty good shape and if so, I could give them a much better home than the bottom of the lake. If they're going to end up as boat anchors, send me a PM instead.