Grassroots Motorsports

Register Log in to GRM
- Community

Birthdays
Recently on the Classic Motorsports forums

Subscribe get a full year for only $23.95 Sample Free Test Drive—no strings attached

Login to post Forums » Grassroots Motorsports » KJet fuel pump with carbs?

Aug. 12, 2011 10:03 p.m. OldGray320i New Reader

Fuel system knowledge greatness needed, for the following:

Can I keep a stock KJet fuel pump and run a regulator to step the pressure down to carb levels (looking at an Aeromotive 13301; http://www.summitracing.com/parts/AEI-13301/)?

Or do I fundamentally mis-understand the workings of a FPR?

System pressure for KJet is 65-75psi, control pressures cold/warm are +/- 35psi to +/- 50psi, respectively, and injectors open around +/- 45psi (according to sources).

It's an e21 BMW, tried to do some cool things with early style manifold and late style air sensor assembly in a decidedly nonstock location, but took three years to get there, and that isn't ideal for KJet. If my fuel distributor is toast, I'm going to run a Weber 32/36 I have, thus the question.

Help.

Thanks!
Aug. 13, 2011 8:25 a.m. MadScientistMatt Dork

Sure, you can do this fine with a bypass type regulator like the one in your link. The pressure is created by restrictions, and the regulator just lets the gas flow past so it can't build pressure.

A deadhead type regulator probably wouldn't have worked.
Aug. 13, 2011 9:32 a.m. mad_machine SuperDork

I think I would keep the mechanical parts.. the intake, injectors, and such.. and just wire up a megasquirt to run it all
Aug. 13, 2011 10:54 a.m. Streetwiseguy Dork

I don't think a Kjet pump would be a very good choice, because to find a regulator that will return 70psi and 100 gallons per hour would probably cost more than a proper pump. It would be overkill on a gargantuan scale.
Aug. 13, 2011 11:24 a.m. MadScientistMatt Dork

The regulator in the link will do the job, though.

K-Jets are mechanical instead of electronic, so you'd need to change out the regulator and injectors if going with an aftermarket controller.
Aug. 13, 2011 12:23 p.m. OldGray320i New Reader

MadScientistMatt wrote:

Sure, you can do this fine with a bypass type regulator like the one in your link. The pressure is created by restrictions, and the regulator just lets the gas flow past so it can't build pressure.

That's what I thought, but not being steeped in fuel system knowledge, wanted to see if I was on the right track. It sounds like I'll just be circulating a boat load of fuel all the time.

Others going from Kjet to carb have talked about surging issues using an aftermarket pump on the stock wiring harness, not sure why that would be. One way or another, I'm going to switch out something, just seemed like it would be easier to throw a regulator on there, plumb it, and go.

I could wait for a known good fuel distributor, and keep the injection, but I'm getting impatient and I have the carb sitting there....

Thanks for the inputs!
Aug. 13, 2011 12:28 p.m. OldGray320i New Reader

Streetwiseguy wrote:

I don't think a Kjet pump would be a very good choice, because to find a regulator that will return 70psi and 100 gallons per hour would probably cost more than a proper pump. It would be overkill on a gargantuan scale.

I probably should look at what a "regular" pump costs; the regulator I looked at was $100 and some, I figured any electric pump would be in the same ball park anyway...

Thanks!
Aug. 13, 2011 12:39 p.m. BoxheadTim SuperDork

The main issue I can see is that you're talking about a huge differential in pressure here - Webers usually want to see around 3-4psi fuel pressure IIRC and you've got an order of magnitude more. That might be hard to control with an FPR.
Aug. 13, 2011 1:03 p.m. benzbaron Dork

I don't see why you'd use a k jet fuel pump as they deliver relatively high fuel pressure needed to operate the mechanical fuel injection system. You'd be using a very high pressure pump for a very low fuel pressure application. I would go to the junkyard and find a low pressure pump for carbs.
Aug. 13, 2011 1:26 p.m. ditchdigger Dork

Mr Gasket brand low pressure carb pumps are around $40 at every parts store in the nation.

Not the best but they do work and will provide a 32-36 with anything it needs.
Aug. 13, 2011 2:40 p.m. wclark New Reader

You can get a 95gph 7psi electric fuel pump and 1-4psi regulator at Summit for under $110.
Aug. 13, 2011 2:55 p.m. mad_machine SuperDork

IF you are insistant on the carbs.. ditch the Kjet pump and go with a carb pump. Overall it is just easier and simpler
Aug. 13, 2011 3:34 p.m. OldGray320i New Reader

Not necessarily insistent on the carb, but atm it's the quickest way to get the car running. Not necessary to get it running as I have a DD, it's just time.

And, trying to find a good fuel distributor is a bit of a pain. Most 320i's that aren't running haven't been for quite a while, so grabbing parts off of one leave me where I already am.

I don't think the carb is THAT inferior to Kjet, though it would be cool to get my set up running.

Time for a trip down to Don's Hot Rod Shop, see what's on the shelf....
Aug. 13, 2011 6:41 p.m. ditchdigger Dork

What is wrong with the CIS system? Being mechanical it is pretty easy to troubleshoot. There really isn't much to go wrong inside a fuel distributor other than it getting gummed up. You can disasemble and clean them when that happens. I is just a simple plunger valve, nothing magical.

The best troubleshooting guide for K-Jetronic is in this

Whatever you do, do not head straight for the "mixture adjustment" 3mm allen screw on the sensor plate. That is the last thing anyone should ever need to adjust yet it is the first thing ham fisted mechanics go for when trying to sort out a CIS car with a vacuum leak.

I like CIS. There is no way I would remove it for a carb if it has any chance of working.
Aug. 13, 2011 10:47 p.m. OldGray320i New Reader

Uh oh. I'm a ham fisted mechanic. But that was when the car was running a bit, richening seemed to help run just a bit smoother, but that was 3 or 4 weeks ago. Since then, nothing.

The plunger is pushing against the air sensor plate, won't allow it to move freely. Some cursory looking about (a "general" bosch guide, KJet and Kjet Lambda - what I have - and a link from bimmer forums) seemed to indicate that cold rest pressure was too high, and that either from a dirty WUR/CPR or something in the FD itself.

However, without the gauges to check pressures, that's just a stab in the dark.

I could (as my wife has suggested as well) have it towed down to my favorite BMW mechanic and have them give it a go, but that is to admit what I aleady know to be true but deny anyway - that I'm not that great a mechanic.... Ow, that hurt...

I suppose I could tear apart the spare FD I have, but it seems a bit of a long shot. I've heard it's not that easy to get them to work. Your reply seems to indicate otherwise?

You'll need to log in to post.