Jetting RC36

[93 VFR750 RC36]

I've finished rebuilding my RC36 (thread: http://www.vfrdiscussion.com/forum/index.php/topic/61955-rebuilding-my-93-rc36/page__p__726428__fromsearch__1#entry726428).

Having got the bike back together, I got it down to GP Performance to do a dyno run and get it MOT'd. Unfortunately, some cracks in a front disc foiled the MOT, but it just attracts an advisory with the baffle out (great news!).

However, the dyno is a different story. It topped out at 97.1hp gearbox ISO (103.9 fwhp and 95.2rwhp) at 10691rpm. So, on the face of it, good news (stock rating is 95.1hp Gearbox ISO for the UK bike). However, it's running at almost 14.5:1 fuel to air ratio to about 11k rpm indicating a very lean condition. It also has a big hole from 7900 to 8400rpm where an audible intake note change is acompanied by a drop in power.

The power curve is:

5000 - 42.3hp

6000 - 52.9hp

7000 - 65.9hp

8000 - 75.9hp

9000 - 89.2hp

10000 -94.8hp

10691 -97.1hp

11000 -96.8hp

The airbox mod involves the front of the airbox (where the stock snorkle fits) being completely opened up and just protected from debris etc by a mesh screen (see below). I'm running Factory Pro 132 main jets and at position 3 on 84j needles.

In the short term, I'm going to put on a stock airbox lid with the snorkle removed to see if that brings it closer as it should help it from going really lean on the top end. Factory have two setups on their sheet with a similar setup (stock airbox with snorkle removed) and one runs 132 mains and the other 135. They both run the 84j/3 on the needle. As the one running the race can is on 135 mains, I've picked up a set of 135s that I'll drop in when I pull the tank off.

In the longer term, I'm kinda keen to get the modded airbox running well. Factory lists one setup with an airbox with 5x 1.5" holes drilled in the front (which would give a similar intake area to my modded airbox). This setup is running 138 mains and 80j/3 needle.

Before I dive into the bike again, does anyone have any experiences with de-snorkled and/or modded airboxes on the 750? I'd be curious to hear your thoughts, good or bad.

Stew

Edited September 5, 2010 by 93 VFR750 RC36

[SEBSPEED]

I'm interested too.

[93 VFR750 RC36]

This is where I admit to having been an idiot/too lazy or a bit of both.

I reviewed the Factory Pro sample installations and realised I should have paid more attention to start with.

Basically, the snorkel on a RC36 gives you 21.5 sq.cm intake area. Removing the snorkel increases this to 31.7 sq.cm, 147% of stock.

Three setups with a stock airbox (snorkel removed) are listed. The exhausts are all different, but the two with Two Bros systems are closest.

One, with a street muffler, was set up with a 132 main and 84j/3 needle. The one with the race can had a 135 main with 84j/3 needle. Both were running stock filters.

Of the four setups, one was more modifed. It had 5x 1.5" holes drilled in the front of the airbox and ran a filter with the screen removed (similar flow to a foam filter). This setup gives a 88.7 sq.cm intake area, 413% of stock. When tuned, it used a 138 main and 80j/3 needle (80j is richer than 84j I think).

When I measured up my modifed airbox, I have 120 sq.cm intake area, minus a bit for the mesh, I'm still at 500+% of stock. I guess starting out with a 132 main jet was probably a bit idiotic.

Anyway, I'm going to drop in a stock airbox with the snorkel removed and 135 main jets. I'll then head back to the dyno and see where I'm at. If I end up rich, I'll start with a couple of 1" holes either side of the snorkel hole and then work up from there. Using holes rather than opening up the stock intake hole allows the internal profile of the airbox to be maintained, with better control of any resonance issues (possibly causing my hole at 8k, as it sounds like resonance in the airbox).

I'll keep you posted.

Stew

Edited August 21, 2010 by 93 VFR750 RC36

[93 VFR750 RC36]

Here's the new airbox lid sat on the old one. The snorkel hole has been radiused all around and the two holes I drilled in the corners are 20mm and the edges radiused. Total inlet area is 38sq.cm against 21.5sq.cm for the snorkel on it's own.

I've got the carbs set up with 135 mains and Factory 84j/3 needles.

I had the bike on the dyno today and here are the results. (Gearbox ISO figures)

Rpm First Airbox Current Airbox Gain

5000 43.3hp 42.9hp -0.4

6000 52.9hp 54.7hp +1.8

7000 65.9hp 67.2hp +1.3

8000 75.9hp 84.0hp +8.1

9000 89.2hp 93.5hp +4.3

10000 94.8hp 101.5hp +6.7

10691 97.1hp 101.6hp (10670rpm) +4.5

11000 96.8hp Throttle rolled off

I'll try to scan in the graphs when I can as they tell the story even better. The big hole at 8200 I mentioned actually saw the hp drop to 73.4hp for the first airbox. The gain at 8200 is a full 12hp between the two curves. It's now running a hair rich, but it's better to be rich than lean. I'll try to adjust the airbox a bit to dial it in a bit closer but it's not bad now and it pulls hard so I don't want to overshoot and mess it up.

On the road, the bike pulls like a train, has great induction roar and the exhaust note is beautiful. Hope to be at Mallory Park on the 23rd for a bit of track time. That should give me a chance to really get a real world (well real track) feel for this new setup.

Now all I need is a 837cc big bore kit, ported heads with 800 valves and a bunch more time :biggrin: .

However, in an effort to ensure my wife doesn't break my helmet over my head, that'll have to wait.

Stew

[VFRSuperfreak]

Here's the new airbox lid sat on the old one. /edit

On the road, the bike pulls like a train, has great induction roar and the exhaust note is beautiful. Hope to be at Mallory Park on the 23rd for a bit of track time. That should give me a chance to really get a real world (well real track) feel for this new setup.

Stew

Fantastic!. Hope after all that it isn't too much trouble to bring a camera and post video to get a taste of audiopr0n!?

[samaki]

I modded the airbox lid on my 3rd gen and had a similar mid range stumble. I ended up returning the airbox to stock by installing an unmolested lid and the stumble went away. On my 4th gen it came with the snorkle removed. With a K&N filter, stubby Delkevic exhaust and pilot circuit richened 1/2 turn out it also had a stumble around 6000. I reinstalled the snorkle and performance improved. There is a lot more to an airbox function than the volume of air it will flow.

[rangemaster]

"I'll try to adjust the airbox a bit to dial it in a bit closer but it's not bad now and it pulls hard so I don't want to overshoot and mess it up."

I'm very interested in what you finally settle on for airbox mods and jetting. I've got a '97 w/ a Staintune and ride at 5000 ft. + in elevation.

[93 VFR750 RC36]

I'll see if I can get my wife's video cam mounted on the bike. Not sure how I'll manage it but it can't be beyond the whit of man to fabricate something that fits to my wingrack mounting kit.

Samaki,

Any significant change to intake or exhaust flow will require some tuning adjustment. The stock carbs run 130 mains. Factory Pro list de-snorkeled airbox setups that use 132 or 135 mains depending on what other mods have been made. All use the 84j needle on setting 3.

I'd go for at least a 132 main + 84j/3 needle with a de-snorkel at sea level. I've gone beyond de-snorkeling with the additional holes so am running 135. I could probably get away with a 132 with this setup, but I'm looking at further fettling of the airbox so would rather be a bit rich to start with. My plan is to add a pair of 15mm holes between the 20mm ones above the main inlet and then re-dyno. If that goes too far, I'll step back with some speed tape.

The hole you had mid-range was probably a function of the main jet being too small. You can probably get the mid range close by shimming the stock needle but without getting the main right, you'll just end up chasing your tail. Unfortunately, changing the main jet involves pulling and re-fitting the carbs, something that takes me a couple of hours, but that's the price of running a V-4.

Rangemaster,

As you're up in less dense air, you've got the equivalent of a rich running condition already in a stock setup (as air density reduces, a lean condition improves and a rich condition gets worse).

With your staintune and a de-snorkeled airbox, I'd get the bike on the dyno with stock jetting and see how it runs. You might be OK. Bear in mind that my RC36 runs the 36mm carbs and yours 34mm so my settings will differ from yours. If it turns out to be a bit lean, you have a couple of options. If you get a Factory Pro kit, you'll get main jets a size above and below stock and a set of adjustable needles (start on setting 3 if using the Factory needle). The other option is to buy a set of genuine Factory or Keihin jets and shim the stock needles.

If you're a bit lean at the top end, go one size up on the main and test again. If it's good, then have a look at the mid-range. If it's lean, shim or go up a setting on the needle and try again. If you're rich, back off a bit.

If you email Factory and give them your local conditions and setup, they can tailor the kit they send you to suit your needs.

Stew

Edited September 5, 2010 by 93 VFR750 RC36

[93 VFR750 RC36]

I've also been in a PM discussion with another VFRD member and am looking into a baseplate setup that uses the complete underside of the tank as the airbox. This may add a small amount to my current setup but would really come into it's own if I go for an over-bore.

Also, a word about tuning kits. My local tuner has all but given up on Dynojet kits. The needles require an irreversible mod to the slide in the carbs and the jet profiling is different to stock Keihin jets.

Factory Pro jets are interchangeable with Keihin jets and their internal cuts/angles are the same. This means the flow charactoristics from low to high flow react the same as stock jets. Dynojet mains don't react the same so if you go for Dynojet, you're then stuck with buying only their mains as you make adjustments.

Grant's dyno testing has shown that Factory Kits are generally easier to fine tune than DJ ones as the stock pattern jets work better. This is due to a phenomenon that contradicts a lot of carb theory but occurs non-the-less. The main jets don't just affect the top end, but also have a significant effect in the transition from the low-speed jets to the needle (around 4-5000rpm ish I think he said). Stock pattern jets are predictable in this but DJ ones aren't. With DJ mains, getting the top end right results in a compromise in that transition or vice-versa. Fine for a bike that is ridden hard and in the upper rev band all the time (track bike) but not so good for us sports-touring types.

With Factory or Keihin jets, you don't end up with this problem and I can now roll on full throttle at 3500rpm in 5th and all I get is a hard pull all the way up. For my normal road riding (moderate to heavy traffic around southern Oxfordshire) that smooth pull is priceless as I can be a gear high or low and it just pulls anyway.

In case anyone is curious, all this testing is being done on a Fuchs BEI251 dyno. It outputs in Gearbox ISO hp (primary) as well as fwhp and rwhp. The dyno curves include all three when printed off. I'm really happy with 97.5hp at the rear wheel in my old 750 and it feels like a new bike.

Stew

Edited September 5, 2010 by 93 VFR750 RC36

[rangemaster]

I'll see if I can get my wife's video cam mounted on the bike. Not sure how I'll manage it but it can't be beyond the whit of man to fabricate something that fits to my wingrack mounting kit.

Samaki,

Any significant change to intake or exhaust flow will require some tuning adjustment. The stock carbs run 130 mains. Factory Pro list de-snorkeled airbox setups that use 132 or 135 mains depending on what other mods have been made. All use the 84j needle on setting 3.

I'd go for at least a 132 main + 84j/3 needle with a de-snorkel at sea level. I've gone beyond de-snorkeling with the additional holes so am running 135. I could probably get away with a 132 with this setup, but I'm looking at further fettling of the airbox so would rather be a bit rich to start with. My plan is to add a pair of 15mm holes between the 20mm ones above the main inlet and then re-dyno. If that goes too far, I'll step back with some speed tape.

The hole you had mid-range was probably a function of the main jet being too small. You can probably get the mid range close by shimming the stock needle but without getting the main right, you'll just end up chasing your tail. Unfortunately, changing the main jet involves pulling and re-fitting the carbs, something that takes me a couple of hours, but that's the price of running a V-4.

Rangemaster,

As you're up in less dense air, you've got the equivalent of a rich running condition already in a stock setup (as air density reduces, a lean condition improves and a rich condition gets worse).

With your staintune and a de-snorkeled airbox, I'd get the bike on the dyno with stock jetting and see how it runs. You might be OK. Bear in mind that my RC36 runs the 36mm carbs and yours 34mm so my settings will differ from yours. If it turns out to be a bit lean, you have a couple of options. If you get a Factory Pro kit, you'll get main jets a size above and below stock and a set of adjustable needles (start on setting 3 if using the Factory needle). The other option is to buy a set of genuine Factory or Keihin jets and shim the stock needles.

If you're a bit lean at the top end, go one size up on the main and test again. If it's good, then have a look at the mid-range. If it's lean, shim or go up a setting on the needle and try again. If you're rich, back off a bit.

If you email Factory and give them your local conditions and setup, they can tailor the kit they send you to suit your needs.

Stew

Thank you Stew for the info. My air box and filter is stock and I'm running the big bore baffle in my Staintune. You are right on the factory jetting being rich for my altitude- with my current exhaust the plugs look great and no popping on decel.

Great thread.

rm

[VFRSuperfreak]

/edit -There is a lot more to an airbox function than the volume of air it will flow.

Yea, part of it is the "Helmholtz resonance" -Standing waves created by the cycling of air going in then valves shutting off. There has to be an airbox+ducts big enough to compensate for this so that these high-then-low standing waves don't starve the intake. -link; see the section "Volumetric efficiency"

[Guest Salty42]

Also, a word about tuning kits. My local tuner has all but given up on Dynojet kits. The needles require an irreversible mod to the slide in the carbs and the jet profiling is different to stock Keihin jets.

Factory Pro jets are interchangeable with Keihin jets and their internal cuts/angles are the same. This means the flow charactoristics from low to high flow react the same as stock jets. Dynojet mains don't react the same so if you go for Dynojet, you're then stuck with buying only their mains as you make adjustments.

Grant's dyno testing has shown that Factory Kits are generally easier to fine tune than DJ ones as the stock pattern jets work better. This is due to a phenomenon that contradicts a lot of carb theory but occurs non-the-less. The main jets don't just affect the top end, but also have a significant effect in the transition from the low-speed jets to the needle (around 4-5000rpm ish I think he said). Stock pattern jets are predictable in this but DJ ones aren't. With DJ mains, getting the top end right results in a compromise in that transition or vice-versa. Fine for a bike that is ridden hard and in the upper rev band all the time (track bike) but not so good for us sports-touring types.

Do you know how the carb slide is modified when using the DJ jet kits?? I think my carbs had a dj kit installed when I got them, and I have been bothered by a funny condition in that rev range...

any help is greatly appreciated.

[gll429]

Do you know how the carb slide is modified when using the DJ jet kits?? I think my carbs had a dj kit installed when I got them, and I have been bothered by a funny condition in that rev range...

any help is greatly appreciated.

the "breather hole" at the bottom of the slide is enlarged slightly.. making it more sensitive to pressure change.. ergo sliding faster..

remove the top of the airbox and filter.. see if all the slides move smoothly.. if they do. start the bike and see if they move together.. if they dont.. you may have a ripped diaphram or one thats not seated properly.

[gll429]

Do you know how the carb slide is modified when using the DJ jet kits?? I think my carbs had a dj kit installed when I got them, and I have been bothered by a funny condition in that rev range...

any help is greatly appreciated.

the "breather hole" at the bottom of the slide is enlarged slightly.. making it more sensitive to pressure change.. ergo, sliding faster..

remove the top of the airbox and filter.. see if all the slides move smoothly.. if they do. start the bike and see if they move together.. if they dont.. you may have a ripped diaphram or one thats not seated properly.

[Guest Salty42]

Do you know how the carb slide is modified when using the DJ jet kits?? I think my carbs had a dj kit installed when I got them, and I have been bothered by a funny condition in that rev range...

any help is greatly appreciated.

the "breather hole" at the bottom of the slide is enlarged slightly.. making it more sensitive to pressure change.. ergo, sliding faster..

remove the top of the airbox and filter.. see if all the slides move smoothly.. if they do. start the bike and see if they move together.. if they dont.. you may have a ripped diaphram or one thats not seated properly.

Do you know what size the hole is stock vs. after modification? I'd like to be able to confirm what I've got.

The diaphragms are installed correctly, that is not my issue. I currently am running a stock setup with no problems whatsoever. However, with the jet kit and full exhaust installed, something is not quite right. I posted a better description on another thread: http://www.vfrdiscussion.com/forum/index.php/topic/63517-cbr900rr-forks-on-3rd-gen/page__view__findpost__p__756681

[Guest Salty42]

Anyone? Anyone?

Bueller? ..... Bueller? ....

[douglasthecook]

Ok, so with an aftermarket air filter such as K&N, or a UNI Filter, a full aftermarket exhaust (Headers, and cannister) and a jet kit, what is the approximate horsepower gain over stock? I have all 3, just haven't installed them yet, as I'm looking at other mods aswell. I'm guessing 5-8 horsepower over stock, but again, that's just a guess.

[SEBSPEED]

Ok, so with an aftermarket air filter such as K&N, or a UNI Filter, a full aftermarket exhaust (Headers, and cannister) and a jet kit, what is the approximate horsepower gain over stock? I have all 3, just haven't installed them yet, as I'm looking at other mods aswell. I'm guessing 5-8 horsepower over stock, but again, that's just a guess.

:lurk:

[douglasthecook]

Ok, so with an aftermarket air filter such as K&N, or a UNI Filter, a full aftermarket exhaust (Headers, and cannister) and a jet kit, what is the approximate horsepower gain over stock? I have all 3, just haven't installed them yet, as I'm looking at other mods aswell. I'm guessing 5-8 horsepower over stock, but again, that's just a guess.

:lurk:

You know what I'm looking for, and what's in the works...