Wont exceed 4,500 rpm then stalls and wont restart - on route to MOT

[totty]

Hi,

Having a bit of an MOT disaster today - It started bogging at around 3,500rpm on route and wouldn't go any higher unless I was very gentle on the throttle then refused to pass 4,500rpm. When I stopped outside it was holding revs, I turned it off while some space was cleared, it then started but ran really flat and wouldn't pull up the hill, stalled and wouldn't restart.
I'm thinking that it possibly overheated, but not sure if the cause is ignition or fuel related (vacuum leak maybe), or it could even be oil flow as I'm yet to trace why the oil pressure light doesn't work.

They're going to leave it to cool for a couple of hours before trying to start it for an MOT and I'm arranging a van to take it home  

Any tips would be great, I've got a strobe so the ignition should be easy to test.

Cheers

Allan

[UK Pete]

Sounds fuel related, i had a problem like this on one of my other bikes, try all the obvious things in fuel system starting with checking enough fuel is coming from tank, check the filter if you have one, mine turned out to be a blocked breather tube in the fuel tank so the petrol only dribbled out.
Pete

[totty]

The good news is it not only started for the MOT but passed and got me home 

Do you know what the flow rate should be or should I just look for a steady stream?

I've sprayed a bit of carb cleaner around to confirm that it's not a vacuum leak. I noticed the symptoms after riding just about 50 yards on the way home so I don't think it could be a symptom of overheating.

[F2 Paul 876]

Hi
Make shaw the fuel cap breather hole is clear and not blocked over time this gets dirt in it and if you use a tank bag it can bloke it off. all so i had once a badly fitted air box rubber this let air in and stopped it reeving and was just like yours was, refitted it correctly and was instance 100% better.
Paul

[totty]

Which bit do you mean by "badly fitted air box rubber" ?
The rubbers on the carb to inlet side are quite hard but spraying carb cleaner around then doesn't affect the tickover so they aren't leaking. I can't get my head around how the carb to aribox or even the join from the airbox to the filter housing could have an effect but I'll definitely check them.
I've also just remembered that the fuel level is quite low as I only put 5L in it, but I did run it with the tap set to reserve.

Cheers

Allan

[totty]

The filler cap breather seemed to be blocked as I couldn't blow through it, but I need to wait of the tax disk arriving in the post before I can test ride it.

Cheers

Allan

[Bryanj]

What exhaust you got now, too much wadding will give exactly that symptom----I have a wide open Marvin on one that needed "Quieting" every year and it only ever got 1/2 way back from MOT

[totty]

It's a Motad. I've got no idea how it should sound so I don't know if it's been messed with.

[florence]

when my bike goes onto reserve it takes ages for the fuel to come through and it stops flowing a long way before it is empty.  I always keep my tank as full as possible to avoid this problem.  If your tank has been low, maybe you have picked up some debris from the bottom of the tank which has blocked jets in you carbs.

If your bike is overheating that is a different matter.  How hot do you mean?  You mentioned that you have no oil warning light. I would advise to find out the cause of that before riding it any further as you may cause damage if oil isn't getting around properly. 

If your bike has electric start, take out the spark plugs to make it spin over easily on the button, remove some tappet covers and run the engine over on the starter motor and see if oil comes up.

[Yoshi823]

Many moons ago when I first started to ride I was out with some friends for the day when the bike started to give all sorts of revving problems & apparent overheating. It ran like a 8-stroke instead of a 4-stroke. I too took the valve/tappet cap off to check if oil was getting around, but the overheating was really cooking the lubricant. I was new to biking so I didn't know what to do next, so I started at the basics & checked the points gap. It had slipped & opened up the gap quite a bit. After regapping it went like a rocket...as far as a Honda 125s could in those days, but a lot better than previously. But I had learnt a lesson to check this sort of thing more regularly, especially as it was my only form of getting to Navy college in those days.

[totty]

The tank had 2L of old petrol in it when I bought it, that was replaced by 5L of fresh before riding it. I've removed the tap and found that the plastic strainer is missing which further increases the chance of dirt getting in. (any idea where I can get one of these at a decent price?)

I don't really have anything to gauge the temp against, overheating was assumed as it felt/smelt very hot and started easily once it cooled down. "8 stroking" would describe how it ran for the final few seconds before it stalled and refused to restart.

On the way home the bogging still occurred in but felt less harsh, more like a restriction. It idled fine and restarted easily when home.

So far for the oil pressure light all I've tested is the bulb and it was not the cause. I'll check for oil reaching the top end as advised and then look at the wiring. I also intend to drop the sump and clean the strainer.

The points had just been replaced so slippage may well be the cause, I've got the cover off and the tools next to it but work has been stopping me from getting any further with it.

Cheers

Allan

[totty]

Oil is reaching the top end, although I've got no idea of how much oil to expect.
The static timing is still spot on so the points can't of slipped.
I couldn't do much else by torch light.

Any idea how your supposed to get the front lower bolt out of the sprocket/pump cover to get to the oil pressure switch? There isn't enough clearance around it to get a spanner or socket on it. I've also found that the gear selector seal is leaking so I'll do that while I'm on.

[florence]

On mine there are screws not bolts holding the cover on.  Sometimes people put a bolt in if the screw is lost/broken.  However, if one does, it needs a washer so that it protrudes enough to get a socket on.

If oil is getting up to the top I am sure it is fine, and if the timing is right, I would investigate the dirty petrol theory.  I have in-line filters on mine.  They are really cheap to buy and keep all the grime from my tank away from the carbs.  Before I installed filters I was stripping the carbs all the time which is a really boring job and they are a pain to get on and off.  

Is your bike a 550 or a 500?

[totty]

The cover on mine has 2 allen key bolts and two normal hex headed bolts. I'm going to try and buy a box spanner or/or thinwall socket at lunchtime to shift the problem one, if they don't fit I might have to drill the head off.

If it's 6mm hose then think I have a spare filter lying around that I could use. I'd like to also replace the strainer in the tank to try and keep the grime out of the tap.
I certainly don't want to frequently remove the carbs, I had a right battle with them to get them back on last time.

The road tax arrived today so at least testing it wont be a problem  

It's a 550.

Cheers

Allan

EDIT - The bolt is now out, had to grind a box spanner down to paper thin walls.

[totty]

I've just had it out for a test ride and it still wont rev  

The fuel flow looked good when I removed the pipe. I'm still running on reserve though as I don't want to fill the tank until I've got a new strainer for the tap.
No fuel filter is fitted (yet).
Cap breather is clear.
Carb rubbers look fine.
I forgot to check the exhaust so I'll have to go back to that.
Points gap and static timing are both spot on.
I'm yet to try the strobe, maybe it's not advancing???
The oil warning light switch is faulty, doesn't earth even when not running. Oil did reach the head when tested.

After adding another 5L of petrol I can now coax it up to 7,000rpm. I can also now clearly feel that anything over 1/4 throttle causes the bogging, it looks like I'm going to have to take the carbs off to clean the mains and emulsion tubes and set the float heights.