ROLY239
New member
If I choose to upgrade the air filter on my 670cc engine, will I need to jet the carburetor, or is it fine to leave it stock? I currently have no upgrades on it but would eventually like to upgrade the exhaust.
Air filter for 670cc
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ROLY239
New member
Is there a way to tell if I’m running rich or lean?
mckutzy
Well-known member
Looking at the sparkplug... Have to look up for a plug chart as to conditions...
ROLY239
New member
How do I jet the carb? Will I have to buy a new jet?
65ShelbyClone
Active member
You could drill out the one you have, but given the nature of the questions you're asking, I recommend just getting spare jets.
The guidelines others have posted already are good for general rule-of-thumb.
In this specific case, reducing restriction upstream of the carburetor will reduce the vacuum signal that helps pull fuel through the jets, so it will tend to make the mixture leaner.
On the other hand, freeing-up the exhaust *can* improve the intake signal and make the mixture richer. Sometimes it does the opposite though...depends on the engine design.
The guidelines others have posted already are good for general rule-of-thumb.
In this specific case, reducing restriction upstream of the carburetor will reduce the vacuum signal that helps pull fuel through the jets, so it will tend to make the mixture leaner.
On the other hand, freeing-up the exhaust *can* improve the intake signal and make the mixture richer. Sometimes it does the opposite though...depends on the engine design.
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ROLY239
New member
I’ve seen videos on YouTube of people drilling the jet but I never really know how much I would have to drill. What size drill bit should I use?
65ShelbyClone
Active member
Depends on what you have and how much more jet is required.
anickode
Active member
Just doing an air filter is unlikely to make it need a rejet.
Once you do an exhaust and (possibly) a governor delete, then you will definitely need one.
670 is a double bore carburetor with 2 separate jets. They will both need to be drilled.
You will need a set of wire gauge drill bits down to #60 for a 670 (down to #80 if you want to do jets on smaller engines) andpin vise (not a drill).
A set of round feeler gauges helps, but you can also use the drill bits to probe the bores and determine what size they are starting out, so you can estimate what size you need to go to.
And probably buy a set of replacement jets anyway. It's real easy to get greedy and overdo it.
Once you do an exhaust and (possibly) a governor delete, then you will definitely need one.
670 is a double bore carburetor with 2 separate jets. They will both need to be drilled.
You will need a set of wire gauge drill bits down to #60 for a 670 (down to #80 if you want to do jets on smaller engines) andpin vise (not a drill).
A set of round feeler gauges helps, but you can also use the drill bits to probe the bores and determine what size they are starting out, so you can estimate what size you need to go to.
And probably buy a set of replacement jets anyway. It's real easy to get greedy and overdo it.
ROLY239
New member
Will both the jets have to be #60 jets? I am going to remove the governor and also add exhaust and an air filter.
bob58o
SuckSqueezeBangBlow
So would you say that switching from a smaller carb to a larger carb of the same style would require larger jets due to the decreased signal? And similarly, swapping for a smaller carb would require smaller jets??
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