390 fuel prob.

kartboy63

Member
we just put a 390 together with a 38mm carb from comp carbs, the engine starts easy and runs good on the stand, we never raced it yet, but it has a lot of fuel coming out the exhaust, I have built a lot of SA 212 predators but not a 390, so is this normal for these big motors and big carb.
 

Jimbo

You can fool some of the people some of the time
Sure
How often are you changing oil?
Sounds very rich to me but it may perform very well on the track under full throttle.
The question is weather it needs to be that rich to perform well on the track.
Im sorry but i can't answer that for you.
To really know if it is performing at peak, the engine would have to put on the dyno and tested.
If there is a gain by having it that rich at idle is the gain big enough to justify the end result?
You never mentioned what brand of carb it is.
 
Last edited:

kartboy63

Member
Sure
How often are you changing oil?
Sounds very rich to me but it may perform very well on the track under full throttle.
The question is weather it needs to be that rich to perform well on the track.
Im sorry but i can't answer that for you.
To really know if it is performing at peak, the engine would have to put on the dyno and tested.
If there is a gain by having it that rich at idle is the gain big enough to justify the end result?
You never mentioned what brand of carb it is.
thanks, we haven't ran it on the track yet to see how it does, its a new build, I have built a lot of SA motors but none with the big tilly, I have had some with a little methanol out the exhaust but not like this one.
 

CarlsonMotorsports

Site Supporter
Doesn't sound like a problem to me.
Tilly carbs need to be overly rich on the bottom end to work right on the track.
You never let a Tilly idle long -- it'll wash the cylinder down with fuel in no time flat.
Personally, I never run an engine up on the kart stand or with no load either. You can't tell anything tuning-wise by running it that way.
Put it on the dyno or put it on the track. That's the way you will need to tune it.

If the carb builder was familiar with the rest of the parameters of your build, and the drill pattern is correct, you should be able to tune it easily by adjusting the needles and possibly the pop-off (only if it were off to begin with.)
This must be done on the dyno or on the track.


-----
🏁Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
Carlson Motorsports on Facebook
30 years of service to the karting industry
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
 

Jimbo

You can fool some of the people some of the time
One of the reasons people make carbs fat on the bottom is that we are asking the carb to do something they normally aren't designed to do.
We are asking the carb to make as much HP as possible on the bottom end so it comes off the corner with out hesitation.
The conventional concept of a carb will usually be said to have the following circuits:
Idle circuit -- Just for idling
Transfer circuit
Mid range
High speed circuit -- for WOT
You might also add an acceleration circuit

In racing for the most part we don't care about idle. We only care about coming off the corner as hard as possible and max HP at WOT.
MOST of the carbs we use for kart racing do not have an accelerator pump circuit so people make the idle circuit overly rich to compensate for it.
Imagine flying down the straightaway and then lifting when you go into the corner.
When you lift the throttle plate or slide closes and you get a very high vacuum (close to 30 inches of HG) in the entire intake tract in front of the throttle plate or slide.
The idle circuit and transfer circuits are both subjected to this extreme high vacuum and fuel pours out of those circuits into the entire intake and even the exhaust tract. That includes the the carb in front of the throttle plate, the intake manifold, the cylinder and the exhaust system.
Talk about washing the oil from the cylinder walls from the excessive fuel (gas or more so methanol)
On a methanol engine a lot of this excessive methanol ends up in the crank case / oil. Change it often!!
On decel you have this massive amount of fuel throughout the entire upper end of the engine just waiting for some Oxygen so it can burn.
If everything is right when you get back on the throttle you feel yourself up against the back of the seat and everything is right again in the world.
The rush you get when that dopamine is released is way better than booze or drugs!
The down side of an overly rich idle circuit is that the engine either doesn't idle or idles very poorly. This overly rich idle circuit can also dilute the oil and foul spark plugs (especially on gas)
Some people build carbs with the idle circuit richer than it needs to be.
It can take a long time on the dyno to get the idle circuit big enough to have max acceleration with out overdoing it.
In this tuning process then you have to make other circuits a little bigger to compensate. Correction: in this tuning process you have to make the other circuits the correct size to compensate for the size you made the idle circuit. ;)
I would say there have been a lot of karting engines blow up because of too much fuel in the oil and or not changing the oil frequently enough.
Idling during a long caution and continually punching the throttle ain't good!!
 
Last edited:

kartboy63

Member
Doesn't sound like a problem to me.
Tilly carbs need to be overly rich on the bottom end to work right on the track.
You never let a Tilly idle long -- it'll wash the cylinder down with fuel in no time flat.
Personally, I never run an engine up on the kart stand or with no load either. You can't tell anything tuning-wise by running it that way.
Put it on the dyno or put it on the track. That's the way you will need to tune it.

If the carb builder was familiar with the rest of the parameters of your build, and the drill pattern is correct, you should be able to tune it easily by adjusting the needles and possibly the pop-off (only if it were off to begin with.)
This must be done on the dyno or on the track.


-----
🏁Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
Carlson Motorsports on Facebook
30 years of service to the karting industry
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
I run the 3910x in all the motors I have, I have had good luck in SA class with them, but I hear a lot about they not hot enough, what would you say is a better and hotter plug, for years, back when we ran flatheads I have ran a W24FSU.
 

CarlsonMotorsports

Site Supporter
The ND 24 plug is a middle of the road plug as far as heat range.
We typically use a 16 for our small plate engines that run extremely rich.
Now, with that said, flatheads have very little compression compared to the ohv stuff like the 390 you have built.
There's no way I'd try running a hot plug (certainly not that hot) in a higher compression engine.
My thought has always been to try to run as cool of a plug as you can for the build and not to try use the plug as a way to build (or take away) heat.

I think there are better plugs than the 3910X (even within Autolite's catalog.)
I totally understand why you would use that plug in a class that is restricted on compression (ie clones & animals,) and it certainly does work in more modified situations, but there are other plugs out there that will do as well (or better) for higher compression engine applications (imho.)
 

kartboy63

Member
The ND 24 plug is a middle of the road plug as far as heat range.
We typically use a 16 for our small plate engines that run extremely rich.
Now, with that said, flatheads have very little compression compared to the ohv stuff like the 390 you have built.
There's no way I'd try running a hot plug (certainly not that hot) in a higher compression engine.
My thought has always been to try to run as cool of a plug as you can for the build and not to try use the plug as a way to build (or take away) heat.

I think there are better plugs than the 3910X (even within Autolite's catalog.)
I totally understand why you would use that plug in a class that is restricted on compression (ie clones & animals,) and it certainly does work in more modified situations, but there are other plugs out there that will do as well (or better) for higher compression engine applications (imho.)
what would you recommend using, or what would you run in such a build.
 
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