Jump to content

79 620 Has Developed a Misfire

Recommended Posts



Out of the blue, my 79 620 L20b / A87 / Weber. It has the oem e12-80 matchbox ignition.

Was running great then it developed a miss after 20k miles since total rebuild, Distributor cap looks clean inside, rotor shows minimal wear. Plugs were odd though, 1 and 4 were nice light tan, 2 and 3 wet. Idles rough at about 400 smoothens out a bit around 2300.

I've replaced the plugs already, checked and readjusted the valve lash and no change. (Lash was slightly on the loose side, but nothing terrible).

Would appreciate any suggestions what should be tried next.





Link to comment
  • Replies 75
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Idle is 750 and should smooth it out. Unless your tach is inaccurate. Reset the idle mixture.


Check that the fuel level is at the mark on the front of the glass window. Could be flooding


Check that the choke is fully off when warmed up


Examine the coil with the high tension lead off. Is there any cracking on the black insulator?


Tonight after it's dark look at the coil and all the ignition system for any blue sparks when revving the engine.

Link to comment

Thanks for the suggestions.

No sparks visible in the dark.

Cylinders 2 and 3 were wet with fuel.

Choke disengaged, have always had the automatic choke set to kick off at a low temp since it's hot here year round.

I don't think there is a window on this weber 32/36 downdraft, the oem hitachi had a window though.


The truck has been running excellent prior to this for 20k miles. When the truck hadn't been run in a couple of days, I have had a single intake backfire occasionally only one time during the first startup thereafter then not at all once running. Truck always starts on only a single or even half a rotation from cold.


Since the recent timing chain replacement about 400 miles ago, the temperature gauge has been reading a little on the low side...but otherwise running really good?

Link to comment

Yes he did, but people say lots of things mistakenly, specially when fuel is expected to be seen there. We recently had someone with a rattle in the engine. One of the first things suggested was the alternator or water pump and to remove the belt to see if the sound went away. Somehow it was forgotten or never answered and a page or two later discussing timing chain blah blah blah.... it turned out to be a lose alternator mount. It happens.


If 1 and 4 are light tan the mixture is good and 3 and 4 should be the same.


If 3 and 4 are wet with unburned gas they would likely be much darker from soot.

  • Like 1
Link to comment

Ok, Had to work yesterday, did a compression check today with warm engine and here are the results:

#1 @ 35lbs

#2 @ 180lbs

#3 @ 185lbs

#4 @ 145lbs


Rechecked lash and made sure 1 and 4 were 'perfect' NO Change in the 2nd set of compression tests.


Interesting that number 2 and 3 had great compression even though the plugs were darker and wet... (on the new plugs as well)

I guess I've got a valve problem... and need to remove the head?

I appreciate everyone's comments!

Link to comment

Also, not losing any coolant whatsoever, motor has only 17k since 'total rebuild', but keep in mind this is the same engine that had to have the timing chain replaced because in my opinion they never replaced it when it was rebuilt. Shop is closed up. I say this, because they listed everything, fluid cleanup fee and part on the invoice, even the oil filter but no mention of a timing kit...

Link to comment

I assume you did a compression test before changing the HG???


Maybe valve got bent when it was off? and why I asked about the cam being turned.


Won't be a hole or you would have seen it when the head was off.


Probably the exhaust.

Link to comment

Sorry, I haven't taken the head off yet, just asking if that should be the next step. I did squirt some oil into the dead cylinder #1 and rechecked compression. It was up from 35lbs to 65lbs. does this indicate a piston problem, hole in piston, cracked ring ?  Looks like I will need to take the head off either way. In the meanwhile driveable, Before disabling my truck, Just trying to figure out how much money I'll need before taking it apart so I have an idea of how much to expect to spend. If this is going to be just a Valve Job or more expensive....


Link to comment

Problem is that if you pull it at TDC then that #1 piston will be all the way up and you won't be able to inspect the wall unless you unblock the timing chain from falling, looks like if you don't find a head issue this will be a bigger job, good luck, hopefully it's a valve seat doing this.

Link to comment

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.