problem with jetting and performance filter swap, mind stumped !!

[rogergendron1]

ok guys i have a cns v3 carb and i have been running a brigs air filter retro fitted to it and all was good, a lil four stroking across the board but good tunning over all.

i recently bought a sbp 90 deg performance air filter
i fuelled up and oiled the bike
installed a new fuel filter
installed the new air filter
let the bike sit for the weekend

well when i got back i took her out for a ride to work and with the new performance filter on it seems like its way too lean and it bogs after 1/2 or 3/4 throttle and has no power up hills and i couldnt figure out what was wrong ! there was no way a simple filter swap would make her run this lean !!!

so i pulled the carb and manifoled thinking it was an air leak or clogged main jet and rebuilt thee carb and installed new gaskets and made shure the carb was seated to the manifold at the clamp nice and tight !

well the same thing happend ! bogging at 1/2 to WOT and no power just like someone cuts of the fuel when i pass 1/2 throttle !

now i am stumped cause i know there is no air leak and the carb was jetted slightly ritch before on porpouse when the other filter was on and it ran great so why does it run so lean and have no power now that i put a 90 deg SBP filter on ?,

thats when it hit me ... well i am running 13:1 comp and require high octain gas as fuel and i filled up at a coner station .... mabey i got ripped off and given 87 oct instead of 93 ! that would explain the motor change of tone and the loss of power the higher up the rpm range i go ! so i drain all the gas and drain the carb and refill the tank with high octaine 50:1 premix fuel from the hardware so i know its good ! TRUE FUEL 50:1 92 oct no ethanol. then i make shure all the fuel is flowing out the petcoxck fast enough and then check to see if it flows out the bowl fast enough and shure as heck it does there is no restriction there ether !

now with the new fuel 92 oct 50:1 and shure there are no air leaks i take her out again with the SB filter on and guess what ??? the same simptoms !!!

now i try my hand at jetting thinking the new filter may flow too mutch air and be leanning it out when the main jet comes into play so i ream it out from .69mm to .74mm and take it out ... well it got slightly better, now it bogs at 3/4 throttle to WOT with the air filter on and still zero power up hills of any kind !!!

well i just swapped back to the old dirty orange brigs air filter and my god what a differance !!! the bike has tons of power and can rip all the way up to rpm's i am not willing to take her to yet LOL and she blast up hills without a single inkling of power loss !!! as a matter of fact she is more powerfull than before !!! i guess because of the rejetting to a .74mm !!! there is a verry slight 4 stroking when not under load but breaks clean into a good 2 stroke when throttled.

when i put the sbp 90 deg filter back on again ...... booom no power and a sharper raspier engine tone and she keeps cutting out/bogging down at around 1/2 or 3/4 throttle !

whats up here ? should i go even bigger on the main jet and see if that helps or should i just run the filter i know works ? why would the 90 deg SBP filter cause sutch a performance loss ?

 

[rogergendron1]

the sbp seems real small compared to the big filters i was running and have the carb tunned to, do you think its not flowing enough air at high rpms and causing the bog ?

or even though its smaller the material its made out of may allow it to flow significantly more air than the bigger ones and now my motor is not getting enough fuel at high rpms ?

what is going on here ?

all i know is the bike took off like a rocket and has a lot of power once i put the dirty old orange filter back on ! with the old orange filter there is no bog and it will continue to build up speed and rpms untill she shakes herself appart at 45mph !!!

slap on the SBP filter and i cant go over 25mph with out it bogging out and having to let off the throttle !

 

[a.graham52]

Iv got the same carb. I know what's going on. Also with my experience with motorcycles... A "bog" is too rich while a lean concern is just lacking power without the bog. Also if it's lean you will be able to make it better by adding small amounts of the choke (give it more fuel).

First thing I did to my carb was install a bigger filter. Used a K&N style oil breather ( for a truck or car) from eBay. That opened up the carb very nice. Still had to reduce my Main and pilot jets to make it run 100%. Used the saudering and drill method.

My bet is the new filter is too small. Period.

 

[maniac57]

Your old orange filter has several times the surface area, not to mention that restrictive elbow. I'm sure it flows far more air than the cone filter.
Why would you switch to a smaller air filter anyway? The engine is clearly telling you what it prefers. Either of the second two look to be far better choices as far as airflow capacity.

 

[16v4nrbrgr]

I'd do some plug chop runs to figure out for sure if you're lean or rich when you full throttle it on the main jet. I concur with Maniac's assertions on the air filter, I had problems with an aftermarket air intake on my car that had a similar elbow, the 90 degree bend causes all sorts of turbulence in the intake and drops flow to about 50% of the cross section around the corner. When I put the standard V8 style air filter housing that uses a paper air filter surrounding the intake and allows air to swirl in all sides like a toilet flushing, it alleviated all the weird drivability and torque loss issues I had.

 

[Ghost0]

I doubt the 90 degree elbow is causing much of an issue here as the intake velocities on these engines is pretty slow compared to a V8. I would do as 16v4nrbrgr suggests and try some plug chop runs to see what is really going on. Rich and lean can have very similar symptoms. Our filter offers much improved air flow over the stock filter as it has many times the surface area to allow air flow. Now compared to the other filters you are using it has much less surface area and will probably allow a little less air flow. In any case your performance will only be improved when the air/fuel mixture is correct so it really doesn't matter which filter you use if you get the right mix you will be producing max power. Now with that said if you can get more air then you can use more fuel but without some additional help, supercharger, turbo etc you can only pull in so much air on a piston port engine no matter what filter you use. My guess is that no matter which filter you use you will find the performance to be very close between them all if you tune your engine to run correctly with each filter.

 

[16v4nrbrgr]

Here's a visual representation of what I'm talking about, the problem only gets worse with increase of airflow, so the problem will be most prevalent at high engine speeds and at WOT.

 

[Ghost0]

Yes this can happen at higher velocities but considering the size of the elbow compared to the intake on the carb the velocity as I mentioned is VERY slow in the tube and only speeds up as it goes into carb. The other thing to take into account is the air is being pulled through the tube by the engine and not being forced in under pressure which completely changes the air flow characteristics. Also there is a huge difference between a fluid which cannot be compressed and a gas (air) which can be compressed, two completely different dynamics going on.

 

[16v4nrbrgr]

That's a pic from a CFD program modeling intake air flow, air is modeled using Computational FLUO-Dynamics, air is compressable, indeed, but you don't want air to pack up and compress in your intake around bends. As stated in the picture, this causes a pressure drop downstream, the reverse of supercharging. If the diameter of a long intake is too big and the airflow velocity is too low where smooth fast airflow doesn't occur, that's a problem in itself when coupled with a restrictive filter, since airflow velocity is coupled with intake pressure, if the velocity is too low the flow through the filter material and pipe will suffer.

Bends are bad, filter area is good, HT's flow quite a bit of air, and with small engines small details make big differences in how they run.

 

[Ghost0]

Well that was kind of my point that the engine is pulling air in so it cannot pack up, it will only be pulled at the rate the engine is using it. There are very different dynamics going on depending on if air is being forced through a tube versus being pulled through a tube. Case in point the intake manifolds on race cars are treated different than exhaust manifolds, intakes are under vacuum (unless pressurized by turbo or supercharger) and exhausts are under pressure. No matter really we just need to get the OP to tune his carb for whatever air filtration system he chooses. As the seller of one of the filters in question I am just trying to point out some of the reasons for his results. Since most customers are going from the marginal stock filter to ours they tend to see an improvement in performance.

 

[16v4nrbrgr]

The difference in pressure around the bend at high velocity causes a low pressure vortice around the elbow, which is turbulence, which causes a pressure drop. The packing up of airflow in its path through the elbow is relative to the low pressure vacuum on the carubretor side. Gradual bends and mandrel bent tubing are used in an effort to reduce this effect which decreases with increase in bend diameter.

Whether the tube is under positive pressure from a blower or under vacuum does not change this effect, just the density of the charge though the pipe. The denser the air flow through the pipe, the worse the airflow separation can occur because the airflow behaves more like a fluid and an ideal gas. That is not to say that boundary layer separation cannot occur under a vacuum, since the downstream vacuum is the driving potential for inflow of higher pressure atmospheric air, the colder the air is, the more airflow separation will occur due to the air density effect. I'm stating commonly known fact, I don't see why there is any debate about this. A tight 90 degree elbow drops airflow by about 50%, positive pressure systems with turbos or superchargers overcome system inefficiencies by upping the overall air density of the intake system, packing more molecules in the pipe at any given time, that's why a straight shot isn't emphasized on basic boost kits, because the pressurizing of the system nets a gain in air molecules packed in the cylinder per intake stroke. Of course this type of system on a two stroke would just blow the mixture right through the motor and screw up the exhaust harmonics, and net a highly inefficient flamethrower of an engine, lol.

 

[Ghost0]

I am certainly not debating anything you are saying. I am an engineer and studied vehicle design and am very familiar with all this. I have studied IC engine design and done extensive wind tunnel testing on race cars so I am very familiar with all these principals. However, us debating CFD related to the intake on a $50 engine is not getting the OP any closer to improving his situation. My basic point is that the carb needs to be tuned correctly no matter what filter he uses and our testing indicates that the 90 degree filter offer significantly better air flow over the stock foam filter even with the bend in it.

 

[16v4nrbrgr]

The stock skull filter is a POS for sure, lol. I put a UNI foam filter on my HT back in the day and picked up 5 mph top speed because the engine could breathe enough to rev much higher. I never messed with the jetting because the plug chops and eliminated four stroking indicated that it corrected a rich high rpm condition that limited the motor's output a lot. Just taking the cone filter off the bent intake and placing it directly on the end of the carburetor will be better than using the bent intake pipe, a plenum would be beneficial if placed over the filter if it is the correct volume, and will silence the intake noise. On my BMX the intake was so noisy that I put a plastic coffee can over the foam filter directly facing the pumper carb, and I used a cut off end of a hand grip as a velocity stack to aid the airflow into the plenum. I found the best results of stack placement was not with it facing directly forward, because the ram air effect at high speed exceeded the high speed needle's richest setting, and caused it to lean out, instead I turned the canister so that the stack is 45 degrees to oncoming airflow so that the stack didn't receive pressurized air, turning it backward or 90 degrees to oncoming airflow caused a vacuum in the intake from Bernoulli effect which hurt the power and tunability.