Tuning questions

[americium]

I’ve been slowly but surely working on this 2800 to get it road ready and reliable. I changed the oil and filter, installed new plugs, wires, cap and rotor. Had new points in it but changed to Pertronix. I took it out for a drive when it had points and it was running ok but after a while it was acting like it was running on fewer cylinders and would come back to full power randomly. That’s when I changed to the Pertronix and also put in a new fuel pump and gas filter. I test drove it again and for a while it seemed ok then it started dropping power again.
I pulled the air cleaner off and followed the carb tuning procedure (still has the Solex carbs) of adjusting the idle speed and idle mixture screws to sync the them. I noticed that the idle speed on the front carb has no effect but the rear does. The mixture screw had some effect but it was most significant on the rear, the front only had a minor effect. I have an air flow gauge and I could see changes in the air flow when adjusting the rear carb but nothing changes the front. I could get the ball to float about 3/4 of the way up the scale on the rear carb but could not get the ball to drop at all on the front.
I did observe that both chokes are fully open when I did the adjusting.
I would appreciate any insights or advice as to next steps. I’d like to change to the dual Weber’s that would bolt on the existing manifold but I won’t be able to do it for a while so in the meantime I’d like to get it running with the Solex carbs if possible.

 

[Stevehose]

Sounds like linkage. Have you disconnected each carb from its respective threaded rod in order to be able to zero out the throttle plates on the screws? Webers are a downgrade ot properly functioning Zeniths.

 

[americium]

Haven’t done that Steve. I’ll give it a try tomorrow.

 

[Stevehose]

Also check that your secondary barrels aren’t leaking

 

[americium]

How do you check that? is It a visual check?

 

[Stevehose]

What kind of airflow meter are you using?

 

[Baikallackierung]

It might help to give the carbs a dose of chemicals to clean them. The risk is "cleaning" gunk into passages that were otherwise free before though. I have been currently putting an ounce of Seafoam per gallon of gasoline that will be used in all of my carbureted power equipment. My Webers are off of my E3 being rebuilt, but I ran the tank dry and then put some $20.00 a gallon pure power equipment fuel in my tank for storage. Modern gas is a formidable force to our carbs not to be taken lightly.

 

[Ohmess]

With the air cleaner off, and the engine at idle, cup your hands over the front carb and see how much the idle drops. Then let it come back to idle. Then cup your hands over the rear carb and see how much the idle drops. If you hear a distinct difference between the two, you probably have a vacuum leak around the carb that had less influence on idle speed.

 

[americium]

Here is the airflow meter I’m using

 

[americium]

Ohmess, I’ll try the method you mention later today and report back.

I sprayed a bit of B12 in the carbs while it was running. Also sprayed into the two orifices next to the throats. It didn’t seem to make a difference after settling down.

 

[Stevehose]

Yes and with this being said, you can put your palm over each secondary bbl and compare the idle drop. A synchronmeter is a better option than the uni-syn, it allows you to get an idea of both bbls on each carb.

STE Synchrometer Type SK Air Flow Meter - Weber SU Solex Dellorto Carburettor | eBay

Find many great new & used options and get the best deals for STE Synchrometer Type SK Air Flow Meter - Weber SU Solex Dellorto Carburettor at the best online prices at eBay! Free shipping for many products!

www.ebay.com

I'm not sure a Uni-syn type device is sensitive enough to detect the secondary airflow. You could try making an adpater like the one you have to fit the individual barrels of the Zenith.

Often times the secondary is more open than it should be, allowing too much air in and rendering idle metering innefective. Checking this is always a good idea when synching. And as Chris said, check for vacuum leaks.

Once you know the secondaries aren't leaking then we can check other parts.

With the air cleaner off, and the engine at idle, cup your hands over the front carb and see how much the idle drops. Then let it come back to idle. Then cup your hands over the rear carb and see how much the idle drops. If you hear a distinct difference between the two, you probably have a vacuum leak around the carb that had less influence on idle speed.

 

[americium]

Ok thanks. I’ll see if I can adapt the Uni-syn to one barrel at a time. I’ll be getting back into it this afternoon and will let you know what’s happening.

 

[americium]

Steve hose, earlier you said that Weber’s would be a downgrade to properly functioning Zeniths. I haven’t heard that before, can you say why they would be? I was thinking the Webers would be better and also have better parts availability.

Sounds like linkage. Have you disconnected each carb from its respective threaded rod in order to be able to zero out the throttle plates on the screws? Webers are a downgrade ot properly functioning Zeniths.

 

[Stevehose]

I say that because back in the day instead of figuring out why the Zeniths weren't working correctly (often from warping top sections), mechanics threw them away and put the 32/36 DGAV on. Including me in 1985. You will lose top end for sure, because the bores on the Weber are 32mm primary / 36 secondary, the Zeniths are 35mm primary / 40mm secondary so you choke down airflow. My old one and my friend's E9 with DGAV's thin out at higher revs.

Also if not installed correctly they can go over center and lock open into WOT. Ask me how I know.

The air cleaner doesn't fit the same, will need to be drilled for different mounting holes, and will either be farther back or higher, leaving an imprint on your hood insulation.

Yes they idle well and are reliable and overall a solid carburetor but a properly sorted Zenith (it might take some work and commitment) will out perform them. And you keep the flame of oem alive

Just my $.02

Steve hose, earlier you said that Weber’s would be a downgrade to properly functioning Zeniths. I haven’t heard that before, can you say why they would be? I was thinking the Webers would be better and also have better parts availability.

 

[Ohmess]

Weber conversions were popular not because the carburetors were necessarily superior. Rather, they done because Webers were cool. They were used on race cars and on a number of fast, expensive Italian road cars. In addition, there was a large pool of mechanics in the US who knew how to work on them who didn't want to take the time to learn about Zeniths.

I've not had Zeniths, but they were specifically designed to make our cars perform well in a wide variety of conditions. And, you have factory specifications and procedures set out in the manuals to follow.

 

[americium]

Good points on the Zenith vs. Weber. I bet that WOT run was exciting. It’s interesting how things like that become popular. It’s probably way worse now because there are less mechanics willing to tweak carburetors. I’m willing to learn the ins and outs of these things based on your inputs above.

Having said that, I went out and fired up the 2800 earlier. I made a half arsed attempt at making the Uni-syn work for each barrel and I realized that was going be more of a project so I went with the covering each barrel to see the effect. Covering the secondaries had little effect, covering the the front primary dropped the idle a little but covering the rear primary dropped it a lot. I also had a vacuum gauge hooked up and it was reading around 10in. I looked around the front carb for any possible vacuum leaks. I saw that the brake booster vacuum line is connected there and it didn’t look good. I had bought a new check valve for that line and had plans to replace the line and the check valve but didn’t because the hose clamp at the booster end was rusted solid. I figured I would take a dremel to it later. I pulled the tube off at the carb and plugged the port. I started the car and saw that it was running better and had gained vacuum to 15in. I was able to adjust the front carb to match the airflow of the rear carb. I decided to at least replace the check valve in the line.

I could not get the valve out of the tube on either end so I cut the old one out. The old valve was flowing air in both directions, one more so than the other. I put the new valve in the now shorter tube and reattached it to the carb. I think that cutting off the ends of the old tube may have helped along with the new check valve.

I took it on a test drive and it was markedly improved. It was no longer cutting out and seems to have consistent power, running on all cylinders. It also didn’t die when I came to a stop, which it did previously. It’s still idles a little rough so I’m sure I will need to do some more tuning. But hey, I think we succeeded!

Thanks for the advice and support, it helped a lot.

 

[Stevehose]

That's good, you shouldn't get much change in idle from covering the secondaries because they should be all but closed. Replace all vacuum lines and cap anything unused at the carbs. You're on the right track!

 

[americium]

Will do! Are rebuild kits still available?

 

[Stevehose]

Yes, Royze and Walker, Royze is preferable if you can find them. You'll likely end up using your existing needle valves as the new ones are not confidence inspiring. Trim any off-size gaskets to match the existing ones, especially the one by the float. Be careful installing the accel pumps with the new skirts on them, they tear easily. When assembling the carb with the 4 main bolts, tighten them up then wait 24 hours and snug them down again after the gaskets compress.

Will do! Are rebuild kits still available?

 

[americium]

I’ll start looking for kits, I’m sure these will need it.