No vacuum on front carb

[dutchzcarguy]

18 minutes ago, SteveJ said:

I did the test with the engine cold and 1 spark plug removed at a time.

Always all sparkplugs out, this way it's much easyer for the starter (and the battery) as they don't need to work hard to get compression in all 6 cylinders. 

[Captain Obvious]

If you had one carb completely off, all six cylinders had a pretty clear path direct to atmosphere. Three of them (obviously) direct, and the other three almost as good to atmosphere through the balance tube.

Point is... I don't think you have to run the test again simply with the throttle held open. I think your previous test already took care of that.

[mudkip777]

Well I just redid the test with all 6 plugs removed, spinning it until it stopped building compression, and with the rear carb throttle held open for good measure. 

1. 120 psi

2. 125 psi

3. 125 psi

4. 120 psi

5. 118 psi

6. 122 psi

They went from being within 20 psi to within 7 psi but they are still a little low. I haven't tried adding oil to the cylinders, because I'm not sure how much or what kind. I'll have to see if any of my friends have a compression tester. The one I'm using is a rental from O'Reilly.

Edited August 23, 2023 by mudkip777

[Patcon]

https://dannysengineportal.com/wet-compression-test-how-to-do-it-what-will-it-tell-you/

[Diseazd]

Seems to me if you have an average 120 lbs per cylinder in the first 3 cylinders, they are getting air from somewhere other than through the butterfly. Is it possible you have worn shaft bushings or a bad manifold or carb gasket. At first, burned valves came to mind, but with 125 lbs pressure its getting air in there from somehow. 

[mudkip777]

 

2 hours ago, Diseazd said:

Seems to me if you have an average 120 lbs per cylinder in the first 3 cylinders, they are getting air from somewhere other than through the butterfly. Is it possible you have worn shaft bushings or a bad manifold or carb gasket. At first, burned valves came to mind, but with 125 lbs pressure its getting air in there from somehow. 

Not sure if you read all of my previous messages, but the front carb is removed, so the front manifold was completely open during the compression test.

[Captain Obvious]

I'm not really concerned by the compression test results. They're all consistent, but a little low. Sounds to me like it's an old Z engine in typical condition. No problem.

As for your carb issues... It's all in the adjustment of the carbs. The front carb has no vacuum at idle because it's all the way closed shut. And when it's all the way closed, it doesn't contribute anything to the running of the engine. You're idling completely on the rear carb.

Once you get off idle and the linkage opens the front carb, it runs off both carbs and you're OK. But at idle, the front carb butterfly is adjusted completely shut and that carb isn't doing anything.

Bottom line? Close the rear carb some and open the front. Adjust them so both carbs are doing something, even at idle.

[mudkip777]

I dont think its doing much even under load. Based on the fact that I took a look in the fuel bowl and it was full of old yellow fuel. I've drained my fuel tank and have had new gas in it for a while and have been driving for at least 100 miles since. Plus like I said at the beginning, it struggles to rev past 4k, which leads me to believe the vacuum advance isn't working (it's run off the front carbs vacuum). I don't think the issue lies in only idling.

[Captain Obvious]

Oh, OK. I saw the note about struggling above 4K and I was attributing that to other stuff that would be dealt with later. 

So it sounds like you might have multiple things going on. Let's back up a little... Have you run a float level test using a clear tube to verify that the bowl levels are correct? If not, I would start there.

[SteveJ]

You talk about old gas in the front carb. How old? Has this car been sitting for a long time? In addition to the float level, the path between the float bowl and nozzle could be clogged. I saw that with my 240Z that sat for a long time. How old are the hoses that go between the float bowl and nozzles? (I thought I posted about cleaning out the float bowl, but I don't see the post here or in my history.)

[siteunseen]

I've had problems with mine before that were fixed by tightening all the visible screws. My float chamber lids weren't tight. 

[mudkip777]

When I took apart the front carb, I checked every fuel passage from the inlet nipple to the metering rod and found no blockages or debris. The only thing I did notice was the needle and seat seemed a little sticky, which was solved by removing it and reinstalling it. The hose between the bowl and the nozzle doesn't seem very old (its not cracked or very hard). I don't know how old the fuel was because I got the car in December and it already reaked and was very yellow. I drained and refilled the tank in January (refilled it with 5 gallons of premium and a bottle of seafoam to stabilize it). I only got it back running earlier this month. I know the previous owner "rebuilt" the carbs, because he gave me a box full of old and new carb parts and gaskets. Im not sure what all he changed, or adjusted. I'm also not sure how to measure or adjust float level.