Fuel starvation then high revs.

[Watfordwhite]

For all of you not at Scarborough. Just rebuilt my S2, complete ground up rebuild. Runs and starts fine. After running about 20 minutes it seems to suffer fuel starvation then if you pull the clutch in it revs really high like the choke is on full?
Turn off the ignition and restart and all is good again until the next time. The fuel tank was cleaned and there is an in line fuel filter. What's going wrong?
Rapido 225 with MB shorty reed and MB Clubman. 28mm Delorto carb with Ramair filter.

[MickYork]

"it revs really high like the choke is on full?"

The choke on full usually bogs the engine and rev's drop ?

From your other description i'm thinking an air leak ? this would cause the engine to over rev.....worth doing a leak test.
The air leak might only be apparent when the engine is warm (something expanding ?).

Alternatively it could be fuel shortage. Have you checked the flow rate from tank ? the petrol tank breather hole might also be a culprit....

[Wack]

I'd throw the inline filter away for a start,no matter what they say they restrict the flow.

[Watfordwhite]

Yes, it only happens when warm. But as soon as you stop the engine, it will restart 1st kick and run well again until the next time. Rode for 4 hours today and it happened 3 times.
I will remove the in line and clean the carb out tomorrow. If it was an air leak surly it would be difficult to restart again?
Will let you know what happens. Thanks for your replays guys.

[Warkton Tornado No.1]

For all of you not at Scarborough. Just rebuilt my S2, complete ground up rebuild. Runs and starts fine. After running about 20 minutes it seems to suffer fuel starvation then if you pull the clutch in it revs really high like the choke is on full?
Turn off the ignition and restart and all is good again until the next time. The fuel tank was cleaned and there is an in line fuel filter. What's going wrong?
Rapido 225 with MB shorty reed and MB Clubman. 28mm Delorto carb with Ramair filter.

It could be a sticking float needle, cra*p/sediment in the float bowl causing intermitent blockage or even a blocked fuel tank breather...

[col68]

Have you checked that your jets are clear could be simple blocked idle jet would do that at idle . Have you got good flow on fuel have you got the right fuel jet in to small will not let flow as fast on my phbh had to go up to a 350 jet. col

[Watfordwhite]

Just to clarity, the fuel starvation only seems to happen in 4th, mid/ low revs. It Boggs down to a crawl. Then if you pull the clutch the tick over revs start to climb very high.. As this is still a relatively new engine I am still taking things easy, so no excessive speed done at the moment..
Having read the replays, I'm now thinking, maybe it's not fuel starvation but could be excessive fuel/ flooding. So will check the float bowl and clean out the entire carb today. I have just replaced the rubber mount on the reed valuve due to a split.
If the reeds were damaged, what would the symptoms be? ( never had a reed engine before).

Starts 1st sometimes 2nd kick, runs great apart from this.

[CHRIS in MARGATE]

Ditch the filter, it also eliminates that from the available evidence. If you've ever turned the fuel off and let it stall that way you'll have noticed that the engine always attempts to rev on a bit more before dying. It's the similar thing. Personally and I might well be wrong, I agree with all the other knowledgeable posters in that it sounds very much like fuel starvation and not an excess of fuel. Check fuel circuit starting with breather in cap, fuel tap flow, discard in line filter, check float assembly and finally blow out all jets.
What colour is spark plug ?
Buy a mountain bike inner tube from Asda 4.99, cut it in half opposite valve and mount on inlet and exhaust and pump to 7 psi and watch and listen and brush everywhere with soapy water.
Finally good luck.

[Watfordwhite]

So completely striped the carb, cleaned and removed all jets and blow through. Replaced in line filter. I only managed to go for a 10 mile run but all seems well. Will check the plug colour tomorrow.
Lots of s**t in the carb, so hopefully this has cured the problem. Will let you know tomorrow after a good run.

[Watfordwhite]

Right , so found the problem... The new fast flow tap with low fuel warning light is not so fast flow after all. It won't even keep the in line fuel filter full. Hence why it was spluttering about every 20 minuets.
Can anybody recommend a good quality fast flow tap with indicator light please?
Why is everything you buy for a Lambretta so shit? And expensive....
I will also change the fuel line as this was only 6mm bore and looks a little restrictive.

[St George]

As you said on an earlier thread. " Fighting you all the way" Keep at it mate. Mine will soon be ready for MOT, it is flying otherwise thanks to Scootopia bits.

[Fast n Furious]

I've had these so called fast fuel taps as well.
I dismantle them, drill the holes out in the main block and make a new rubber seal from E10 petrol friendly sheet rubber. Then they flow properly.

[Wack]

I've repaired a few taps recently where the rubber seal had crumbled and blocked the drillings so bought some 2 mm viton sheet from eBay for a couple of quid and made new seals. Hopefully they will last longer.

[Watfordwhite]

Thanks Fast. I may try that with the old tap as I have ordered a Camlam one. Hopefully cure the problem.

Colin, good luck with the MOT mate.

Apart for a few teething problems it runs like a dream. The plug is a slightly dark chocolate. Starts first time (almost) every time. Joy to ride. Surprised how comfortable the saddles are. Looking forward to a longer ride out.

[Watfordwhite]

So, quick up date. Still not right. Haven't had time to play recently so in the garage today. It's been sitting for a while. Fuel filter empty? Turned on fuel tap filled ok. Started second kick and left on tick over for about 10 minutes. All of a sudden it started to splutter and died. Fuel starvation. Fuel filter full. Checked plug. Black and wet.. Normally mid/dark brown.
Will not restart.
So far replaced tap, in line and fuel pipe. Blown out jets and cleaned carb with carb cleaner.
So thinking, could this be a problem with the float bowl? As the filter was empty when returned to the scoot. Messing with carbs is not my thing.
This is a Rapido 225 with MB Shorty reed. What symptoms do a faulty or broken reed give you?
How do I check the float. Some of the forums on here talk about a float needle set at different levels. What does this do and where should mine be?
Does anybody have a carb breakdown exploded view to show what bits are what? Delorto 28PHPB
Any other ideas on what to check?

[Storkfoot]

As has already been suggested twice, get rid of fuel filter. You have a filter in a PHBH carb anyway. If you have a problem with excessive muck coming from your tank then you need to address that too.

Have you properly checked for an air leak?

Is the carb correctly jetted?

[Warkton Tornado No.1]

You have a problem with the tank, that much is obvious.

I would recommend you obtain a pack of paper ‘paint’ type filters that fit in a funnel. You can leave one permanently in your funnel(s) & re-use them. I leave them permanently in place.....

I would also recommend you drain your tank entirely, remove the tap, strip it & clean it then reassemble in the tank with just a fibre/nylon washer to seal but no in-tank filter.

Run your (filtered) fuel through again after sloshing the fuel about within the tank (neighbours are often bemused by Lambretta owners shaking their bikes about....)

Do it as many times as you think will loosen the cr@p within the tank, preferably with neat fuel. (You can add two-stroke oil later to any such filtered, neat fuel & re-use)

If you do all of this before finally removing, stripping/cleaning & refitting the tank tap c/w tank filter, you stand a fighting chance of clearing the loose debris within your tank & should be able to discard the in-line filter.

Like everything else Lambretta, you will need to monitor the situation & if your tank is so thoroughly rusty inside, take measures to fix that.

That’s another story though....


See how you go...

[Watfordwhite]

I know I have an obsession with in line fuel filters.. Will remove it. I don't think it's the tank as there is no muck in the in line at all. Also this is a brand new tap. Could of only done 10 miles. Seems a little odd that the filter was empty when it had been standing for a week or so, but it was full when parked up? This led me to believe a possible problem with the float bowl?
As stated, the plug was black and wet after stalling. If there was an air leak would this not of been white? In normal operation the plug is a mid-dark brown. So would suggest jetting Okish?. I want to get this dynoed but need to make it to a dealer first. So far only managed runs of 10-15 miles. No dealer even near to Watford.... Since AF left in the 60's.
How would you check for an air leak?
As a carb novice, I don't know what jet is what? I have always avoided playing with carbs. You know what they say. Tinker, tinker tinker, fuck.
This is the first engine I have had with a reed fitted. What's the symptoms of a faulty or damaged reed?

[Watfordwhite]

My S2 has only just finished its restoration, this was a complete nut and bolt job. The tank was cleaned after powder coating and has been fine for the last 4 months running the engine (however different engine!). I have been through at least 15 ltrs fuel and the fuel in it now is only 1 week old. So sorry but I don't think it's a tank issues. The in line was put on to catch any last bits of crap that may work loose after the clean out. There is no crap in it. Also a brand new tap and fuel line.

I am convinced it's now a carb or reed problem. The carb was stripped and cleaned, all jets blown out all with a spray of carb cleaner. Since then it's seems to have gotten worse. I will re strip the carb today and write down the jet sizes.

When it was running, I blocked the exhaust with a rag and this killed the engine after a short while, so I know there is no leaks on that side.

[Wack]

If it was an air leak or fuel starvation I'd expect it to rev higher then cut out, just as it tends to if you turn the petrol off when ticking over. To splutter then cut out sounds more like over fuelling so check the float, needle and idle circuit. If your pilot is too big it would tend to choke up after ticking over a while as well. Dry the carb venturi then turn fuel on and with a torch look down the bore for a weep from the idle holes under the slide and after, you may see fuel running out and into the manifold.