Cascade Crew

Twilightoptics · Posted: Thu Dec 01, 2011 7:53 am Post subject: Carb tuning issue n theory...

In carb land I've never had this issue before so I'm not sure how to go about curing it.

Say you're cruising on the freeway in gear (stick). You let off the throttle (throttle now closed, rpms high) and when you get close to your exit you push in the clutch. When you do this, the rpms drop down to actually below idle and the engine runs very lean and stumbles for literally 30 seconds, if not stalls out completely.

Idle mixture, timing, and idle speed are all consistant with factory spec as it's a stock motor, and seem to function just fine with the exception of the above condition.

What is this indicitive of?

blue89 · Posted: Thu Dec 01, 2011 12:14 pm Post subject:

When the throttle is closed the fuel only comes from the idle mixture and transition slot (or hole). How closed is the throttle when its closed? And what carb?

Twilightoptics · Posted: Thu Dec 01, 2011 12:26 pm Post subject:

blue89 · Posted: Thu Dec 01, 2011 12:47 pm Post subject:

Im thinking that if it is too open you arent pulling enough vacuum to pull fuel through the idle and transition circuit. Just a thought. I would try to measure engine vacuum with a gauge and then set the idle speed and mixture to produce the most vacuum.

Twilightoptics · Posted: Thu Dec 01, 2011 1:00 pm Post subject:

Done that. I get 18inhg max.

Good idea though. I've been trying to open the blades lately and it does seem to be getting a touch worse.

Get this too... summer time, doesn't have the problem. Only when it's under about 65deg out LOL.

blue89 · Posted: Thu Dec 01, 2011 2:45 pm Post subject:

Does it run normal after a couple if throttle blips? I wonder if you have a clogged transition slot or if it is improperly sized.

For manual transmissions ive always been curious how the system will adjust the idle while it hunts for the set speed after a high rpm drop like that.

aaron_sK · Posted: Thu Dec 01, 2011 9:09 pm Post subject:

Couple of thoughts:

Does it idle lean in general?

Do you have a WB on it or are you just hearing the lean pops?

Are those carbs like a Holley where you can have the plate in a position for good idle but have the transfer slot covered?

chevymad · Posted: Thu Dec 01, 2011 9:57 pm Post subject:

Dont have any good idea.. but a couple thoughts. Did this carb have or once have a deceleration valve that cut off the fuel when you are slowing down? Most of the jap carbs did this to keep the exhaust from popping from being too rich due to the high vacuum signal on the idle circuit.

Guess I only had one thought.. I really try to stay away from the jap carbs because I have virtually no success with them.

aaron_sK · Posted: Thu Dec 01, 2011 10:01 pm Post subject:

Twilightoptics · Posted: Fri Dec 02, 2011 8:49 am Post subject:

It's WB.

When it's doing this stall out, if I blip the throttle it usually kills it. Accelerator pump is working right.

Also, when it stalls, it's hard to start. I have to pump the gas a few times for it to catch. Like it's not getting fuel.

If I shut the car down when it's idling correctly. I can turn the car off and start it again with out throttle, and a very short crank time.

Idles anywhere from 13 to 14.7. Often it will start at 14.7 and richen up to the stable Idle I set it at.

I'm starting to think something must be plugged or blocked in the carb. It's just gotten worse. Tried a lower idle amd set the mixture for the best idle and it still didn't want to catch itself.

Think of this carb like this: USA makes a killer VCR. Japanese take the design, make it plastic, cheaper, and sell it back to us as a disposable. Seems like that's what they did with the Quadrajet and spit out this carb.

Time to stop dicking around and shell the $ for EFI.

aaron_sK · Posted: Fri Dec 02, 2011 8:51 am Post subject:

SR20DET

Twilightoptics · Posted: Fri Dec 02, 2011 9:08 am Post subject:

Well here is the trouble with that: It's $500 more intial cost than a 13BT. Then the SR I have to ghetto rig a speedo from a 240, hope my clutch master is big enough, build an oil pan, hope I can route the exhaust as it exits on the opposite side of the motor, and might have to work my nicely painted hood over.

The 13bt All I have to do (aside from fuel system which both would need) is wire it in, and swap the engine crossmember I already have....

And It just so happens I have a non T motor sitting on the floor with the early EFI. I'm going to try and get it running here soon and see if everything is in good shape. If it is, that all gets swapped over to start.

blue89 · Posted: Fri Dec 02, 2011 10:29 pm Post subject:

Don't those nissan carbs come with a speghetti mess of vacuum hoses?

Twilightoptics · Posted: Sat Dec 03, 2011 8:49 am Post subject:

Yeah but they control emissions crap and are gone.