Rough running engine

[cjTango]

Got one for the carburetor specialists out there…

Here’s the situation:

Starts up easy.
Idle is good, both cold and warm.
There’s a band of “goodness” from idle (900 RPM) to about 2500 RPM with no engine load (i.e. I’m pulling on the throttle cable on the carb manually).
At 2500 RPM, the engine starts to sputter and drops RPM suddelny, like it’s going to stall, but doesn’t. It runs crappy with no power until you give it enough gas to land it at - I’m guessing - about 4000 RPM.
It seems to be okay at the high RPMs, but I didn’t dwell long there.
When I drive it, the same thing happens, except as soon as I load the engine. I have to give it a little gas before let out on the clutch to keep from stalling.

Here’s a link of the engine running and the sounds it makes: https://photos.app.goo.gl/tgWKxO89QJFQ2ut53

It seems like some sort of fuel-air mixture, but I don’t know carburetors…at all really. I just rebuilt this on because of the same issue, but it didn’t fix it. Note: the video is after the rebuild.

Any ideas?

 

[Posi]

Need some info:
When was the last time you did a tuneup? What was replaced and adjusted?
Was anything done to the carb?
What is your idle vacuum reading?
What is the fuel pressure?
Condition of the fuel cap seal and two way check valve?

 

[cjTango]

No problem:
I recently did quite a few things. Let me start with perhaps the most important: I took off the air injection system. I brazed the holes in the exhaust manifold that the pipe bolts were in. That, alone, is why I suspect a fuel/air ratio is now off.

I just did a rebuild this last week. I bought a rebuild kit and replaced all the gaskets, the accelerator pump diaphragm, adjusted hot idle, cleaned everything real good. Could one of the metering jets be off?

I also re-routed and replaced all the vacuum hose. Some of the hoses were incorrectly routed. The engine runs way smoother now.

Idle vacuum is around 18-20" Hg.

Haven't tested fuel pressure yet.

New gas cap and the 2-way check valve is on order. Non-existent before. Note: I'm assuming you are referring to the "liquid check valve" as the service manual calls it. Basically the thing that vents back to the charcoal canister?

I'm planning on running a compression test on it today.

Thanks for helping Posi.

 

[Posi]

Fuel cap has a 2 way check valve. It opens do to a preset pressure and vacuum.

You say vacuum is between 18-20". Does this mean you don't have a steady needle and it fluctuates?
What was your method of adjusting the mixture screw/screws?
What did you set the fuel float to?

 

[Lumpy Grits]

When you rebuilt the carb, did that include a new fuel filter?
Check your float level.
If you're run'n 2 idle mix screws-make sure BOTH are set the same.
LG

 

[cjTango]

Okay, allow me to share this experience with the community.

Let me start by saying that I don't need to SMOG this vehicle. So in the interest of simplifying some things, and also to get rid of non-working parts (read: save money), I ripped out the air injection system and brazed shut the holes where the pipe bolts went.

Then, I re-routed the entire vacuum system based on the service manual. It was incorrectly routed prior to this. Of course, I also eliminated any vacuum references to the air injection system. During all this, I also noted that the EGR valve was functioning as expected. In fact, everything was checking out when I put a vacuum tester on it. Ignition timing was good, fuel pressure was good. Nothing was making sense, and the situation described in the original post was persistent. So... WTF?

While in the bathtub of despair, and with a little help from a local mechanic, it was noted that the sputtering coincided with the EGR valve opening. We found this out by pulling vacuum lines when the sputtering started to see if anything improved.

You see, in the absence of the air injection system, the EGR valve was throwing off the fuel air mixture off. Not at idle. Not at high engine RPM (well, probably but you couldn't tell when it's getting that much gas). Only when starting out and after shifting gears.

Disconnecting the EGR valve completely solve the problem. Instantly.

I hadn't thought about this earlier because the EGR system was working as it should and I wanted to keep that system because it is supposed to cool the engine and improve performance. (All the salt dogs out there are probably rolling their eyes right now.) HOWEVER, because I had taken off the air injection system the EGR valve wasn't putting in the proper amount of exhaust gas. It was putting in too much. So I had too little fuel, and the engine with sputter.

So, the moral of the story: I got a little too involved in thinking that because all the components of the system worked correctly, the system as a whole should work correctly. Negating, of course, the fact that I had changed the fundamentals of the system when I took the air injection system off.

I also know better than this, but I guess I need another reminder. We're all human in the end.

Hope this helps someone else. Keep cranking everyone.

-cjTango

 

[Lumpy Grits]

No SMOG-You must live in Imperial County. Very jealous-
TNX for the update.

LG