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  #16  
Old 12-16-2009, 06:13 AM
t walgamuth's Avatar
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That is good advice.

Also check your vacuum with an old fashoned vac guage. A small leak will make it run lean and buck on takeoff.

And be sure to check for accelerator pump function by looking down the carb throat and observing the little squire of fuel when you open the throttle.

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..I also have a 427 Cobra replica with an aluminum chassis.
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  #17  
Old 12-16-2009, 11:23 AM
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Great discussion. Thank you all. I am running through the list of possible causes.

Last night I pulled the valve with screen (parts 77-79 on above diagram) and cleaned them with carb cleaner and compressed air. Still not working right...

The car seems to run properly with the choke pulled out. It also runs properly taking off up to about 2500 rpm, then I have to pull the choke out. With the choke out (fully engaged) it drives normally. The engine sounds strong and pulls the car without hesitation.

Next I took the tops off the carbs. The fuel cut-off valves seem to be working correctly and the float is positioned to allow bowls to fill before cutting off.

The carbs have already been tuned and synchonized. I can see both carbs squirting fuel, till they stop squirting fuel. It acts like it is running out of gas, like I have a flow problem.
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  #18  
Old 12-16-2009, 01:05 PM
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Having fixed my share of carburettor problems on my MG V8 (SU HIF6) recently, here are my suggestions:

1 - Check the float needle (part 156). I have had those sticking on multiple occasions, especially if the car has been sitting for a while. Clean them out with royal doses of carb cleaner. Air won't do it. Carb cleaner (and fuel) need to squirt out massively. I had to clean a needle twice the other day before my SU carb worked again (and the needle was NOS!).

2 - Check whether your float (and its shaft) are sticky. Normally, fuel pressure from pump will blow it open, but if float can "float" over its shaft, it may become sticky. Very rare, but it can happen.

3 - Make sure your float level is adjusted correctly as Tom said. In general, the workshop manuals seem to make a bigger deal about the precision than is necessary (in my opinion). The bottomline is that you want to make sure it closes, but at the same time it should allow enough fuel in the bowl. I have had to adjust the floats of one of my previous MGs also because it seemed to starve a bit.

4 - Vacuum/"false air" leak would be my next chase, although if you have new gaskets, etc, I can only think about having forgotten to close off some orifice in the carb or so as you problem. Check what the starvation symptoms are under different load conditions. It may give you a clue as to whether it consistently runs out of gas after a certain time (float filling problem), or whether a sudden increase in load causes the engine to starve. The latter is more typical of an air leak (or a clogged carb jet).

If possible, try to check air flow/tuning on each carburettor to see which one is the problem case. I had 3 weeks ago an interesting problem (when trying to get my MGB V8 going again after sitting for a while) that adjusting one of my SUs never changed the engine speed (!). Whatever I did to the set screw, nothing changed. Turned out the needle seat was stuck, followed by sticky float needle, followed by yet another (NOS) float needle that was sticky, followed by succes which is a incredible smooth humming 3.5 liter V8 with no blue fumes or barking (starving) exhaust anymore. But it did take 3 disassemblies of the carburettor. If both carbs act the same way, poorly working fuel pump would be my next bet. It may reach pressure for a while, but may not be able to sustain it and necessary flow rates.

And change the fuel filter in case it is old. And I have seen one case where the lining in a rubber fuel hose started separating and clogging (like in MB brake hoses), so if they are old, just put some new ones on (for safety too).

Good luck,

Bert
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  #19  
Old 12-16-2009, 02:16 PM
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Bert,

Thank you very much! I will probably read your post and make a check list before I attack it again.

This carburetor is a two-barrel. The second stage seems to be where the "starvation" occurs. The thing that rules out float needle/level issues is that it runs perfectly with the choke pulled out. Without choke it drives strong to 2500rpm, then acts like it is starving. If I have the engine running at 4000rpm then push the choke in, it will maintain the 4000rpm but not increase rpm.
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  #20  
Old 12-16-2009, 04:26 PM
t walgamuth's Avatar
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It sounds like the secondary barrell is not getting its fuel.
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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.
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  #21  
Old 12-16-2009, 09:58 PM
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Quote:
Originally Posted by t walgamuth View Post
It sounds like the secondary barrell is not getting its fuel.
I agree. Or it has a massive air leak that the choke can only compensate for so much (4000 rpm).

Bert
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  #22  
Old 02-03-2010, 02:26 PM
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Update:

The car is running fantastic now. I am not entirely sure why the clean carburetors where not functioning correctly before, but the clues you all offered in this thread certainly helped me arrive at my satisfactory conclusion.

I had an extra set of carburetors for the car sitting in a box in unknown condition. I completely dismantled the carburetors and soaked them in cleaner then made sure every port was clear of obstruction and scale removed. Going the extra mile, I boiled the carbs with water and a touch of vinegar to really get them clean. I bought a "Royze" brand carburetor kit, and installed all the new bits with gaskets. The choke function does indeed have two 180 degree possibilities and I was careful to follow the flow pattern and installed it correctly.

I adjusted the valves to specification. The timing had been set at zero degrees, which was way off. I set it to five degrees instead of the eight degrees specified because I want to run regular unleaded instead of premium unleaded gas.

The vacuum tube that goes from the carburetor to the distributor was plugged up and stopped the system from advancing or retarding by vacuum. Now it is working correctly after a cleaning.

The car is running great with lots of torque. I can easily keep up with traffic now.

The mystery had several villainous components that robbed the car from optimal performance. Thank you all for sharing your wisdom and experience!
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1962 220Sb ~ The Emerald Bullet http://www.youtube.com/watch?v=hx6tN1W48_o
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The Universe is Abundant ~ Life is GOOD!http://www.classiccarclock.com
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  #23  
Old 02-03-2010, 07:19 PM
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Quote:
Originally Posted by JeffreyNMemphis View Post
Thank you very much! I have a few possible things to check now. I will report back when I figure out what fixes it.

Does your model have one or two carbs? Did you synchronize them, if you had two?
That was a PITA in my old 230S, believe me.
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  #24  
Old 02-03-2010, 08:26 PM
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Yes, my carburetors are synchronized. That was easy with the correct tool.
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  #25  
Old 07-10-2010, 11:00 AM
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Very helpfull thread. I will be rebuilding two Zeniths very soon. Looks like Royze kits are the way to go. Leary about the Walker kits.
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  #26  
Old 07-10-2010, 12:08 PM
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Ritchie,

Which car has the two Zeniths? I had two solex carbs.
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1962 220Sb ~ The Emerald Bullet http://www.youtube.com/watch?v=hx6tN1W48_o
1957 Ponton 220S

2001 S600 Daily Driver
The Universe is Abundant ~ Life is GOOD!http://www.classiccarclock.com
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  #27  
Old 07-11-2010, 09:20 AM
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Sorry for the confusion Jeff. i was searching on "royze". found this thread. my zeniths have the same symptoms.
was searching to see if anyone had issues with Royze kits. could not find anything but good comments. I have use Walker kits for a Holley and I did not care much for them.
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Last edited by Ritchie; 07-11-2010 at 09:20 AM. Reason: typing o
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  #28  
Old 07-11-2010, 07:51 PM
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If we'd all pitch in ten cents every time there is a screwed up carburetor thread on here in a year we could hire a guy to fly around the country and convert them all to fuel injection.
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  #29  
Old 07-11-2010, 09:30 PM
Brian Ostosh
 
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Don't despair, carb owner's
when perfect they are......
still, just simple mechanical analog devices

I have a fellow hot rod bud from the mid 1960's who dreams, knows, totally understands fuel flow through carbs. Send one in and we can send it out perfect. We just did the Holly 3310 on the hot rod. Any carb on any car, distributor gets the treatment too.

This Man knows Motors

But I love my Bosch fuel injection M127 and M130 and M117 and M100's

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