Puzzle under the Hood

[ryanonthevedder]

Thanks guys, I will have a closer look at the cap and rotor. I have checked and overridden all of the connections in the ign circuit with no change. The little ground strap under the cap is OK. It also does not loose spark when I retard it. I checked it w/ my spark tester as I changed the timing (sacrificed #1 cyl). I also verified the spark before I changed the timing initially. It looked all good at 8 degrees advance. I began to believe that someone may have tampered with my fuel because all the ingredients for combustion appeared to be there. So I drained the tank and put in new gas... with no change. I also checked my egr today and it is OK although the temp switch is pooched it is not the cause of my frustration.

Thanks for commenting

[Captain Obvious]

I also checked my egr today and it is OK although the temp switch is pooched it is not the cause of my frustration.

I don't know how your temp switch pooched, but on the 240 and 260, the temp switch is supposed to be open when cold and closed once the engine is warmed up. If the temp switch failed open (cold) then your EGR will never actuate. If the temp switch failed closed (warm), then the EGR system will always be active. Important to note however, that even though the EGR system may be "active", it does not necessarily mean the EGR valve is "open". That's because the EGR uses a port vacuum source off the rear flat-top.

You can reach up under the EGR valve hat and manually lift the diaphragm which should open the valve. If it's already open at idle, then you've got a problem. Conversely, if you lift it and there's no effect whatsoever, then your balance tube is plugged with crap from the EGR system (that's what I've got). You can leave it that way if instead of removing the EGR system. Looks perfectly stock, but doesn't do anything.

One other thing to consider though... I've got a 74 manual trans which has the single electronic pickup in the distributor. You've got a 73 and you have points. How many sets of points do you have? One or two? I haven't refreshed my memory on the 73, but IIRC, the temp switch controls which set of points are used. Just seems to possibly correlate with the timing issues you're having.

I think I am going to get rid of the flat tops. I ve poured my heart into those things but like any bad relationship, this one might be over... sob.

Yeah, I understand about the flat-tops. I've done extensive work to mine as well and after a long fought battle, I finally prevailed. That's why I just can't see anything that could possibly happen with the carbs (round tops OR flat tops) that could cause ON-OFF running like what you described. I mean, if you've got fuel in the bowls verified through the sight glasses, then there isn't anything that could happen inside the carbs that could cause that. People get all uppity about the flat tops, but inside, they really aren't that much different than the round tops.

[ryanonthevedder]

Thanks again for sticking with me there Capt.

My car has a single point distributor. The temp switch failed open so the egr system never got activated. I just don't know what else could be causing my issues other than the fuel system. I had good spark no matter where the distributor was set. I re checked the mechanicals today: verified TDC on the dampener, double checked the dizzy drive shaft position/angle at TDC #1, I am not going to recheck the compression because I am sure of the values, I ve already verified that the cam timing hasn't moved a bit, so I am confident that the rotating assembly is good. I went through the ign system a couple of times, I did a re & re on the distributor to make sure the centrifugal advance was OK. I bypassed every wire in the system and replaced the coil and condenser (I have already returned the coil in favour of the 280zx ign). So in reality there is only one thing left in the equation: fuel. The tank was drained and filter was checked, the pressure was around 4 psi, and there was no water or debris in the front carb when I split it open. There didn't appear to be any bad vacuum leaks, the emission control stuff was inactive/OK, PCV was fine.

I can't think of anything else that I have missed. Even a novice should be able to get a car w/ points going, and when I pick my head up out of my hands those square eyed carbs are all that there is left to check. I am going to miss drilling out the carb body to get my needle lever pins out....

[Captain Obvious]

My temp switch failed closed (warm), so on my 74 the EGR system is always active. But since the passageway in my balance tube under my EGR valve is caked shut with almost forty years of crud, my EGR system isn't doing anything anyway. It's on the list of things to take care of, but it's lower priority than some other items.

OK, so with just one set of points, there aren't any intentional temperature related timing changes.

I'd like to back up a little bit because I'm a little unclear... What problem exactly is it that you're having right now?

I understand that the car was working great for the last few months and then one recent day, you left work for lunch and some problems started. The problems got worse after lunch and when you got back to work, it died and would not restart. That much, I got.

So is the car still sitting in the parking lot at your work?

I know you can get it started by severly advancing the timing, but does it even run well enough to drive? Or does it just sit there and idle roughly until it sputters and dies?

Even a novice should be able to get a car w/ points going, and when I pick my head up out of my hands those square eyed carbs are all that there is left to check. I am going to miss drilling out the carb body to get my needle lever pins out....

Haha! I hear ya on those float pins. I hated that design flaw so much that I modified mine so I would never have to go through that again. That's just one of the improvements I made during my long fought battle with my flat tops.

[ryanonthevedder]

I drove the car home which is not more than 3 or 4 blocks. So it will drive marginally... I was driving the car last summer and it quit last August. It has been sitting in the garage for the last few month while I was at Uni. I have been around the house more so I have made some time to address the 240's needs. So this is round two. The first was last summer, so I it is safe to say that I have been pondering the problem for some months now, and not just the last few weeks.

How did you deal with your power valves? Did you get them working or did you eliminate them? I did a bit of drilling and some adjustment of the springs and I dug out the adjustment screws, and they worked OK after all that; but I also know that I need new floats and I think I am going to invest the money into some round tops.

Edited February 28, 2012 by ryanonthevedder

[Captain Obvious]

Well I still can't explain ON-OFF shuddering with any carb problem. Especially since you've got the correct fuel level in the bowls. I know from experience that once my bowls are up to the proper level, my car will idle for maybe 30 seconds to a minute or so with just the fuel that's in the bowls. And the when it finally reaches the point where the fuel has been sucked down to the point where the nozzle just can't pick up fuel anymore, it sputters a little bit until it just can't sustain running any longer and finally stalls.

In other words, even under that situation, I get a slow painful gradual death. Not a "lights out - no wait... lights back on... lights out again" kind of thing.

Once you've got fuel in the bowls, you've already made it past the smallest of the orifices. The fuel is pretty much home free by that point.

I dealt with my power valves by eliminating them and I do think I'm a little lean at WOT because of it. I'm running early round top needles in my flat tops because they're a tiny bit richer, but not enough to make up for the loss of the power valve at WOT. The car starts instantly and runs great and if I put my foot to the floor, it doesn't fall on it's face, but I think it should go a little better. So again, like my plugged EGR port, it's a lower priority for me right now.

What makes you say you need new floats? You mangled them a little trying to get that $*%@#@#$ float pin out?

[ryanonthevedder]

I know the floats are hooped because I had to set them way lower to get the correct fuel level in the bowls. They dont float the way they used to.

I am not convinced that it is a fuel problem either, but having eliminated everything else I am preparing to bail on the flat tops.

The power valves were not too hard to figure out and they do help under acceleration. You just need to make sure that they both have the same sized spring inside, then dig out the epoxy on the nipple and expose the adjustment screw. Turning the screw in makes the valve activate with less of a drop in vacuum. I adjusted them so they came on at idle and then backed off 1/2 a turn. This way they come on any time you touch the gas same as an accelerator pump. What I really want out of those round tops is to eliminate all the idle mixture/transfer tubes that run all over the place, they are a pain. The car was running well when it did, but my hope is that with a little massage those round tops will increase the performance as well as simplify the whole mess.

Edited February 28, 2012 by ryanonthevedder

[Captain Obvious]

Yeah, I've been all through the ins-n-outs of the power valves as well. Dug out the epoxy and played with the screws and all that. The reason I took mine off was that I was running very rich and I wanted to eliminate as many variables as possible. Then once I had them off, I realized that my car (other than foot hard on the floor) runs pretty good without them if you use the right needles. I mean, after all, without the power valve, the flat tops function exactly the same as the round tops. I tried round tops as well, and my performance was identical. YMMV.

As a matter of fact, I modified a set of round tops to be a direct drop-in to my 260 so I didn't have to muck around with the round top linkages and return springs hooked to the heat shield.

Granted, the extra idle tubing is a little cumbersome with the flat-tops, but there are advantages in that as well. The engineer in me sees the beauty in the functionality. But we're getting a little off course here with all the flat top talk...

So, the bottom line is that unless there's some interpretation discrepancies on what your car is doing, I don't think round tops will solve your problem, but it will certainly simplify the engine compartment and hopefully make the problem easier to find.

The timing issue is really perplexing...

The only way the car will start and run is if I advance the timing to 30 or 40 degrees. When I get it running I cant retard it past 20 degrees or it sputters and dies.

What RPM is the engine spinning while your setting the timing? Is it as a "reasonable" idle speed (like below 1000 RPM)?

[tlorber]

I'm not trying to start anything here, but is there any way you can put on some round tops and an old balance tube and ditch the egr, or do you need it to pass smog up there? The more you can simplify the system the better. Z therapy is not that far away.

[ryanonthevedder]

The timing values were taken around 1000 rpm, maybe 1250. Even when I had one hand on the throttle as I retarded it, it would fail no matter how much gas I gave it. So I still agree that the carbs may not be the culprit. My ZX distributor just showed up in Sumas WA so I hope to get it in this weekend. That depends on my radiator too, its getting re-cored. So I will see how she likes it, but I think those round tops might work well on the motor.

I do have to pass "air care" up here because I am a little too close to Vancouver.... but there are ways around it. As well, I don't think the techs will notice anything out of the ordinary. I plan on keeping the egr and the SUs I'm getting have an adapter plate to work with the 73 air box so most of it will look stock-ish.

I believe the round tops will also help with resale. The owners that champion the flat tops are far and few between.

[Captain Obvious]

The reason I was asking about the RPM while timing is that I'm wondering if the wacky numbers you're getting are partially caused by the vacuum advance. The vacuum advance signal comes off the front carb, and it's a port driven signal which should be zero at idle, at it's highest vacuum a little above idle, and then fall off from there as the engine load increases and manifold vacuum decreases.

At 1250 RPM, it's a toss-up if you're still running on the idle tubing feed to the balance tube or not. At idle, your flat tops should be dead against the factory set linkage stops on the front (radiator side) of each carb. The epoxy plugged Phillips head screws that the manual warns you not to mess with? You're still up against those stops at idle, right?

I'm assuming when you said "TDC verified on the dampener" that means that your manually rotated your crankshaft until you had #1 piston at TDC and then verified that the timing mark on the dampener was in fact in lined up with the "0" mark on the timing gauge. Is that correct?

So it won't even run at all unless your idle timing is set somewhere above 20 degrees BTDC.

That's just not right.

[ryanonthevedder]

That's just not right.

, you got it mac.

TDC was verified the old fashioned way; rotating and probing. Its somewhere in the ign or fuel system. I had to order a new magnet for my zx distributor so I wont get it in until next week. I will report back then. Meanwhile I ve got some Skinners to rebuild.

Cheers