nick9one1
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| posted on 11/9/26 at 09:35 AM |
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Low Vacuum on Cyl 1 (Twin Webers)
Engine Specs: 1.8 Zetec Silvertop running twin Weber DCOEs.
The Problem:
I am trying to balance my carbs using a 4-dial synchronizer, but I'm getting a massive imbalance on the front carb. Cylinders 2, 3, and 4 are
pulling a healthy 10-11 inHg at idle. Cylinder 1 is hovering way down at ~5 inHg.
Symptoms & Observations:
It is firing: The exhaust manifold runner for Cyl 1 is getting hot to the touch.
The "Hand Choke" Test:
Putting my hand over port 2 bogged the engine down heavily and spiked the vacuum (even pulled a bit of oil, which is clearing up).
Putting my hand over port 1 made very little difference to the engine RPM. Crucially, the vacuum gauge on port 1 did not spike when I completely
blocked the trumpet.
What I’ve tested/ruled out so far:
Compression: Did a compression test. Cyl 1 actually has the highest compression of the four, so the rings and valves seem perfectly healthy.
External Leaks: Sprayed carb cleaner everywhere. No leaks found.
Mounts: Misab plates and mounts are in good condition and not overtightened.
Carb Settings: The air bypass screw on barrel 1 is completely closed.
Manifold: No brake servo or external vacuum lines are connected to runner 1
Cleaned: stripped the idle circuit, cleaned all passages and swapped the jet and needle with its neighbour - no difference.
https://youtu.be/E4EQWU_5Ky4
[Edited on 11/9/26 by nick9one1]
https://sscstylus.blogspot.com
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nick9one1
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| posted on 11/9/26 at 11:34 AM |
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my own fault!
I swear the first thing I did was swap the guages over to check if it was faulty equipment.... doh!
https://sscstylus.blogspot.com
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jollygreengiant
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| posted on 29/9/26 at 11:35 PM |
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quote:
Originally posted by nick9one1
Engine Specs: 1.8 Zetec Silvertop running twin Weber DCOEs.
Putting my hand over port 1 made very little difference to the engine RPM. Crucially, the vacuum gauge on port 1 did not spike when I completely
blocked the trumpet.
https://youtu.be/E4EQWU_5Ky4
[Edited on 11/9/26 by nick9one1]
I remember going back to my sixth form the year after I left and when I went around the computer lab, one of the guys grabbed me and asked me WHY? his
program would not compile. In this case the answer was in the program, he had labled "X" as both a variable AND as a output result.
I suspect that maybe you are running a vacuum gauge off of Cylinder , this is not a good idea with twin weber DCOE's. A) it destroys the gauges
with constant peaks and troughs of vacuum. B) It provides a source of a vacuum leak when it is running. Even with a restrictor and a one way valve. I
KNOW this because I went though it back in 1980 when I built a 1662cc Full Race Xflow. I never did need a vacuum take off so I blocked it off, then
just used A gunsons carb balancer to get it tuned, the end result was reving to 8K, and 30mpg @ a steady 70mph (OK so about 18 to the gallon being
silly round town. BUT, once out of town, I tended to keep my foot still and vary my speed by changing gear and 70mph was about 3700rpm, the sweet spot
on the cam. Oh and I used 4 gunsons spark plug adapters (1 per cylinder) to set the mixture. JD
Beware of the Goldfish in the tulip mines. The ONLY defence against them is smoking peanut butter sandwiches.
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