Engine cutting out from full throttle to idle

[Paul1098]

I've started to use my bike a bit now and really enjoying it but have a persistent issue which I first thought was fuel starvation due to Indian tank not venting but now beginning to wonder if it's someting else.

Bike starts and idles OK.
No issues accelerating all the way thru' the rev range.
Bike pops on a closed throttle but stops if throttle cracked slightly.
First time I noticed the problem was after filling fuel tank and riding it about 3 miles, bike cut out and stopped at side of road, wouldn't start. I removed and refitted petrol cap & bike started straight away and no issues on rest of ride.
I thought i'd fixed it by drilling a vent hole in cap but it leaks fuel so blocked hole for now until I sort out a baffle arrangement on cap.

Not had an issue for a while but recently had the bike up to full throttle for a while doing approx. 60mph ish and as soon as I came off the throttle the bike cut out but started again once I started blipping the throttle. This has now happened a couple of times so wondering if anyone has any ideas?

Bike is 450 R/T on scrambler road gearing.
Electronic ignition (Electrex STK-164)
Original carb.
Pod filter (K&N copy)
Spark arrestor fitted to end of open exhaust.

I've pulled the plug (NGK B7HS) and it looks normal with a light brown insulator & electrode looks OK.
I did wonder if the cone filter was too small and although the flow area is a lot less than standard it should be big enough from the calcs I've done.
The carb stub stack does butt up against a shoulder in the cone filter so air flow will be pretty rubbish going into the carb but not sure it it'd be any worse than the convoluted hose of the original setup?
Debating about replacing the air filter with either a pukka K&N pod filter or standard/scrambler paper filter and housing.
Not sure how much back pressure the spark arrestor is causing but not sure I'd get away with open exhaust on the road even for a short time.

Any ideas?

[Bevel bob]

Sounds exactly like fuel starvasion .Be carefull .you can seize up.

[George]

Sounds exactly like fuel starvasion .Be carefull .you can seize up.

I agree with Bob. First thing to do is sort tank vent. With restricted air flow to carb and open exhaust I would have thought plug would show as rich. It would do no harm to remove filter and see how it runs. If you have a colour tune kit it would pay to check through rev range.
Good luck.

[ranton_rambler]

Is it the black “Bakelite” cap? There is a a moulded rubber anti-splash piece which is available from one of the online specialist suppliers. I find that with one of those fitted the fuel only spits out of the vent hole when the tank is very full.
Ian

[Paul1098]

Sounds exactly like fuel starvasion .Be carefull .you can seize up.

I was thinking it was fuel starvation but surprised by colour of plug. Hadn't thought of it seizing.

[Paul1098]

Sounds exactly like fuel starvasion .Be carefull .you can seize up.

I agree with Bob. First thing to do is sort tank vent. With restricted air flow to carb and open exhaust I would have thought plug would show as rich. It would do no harm to remove filter and see how it runs. If you have a colour tune kit it would pay to check through rev range.
Good luck.

Thanks George you've just reminded me that I have a colourtune kit stashed away from years ago using it on mini's. I'll dust it down and use it.

[Paul1098]

Is it the black “Bakelite” cap? There is a a moulded rubber anti-splash piece which is available from one of the online specialist suppliers. I find that with one of those fitted the fuel only spits out of the vent hole when the tank is very full.
Ian

It's an indian cap that came with the tank, the original cap doesn't fit and not sure a bakelite one would either. I'll have a look to see if I can see an anti splash device online.

[Paul1098]

Tank vent sorted and bike was running fine on last outing of 90 miles.
Bike went into garage with fuel taps off and piston taken to top on compression stroke to stop any fuel leaking in.

New problem of no compression when I went to start it this morning. Expected it to be the decompressor sticking but after cleaning that up still no compression. Fitted a screw and copper washer in place of decompressor but still no compression. Engine can be turned over by hand on kickstart.

Couldn't get compression tester to show any reading but that probably down to cheap tester.
Might have done someting wrong but couldn't get leak down test to work with 40-100psi pressure as it was turning piston over (due to going over tdc?)
Tried leakdown at 20psi and inlet seems OK, lots of noise from exhaust and a little bit from the oil plug hole.
The test was with cold engine as it won't start so I'm wondering how much difference a warm engine would make to rings sealing.

I need to check valve clearances but opening shims can be spun easily by hand so it's not overly tight clearance keeping the exhaust valve open.

Am I missing something before I pull the head as i'd have expected to notice this drop in compression when riding it?
I'm assuming that my riding with a lack of fuel has either burnt ot warped the exhaust valve but I'd have expected to notice that before.

[Duccout]

You may have a lack of clearance in the exhaust valve guide, so that with the extra heat the valve may have seized in the guide..


Cheers,

Colin

[Paul1098]

You may have a lack of clearance in the exhaust valve guide, so that with the extra heat the valve may have seized in the guide..


Cheers,

Colin

Would the valve stay seized in the guide once the engine cooled down?
Does that mean new valve and guide if that's whats happened?