Carb problem

[Mr_Roboto]

Thanks, Keith. The adapter is working beyond my expectations. Usually a fab job like this ends up with a piece that sort of works, but actually shows how to make a better one. So, I have to start all over again typically. But this one came out right the first time. By the way, that adapter doesn't look like much, but it took me about 9 hours to make it.

I did some more tuning on the carb today and got the idle a bit nicer, but I've got a lot to learn about dialing in a carburetor. The idle isn't where I want it, but I have to say that this carb makes starting the engine the easiest that I've yet to see. Even when it's sat overnight, I barely have to hit the starter and the engine lights with a little pedal pressure. I've noticed that throughout the day, if the engine is the least bit warm, no throttle pedal pressure is needed for a start; just turn the key and the engine is running, like fuel injection. That could be because the carb is rated at 465 cfm and is a bit small for this application--Holley recommends 570 cfm.

I wondered if the smallness of the carb would cause problems at 4 lane speeds, so I did about 35 miles of turnpike driving today. Unfortunately, I was bucking a gusting head wind that pushed the van all over the road. I wanted to see how the engine responded at high cruise speeds, but the wind made that hard. But I think the engine was a bit lean. I've got #56 jets in the carb and think that I'm going to try using #58 jets just to see what'll happen.

Do you have a vacuum gauge? If the carb is pulling vacuum at the top end it's too small. I don't think it'll really hurt anything except the top end power.

[SafariRob]

Robo, with this Holley I read 2" of manifold vacuum at wide open throttle; with the Demon I was hard pressed to see any manifold vacuum at WOT.

Engine starting continues to be a pleasure. Like I mentioned earlier, if the engine has been run at all during the day, just bump the starter and the engine is running, no pedal needed at all. The Demon started easily, too, but not like this Holley does.

Haven't tried increasing the jet sizes yet, though. Wanted to get to it tomorrow, but I think Mother's Day will take presidence over it!

[peter]

Robby, does the Demon have a power valve like the Holleys? That could be the issue with the bogging at 5".
As far as the secondaries/jetting goes, it takes some trial & error to tune them for your particular application. Back in the 70's, Holley sold 3-4 different secondary springs that would/could wake up an otherwise boring engine. I've found that Holley usually sent their carbs with rather stiff secondary springs, prolly to avoid bogging, espeically in an auto stick.
For jetting, you need clean plugs and full throttle blasts followed by engine shutdowns and plug reading to tune primary, then secondary jets. Mebbe play with the power valve afterwards to fine tune midrange fuel delivery.
As well, slight bogs off the line or on acceleration can be eliminated with proper selection of accelerator pump cams and/or squirters

[SafariRob]

Peter, I didn't see this posting of yours until today, May 20th. Don't know why it wasn't flagged as new back when you posted it.

Yes, the Demon has a power valve like a Holley does--in fact, it exactly the same valve because a Holley one will fit right in. I also wondered if it was the culprit, so I installed a new one (also a 6.5, like the original), but nothing changed...still had the ferocious bog.
I then wondered if the bog was caused by the secondaries starting to open. Took the engine cover off and tried to watch the vacuum pod's arm as I nailed the engine, but watching the carb while trying to drive is a little rough. Didn't see any action on the secondaries though I made several attempts. Really made me wonder if the secondaries were opening at all ever. I installed a very light spring in the pod and could then see the rear barrels opening, but only for a short while. Didn't affect the bog, though.

I then installed the 465 cfm Holley I'm now running. It also has a slight leaning at 5" but it isn't a problem. It's much easier to watch its vacuum pod open the secondaries, too, as it's a model 4160 (the Demon is a clone of a Model 4150) and the rear bowl doesn't jut out as far.
Keeping an eye on the road and one on the vacuum pod showed me that the secondaries don't just open like I thought they would; it's more like they are modulated by the pod as engine load changes slightly. I'd like to be able to rig up a sensor on the pod's arm so that I could watch (safely!) an indicator on the dash to see what's going on with it. That way, I figure I'd soon learn to tell by engine sound what's going on.
I have some issues with this Holley; there's a bit of a rough idle that I haven't been able to tune out yet and I installed a manual choke kit from Holley that refuses to come off by pushing in the choke cable, but all in all, I like the way the engine runs. And I hate to beat this topic to death, but starting the engine amazes me to no end--it really is like fuel injection and I've never had a carbed engine start like this before. After I changed the jets, I filled the bowls by jogging the electric fuel pump, and then hit the starter--didn't pump the pedal at all--just to see if the engine would start. It did, but died almost immediately. I then hit the starter again, still with no pedal pumping, and the engine lit and continued to run. It wasn't happy, there was no choke, but it ran at a fairly steady 450 RPM. At that speed, I swear I could hear the timing chain working when I stood in front of the van with the hood open. Would have loved to have had timing gears whining right then. As the engine warmed up, the idle speed increased to 800 RPM, where it usually sits when in Park.
I changed the plugs today and noticed that all of them were a little lean looking. I upped the main jets from size 56 to size 58 and will check a plug to see if anything has changed.
I did do a test drive but didn't notice anything different, though.

One final thought--and most of you will think that I'm a retro-caveman, but--carburetors are elegant fuel mixers; they are really an analog computer of sorts that works on fluid flow with vacuum circuits calling the tune. Actually, from what I've read, vacuum isn't the right word to use, the proper phrase would be atmospheric pressure, as that's what's doing the work of pushing fuel into the cylinders.