WO carb butterfly question

1941 - 1945, MB, GPW Technical questions and discussions, regarding anything related to the WWII jeep.
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Chuckman84
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WO carb butterfly question

Post by Chuckman84 » Sat Jul 19, 2014 4:31 pm

ok, so Im on week 3 of trying to diagnose a no idle issue on my GPW. got the fuel regulated down to 2 PSI from 5+. Got good spark. Timing right. Plugs and wires fresh.
So we are looking more closely at the carb as to why it won't idle. I took it all apart and was studying things closely. On the lower butterfly, it has sharp 90 degree angled edges. On the newer ones (read 4bbls) that Ive messed with, they are machined with leading/lagging angle to match the bore more closely when shut. So when I reinstall the blade on the shaft loosely, and then tighten as its set in its bore and close, it does not come close to sealing off in the bore- theres tons of light around the perimeter. I can only imagine that this much light is the same as so much air and maybe isn't sucking air/fuel through the idle port as required. Is this a poor repop butterfly or is it supposed to not seal 100% tight?

the saga continues
83 CJ7 full-bore rock crawler
53 CJ3A farm/hunting rig
45 GPW being restored


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Re: WO carb butterfly question

Post by artificer » Sat Jul 19, 2014 4:38 pm

The throttle plate in this carburettor has never had leading or trailing edge closing surfaces.
Did you have the 2 throttle plate screws loosely fitted then fully closed the throttle before tightened them?
John GIBBINS Member Institute of Automotive Mechanical Engineers [Ret], ASE Master Medium/Heavy Truck & Auto Technician USA -2002 Licensed Motor Mech NSW MVIC 49593 Current 2015
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Re: WO carb butterfly question

Post by andys » Sat Jul 19, 2014 5:03 pm

Now that my resto is almost complete, I've been taking short drives and sorting out issues that crop up. I had a "no idle issue" happen a few weeks ago, so maybe you have a similar problem?

To answer your original question first, my WO carb throttle butterfly disk looks like it was stamped out of sheet metal with square edges. It will close off the carb throat fairly tightly, but it is prevented from closing by the idle adjustment screw.

The low speed circuit has a slot shaped opening that is near the edge of the throttle butterfly when it is in idle position (almost closed). The almost closed position is what creates the vacuum that causes the low speed circuit to work. When the throttle opens enough, vacuum at the low speed circuit drops to zero and the low speed circuit stops working.

The high speed circuit is independent, so if there is a problem in the low speed circuit, the jeep will still run if you rev it. This is what happened to me...

So... Assuming that you are sure you have good spark, and your timing is correct, and you checked the vacuum at idle and it indicated no significant leaks (this btw was my set of symptoms), I would look to a blocked low speed circuit.

Disassemble the carb, take out the plugs in the low speed circuit and spray in carb cleaner then blow out the passages with compressed air. Reassemble. After this treatment, my jeep would idle again.

In my case, I painted the inside of my fuel strainer when I restored it. The gasoline dissolved the paint and sent it through the carb. The paint burnt fine until I parked the jeep for a few days... Then the dissolved paint settled to the bottom of the carburetor fuel bowl. The low speed circuit intake jet is located there, so it became plugged with paint. Due to its location at the bottom of the fuel bowl and the lack of true filtration in the OEM fuel system, I think the low speed jet is vulnerable to plugging up. You can still start the jeep due to the choke, and still run it by revving it.

Of course I haven't been following you whole case so I could be off base, but maybe this could help. Sorry to write so long... I think I got carried away :D

Regards, Andy

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Re: WO carb butterfly question

Post by artificer » Sat Jul 19, 2014 5:15 pm

Andy wrote:it is prevented from closing by the idle adjustment screw.
There is an idle adjustment screw that influences the closed throttle position by altering the linkage position/s....there is also an idle mixture adjustment screw that influences what it indicates. This screw should not influence the position of the throttle plate in any way.
When adjusted properly this mixture screw is turned in lightly till bottomed then out 1.5 turns [on the Carter].
This will be nearly perfect with minimal fine adjustment required.
Andy's description is good but I think he may have missed a small bit....
as the throttle butterfly is opening vacuum is also applied to another small port above the plate drawing in a little more fuel from a port called the progression port.
John GIBBINS Member Institute of Automotive Mechanical Engineers [Ret], ASE Master Medium/Heavy Truck & Auto Technician USA -2002 Licensed Motor Mech NSW MVIC 49593 Current 2015
TO DIAGNOSE, TROUBLESHOOT OR FAULT FIND ANY AUTO SYSTEM....
Understand how system parts interact with one another. GOOD parts can then be established & the NOT GOOD problem/s part/s isolated for repair or replacement.

Chuckman84
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Re: WO carb butterfly question

Post by Chuckman84 » Sat Jul 19, 2014 5:37 pm

thx for the input guys, at least thats one more issue (shape of the butterfly) that I can cross off. Took carb down today and cleaned the whole thing, nothing noticed wrong, and no change after the cleaning/inspection.
83 CJ7 full-bore rock crawler
53 CJ3A farm/hunting rig
45 GPW being restored

gareth dyas
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Re: WO carb butterfly question

Post by gareth dyas » Sun Jul 20, 2014 6:53 am

just to chip in, had same problem on mine, pcv valve kaput, blocked it off, good to go!!

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Re: WO carb butterfly question

Post by artificer » Sun Jul 20, 2014 1:58 pm

Best to get another, they are not expensive or alternatively clean the one you have unless bits are missing.
John GIBBINS Member Institute of Automotive Mechanical Engineers [Ret], ASE Master Medium/Heavy Truck & Auto Technician USA -2002 Licensed Motor Mech NSW MVIC 49593 Current 2015
TO DIAGNOSE, TROUBLESHOOT OR FAULT FIND ANY AUTO SYSTEM....
Understand how system parts interact with one another. GOOD parts can then be established & the NOT GOOD problem/s part/s isolated for repair or replacement.

Chuckman84
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Re: WO carb butterfly question

Post by Chuckman84 » Sun Jul 20, 2014 4:27 pm

John,
do I read your last post as to replaced/clean the PCV valve or the carb (or parts of carb)? It so wants to run. It will immediately fire, cough twice and then nothing. When I do rarely get it to jump and buck for a few seconds, it won't take gas. Everything so far in my inspection is good to go- except for the fuel pressure- which I have under control now.
83 CJ7 full-bore rock crawler
53 CJ3A farm/hunting rig
45 GPW being restored

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Re: WO carb butterfly question

Post by artificer » Sun Jul 20, 2014 5:18 pm

Faulty PCV valve operation can cause idle issues.
Blocking temporarily, as Gareth says he did, may eliminate an associated issue.
All I'm saying is the PCV valve should be in place & working properly.
John GIBBINS Member Institute of Automotive Mechanical Engineers [Ret], ASE Master Medium/Heavy Truck & Auto Technician USA -2002 Licensed Motor Mech NSW MVIC 49593 Current 2015
TO DIAGNOSE, TROUBLESHOOT OR FAULT FIND ANY AUTO SYSTEM....
Understand how system parts interact with one another. GOOD parts can then be established & the NOT GOOD problem/s part/s isolated for repair or replacement.


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