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Am I diagnosing this weird problem correctly?...


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For reference, this is a 1985 720 with a stock z 24 motor, carb version. I have replaced the timing chain ad cylinder head, among other things. So it got a top end rebuild several hundred miles ago. I have reported a hesitation in the acceleration, which leads to a bit of a lurch forward, when everything gets to going. It feels like a sticky throttle, but it is not. It comes and goes, and seems to be a little worse when things are warmed up. Several times, I thought I found the problem, only to discover it came back again.

 

I hasten to add, the engine timed up  to 3 degrees BTDC and it ran fine, except for the persistent hesitation issue.

 

So I am trying to fall asleep the other night and I get stuck on the idea that it must be some kind of vacuum leak, or something that is somehow not helping advance the distributor through the bellows. And it occurs to me to double check the EGR valve mounting again. I had a hard time getting it back on and that might be where the leak is happening.

 

So I check and sure enough, when reassembling, I had flipped the stud and the bolt so that the bolt side snugged down fine, but the stud side was a thread or two short of snugging. The mounting into the intake manifold has one side threaded through, and the other side is not, making it possible to make this mistake.

 

So I flipped them and remounted the EGR valve snugly. Then, all hell broke loose. 

 

First, the idle setting and the auto choke were now a bit out of whack. Not a problem, I sorted that out. But the engine still ran weird, so I figured that maybe the timing was off a bit now that the true and full vacuum had been restored. I checked it and now my timing was maybe 5 degrees AFTER t5op dead center. by cranking the distributor clockwise all the way, I can get it to just about zero. I did not expect to find such a dramatic change!

 

Do I need to drop the oil pump and rotate the shaft one tooth to get the distributor to be in the right spot for timing? It feels like that is what I need to do, but before I did it, I thought that I would ask the collective Ratsun inteligencia if I was diagnosing the problem correctly.

 

MANIFOLD%20CONNECTION_zpsp9xhkpmu.jpg

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I hasten to add, the engine timed up  to 3 degrees BTDC and it ran fine, except for the persistent hesitation issue.

 

 

So I flipped them and remounted the EGR valve snugly. Then, all hell broke loose. 

 

First, the idle setting and the auto choke were now a bit out of whack. Not a problem, I sorted that out. But the engine still ran weird, so I figured that maybe the timing was off a bit now that the true and full vacuum had been restored. I checked it and now my timing was maybe 5 degrees AFTER t5op dead center. by cranking the distributor clockwise all the way, I can get it to just about zero. I did not expect to find such a dramatic change!

If you had +3 and now have -5 now, one of these times it was checked or set wrong. The timing is not affected by anything you did and certainly not by manifold vacuum.

 

 

 

SEVERAL THINGS...

 

To properly check the timing, pull the vacuum advance hose off the distributor, (plug it) warm engine, idle around 750. Timing should be set to 3 degrees +-2 degrees. This is the only way to be sure you have the timing correctly set. While the hose is off pull the distributor cap off and suck on the distributor hose. You should see the rotor turn clockwise as it advances. It should hold until you release it. It should NOT stick or be stuck.

 

 

If the idle is too high, some mechanical advance may have been introduced into the setting, this is why the idle should be close to 750 to eliminate that. Mechanical advance can begin as low as 900 RPM.

 

If the vacuum advance hose was connected, and the idle too high, some vacuum may have advanced the timing and the setting would be altered. Here's why. The vacuum advance is a ported vacuum signal from the carburetor. There is a small port opening just above the throttle plate. At idle, the port is above the throttle plate and reads normal air pressure. When the throttle begins to open, it rises above the port and little by little more and more intake vacuum below the plate is sent to the distributor and it advances. This is why the idle speed must be at 750, and just to be safe, the hose disconnected so the true static engine timing can be set.

 

EGR...

 

This is introduced inert exhaust (burnt fuel and air with almost no free oxygen in it) added to the air/fuel mix while driving. EGR reduces combustion temperatures and reduces emissions. There is NO EGR at idle or at full throttle, or if the engine is not up to full warm condition. Just like the vacuum advance, the vacuum signal to open the EGR valve is ported from the carburetor. EGR at idle was cause it to be very unstable so there is none. If the EGR valve is dirty or sticking and does not close properly the idle will be affected. If the SS line that draws exhaust gasses from the exhaust manifold to the EGR is rotted and leaking oxygenated air in, then the combustion mixture will run lean. You can disconnect the vacuum signal from the carb to the EGR valve and this will deactivate it and eliminate it as a possible cause of bad running.

 

NOW...

 

There are three vacuum ports on your Z24 carb. The two that matter the most are the front two. The one closest the front is the vacuum advance to the carb. (it also goes elsewhere but if you follow it far enough it goes to the distributor) The port in the middle, or second closest to the front, goes to the rear and to the EGR valve. Maybe these are reversed or using the third port on the carb?

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If you had +3 and now have -5 now, one of these times it was checked or set wrong. The timing is not affected by anything you did and certainly not by manifold vacuum.

 

 

 

SEVERAL THINGS...

 

To properly check the timing, pull the vacuum advance hose off the distributor, (plug it) warm engine, idle around 750. Timing should be set to 3 degrees +-2 degrees. This is the only way to be sure you have the timing correctly set. While the hose is off pull the distributor cap off and suck on the distributor hose. You should see the rotor turn clockwise as it advances. It should hold until you release it. It should NOT stick or be stuck.

 

 

If the idle is too high, some mechanical advance may have been introduced into the setting, this is why the idle should be close to 750 to eliminate that. Mechanical advance can begin as low as 900 RPM.

 

If the vacuum advance hose was connected, and the idle too high, some vacuum may have advanced the timing and the setting would be altered. Here's why. The vacuum advance is a ported vacuum signal from the carburetor. There is a small port opening just above the throttle plate. At idle, the port is above the throttle plate and reads normal air pressure. When the throttle begins to open, it rises above the port and little by little more and more intake vacuum below the plate is sent to the distributor and it advances. This is why the idle speed must be at 750, and just to be safe, the hose disconnected so the true static engine timing can be set.

 

EGR...

 

This is introduced inert exhaust (burnt fuel and air with almost no free oxygen in it) added to the air/fuel mix while driving. EGR reduces combustion temperatures and reduces emissions. There is NO EGR at idle or at full throttle, or if the engine is not up to full warm condition. Just like the vacuum advance, the vacuum signal to open the EGR valve is ported from the carburetor. EGR at idle was cause it to be very unstable so there is none. If the EGR valve is dirty or sticking and does not close properly the idle will be affected. If the SS line that draws exhaust gasses from the exhaust manifold to the EGR is rotted and leaking oxygenated air in, then the combustion mixture will run lean. You can disconnect the vacuum signal from the carb to the EGR valve and this will deactivate it and eliminate it as a possible cause of bad running.

 

NOW...

 

There are three vacuum ports on your Z24 carb. The two that matter the most are the front two. The one closest the front is the vacuum advance to the carb. (it also goes elsewhere but if you follow it far enough it goes to the distributor) The port in the middle, or second closest to the front, goes to the rear and to the EGR valve. Maybe these are reversed or using the third port on the carb?

 

Very thorough Mike. Lots of valuable information most people wouldn't take time to type.  :thumbup:

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