Tuning help

[Donut 77]

So today I had a play at solving the grumble at low revs - with the bike warm I had a go at tweaking the pilot screws (very awkward and very hot hand) also very hard to accurately do, but I’m learning a good process. I think they are happy somewhere near 2turns front, 2.5rear but it hasn’t totally solved the issue. Basically, from idle if I blip the throttle it stumbles briefly then catches up - more of a ‘guguuun’ than a smooth ‘vroom’ as though something is slow to respond, or a lack of fuel - sometimes with a slight clanking sound. The revs also don’t return immediately - although not really slow, although I have to admit I don’t know what the characteristics should be to compare it against. If I throttle slowly then it’s all fine. I also had a couple of very mild pops/backfires at about 6-7k revs too.

Hopefully someone reading this might recognise the symptoms and point me in the right direction. There is a fuel smell too so I’m wondering if it’s too rich? Also, when I blip it the fuel light flickers on. Could it be residual dirt in the tank? I only put a gallon in and it’s the first fuel it’s had? I didn’t clean the tank. I inspected it carefully and hoovered out loose rust and I did put a new filter in. Could that possibly be blocked as the loose rust cleans off? I’m happy to check all possibilities but obviously it takes time to take things apart so it would help to have a steer in case I’m barking up the wrong tree..

I haven’t balanced carbs yet as I don’t have kit… it’s on its way. I was planning to do this last, once each carb is set up. Does this sounds a reasonable order?

Appreciate any suggestion are very welcome.

[Donut 77]

Anyone any ideas? Got an MOT booked in 8 days really running out of time to strip it back too far.

[tony.mon]

If you think there might be more rust left in the rank it's very easy to pull the tank off and empty it I to a container to see if there's any more. ( There will be, you can't just hoover it out) and get it all.

Even easier, use a remote reservoir to fuel it with the tank off. If it runs ok on that, it's your tank and fuel filter causing the problem.

Mostly, just ride it, they are an old bike now, so don't expect it to ride like a new one.

[Donut 77]

Thanks Tony, I’m rigging up a temp tank as we speak. I know that you have to try stuff to find the issue - i was just hoping it might jump out as obvious to someone that I’m either too lean/rich/un balanced etc?

One thing I did want to know from your experience - if I get the tweaking/pilot/balancing done with the airbox off - how much if any will those settings likely to be out when the box is replaced? Just out of interest.

Cheers

[tony.mon]

Balancing will be the same, but for the rest, just rest the lid on with a brick on top rather than screwing and unscrewing it every time.

[Donut 77]

Rigged up a temp fuel tank, ran it first with airbox still on to eliminate the fuel tank/petcock variable - Result was the same.

Then removed airbox and ran it. It wasn’t very happy to run like this - backfires etc. obviously expecting suction on hoses and airflow etc.

One thing I did notice was that the slides were twitching when opened. Is this normal? I thought they just moved in a linear way with the throttle, and the vacuum held them relatively still.



With the engine warmed up - the very end of these next two vids shows how it doesn’t respond well to being blipped - seems to have a lag in the slide response.





On a positive, I now had access to the 5mm vacuum take off on the front, so (after manufacturing another special tool) that is all ready so I can balance it up.

So, plan is to put it all back and tweak pilot screw/balance and just not worry about the slight hesitation when I crack the throttle until I’ve road tested it. Maybe the other adjustments might help anyway.

The strong fuel smell might’ve been down to the crap replacement fuel cap.£20 eBay. I lost 5L fuel just sitting in the garage and when it was upside down it leaked all over the tank, damaging the paint. Be wary of these. I’ve ordered a hand screw, non locking one now.

[tony.mon]

Slides are normal.
They won't work in the same way as they do with a sealed airbox, because the chamber above each slide diaphragm is open to atmosphere.

Lesson #37: don't turn the tank upside down with fuel in it

[Donut 77]

Ok, that’s reassuring. Thanks Tony.

Would be nice to find out why the lag when blipped?

I don’t expect the same response as a 4cyl, but there’s a definite delay. Do other people’s storms respond nicely to blips from tickover?

[Donut 77]

Lesson #37: don't turn the tank upside down with fuel in it

YEP

[fabiostar]

option 2.

balanace the carbs well then, here comes the technical bit bring it out and give it a dam good thrashing to clear its throat .


also, while your at it make sure the choke nuts are tight on the carbs but be carful because they break faster than you can look at them.. or have it ticking over and spray some easy start round the carb rubbers and chokes to see if the revs rise to rule out air leaks

[Donut 77]

option 2.

balanace the carbs well then, here comes the technical bit bring it out and give it a dam good thrashing to clear its throat

I plan to as soon as it’s Mot’d next week. Just trying to be careful with my limited knowledge before then..

Good point on the choke nuts.. I hadn’t thought of air leaks. You see, novice!

[fabiostar]

option 2.

balanace the carbs well then, here comes the technical bit bring it out and give it a dam good thrashing to clear its throat

I plan to as soon as it’s Mot’d next week. Just trying to be careful with my limited knowledge before then..

Good point on the choke nuts.. I hadn’t thought of air leaks. You see, novice!

a choke nut loose or a choke stuck in the carb body maybe but be SO gentle with them, they are 20 plus years old plastic

[Donut 77]

I replaced the choke cables - old ones were seized solid.. Cleaned up the needle bits and where they attach to the carbs. Plenty of oil down the outer cables too. The choke doesn’t feel very positive - ie it just flaps about - there’s little resistance to pulling or pushing, but it does kind of work. I think they’re in ok though.

[tony.mon]

If the backnut behind the knob won't tighten enough to give some stiction to the choke knob (i.e. hold it in position when you want it to stay out) you can simply use a wooden clothes peg.

[Donut 77]

The bike is all back together now. Still having trouble.

It’s proving very awkward to tune. Here’s what I did:

1) Bike warmed up.
2) pilots both at 1.5 to start
3) Idle screw set to high idle - 1800rpm
4) Front - turned pilot out slowly to see if rpm increased - it did not. So, I turned it in and the revs kept increasing until it was fully closed, where it happily revved away - presumably with no pilot on that cylinder? This is opposite of what I expected - I thought it would stall when nearing fully closed?
5) I gave up and put them both back to 1.5 turns out.
6) Balanced it at that, just to try out the manometer I made and vacuum take off etc. no problem with that part.

I can tell it’s not happy at the current settings. I’m starting to wonder if the pilot jets are bad/dirty/wrong. Would that account for the stumble on low revs - when the bike is quite happy after 2k rpm. It’s just the initial blip that it doesn’t cope with. Or maybe an air leak - as the revs do hang a bit.

If so, what do I spray to test for that and where to spray?

Thanks good people. I want to solve this myself, mainly for the experience.