Power dropping off in 4th

[scootersi]

Hi all,

I have a frustrating issue with my SIL200 which I run with an AF Clubman and bored-out Jetex. Going through the gears is fine and the revs progress nicely in 2nd & 3rd. However in 4th the revs seem to plateau and the power drops off with the occasional surge.

I thought it might be stator issues so replaced the original SIL with a BGM which has improved starting but the issue at the top end of the range remains.

I'm pretty sure the carb is delivering enough fuel as it's been on the scooter for a while and this is a relatively recent issue.

Any thoughts welcomed.

Regards,
Simon.

[mainstand]

Have a similar issue with me (sorry if you clicked expecting to find an answer ) I have messed about with different pipes & gearboxes & sprockets, goes through the gears great, but as soon as I am in 4th & meet any kind of resistance, hill or headwind, I may as well get off & walk.

[coaster]

....I'm pretty sure the carb is delivering enough fuel as it's been on the scooter for a while and this is a relatively recent issue.

I see no reason to assume that there is nothing wrong with the carb just because it WAS working fine It is quite possible that the problem is related to fuel starvation which if only marginal would only manifest itself when the demand for fuel is at a peak i.e. flat out in 4th. Things to check would be the filter, float valve and also the float itself in case it is rubbing on the side of the float bowl. Also disconnect the fuel pipe and check the fuel flow rate into a jar, should be about a pint a minute I think.

What is the plug colour looking like btw??

[CHRIS in MARGATE]

Agree ..... Strip clean and check carb first. The fact that it's been on there for a long time is a very good reason to overhaul it. Problem does sound fuel related.
If no better.... borrow another CDI and try

[MickYork]

Stuck piston rings could be a cause and possibly a slipping clutch (sudden surge in power).

[scootersi]

Thanks for the helpful replies. I'll spend some time on the carb and clutch and see if that cures it.

[Knowledge]

The issue might be related to the tune of the scooter. There might not be enough torque at road speeds to overcome the gap between 3rd and 4th gear. If there isn't enough torque, then you will be fighting a losing battle in 4th as the power slips away, leaving you hunting for 3rd gear to get up inclines and hills. If it seems OK (and even great) once back in 3rd, then it might be an issue with the tune rather than the settings.

You can reduce the symptoms by down-gearing, but you will reduce the top speed or run at higher revs at speed. This may not be desirable.

I have written about this very problem in the next edition of Jetset, which is with the printers.

[mainstand]

The issue might be related to the tune of the scooter. There might not be enough torque at road speeds to overcome the gap between 3rd and 4th gear. If there isn't enough torque, then you will be fighting a losing battle in 4th as the power slips away, leaving you hunting for 3rd gear to get up inclines and hills. If it seems OK (and even great) once back in 3rd, then it might be an issue with the tune rather than the settings.

You can reduce the symptoms by down-gearing, but you will reduce the top speed or run at higher revs at speed. This may not be desirable.

I have written about this very problem in the next edition of Jetset, which is with the printers.

I know it is that for me, (sorry to hijack the thread) as I have tried every other possibility. When it was static timing with a 25mm carb & a clubman, it would sit at 60 no bother. (it's a tuned stage 4 with 67.5mm piston), I put a taffspeed copy on, augusto 6000, reed block with the piston done & a 28mm carb & in top gear it's got worse.

[Adam_Winstone]

^... What is your starting ignition point (degrees btdc) and your max retard? If you're retarding too fast too soon all that will happen is that you'll lose power as the revs rise.

Adam

Edit: in your shoes I'd test this series by taking the Agusto off and setting the static timing to 17 dbtdc and see if you instantly get the power back then you know it is an issue with your Agusto settings.

[CHRIS in MARGATE]

I think everybody is at cross purposes here. The original posting relates to SIL 200 fitted with a Clubman exhaust. State of tune is probably Indian Stage 4 and should run very well with the exhaust. I make the assumption that this is a problem that has developed rather than been there from the outset. Refetences to gearing, timing, etc are for another debate. Let's not confuse people and get them half stripping the fuel, electrics and transmission.

Don't mean to upset or offend anybody I promise.

[Adam_Winstone]

Agreed. Mainstand's post is triggering responses that is taking the thread off its original course... But would be worth a thread on its own. I understand that symptoms are similar but the motor spec is very different.

Adam

[St George]

My SIL 200 stage 4 is the same, great in 1st 2nd and 3rd dies off in 4th. New 22 Jetex and standard new GP 200 exhaust. Will put my Clubman back on and try again. It is a newish rebuild. Will watch your thread with interest ?

Colin

[scootersi]

Thanks again for all the replies.

Over the w/e I had a good check on carb and fuel flow which all check out fine. I put a spare CDI on which I have had for a while and it seemed to improve things a little. I shall order a new one to be sure. I have also renewed clutch plates and springs as these needed doing in any event.

I also bought a cheap digital tacho to help me see what was going on. In 2nd I can hold the motor open at circa 8000 rpm at which point it is screaming in agony but there is no problem with power delivery. In 3rd I can hit 7200-7300 with a very occasional sense of power outage but only very intermittent and short-lived. In 4th I can hit 6200-6400 (GPS app shows max 57mph on the flat at this point) but the power delivery is still off by which i mean it feels like the engine is somehow being held back from reaching full potential. Very occasionally the issue (whatever it is) will clear and the speed will pick up but only for a moment.

Hopefully a new CDI will cure it but in case not i'm wondering if an over-rich main jet would cause an issue like this. I run the Jetex without a filter (as starting is rubbish with a filter on) with a 145 main due to previous seizure issues (albeit with a Scootrs pipe which I have taken off for the time being). Plug check is showing it is rich but not ridiculously so.

Regards,
Simon.

[MickYork]

My first thought was that your gearing is too tall but to rev at 6400 rpm and get 57 mph shows a short box giving about 5.75 final gearing. It may be worth looking at your gearbox and checking what your running. An indian gp200 box should be about 5.1 which would give you 60mph at 6000 revs (65mph-ish at 6500rpm).

....all seems a bit odd

[Wack]

Have you checked the top end over?

[Hoffman Fisher]

Mine was lacking power in the same way you describe, was running jets at 43 pilot 100 main and 50 starter, changed the main jet back up to 118 and hey presto! power is back. I'm not saying this will be the same for you, I just went back through (in my head) the things (a fair few) that I had done since it last ran without any issues. I got lucky, as my thinking was not enough juice getting down to the combustion chamber when it was needed, fuel starvation as it is. hope you get it sorted.

[scootersi]

An update but no cure as yet.

I've been through the fuel system and all checks out ok. I've been up and down main jet and I've swapped out every part of the ignition system with no joy.

I think I need to pull the top-end down to see what's going on.

I put a three ring Gol piston in a while ago as it was all I could get hold of in a hurry. I wonder if this is the cuplrit?

Regards,
Simon.

[scootersi]

Just in case anyone other than me is interested in this issue!

I have I think cured the problem by pulling the top-end down, replacing the three ring Gol piston with a two ring Goetze and bringing the squish down to circa 1.3mm by leaving out the head gasket. I can now hit 63ish on the flat and topped out at 70.1mph on a long downhill stretch today.

Thanks again to all those who replied with suggestions.

Regards,
Simon.