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Old 01-26-2013, 05:17 PM   #1201
rperedel
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Advice before Opening+Cleaning+Overhauling my Mikuni BST40

Hello,

I own an Aprilia Motó 6.5 (Rotax engine + BST40 Carb) bought it second hand with around 5000 miles. Manufactured in 1995 so it has low mileage and seems to have been sitting for long periods

Not long ago got some spares from Motolab in the event of having to clean / rebuild the carb at anytime

My main problem seems to be just cold starting the engine and keep it running until it warms up. Then it just runs fine in any throttle/speed combination

As I've said, firing it up when it has been stopped for one or many nights is a nightmare.

The sequence is to pull the choke fully, and keep it starting like 6-8 times until it runs.... then I should not touch the throttle or push the choke for about 7 or 10 minutes until the engine gets really hot (fan starts cooling the radiator) and then slowly I must push the cable operated choke until it estabilizes without choke and the it runs normally. If I stop the engine and start it again before 1 hours it starts and keeps running normally

I've read somewhere that choke should be used just for cold starting for about half a minute or so. If I don't keep it longer, the bike doesn't keeps running and so doesn't warms up to an "stable service temperature"

I'm about to dismantle the carb and try to clean it and replace what it may be dirty or worn out but before I touch anything, would like to hear from someone of you what may be possibly wrong and what I should take care of mainly to solve the problem.

As I said at the beginning, the bike may have been sitting stopped in a garage for a long time (not sure but looks like that because of the low mileage) also I live in a tropical region, so when I say "cold start" outside temperature is somewhat warm (23C degrees) the whole year (that should be around (75Fahrenheit more or less)

Thanks in advance for your help
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Old 01-27-2013, 12:15 AM   #1202
motolab
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Quote:
Originally Posted by rperedel View Post
Hello,

I own an Aprilia Motó 6.5 (Rotax engine + BST40 Carb) bought it second hand with around 5000 miles. Manufactured in 1995 so it has low mileage and seems to have been sitting for long periods

Not long ago got some spares from Motolab in the event of having to clean / rebuild the carb at anytime

My main problem seems to be just cold starting the engine and keep it running until it warms up. Then it just runs fine in any throttle/speed combination

As I've said, firing it up when it has been stopped for one or many nights is a nightmare.

The sequence is to pull the choke fully, and keep it starting like 6-8 times until it runs.... then I should not touch the throttle or push the choke for about 7 or 10 minutes until the engine gets really hot (fan starts cooling the radiator) and then slowly I must push the cable operated choke until it estabilizes without choke and the it runs normally. If I stop the engine and start it again before 1 hours it starts and keeps running normally

I've read somewhere that choke should be used just for cold starting for about half a minute or so. If I don't keep it longer, the bike doesn't keeps running and so doesn't warms up to an "stable service temperature"

I'm about to dismantle the carb and try to clean it and replace what it may be dirty or worn out but before I touch anything, would like to hear from someone of you what may be possibly wrong and what I should take care of mainly to solve the problem.

As I said at the beginning, the bike may have been sitting stopped in a garage for a long time (not sure but looks like that because of the low mileage) also I live in a tropical region, so when I say "cold start" outside temperature is somewhat warm (23C degrees) the whole year (that should be around (75Fahrenheit more or less)

Thanks in advance for your help
Make sure that the little brass cold start enrichment jet that is pressed into the cold start enrichment feed pipe portion of the float cage and the brass cold start enrichment stand pipe are clear.

Set the float height to 14.7mm:

Hold the carb in one hand and hold the base of the float cage tightly against the carb body with the index finger of the same hand. Rotate the carb so that the float tang just contacts the float needle's spring loaded plunger, but does not depress it. With the float height setting tool set to the desired height, use the other hand to drop it down over the float so the tool's posts contact the bowl gasket surface and the posts are square to the body (front to back - the tool takes care of side to side). Flip the tool to check both sides (as the float may have some twist in it). Bend the float tang until the tang on the float height setting tool just contacts the highest point on the float, but does not depress it.

Replace the pilot jet.

Set the idle mixture:

Start the engine and warm it up. Lower the idle speed below the factory spec. Starting from a setting that is known to be lean (1-1/2 turns is likely but not guaranteed to be), adjust the fuel screw to obtain the highest idle speed. Adjust to 1/8 - 1/4 turn richer than that. Then, adjust the idle speed back to 1300 rpm.

Regards,

Derek

motolab screwed with this post 01-27-2013 at 10:07 AM
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Old 01-27-2013, 01:48 PM   #1203
rperedel
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Quote:
Originally Posted by motolab View Post
Make sure that the little brass cold start enrichment jet that is pressed into the cold start enrichment feed pipe portion of the float cage and the brass cold start enrichment stand pipe are clear.

Set the float height to 14.7mm:

Hold the carb in one hand and hold the base of the float cage tightly against the carb body with the index finger of the same hand. Rotate the carb so that the float tang just contacts the float needle's spring loaded plunger, but does not depress it. With the float height setting tool set to the desired height, use the other hand to drop it down over the float so the tool's posts contact the bowl gasket surface and the posts are square to the body (front to back - the tool takes care of side to side). Flip the tool to check both sides (as the float may have some twist in it). Bend the float tang until the tang on the float height setting tool just contacts the highest point on the float, but does not depress it.

Replace the pilot jet.

Set the idle mixture:

Start the engine and warm it up. Lower the idle speed below the factory spec. Starting from a setting that is known to be lean (1-1/2 turns is likely but not guaranteed to be), adjust the fuel screw to obtain the highest idle speed. Adjust to 1/8 - 1/4 turn richer than that. Then, adjust the idle speed back to 1300 rpm.

Regards,

Derek
Hello Derek

Thanks for your support

Is there any other detailed info (step by step) about "Setting the idle mixture" (last paragraph) I could be reading to better understand the process?

Regards
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Old 01-27-2013, 02:42 PM   #1204
motolab
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Quote:
Originally Posted by rperedel View Post
Hello Derek

Thanks for your support

Is there any other detailed info (step by step) about "Setting the idle mixture" (last paragraph) I could be reading to better understand the process?

Regards
Is there a specific part of the instructions you are having trouble with?

Regards,

Derek
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Old 01-27-2013, 02:55 PM   #1205
rperedel
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Quote:
Originally Posted by motolab View Post
Make sure that the little brass cold start enrichment jet that is pressed into the cold start enrichment feed pipe portion of the float cage and the brass cold start enrichment stand pipe are clear.

Set the float height to 14.7mm:

Hold the carb in one hand and hold the base of the float cage tightly against the carb body with the index finger of the same hand. Rotate the carb so that the float tang just contacts the float needle's spring loaded plunger, but does not depress it. With the float height setting tool set to the desired height, use the other hand to drop it down over the float so the tool's posts contact the bowl gasket surface and the posts are square to the body (front to back - the tool takes care of side to side). Flip the tool to check both sides (as the float may have some twist in it). Bend the float tang until the tang on the float height setting tool just contacts the highest point on the float, but does not depress it.

Replace the pilot jet.

Set the idle mixture:

Start the engine and warm it up. Lower the idle speed below the factory spec. Starting from a setting that is known to be lean (1-1/2 turns is likely but not guaranteed to be), adjust the fuel screw to obtain the highest idle speed. Adjust to 1/8 - 1/4 turn richer than that. Then, adjust the idle speed back to 1300 rpm.

Regards,

Derek
Hello again Derek

1.- Do I lower the rpm with the idle screw just before leaning the mix with the fuel/air screw or, first of all I lean the mix and then go for a lower rpms?

2.- ... also, which way do I richen/lean the mix turning the fuel/air mix screw, clockwise or counter?

Thanks
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Old 01-27-2013, 03:07 PM   #1206
motolab
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Quote:
Originally Posted by rperedel View Post
Hello again Derek

1.- Do I lower the rpm with the idle screw just before leaning the mix with the fuel/air screw or, first of all I lean the mix and then go for a lower rpms?

2.- ... also, which way do I richen/lean the mix turning the fuel/air mix screw, clockwise or counter?

Thanks
Lean the mixture first. The rpm should drop some by themselves. The BST40 has a fuel screw, so screwing it in leans the mixture and screwing it out richens the mixture. The screw has a right-hand thread, so in will be clockwise and out will be counterclockwise.

Regards,

Derek

motolab screwed with this post 01-27-2013 at 03:13 PM
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Old 01-27-2013, 03:09 PM   #1207
rperedel
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Quote:
Originally Posted by motolab View Post
Lean the mixture first. The rpm should drop some by themselves. The BST40 has a fuel screw, so screwing it in leans the mixture and screwing it out richens the mixture. The screw has a right-hand thread, so in will clockwise and out will be counterclockwise.

Regards,

Derek
Great help

Thanks once more for your support
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Old 02-03-2013, 05:38 PM   #1208
rperedel
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Quote:
Originally Posted by motolab View Post
I'm sort of confused about this now, because as far as I can tell, KTM lists the same seal/gasket kit for all years of BST40 equipped models. However, I've never taken one apart yet that didn't have rubber lip seals in it. Has anyone else?

Regards,

Derek
Aprilia's Moto 6.5 BST40 do have felt gaskets seals on the shaft
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Old 02-03-2013, 06:51 PM   #1209
motolab
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Quote:
Originally Posted by rperedel View Post
Aprilia's Moto 6.5 BST40 do have felt gaskets seals on the shaft
I had figured that out some time ago, but thanks for the confirmation.

Regards,

Derek
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Old 02-09-2013, 06:43 PM   #1210
mountainduro
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TM28 carb issues

Hi I would like te recieve a good answer here as I can see you guys here are very good in carbs stuff;my problem is this: ,I have a Suzuki Drz 250 it runs ,accelerates and idles very good,the problem is when I do big stops or going down on steep hills the bike decreases acceleration,like if it chokes and turns off,but theres no trouble starting it again .What can it be?
its a TM28 pumper carb.
Thanks.

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Old 02-09-2013, 09:07 PM   #1211
bmwktmbill
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Quote:
Originally Posted by mountainduro View Post
Hi I would like te recieve a good answer here as I can see you guys here are very good in carbs stuff;my problem is this: ,I have a Suzuki Drz 250 it runs ,accelerates and idles very good,the problem is when I do big stops or going down on steep hills the bike decreases acceleration,like if it chokes and turns off,but theres no trouble starting it again .What can it be?
its a TM28 pumper carb.
Thanks.
I'd check the float level.
bill
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Old 02-09-2013, 09:38 PM   #1212
motolab
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Originally Posted by bmwktmbill View Post
I'd check the float level.
bill
Sound advice!

However it's worth pointing out that increasing the float height lowers the fuel level, and decreasing the float height raises the fuel level. The term "float level", can therefore be quite confusing because it is an amalgam of the terms "float height" and "fuel level".

Regards,

Derek
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Old 02-09-2013, 09:49 PM   #1213
motolab
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Quote:
Originally Posted by mountainduro View Post
Hi I would like te recieve a good answer here as I can see you guys here are very good in carbs stuff;my problem is this: ,I have a Suzuki Drz 250 it runs ,accelerates and idles very good,the problem is when I do big stops or going down on steep hills the bike decreases acceleration,like if it chokes and turns off,but theres no trouble starting it again .What can it be?
its a TM28 pumper carb.
Thanks.
I don't want to discourage you from asking questions, but do keep in mind that this is intended to be strictly a BST40 thread. I think it would be more elegant if you were to find a DRz250 or TM28 thread to post in, and if you couldn't find such a thing, to start one yourself. If you got no responses, I think it would then be perfectly acceptable to post here asking for people to check it out.

Regards,

Derek

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Old 02-10-2013, 12:43 AM   #1214
warewolf
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Quote:
Originally Posted by mountainduro View Post
its a TM28 pumper carb.
I had the same bike and carb. The carb was serviced by 2 different Suzuki shops trying to eliminate this problem. Also, I couldn't open the throttle fast - as befits gnarly off-road situations - lest it stumble. I reckon there is some inherent tuning/operating problem with the accelerator pump. I could improve mine by polishing the AP piston to reduce stiction and adjusting the throw but could never cure the problem. That was one reason I got rid of that bike. Once my research found the kind of in depth craziness WR4xxF owners went to in order to tune the AP action, I gave up since there were far fewer DR-Z250s pushed to that kind of performance that the likelihood of a guru publishing a solution was very low... and I wanted more bike so wasn't prepared to resolve it myself. Best of luck with it.
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Old 02-10-2013, 05:47 PM   #1215
joefromsf
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Hi Derek,

Do you have a Suzuki part number for the slide guide o-ring for 2006 DR650? I don't see it in the online fiche diagrams.

Also, if the CA emissions canister is removed, then the tube on the bottom of the carb should be capped, correct?

Thanks.
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