Exhausts & carbs

[tweaker]

My NTV650-P has the stubby exhaust. Does anyone know what the longer exhaust does for the bike (apart from add weight!!).
From my previous life, I expect the longer exhaust results in more mid range torque - but you guys tell me .

ON the carbs front, I noticed that in the 650-P the size of the front cylinder main jet differs from that in the rear carb - does anyone know what honda were playing at with this arrangement, and would I be better changing the jets to be equal front and back? - Was the difference in front and back supposed to add "character" (for which read "vibration"?

Tweaker - in hope of wisdom

[hovis]

Hello mate,
from my limited experience,the exhaust makes little difference appart from noise,these bikes are bread and butter bikes and do not respond well to tuning,
Your right about your main jets,although i think from memory a couple of the models went with same size main jets on both cylinders(i may be wrong,and often am !!!!!!),why they did this i don't know,i would guess probably due to emissions ??,as i dont think it altered BHP ,again i may be wrong !!!!!!!!!!!!,sure someone will put me right !!!!!!!!!


Hovis

[dpmiller]

I believe the exhaust length change was for no reason other than styling. Main jet sizing might be because of the odd firing pattern, or it could be purely to balance cooling loads, dunno. I can tell you tho that my '97 NTV carburates very nicely on the aftermarket Micron 2-1...

[jonsan]

The cooling seems a likely reason, provided the rear cylinder has the larger jet size, slighlty more fuel as a gas bound "coolant" to the rear (hotter?) running cylinder. I've heard of this before from friends who work at Honda R&D Swindon, and some in other Japanese car companies!

Does the "long" system have the silencer/collector box under the bike the same as short stubby silencer system? If not then the "effective" system length may be the same.

QUOTE "from my limited experience, the exhaust makes little difference appart from noise, these bikes are bread and butter bikes and do not respond well to tuning"

I read somewhere, that gas flow is restricted on the inlet side by port/valve size and camshaft timing therefore a "sporty" exhaust system on a std bike has no positive effect apart from the sound.

Also remember reading that fitting Africa twin 750 barrels and pistons without any mods can result in no gain!

easier to buy a VTR 1000.....tehehe

[dpmiller]

Yes, you need the Africa Twin's cams too, AFAIK. A friend of a friend had (has?) a race-prepped Bross w/ the AT conversion. Evil nasty hairy thing it was...

Both standard pipes have the expansion box underneath.

[hovis]

Don't think the jets are anything to do with cooling as latter models as i thought ran same size main jets,still think it was to do with emmissions and exhaust design,longer system on latter bikes,although as jonsan says exhausts maybe of equal length with stubby earlier one with silencer/collector box under engine which blows my theory away !!!!!!!!!!!!!!!!!!!!!

Hovis

[tweaker]

This carb jet sizes thing baffles me ..

Honda had different size main jets in the 600-J and K models and then put the same jets front and back on the 600-M model.
When the 650 came along (650-P), they reverted to a larger jet for the front cylinder carb with a smaller jet for the rear cylinder carb. On the 650-S/T/V models they fitted equal size jets for front and back.
The DV uses same sized jets front and back.

Aside from the fact that they couldn't make up their minds what to do I thought they might be loading the rear pot less because it would get hotter being at the back.

Unless, as I alluded to at the top, the jet sizes were unequal to add character (i.e. vibration) and they decided that bikers would actually prefer fewer vibes, ... or unless they just got things wrong with the unequal sized jets (unlikely?), ... the only reason I can think for them moving over to equal jets is so that the possibility of having the carbs on the wrong way round is eliminated - i.e. the unequal jet arrangement is the ideal one, but having equal jets makes the manufacturing easier / more foolproof.

- course, I'd like that to be the case since I have a 650-P with the unequal jets , but I was interested to know if the "character" had any truth in it. I know you shouldn't believe everything you read in a Haynes manual, but my idea that the unequal carbs were there for "character" was sparked by the text of the Haynes manual introduction which says the designers went to a lot of trouble not to eliminate vibration completely ... (3rd paragraph of the "The Honda NTVs" section.

Exhausts - If the exhaust length makes no difference I'm keeping the stubby! - shows off the pro-arm, makes a nice noise, and there isn't a weld seam in the polished section. Yeah - it's an acquired taste - thought it looked daft first time I saw it, but now (what with mass centralisation being all the rage ..) it's strangely trendy!!

- Tweaker

[ntvnick]

In the same section of the Haynes manual, were they say that "Honda went to great trouble not to eliminate vibration completely" they also state that the NTV is a 90 degree V Twin.
I reckon that the same design ethos must have been applied to my two other bikes an XBR500 and a Laverda Jota. The XBR does have a balance shaft but at sustained speed the family jewels can go to sleep, on the other hand the 180 degree crank in the Laverda isn't that bad. The NTV by comparison is very smooth.

On the subject of silencers, could it be down to changing noise regs? If you look at the Deauville, it has the longest silencer of the lot.

[hovis]

Right,been looking at this main jet thing,have been looking at Transalp sites and noticed those guys DO change the main jets to give better throttle response and a more linear torque curve,don't know if it would work on the NTV ,cant see why not as its the same engine,so maybe Tweaker it's worth a go ??,you can report back to us !!!!!!!!!!

Hovis

[derbyshirebill]

Tweaker- I can't imagine that anyone changes jet sizes to affect engine vibration. The only things that usually make them use different ones are if the induction tracts are different shapes (they might be slightly different on the NTV, I don't know) or as a crude way of regulating cylinder temperatures (not unknown on in-line fours to have richer jets on the inside pair).
Vibration levels on a properly running engine are a function of piston, con rod, crank and flywheel arrangements. Honda went to alot of trouble with the VT and NTV to make a narrow angle V twin with staggered crank pins. I bet the computer scratched its head. Although smoother than a 45 degree twin, I bet that the vibe levels on Aunties are about the best they could do, and the press release about including some vibration to add 'character' was just a piece of marketing guff. The old VT500 was a tad smoother and revved more but, as I remember from old Brit vertical twins, a 500 of the same design will always have alot less vibes than the 650 version. Perhaps Honda got the wind up when they bored out the 500 and found it shook a bit.
However, to put it in proportion, I had a go on a Yamaha MT01 a bit back. I could not believe that anyone would make an engine that vibrated as badly . Aunty Vera felt like a GoldWing after that.
As far as exhausts are concerned, I have today discovered the first pinhole rot in my 'stubby'. I'll keep it going with alloy and pop rivets for a while no doubt, but if anyone's got a Motad they don't want. sling it my way

[tweaker]

Hovis - Don't think I'll be getting in that far for a while! In a few weeks time i'll probably be checking/ setting valve clearances, checking/re-gapping or renewing plugs and doing a carb-balance job.

Hovis/Bill - doubt we'll get to the bottom of why Honda started off with different sized jets on both the Revere and the NTV650 and on both models changed to equal sized jets later in the production run - mysteries!

Thanks for all you comments.

- Tweaker

[honda]

The front exaust has a reasonable flow passage from the head but the rear is restricted. A bit of playing on a dyno will mostly result in differant jet sizes. In other words the front cyl produces slightly more power than the rear due to it breathing more easly. Other V twins with one poor exhaust route also suffer the same problem. One can only presume Honda designed a bike for a purpose and as always there were compromises. It is a compact design which suits average size people down to the short. (and that's not me, I have a higher seat and thurn lower peg's to stop me knee's seizing) It would not have been easy to locate all the components if the rear cyl pipe were to go further back with a large radious bend or come down behind the axis of the swing arm. H

[everydaybiker]

Good answer Mr Honda! (Schociro?)

[tweaker]

Thanks for that Honda - I agree with everyday biker - good answer - seems credible to me anyway - think you might have hit it on the head!!

- Tweaker