YB100 sweet at idle - struggles under throttle

[VonR]

Hello, I'm looking for some advice and best plan of attack

Bought a 1981 YB100 today.

It starts first kick every time (without pulling the choke knob) and idles really nicely with about 10K miles on the clock.

When you try and ride it, the engine struggles as if it's all clogged up. You can go through the gearbox but 20mph is about the best it will do.

I know nothing about small Yamaha 2 strokes (Haynes Manual on Order).

However, notices that the choke knob stands about 10mm proud of the carb cover - is this normal? Just wondering if the carb has been assembled wrongly and the engine is flooding. Examined the plug (which previous owner said was new). It was a bit dark around the electrode and a bit wet around the perimeter but nothing too drastic (possibly because the bike hasn't been very far).

Can anyone offer advice on sequence of checks:

a) Should I go for the carb/choke first?

Ignition timing - too far advanced maybe?

c) Oil pump requires adjustment?

d) Check state of air filter?

e) Anything else?

Many Thanks

[davey1825]

sounds very much like it's running too rich, you should really need the choke to start from cold, not having to use it suggests it's either stuck partly out or the Pilot mixture screw is set incorrectly

[Airhead]

Does sound like choke problem yes,

Does the choke knob feel right with a definite two positions to it?

[Pittsy3000]

I had the same problem on my old scooter.....

It was to much petrol and not enougth air...

[VonR]

OK thanks all for the advice.

The choke does have 2 definite positions. You can pull it upabout 1cm and it does stay there. In it's fully down position, however, you can still see about 10mm of bronze stem above the black plastic base - just doesn't look right.

It also occurred to me that exhaust baffle may be coked up - so I will look at that first and then, when my Haynes Manual arrives, I will venture into the carb.

Brgds

[Airhead]

OK thanks all for the advice.

The choke does have 2 definite positions. You can pull it upabout 1cm and it does stay there. In it's fully down position, however, you can still see about 10mm of bronze stem above the black plastic base - just doesn't look right.

It also occurred to me that exhaust baffle may be coked up - so I will look at that first and then, when my Haynes Manual arrives, I will venture into the carb.

Brgds

Exhaust baffle would be a good place to start as its one of the routine maintenance operations, you will possible find a hole in the side of the silencer, its an access hole for the screw that holds it in, Remove the screw and the baffle can be twisted side to side as it is withdrawn. If its blocked, use a blowtorch and wire brush to clean it up

[JimR]

OK thanks all for the advice.

The choke does have 2 definite positions. You can pull it upabout 1cm and it does stay there. In it's fully down position, however, you can still see about 10mm of bronze stem above the black plastic base - just doesn't look right.

It also occurred to me that exhaust baffle may be coked up - so I will look at that first and then, when my Haynes Manual arrives, I will venture into the carb.

Brgds

The Choke is correct i.e 10mm of brass showing, carbon build up sounds more favourite. As well as the baffle remove the whole exhaust and have a looksee up the exhaust port you may find a build up there. If the exhaust system seems to weigh a 'ton' that will need decoking also .. various methods to do that also use one that suits you best, I use a fairly brutal method but it's effective, but however you do it undo the gland nut and remove the front pipe, best way to decoke the front pipe is a circular wire brush on a flexi drive. Ignition Timming should be set at 1.8mm btdc at this point the points should just open and not just gapped at 12 thou (you will need to use a dial gauge and ohm meter to do this correctly). Mixture screw set 1 & half turns from fully home, On the year you state there were a batch of YB's that the oil pumps minimum stroke were set incorrectly (I would doubt that ur bike sliped thro the net) but maybe worth checking while the cover is off, if you want a walk thro on that let me know.

Regards Jim

[VonR]

The Choke is correct i.e 10mm of brass showing, carbon build up sounds more favourite. As well as the baffle remove the whole exhaust and have a looksee up the exhaust port you may find a build up there. If the exhaust system seems to weigh a 'ton' that will need decoking also .. various methods to do that also use one that suits you best, I use a fairly brutal method but it's effective, but however you do it undo the gland nut and remove the front pipe, best way to decoke the front pipe is a circular wire brush on a flexi drive. Ignition Timming should be set at 1.8mm btdc at this point the points should just open and not just gapped at 12 thou (you will need to use a dial gauge and ohm meter to do this correctly). Mixture screw set 1 & half turns from fully home, On the year you state there were a batch of YB's that the oil pumps minimum stroke were set incorrectly (I would doubt that ur bike sliped thro the net) but maybe worth checking while the cover is off, if you want a walk thro on that let me know.

Regards Jim

Thanks Jim, appreciate all of these points and clarification. Work has stopped play until later on this week - will feedback then.

Brgds

[VonR]

Thanks Jim, appreciate all of these points and clarification. Work has stopped play until later on this week - will feedback then.

Brgds

OK - I've done the exhaust (obviously been recently cleaned), checked the carb settings and ignition timing and all OK - so I'm suspecting the adjustment of the oil pump. Will gladly take your follow-up advice on offer.

Thanks,

Ian

[JimR]

OK - I've done the exhaust (obviously been recently cleaned), checked the carb settings and ignition timing and all OK - so I'm suspecting the adjustment of the oil pump. Will gladly take your follow-up advice on offer.

Thanks,

Ian

Hi Ian,

How much did the silencer wiegh ? it should be about half a kilo if that any more than that it'll need some serious decarbonisation and I guess you checked the exhaust port, most over look that, as it's difficult to look at and work out if the port is half the size it should be ! The oil pump is dead simple to set up .... check that the marks on the grey plastic wheel( the one the cable runs thro) align at the correct point (this is dependant on modal year, some have a mark on the throttle slide which should be set to the top of the venturi and the marks should align else the lines should align at throttle shut (easy to work out which set up suits). Then you have minimum pump stroke to check do the following :-

Run the engine with the carb cover removed and at idle (after cable adjustment)

watch the plunger of the oil pump cycle in & out, stop the engine when the plunger is fully out (this takes a while to stop the engine at the right time, you may have a few goes at it). Then measure the gap between the grey cable wheel and the washer on the end of the pump piston with the throttle fully shut it should read between 0.10mm & 0.15 mm, if its out of spec remove the nut on the end of the plunger/piston & washer underneath will be some brass shims add or remove these to get the correct clearence, I guess you may have problems getting the correct shims because of the age of the bike but normally if the minimum stroke is set correct it well not alter over the life of the pump, some modals were set incorrect from the factory but i would think that your bikes original owner would have complained about either

1) The mess seeping from the rear of the silencer

2) The mess dripping from the gland nut between the exhuast pipe & silencer

3) If ridden hard excessive oil consumption

if this makes no sense just drop the bike over & I'll test it out

regards Jim

[JimR]

Hello, I'm looking for some advice and best plan of attack

Bought a 1981 YB100 today.

It starts first kick every time (without pulling the choke knob) and idles really nicely with about 10K miles on the clock.

Many Thanks

Hi Ian,

forgot this bit the bike will start without choke if you leave the fuel tap in the on position as everything is a bit worn and the machine will 'flood' the disc valve so the machine will start because of excess fuel in its primary compression phase turn off the tap & you may have to use the choke

Regards Jim

[VonR]

Hi Ian,

How much did the silencer wiegh ? it should be about half a kilo if that any more than that it'll need some serious decarbonisation and I guess you checked the exhaust port, most over look that, as it's difficult to look at and work out if the port is half the size it should be ! The oil pump is dead simple to set up .... check that the marks on the grey plastic wheel( the one the cable runs thro) align at the correct point (this is dependant on modal year, some have a mark on the throttle slide which should be set to the top of the venturi and the marks should align else the lines should align at throttle shut (easy to work out which set up suits). Then you have minimum pump stroke to check do the following :-

Run the engine with the carb cover removed and at idle (after cable adjustment)

watch the plunger of the oil pump cycle in & out, stop the engine when the plunger is fully out (this takes a while to stop the engine at the right time, you may have a few goes at it). Then measure the gap between the grey cable wheel and the washer on the end of the pump piston with the throttle fully shut it should read between 0.10mm & 0.15 mm, if its out of spec remove the nut on the end of the plunger/piston & washer underneath will be some brass shims add or remove these to get the correct clearence, I guess you may have problems getting the correct shims because of the age of the bike but normally if the minimum stroke is set correct it well not alter over the life of the pump, some modals were set incorrect from the factory but i would think that your bikes original owner would have complained about either

1) The mess seeping from the rear of the silencer

2) The mess dripping from the gland nut between the exhuast pipe & silencer

3) If ridden hard excessive oil consumption

if this makes no sense just drop the bike over & I'll test it out

regards Jim

Thanks Jim - will continue with diagnosis this coming weekend. I still have the silencer off so will weigh it. It did feel heavy, come to think of it. It also appeared to have been patched up with gun gum around the fixing bracket, so investment in a new silencer might be worthwhile. Looked into exhaust port with torch and felt around - all felt clear but will double check. Will then proceed to look at oil pump. Have been turning off petrol tap in between running. Always starts first kick with no choke.

Brgds,

Ian

[JimR]

Thanks Jim - will continue with diagnosis this coming weekend. I still have the silencer off so will weigh it. It did feel heavy, come to think of it. It also appeared to have been patched up with gun gum around the fixing bracket, so investment in a new silencer might be worthwhile. Looked into exhaust port with torch and felt around - all felt clear but will double check. Will then proceed to look at oil pump. Have been turning off petrol tap in between running. Always starts first kick with no choke.

Brgds,

Ian

Hi Ian,

what's the air filter like ? The machine shouldn't start with no choke from cold. but 10 secs and it's time to knock it off, so I would guess it's running a little rich but a blocked silencer can cause over richness, try running the bike for a bit without the silencer run it up the road & see if you have a performance gain if so job done. With the silencer in that nick a new one would be a good idea, but to clear the old one tempory you could fill it with a caustic solution and leave it for a day or 2 but with a repair the caustic may eat thro it ! other alternative is to leave the silencer stood upright (gland nut downwards) until all the 'wet' gunge has droped out then shock the silencer (with a hammer or by bashing it on the floor, don't forget to remove the baffle) and shake out the dry carbon deposits. Had a YB once that would only do 20mph I removed double the weight of the silencer in carbon from it and performance was restored, this bike was a trade in the previous owner thought it was dead, he was a bit sick when he saw it wizzing round at 60 two days later !!

Regards Jim

[Airhead]

Wouldnt use caustic on a repaired silencer, its dangerous stuff and will give you severe burns if you get it on you!!

[VonR]

Hi Ian,

what's the air filter like ? The machine shouldn't start with no choke from cold. but 10 secs and it's time to knock it off, so I would guess it's running a little rich but a blocked silencer can cause over richness, try running the bike for a bit without the silencer run it up the road & see if you have a performance gain if so job done. With the silencer in that nick a new one would be a good idea, but to clear the old one tempory you could fill it with a caustic solution and leave it for a day or 2 but with a repair the caustic may eat thro it ! other alternative is to leave the silencer stood upright (gland nut downwards) until all the 'wet' gunge has droped out then shock the silencer (with a hammer or by bashing it on the floor, don't forget to remove the baffle) and shake out the dry carbon deposits. Had a YB once that would only do 20mph I removed double the weight of the silencer in carbon from it and performance was restored, this bike was a trade in the previous owner thought it was dead, he was a bit sick when he saw it wizzing round at 60 two days later !!

Regards Jim

OK Jim, thanks again for the advice. Have just ordered a new silencer on Ebay anyway for a reasonable price but will try a quick blast up the road with just the down-pipe - didn't realise you could do this in my ignorance of 2-strokes (thought the engine needed the back-pressure). At least it might prove the point before I fit the new silencer. Air filter looks very clean -presumably previous owner has already taken the obvious measures. It will be intereting to compare the weight of old and new silencers based upon what you have said - will let you know.

Regards,

Ian

[VonR]

Hi Ian,

what's the air filter like ? The machine shouldn't start with no choke from cold. but 10 secs and it's time to knock it off, so I would guess it's running a little rich but a blocked silencer can cause over richness, try running the bike for a bit without the silencer run it up the road & see if you have a performance gain if so job done. With the silencer in that nick a new one would be a good idea, but to clear the old one tempory you could fill it with a caustic solution and leave it for a day or 2 but with a repair the caustic may eat thro it ! other alternative is to leave the silencer stood upright (gland nut downwards) until all the 'wet' gunge has droped out then shock the silencer (with a hammer or by bashing it on the floor, don't forget to remove the baffle) and shake out the dry carbon deposits. Had a YB once that would only do 20mph I removed double the weight of the silencer in carbon from it and performance was restored, this bike was a trade in the previous owner thought it was dead, he was a bit sick when he saw it wizzing round at 60 two days later !!

Regards Jim

Jim,

Latest on this one! - Replacement Exhaust made no difference at all. Weight was almost identical anyway. As often the case, looks like multiple factors at work. Oil pump setting slightly out. An adjustment means that bike pulls much better until it gets hot then it exhibits the same symptoms (nevertheless, a step in the right direction). Took it to a local bke specialist. They have re-checked timing/tuning. They suspect a faulty magneto winding and/or condenser but it's a bit trial and error - and of course no spare parts readily available. Do you know of anyone that stocks these items or suitable replacements?

Thanks

Ian

[JimR]

Jim,

Latest on this one! - Replacement Exhaust made no difference at all. Weight was almost identical anyway. As often the case, looks like multiple factors at work. Oil pump setting slightly out. An adjustment means that bike pulls much better until it gets hot then it exhibits the same symptoms (nevertheless, a step in the right direction). Took it to a local bke specialist. They have re-checked timing/tuning. They suspect a faulty magneto winding and/or condenser but it's a bit trial and error - and of course no spare parts readily available. Do you know of anyone that stocks these items or suitable replacements?

Thanks

Ian

Hi Ian

Ok I now have a workshop set up in South Worcestershire, which you are welcome to have the machine fixed at !!! but without more info of the bike it would be difficult to diagnose the problem. I guess the timming was set @ 1.8mm btdc by the shop & the min stroke of the oil pump corrected ? If you could post a video of the bike running showing the exhaust outlet/engine sound at idle that may help. If the Condenser is up it the bike will misfire (cut out) when the lights are switched on or if you look thro the mag with the engine running you will see "sparks" (arcing) acroos the points.. either will show the condenser down (but not normally both). The primary winding (source coil, in the mag) was very rare to give problems, if it did the bike ended up with no spark at all so I may discount the source coil from the problem. Just thinking was the oil pump cable adjusted or the minimum pump stroke set ? coz you could get the problem with both cable adjustment & minimum pump stroke i.e cable adjustment makes it better min pump stroke fixes it !!!

Regards Jim

[VonR]

Hi Ian

Ok I now have a workshop set up in South Worcestershire, which you are welcome to have the machine fixed at !!! but without more info of the bike it would be difficult to diagnose the problem. I guess the timming was set @ 1.8mm btdc by the shop & the min stroke of the oil pump corrected ? If you could post a video of the bike running showing the exhaust outlet/engine sound at idle that may help. If the Condenser is up it the bike will misfire (cut out) when the lights are switched on or if you look thro the mag with the engine running you will see "sparks" (arcing) acroos the points.. either will show the condenser down (but not normally both). The primary winding (source coil, in the mag) was very rare to give problems, if it did the bike ended up with no spark at all so I may discount the source coil from the problem. Just thinking was the oil pump cable adjusted or the minimum pump stroke set ? coz you could get the problem with both cable adjustment & minimum pump stroke i.e cable adjustment makes it better min pump stroke fixes it !!!

Regards Jim

Jim,

Might well enlist your services if you are up and running businesswise as I could easily spend another 20 years messing around with this (in fact I'd only spend another couple of months before setting fire to it at the bottom of the garden) - if you email me your details to [email protected], I'll arrange to trailer it up sometime.

Thanks