79 620 Has Developed a Misfire

[willz]

If the carb was leaking fuel into the motor those would be the two cylinders that would get the majority of the flow. So you might be running rich on those two. Now for the 35psi on #1 cylinder, that sounds like a bent valve. I highly doubt you have blown rings or a hole in your piston. Those are a bit more obvious than a slightly bent valve.

 

Good thing it's easy to pull the heads on these cars lol

[willz]

Ah, I see you already said bent valves were the culprit lol

[Mitchell]

Update, I received the old valves with the machined and repaired head and they are bent, you can see it by spinning them in hand. Looking again very carefully, I could see a small depression in each piston below where the exhaust valves had hit and made a mark. So I went ahead and finished putting the head back on, all went smoothly, timing seems correct, made sure cam gear groove, slightly right of tower mark / crank at 0 / cylinder 1 at TDC and 11:30 on the dizzy. Started right up, valve train is quiet, even idles at 800 rpm right away. Unfortunately, Still has the miss from before all of this, bent exhaust valves replaced w/valve job. Checked with a timing light and timing light showed exactly on 0, so adjusted advance to 12 BTDC.  Did a compression check and all 4 cylinders are above 165 to 175, big improvement as cylinder 1 was only about 50 before replacing the valves, haven't done my warm lash adjustment yet. I'll check tonight again to see if there's arcing anywhere once in the dark. I'm going to drive it a bit to color up the plugs to see if I can get a clue from them.

 

Here are the timing mark photos...

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[Racer X 69]

 

You can also see where the valve kissed the piston to the left of the word 'Depression' in the photo.

 

Did you miss a shift and over rev the engine?

[Mitchell]

Don't think so, this engine is in my 620 pickup and not driven that 'briskly', but the timing chain was rattling pretty bad before I had replaced it... It did backfire on a few 1st startups of the day from overnight.

[Racer X 69]

Don't forget the cam tower shims to account for the head getting shaved.

[Mitchell]

Aren't they only needed if too much has been shaved off, making it impossible to get a lash adjustment?

[Cosmonaut]

Late in game have not read entire post. Have you checked the distributor for shaft play?

[Racer X 69]

Aren't they only needed if too much has been shaved off, making it impossible to get a lash adjustment?

 

Been a couple days years since I did any, but as I recall the shims are used to maintain the valve train geometry to compensate for shaving the head. So if .003" is removed from the head, a .003" shim goes under each cam tower.

 

Keeps the end of the cam follower centered on the lash pad, and the cam lobe centered on the part of the follower it rides on.

 

The shims also keep the centerline distance between the cam and the crank the same.

[Mitchell]

Thanks for your reply, since the miss which is very similar to whats going on now, was already happening before the latest valve machine work, I don't think the very light resurfacing cleanup is the cause of the miss, which I told the machine shop to be careful. If the geometry was off on the valve-train, wouldn't it just reduce overall performance and before getting to the point of causing a miss? I'm going to read up on how to check the end play of the distributor and possibly replace it just in case there is a short within it or possibly the magnets are weakened. It's a 79 so it has the factory matchbox distributor. Does anyone know of a good factory electronic Distributor symptom / diagnosis post within Ratsun?

[datzenmike]

Bent valves (all 4 exhaust) can only be from the cam installed wrong and the engine rotated with starter, striking the pistons. Cam was then corrected and luckily the engine ran. Only on the exhaust stroke at or near TDC are either valves partly open. The exhaust is almost closed (14 degrees ATDC) and the intake is just opening. (16 degrees BTDC). Whoever assembled this engine 20K ago did this. There's no way either valves will hit the piston any other way.

 

Over revving? That's rather ambitious for a stock L series.

[Racer X 69]

Thanks for your reply, since the miss which is very similar to whats going on now, was already happening before the latest valve machine work, I don't think the very light resurfacing cleanup is the cause of the miss, which I told the machine shop to be careful. If the geometry was off on the valve-train, wouldn't it just reduce overall performance and before getting to the point of causing a miss?

 

I didn't mean to imply that the miss was caused by incorrect geometry.

 

Your engine is high miles, isn't it?

 

Either way you are past all that now. It appears that the cause of the misfire was related to the bent valves, and now with a freshly shaved head you are moving in the right direction. Since the engine is nearly 40 years old this likely isn't the first time the head has been milled.

 

If it were me, I would find the spec on the cylinder head to cam tower surface distance and check it, and shim accordingly. Then I would check the wipe pattern on the cam lobe to follower surfaces, and the follower to lash pad area. You may also need to get a set of various thickness lash pads to get the whole thing right.

 

Another thing to consider is the valve seats. Each time they are cut or ground the valve will sit deeper in the head and the stem will sit higher in relation to the cam follower and cam. This also affects the geometry of the valvetrain.

[datzenmike]

An uncut L head is 4.248" from valve cover surface to block surface.

[Mitchell]

Wow, Thanks for your replies, that's some really interesting information. Since I am not racing this truck and on a fairly tight budget, if all 4 cylinders are measuring above 170 lbs compression, will getting the geometry corrected/optimized be necessary? In other words, with 170+ could the geometry being off, still be a likely cause of the misfire which seems mid range? I was thinking of trying a remanuf distributor to see if something is going on within the ignition? I really appreciate all the help you guys have been giving! Thanks to You's All!

[Mitchell]

I'll measure the head thickness tomorrow...

[datzenmike]

I thought this mystery miss fire was cured by replacing 4 bent valves.

 

If still missing..

 

 

Are you using NGK spark plugs???

[Mitchell]

Yes, always, the regular resistor I think they're BPR6ES

[Mitchell]

The miss has improved particularly the idle is smoother

[Racer X 69]

You can check the geometry with some gear marking compound. It should show where the cam is wiping on the follower, and where the end of the follower rides on the lash pad. If it is close to centered, and you are able to get the valve lash set correctly I wouldn't get too worked up over it. Also check that the chain tensioner isn't sticking out too far, as that is another complication of not maintaining the correct cam to crank centerline distance.

[Cosmonaut]

Do yourself a favor, pull the distributor and check for play. By hand can be all thats needed

[Mitchell]

Measured Head thickness is at 4.230"

 

The distributor has some play but does not seem terrible. I think I replaced it about 40k ago... Before the complete rebuild.

 

New information, I did an infrared temp check near outlet of exhaust ports at approximately 1" from head (tubular header) with cylinders 1 & 2 at 250 degrees F and cylinders 3 and 4 at 160 degrees.... Plugs appear symetrical, light gray but they only have about 100 miles on them, I guess it's fuel delivery since compression is even 170 lbs compression at all cylinders? I haven't made any changes to the intake setup or carburetor, a 32/36 downdraft which was purchased new but has at least 70k of use.

[datzenmike]

Half a mm is not going to upset timing chain. If cam sprocket mark is correct, move on.

 

 

 

 

I'm assuming this 'miss' is random??? Put an inductive timing light on each plug wire and watch it while idling...

 

When you hear a misfire does the lamp flash or not?

 

If this on one plug or random on any plug?

 

Now try the coil wire. Does the lamp flash every time or stop when there is a miss fire???

[Mitchell]

Eureka! Mike you were right. After all of this, it ended up just being my distributor cap. I know I feel like an idiot, but at least admit to it. After doing exactly what you said, placing the timing light on each wire, #2 wasn't getting any fire. So fortunately I had a set of spare old wires, cap and a rotor. 1st Swapped the plug wire to #2, then tried swapping the cap and that was all that was wrong. Again thanks so much for the advice!!! Appreciate your patience...

[datzenmike]

Easy fix, I like that.

[Mitchell]

At least now it is and easy fix, Sort of, that is if the bent valve situation was bad enough in the beginning to have even needed to be done. I guess we'll never know. But running excellent now and again thanks so much for all of the help through this.