Impco straight gas too lean at idle.

[Ando81 2,949]

Hi all, I've recently had my freshly built crossflow put onto impco straight gas but it's too lean at idle. I put a 225 mixer onto an efi throttle body and manifold which makes pretty good power from 1500rpm onwards but it runs really rough at idle and the local mechanic said its running very lean only at idle. He said the mixtures were great during normal driving but couldn't improve it at idle. I tried putting a balance pipe between the atmosphere breather on the new impco model L converter and the clean air side of the mixer but as soon as I tried to accelerate the engine would stall. Will post some photos tomorrow. Any ideas would be greatly appreciated.

[deankxf 20,287]

is the idle air control(IAC) valve got anything to do with it.. i thought on gas they cut the wire so it stays closed or something..
 

[Ando81 2,949]

It has been removed altogether because I didn't have the efi wiring loom under the bonnet so hopefully that is not affecting idle.

[deankxf 20,287]

It has been removed altogether because I didn't have the efi wiring loom under the bonnet so hopefully that is not affecting idle.

is the port on the throttle body plugged then, would assume so as its a big hose..

the issue might be an air leak..

 

is the converter a 225 CA or 225FB (usually there is a sticker on the side) the FB ones run rich at idle and are trimmed with the impco commander computer thingy..   not sure where i am going with this, but if you have an FB(feed back) it should be rich, and i'd be looking for an air leak somewhere or possible damage to the mixer

[gerg 10,871]

What kind of pcv are you running?

[Ando81 2,949]

Not sure if it's a free air or feedback mixer, the port on the throttle body has been plugged but I could check it properly. As for the pcv, I wouldn't have a clue, looks like the original efi one going into the top of the rocker cover at the rear. If the dipstick seal isn't sealing could that affect it?

[Ando81 2,949]

Here's a pic of it, the only one I have on my phone. Note that the oil breather hose near the filler cap isn't really connected, it's just going over a piece of aluminium on the throttle body.

[PRO250 1,506]

I cant help with this buy my white car did the same thing i looked at it a few times but could see what the issue was I fixed mine with gasresearch a 5yo could tune that shit, must be one of the reasons I use it

you have fitted a whole new system? shame you did not say i still have all mine in the shed, you could of had it as ill never use it but if a job ever came in that needed any of it it would just be there to use

[Ando81 2,949]

Thanks for the offer Dave

[Banno 479]

Is it a xe throttle body or a xf? Cause they are different. As what I can see but hard from the photo you have pipe from throttle at top going directly to plenum chamber which would constantly draw air even with butterfly closed so would may cause lean out. Normally that for a XE throttle body it ran through a bi-metal valve that would open and close with engine temp for cold start and hot when the efi system is running not needed I think for straight gas. I am pretty sure I have seen a members car with them two ports blocked.

[Jiminy Kriket 133]

Is the rocker cover breather hose connected securely? The elbow appears to be off the throttle body in that photo you posted.

 

And the idle control hoses are connected together? Just blank those off with some electrical tape for a trial, i think that'll be where you are leaning out at idle.

 

The port on the back of the throttle body is before the throttle blade, then you have it going straight to the plenum chamber. That would be one hell of a vacuum leak.

[Ando81 2,949]

Thanks for the tip Banno, I'm not sure what throttle body it is as I bought the engine off a fella second hand. Block is an 83DA I think, not sure if that means much though.

[Banno 479]

Also the bottom port on the throttle body for the fresh into engine should be blocked off at the TB and the fresh air ran through either filter housing or port on the mixer

[Ando81 2,949]

Is the rocker cover breather hose connected securely? The elbow appears to be off the throttle body in that photo you posted.

 

Yeah, I realised that after I posted the photo, I secured it properly after I took the photo.

[Banno 479]

Dont run fresh air port to TB otherwise gas will be drawn into engine. Then it would get interesting

[Ando81 2,949]

Dont run fresh air port to TB otherwise gas will be drawn into engine. Then it would get interesting

 

I think the breather hose is going to the idle adjustment screw into the throttle body so hopefully it's ok. Thanks heaps for the help lads, I've got plenty to look at when I finish work this arvo

[Banno 479]

If not sure if you have a XE or XF. The XE has two small ports that two vacuum hoses on TB at rocker cover side connect to, a red and green. One is for vacuum advance and the other is for control of EGR. The XF TB dont have these. The XE  TB would be drawing air at Idle.

[Ando81 2,949]

Oh right, I saw those hoses and thought they were linked to heater but there was a bracket near the coil for it and it had vacuum lines on it so I guess you're spot on. Cheers

[gerg 10,871]

As what I can see but hard from the photo you have pipe from throttle at top going directly to plenum chamber which would constantly draw air even with butterfly closed so would may cause lean out.

 

Yeah I saw that but if you think about it, it draws air after the mixer which would see it as it an idle bypass, the diaphragm would open accordingly and supply more fuel. The pcv is somewhere else I'd look as I've chased carby problems on mine when it was a weak pcv spring. His might be jammed open, drawing too much air at idle.

[Ando81 2,949]

So I was time poor tonight and all I could do was try to kink off the hose from the throttle body that goes to the inlet manifold and no change, I lifted the pcv valve out of the rocker cover at idle and it had slightly slowed the idle up a bit. No bloody time to get to eliminate all options due to lack of time. Won't do any more now as I don't want to disrupt the neighbours.

[Banno 479]

Did you solve the problem with gas system?

[Ando81 2,949]

It has improved but isn't perfect. I had a bad vacuum leak where the egr pipe used to be and it idles happier now but I can't measure fuel mixtures at home.

[Banno 479]

Thats all good then problem solved. Not that this the problem you had but I reckon,too many bolts and mating surfaces with X-series efi intake runners. Makes you wonder they dont leak more? I am convince they do after 30 something years of being taking off and on when not torqued right.