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DT125MX big end bearing


Benjersey
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Evening all, 

Noticed the DT was running rough on the way home from work, vibrations and a worrying rumbling noise lead me to believe it was worth taking a closer look.

Pulled the head and barrel off and found lateral left to right play in the conrod caused I'm guessing by a knackered big end bearing. I'll split the case and get the crank sent off to have the bearing replaced. Can anyone recommend somewhere that would do a good job of this? no where in jersey can do it so it will be sent to the UK anyhow.

Thanks,

Ben

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Yip, its Grampian Motors,   liverpool.    just send the crank,   they have all the parts,   same with a rebore, just send the barrel,  they measure true, and bore the size required , :welcome:

http://www.grampianmotors.co.uk/

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Nice one thanks very much to you both for the recommendation. Will give them a ring for a chat tomorrow. Bore on the cylinder is like a mirror so doesn't need any work, am thinking about building it back up as a 175 , stay tuned B)

Cheers,

Ben

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  • 3 weeks later...

So before I embark on a conversion to a 175 top end I have a couple of questions that I would greatly appreciate help with.

Firstly I cannot seem to get a definitative answer as to what the standard carb on the DT125MX was some places I look say VM24 and some say VM26 can anyone shed any light please or a way to check.

Second question besides the head, barrel, piston, rings, small end bearing and new gasket set is there anything else that needs changing? would I for example need a 175 CDI this is assuming the carb is in fact a VM26 which is the same as what the 175's came with.

Thanks as always

Ben

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Carb is model dependant. Some even had a 22mm. Same type of carb.

Cdi may or may not be different. I would expect so but not by enough to matter in the real world. You really have 3 options. Some 125s were points. On the 175 you had 2 different cdi and you need the matching mag. 

It is generally accepted that the early cdi (79) with the later airbox works best. That does need a matching oil tank and moving the voltage regulator.

The 175 does have an extra friction plate in the clutch. You may or may not need it. In good nick i see no reason for a 125 clutch not to handle the extra 3hp.

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Fab thanks Cynic sounds like we are on, I know mine isn’t points so that rules out that option. I will build up with the existing CDI and see how she runs. Will take a gamble on the carb and just adjust the jetting as best I can to get the mixture correct. If not I’ll purchase a 175 CDI and mag if still not running right.

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  • 1 month later...

Quick update from me, not much has happened i'm still waiting on a piston to come down but have all the other parts ready. I'm still torn on whether to buy a new VM24 Mikuni rather than try to use the VM22(4J300) currently on the bike, this will involve drilling on oil injection port as it seems modern reincarnations do not come with them and originals are rare as hens teeth.

Fingers crossed the postman comes this week so I can get cracking this weekend and give you an update with more substance.

Ben

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If it were me, stay with the 22, until the very least you get it running. It works, you can trust it.

A new mikuni will be money that's wasted in my humble view.

The 22mm will not really restrict the engine in any way as they don't rev well, it may even improve low/midrange rpm performance. You need revs to make use of the bigger carb venturi.

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  • 2 weeks later...
On 23 April 2018 at 9:06 PM, Cynic said:

If it were me, stay with the 22, until the very least you get it running. It works, you can trust it.

A new mikuni will be money that's wasted in my humble view.

The 22mm will not really restrict the engine in any way as they don't rev well, it may even improve low/midrange rpm performance. You need revs to make use of the bigger carb venturi.

Cynic I am happy to take your advice and stick with the 22mm it's a 4J300, do you think it would need any increase in jetting for the extra 50cc?

Main Jet - 130

Main Air Jet - 0.5

Jet Needle - 4h16

Clip position - 2nd

Needle Jet - 0-2

TS Cutaway - 2.5

Pilot Jet - 22.5

Pilot air screw turns - 1 1/4

Starter Jet - 20

 

Thanks as always, 

Ben

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4 hours ago, Benjersey said:

Cynic I am happy to take your advice and stick with the 22mm it's a 4J300, do you think it would need any increase in jetting for the extra 50cc?

The 4j4 is the 175 version of your bike. Carb 3u500

Main Jet - 130 160

Main Air Jet - 0.5

Jet Needle - 4h16  4j6

Clip position - 2nd  3rd

Needle Jet - 0-2   0-8

TS Cutaway - 2.5  1.5

Pilot Jet - 22.5   20

Pilot air screw turns - 1 1/4  1 1/2

Starter Jet - 20  20

 

Thanks as always, 

Ben

I do have some  spare carb parts that are 175, need cleaning up and such, no promises their 100%  but you can have em for the postage. Will have a look in my bits box.

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  • 2 weeks later...

Plot thickens further guys, the reproduction cylinder barrel although looking well made has a different bolt spacing and size inlet port, new spacing is 70mmx52mm which is the same as a DT250 so I’ve managed to find a reed cage and carb boot from a DT250. Should be here this week and the fun can start.

Can anyone shed any light or advice on if the different reed cage/inlet port size will change the required specifications for the carb. I am attempting to stick with the original VM22 still using the specs posted above by Cynic.

Thanks,

Ben

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