More VC625 carb voodoo/experimentation

[oldphart]

I spent some time yesterday afternoon screwing around with my spare (so far, unmodified) VC625 carb. First, I screwed around with compressed air and aerosol carb cleaner to make sure the passages were set up conventionally and not like the later Holley reverse-idle POS 4 bbls. It IS set up quasi-normally....the idle mixture screw passes air and only air.....the plastic primary or idle jet meters fuel only.....100% independent of where ever the idle screw is set. The supposed "three circuits" in this carb are 1) the main jet and it's emulsion tube (in the center, with the float bowl removed), 2) the second emulsion tube offset from the center main jet (with a brass blank plug retaining it, and 3) the primary or idle jet. All three of these circuits pick up fuel from the main jet well. So, I put my little peanut brain into "pretend it's a Mikuni" mode. Now that I have adjustable air, I need more fuel at and off idle. I pulled the plastic primary, or idle jet out and attempted to measure it. It's smaller than .03 mm which is the smallest on in my set. Next, I measured the primary jet that came with my EC adjustable main jet kit that is supposedly for a hot-rod GX390 clone on gasoline. It's .65mm (my bits are in .05 mm increments.....and .7 mm wouldn't go in...). So....I pulled the carb off my cart (that I've already dug out the epoxied-in adjustment screw and replaced as per Shepard's instructions....and replaced the plastic abortion of an idle speed screw with a 'real' slotted screw and spring...but have NOT yet added the adjustable main jet)) and stuck the .65mm one in. Starts cold without the choke, no lean-sneeze off idle, but it's obviously too rich to get a really good idle after warmup......but it's one helluva' lot better than the stock way-too-lean setup. After that, I drank beer and thought about it..... This morning, I pulled it apart again, re-measured jets again just to make sure I was right, and drilled the stocker to .3mm. MAJOR improvement.....I have an idle close to 1000 rpm, it responds to the mixture screw rather nicely, and still no stall or 'lean sneeze' after warmup.

I think I might be on to something here. I'll post more as experimentation continues. I still think a little 'more' might be even better, but I gotta' drive it for a few beers.

​Jim

[Shepard]

When I looked into the "third" circuit I found it to meter off of the main jet. Being smaller in diameter and shorter in height - it would required less air speed over the emulsion tube tip to make it function. This overcomes the off idle cough and sputter. I wish this carb had an accelerator pump.
Oldphart - do you use a pedal start setup? The carburetors I build are for key start machines that have to run on the own and be drive able like a car/truck -- like I said -- wish there was an accelerator pump.

[oldphart]

Nope. Mine's a keystart also. Please.....lemme' know any input. I'm trying to be scientific about figuring this out.....and I appreciate anything that will help. You've probably forgotten stuff I don't know. Thanks!

Jim

[csason]

Looks like I found the right thread

I just started with a stock carb and VC 625 new in swap out about two weeks ago.
I opened the main jet a little it idled better but needs work.

That isn't the major issue I am wondering why yesterday after running fine for weeks now, (new swap out) gas is pouring out of the overflow by the block ?

Why would it suddenly do that ? I drilled the main jet couple weeks ago

[raydhd]

Are you using a return fuel line ?

[csason]

Originally Posted by raydhd

Are you using a return fuel line ?

No. Is there somehow supposed to be one not included with the engine ? I bought the full kit, it installed fine, worked fine, then gas started pouring out by the block, I pulled the carb that is the overflow. It is coming from some overflow of some sort not sure why and it isn't consistent. I can start it and it seems to run fine then bang gas starts pouring out, or it seemed to do it the first time when I was out testing it, I am literally just testing it out pretty much lol its that new. I don't have time to run it every day or spend hours with it, I am a working man lol

[raydhd]

The clone carbs are built as gravity fed units pure and simple. Some people get away with not running a return but many do not.

[slonomo]

Originally Posted by oldphart

After that, I drank beer and thought about it..... This morning, I pulled it apart again, re-measured jets again just to make sure I was right, and drilled the stocker to .3mm. MAJOR improvement.....I still think a little 'more' might be even better, but I gotta' drive it for a few beers.

Beer fixes everything doesn't it?

[oldphart]

Return lines.....here's the hands-down easiest and cleanest way to add a return line. A Wix 33040, or a Napa 3040 filter. 5/16" in and out, 1/4" return. Get a fitting for a plastic lawnmower tank with a rubber grommet that presses into a drilled hole in the tank, and you're done.

Jim

[oldphart]

Shepard's post above got me thinking....I checked a stock secondary emulsion tube; the fuel metering holes (the ones at the bottom...there's four) are over .65 mm, but not quite .7 mm. So, I opened one up to .8. I'll put that in and see what that does. It still needs some more fuel at off-idle and low throttle openings, but I'm hesitant to open the pilot up any more just yet because of the dramatically improved idle quality. Stay tuned....

Jim