Another 720 won't idle

[northoak]

Hey y'all.  I've got a 720 that is refusing to idle.  It has gradually been getting worse.  At first it wouldn't stay running until it fully warmed up and not it will not idle at all.  I fire it up and the truck will die unless I'm giving it a good amount of gas.  I believe it is a Hitachi and I'm here in California so I ave the 6 pin plugs with the feedback carb.  There seems to be two sets of the 6 pin plugs,  is this normal?  

 

Here are some things I tried already:

 

Removing the idle cut solenoid and spraying carb cleaner in there.

 

Removing the pin and spring from the idle cut solenoid and reinstalling it.   The truck ran for a few minutes and then the problem returned.

 

Replacing the fuel filter.

 

checking for loose screws on the carb top.

 

I haven't been able to find any vacuum leaks yet.  I'm thinking maybe a broke wire in the 6 pin feedback connector?  Any advice is much appreciated, thanks!

 

Nick

 

Edited July 24, 2018 by northoak

[datzenmike]

The double plug is worrisome but if it did run only to worsen then lets assume the wiring is.... OK for now.

 

 

 This seems to be a problem in or about the idle circuit, so I'll concentrate on it rather than the wiring on the feedback carb which is more for driving speed. In no particular order but perhaps easiest and most likely to the most difficult and unlikely...

 

While idling, pinch any vacuum line connected to the intake as close to the intake as you can and inspect all the others for cracks or bad fit.

 

Check set the valve lash to 0.012" on all with the engine HOT. A tight valve will not seal properly.

 

Check set the timing to 3 degrees BTDC. Timing at idle is critical.

 

Confirm that BOTH intake and exhaust side plugs are firing. Carefully take one each of (int. and ex.) plug wires off, put an old plug in the end, place on a grounded surface and turn engine over with starter. Got spark? If the exhaust side is not working check the fuse on the far left of the fuse box closest the driver's door. Half the plugs working, the enhine will run like the timing is retarded.

 

Check condition of the plugs, wiring, cap and rotor.

 

Make sure the choke is OFF and the butterfly fully vertical or open when engine is warm. Choke on will run over rich and carbon up the plugs.

 

The  right side of the carb base has an idle mix adjustment that is set at the factory and then sealed with an aluminum plug. You can drill a small hole in it and twist a wood screw in and pull it out to get at it. Take it out but count the turns so it can go back in the same position. Remove the idle cut solenoid again and use spray carb cleaner with a straw. Spray in both ends to flush out any debris. Put the mixture jet back in and see if there is any improvement. On a warm engine with the choke off... try adjusting the mixture screw in or out looking for any improvement in idle quality. Keep track of the turns so it can be put back. This may fix the problem but before the next emissions test, that mixture screw must be sealed up again. Maybe JB weld the hole in the plug and press back in place? They won't like it if they see it was tampered with so be sneaky.

 

Remove the EGR valve and wire brush the exhaust deposits off of it. Deposits can prevent the valve from closing tightly. EGR should NOT be happening at idle. If exhaust is leaking in it will destroy intake vacuum (vacuum leak) and it will dilute the air and gas mixture making it weaker and less able to idle.

 

 

 

 

[northoak]

Datzenmike Thank you once again for your insight.  I've got the day off tomorrow and will start tackling these tomorrow!  Does anyone know of a good write up for adjusting timing or valve lash for the z24?  I've only done timing/valves on a old honda s90 motorcycle so I'm a little bit lost.

Edited July 25, 2018 by northoak

[northoak]

ok, gonna reply to my own request for a write up on setting the valve gaps and say that this post about finding tdc:

 

and this post about setting the valve lash should suffice

 

still searching for the best thread on timing

Edited July 25, 2018 by northoak

[datzenmike]

You only need proximate TDC on the compression stroke. With the valve cover off turn engine till the #1 cylinder cam lobes are pointing downward at 8 and 4 o'clock as viewed from the front. You might look on the crank pulley for the timing mark at or near the 0 on the timing scale also. With the lobes down on #1, the valves are closed and you can set the clearance or lash. In fact you can set both #1 valves, intake on #2, and exhaust on #3. Now turn the engine ONE full turn back to TDC on the exhaust stroke. Now adjust the exhaust on #2, the intake on #3 and both valves on #4. Done this way you don't have to waste time bumping the engine over with the starter setting each valve individually and each valve can be set in well under a minute, more than enough time to do before it cools down.

 

Setting a cold lash assumes that when hot, the clearances are correct. I find this is close enough for a rough setting but hot is where the engine actually runs. There's no assuming if set hot.

[northoak]

Well I wen't ahead and started with the easier things first.  I took off the EGR and cleaned it out with a toothbrush and some carb cleaner, removed the idle screw and idle cut solenoid and sprayed a bunch of carb cleaner in those passages, and went ahead and changed some worn vacuum lines even though I couldn't find any leaks.  The truck is idling now and I can drive around once again!

 

I did notice that it would die when I gave it gas and I had to let off of the peddle for a second and then apply it or the truck would just kinda bog out.  The idle screw was two turns out and I took it to about 3 full turns out and that problem went away.  It still idles a bit sporadic for my liking but I'm going to try to get into the valves and timing this week.   We will see how it does going on a longer drive ?

 

Also I noticed that there is a hole in my valve cover where a weld from the bolts that hold down the air cleaner housing was! uh oh.  Do you think this is effecting things?

[datzenmike]

No effect but you may notice a smell.

[northoak]

thank you once again for sharing your wealth of knowledge

[Charlie69]

Find a replacement valve cover as that is a great source to get foreign matter in to your engine.  Replace the valve cover gasket at the same time.

[datzenmike]

This. Just put it on your list of things to look for in a junk yard. Like the tach and clock options.