The Ford Barn

Ken Anderson · 09-02-2016, 04:09 PM

I am trying to help a friend with his 30 Model A. How much of a vacuum leak does it take to have a significant third gear power loss? Car cranks right up and does OK if 1st and 2nd but as soon as you go into third it looses power. New distributor, timing set by test light, good carb - sudden onset problem.

DJ S · 09-02-2016, 04:19 PM

Spray carburetor cleaner where the manifold meets the block. If the idle increases, you have a vacuum leak. Try tightening the nuts (not too tight). If this doesn't solve the issue, remove the manifold and check for straightness.

Mitch//pa · 09-02-2016, 04:26 PM

If it is hitting on all 4 at idle and running good until 3rd gear I would look else where..
Could be timing, fuel starvation, weak spark, restricted exhaust etc...

Safewiz · 09-02-2016, 04:31 PM

x2 Mitch sounds correct. Doesn't sound like Manifold problem.

Step-down · 09-02-2016, 04:43 PM

Check color of the plugs .

Jacksonlll · 09-02-2016, 09:12 PM

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Make sure your points are not floating due to a broken spring. Just feel the spring load with your finger. Should be pretty tight.

ddweave · 09-02-2016, 10:08 PM

It sounds like it's running lean. Have you tried opening the GAV a bit - or pulling the choke?

MikeK · 09-02-2016, 10:40 PM

Fuel starvation. Restrictive crud somewhere or perhaps fuel line pushed too far in at carb connection? If it is a stock Zenith carb try removing the single bolt that holds the bottom half. With the bottom half removed place a pan underneath it and open the gas valve. If you do not get at least 1 pint per minute you have a starvation problem.

Of course you do this test with the engine/exhaust cold and outside with an assistant and a water hose handy. Expect to slobber some fuel.

Ken Anderson · 09-03-2016, 07:58 AM

Update: Decided to remove the manifold. Took the front nut off by hand, the stud came out with the rear, middle two seemed tight, gasket was split top to bottom between #4 port and rear intake. Have ordered new studs and gasket from Mike's. Even if this doesn't fix the problem it's still a fix that needs to be made. My buddy is upset enough with the way the car has been running that he is threatening to sell her and I would hate to see that happen - everything else is good and I need another "A" to run around with!

Mitch//pa · 09-03-2016, 08:48 AM

If the intake manifold was sucking air past the gasket , then it was not running properly at idle either

thanks for the update

Brentwood Bob · 09-03-2016, 11:38 AM

Hope you put a straight edge on the assembled intake and exhaust to see how straight the assembly is flush to the block.
Bob

09-02-2016, 04:09 PM	#1
Ken Anderson Member Join Date: Aug 2013 Location: Panama City, Florida Posts: 54	Manifold Vacuum Leak Question I am trying to help a friend with his 30 Model A. How much of a vacuum leak does it take to have a significant third gear power loss? Car cranks right up and does OK if 1st and 2nd but as soon as you go into third it looses power. New distributor, timing set by test light, good carb - sudden onset problem.

09-02-2016, 04:19 PM	#2
DJ S Senior Member Join Date: Aug 2014 Location: Southwick, MA Posts: 590	Re: Manifold Vacuum Leak Question Spray carburetor cleaner where the manifold meets the block. If the idle increases, you have a vacuum leak. Try tightening the nuts (not too tight). If this doesn't solve the issue, remove the manifold and check for straightness. Last edited by DJ S; 09-02-2016 at 04:19 PM. Reason: Typo

09-02-2016, 04:26 PM	#3
Mitch//pa BANNED Join Date: May 2010 Location: Bucks County, PA Posts: 11,454	Re: Manifold Vacuum Leak Question If it is hitting on all 4 at idle and running good until 3rd gear I would look else where.. Could be timing, fuel starvation, weak spark, restricted exhaust etc...

09-02-2016, 04:43 PM	#5
Step-down Senior Member Join Date: May 2012 Location: Hat City (Danbury CT) Posts: 647	Re: Manifold Vacuum Leak Question Check color of the plugs . __________________ "I can't cry as long as I know the Lord has risen "

09-02-2016, 09:12 PM	#6
Jacksonlll Senior Member Join Date: Jul 2012 Location: Michigan-- Member of Oakleaf of MARC Posts: 1,686	Re: Manifold Vacuum Leak Question Sponsored Links (Register now to hide all advertisements) Make sure your points are not floating due to a broken spring. Just feel the spring load with your finger. Should be pretty tight.

09-02-2016, 04:31 PM	#4
Safewiz Senior Member Join Date: May 2016 Location: South Florida Posts: 147	Re: Manifold Vacuum Leak Question x2 Mitch sounds correct. Doesn't sound like Manifold problem. __________________ Coral Springs Florida

09-02-2016, 10:08 PM	#7
ddweave Member Join Date: May 2010 Location: Waterloo, IL Posts: 81	Re: Manifold Vacuum Leak Question It sounds like it's running lean. Have you tried opening the GAV a bit - or pulling the choke? __________________ Doug in Waterloo

09-02-2016, 10:40 PM	#8
MikeK Senior Member Join Date: May 2010 Location: Windy City Posts: 2,919	Re: Manifold Vacuum Leak Question Fuel starvation. Restrictive crud somewhere or perhaps fuel line pushed too far in at carb connection? If it is a stock Zenith carb try removing the single bolt that holds the bottom half. With the bottom half removed place a pan underneath it and open the gas valve. If you do not get at least 1 pint per minute you have a starvation problem. Of course you do this test with the engine/exhaust cold and outside with an assistant and a water hose handy. Expect to slobber some fuel.

09-03-2016, 07:58 AM	#9
Ken Anderson Member Join Date: Aug 2013 Location: Panama City, Florida Posts: 54	Re: Manifold Vacuum Leak Question Update: Decided to remove the manifold. Took the front nut off by hand, the stud came out with the rear, middle two seemed tight, gasket was split top to bottom between #4 port and rear intake. Have ordered new studs and gasket from Mike's. Even if this doesn't fix the problem it's still a fix that needs to be made. My buddy is upset enough with the way the car has been running that he is threatening to sell her and I would hate to see that happen - everything else is good and I need another "A" to run around with!

09-03-2016, 08:48 AM	#10
Mitch//pa BANNED Join Date: May 2010 Location: Bucks County, PA Posts: 11,454	Re: Manifold Vacuum Leak Question If the intake manifold was sucking air past the gasket , then it was not running properly at idle either thanks for the update Last edited by Mitch//pa; 09-03-2016 at 09:42 AM.

09-03-2016, 11:38 AM	#11
Brentwood Bob Senior Member Join Date: May 2010 Location: brentwood, ca Posts: 4,244	Re: Manifold Vacuum Leak Question Hope you put a straight edge on the assembled intake and exhaust to see how straight the assembly is flush to the block. Bob