1986 318 - questions about manifold vacuum + carb tuning
#1
06-18-2018, 08:35 PM
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1986 318 - questions about manifold vacuum + carb tuning
What's up guys! I've been doing a ton of work on my 1986 B250 and i'm hoping you can clear up some information about manifold vacuum.
This van runs on factory propane, and I just replaced & tuned the carb. My understanding was that you're supposed to adjust the idle mixture screw until you get the strongest idle with the highest manifold vacuum, then you adjust the idle speed screw back to factory settings. This took a lot of back and forth, but I finally got everything right! I pull just a hair under 19 inches of mercury at idle, and the vacuum is nice and steady.
Once I drove the van, I noticed a few odd things. First, when you shift it into gear, the idle speed drops (to around 450 rpm) and the vacuum drops to about 15." It's not super steady - the needle bounces a bit just like it did when I was too lean or too rich on the mixture.
In park everything works as expected. A quick blip on the pedal causes the vacuum to drop, then spike up to around 22, then drop back down to idle. But under load, vaccuum is very low. If my foot is on the throttle at all, vacuum drops to below 10" even just cruising. If I put the pedal to the floor while going up a hill, it dropped almost to zero. The only time I get a reasonable vacuum reading is when cruising down hill with no throttle.
So my first question is: what should the vacuum be doing under load? Is this normal? If not, what kind of problems does this indicate. My thoughts were either that my exhaust has poor flow or is plugged, or that the little air intake isn't large enough for such a large engine. This was never a performance vehicle, so it's hard to tell if my vacuum readings are by design or indicative of a problem.
My next question has to do with setting the idle. The idle speed drops when in gear. Should I be tuning the carb for the idle in gear, or in park? Since performance doesn't really matter in park, i'm thinking that maybe I should have tuned it in gear.
Propane is a little tricky to tune because you can run it really rich or you can run it really lean. It also supports very low idle speeds (my van runs fine as low as 300 rpm) so you can't really go off the roughness of the engine.
This van runs on factory propane, and I just replaced & tuned the carb. My understanding was that you're supposed to adjust the idle mixture screw until you get the strongest idle with the highest manifold vacuum, then you adjust the idle speed screw back to factory settings. This took a lot of back and forth, but I finally got everything right! I pull just a hair under 19 inches of mercury at idle, and the vacuum is nice and steady.
Once I drove the van, I noticed a few odd things. First, when you shift it into gear, the idle speed drops (to around 450 rpm) and the vacuum drops to about 15." It's not super steady - the needle bounces a bit just like it did when I was too lean or too rich on the mixture.
In park everything works as expected. A quick blip on the pedal causes the vacuum to drop, then spike up to around 22, then drop back down to idle. But under load, vaccuum is very low. If my foot is on the throttle at all, vacuum drops to below 10" even just cruising. If I put the pedal to the floor while going up a hill, it dropped almost to zero. The only time I get a reasonable vacuum reading is when cruising down hill with no throttle.
So my first question is: what should the vacuum be doing under load? Is this normal? If not, what kind of problems does this indicate. My thoughts were either that my exhaust has poor flow or is plugged, or that the little air intake isn't large enough for such a large engine. This was never a performance vehicle, so it's hard to tell if my vacuum readings are by design or indicative of a problem.
My next question has to do with setting the idle. The idle speed drops when in gear. Should I be tuning the carb for the idle in gear, or in park? Since performance doesn't really matter in park, i'm thinking that maybe I should have tuned it in gear.
Propane is a little tricky to tune because you can run it really rich or you can run it really lean. It also supports very low idle speeds (my van runs fine as low as 300 rpm) so you can't really go off the roughness of the engine.
#2
06-19-2018, 09:13 AM
