Vacuum operated petcock problem

[AllanB]

After discovering a diaphragm problem on my NV400 petcock, resulting in a high rpm misfire, I have decided to turn it into a manual type by removing the pressure spring. This will allow the flow to the carbs when the petcock is either in "normal" or "reserve" and if the float chamber valves work all will be well. To be on the safe side I will need to remember to set the petcock to "off" when I leave the bike. Plus, of course, I will blank of the vacuum connection on the engine and resolve the misfire problems caused by the air leak.

Now to me this seems a simple solution, until such time that I can source a new diaphragm, if indeed I can, but does anyone know of a reason to avoid doing this, as of course petrol is involved?

Or is there a spray available to repair the perforated diaphragm?

[Wong!]

I had a similarly crappy petcock on my old VF750. See if you can find a rebuild kit on ebay or through an specialist forum. If not, you'll be ok using a standard petcock.

[Gsxrnz]

As I understand it the vacuum hose only allows the fuel to flow if it has venturi pressure that moves the diaphragm. Your solution will work, and assuming the float levels are OK and your bowl gaskets are sound, then even leaving the fuel turned on should (in theory) not allow the fuel to flow anywhere. However, best to turn off when stationery just in case.

Rebuild kits are cheap enough on ebay as somebody has suggested. But have a look at the diaphragm when you pull it apart - maybe you can figure out a way to fix it.

How did you manage to figure out it was the diaphragm allowing air (and/or fuel) into your carby via the hose?

[AllanB]

We had already cleaned, checked and set up the carbs, fitting new diaphragms so we knew all was well there. We then had two problems, a high rpm misfire, which meant an air leak, or fuel starvation (or possibly an HT fault) and a serious hot starting problem, possible a hot running cut out, which could only be down to fuel.

We hadn't checked the petcock, so that was first on the list. Upon stripping it, I found an almost total block on the vacuum pipe within the petcock body, the cause of the fuel starvation, which I cleared, but when I sucked on the vacuum pipe, fuel flowed normally, but I couldn't hold the the vacuum with my tongue.

On reflection perhaps I should have tried it prior to removing the spring, as I couldn't see and diaphragm perforation and the misfire could have been fuel starvation rather than a air leak. So I may put the spring back and run it.

Will take a look on Ebay, if the guy in Khon Kaen doesn't have one, but I have read these systems are problematic, but with no fuel pump on the bike (which I like), kind of necessary.

[Garry]

I had a GPZ1000RX many years ago with a similar problem. I knew it was the vacuum side, because as soon as I selected reserve, the fuel started flowing again with the engine running. Selected back and fuel speed flowing.

[Garry]

What I was alluding to, is that 'reserve' is normally designed to bypass the vacuum side of the petcock

[thaicbr]

your idea will work.. long term fix it..

http://www.nrp-carbs.co.uk/shop/index.php?route=product/manufacturer&manufacturer_id=22&sort=pd.name&order=DESC&page=23

[thaicbr]

your idea will work.. long term fix it..

http://www.nrp-carbs.co.uk/shop/index.php?route=product/manufacturer&manufacturer_id=22&sort=pd.name&order=DESC&page=23

[AllanB]

What I was alluding to, is that 'reserve' is normally designed to bypass the vacuum side of the petcock

I don't think mine works that way, but will check, pretty sure that the output port to the carbs is the one vacuum actuated.

A video I saw of a Yamaha Virago worked that way, but that had a fuel pump.

[Garry]

I had a GPZ1000RX many years ago with a similar problem. I knew it was the vacuum side, because as soon as I selected reserve, the fuel started flowing again ●without● the engine running. Selected back and fuel ●stopped● flowing.

Correction in my post above That will teach me to swipe type with my Android without paying close attention

[Garry]

What I was alluding to, is that 'reserve' is normally designed to bypass the vacuum side of the petcock

I don't think mine works that way, but will check, pretty sure that the output port to the carbs is the one vacuum actuated.

 

A video I saw of a Yamaha Virago worked that way, but that had a fuel pump.

Let me know what you find out, as we know not all bikes are created equal.

[AllanB]

What I was alluding to, is that 'reserve' is normally designed to bypass the vacuum side of the petcock

I don't think mine works that way, but will check, pretty sure that the output port to the carbs is the one vacuum actuated.

A video I saw of a Yamaha Virago worked that way, but that had a fuel pump.

Let me know what you find out, as we know not all bikes are created equal.

Yes, they are both connected to the same port, as with the Virago, so both N and R are vacuum operated.

I think it has fixed high rpm misfire, but the hot start is still a bitch, just had it stop at the lights and would only start after a 5 minute cool down. I am assuming a vapour lock of some kind, but would welcome any other suggestions.

After I got it running and arrived back home, I noticed as I pulled up the idle was around 1000rpm and after 5 seconds it dropped to 700, so it sounds fuel related.

[Gsxrnz]

What I was alluding to, is that 'reserve' is normally designed to bypass the vacuum side of the petcock

I don't think mine works that way, but will check, pretty sure that the output port to the carbs is the one vacuum actuated.

A video I saw of a Yamaha Virago worked that way, but that had a fuel pump.

Let me know what you find out, as we know not all bikes are created equal.

Yes, they are both connected to the same port, as with the Virago, so both N and R are vacuum operated.

I think it has fixed high rpm misfire, but the hot start is still a bitch, just had it stop at the lights and would only start after a 5 minute cool down. I am assuming a vapour lock of some kind, but would welcome any other suggestions.

After I got it running and arrived back home, I noticed as I pulled up the idle was around 1000rpm and after 5 seconds it dropped to 700, so it sounds fuel related.

The idling drop suggests that the carbs are possibly sucking air somewhere, or that the mixture is too lean and the butterflies (or sliders) at idle are too far open to compensate for insufficient fuel being delivered. The return to idle speed is therefore delayed and causes the overrun that you describe.

That could also be the cause of the problems hot starting - definitely fuel/air related.

Did you do a full strip down of the carbs? Incorrect float levels can also be a contributing factor to poor starting and mixture setting accuracy.

[AllanB]

Yes, we did a proper job on the carbs and corrected quite a few faults, completely blocked jets, partially blocked jets, a hole in one diaphragm, one float chamber possibly empty. Though a thorough test was not carried out, only a simple mouth blow test. There is no over-run though GXSRNZ, I meant while the engine is running the idle rpm drops from 1000 to 700 when warm, when the ignition is switched off everything dies immediately, as it should and according to plug condition, the mixture is perfect.

It seems the more things we find and fix the worse it runs, after a thrash the high rpm misfire is still there, perhaps worse than before as is the stalling at the lights and refusal to start. Though last night, after a thrash, I didn't notice that my mate had switched to "reserve" and I ran out of petrol at the lights in the busy city centre, though with the hot start problem, it took me while to realise that.

But she starts without a problem and within the limited rev range, goes like stink, with no smoke of any kind.

So I am now thinking "electrical"....So how to test?

....One thing I did do that was none standard, was to make my own air filter out of the old one, due to unavailability. I removed the paper element and against the remaining steel mesh, wrapped a thin foam sponge. 10 baht from a hardware store. I assume I am getting sufficient air-flow...clutching at straws here, I think, but this engine does run to 10,000rpm and produces 43bhp at 9,500?

Edited October 30, 2013 by AllanB

[AllanB]

So we left the fuel system and starting looking at the electrics, easy to check without a DVM with a twin spark system and found a faulty coil, upon it's removal found a split casing. Bought a s/h one from a Steed and expected all to be well, but if anything it was worse....<deleted>.

It was pretty noising with the straight through silencer and to avoid annoying the neighbours and wife revving the <deleted> of it, I repacked the MOT passed baffle box and took it for a run. This made a big difference and the misfire is gone I think and...I think.... the hot start problem too.

What I do notice is that the exhaust gets hotter and it still sounds a little crude, so now I plan on "playing around" with both baffle boxes to see If I can refine things with a bit of trial and error. So any tips would be appreciated, as exhaust technology looks a little complicated. I think I need a "dead end" chamber to kill the pulses, without causing the resistance which results in the heat gain. Sorry if that sounds naive.

I think straight through exhaust systems can cause a lot of problems and even damage engines, especially small ones.

[AllanB]

I think there is an inherent problem with this model and it this.

My old NV400 was designed to produce 43hp and that is a lot from 400cc, the similar engined Steed is only 30hp, my mate's Yam 650 just 40hp and even his 1100 a mere 65hp, meaning that there is some delicate engineering I am dealing with. This would also extend to the exhaust system and my clumsy efforts to build a baffle box need a bit more sophistication.

So I left with a good deal of trial and error, or getting one from Japan, I think some serious research is necessary, so any tips would be useful to get her running sweet.

This could explain why production only lasted 3 years............

Edited November 2, 2013 by AllanB

[VocalNeal]

Allan,

I take it the original exhaust is broken or has fallen apart? How much do you have left to work with?

Not relevant to any solutions but is it a 400 custom (Chopper) or a 400 SP ?

Edited November 2, 2013 by VocalNeal

[AllanB]

I bought the bike with a straight through empty baffle silencer which failed the Thai MOT on noise, plus it ticked over at about 2000rpm, so not a good staring point.

In order to get it through the MOT and get the green book done, they told me to take it to an exhaust shop who put in a simple 3 stage baffle packed with wire wool for 500 baht and they also dropped the rpm to 700ish.

So two incorrect silencers, what I need is somewhere in between the two, in terms of back pressure.

I have been searching for simple design information and haven't found anything remotely useful.

[KRS1]

to increase back pressure take the tube out and put flaps in it, imagine a toilet paper roll when you poke your finger into it, a small flap will bend down.

you can also pinch parts of the tube, towards the front will have an effect on low end, towards the rear will have an effect on top end.

pinching techniques are also different, you can make the entire length of the tube look like a " + " or just different sections, with various pinch depths.

if its a solid tube thats been drilled, put more holes in it.

on some bikes with air injection systems (emissions), when you change the exhaust it can create misfiring, the usual thing to do is block it off. Hot start problem can also come from a hot/faulty CDI.

Edited November 2, 2013 by KRS1

[Garry]

to increase back pressure take the tube out and put flaps in it, imagine a toilet paper roll when you poke your finger into it, a small flap will bend down.

 

you can also pinch parts of the tube, towards the front will have an effect on low end, towards the rear will have an effect on top end.

 

pinching techniques are also different, you can make the entire length of the tube look like a " + " or just different sections, with various pinch depths.

 

if its a solid tube thats been drilled, put more holes in it.

 

on some bikes with air injection systems (emissions), when you change the exhaust it can create misfiring, the usual thing to do is block it off. Hot start problem can also come from a hot/faulty CDI.

You haven't worked on Webers by any chance? Air jets, emulsion tubes & fuel jets.

[VocalNeal]

You use the word silencer so I assume it is a chopper style 2-into-one. My thinking is a Supertrapp. But the next question is where from. But I do like the bent tab idea to increase back pressure.

Similar to this idea

Doesn't have to be so complicated but...

The question is are you doing it yourself or having other do it. Having others do it will get time consuming and maybe frustrating.

The SR400 crowd in Bangkok seem to favour the Supertrapp approach so they must be here somewhere. Supertrapp is adjustable.

[KRS1]

vocalneal...man i have always wanted to try making something like that exhaust flap.

Garry, nope sorry Rochester Quadrajet is about as close as ive come...and thats not even close.

[AllanB]

Adjustable back pressure, great idea, but with my limited metalworking facilities a bit difficult, so I am busy drilling holes to lower the silencer resistance.

I have two baffles to work with, so...

,

Baffle box A. I started by removing an upstream end cover plate, held on with a couple of tack welds, revealing 8off 9mm holes and that helped a lot in terms of cooling the exhaust pipes, which were showing signs of discolouration, but I think there is a little more to go.

Baffle box B. This other one is a "straight through", so I plan on working from the other end on that, by increasing the back pressure by converting it to none-straight through.

Somewhere along the line I hope to achieve the optimum, as both are or will be easily removable and the Khon Kaen ring road is not too far away for testing,

A friend of mine has a Steed with straight through exhausts, but has fish tail ends, that sounds great and runs well too, but although the engines are similar, there must be some differences as mine is designed to produce 43hp and his 30hp. So I assume mine is more sensitive to these sort of things.

Still haven't found another NV400 around and there must be a few.

[VocalNeal]

Baffle box B. This other one is a "straight through", so I plan on working from the other end on that, by increasing the back pressure by converting it to none-straight through.

I have read that some scooter perform worse if the dB killer is removed from the silencer/muffler. So on your straight through one the simplest way would be to reduce the outlet size until you get what you want. For testing just insert some thin wall pipe.

Another thing that just occurred to me is that a lot of PCX 's here have an exhaust the size of which would not be out of place on a 1000 bike let alone 125 or 150.

So a trip to a local accessory shop might yield results for a 400! About 7000 baht down to a few thousand.

If you find one with a db Killer you can play with that. Some dB killers are simple some are complex, the ultimate design would be up to you.

[AllanB]

Baffle box B. This other one is a "straight through", so I plan on working from the other end on that, by increasing the back pressure by converting it to none-straight through.

I have read that some scooter perform worse if the dB killer is removed from the silencer/muffler. So on your straight through one the simplest way would be to reduce the outlet size until you get what you want. For testing just insert some thin wall pipe.

Another thing that just occurred to me is that a lot of PCX 's here have an exhaust the size of which would not be out of place on a 1000 bike let alone 125 or 150.

So a trip to a local accessory shop might yield results for a 400! About 7000 baht down to a few thousand.

If you find one with a db Killer you can play with that. Some dB killers are simple some are complex, the ultimate design would be up to you.

Nice idea, but that blows the budget for me, this is Cafe Racer mentality, ton-up for no money. I already splashed out 27baht for a drill bit.

Sorry, I didn't mention I am a tight arse.

[AllanB]

Something interesting I found out yesterday, friend of mine bought two lots of exhaust gasket rings (engine to exhaust) for his Steed and gave the second set to me, when I found mine were missing too. That is 3 Honda V2/400 bikes all without exhaust gaskets.

Anyway it has made quite a difference in the running, especially with the slow running, which is now steady. It surprised me, as I didn't suspect a leak there at all..