Thread: Carb Jet Variations
03-03-2011, 02:37 PM #1
Carb Jet Variations
Ok carb experts - noticed quite a remarkable variation in carburetors today. Was wondering if anyone could chime on on the reasoning/benefits (if any) to the differences.
On the 1200 engines, it is apparent that depending on what boat the 1200 came from, the carb jetting is different. this makes sense. What doesn't make sense is how the jets differ. Let me explain.
In 1 of the 1200 carbs, the jets seem to get smaller as you get farther from the gas tank. The mag cylinder has a 142, the center a 140, and the pto a 135.
However, in another carb recently purchased, the smallest jet is in the front and they get larger toward the back.
Not leaning toward any foul play, but just wondering why anything would be done like this? Also - would there be any consequence to just modifying a carb to have all three of the same size jets? Of course in comparing these carbs to carbs we already have bolted onto machines (and work perfectly) we are finding different sized needles across the board in different locations.
Another reason I ask the question is regarding air/fuel flow to begin with. Would a 38mm carb work on a 1200 (stock is 40mm i think) if you change out the jets to allow more fueld and then tweak the brass from there? etc...
03-03-2011, 03:22 PM #2
- Join Date
- Jul 2007
- near Toronto, Canada
Has a lot to do with how the exhaust manifold allows each cylinder to 'breathe', among other things. Different exhaust systems affect each cylinder differently. Polaris used a bunch of different manifolds (on the 'same' engine) depending on the hull model, the year, and other factors.
Sometimes a cylinder will have a tendency to run hot or lean, so the production carb jetting is tweaked to compensate.
The ignition advance curve in the CDI also varies from model to model and year to year, which can change how much fuel a given cylinder needs to remain in the 'safe' zone for fuel to air ratios and combustion temperatures.
Sometimes the same carburetor has manufacturing differences other than the throat diameter. These differences can cause the fuel flow to differ between carb versions, even with the same size jets installed. For an example, see the Pro 785 Mikuni carbs with 2 or three 'Low Speed' orifices in the carb throat near the throttle plate.
03-03-2011, 05:01 PM #3
^^ this makes sense.
Obviously many of us have made purchases on carbs we thought were for the same model/engine etc. It just worried me that two carbs made for the same motor would have two highly varying jets and settings. Don't want grenade a motor because a carb isn't right.
Granted I understand we are talking about very small differences in sizing - I guess what I should take from it is that as long as I can get a nice piston wash, etc and tune the carbs properly, i shouldnt put much stock in the size of each jet in each carb?
03-04-2011, 01:02 PM #4
log manifold and pipe vs red pipe and manifold.on all red pipe 1200's the mag is a 140,the cen a 142,and the pto a 135.water routing plays a part too.so any changes to water flow could result in a lean condition such as replacing the water manifold with a more restricted brass fitting for pissers.
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