I rebuilt a l28 from a 75 but swapped to flat top pistons with a mild cam. Ran good but had issues with the su’s so went to a set of triple webers. Now I am having issues with vacuum, it bounces all over the place but I’ve sort of hit a wall. Was planning on removing the cover this weekend and making sure I don’t have a valve issue but really not sure what else to look into. Any suggestions would be greatly appreciated.

How's all around performance , off idle , accelerating, cruise , w/mild cam ? Would the "mild cam" have a not so smooth idle , and wouldn't a combo / situation like that give a flaky vacuum reading especially if a very responsive gauge ? Does the "bounce" begin to go away as you come off idle? Hitting over 1000 revs? over 1500 revs ? Explaining more specific the "issues" , and even issues with the seemingly non-connected points like the previous "issues" w/ Webers may give a more complete scenario here , for diagnostic opinions.

Sounds good and has a great idle. Carbs burp and fart back at you under some heavy acceleration. The vacuum is 5 to 20 and very dramatic peaks and valleys, like as fast as the needle can move.

Does it perform well all around - sounds like it CAN do "heavy accelerations" - BUT the " burping and back-farting" , akin to backfiring through the intake ? , Like leaky or bent valves ? That could in time become a carb fire ? But sounds like very recent engine work or rebuild done . And maybe if it's just sort of a Beast right now , hasn't burned to a crisp , just break it in ya know? Valves taking a better and better "seat" after 500 mi . Finally or better yet first off re-check valve lash/clearances ,for adj. , if any may have valve(s) not fully closed ,all or just some. Or cam is too "Wild" in profile as in baseline,duration overly wide for springs, but more likely too high lift ?? Who knows.

Thanks, figured I’d start with checking the valves again and doing a leak down test on it. Everything comes back thru the carbs when it does happen but it’s not a real constant thing. Engine has maybe 3or 4 hours of run time on it, been about a 2 year project and just ready to start enjoying it.

Webers come in a thousand different configurations, if not jetted and kitted to fit that particular motor's demands you may NEVER get them right.

Worked on it over the weekend and figured I post with what I found. First I was concerned about the vacuum for the advance on the distributor. After looking at the carb noticed it only pulls vacuum off one of the carbs which would make the jumping make sense. Assuming with these carbs they are only concerned of pulling vacuum for the brake booster.

Took it on the road and wideband o2 sensor runs super rich at idle and goes painfully lean on rpm, worse under load. Dig into the carbs and noticed the emulsion tubes are terribly miss sized. By their calculation for a 2.8 I should be running f2’s when what is in it are f11’s. Seems it is sized up for a 1.9 liter. Going to order them up and see where it takes me but carburetors are not my strong point.

You can calculate till the end of the earth and still be wrong on things like airbleeds and emulsion tubes. You need to find someone that builds the carbs to specific combinations and find out what that engine requires, for instance if the main venturi is mis-sized then everything else you can do will pretty much be wasted work.

Emulsion tubes complement the main air bleed that goes with it and can pass no more mass air than the bleed gives. Main bleed is more important. There is no '1.9' emulsion tube just like there is no '2.8' one. It is the close combination of air bleed plus main jet plus emulsion tube plus venturi size and the metering there at pullover that makes the setup work across a wide range.

Vacuum to advance; you need to know the total mechanical amount to determine the vacuum amount and need to know whether it is spark ported vacuum or constant, which determines where on each carb you pull the vacuum from, there should be at least two locations.

I used to tune 8 bbl. tunnel ram race engines as well as stuff like 340 and 440 Mopar 6 Pack 3X2 bbl. You have no idea of how much trouble you can get into with the wrong carbs there although the wideband will help you some. I strongly suggest you find an expert somewhere to give you some idea of the correct Webers or parts that work much closer there or you will have utter fits and never get it solved. Simply fouling plugs due to the super rich idle you have can be rocking the vacuum needles. You if the idle got jacked up too far (VERY common rookie mistake) will be into the transfer slots or holes past the butterfly and why you likely are so rich off idle.

http://datsunzgarage.us/weber/

40s on a 2.8.........

Hoping you have 40s. The 'progression holes' he speaks of are the transfer holes I was talking about. My word venturi is same as Weber word choke. Need a header on the engine or you are basically overcarbed with Webers, he addresses that and I second it bigtime. He goes over the basic 40 tune parts and you already have the emulsion tubes you need there, what did I tell you? You may need to curve the distributor like said to drop vacuum entirely, we used to do that on all race cars.

Thanks amc, that link is exactly what I was hunting. Just adjusting what came with the carbs helped immensely, feels like I have a turbo car now. Have some work to do to just even it out some but when that power hits it feels amazing.

You are welcome.