Carburetor issues

[TR7Aaron]

Well, I pulled the tops of the carbs off this morning and the diaphrams looked like new. The damper oil was low so I added some and cleaned up the linkage and carb throats. When operated manually, the carb pistons have about equal resistance when pushed up and they drop down when released. I did find a small vacuum line that was hanging loose, so that added to my erratic idle problems.
The car fired up immediately and seemed to idle fine. I did notice that one of the manual choke cables binds up just a bit so that would also effect the idle if it doesn't retract smoothly into it's sheath. That will be for another day as it doesn't do it all the time.
As I operated the linkage manually and watched the carb pistons, they are still not moving evenly and in unison. The front carb piston is much more responsive to throttle changes and opens much more than the back carb; that seems a bit sluggish and doesn't open nearly as much.
I assuming (maybe mistakenly) that these carb pistons are vacuum actuated. What could be causing the back carb to be sluggish - is there a vacuum line or passage in particular I should investigate?

Also, I noticed that I can move both carbs just a bit when I lift on them - they can rock up and down just a fraction. Their gaskets both have cracks in them. I did this while the engine was running and didn't notice an RPM change, so I'm guessing that they are not leaking at this time. Unless this is normal? See pics below.
If the carb gaskets need to be changed, are there any secret methods to getting these carbs off as the the hold down bolts are BURIED and look very difficult to access.

Anyway, I took the car out for a thrashing and it seems to be accelerating better (maybe just wishful thinking) but it's no where what I'd consider fast. I'd say it's 'adequate'. It pulls smoothly at low rpms but really doesn't have anything left by 3500 - 4000, so I shift then. I'm not looking for a drag racing car (been there, done that), but spinning the tires now and then is a lot of fun as is the occasional 2nd gear tire chirp.
Any advice or ideas regarding the carbs?
Thanks,
Aaron







Relax, all right? My old man is a television repairman, he's got this ultimate set of tools. I can fix it.

[jeffremj]

Hi, the rubber mounts should be OK. Have you balanced the carbs? Lack of balance would cause one carb to rise quicker than the other - it already has a head start. Lots of stuff on the net about carb balancing. My balancer is like this:

[Workshop Help]

Yes, Aaron, those are the carburetor mounts, not gaskets. Do not lean on them. At this stage, the cracking is to be expected and replacing them is one full bucket of work. As a preservative step, apply a thin coat of silicone sealant to the cracks to slow down the deterioration.

Try lubricating the choke mechanism on the carburetors to loosen up their movement.

As for the unequal rising of the air valve pistons, you must adjust the idle speed screws AND the balance bar screws to achieve unison of movement. Once the air valves are balanced, your acceleration will improve to its expected design level. Which is to say, less than Warp Factor 9 but somewhat more than it is now.

As our friend Jeffremj says, using a balance tool such as his makes life easier and more accurate. However, if you have not acquired the recommended tools, a rubber hose will suffice to start with. Yes, that's correct, about 18" of 3/8" diameter fuel line hose with one end stuck in your ear and the other end poking into the mouth of the carburetor when the engine is idleing. This is where the expression, 'Tuning an engine', comes from. You will be adjusting the idle speed screws and balance bar screws to 'tune' the intake hiss to be the same.

Once the engine has reached it's hot operating temperature, remove the air cleaner assembly. Restart the engine and with the hose in your ear, listen to the hiss from each carburetor. The softer hiss will need to be speeded up, while the louder hiss will need to be slowed down to be balanced at an idle engine speed of about 800 to 900 RPMs. This is done by adjusting the idle speed screws on the rear sides of each carburetor.

Once harmony of hissing is achieved at idle, accelerate the engine and observe the air valve pistons as they rise. They should rise in unison. If not, the two balance bar screws must be adjusted, one in and the other out.



The triangular metal piece between the carburetors with the two screws at the top is the balance bar. In this photo behind the two springs on the sides of the carburetors are the idle speed screws.

These steps are not a comprehensive instruction. Please search the forum threads for my complete article on carburetor tuning for a more complete story. Also, please obtain the various publications relating to Z-S tuning. For we who intend to keep these cars forever, having the correct tools, particularly for the adjusting of the carburetors, is essential. A rubber hose in the ear is a poor, tho necessary, item for your tool box. Please start collecting the correct tools to keep your TR7 running well.

Mildred Hargis

[TR7Aaron]

Mildred,
I have read your very thorough and informative posts regarding carb tuning and have learned a LOT about these carbs, thanks to you.
That triangular piece you mention is what I use to rev the engine manually. You had me stumped when you mentioned a 'balance bar', so I studied the photo you provided and compared it to my car. I was shocked with what I saw and I'm disappointed with myself that I didn't notice it before...



My balance bar adjustment lever is missing one of it's adjusting screws!
About 5 minutes ago, I manually moved the lever and watched as the front carb linkage moved immediately but the back one compresses it's tension spring until it bottoms out, then the back carb's linkage begins to move! Problem solved!! Thank you so much, Mildred!

Now.....where do I get one of these only-use-on-the-entire-planet-is-this-application screws?? [:)]


Aaron
1976 TR7 FHC (an ongoing project)

[jclay (RIP 2018)]

My lever is not shaped like your, but I do have two screws that go into it. They are Philips headed screws. Would you like to have them?

jclay

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[TR7Aaron]

Thanks for the offer, Jclay. And thanks to the two guys that emailed me about them too! I took one off my parts car - it too had a different shaped lever, but the screw fit perfectly!

I'm running with both carbs now - watch out!! [:D]



Aaron
1976 TR7 FHC (an ongoing project)

[Workshop Help]

You're welcome! Anything to help a friend in need. Now, stick that hose in your ear and balance the air flow thru your Z-S carburetors along with their synchronzation with the balance bar adjuster.

Once this is done, you will likely be amazed at the new found character of your engine.

Mildred Hargis

[TR7Aaron]

Mildred,
I won't have a chance to do the balancing for a few days, but this did occur to me...
The center lever of the balance bar is what controls the opening of both carbs, so if it is adjusted to contact each set of carb linkage at exactly the same time, would that not be another way of balancing the opening of the carbs?
I ask because since my car was missing one of the adjustment screws, I had to insert it and set it somewhere as a starting point. By watching the center lever's movement, I adjusted the 'new' screw so that it matched the contact point and movement of the 'old' screw.
Am I way off base here?
Thanks again,

Aaron
1976 TR7 FHC (an ongoing project)

[FI Spyder]

That would be your starting point but you "fine tune" to make up for any differences in carbs/linkage by the "sound" equalization which is the desired end result.


TR7 Spider - 1978 Spifire - 1976 Spitfire - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra