NC35 - Cutting out when hot - Help please

[subzero]

Afternoon all.

I was hoping the collective brain trust of this forum might be able to shed some light on the faults im having with the bike. Ive read up on a few other peoples issues of a similar nature, but it think ive tackled most ideas. Please let me know what you think.

The bike is a 1996 Honda RVF 400 of unknown mileage. Speedo doesn’t work, but it reads 50k miles.

The bike cuts out when hot and is then extremely difficult to restart. If it does restart you don’t get very far before it cuts out again.


THE STORY SO FAR:

The first time I rode the bike was when we bought it in October. I was told the bike was a stolen recovered (which also showed up on the HPI check) and had sat for at least 6 months.

It had new tyres, new braided brake lines and the fairings had been re-sprayed by House of Colour. There was of course only one key, not much in the way of paperwork, but the bike looked straight and was in our price bracket. It also had a year’s MOT.

The seller appeared to be a genuine bike guy (enthusiast) with large professional premises, a huge selection of stock and a large workshop out the back with all manner of 2-wheeled machinery in various states of repair.

It rode fine (this was my first experience of an RVF400), but I noticed a hesitancy in top gear as you reached 4-5000 rpm. I was worried I was running out of fuel as I had been told before I set off that it was low, but when I pulled over to check I could see the tank still had a good litre or more present. My return journey showed the same fault.

Back at the garage a look over the bike showed a weeping brake banjo, a slight oil leak, a few missing clips and other minor issues of that nature. A quick chat with the bloke and we both agreed there was nothing really wrong with it, but it needed a good run and a little bit of tlc as it had been put back together in a bit of a rush. He knocked a couple of hundred quid off, we shook hands and I took it away on my trailer.

The girlfriends first ride the next day and with a full tank of fuel she rode approx. 25-30 miles before it cut out for the first time. The symptoms she described sounded very much like it had run out of fuel. She assured me it was full up, but tried filling it again and under instruction from me, rode it home and revved the sh*t out of it (she has only just passed her test and i was thinking she was being a bit timid which might not help). Possibly dirt in the fuel, gummed up carbs or plugs were my first thoughts.

Once home I took it out for a ride myself (she had cut out a couple more times, but had eventually had a decent run of 15+ miles). I didn’t get very far at all. Twiddling the fuel tap seemed to restart it, but this could have been because I was sat at the side of the road on a cold day for a few minutes and it cooled down. Each time I got it going, I would get maybe a mile or 2 before it cut out again. The worst thing was it cut out even on wide open throttle at speed and even a dip of the clutch couldn’t bump it in to life.

I stripped the bike back and have carried out a fair bit of work on it. Thanks to a delay in parts, Xmas and weather, this last week was the first time she got to take it back out again. I had previously tested the bike for around 10 miles and had not even the hint of a failure. The girlfriend went out and once again with a full tank of fuel and only 15 or so miles from home it cut out when on wide open throttle. Unfortunately this time she killed the battery trying to restart it and had to be recovered.

So I’m at a bit of a loss as to what to do next.


FAULT

Engine cuts out when hot.

Feels like it has no fuel, even though the tank is full.

Wont restart straight away, but wait a couple of minutes and it fires up.

When it cuts out on open throttle, you cannot bump start the bike when rolling.


WORK SO FAR

New oil and filter.

New air filter.

New plugs.

New coolant.

Replaced one worn plug lead.

Replaced engine to tank vacuum line.

New airbox (2nd hand).

Gave carb a general clean (removed from the bike first), but did not strip down.

Adjusted all throttle cables.

Drained and cleaned fuel tank.


OTHER POINTS OF NOTE

Electrical harness appears to be a bit of a mess.

The thermostat on the radiator has nothing plugged in to it.

I checked the fuel tap operation and with a good vacuum it works fine.



WHAT NEXT

My first thoughts are to strip the carbs and clean them properly with new gaskets and set all the float heights etc. That can then be ruled out.

My main worry is that it is something electrical and I don’t have the luxury of another RVF 400 owner nearby with another bike, which works so I can swap things across.


Can anyone offer any ideas? Something glaringly obvious which I’ve missed? Does the thermostat control anything other than the fan?


Many thanks,

Robin

[CMSMJ1]

Running rich and I suspected a dodgy vacuum line, but you noted that as replaced?

Sure that the tap has a working vacuum function?? Filters in tank blocked?

[arsey30]

Does fuel flow full bore from tap with vacuum applied.
I had one that worked, but just a fast trickle that lowered the float bowl levels.

If it cuts out, then not likely carbs, plugs, coil, leads, as all 2 or all 4 won't fail at once.
Does the engine spin over fast on the starter, [rectifier/charging], if so then check for spark at a plug when
it cuts out to prove ignition or fuel.
If wiring or connectors are bad vibration may break the LT circuit
CDI controls 4 cylinders, but unlikely to fail when hot, more common to be fuel supply
Tank breather pipe, tap diaphragm, filter, tap drillings.

Check tap outlet flow rate and spark at plug.

[vfrman]

Does it have the fan on the lower radiator? That thermoswitch that isn't hooked up trips the fan, and if it isn't being tripped than your bike is more than likely overheating and shutting down. I used to have an air cooled Yamaha Seca 600 that did this to me. You could also be running rich, as CMSMJ1 pointed out. Did you happen to check the float heights, main jet sizes and pilot screw settings?

Another thing to look at is the tank breather....

I would invest in a Haynes manual if you don't already have one.

One last word.... welcome to the fourm!

[speedy231278]

Could be the float bowls emptying. Also check to make sure the tank to carbs fuel hose isn't kinked.

[subzero]

Tap drillings? What do you mean by this?

Fuel flow from the tap seemed strong to me, but i did not measure the rate. What should it be roughly?

Tank filter, now this seems a good lead. Is this located above the fuel tap? Remove the tap and there it is? A simple gauze or a paper type? I have the tank off so i will be checking this asap.

I have not taken the carb apart. Just checked operation, gave it a general clean and refitted and justed throttle cables and idle.

Vacuum line is new.


Appreciate the quick response guys, thanks very much.

Robin

[arsey30]

Fuel flow should be full bore, if so then filter is OK, it fits in the top of the tap.
Drillings are just the fuel feed holes through the tap body, if flow is slow remove tap covers and diaphragm then poke a stiff wire from each end, then compressed air both ways.

Do check for spark first to eliminate ignition.

[speedy231278]

Tank filter, now this seems a good lead. Is this located above the fuel tap? Remove the tap and there it is? A simple gauze or a paper type? I have the tank off so i will be checking this asap.

Yes, it's moulded into the top of the pipes inside the tank that feed the tap. It's a bit like the stuff they use for the filter in kettle spouts if they're too pikey to use a wire one.

[vfrman]

Yeah but if the bike is cutting out with the tank full, I am betting the tank breather is clogged, not the tap filter.

[subzero]

Apologies for the delayed response. As usual there is always something else which needs doing (in this case a mk1 escort and a GSXR 750).

I took the tap apart, gave it a good clean, replaced all the seals and diaphragm and put it back together. I also cleaned the filter and made sure the breather holes are clear.

I haven't run it yet, but there was certainly nothing obvious amongst those parts that screamed FAULT!!

What i did note was after sucking on the vacuum line in to the tap and checking the fuel rushed out, if i opened the filler cap there was a slight pressure release. Is this right? Surely the breathers should eliminate this?

I cant remember if this was before or after i tried to clean the breather out though. I ended up squirting brake cleaner down the holes and it flowed with no issue. I'll check this again in the morning.

Many thanks again for your help guys.