Jump to content

4K Carb Setup


Jackson

Recommended Posts

G'day fellas.

 

I got two 4k carbs here, one with a manual choke, one with an auto. I have a 4k-C in a Suzuki sierra, done by someone else, at the moment it has the auto choke carb on it and I think some of the things like the dash pot, hot idle compensation and auto choke isn't working. I think I would prefer to use the manual choke one but I have ordered a kit to put through whichever one I decide on.

 

Anyway I have been looking at this in the Wiki section and I have a few questions.

 

The distributor line. Does it go into the plate between the carb and the manifold or does it go into the carb somewhere. Also I have an electronic distributor, the wiki section says to just T it to the other one, is that all I have to do? For some reason mine runs into that BVSV. Also I don't have that loop of the side of the dizzy line with the VSV and VCV what is the actual purpose of that?

 

Also the picture shows 5 hoses on the right hand side of the carb. I can only find four on the manual one, however the auto choke one has the four plus a bigger (than all the other pipes) coming out of the very top plate, just next to the pump plunger I think it is in the top of the fuel bowl, however the manual choke carb doesn't have one. Is this bigger one meant to go to the canister?

 

If anyone can answer any of that, it'd be a big help

 

cheers.

Link to comment
Share on other sites

Members dont see this ad
Does it go into the plate between the carb and the manifold or does it go into the carb somewhere.

 

Depends on how you want to run your dizzy. The plate should be manifold vacuum, so when the throttle is shut on idle it is exposed to maximum vacuum. On acceleration it has very litle vac, and on cruise the vac goes up again as the throttle closes and the ignition advances.

 

The carb one normally used is ported vac, the drilling enters the carb throat right under the throttle plate edge and is closed off when the throttle plate is closed over the top of it. So zero vac at idle, very little vac at acceleration and good vac at cruise.

 

With manifold vac you will set the timing to 10deg, and nearly all of that will be vac supplied. So as soon as you open the throttle it collapses the vac and the timing goes back to zero for acceleration and it advances on the mechanical weights..

 

With ported vac you set the timing to 10deg and none of that is vac supplied, so when you open the throttle the timing stays at 10deg then climbs with the mechanical advance weights.

 

At other throttle times they behave the same. Cars used manifold vac in the old days, then ported vac later on, say 1980s on.

 

Personally, I would mark the pulley out to 50deg with a pair of dividers and nwhite paint, then generate an advance curve of revs V advance every 500rpm up to 3500rpm. That is your starting point. Then swap carbs with no vac advance, check the curve and try each of the two ports ane decide which gives you the best driving.

 

I'd love to see the 3 curves!

 

http://www.rollaclub...594#entry511594

 

Here are the two layouts, auto box & manual.

post-7544-0-42565600-1315624221_thumb.jpg

post-7544-0-27417000-1315624307_thumb.jpg

Link to comment
Share on other sites

I'm not sure but I think the plate under the carb runs up into the carb and comes out into the throats just above the primary butterfly. So does that mean it is ported? I did a quick vac advance test and upon increasing revs the advance increases. Then is it just that all the other things need some sort of vacumn so they can just go on whichever ones of there is some specific positions for each hose?

Link to comment
Share on other sites

I did a quick vac advance test and upon increasing revs the advance increases

 

That might be the advance weights doing that. The way to test is watch the advance as you pull the vac hose off at the dizzy and put your finger over the end to seal it. The vac effect will stop and the timing advance drop back. When you put the hose back on the advance will jump up again. That difference is the amount of vac advance you have.

 

So does that mean it is ported?

I'm pretty sure it is.

 

There will be a different vac amount at different revs. Asian could have different amounts of vac on different ports depending on where they go into the carb throat, but mainly anything "ported" will have zero at idle and "manifold vac" will be at max at idle. I borrowed a mate's vac gauge and ran it through the firewall and we watched it as we drove, which taught me the vac at 3000rpm sitting in the garage is nothing like the vac at 3000rpm on the road when the motor is working..

 

All the vac advance does is give you extra advance at cruise, where there is little throttle opening and the extra spark advance allows you to burn all the weak mixture and reduce emissions. When you boot it the vac collapses and the timing retards to give you acceleration, then when you lift off it advances again.

 

I don't know which goes where from the diagram. When I ran the stock carb years ago I blocked all the ones marked in red using a bit of 4" nail stuffed up each one and kept the two marked in green. They do the vac advance and the charcoal canister, the rest don't matter for performance as far as I know. If you have all the lines, block them and swap the dizzy vac from port to port and measure the timing on each one. It would help everyone if you could report any difference between each port on the carb.

Link to comment
Share on other sites

  • 4 weeks later...

So I have rebuilt the carb but stumbled across something interesting. I left the secondary vacuum diaphram sitting upside down on the bench. The next morning it was sitting in a nice little puddle of black sludge. :bash:

 

I want to try get it all out before I put it back on, as it might be a reason the secondarys stop opening.

 

Anyone got an idea of how to get it out?

 

Also the bits in those diagrams that says "jets" are they little plastic cylinder things in the vac lines?

Edited by Jackson
Link to comment
Share on other sites

I'd get an aerosol can of carb cleaner or brake cleaner & make sure it has a little extension hose on. Use that to blow through all the carb drillings. Following up immediately with an air compressor would be good.

 

Those 'jets' are the plastic cylinders. Sometimes they delay the onset of vacuum, or limit it, or are one-way valves, I've never worried about what those ones do. Might not pay to get them in backwards though!

 

Good luck!

 

 

Link to comment
Share on other sites

Yo Altezza, do you know what the power valve is all about? Having a look it is just like another jet that opens up with vacumn, it looks like it might come out the primary barrel in the same but the primary jet feeds. But in the diagrams it looks like it is linked up to the idle bit (it comes out of that port on the bottom edge in the pics).

 

Is it called a power valve cause it allows more fuel when you plant it? Isn't that what the secondary is for? Or is it something to do with when she is idling?

 

Cheers.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...