Go Back   The Ford Barn > General Discussion > Model A (1928-31)

Sponsored Links (Register now to hide all advertisements)

Reply
 
Thread Tools Display Modes
Old 11-01-2019, 01:23 PM   #1
TinCup
Senior Member
 
TinCup's Avatar
 
Join Date: May 2010
Location: OKC / Tonkawa, Ok.
Posts: 1,964
Default Stubborn Zenith

I have a very nice Zenith 1 that runs great at speed but tends to struggle to idle. I have gone through carb completely as I have done many others but have never found one that I could not adjust the idle air mixture. The passage is clear with the correct length air mixture needle driver. Turning it makes no difference in the idle of the carb whatsoever. No leaks around the manifold or gaskets to allow extra air into the circuit. It used to require more GAV adjustment than most but the correct #20 jet fixed that. Float height is spot on. My last thing to try is a different secondary well. There were 3 different wells and I have a version of all 3 so that is next on the list.
__________________
Sooner Model A Club of Oklahoma
TinCup is offline   Reply With Quote
Old 11-01-2019, 02:24 PM   #2
Brentwood Bob
Senior Member
 
Join Date: May 2010
Location: brentwood, ca
Posts: 2,866
Default Re: Stubborn Zenith

How about air leak from throttle shaft?
Brentwood Bob is offline   Reply With Quote
Sponsored Links (Register now to hide all advertisements)
Old 11-01-2019, 02:34 PM   #3
alexiskai
Senior Member
 
alexiskai's Avatar
 
Join Date: Apr 2018
Location: Mebane NC
Posts: 365
Default Re: Stubborn Zenith

Quote:
Originally Posted by Brentwood Bob View Post
How about air leak from throttle shaft?
How would one diagnose that?
alexiskai is offline   Reply With Quote
Old 11-01-2019, 02:47 PM   #4
TinCup
Senior Member
 
TinCup's Avatar
 
Join Date: May 2010
Location: OKC / Tonkawa, Ok.
Posts: 1,964
Default Re: Stubborn Zenith

Added a bushing there and it's very snug. I sprayed starting fluid around it and it didn't surge on me.
__________________
Sooner Model A Club of Oklahoma
TinCup is offline   Reply With Quote
Old 11-02-2019, 01:33 AM   #5
redmodelt
Senior Member
 
redmodelt's Avatar
 
Join Date: May 2010
Location: Portland OR
Posts: 4,658
Default Re: Stubborn Zenith

Put some grease around the shaft, if things get better the shaft and /or bore is worn.
__________________
I know the voices aren't real but damn they have some good ideas!
redmodelt is offline   Reply With Quote
Old 11-02-2019, 02:49 PM   #6
Gary WA
Senior Member
 
Gary WA's Avatar
 
Join Date: May 2010
Location: Clinton,WA/Whidbey Island
Posts: 2,959
Default Re: Stubborn Zenith

no gasket on idle jet. check if on there remove.
__________________
www.whidbeymodelaclub.com
Gary WA is offline   Reply With Quote
Old 11-02-2019, 03:26 PM   #7
rocket1
Senior Member
 
Join Date: Mar 2014
Location: Midland Park,N.J.
Posts: 839
Default Re: Stubborn Zenith

I would start with the basics,check the cylinder compression and tappet clearance,I have had the same problem,one time worn lifters with huge valve clearance and another time low compression in one cylinder.Engine ran fine but no idle,died when I would stop.
rocket1 is offline   Reply With Quote
Old 11-02-2019, 03:36 PM   #8
johnbuckley
Senior Member
 
johnbuckley's Avatar
 
Join Date: Jul 2010
Location: United Kingdom
Posts: 1,118
Default Re: Stubborn Zenith

tried substituting a different idle jet?
johnbuckley is offline   Reply With Quote
Old 11-02-2019, 09:22 PM   #9
GRutter
Member
 
GRutter's Avatar
 
Join Date: Sep 2017
Location: Mint Hill, North Carolina
Posts: 84
Default Re: Stubborn Zenith

It may be that your engine is idling too fast for the air mixture adjustment to have any effect. If you can't reduce the idle speed, check the comp jet and GAV adjustments. Until you can reduce idle speed and have the main throttle plate closed, you won't see much effect with the idle air adjustment.
GRutter is offline   Reply With Quote
Old 11-03-2019, 10:43 AM   #10
katy
Senior Member
 
katy's Avatar
 
Join Date: Jun 2015
Location: Alberta, Canada
Posts: 2,122
Default Re: Stubborn Zenith

What camshaft are you running? Stock? Or? If a stock camshaft, what shape is it in? A badly worn camshaft can cause idle problems.
__________________
Don't go away mad, just go away.
katy is offline   Reply With Quote
Old 11-03-2019, 11:06 AM   #11
30 Closed Cab PU
Senior Member
 
Join Date: Aug 2018
Location: Wisconsin
Posts: 1,606
Default Re: Stubborn Zenith

Had an idle issue, blamed the carb/ignition, etc. Spent weeks on the issue. Finally one day I ran a metal dryer vent hose attached/sealed to the carb intake, hung the hose from the radiator support rod to up by the radiator, Issue disappeared. Found a small exhaust leak at the exhaust manifold/muffler clamp. Was getting dirty air.
30 Closed Cab PU is offline   Reply With Quote
Old 11-03-2019, 02:03 PM   #12
alexiskai
Senior Member
 
alexiskai's Avatar
 
Join Date: Apr 2018
Location: Mebane NC
Posts: 365
Default Re: Stubborn Zenith

I'm also dealing with this right now. Yesterday ran a vacuum leak test, squirted starting fluid pretty much everywhere with no effect.

Took the carb apart, used compressed air on each jet and port. Everything looked fine. Float height normal.

Took the manifold off, looks like there's warping at either the first exhaust or the first intake, hard to tell. Also major pitting on the internal walls. Soot all over the exhaust manifold (although that might be normal). I've ordered a nice used combo manifold – gonna put it on with copper gaskets and see if that fixes things. Also planning to seal the joint at the muffler clamp. I also cleaned and re-gapped the plugs.

Such a frustrating problem to troubleshoot, could be just about anything causing it.
alexiskai is offline   Reply With Quote
Old 11-03-2019, 02:16 PM   #13
redmodelt
Senior Member
 
redmodelt's Avatar
 
Join Date: May 2010
Location: Portland OR
Posts: 4,658
Default Re: Stubborn Zenith

The copper gasket/gland rings are not a fix for warped leaking manifolds. All the ports need to be flat and level with each other. Check the used one your are getting before you install. And no there should not be soot around the ports.
__________________
I know the voices aren't real but damn they have some good ideas!
redmodelt is offline   Reply With Quote
Old 11-03-2019, 03:13 PM   #14
Jacksonlll
Senior Member
 
Jacksonlll's Avatar
 
Join Date: Jul 2012
Location: Michigan-- Member of Oakleaf of MARC
Posts: 1,272
Send a message via ICQ to Jacksonlll
Default Re: Stubborn Zenith

Next time you have the carb off, hit the mounting flange a big file and make sure it is flat and not warped a bit. Can’t have any low spots.
Jacksonlll is offline   Reply With Quote
Old 11-03-2019, 08:01 PM   #15
alexiskai
Senior Member
 
alexiskai's Avatar
 
Join Date: Apr 2018
Location: Mebane NC
Posts: 365
Default Re: Stubborn Zenith

Quote:
Originally Posted by redmodelt View Post
The copper gasket/gland rings are not a fix for warped leaking manifolds. All the ports need to be flat and level with each other. Check the used one your are getting before you install. And no there should not be soot around the ports.
I'm buying it from Bert's, so... probably it's OK... but I'll definitely check and have it surfaced if needed. I would've had the existing one surfaced, but the rust was so bad I decided to just replace.
alexiskai is offline   Reply With Quote
Old 11-04-2019, 10:29 AM   #16
TinCup
Senior Member
 
TinCup's Avatar
 
Join Date: May 2010
Location: OKC / Tonkawa, Ok.
Posts: 1,964
Default Re: Stubborn Zenith

I spent Sunday afternoon going through things again. Changed the idle jet, Changed the secondary well, and put a bushing on the throttle shaft. I also hooked up a vacuum gauge so I could watch the effect of adjusting the air/idle needle. I used starting fluid to spray around the intake and the carb itself to see if I have an air leak I missed and all is good on that end. I am starting to look more to the condition of the intake valves. I know when I was going through them this spring they were a little off most measured around .014 and with original solid tappets I left them alone. I think I have gotten as good as I am going to get with this carb. Another known good carb acts almost identical to this one which with the absence of a manifold leak would tend to point to valves. I really liked using the vacuum gauge when setting the idle jet. While I was taught to tune it by ear watching the impact on the vacuum gauge was really helpful.
__________________
Sooner Model A Club of Oklahoma
TinCup is offline   Reply With Quote
Old 11-04-2019, 12:36 PM   #17
Brian T
Senior Member
 
Brian T's Avatar
 
Join Date: May 2010
Location: San Diego,Ca
Posts: 1,330
Default Re: Stubborn Zenith

Hi Tin Cup,
Be sure that the idle speed is down below 6-700 RPM or lower , and the GAV is almost closed, then set the idle air screw at 1.1/2, - just a thought.
__________________
Nothing can be made foolproof, ---- fools are ingenious bastards.
Brian T is offline   Reply With Quote
Old 11-06-2019, 07:36 AM   #18
ronn
Senior Member
 
Join Date: Feb 2011
Posts: 3,637
Default Re: Stubborn Zenith

sell it- buy a rebuilt from Berts and be on your way!
ronn is offline   Reply With Quote
Old 11-06-2019, 10:27 AM   #19
katy
Senior Member
 
katy's Avatar
 
Join Date: Jun 2015
Location: Alberta, Canada
Posts: 2,122
Default Re: Stubborn Zenith

Quote:
Another known good carb acts almost identical to this one which with the absence of a manifold leak would tend to point to valves.
Have you done a compression test and a leak down test? A bad valve will cause a poor idle but will be masked at speed.
__________________
Don't go away mad, just go away.
katy is offline   Reply With Quote
Old 11-15-2019, 09:18 AM   #20
Ed in Maine
Senior Member
 
Join Date: May 2010
Posts: 610
Default Re: Stubborn Zenith

I would flow test the idle jet to be sure it is passing 44-48 ml/min. with a 36 in. water column test jig per Steve Pargeter's recommendations in his manual "Zenith Model A Carburetor Restoration Guidelines." I like an idle jet that passes 44-45 ml/min. because it allows the idle adjusting screw to be positioned near the seat. Good luck, Ed
Ed in Maine is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Sponsored Links (Register now to hide all advertisements)


All times are GMT -5. The time now is 08:52 AM.