can lack of OEM airbox effect performance?

[idodishez]

Been working on my 91ZX7. Got it running finally after a melted connector was causing a no spark condition. It appears to be running lean, and falls on its face when given a lot of throttle. Full choke causes it to run better, further supporting the "lean" theory. I took it to the local Kaw shop, and they gave me some suggestions I wanted to confirm.

Some history:The bike is far from OEM. It has an aftermarket pipe. The OEM airbox has been removed, and 4 individual foam type filters were installed in place. This was all done prior to me taking ownership, so I dont know if the cabs have been jetted or not. Now the bike HAS run decent since Ive had it. So its not like the airbox was removed, and it started running like crap. But the guy at the Kaw shop said the missing airbox will cause it to run lean, as it doesnt have the restriction or forced air of the OEM air box. Is this true? I know on a car it makes no difference if you have the air cleaner on or not, unless your midstream a MAF sensor, vacuum, etc. I understand the OEM airbox gets its fresh air from the ducting on the cowling/fuel tank, but is it actually pressurized where it could cause performance issues?

If all this proves to be NOT the issue, where do I find out what jet kits would go with my particular pipe? (Assuming that would be the next step)

[StarGate]

Any time you increase or decrease the air flowing thru the engine enough it will affect the performance. Typically people will add a 4-1 pipe or pod filters that requires them to rejet to a larger main and pilot jet. I don't know enough about your particular bike to answer your "pressurized" question though. Hopefully someone has first hand knowledge of your setup and what a good starting point would be for rejet. Either way, if you are going to stay with the pipe and filters you will need to know what jets you have installed.

[hayward6]

If its only running a tad lean you might just want to go up to the next size jet. Airflow on a motorcycle is critical. If the bike has new pipes and new high flow filters and the jets weren't done your bound to be running lean. Another thing to check if your cutting out at full throttle is the float levels and anything else that might be causing fuel restriction.

[idodishez]

Thats the part that confuses me. Its running lean, but its got the same carbs, pod filters, and pipe that it did when I got the bike. Granted, it always did have a very minor flat spot on accell, but noting like it is now. It will flat out fall on its face now when goosed. The only thing that is different is the fact that the carbs were "rebuilt" by a local shop. (Not the Kaw shop) So, I totally agree that the jets may not be the right size, esp since I always did have a minor hesitation on accell. I just wich I knew what has changed to case it to run THIS lean, with no equipment changes. Maybe the guy didnt rebuild correctly?

Im open to whatever is easiest/quickest. If it means going back to stock jets and the OEM airbox, so be it. But then wouldnt I have to replace my pipe w an OEM pipe?

Or, is there any way to say "you need THIS jet kit w THIS pipe"? Any resurce esix like that where i could match the jet kit to my pipe? (and POD filters)

Thanks

[StarGate]

I'm not aware of any site that has a "chart" of jets to use with xxx mods on xxx bikes. I would think that compiling a list like that would take some doing since the possible combinations of bike size/model/carb type/exhaust type/ and other mods could all effect what jets you needed.

As for the lean running now, there are lots of reasons it could happen. It could be something was done wrong at the shop. Have you taken it back to the shop that did the work to see what they had to say? The original owner may have had the mixture set too rich to compensate for the leaness, trash in the fuel dislodged and is partially blocking jets or fuel flow now. First thing I would do is check for trash in the carbs and while in there check the main/pilot jet sizes. Check the factory jet sizes and see if they have been changed. If it still runs bad try to find someone with a similar bike/setup and see what size jets they are running. Try a larger main or pilot jet depending on the exact symptoms.

[ari_one]

Does it have the emissions stuff in the front right corner (a couple of vacuum lines running to it)?

[idodishez]

Originally Posted by ari_one

Does it have the emissions stuff in the front right corner (a couple of vacuum lines running to it)?

Running to what? As far as I can see, all emmission components have been removed. The air box is gone, so obviously no emmission piping to that. Only thing coming off carbs is 4 vacuum lines that are plugged off. I think they are used for syncing, and someone just put them on there for easier access.

Stargate, thanks for the tips. And as far as taking it back to the shop that did the carbs, they have pretty much washed their hands of it, which is why Im tackling it now. And no bike shop wants to mess with it because it is not OEM.

[elr658]

First, the only PROPER way to rejet carbs uses an exhaust gas analyzer, a dyno, and an experienced operator. Now I'm about to voice an unpopular opinion based on thirty years of dealing with CV carbs. Everyone thinks that you automatically go rich when you install a pipe. That is not always the case. Many aftermarket exhausts do not flow as well as a stock exhaust.

I suggest a stock carburetor calibration with a stock airbox. Use your pipe.

Adjust your valves to the LOOSE side of the tolerance.

[ari_one]

The reason I ask is because on my 95 ZX-7 there is (was) an emissions vacuum valve that variably pressurized the carbs and caused some running problems when it wasn't working correctly. The fix was to bypass the valve and run a line directly from the OEM air box to the rail on the front of the carbs. However, I'm not sure if a 91 works the same way. The vacuum hoses seem to be very important on mine and was wondering if perhaps something didn't get hooked up that was hooked up before the carb work.

[wiredgeorge]

I believe this isn't a jetting issue but the fact that the airbox was removed and the pressuring mechanism disconnected could well be the source of the problem. I think the ZX-7 carbs have pressurized bowls. I don't know a lot about this model but believe ari_one is correct. I believe this model has a user group and would find it and find out more on this subject.

[JimmyD]

Originally Posted by elr658

First, the only PROPER way to rejet carbs uses an exhaust gas analyzer, a dyno, and an experienced operator. Now I'm about to voice an unpopular opinion based on thirty years of dealing with CV carbs. Everyone thinks that you automatically go rich when you install a pipe. That is not always the case. Many aftermarket exhausts do not flow as well as a stock exhaust.

I suggest a stock carburetor calibration with a stock airbox. Use your pipe.

Adjust your valves to the LOOSE side of the tolerance.

I was expecting a less civilized response when I saw the thread title!

[idodishez]

Originally Posted by JimmyD

I was expecting a less civilized response when I saw the thread title!

Huh? Did I miss a spellcheck? (effect)

[elr658]

Jimmy, my claim to fame around here is insightful and EXTREMELY accurate troubleshooting delivered on a baseball bat.

[JimmyD]

Originally Posted by idodishez

Huh? Did I miss a spellcheck? (effect)

No, ELR usually gives advice like a drill sergeant gives compliments. (LISTEN HERE MAGGOT!) Your spelling is excellent.

[A2rider]

Originally Posted by JimmyD

No, ELR usually gives advice like a drill sergeant gives compliments. (LISTEN HERE MAGGOT!) Your spelling is excellent.

You have to admit he's getting better.

[elr658]

After getting a couple of threads blown out by the moderators around here. Greetings ladies and gentlemen.

[idodishez]

Originally Posted by JimmyD

No, ELR usually gives advice like a drill sergeant gives compliments. (LISTEN HERE MAGGOT!) Your spelling is excellent.

My question was because you mentioned the title of the thread. Maybe a noobie question?

[A2rider]

Originally Posted by idodishez

My question was because you mentioned the title of the thread. Maybe a noobie question?

We've all been noobs at something, and there are no dumb questions on this board. As long as you listen to the baseball bat wielding mechanic, and can prove him wrong, you'll be OK!

[idodishez]

Originally Posted by wiredgeorge

\I believe this model has a user group and would find it and find out more on this subject.

On this forum or otherwise?