Diagnosing a misfire

[Mrturner1]

Originally Posted by davepl

165 vs 180 seems like a lot of variation, but I guess it's only about 10%.

Did you have the throttle blocked open when you did it? You'd imagine that forgetting to do that would reduce them all by the same proportion, but it doesn't always seem to work that way.

Yep I wired it open all the way. What's the normal scale for SBC compression?

[Mrturner1]

Well I've gone through 3 sets of Accel header plugs, and after reading about them I decided to try a set of Autolite 26's which are a bit hotter. I let it idle for about 10 minutes and pulled a plug, same thing, black and sooty. This time I had the mixture screws 1 1/2 turns out.

The timing is all set perfect, 18* initial and 36* total at 3000rpm. I put a new power valve in because it's only making 7"Hg vacuum, power valve is a 4.5

The haze of unburnt fuel seems to get worse the longer it idles, maybe because the plugs are fouling further?

[Mrturner1]

I just watched a video on tuning Holley carbs with long duration cams and I have a MUCH better idea of what's going on inside the carb. I'd be willing to bet that my transition slot is showing way to much but I'll have to confirm tomorrow when it's light outside. The guy on the video showed a way to close the throttle blades a bit by drilling small holes, which allowed him to close the throttle blades exposing less of the transition slot.

[geezer#99]

Originally Posted by Mrturner1

The guy on the video showed a way to close the throttle blades a bit by drilling small holes, which allowed him to close the throttle blades exposing less of the transition slot.

That's the best way to render a 500 buck carb into a useless one.

[bean]

X2 I would really think long and hard before doing that

[davepl]

Originally Posted by Mrturner1

I just watched a video on tuning Holley carbs with long duration cams and I have a MUCH better idea of what's going on inside the carb. I'd be willing to bet that my transition slot is showing way to much but I'll have to confirm tomorrow when it's light outside. The guy on the video showed a way to close the throttle blades a bit by drilling small holes, which allowed him to close the throttle blades exposing less of the transition slot.

I've done that... once. Actually I didn't do it, it was done to my carb by someone I figure knows what they're doing, but I still am not sure it was the right approach. That was for a ZZ502 with a Q-Jet.

I think 99% of the time when people want to start drilling holes in throttle blades to get them to close further so that they can uncover the transfer slots they're actually band-aiding some other problem, like a vacuum leak that they don't know about.

And 1% of the time it's actually the right fix... but I think it's uncommon.

[1969c10joe]

does your engine stall when you screw your idle mixture closed? check your fuel pressure and or install regulator. I wont run without a regulator anymore dont ruin your carb

[Mrturner1]

Well I made some good progress. I decided to take the rocker arms and pushrods out to check everything because I started to suspect a cam lobe flattening causing the rockers to loosen up after I tightened them. Turns out the tips of a few pushrods were wearing down and this was causing the rocker to become loose. I got a new set of pushrods and reinstalled everything, and set lash at .017". When I took it out for a drive it misfired again, brought it home and set lash at .020" cold, and still a slight misfire. Tried again and set it to .020" hot and it runs fine now. I don't understand why the lash can't be fooled with on this cam, but I know it can't. Maybe some cams are different and have different take up ramps and lobe characteristics, and those can be set tight or loose?

[davepl]

Maybe I missed it, but I assume this is a solid cam if you're quoting lash clearance?

[Mrturner1]

Originally Posted by davepl

Maybe I missed it, but I assume this is a solid cam if you're quoting lash clearance?

Yep I took that 280H out because the way the valve events happened it built so much compression that I had to mix a lot of race gas in at every fill up. And even then it still pinged pretty bad.

The cam that's in it now is a 282s .496" 236@.050" 106 LSA. Much less compression, lot more power down low and mid but I'm not 100% sure I like it. It's staying in though, no way I'm doing a cam change I just wanna drive it for a while.

[Mrturner1]

Made some more progress with the pig rich idle condition. So this is embarrassing but ya gotta learn somehow so what the heck lol

My idle was so rich that there was black smoke everywhere and my plugs kept fouling after 10 mins of running. I tried all kinds of tricks and adjustments on the carb to no avail, and finally I took a picture and by a stroke of luck somebody saw it and said my PCV was plugged in wrong. I had the PCV going from the valve cover to the air cleaner, so all that air was going through the carb and pulling more fuel with it.... = rich idle. So I plugged the PCV into the back of the Holley carb on the big port and my idle went way up immediately. This allowed me to turn the idle speed down closing the throttle blades, and that left less of the idle transfer slots exposed, pulling even less fuel. The other problem was that my idle mixture screws were 2 turns out, and after I hooked up the vac gauge I found that the highest vacuum was with mixture screws turned out only 1 turn, again now I'm pulling even less fuel through at idle. Now I'm still not making great vacuum by any means at 7"Hg, but my cam is a long duration 236@.050" cam and its on a 106 LSA, so lots of overlap which equals less vacuum. Thing is, it's MUCH more manageable now and tuneable, and my mixture screws actually change things when turned in or out which was non existent before all this work.

I'm not quite done yet, but I think I'm close. I need a little more air at idle but I'm not going to drill holes in the throttle blades, and I learned a trick that will safely help the situation. Take the carb off and turn out the secondary blade screw 1/4 turn, reinstall and adjust mixture screws for best vacuum again, then turn down idle speed more. If that's not optimal, I'll take the carb back off and give the secondary screw anothe 1/4 turn out and adjust everything again and see how that runs.

I'm getting close, and listening to you guys and applying is working great. As far as solid cam lash goes I adjusted it to .020" which is what the cam card says, and although it doesn't sound as wicked or pull as hard on the top end, it does run better and there's NO misfire. This thing had a misfire at .018" lash which I don't get but for now I don't need to, I'm just going with what works and drives.

More later,
Matt

[geezer#99]

Musta been some old guy noticed your pcv was hooked up wrong!!
You don't need to pull the carb to adjust the secondaries. Just make or buy a 90 degree small flat blade screwdriver. Or maybe the newer carbs take an allen wrench. Stick a mirror down there to check it first.

[Mrturner1]

Originally Posted by geezer#99

Musta been some old guy noticed your pcv was hooked up wrong!!
You don't need to pull the carb to adjust the secondaries. Just make or buy a 90 degree small flat blade screwdriver. Or maybe the newer carbs take an allen wrench. Stick a mirror down there to check it first.

It sure was lol I'll look into that screw driver thanks!