I have an R25/3 and am using an appropriately sized Mikuni carb. Normally the bike runs just a little rich, however on a recent ride of about 30 km with a gentle upward slope and the last couple of km fairly steep, the bike lost power and ran erratically. After letting the bike cool, I checked the plug and it was obviously running very lean, at least at the end ofthe ride. Question: what would cause the bike to run slightly rich and then lean? Subsequently I tried a different plug (no changes to the carb) and it ran so lean that after only a hundred meters or so, the bike was exhibiting the symptoms above only more pronounced. I switched back to the original plug. The bike normally runs very well but the change to lean after an uphill ride has me concerned. I would appreciate any help offered. And you may e-mail me directly at pgw1@yahoo.com Thank you.
Peter Waddell
Carburetor Question
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Peter Dunn
Re: Carburetor Question
Peter,
You describe the symptoms occuring as you rode up a fairly steep hill. Could it be that the bike was suffering fuel starvation at that point ? How much fuel did you have & was this successfully getting into the fuel line & then carb ? You could have debris / rubbish in your tank or fuel tap causing this problem.
Pete
You describe the symptoms occuring as you rode up a fairly steep hill. Could it be that the bike was suffering fuel starvation at that point ? How much fuel did you have & was this successfully getting into the fuel line & then carb ? You could have debris / rubbish in your tank or fuel tap causing this problem.
Pete
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Allan Atherton
Re: Carburetor Question
I wonder if this is a mixture problem at all.
I have heard of a bike starving and dying on a long downhill run - it was about to go on reserve and the gas went to the front of the tank. But gas goes to the rear of the tank when going up a hill, increasing the availability of gas above the petcock.
It is possible that the 30 km uphill ride overheated the engine, and then the steep hill of 2 km caused the engine to begin to seize?
The R25/3 had a problem of running hot. Below I have pasted some comments that Mark Huggett made in this forum several years ago. If you find no other cause for your problem, you may want to remove the cylinder and look for signs of seizure.
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"All the R25 series had heating problems for a number of reasons. Lubrication to the piston and cylinder head where inadequate. The cooling fins on the cylinder head were inadequate, and the cylinder and cylinder head where hidden from the airflow by a large flared front mudguard. The measures that BMW undertook in those days where as follows: As of motor number:
260 322 Ignition timing is changed to 5 degrees before TDC.
261 301 The carburetor main jet is changed from 105 to a 110.
263 051 The cylinder head and valve covers were painted mat black (Customers with older R25 models could get a free tin of lead based heat resistant black paint from their BMW dealer.
263 651 The conventional tappet cover gasket was replaced with an aluminum gasket in order to improve the heat transfer from cylinder head to tappet cover in the hope that the heat dissipation would be improved.
Even the R25/3 had a black painted cylinder head. BMW really solved this problem for the first time with the R26 in two ways. First, the surface area of the cooling fins on the cylinder head was massively increased by about 60 % and the size of the front mudguard decreased allowing a better flow of air to the cylinder head. Second, a major improvement was made in the lubrication by a 3mm hole drilled in the side of the cylinder hole in the aluminum engine block to connect up with the main oil way that leads up into the cylinder and thereafter cylinder head. A 1,6mm hole was then drilled in the corresponding position through the cylinder wall below the cylinder foot in order that oil would be pumped directly into the cylinder and the piston. With the R27, BMW turned an oil groove on the out side of the cylinder under the cylinder foot and drilled a second hole on the opposite side in order that the piston received double lubrication. This is a modification that we make to all of the R24 to R25/3 series. Regarding the dropped valves, excessive heat means excessive wear. The most common is a broken off exhaust valve head. This is due to excessively worn valve guides and valve stem. During operation, the valves can rotate slightly. If the valve guide has too much play the valve can close at a slightly different angle each time. This causes the valve head to be bent backwards and forwards by a minute fraction each time and therefore weakening the point where the valve head and stem come together resulting in a break. Luckily, valve materials have since massively improved since the 1950's and this problem seldom happens with modern valves in these older machines."
I have heard of a bike starving and dying on a long downhill run - it was about to go on reserve and the gas went to the front of the tank. But gas goes to the rear of the tank when going up a hill, increasing the availability of gas above the petcock.
It is possible that the 30 km uphill ride overheated the engine, and then the steep hill of 2 km caused the engine to begin to seize?
The R25/3 had a problem of running hot. Below I have pasted some comments that Mark Huggett made in this forum several years ago. If you find no other cause for your problem, you may want to remove the cylinder and look for signs of seizure.
--------------------------------------------------
"All the R25 series had heating problems for a number of reasons. Lubrication to the piston and cylinder head where inadequate. The cooling fins on the cylinder head were inadequate, and the cylinder and cylinder head where hidden from the airflow by a large flared front mudguard. The measures that BMW undertook in those days where as follows: As of motor number:
260 322 Ignition timing is changed to 5 degrees before TDC.
261 301 The carburetor main jet is changed from 105 to a 110.
263 051 The cylinder head and valve covers were painted mat black (Customers with older R25 models could get a free tin of lead based heat resistant black paint from their BMW dealer.
263 651 The conventional tappet cover gasket was replaced with an aluminum gasket in order to improve the heat transfer from cylinder head to tappet cover in the hope that the heat dissipation would be improved.
Even the R25/3 had a black painted cylinder head. BMW really solved this problem for the first time with the R26 in two ways. First, the surface area of the cooling fins on the cylinder head was massively increased by about 60 % and the size of the front mudguard decreased allowing a better flow of air to the cylinder head. Second, a major improvement was made in the lubrication by a 3mm hole drilled in the side of the cylinder hole in the aluminum engine block to connect up with the main oil way that leads up into the cylinder and thereafter cylinder head. A 1,6mm hole was then drilled in the corresponding position through the cylinder wall below the cylinder foot in order that oil would be pumped directly into the cylinder and the piston. With the R27, BMW turned an oil groove on the out side of the cylinder under the cylinder foot and drilled a second hole on the opposite side in order that the piston received double lubrication. This is a modification that we make to all of the R24 to R25/3 series. Regarding the dropped valves, excessive heat means excessive wear. The most common is a broken off exhaust valve head. This is due to excessively worn valve guides and valve stem. During operation, the valves can rotate slightly. If the valve guide has too much play the valve can close at a slightly different angle each time. This causes the valve head to be bent backwards and forwards by a minute fraction each time and therefore weakening the point where the valve head and stem come together resulting in a break. Luckily, valve materials have since massively improved since the 1950's and this problem seldom happens with modern valves in these older machines."
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Peter Waddell
Re: Carburetor Question
Thanks Allan. I know that this model gets hot. I'm living in Bali and people here often run a little rich, as it's said that this keeps the heat down. I'll look into the modifications that Mark suggests.
But I'm curious as to why when, later, I tried a different type plug that the bike ran so lean that it was acting up after only a hundred meters or so. This was on level ground. The plug was supposedly a medium heat plug. The plug I normally use is a Bosch with four prongs surrounding a central electrode, I don't remember the number.
This bike is my only transportation and I use it on a daily basis.
But I'm curious as to why when, later, I tried a different type plug that the bike ran so lean that it was acting up after only a hundred meters or so. This was on level ground. The plug was supposedly a medium heat plug. The plug I normally use is a Bosch with four prongs surrounding a central electrode, I don't remember the number.
This bike is my only transportation and I use it on a daily basis.
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Allan Atherton
Re: Carburetor Question
Plugs do not affect the mixture. A plug cannot make the mixture rich or lean. Plugs do come in heat ranges.
A plug with four electrodes doe not seem like the proper plug for a vintage bike. I think a traditional Bosch single-electrode might work better. These are short reach plugs, suitable for my R27 and I assume for your R25:
Bosch W4AC (former W 240 T1) hot for running in
Bosch W3AC (former W 260 T1) cool for regular use
A plug with four electrodes doe not seem like the proper plug for a vintage bike. I think a traditional Bosch single-electrode might work better. These are short reach plugs, suitable for my R27 and I assume for your R25:
Bosch W4AC (former W 240 T1) hot for running in
Bosch W3AC (former W 260 T1) cool for regular use
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Peter Waddell
Re: Carburetor Question
Again, thanks. I, of course understand that the plug can't affect the mixture. I suppose what I was trying to get across was the fact that plug was extremely hot and white as opposed to a more natural temp and color (of course the performance was unacceptable) I think what was surprising to me was that there was such a difference between the plugs such that one was unusable, with the carb unchanged.
I will try and locate the plug you mentioned.
Also, do you know if/where I might find drawings of the modification that Mark speaks of in your initial response to me. ( "With the R27 BMW turned an oil groove ...") I don't have access here to an R27 manual, which I suppose would show what Mark is speaking about.
Again, Thanks.
I will try and locate the plug you mentioned.
Also, do you know if/where I might find drawings of the modification that Mark speaks of in your initial response to me. ( "With the R27 BMW turned an oil groove ...") I don't have access here to an R27 manual, which I suppose would show what Mark is speaking about.
Again, Thanks.
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Garnet Grylls
Re: Carburetor Question
Hi Peter
Do not forget to check the rest of your basic tuning as well. Have the points closed up causing a late spark? Have you gotten some dirt in the fuel and partly blocked the main jet? Has the needle in the slide vibrated lose and moved down causing a lean midrange?
Perhaps your main jet is too small but you never noticed before because you never had the throttle that far open for so long.
Try the address below. It is the English forum of a German site that has been very helpful to me as I am rebuilding my R26. This particular thread shows all the steps to upgrade your cylinder to R27 spec.
Garnet
http://www.bmw-einzylinder.de/forum/ind ... pic=1604.0
Do not forget to check the rest of your basic tuning as well. Have the points closed up causing a late spark? Have you gotten some dirt in the fuel and partly blocked the main jet? Has the needle in the slide vibrated lose and moved down causing a lean midrange?
Perhaps your main jet is too small but you never noticed before because you never had the throttle that far open for so long.
Try the address below. It is the English forum of a German site that has been very helpful to me as I am rebuilding my R26. This particular thread shows all the steps to upgrade your cylinder to R27 spec.
Garnet
http://www.bmw-einzylinder.de/forum/ind ... pic=1604.0