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IDF idle/mixture problem

Posted: Sun Nov 11, 2012 4:12 am
by JonR400
I've got an issue with a Spanish 44IDF which I've got installed on 3 & 4 cylinders.

The carb runs OK with the linkage disconnected and the idle speed screw all the way out. Also, when I try and set the idle mixture, I can seat the screws all the way in without the engine stumbling. The carb on 1 & 2 seems to work fine, with the idle speed and mixture screws having 'normal' effects, the engine stalling when they are turned too far.

To try and sort this, I've removed the carb, taken out the mixture screws which looked fine, checked the butterflys were closing, removed the idle jets and blown air through the mixture screw holes, the idle jet holes. and the holes in the top of the carb. I checked there was air coming out of the remaining hole when I blocked 2. Idle jets are clear. I've also changed the idle jets around between carbs.

Carefully refitted the thing, and there seems to be no change in behaviour. Any suggestions before I ditch the things and fit some 40DRLAs I've got handy?

If it makes any difference, it's a 1914 with 40x35 CB round port heads, jetting is 175 air, 50 idles, 150 mains. I know the jetting is probably out, but I'm looking to get the idle sorted first.

Re: IDF idle/mixture problem

Posted: Sun Nov 11, 2012 4:23 am
by Piledriver
fuel pressure
float height set
all idle jets same actual size, not just same numbers stamped on.

Re: IDF idle/mixture problem

Posted: Sun Nov 11, 2012 4:27 am
by JonR400

Sorry, should have covered that. I've set the floats to 11mm (they were way out). I've also swapped the idle jets between carbs so they don't seem to be the issue. I'm using a CB rotary pump, so I think fuel pressure should be OK too..

Re: IDF idle/mixture problem

Posted: Wed Dec 05, 2012 7:22 am
by ainokea
You don't have mixture problem, you have a jetting problem. 1.50 mains is ok. 1.75 air is to small, should be around 1.95 or 2.00, 1.75 will cause it to richen up. .50 idle jets to small should be .55 or even .60. Are your air bleed screws in all the way and the jam nuts snugged down. Heres some basic stuff, most of your driving will be done on the idle jets from idle to 3000 rpm. From 3000 to 4000 rpm, it's the main jets. 4000 rpm on up it's the air correction jets. basic bench adjustments, idle speed screw 1/2 turn in after contact with throttle arm. Idle mixture screws 1 1/2 turns out. start engine, run to op temp then synchronize both carburetors by bringing the high flowing carburetor even with low flowing carburetor.Now adjust for best idle mixture. More than 1 1/2 turn out, it's to lean. Less than 1 turn out, it's to rich. Hook up a tach and adjust idle speed keeping them balanced. After all that, connect the linkage and check the balance again.I hope I have all this stuff right, it's been awhile since I did it.


Re: IDF idle/mixture problem

Posted: Wed Dec 05, 2012 12:29 pm
by Marc
Are the idle bypass screws set the same on both carburetors?
(#37 in this diagram: )
Generally these are left all the way in to shut off the passage which bypasses air around the throttle plate.

Re: IDF idle/mixture problem

Posted: Wed Dec 05, 2012 2:17 pm
by JonR400
Bypass screws are fully closed.

Re: IDF idle/mixture problem

Posted: Thu Dec 06, 2012 9:54 pm
by Dale M.
Sounds like left side carb has slight bind in it near full closure.... You should be able to choke off engine with throttles full closed ( Idle speed screw backed all the way out) and it does not seem to do this... Also you may have vacuum leak on 3/4 side...