BiKenG

Puzzling that despite what JZH's picture seems to show, those TBs on the right will NOT fit into the intake rubbers for the carbs on the left as the TBs are so much bigger. Really doesn't look that way in the photo. Mind you, I was trying it with 6th Gen. TBs. Could they be bigger? But it does explain from where I got my previous optimism about splitting the TBs. The 5th Gen. TBs as shown above are the ones I first thought about splitting and it looks entirely feasible. The 6th Gen. ones, not so, hence my more recent less optimistic, post. Not only are the castings less favourable to splitting, but also the upper intake connection to the airbox is siamesed on the 6th Gen, but separate on the 5th Gen. which is far preferable. No reason to not use 5th Gen. TBs on this 750 project so I just need to find some 5th Gen. TBs to cut. Don't 'spose you want to sell yours John? As far as I know the 6th Gen got 8 point injectors but then upped it to 12 point in 2006. At least the parts list shows different part numbers for the injectors themselves which supports that. However how can one distinguish between TBs with 8 point injectors and the later TBs with 12 point injectors. Obviously counting the injector holes would be tricky, but is there some other way to identify TBs as early or later 6th Gen? One other question to which someone might know the answer - Is it possible to fit the later multipoint injectors into the 5th Gen. TBs? Usually the whole TB is swapped if you want the later injectors, but would they actually fit? I suspect not, but don't have both here to try it. Anyone know the answer?

Taking a good look at the 800's TBs, the way they're cast is not how I remember from previously and separating them won't be as simple as I thought. May be impossible depending on if there's any internal cross passageways I cannot see. Hope not, but anyway I started looking at alternative TBs. First of all I discovered that the 929 FireBlade seems to have 2 pairs, just like all the other Hondas and not the 4 separate units as I have seen stated and as I mentioned in a previous post in response to Mohawk. So whoever wrote that is simply wrong. As Durbahn discovered, the GSX-R750 is a good candidate with 4 individual TBs. But at 45mm intake ID they'll make the rubber matching problem worse. Unfortunately, although the GSX-R600 uses smaller TBs that might fit nicely, they also use ECU controlled secondary butterflies and that's just not gonna work with the VFR. So nothing's falling into place at the moment:-( Just have to keep looking.

Fallen at the first hurdle unfortunately. It would appear the 800's throttle bodies are significantly larger than the 4th Gen carbs :-( There's no way that I can see the injection TB fitting into a 4th Gen intake rubber. I pushed hard but it's not even close. Also the internal shape is wrong as it partially obscures the actual injector. The 800's intake rubbers are shaped around the injector to not obscure the fuel pulses, but of course the RC36 intake rubbers don't have that. I also tried to fit the 800's intake rubbers onto the 4th Gen intakes. It's closer, but not ideal. They slip on very easily of course as they're basically too big and they don't grip the intakes at all. There's a mm or more of movement side to side - until you tighten the clamp screw. It is actually possible to squeeze the rubber onto the intake stubs. They're then quite firm although it is possible to twist them around if you grip them well and try hard enough. The stubs seem longer than the 800's as the rubbers are not snug up against the head. There's a gap. However, that wouldn't matter if the rubber can form a good tight seal onto the stubs. I think a better type of screw clip would have a better chance of getting a good seal. But I'm not sure of the long term implications of the rubber being compressed like that and whether any possible good seal would last. So, 4th Gen intake rubbers are a non starter (damn, just wasted £25 on them) but 6th Gen rubbers are a possibility, although not ideal. So the search is still on... Do any of the pervious carb models have a larger intake/carb diameter than the 4th Gen? That might help.

What's the difference in the intakes of the VFR versions, i.e. the Gen 3 to 6? I'm pondering the possible fit of throttle bodies and intake rubbers. Have 800 TBs varied in size over the years? I know the injectors have changed a couple of times, but what about the intake sizes, have they altered at all? Likewise, did the carb size change between the 3rd and the 4th Gen bikes? I have some 800 TBs (late 5th Gen and a 6th Gen) and will soon have some 4th Gen intake rubbers to compare to. But I've no obvious way of knowing if there were any differences from the items I have, without buying one of everything of course. I suspect all the 800 have the same size intakes and likewise the RC36 models, but can anyone confirm or refute this?

I'm not convinced they would be simpler. I'd have more to do figuring out how to assemble them in the correct rigid layout. Besides, I really think the larger choke diameters would not work in my favour and require far more of a change to fuelling from a basic 800. Probably a better option would be to use those from a CBR900RRY (929). They are similarly separate units, 40mm and also from Honda :-) At the end of the day, I'll probably use whatever best suits the RC36's inlet tracts and rubbers.

First step will be to find out if the 800's throttle body will fit the 750's intake rubbers. But I don't have the 750 engine to hand at the moment so it'll be a few days till I can do that. Another thought. The RC36 intakes changed size did they not, from 3rd to 4th Gen? Are the rubbers also different?

That's what Durbahn used, but he no doubt wanted power so the GSX-R's 45mm throttle bodies were appropriate. I always try to use Honda designated parts if possible and I have a spare set of 800 throttle bodies, so that's a big incentive to make use of those. One reason to NOT use those is that they are cast together in pairs which means trying to change their relative spacing would appear to be harder. But, on the other hand, the cast 'web' between each choke in a pair could be cut and then used as the mount on to which could then be bolted something to space them as required. Since these webs look kinda flat, that might open up the way to use some simple carbon fibre sheet cut to size which would be stiff enough and nice and light. Then something else between the spaced out pairs to suit the V and intake angle of the RC36 motor. A big plus is that with the 800's throttle bodies, injectors, ECU, sensors and wiring, it ought to run. A PC can then be used to clean up the ignition and fuelling. Simples But still just tossing ideas around in my head at the moment.

If the throttle bodies are close enough to the carb's diameter, it ought to be possible to just use the RC36 rubbers. That part number is I think wrong, but if you can let me know the correct part and I can look into getting one for you in exchange for the loan of the carbs in order to get it to first run.

Ah, so clearly the carbs are a larger bore than the injectors. Hmm. Durbahn used GSX-R 750 injectors which are larger and may therefore be better. But, I am more interested in mid range torque and ride-ability than outright power, so the smaller injectors might be a better bet. They flow enough for acceptable power on the 800, so really, they should be enough on this 750. Re-joining the bodies for the different spacing and angle will be one problem, but the intake rubbers will be another. Durbahn had some specially made. That'll be something to consider at a later date. In fact, if I use the throttle bodies with the same OD as the carbs, I could use the 750's original intake rubbers. You know the next question John. Could you possibly measure those ODs? How well would a throttle body fit in the 750 intake rubber? BTW, this is not a short term project. I also have several others I'm working on and this is more a case of as and when I get time. So don't expect rapid progress.

The dash is really simple and the ECU doesn't care about whether it's connected or not. I've not decided on dash for this bike yet, but won't be OEM VFR. I'm building this from a separate engine and frame (that I already have), so I'll just buy the bits I need rather than butcher any existing bike. The FI system requires a CranK Position sensor and a CaM Position sensor. The former will be very accurate to determine the exact position of the crank. The latter will be far less so, but is required to establish on which part of the stroke the engine is on as that cannot be determined from the CKP sensor output which is used to provide the ignition timing. The ECU can simply control the timing of the spark relative to the CKP sensor output, but also knows when it's TDC and when it's top of the exhaust stroke from the CMP sensor output and in this way there's no need for any wasted spark. What I'm not sure about is how it decides when to open the injectors. That's less critical than ignition timing and could use the less accurate CMP sensor output, but I think it's more likely that the CMP sensor is only used to determine the stroke and it's the CKP sensor output that is used to accurately determine when to spark and/or squirt. However, to a certain extent it's irrelevant. There will need to be CMP sensor and I'll need to figure out how to add that. Durbahn's site will help there as he's done it for the RC30 and a 400. In any case, assuming the ignition pulse of the 750 can be used as the CKP sensor output the CMP sensor is the only one that needs to be added to engine. I was thinking of using an 800 fuel tank, which has an internal pump of course. Might drop straight on. Anyone tried this? I'm not imagining any of this will be simply swap and fit from other bikes. I WILL have to manufacture parts for the throttle bodies assembly and the cam will need welding etc, but I have the equipment. There are various ECU options if I don't go with the 800's. Durbahn used Motec and could provide ready made working maps to start from, but there are others, although probably have to start from scratch creating the maps. I still think the 800 ECU and a PC would be the simplest.

I thought about and looked into that, but it wouldn't be trivial. Cut the S/A pivot boss off the back of the 800 engine for starters and then most of the mounting points don't line up so quite a lot of reworking the frame to make it fit. Even then you'd have to mess with the airbox, so no advantage there. But also, I like the idea of the 750 engine. Nicely symmetrical compared to the 800 and I think a fun project to inject it. Don't get me wrong about the 800 engine. I do like that also and have other projects going on involving a couple of those, so in some way, I also wanted this to be a 750 to differentiate it.

Hey Keef, I've been telling him that for years now.

I'm thinking about injecting an RC36.2 and hoped we could have a good technical discussion about it here. First of all:- I realise that the RC46 throttle bodies won't just drop on. Durbahn has injected an RC30 and a 400. However I believe he used the Motec system. I have also read (here?) of someone using MegaSquirt on a V4. But... It seems to me that the RC46 injection is way closer to what is required than either Motec or MegaSquirt and MUST be cheaper than either of those. So I'm pondering the possibility of modifying the RC46 throttle bodies to fit the RC36 and use the RC46 ECU, with a Dynojet PC or Rapid Bike module to 'tune' the ECU's delivery to suit the different motor, although I'm thinking that it probably would actually run without either. Not well enough to use, no doubt, but it would sure help development if you have something that runs. The throttle bodies will take a lot of modification, but that surely must be easier than trying to manufacture something from scratch. They'll need to be split apart, mounted in the correct relative position and suitable linkages and fuel pipes made. Tricky, but as I said, has to be easier than starting from scratch and there would be a readily available wiring harness too - RC46. Another very tricky task would be to add a Cam sensor probably requiring accurately positioning and welding something to a cam. However, maybe the existing output from the RC36's Ignition sensor could be used as the CranK Position sensor. Everything else is external and can be simply added to the bike, plugged into the RC46 wiring harness which would have all the right connectors. I notice that the 5th Gen FI system uses an atmospheric pressure (BARO) sensor, but that is no longer used on later VFR's, nor any other bikes I briefly checked. But if the ECU needs it and the wiring harness has the connection (yes to both), then the BARO sensor can be plugged in and used. So there's 3 major issues I see here. 1. Modifying the RC46 throttle bodies to suit the RC36 intakes. 2. Creating a Cam Position Sensor 3. Using a Power Commander to modify the ECU's control sufficiently to suit this engine. I'm pretty sure 1. can be done. 2. can definitely be done, but I'm less sure about 3. I see no actual reason why not, but it would largely depend on the range of adjustment afforded by the add-on management module (PC or other). Please don't ask WHY? No point arguing about that. But apart from that, what do others think about this?

Very kind of you John, but I've found a new axle at decent price, so I'll not have to trouble you for one of yours, thanks anyway. Still need the suspension link though. Meanwhile, arranging to mount the wheel in the lathe and then also on the mill is making my head hurt.

Thanks for that. The RC axle bolt is about 206 long which explains why it looks too long in the NC axle. So in fact it IS too long. Rats :-(

I have determined that opening up the centre bore of the wheel to fit over the full length of axle is the way to go and I'm just working on a way to mount the wheel in the lathe in order to accomplish this. However, the relative lengths of axle have thrown up a problem. If I insert an RC bolt into the NC axle, mount an RC wheel and a spacer, it is clear that the axle is nearly an inch too long. So either the RCs' wheel is much thinner, or their bolt is quite a bit longer and I suspect the latter. Anyone got an NC axle bolt they could measure?

I believe the NC axle will work in the RC36 S/A and I suspect the lack of cush drive is the reason for it being shorter. But you apparently do need to machine the axle slightly to shift it slightly left in order to align the wheel correctly. Mohawk is the expert on this and has it all figured out for when he did it on his bike. In order to use the RC45 conical spacer, the axle needs to extend through the wheel as the spacer centres the wheel to that end of the axle. If I cut the axle shorter to allow the wheel to fit, the conical spacer won't be able to engage on it. The alternative is to enlarge the bore through the wheel so the axle fits through. But my current feeling is to cut down the axle and just use the 400 flat spacer. Of course ti could all change tomorrow.

Yes I have an RC45, but apart from it being tucked away at the moment and hard to get to, I'm not sure I want go that route. Specifically I think it's because I have a couple of 400 spacers and hate the thought of not using one of those and buying something else instead. Unless someone else found they had an RC45 one available... In fact, the raised ridge on the inside of the 400 spacer is an almost perfect fit into the outside of the RC36.2 wheel. Although the wheel has a tapered centre hole like the RC45. the 400 spacer is a snug fit. I reckon with the paint removed it wouldn't even need any machining for the 400 spacer that would grab the wheel nicely when the nut was tightened. So that's probably the route I'll take. I do need to machine out the inside end of the wheel centre bore (currently 50mm) to fit over the axle boss (needs 51mm) however and getting the wheel mounted that way around in the lathe will be interesting. I did measure the axle and the PCDs are as follows:- Drive pins = 85mm Wheel studs = 100mm Disc mount bolts = 110mm I guess using the different PCDs makes sense for OEM, but if they were the same it would make this job a lot simpler.

Excellent info John (JZH) thanks. My concern at the moment is fitting an RC36.2 wheel to the NC30 axle. As you mentioned above, the axle is too large to fit through the wheel. So it's modify the wheel or shorten the axle. Since I'll need to modify the wheel anyway, I'll probably do both. I will need to turn out the wheel's centre bore on the mount side to be a snug fit over that boss and also turn the outer edge of the bore so that the 400 'not cone' spacer fits nicely or I could use an RC45 cone spacer, if I had one. But I don't. I have a couple of 400 type spacers though so that's probably the way to go. Unless anyone has an RC45 cone spacer going spare? Ideally wanting to swap for a 400 one. As has been mentioned before, I will need to plug the original bolt holes in the outer surface of the wheel, but I could then make some large drive pins that would locate into the wheel's bolt holes in the inner mount surface and modify the axle flange to fit these pins at the 100mm PCD, as John has done above with actual wheel studs, although I'll mount my special drive pins instead. It's a pity Honda didn't use 100mm PCD for the drive pins. That would have been just soooo convenient. Hmm, what's the disc mount hole PCD? If that was 100mm... Looking at the pictures it looks slightly larger (damn), but I need to measure it to be sure. That would also be extremely convenient. Next problem is how to mount the wheel in my lathe and that's assuming it's even big enough. Time to find out.

I'm trying to put together a single nut fixing using Honda's central axle bolt. I have an RC45 axle bolt and it appears to fit into the NC30 axle although the splines don't exactly slide in easily and will need some tapping to go fully home (they're totally dry having only just been de-greased). I'm guessing that's normal. However, the bolt then protrudes about 2" from the end of the axle. Should it be this long? Means the wheel and washers have to be much thicker than I would have expected. I've got nothing to which I can compare it. Can anyone recall how much the bolt sticks out of the axle or has a 400 axle they can measure for length? How is a 400 wheel centred? There's no RC45 style cone spacer to centralise it on the axle, so what does? Is the idea that it is a snug fit over the axle and that's sufficient. The fancy spacer under the nut then would be just a big washer. Or does that spacer somehow help even though it's not going to be as good as the RC45's cone spacer. Or does the wheel sit snug on the boss at the centre of the flange, with the drive pins just there to stop it rotating? The RC45 uses its cone spacer for this, but I just cannot see quite how the 400 achieves the same centralisation and I don't have all the 400 parts to be able to work it out. Anyone familiar with the 400s?