No vacuum on front carb

[mudkip777]

I have a 1972 240z with SU carbs. My front carb has no vacuum being pulled through it. Not sure why. The front 3 cylinders are firing (based on new plugs being carboned about as much as the rear 3) and it runs and drives decent (struggles a little to rev past 4k). I went to try balance the carbs and put an airflow meter on the front and it didn't read crap. I can block the throat of the carb with my hand and idle isn't affected. I also don't feel any vacuum being pulled through the vacuum advance (which is hooked to the front carb). If I block the rear carb it kills the engine. I checked valve clearances and all are good. I'm a bit lost to be honest. The slide on the front carb does go up if I give it throttle. And both front and rear slides go up and down at the same rate. They have sae 30 oil in both carbs. Hopefully someone can help.

[dutchzcarguy]

Sorry but it's very contradicting what you say..  If the piston in the carb go's UP.. (when giving it throttle), then there MUST BE some vacuüm pulled through it? Right?

You got 2 airflowmeters and have you switched them.. maybe one is broken?

Does the engine/car pull very well? i assume that the engine is also very weak? Something is blocking the throat of one carb, the engine works on just 1 carb and the other half works via the balance tube? take the inlet apart and see what's going on there!

I can't imagine that that engine runs very well and got over 120hp..

A video of it would be awesome.. a engine that runs on 1 rear carb and no air going through the front carb but the piston go's up anyway.. it's just impossible.. i guess...

I would take the inlet apart, maybe take a look inside the cylinders with a camera.. take a look at the valves.. etc.etc.

[SteveJ]

2 hours ago, dutchzcarguy said:

Sorry but it's very contradicting what you say..  If the piston in the carb go's UP.. (when giving it throttle), then there MUST BE some vacuüm pulled through it? Right?

You got 2 airflowmeters and have you switched them.. maybe one is broken?

Does the engine/car pull very well? i assume that the engine is also very weak? Something is blocking the throat of one carb, the engine works on just 1 carb and the other half works via the balance tube? take the inlet apart and see what's going on there!

I can't imagine that that engine runs very well and got over 120hp..

A video of it would be awesome.. a engine that runs on 1 rear carb and no air going through the front carb but the piston go's up anyway.. it's just impossible.. i guess...

I would take the inlet apart, maybe take a look inside the cylinders with a camera.. take a look at the valves.. etc.etc.

No. There is an easier things to check first that are more relevant.

Edited August 21, 2023 by SteveJ

[SteveJ]

You have the carburetors biased too far to the rear carburetor.

Let's assume that your jet needles are set properly and are straight. Other assumptions include your floats are set properly, the needle valves function properly, and the carburetors are jetted the same.

1. Verify your valve lash! If you haven't adjusted your valves lately, now is the time. This removes a potential confounding factor.
2. Turn your idle adjust nuts fully in (#21 in the first drawing).
3. Back them out 3 turns each. (Okay, this may be a little rich, but let's do it that was for now.)
4. Get the car up to temperature and turn it off.
5. Make sure the first idle setting screw (#7 in second picture) is not pressing on the throttle.
6. Ensure that the balance screw (#3 in 2d picture) is barely in contact with the linkage on the rear carburetor. If it's pushing down on the linkage, that will bias toward the rear.
7. Try starting the car. If it won't idle, then use the first idle setting screw to get an initial idle.
8. Check the balance, and use the throttle adjusting screws (#4 & #6 in 2d picture) to balance the carburetors.

 

[mudkip777]

https://youtu.be/tbbZOlJrnjw

Here is a video of it running. I runs decent, but i know not enough air is getting pulled through that carb because it has old fuel in the bowl. Ive been running and driving it plenty enough with all new gas in the tank where there shouldn't be any old gas left in the system. Also it struggles to rev past 4k so I don't think the vacuum advance is working (not enough vacuum).

The way I found this issue in the first place was because I tried to adjust the carbs according to the service manual. So I had set the idle nuts to the proper turns according to their chart, the first idle screw wasn't touching the linkage, the balance screw on the rear carb was barely touching the linkage, the idle was set the 650 rpm, and when I went to put an airflow meter to the front to measure it, it didn't read anything. I put the same airflow meter on the rear carb and it killed the engine. I tried it with 2 different flow meters.

I have also verified valve clearances between the cam lobe and the top of the rocker. Cold it was set to .20 mm for the intake and .25 mm for the exhaust. I'm not sure if thats the same as the valve lash (if not, I've got no clue how to measure that).

[mudkip777]

I've pulled the front carb off the manifold and looked inside and can't find anything unusual. I don't have a borescope to look deep enough right now. I also haven't checked compression yet, so not sure if the valves are sealing correctly at the moment. Also the oil in the carbs is SAE 20 not 30.

Edited August 21, 2023 by mudkip777

[SteveJ]

56 minutes ago, mudkip777 said:

The way I found this issue in the first place was because I tried to adjust the carbs according to the service manual. So I had set the idle nuts to the proper turns according to their chart, the first idle screw wasn't touching the linkage, the balance screw on the rear carb was barely touching the linkage, the idle was set the 650 rpm, and when I went to put an airflow meter to the front to measure it, it didn't read anything. I put the same airflow meter on the rear carb and it killed the engine. I tried it with 2 different flow meters.

Two things:

1. Post a picture of your airflow meters. Are they the Unisyn type? Personally I don't like them because they block airflow. I like using one like this: https://www.amazon.com/Latest-Rage-5385730856-Carburetor-Airflow/dp/B00CMC57R0
2. Consider getting a meter like this for tuning: https://www.amazon.com/Actron-CP7677-Automotive-TroubleShooter-Multimeter/dp/B0002LZU7K or a timing light with tachometer and advance from Innova or Actron. That way you have your engine speed with you right where you are tuning. 

56 minutes ago, mudkip777 said:

I have also verified valve clearances between the cam lobe and the top of the rocker. Cold it was set to .20 mm for the intake and .25 mm for the exhaust. I'm not sure if thats the same as the valve lash (if not, I've got no clue how to measure that).

Yes, that's the valve lash. You used cam lobe and rocker singularly. I hope you meant you checked all cylinders. Yes, it's pedantic, but I don't like assuming details.

Did you do the other steps I listed? Have you touched the throttle adjusting screws?

[mudkip777]

These are the 2 flowmeters i have. I tried both on the front (both read nothing) but i only tried the silver one on the rear. Yes i checked and set all 12 valve clearances to the same specs. The throttle adjustment screws were both set to 2.5 turns out from the bottom.

 

[mudkip777]

I just compression tested all 6 cylinders. I did the test with the engine cold and 1 spark plug removed at a time. I had the front carb removed, not sure if that made a difference or not. I didn't have the throttle open on the rear carb either. If it will make a big difference to have engine warm, I'll have to do some work to get it back to running shape. So I won't be able to get those results very quickly.

Cyl 1: 105 psi

Cyl 2: 125 psi

Cyl 3: 115 psi

Cyl 4: 115 psi

Cyl 5: 105 psi

Cyl 6: 110 psi

To me that seems a bit low, but they're consistent at least. Hopefully these might show something to someone else.

Edited August 22, 2023 by mudkip777

[Patcon]

They do seem low but consistent.

I would expect 150-175. Maybe 135 at a minimum

Did you crank the engine until the pressures quit rising?

Yes the throttle should be wide open...

All 6 plugs should be out too!

If the numbers are still low, you can add a little oil to each cylinder. If the numbers rise on the "wet" test then the rings are worn or damaged

[dutchzcarguy]

btw.. i never open the throttle.. and get 150+ psi anyway.. so i don't bother to open the throttle.. a good engine will pull that air anyway.. try it..

[SteveJ]

8 hours ago, mudkip777 said:

I just compression tested all 6 cylinders. I did the test with the engine cold and 1 spark plug removed at a time. I had the front carb removed, not sure if that made a difference or not. I didn't have the throttle open on the rear carb either. If it will make a big difference to have engine warm, I'll have to do some work to get it back to running shape. So I won't be able to get those results very quickly.

Cyl 1: 105 psi

Cyl 2: 125 psi

Cyl 3: 115 psi

Cyl 4: 115 psi

Cyl 5: 105 psi

Cyl 6: 110 psi

To me that seems a bit low, but they're consistent at least. Hopefully these might show something to someone else.

Repeat as per @Patcon's instructions. Also keep in mind that not many of these gauges are calibrated. You may want to borrow a friend's gauge if you can to compare readings.

Also make sure your battery is fully charged when you do a compression test. Cranking the engine can take a lot of power out of the battery.

As for the air flow meters, I prefer the second one for the reason you stated earlier. The Unisyn type blocks the air flow, so you have to work fast. I do better working methodically. The "snail shell" style lets you see the effects of adjusting the screws as you turn your screwdriver.