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Help please!! - B175 Dieselling
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starider
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September 2, 2020 - 6:51 pm
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Hi, it's a while since I was on the forum, but today I decided to have a good go to get my 175B running properly. Since restoration it has been very difficult to start and would not reach any sort of speed even after a carb rebuild,new contacts and condenser,accurate setting of timing. Basically I was so fed up I have not touched it for some time.I saw the advert in the magazine for an electronic ignition conversion which I purchased. My son fitted it today,added fresh fuel. It fired up almost instantly and ran better than ever had, so reset tyre pressures checked everything and my son disappeared in a cloud of blue smoke, he came back at speed and as he passed our gate there was a phutt and the engine cut. It started instantly and we set about setting mixture,tick-over when suddenly the engine revs increased alarmingly, the cut out button wouldn't work,pulling off the plug lead wouldn't stop the engine revving but a hand over the carb. did. We started it again after several minutes,it ran for a while then the same thing happened. Any ideas please. We are not sure if the new piston had nipped but there were no signs of this.  HELP!!!  starider

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mikef
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September 2, 2020 - 8:10 pm
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Hi.
    The revving problem seems quite a common issue, I have not heard of it happening on Bantams with a monoblock carburettor only D10s, D14s and B175s with the Concentric carburettor. My theory is that it's caused by a worn carburettor body and weak throttle return spring. Air passes between the slide and body of the carb and somehow this causes the slide to lift, this causes the engine to rev.
I have never proved this theory but can't think of any other explanation. For an engine to rev like that fuel is needed as well as air and about the only way for it to get the fuel is from the needle jet so the slide must be lifting. The revving problem only seems to happen when the engine is hot and some how the mixture is ignited even without a spark, maybe dieseling causing the ignition.
As you have found out, choking the engine is the best way to stop it.
If the carb body is worn, then the only solution is to buy a new one.
No idea why the engine should have cut out, if it nipped up I think you would have felt the rear wheel lock, could have just been a temporary fuel blockage.

Good luck.
     Mike.

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starider
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September 3, 2020 - 12:08 am
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Thanks Mike, I'll look into buying a new carburettor. My son had also heard of dieseling but we couldn't work out how it could happen.  Tony

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cocorico
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September 3, 2020 - 8:15 am
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If it's the carb body around the slide that is worn, I believe there is someone who can re-sleeve it. I'm pretty sure there are posts about it, ** Please log in to view **

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starider
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September 3, 2020 - 11:29 pm
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Thanks Cocorico, I've looked at the posts you suggest, but think a new carb is the best option. It's impossible to check all the wear in the carb.  starider.

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Sponge
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September 4, 2020 - 9:55 am
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I have had a similar issue so please forgive the intrusion but when the bike was revving out of control was there a lot of smoke coming out of it ? 

SPonge

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GlenAnderson
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September 4, 2020 - 1:09 pm
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You pulled the plug lead off and it still kept running? That would suggest something in the cylinder is hot enough to ignite the fuel. 

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cocorico
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September 4, 2020 - 1:57 pm
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Has the oil level in the gearbox dropped at all? If it's smoking a lot you may have a seal going, letting gearbox oil into the crankcase. dunno

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SpacedMarine
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September 4, 2020 - 7:12 pm
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GlenAnderson said
You pulled the plug lead off and it still kept running? That would suggest something in the cylinder is hot enough to ignite the fuel.   

Tons of smoke?Massive carbon build up causing high compression leading to self ignition?Or as coco says maybe drawing gearbox oil in?I've known turbo diesels to empty the sump and explode spectacularly once they get going but a bantam?

I'd be tempted to pull the head off and see if it needs a decoke unless you're sure it's clear.

What's 7/16 in mm again?

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Colin1
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September 17, 2020 - 7:49 pm
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I

starider said
Hi, it's a while since I was on the forum, but today I decided to have a good go to get my 175B running properly. Since restoration it has been very difficult to start and would not reach any sort of speed even after a carb rebuild,new contacts and condenser,accurate setting of timing. Basically I was so fed up I have not touched it for some time.I saw the advert in the magazine for an electronic ignition conversion which I purchased. My son fitted it today,added fresh fuel. It fired up almost instantly and ran better than ever had, so reset tyre pressures checked everything and my son disappeared in a cloud of blue smoke, he came back at speed and as he passed our gate there was a phutt and the engine cut. It started instantly and we set about setting mixture,tick-over when suddenly the engine revs increased alarmingly, the cut out button wouldn't work,pulling off the plug lead wouldn't stop the engine revving but a hand over the carb. did. We started it again after several minutes,it ran for a while then the same thing happened. Any ideas please. We are not sure if the new piston had nipped but there were no signs of this.  HELP!!!  starider  

Interestingly I just had a similar problem today. Bike engine has just been rebuilt, so definitely not carbon build up. My revs shot up after finding a false neutral and would not shut off until dropping down or up a gear. Engine was warm.  I have also noticed when ticking over, if I blip the throttle, the revs stay up for a few second rather than immediately dropping, also when hot. Perhaps some can explain this? I guess it could be drawing air from somewhere but I can't see where. I have a good strong throttle spring and the slide snaps back. 

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GlenAnderson
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September 17, 2020 - 8:03 pm
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Air leak from somewhere I’d say. Maybe the carb to barrel flange, a worn slide or air leak into the crank space from a poor joint. 

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Number6
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September 18, 2020 - 6:14 pm
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Interestingly I just had a similar problem today. Bike engine has just been rebuilt, so definitely not carbon build up. My revs shot up after finding a false neutral and would not shut off until dropping down or up a gear. Engine was warm. I have also noticed when ticking over, if I blip the throttle, the revs stay up for a few second rather than immediately dropping, also when hot. Perhaps some can explain this? I guess it could be drawing air from somewhere but I can't see where. I have a good strong throttle spring and the slide snaps back.

I had this years ago, on my 850 Moto Guzzi and also, coincidentally, on my BMW R80. What I was doing wrong was setting the slide stops too high to get a tickover, instead of getting the idle mixture screw right (which it wasn't). The result was reluctance for revs to return to idle speed whenever blipped or revved, especially when hot. Later, after reading the instructions properly (says, starting at 1.5 turns out [idle screws], which is not necessarily what they must be) and getting the idle screws right, as it's them that basically set the idle (air bleed), so then the slides could be set lower and it wouldn't conk out, all was hunky-dory after that.

May or may not be something like that – compounded by the fact those bikes won't idle anyway until hot, so easy to have the sldies too high, by trying to get it to keep idling when setting up in the back garden.

 

 

 

 

-

Mike H --

Murphy's 4th law of motion states that any small object that is accidentally dropped will immediately hide itself under a larger object.

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chippy
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September 18, 2020 - 7:26 pm
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Sometime ago my D14 did it , I screwed the pilot screw in as I recall, Has'nt done it since.

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Colin1
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September 18, 2020 - 8:17 pm
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Number6 said

Interestingly I just had a similar problem today. Bike engine has just been rebuilt, so definitely not carbon build up. My revs shot up after finding a false neutral and would not shut off until dropping down or up a gear. Engine was warm. I have also noticed when ticking over, if I blip the throttle, the revs stay up for a few second rather than immediately dropping, also when hot. Perhaps some can explain this? I guess it could be drawing air from somewhere but I can't see where. I have a good strong throttle spring and the slide snaps back.

I had this years ago, on my 850 Moto Guzzi and also, coincidentally, on my BMW R80. What I was doing wrong was setting the slide stops too high to get a tickover, instead of getting the idle mixture screw right (which it wasn't). The result was reluctance for revs to return to idle speed whenever blipped or revved, especially when hot. Later, after reading the instructions properly (says, starting at 1.5 turns out [idle screws], which is not necessarily what they must be) and getting the idle screws right, as it's them that basically set the idle (air bleed), so then the slides could be set lower and it wouldn't conk out, all was hunky-dory after that.

May or may not be something like that – compounded by the fact those bikes won't idle anyway until hot, so easy to have the sldies too high, by trying to get it to keep idling when setting up in the back garden.

 

 

 

 

-  

Just took a look at my throttle slide and yes it's a lot higher than I'd expect. So you could well have the answer. Thinking about my setting up of the carb, I don't think I gave it time to fully warm up. The pilot screw didn't seem to be doing much. I should known better . . . But I wasn't thinking. Too keen to get it out on the road. Thanks for you reply. I think you are spot on. 

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Number6
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September 19, 2020 - 8:37 pm
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Would be nice if I was! laugh

Mike H --

Murphy's 4th law of motion states that any small object that is accidentally dropped will immediately hide itself under a larger object.

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Sponge
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September 21, 2020 - 11:37 am
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Just a thought........if your needle was wrong (say a monobloc needle in a concentric carb) you would get exactly that effect. If you are stripping teh carb down I would urge you to check absolutely everything.  Happy to verify measurements etc if you have them. 

Sponge 

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