RB30 Bad Cold Idle

[CaptainNewman]

Hi all,

I've got a 1990 RB30 GQ that's in great nick, but having a problem idling cold. Here's what's happening:

Cold start - starts perfect first time, but once its going need to pull hand throttle out a fair way so she idles. Idles rough at 1,000rpm, nicely at 1,300rpm.
... after a few minutes at 1,300, the revs start to drop (as the choke opens up I assume), so a bit more hand throttle is required for about 10 seconds to keep engine from stalling.
... after this, the hand throttle can progressively go in as you drive along, and the car idles nicer and nicer until the engine is warmed up and the hand throttle is back to normal (in).

So basically she idles absolutely perfectly (850rpm) when HOT (so 10 mins of running), but needs hand throttle when cold.

Any ideas what the problem could be? When the car is a about 5 minutes warm, sitting on the brakes too long (down a hill) makes the engine stall, as does switching the A/C on. When its hot it's fine no probs at all with either of these.

Have recently sprayed all the linkages and springs etc. around the carby with WD40.

The choke valve is fully closed when cold, and open full when hot (as it should be... ?)

Cheers for any help you can give,

Chris :)

[mudnut]

Hello, and welcome to the forum. It sounds as if you may have a vacuum leak. Check all of your hoses, and failing that spray wd40 around the inlet manifold gasket with the engine idling. If there is a leak, the the revs will pick up slightly as the spray is sucked in and burnt. have a read through this thread.

http://www.nissanpatrol.com.au/forums/showthread.php?28517-RB30-Facts-Figures-and-Helpful-Hints


Some other easy things to check are, spark plugs (gap and condition) and leads, condition of the distributor cap (Cracks and worn centre brush etc.), fuel filter, emission control gear (egr and stove pipe), and also timing.

[TroutNut]

Check that the choke opens a few mm as the engine starts, and have a look at the carbi float level too, assuming it's got one.

[dom14]

It's not clear whether it's on LPG, petrol or dual fuel.
Is the idling issue appearing on petrol or LPG or both?
If it's on LPG and the tweaking of other parts(electrical connections, distributor,spark plugs, vacuum leaks, LPG system electrics and plumbing, etc)
doesn't fix the problem, then it must be the carby and it may be time for a rebuild.
LPG tend to wear out the Nikki carbies lot quicker than petrol.
I have(had) the same problem with these carbies.
Now, I keep a second one as a back up. The back up one is still not perfect. But, still cheaper than spending around $400 on a brand new carby.
You can have it rebuilt professionally. Only downside is that, it still costs around $250 for that.
If you can get the carby rebuilt for less than that, I would go for it(provided all the other possible issues are eliminated).
One thing is certain. These Nikki carbies on LPG need a professional rebuid(or a new one) at some stage.
Ones that have been on petrol may be able to get away with by putting a rebuild kit through it yourself.

If I understood the symptoms properly, it sounds like more than one problem to me. There must be an electrical/vacuum issue, if the brake pedal can stall the engine idling. The brake booster vacuum lines are connected with the vacuum lines of the fuel system(if I'm right). That may be an indication of a vacuum leak, either in brake system/brake vacuum lines or fuel system vacuum lines.

Vacuum leaks are notoriousl for manifesting in all kind of weird ways. So, it's worth a good check.

[dom14]

If you check my recent post,

http://www.nissanpatrol.com.au/forums/showthread.php?32639-Humming-noise-from-the-LPG-converter

I was blaming all kind of other parts, before the LPG converter completely died and I had it replaced.
So, it can be pretty tricky when you have to diagnose a problem by yourself, without having access to professional expertise or tools. :)

Sometime, it can drive you nuts, when there's more than one issue's causing all the symptoms.:1087:

[CaptainNewman]

Cheers everyone for the help. Mudnut I found a few split vacuum hoses under the air cleaner box, replacing these has helped the problem a lot. Also mate your RB30 "Helpful Hints" page has been absolutely useful, I actually discovered it months and months ago, and refer to it all the time!

TroutNut I checked the choke valve, it sometimes gets "stuck" open, so on a cold morning just need to blip the throttle before starting to reset the valve to the closed position. Sticky linkages ? !

Dom14 its only on Petrol, and I'm going to look over the vacuum lines for brakes soon too.

Cheers everyone again. :)

[mudnut]

I checked the choke valve, it sometimes gets "stuck" open, so on a cold morning just need to blip the throttle before starting to reset the valve to the closed position. Sticky linkages ?

That is how my auto choke worked. A quick dab on the go pedal for the butterfly to close and give the engine a squirt of juice before starting.

[CaptainNewman]

Its good to know its not just my choke then ! Cheers :)
Yesterday gave the whole carb a good spray with Carb & Throttle Body Cleaner, down the throat, choke valve, inside & out, with the engine revving. Some black smoke from exhaust could just be the carb spray burning off or... the carbon deposits being cleaned out of the carb! (hope its the latter ! ).

[mudnut]

After all the easy stuff has been checked, I recommend putting a kit through the carby. Since having that done the old dirty 30 has better economy and gained a few ponies.

[dom14]

The electrical(bi-metal) part of the Nikki carby choke rarely fails IMO. Even if there's any wear and tear on the bi-metal tension spring, it can be easily adjusted by turning clockwise or anticlockwise, after loosening the three little bolts holding the choke unit. Moving parts, shafts can get stuck if there's too much grime builds up over time, but usually not caused by LPG grime, but from the lack of "liquid effect" of the LPG, IMO.
I'm about to redo my backup Nikky carby. It idles too high, and looks like something gets jammed related to the idle circuit, throttle valve, etc.

[GQ TANK]

Quite often idle issues on the rb30 or tb42s carby point to a need for a carby rebuild. (both carbys a nearly the same and use the same rebuild kit.)

I have experienced issues with the rb30 and tb42 of poor idle / lost of power being caused by the middle part of the carby coming loose. Simple way to check, remove the air cleaner and lpg mixer (if fitted), Grab the top of the carby and give it a shake - there should be no movement. If there is movement, pull the carby flip it and tighten the 3 brass screws.

[dom14]

Quite often idle issues on the rb30 or tb42s carby point to a need for a carby rebuild. (both carbys a nearly the same and use the same rebuild kit.)

I have experienced issues with the rb30 and tb42 of poor idle / lost of power being caused by the middle part of the carby coming loose. Simple way to check, remove the air cleaner and lpg mixer (if fitted), Grab the top of the carby and give it a shake - there should be no movement. If there is movement, pull the carby flip it and tighten the 3 brass screws.

Yep. After a while(especially after installing a rebuild kit), it's a good idea to pull the carby out & re-tighten the three bolts.

The float needle of the backup carby's looked pretty strange when I pulled it apart. The needle is actually a ball end. It works fine, but I haven't seen a ball end float valve before.
Google confirmed they do exist and not that uncommon. Uncommon for a Nikki carby for sure.

I went to the trouble of making my own gaskets for the float chamber as well. Quite an experience with craft work. :)
I didn't want to spend $80 on a rebuild kit, just to get the gaskets( No body sells the gaskets).

With carby's, each time you pull apart the same one, you learn something new and valuable.
It's too bad they are out of date now.

I still think a backup carby is more reliable than not being able to fix parts in an EFI system, when I'm stranded in the middle of nowhere.
That's the reason I haven't been that proactive with the EFI conversion of my RB30.
But, many pro mechanics disagree with me. Carby's are way too much trouble they say.
I think, if I learn to fix them, it's not that bad at all. These skills come very handy in the bush.