TOPIC:

I have a 82 Gpz 750 with stock set-up for jetting and stock airbox. It has an aftermarket 4-1 header that flows into a Supertrapp exhaust. I know I can get more out of this motor with some jetting tweeks but don't want to go the pod route. I love the looks of the pods but worried about getting the jetting right. Wondering if drilling into the airbox will provide the needed extra air if I go bigger on jets and possibly raise the needle. Anyone done this?

Drilling holes in the airbox is a mistake!!! Consider the fact that air is drawn into the airbox through the cap and then passes through the filter on the way to the carbs. If you drill holes in the airbox the air that passes through the holes will not be filtered at all, so dust, grit, etc. will be sucked directly into the cylinders and will accelerate wear. This is a bad thing for sure.

IF (big if) you can find another airbox cap you could experiment with it by either making more holes in it or shortening the tube, but I highly recommend NOT doing that to the cap on your airbox because there' no going back if you find it hurts performance. Frankly, I seriously doubt you will gain any performance on a stock motor by letting more air in regardless of the jetting. The stock airbox was designed to give the engine an appropriate amount of air give the engine's displacement, valve sizes, etc.

Also, keep in mind that your bike came with CV type carbs, and they are VERY sensitive to changes in the airflow since the vacuum inside the airbox affects the opening of the carb slides, etc. You may very well end up losing power at low to mid rpm if you permit greater airflow into the carbs, and let's face it, most street riding is not done at or near redline. Ed

Might not be the best method but I have had some luck doing it this way. You want to reduce the restriction to the air filter and increase jet size . After a point though it will just make the motor lose efficiency ( Low MPG) and power but they are jetted lean stock so who knows you might be able to get some gain, Increase your jets a step and then use a piece of foam of different thicknesses to find the correct restriction. Jetting will be a lot easier with the air box. Just simply applying the foam to the air box works well. Like Ed said the bike needs restriction to operate properly. So any gain may be very minor or could even cause loss if the bike is already at maximum allowed flow rate for proper operation.
Edit: Damn edged out of first

Could think about a K and N filter but would be a minor diff if power if at all...... "99% sure but not positive" a 1983-85 GPZ cam is a little hotter and you could pick up a few HP there although check on that first and it may be more work and cost than you want. I went with a open air box header and jetting with my bike for a while and open 29mm smooth bores for a long time and I think the power increase was very minor and more in my head than in actual use.

I had 28's on my bike and a 903 bore. I went to 1015 and had pods. I noticed different problems. Went back to the air box. Got 33's modified the boots to take the larger ones in the air box. I have smooth dependable power rain or shine, windy or not.K&N air filter.
Steve

650ed wrote: Drilling holes in the airbox is a mistake!!! Consider the fact that air is drawn into the airbox through the cap and then passes through the filter on the way to the carbs. If you drill holes in the airbox the air that passes through the holes will not be filtered at all, so dust, grit, etc. will be sucked directly into the cylinders and will accelerate wear. This is a bad thing for sure.

IF (big if) you can find another airbox cap you could experiment with it by either making more holes in it or shortening the tube, but I highly recommend NOT doing that to the cap on your airbox because there' no going back if you find it hurts performance. Frankly, I seriously doubt you will gain any performance on a stock motor by letting more air in regardless of the jetting. The stock airbox was designed to give the engine an appropriate amount of air give the engine's displacement, valve sizes, etc.

Also, keep in mind that your bike came with CV type carbs, and they are VERY sensitive to changes in the airflow since the vacuum inside the airbox affects the opening of the carb slides, etc. You may very well end up losing power at low to mid rpm if you permit greater airflow into the carbs, and let's face it, most street riding is not done at or near redline. Ed

B.S. All depends on the design of the air box. More of the unknowing misleading the uninformed. Here is a link to help folks to understand how to jet cv carbs with individual filters.. Bottom of page 12.

www.dotheton.com/forum/index.php?topic=39120.0


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Not BS. Ed is right about the design of the 650/750 airbox feeding air in through the cap to the inside of the filter. Drilling holes in the box will bypass the filter completely.

I agree that it is possible to rejet CV carbs, but it's true that they are sensitive. Dynojet sells jet kits with a different needle taper which works well. I doubt there is a meaningful amount of power to be gained though by modding the airbox system though.

You can drill one hole in a kz750 air box and still be using stock or k&n air filter.

I would check the spark plugs. install new if necessary to get a better handle on their color. The bike was probably jetted lean from the factory & the aftermarket exhaust may flow more air, so it may be even more lean. If the bike is new to you, their may be basic maintenance & tuneup stuff that needs to be done. maybe new ignition coils & spark plug wires. new correct spark plugs, properly gapped. service the timing advancer. valve clearance check. that sort of thing.

I think the stock 750-4 has about 75 hp. The ZR750 has a more complex airbox, but its hp is about the same. (Zephyrs & ZR-7 air cooled models)

[b]Nebr_Rex[/b] wrote: 650ed wrote: Drilling holes in the airbox is a mistake!!! Consider the fact that air is drawn into the airbox through the cap and then passes through the filter on the way to the carbs. If you drill holes in the airbox the air that passes through the holes will not be filtered at all, so dust, grit, etc. will be sucked directly into the cylinders and will accelerate wear. This is a bad thing for sure.

Nebr_Rex wrote: B.S. All depends on the design of the air box. More of the unknowing misleading the uninformed.
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YOU are the unknowing misleading the uninformed! Before you call BS take a look at the design of the airbox on the GPZ 750. If you drill a hole in the airbox air WILL come in through it and that air WILL NOT pass through the filter. You only need to look at the design of the airbox and you will see that I am correct and you are wrong, so don't be calling BS until you know the facts! Below is a diagram of the 1982 GPZ750 -R1 airbox. Notice the part marked #11012. That is the cap, and that is where the air enters the airbox. Now notice the part marked #11013. That is the air filter element through which the air passes and is filtered. Now notice the parts simply marked "A". That is the airbox. If you drill a hole in it the air passing through that hole will NOT pass through the filer element. Instead, it will be sucked through the boots marked #14073 and will pass directly into and through the carbs and into the combustion chamber, and as I stated dust, grit, etc. will be sucked directly into the cylinders and will accelerate wear. No BS! Ed

If you want to "increase" (I don´t say it will) air flow with stock airbox, you could modified part nº 13091 named "Holder Inlet Pipe" and open it up by cutting a hole. That attaches to a 6mm rubber hose, it is supposed to drain any water/fluid inside the airbox. You could also put a "snorkel" there too (not to disturbe std air flow). That way your filter will be receiving air from top and bottom. I have only seen this mod once, but it was so well made it looked OEM.

Sorry but Ed is right holes any where else is gonna by pass the filter except for where Undiablo says or the snorkel....... Maybe the K and N and a few more holes in the 11012 snorkel...... thinking (but not sure) if you went with the K and N you could get rid of the cage 13091A too....... trying to out guess the designers at Kawasaki might be a loosing battle though.