The Ford Barn

tiger.1000 · 04-08-2014, 11:44 AM

Model B has a severe flat spot with the GAV closed even on a hot engine. If the GAV is open half a turn it runs fine, idles evenly and no flat spots.

Is it OK to drive with the GAV open?

I know we've been down this path before ?

Mart · 04-08-2014, 12:03 PM

Sorry, Tom, what's a GAV??

(Something valve I guess??)

Mart.

tiger.1000 · 04-08-2014, 12:14 PM

Tom? .....Who's Tom? ......GAV = Gas adjusting valve

Mart · 04-08-2014, 12:35 PM

Oops my mistake. Thanks for the info.

Mart.

1932BB · 04-08-2014, 02:28 PM

My BB wil not run well with the GAV off, exibiting lean run characteristics. Runs best 1/2 to 3/4 turns open. Seems to me that a GAV should be able to turn the mixture off if closed and working properly.

tiger.1000 · 04-08-2014, 03:16 PM

Sponsored Links (Register now to hide all advertisements)

BB....... I don't think the purpose of the GAV is eliminate the supply of petrol. I was under the impression that it was for cold start purposes only?

Does anyone know its' true function?

DavidG · 04-08-2014, 03:25 PM

David,

Its original purpose was indeed for cold starts only. That's confirmed in the owners' manual. The Bs that I've had/have run fine with it closed once the engine warms up.

tiger.1000 · 04-08-2014, 03:48 PM

David,

That's what I thought, so any clues why I need the GAV open when running? I hasten to add that it was running fine with the GAV fully closed and this fault has appeared
"overnight".....?

jimTN · 04-08-2014, 03:53 PM

Sounds like some debre has clogged a passageway

FlatheadTed · 04-08-2014, 05:20 PM

Tigger .I know on the A model you adjust the Mixture with that screw .

DavidG · 04-08-2014, 07:08 PM

David,

I agree with JimTN as to the likely cause, namely a foreign body restricting or obstructing the idle jet, B-9542, or the passages in the upper and lower castings leading to and from that jet. (A friendly reminder from one of the Service Bulletins was to not use anything metallic to clean a jet, but rather use "broom straw" so as to not distort the opening in the jet.)

FlatheadTed · 04-08-2014, 10:54 PM

I have had good results with compressed air .

1932BB · 05-18-2014, 11:14 AM

I remain quite confused about the function of the GAV on the Model B carb. It has often been stated that the valve should be closed once the engine is warm. Mine has never worked that way, it runs better with the GAV open slightly like the original post in this thread. I dismantled the carb to check the 9542 (idle jet) as suggested and found no problems. My question is, why should the "dash controlled main jet adjusting valve" be closed when running? Additionally why should the main jet adjusting valve affect the operation of the idle jet? I'm thinking the air adjustment screw affects the idle jet.

Help me out here! There must be something I'm missing!

1931 flamingo · 05-18-2014, 11:45 AM

As DavidG states probably a plugged jet.
Have you flow tested your jets??
Most jets available from the vendor's are not right. Renner's has GOOD flow tested jets for the A's and B's. Reasonably priced. FWIW
Paul in CT

Moose · 05-20-2014, 09:55 AM

Cleaning a B carb in good condition. Does anyone have a exploded diagram of the B carb? Also how do I add a picture with my post?

Bruce Lancaster · 05-20-2014, 10:42 AM

David is likely right with restriction in gas delvery, less likely possibility is a leak allowing unmetered air in.

tiger.1000 · 05-21-2014, 05:08 AM

Moose....hope this helps ?

04-08-2014, 11:44 AM	#1
tiger.1000 Senior Member Join Date: Apr 2011 Location: Hertford (UK) Posts: 905	Model B flat spot Model B has a severe flat spot with the GAV closed even on a hot engine. If the GAV is open half a turn it runs fine, idles evenly and no flat spots. Is it OK to drive with the GAV open? I know we've been down this path before ?

04-08-2014, 12:03 PM	#2
Mart Senior Member Join Date: May 2010 Location: Solihull, England. Posts: 8,755	Re: Model B flat spot Sorry, Tom, what's a GAV?? (Something valve I guess??) Mart.

04-08-2014, 12:14 PM	#3
tiger.1000 Senior Member Join Date: Apr 2011 Location: Hertford (UK) Posts: 905	Re: Model B flat spot Tom? .....Who's Tom? ......GAV = Gas adjusting valve

04-08-2014, 12:35 PM	#4
Mart Senior Member Join Date: May 2010 Location: Solihull, England. Posts: 8,755	Re: Model B flat spot Oops my mistake. Thanks for the info. Mart.

04-08-2014, 02:28 PM	#5
1932BB Senior Member Join Date: Jan 2012 Location: Marquette Mich. Posts: 125	Re: Model B flat spot My BB wil not run well with the GAV off, exibiting lean run characteristics. Runs best 1/2 to 3/4 turns open. Seems to me that a GAV should be able to turn the mixture off if closed and working properly.

04-08-2014, 03:16 PM	#6
tiger.1000 Senior Member Join Date: Apr 2011 Location: Hertford (UK) Posts: 905	Re: Model B flat spot Sponsored Links (Register now to hide all advertisements) BB....... I don't think the purpose of the GAV is eliminate the supply of petrol. I was under the impression that it was for cold start purposes only? Does anyone know its' true function?

04-08-2014, 03:25 PM	#7
DavidG Senior Member Join Date: May 2010 Location: southeastern Michigan Posts: 10,101	Re: Model B flat spot David, Its original purpose was indeed for cold starts only. That's confirmed in the owners' manual. The Bs that I've had/have run fine with it closed once the engine warms up.

04-08-2014, 03:48 PM	#8
tiger.1000 Senior Member Join Date: Apr 2011 Location: Hertford (UK) Posts: 905	Re: Model B flat spot David, That's what I thought, so any clues why I need the GAV open when running? I hasten to add that it was running fine with the GAV fully closed and this fault has appeared "overnight".....?

04-08-2014, 03:53 PM	#9
jimTN Senior Member Join Date: May 2010 Location: Powell, TN Posts: 2,509	Re: Model B flat spot Sounds like some debre has clogged a passageway

04-08-2014, 05:20 PM	#10
FlatheadTed Senior Member Join Date: May 2010 Location: Auckland Posts: 4,705	Re: Model B flat spot Tigger .I know on the A model you adjust the Mixture with that screw . __________________ http://www.flatheadted.com Flathead Ted brake Floaters ,

04-08-2014, 07:08 PM	#11
DavidG Senior Member Join Date: May 2010 Location: southeastern Michigan Posts: 10,101	Re: Model B flat spot David, I agree with JimTN as to the likely cause, namely a foreign body restricting or obstructing the idle jet, B-9542, or the passages in the upper and lower castings leading to and from that jet. (A friendly reminder from one of the Service Bulletins was to not use anything metallic to clean a jet, but rather use "broom straw" so as to not distort the opening in the jet.)

04-08-2014, 10:54 PM	#12
FlatheadTed Senior Member Join Date: May 2010 Location: Auckland Posts: 4,705	Re: Model B flat spot I have had good results with compressed air . __________________ http://www.flatheadted.com Flathead Ted brake Floaters ,

05-18-2014, 11:14 AM	#13
1932BB Senior Member Join Date: Jan 2012 Location: Marquette Mich. Posts: 125	Re: Model B flat spot I remain quite confused about the function of the GAV on the Model B carb. It has often been stated that the valve should be closed once the engine is warm. Mine has never worked that way, it runs better with the GAV open slightly like the original post in this thread. I dismantled the carb to check the 9542 (idle jet) as suggested and found no problems. My question is, why should the "dash controlled main jet adjusting valve" be closed when running? Additionally why should the main jet adjusting valve affect the operation of the idle jet? I'm thinking the air adjustment screw affects the idle jet. Help me out here! There must be something I'm missing!

05-18-2014, 11:45 AM	#14
1931 flamingo Senior Member Join Date: May 2010 Location: new britain,ct 06052 Posts: 9,390	Re: Model B flat spot As DavidG states probably a plugged jet. Have you flow tested your jets?? Most jets available from the vendor's are not right. Renner's has GOOD flow tested jets for the A's and B's. Reasonably priced. FWIW Paul in CT

05-20-2014, 09:55 AM	#15
Moose Junior Member Join Date: Nov 2010 Location: Williamsburg Virginia Posts: 6	Re: Model B flat spot Cleaning a B carb in good condition. Does anyone have a exploded diagram of the B carb? Also how do I add a picture with my post?

05-20-2014, 10:42 AM	#16
Bruce Lancaster Member Emeritus Join Date: May 2010 Location: Madison, NJ Posts: 5,230	Re: Model B flat spot David is likely right with restriction in gas delvery, less likely possibility is a leak allowing unmetered air in.