Jump to content

Idle revving between shifts...Engine smoking...


Recommended Posts

I got two problems on my '86 720

1-engine revving like crazy

2-Smoke from Cat converter?


Been working on my Carb.  adjusted the throttle screw. took it for a drive around the neighborhood.  Top speed 30mph. drove about 6 blocks. Felt pretty good but pulled over to check a different adjustment and noticed that there was smoke coming from what looks the catalytic converter. I let it cool down for about 5 minutes then started to drive back home. Then it started to rev real high between shifting. my rpm tach doesn't work but I'd guess it reved to about 5,000-6,000 for about 3-7 seconds before coming back down to "normal idle". I still consider myself new to mechanical work. I know enough to get into trouble. LOL. I checked the oil thinking that it was low.  It is fine.  Dirty. but it is full of oil.  Any idea what the heck is going on.  I'm not sure it will make it on a drive to my mechanic without catching fire. Thanks for all the help.This forum is a fantastic resource. 

  • Like 1
Link to post
  • Replies 26
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Stop driving it.


An over rich carburetor adjustment or several other reasons for a rich mixture will force the catalytic converter into over drive trying to burn the extra gas off producing LOTS of heat.


I'll check into this tomorrow.


For now explain everything you did to the carburetor.

  • Like 1
Link to post

-The Smog check guy said it was running rich and suggested rebuilding the carb. So I rebuilt the carb with new gaskets. carefully and methodically.

-Choke connecting rod had a jury rigged connection on it from previous owner so I bought a used carb off eBay for the part and replaced it. 

-Auto Choke coil wouldn't keep a tight connection to the connecting rod/flap so I took it off and stretched out the coil. That seemed to work but once the heater got fully hot the little metal notch would slip out of the hook. So I took it off and replaced it with the coil from the ebay carb. soldered that blue wire connection back to it while the engine was running and a couple sparks went flying. It was exciting! 

-ground down the metal notch on the auto choke casing so i could adjust it Lean or Rich.

-turned the idle adjusting screw 1.5 turns out

-tightened Throttle adjusting screw about 5 turns until it the engine stopped bucking.

-played with the Idle compensator screw. it didn't seem to do anything.

-The carb is definitely running too rich so the Cat into overdrive makes sense. I can't figure out how to lean the fuel mixture out.


I think that is it.


  • Like 1
Link to post

There should be a sight glass on the front with a line or a dot to indicate fuel level. Where is your fuel level????



Remove top of air filter and look. Choke is probably partly closed. Step on gas it should snap closed. The choke adjustment should be just enough to close the choke plate. Start engine. When the choke closes it also engages the fast idle cam so it will likely rev up slightly 1,800-2,400 depending how cold it is.

When warmed up (8-10 min) the choke plate should be vertical and fully open. Remove top of air filter and look. Is it?


Check that the throttle cable is properly adjusted so it closes when released and does not hang up.

  • Like 1
Link to post
  • 3 weeks later...

The choke does fully opens after warm up.



On 3/30/2021 at 4:03 PM, datzenmike said:

It's more important that it opens fully.




For adjusting the carb,  You datzenmike wrote this from May 24 2012 in the forum but I still have a question. 


On 5/24/2012 at 5:57 PM, datzenmike said:

There's no set amount. What I do is warm up thoroughly and turn the idle down or set a nice idle speed by ear. Turn the idle mixture screw in until the idle quality drops off and it starts to stumble. Now turn it out until you get the fastest smooth idle speed. Turn the idle speed down to 650-750 Now repeat but turning in until the idle quality drops, then out until the idle quality drops. Usually about 3-4 turns. Pick a spot roughly in the middle between these two points. Idle speed and quality may improve again from the first try, so turn the idle speed down and repeat the in and out and setting in the middle. Keep going untill you cannot improve the idle speed and quality. You're done.


Before setting the idle mixture always check and set both the timing and the valve lash. You can set the mixture in order to get the motor running to set the timing and valves but always go back and re-adjust the carb. Improper timing and valve lash can make a big difference in idle quality.



To adjust the carb I turn two different screws? first the throttle adjust screw to a nice idle (my dash doesn't have an RPM tach)...then idle adjusting screw (this is the screw you refer to in the 5/24/2012 post)? or is the idle adjust screw the only screw that adjusts the fuel air Mixture?


I do have the 86 Factory service manual on Hand and I've been studying it.

Possibly my Air Fuel Ratio Solenoid is bad? Maybe I need to adjust my fast idle adjust screw?

Link to post

The idle circuit is separate from the air/fuel ratio solenoid.


There is an idle speed screw and an idle mixture screw. The speed screw raises or lowers the idle speed and the mixture screw adjusts how much fuel is mixed in with the air.


The fast idle screw is only for when the choke is ON and will never need adjusting.


Link to post

So she is running super rich. Screwing the idle mixture screw out will give her more air making her leaner right?

  • Like 1
Link to post

Okay so I was wrong. The auto choke isn't opening up all the way. I just disconnected it and will wire it open for now.

I checked the resistance between the choke heater connector and the carb body and it in spec.  

Then I checked the Heater circuit at the plastic plug for continuity and voltage and got nothing.  My plug looks different than the one in the Manual. Perhaps I checked the wrong holes in the plug? here are the pictures of my plug and the holes I used with the multimeter compared to the manuals plug.


Also, She is running very smooth now.
 My Throttle adjustment was way cranked, so i backed that off.
The idle adjusting/mixture screw is almost all the way screwed out at this point.
I will take it to a smog check soon and see how I did. It still has a bit of black smoke when I rev the engine. But, the Catalytic Converter isn't smoking anymore.  Hooray!
New problem. When I press the gas pedal in first gear (and sometimes in second), there is a delay of a second or two before the gas really hits and I can accelerate. What could be going on?  
  • Like 1
Link to post

There are two adjusting screws.


Idle speed and idle mixture. The idle mixture is set and plugged at the factory. You can un- seal it and make an idle mixture adjustment but this may be a smog fail if the inspector sees the plug removed.




Above is the idle mixture screw. It's on the right or passenger side of the carburetor. If not uncovered I would leave it alone.



Sudden bog when stepping on the gas is a problem with the accelerator pump.

  • Like 1
Link to post
22 hours ago, datzenmike said:

Don't bent it.


Move it over to the right to another hole. 

I moved it to the other hole.  It worked great for about 10 minutes of driving.  Then it started to bog a bit again...and star

ted blowing black smoke out the tailpipe. I have adjusted the throttle and mixture screws out. No change to the exhaust.

  • Like 1
Link to post

The mixture screw only adjusts for IDLE. As soon as you drive off it has no effect.


Black indicated too rich. Next time it blows black, as fast and as safely as you can, get the ignition OFF and clutch in and out of gear. What you want is to stop the engine as fast as safely possible.... and pull over to the side of the road. Now look for the fuel level in the little glass window on the front of the carburetor. I'm wanting to know if the carburetor is over filled and flooding so the engine must be stopped as quickly as possible to preserve this 'evidence'.

  • Like 1
Link to post

Have to eliminate the carburetor flooding. Could be dirt in the needle valve, float setting and several other things.


If not flooding, it could be the ECCS carburetor. There's a default to a 'limp home' mode that prevents the mixture solenoid from mistakenly running a dangerously lean mixture if there is a failure in the system. Limp home is characterized by an over rich condition.

  • Like 1
Link to post

I haven't checked the float yet but the "limp mode" might be something. I had disconnected the electric harness from the carb, forgot to reconnect it and was driving around and it drove great. No black smoke. Drove wonderfully except the bogging. Then I parked in the driveway opened the hood and saw the harness disconnected. I reconnected it and took it out the next day only to discover the black smoke was back.  I'll take it for a ride later disconnected from the electric harness again to check it out. 


So how would I know if it is in Limp mode? And how would I reset it?

  • Like 1
Link to post

No idea. This was discussed on a 720 forum well over a decade ago.


The ECC carburetor adjusts the air/fuel with feedback from an O2 sensor, so there is the real risk of a extreme lean condition if there is a failure in the system. Under a failure condition a rich condition is built in as a safer fallback mixture. It's assumed that the owner will notice the over rich and horrendous fuel mileage and attend to it.

  • Like 1
Link to post

Well shoot King Rat! That sucks.  I have already changed the O2 sensor about 6 months ago.  I drove it around today without it plugged in and once again no black smoke.  Is it worth it to go back and find the 'limp Mode" discussion?

Link to post

So plugged in black smoke and unplugged it runs 'fine'? This is a California carburetor. I am not too familiar with them.

Link to post

Yes.  It might be  little rich with the carb unplugged but it is no way near the amount of fuel burning through it before.  CA is the worst. They are holding my title hostage until I can get them a clean smog check. Can I just put a Weber on this thing and Pass Smog? LOL 

I'll be perusing through the old posts looking into limp mode.  Once again I way outta my element...or just at the end of my fuse...hahah. I love doing this kind of stuff though!


I'll check my float tomorrow to see if it is drowning is gasoline just in case.


  • Like 1
Link to post

I pulled over and Checked the float. The fuel level is correct in the reservoir. Right at the dot.

Could it be a BCCD situation?


I screwed the idle mixture screw in tight and the engine didn't seem to react much. 

I screwed the idle speed screw all the way out and not much change either. Screwing it in tighter does bring the idle up. 



Link to post

The mixture solenoid isn't working at idle so the mixture screw and the speed screw should adjust the idle.


Is it revving too high at idle?

  • Like 1
Link to post

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.