Haggisbash

The SBC 350 in my 70 coupe has developed a miss fire and is running unevenly especially at idle. The problem is only on the RH cyl bank.
Checked the AF ratio - at idle is 13.8 - 14.1
Checked idle vacuum - steady at 17in Hg.
Checked plug leads, points, all okay.
Compression - all 120 - 140 psi (approx as only have push in gauge and hard to hold in plug hole).
Have found that pulling the lead to No 8 makes the miss fire stop and the uneven running gets no worse.
Engine sounds mechanically quiet.
I'm stumped, could it be a lifter? I have no experience with hydraulic lifters. Or worse a cracked head or gasket problem?
Only other thing I noticed is that pressure builds in the radiator very quickly once the thermostat opens but there doesn't seem to be any fluid coming out overflow. Not sure how quickly the pressure normally builds as have never had the occasion to look before!
No sign of water in the oil or vice versa.

wajulia

Start with the easy stuff, dist cap, spark plug wires, spark plug

garygnu

after you check the cap/rotor ,plugs and wires.pull the valve covers and check out the valve train for any thing loose .do a leak down test .

TimAT

Check the resistance on that #8 plug wire.

Haggisbash

Cap, rotor, points, condenser, leads & plugs all check out, I will pull the valve cover tonight and have a look. I don't have the equipment for doing a leak down test but will look into getting something!
Funny, when I was out driving, I was just thinking only minutes before the car first developed this problem how I was starting to trust it more after sorting numerous issues since I bought it just over a year ago. Should've known better eh!

Duane4238

I know you've already checked the cap, rotor, points, condensor, plug wires, and plugs, but from your description of how it acted when you removed #8 plug wire, I'd suggest you go back and put in a new spark plug at #8 and put on a new plug wire, too, and see what you have from there. It's usually the simple things and sometimes we get carried away looking for the complex.
Before you replace the #8 plug and wire, connect your inductive lead timing light to it and see if it is firing evenly at idle. If not, replace them. Good luck!
Also, when you pull the valve cover, check out the rocker arms for #8. You could have a broken one that would leave you with a valve not opening at all. I had one break right through the bottom of the rocker arm and the engine had a horrible miss.
Duane

BKbroiler

Try swapping another plug to the #8 location, then try a wire. Something as simple as a cracked porcelain insulator on the plug, which is hard to see, will cause your problem.

Haggisbash

Already checked the #8 plug wire by substitution with a known good one, will verify the firing with the timing light - good idea thanks.
Just as you fire the engine up from cold the misfire is not there for a few seconds, I'm thinking cracked head which opens up as the engine warms. (comp test was done on barely warm motor and showed nothing). I am looking for leak down test gear here.

TCracingCA

Most of what everyone is jumping on applies to a Miss (something not firing!). A mis-fire is different in that it is firing, but at the wrong time! Your initial post says Mis-fire, thus gas lingering in the combustion chamber and it is getting ignited out of sequence!

Misfires, something wrong with that cylinder-- physical problem with the head/valve, crossed wire that is firing another on that side, plug gap etc., etc., etc.. What do all of your plugs look like (coloration?), that would tell you a lot till you get a leak down gauge.

Haggisbash

Plugs look good but the one from No8 is lighter on one side of the insulator nose than the other, I know this means something but cant remember what it is!

TCracingCA

How many miles on this engine? Something dramatic happening should be more obvious on the plug readings, unless they have only been in there a short period, but then I think you said that you have caught this early, as it just started happening. I had hoped for troubleshooting sake that it was something noticeable. The plug color being lighter without a reference to the others didn't help much without pictures. Thus I assume that all have a nice light brown color on the white, not black, no pitting in the electrode or anything! And the one is more white still than the others????????? I have seen guys burr a thread and in tightening them, they feel some tension and don't get them threaded all of the way down, so the plug isn't in the combustion chamber. Thus bad fire, leaving unburned gas, which goes off as a misfire! Are these aluminum or iron heads???? Any detonation prior to the misfiring???

TimAT

I've seen brand new plugs bad right out of the box too. New plug, gapped, installed and no fire.

Haggisbash

Milage of engine is unknown and not original engine. Definitely not a plug, I have tried several and swapped round but problem does not follow plug swap. There are no heli-coils in the heads which are cast iron but are a model acknowledged as prone to cracking. Tried timing light on leads, I'm satisfied it is not an ignition issue. Down to mechanical faults now, talking to someone here they mentioned that they had same symptoms with an engine that wiped a lobe on No 8 cylinder. I will try resetting the valve lash to see if any change.

Haggisbash

I have found an ex drag race and Corvette guru here who will have a listen to my engine tomorrow, wish me luck! On a positive note my 200R4 and associated bits arrived from the USA yesterday, so fingers crossed I can get the engine sorted and then get onto the 4 speed conversion:-)

fishslayer143

Same here.. funny, they were ALWAYS Champion Plugs ? I Quit using Champion.

CA-Legal-Vette

I think that's a good idea. It always helps to have a fresh set of eyes on an issue.

Do yourself a favor and recheck your firing order. It's painfully easy to transpose a pair of wires in the firing order or the wires for #6 and #8. It always amazes me how well these motors run with this error. Just insure that you haven't solved the original problem and replaced it with another one.

You'll absolutely love the 200 4R. I think this was a better mod on my car than adding power to the engine.

fishslayer143

..if is not spark related, you may have a burnt valve , bad lifter, worn cam lobe, or loose or broken rocker arm.. pull the valve cover and observe the movement of #8 rockers.. check lash adjustment . Connect a vacuum gauge to manifold, observe the needles behavior. It can tell you a lot. Good luck

63mako

My bet is it is a wiped cam.

Haggisbash

Vac gauge stable, valve lash is okay, valve movement with cover removed seems okay. As far as spark plugs go, I'm not a great fan of Champion plugs myself. I have had a better run with NGK's from way back when I used Champions in one of my Nortons, they seemed to foul more easily than the NGK's but looking back perhaps they were slightly colder than the equivalent NGK?

Haggisbash

Checking the valve lash with the engine running, I backed off the adjuster until the valve started to clatter and then started to tighten ( I was advised to tighten 3/4 of a turn) but when I did this the engine revs started to drop a lot so I quickly backed the adjustment off again and settled for a little under 1/2 a turn, is this normal??