Air cutoff valve (Honda Bros)

[bogan]

One for the super-knowledgeable perhaps!

I've been having mpg problems, and the occasional slight bogging at 100kmhr cruise for a while, the plugs and exhaust are looking pretty rich. 6 months ago I did a full tune up, and went with factory pro's tuning guides, so I'm pretty sure the Main, Needles, float height are all set, I left the pilots stock.

In my research, I decided that the ACVs might be operating when it's at cruise (1/6th and 3800rpm), so I shimmed the ACV springs a few mm to increase preload, which has affected the low end idle and cruise, with better throttle sensitivity but the 1/4 throttle and up is still good. The first test run had a lot of pop and crackle from the exhaust (which the ACV is designed to eliminate) but otherwise great, then I retuned the fuel screws an extra turn out, and went for another quick ride, and it just does a bit of a burble on heavy engine breaking (>4krpm) bot not popping like an overly lean condition.

So a couple of theories I have:
ACV springs have lost some spring, causing it to come on at a lower vac
New exhaust creates reversion pulses that cause extra vac in the intake manifold
This fix seems to address the problem, but I've got the root cause wrong

comments/thoughts?

[ducatilover]

Did you end up trying smaller pilots in it?
It could well be your exhaust, or I've jizzed in the intake (sorry).

To my very limited knowledge on ACOVs, they're only meant to operate during deceleration to enrich the pilot mixture to avoid bitchin' popping on the overrun? If yours is poo, it's probably leaking.

Is it possible that the new exhaust has more back pressure? That'd do it (in my mind)

[Grumph]

Ohhh shit, tuning CV's - where do I start. Firstly, stop dicking round with the acv - it only admits air on closed throttle overrun.

If your problem is a midrange bog or stumble and you've changed the pipe on the Bros then there's your answer. usually on the Bros it's just a tailpipe and mufler change - yes ?

If so the motor will be seeing a different vacuum signal at the carbs and something must change to compensate. Usually - and I've done just a few Bros...the vacuum sensing hole in the bottom of the slides will need enlarging. For some reason pretty much any pipe change reduces the vacuum seen at the carb so to lift the slides as fast as standard the vacuum holes go bigger.
from memory the Bros holes are 2.80mm std - go up in .10mm steps until it transitions smoothly through the range.

The original CV carbs - SU's - had a range of springs available to tune response - nothing's available for these so it's drill I'm afraid.

[bogan]

Didn't try any different pilot sizes, as couldn't find the right sort, and have never heard of anybody going down a size with an exhaust upgrade anyway. Will be less back pressure I'm pretty sure, the stocker exhaust is pretty inhibitive.

The ACV is also doing something at idle, because when I shimmed the ACV springs the fuel screws needed to come out a bit to idle nicely, and it's conceivable that at 3.8k and 1/6th throttle the manifold vac would be a similar amount, so the ACVs might be active at 100k cruise.
Changed to pods, and a muffler, very long mid-pipe. Similar mods have been done pretty often to these bikes. Jet kit recommended a stretch and bake of the springs, so did that, others have had success with this instead of drilling. Also, if it is slow/low lifting slides, wouldn't that cause a lean condition?

It feels better to ride, and the plugs look better, though under heavy and sustained engine breaking there is a bit of crackle and pop, but I think I'll see what mileage I get out of this tank, hopefully back around 18k/L

[Grumph]

if it's slow lifting slides, this shows as a rich condition - the airflow stalls to a certain extent but fuel continues to flow.
It's the same as too low a cutaway on slide carbs - too much fuel, not enough air.

[ducatilover]

Could possibly be the slide springs then?
Want to try some of my spare springs as a comparison?

[bogan]

if it's slow lifting slides, this shows as a rich condition - the airflow stalls to a certain extent but fuel continues to flow.
It's the same as too low a cutaway on slide carbs - too much fuel, not enough air.

That makes sense, for now anyway Next one will be EFI I reckon!

Just did a fuel efficiency ride to balance bridge and back over the pahiatua track, got a whisker under 16k/l which is up from the 14ish I was getting before, but down from the 18 I used to get before all the mods. However it wasn't the most sedate ride, so I reckon it's in the ballpark I'm happy with now. Might drill the holes out one size and drop a main size on the rear next time I have the carbs out unless I make it to 18k/l before then.

cheers for your help

[Brayden]

I got a new exhaust on my VT250 and love that burbling noise, is it bad to have? Sounds great engine breaking around 6-5 rpm

[ducatilover]

I got a new exhaust on my VT250 and love that burbling noise, is it bad to have? Sounds great engine breaking around 6-5 rpm

It means your engine is running lean.
You may be able to put a 0.5mm washer under each carb needle and get away with it

[bogan]

Do VT250s have air cutoff valves?

If you've changed the exhaust to a more free flowing one, it's a good idea to check and possible change the jetting, from top down. Start with the main jet size.

[ducatilover]

Do VT250s have air cutoff valves?

I don't believe mine did, with a new can/no baffles mine was happy wit the needles raised

[Brayden]

I don't believe mine did, with a new can/no baffles mine was happy wit the needles raised

Yeah its just a straight through done by Neptune in Wellington. No baffle sounds great engine braking in second or third gear oh my god worth every penny man. Sexiest thing since Angelina Jolie. Will put a vid up tomorrow been meaning to do it for ages!

Heard re jetting is a bit of a mission so maybe I will pass up, maybe do it in a few months I have a strong mechanical retardation !

I've noticed since I've got the new exhaust when sitting at 110kmh it revs about 2k more higher, I thought my 6th gear was broken for a while haha.

Sorry for hijacking your thread btw

[ducatilover]

If you're up this way, I can jet it for you if you supply the jets. I can't remeber the sizes off my head, somewhere around 98s I think.
It's easy to do when you've done it countless times

They do sound neat though

It shouldn't be revving higher. Standard was 8500 at 100 indicated I think.