Hi guys,

I've recently replaced the carburettor on my bike. The replacement carburettor I fitted was from a Yamaha TZR125 which, in essence, has the same overall engine layout and design as the DT125R. The carburettor I fitted was a Mikuni VM26SS which is what was already fitted to the DT125R before. The problem now is that when I try to kick start it, it starts for about three seconds before cutting out. When I kick start it, I can smell the fuel strongly so that means that the fuel is getting to the cylinder but isn't igniting properly. The spark plug hasn't been changed in at least a year so I was wondering if this could be the solution. If I replace the spark plug with a brand new one, will it start, and run, consistently?

Thanks

Heavy fuel smell implies it's flooding - overflowing float bowl? Jets the size of the Channel Tunnel?

Was it running OK before you swapped carbs?

Why replace one Mikuni VM26SS with another one?

I'd guess that the TZR is in a higher state of tune than the DT, so can flow far more fuel (probably needs to flow more), so bunging the carb from one onto a DT is like forcing your "one dry sherry at Christmas" granny to chug down a yard of ale ... not a pretty sight!

Or I could be wrong and you just need a new plug ....

Standard main jet size for a DTR (3MB Models) is a 210, for a TZR (2RH Models) its a 180.

Try the plug and swap the jets over from the DTR carb. Although i think it should still start with the 180 in.

Also you may want to consider giving the TZR carb a good clean through as it could be the floats are stuck or a few other dirt in card related reasons.

Edit: The power jet on the TZR is 110 where as the one on the DTR is a 60, this could be the source of your poor-running. If possible, swap the float bowl over from the DTR carb and see if that helps. ( the power jet is part of the float bowl and not changeable as far as i know.

Asking for a guaranteed answer over the internet from people who haven't got any direct contact with the bike is asking alot. We can only take a guess with the information given by yourself, which can never compare to actually being there with the bike.

The previous carburettor had a broken choke, a broken float valve and it kept pouring with petrol whenever I turned the fuel tap on. It needed to be replaced. I still have the old carburettor for spares. If I replace the spark plug and it still doesn't kick start properly then I'll take the carburettor from the TZR; strip it down, clean it thoroughly, and replace the jets with the ones from the DT carburettor. Surely this should have the same effect as fitting a new DT carburettor. After fitting the refurbished carburettor, the cylinder should have no reason for flooding and, with the new spark plug fitted, it should ignite properly aswell..... theoretically.

The previous carburettor had a broken choke, a broken float valve and it kept pouring with petrol whenever I turned the fuel tap on. It needed to be replaced. I still have the old carburettor for spares. If I replace the spark plug and it still doesn't kick start properly then I'll take the carburettor from the TZR; strip it down, clean it thoroughly, and replace the jets with the ones from the DT carburettor. Surely this should have the same effect as fitting a new DT carburettor. After fitting the refurbished carburettor, the cylinder should have no reason for flooding and, with the new spark plug fitted, it should ignite properly aswell..... theoretically.

Edit: The power jet on the TZR is 110 where as the one on the DTR is a 60, this could be the source of your poor-running. If possible, swap the float bowl over from the DTR carb and see if that helps. ( the power jet is part of the float bowl and not changeable as far as i know.

You don't even need to take the carb off. I would try that 1st tbh. 

I had a power jet block when i rebuilt my first DTR, gave very similar symptoms to yours iirc. Took me weeks to figure out what it was. 

Hi folks,

I fitted the new spark plug three days ago and it still hasn't made any difference at all. It still has good compression but it isn't igniting the fuel so now I can only assume that the cylinder is flooding. Next, I'll thoroughly clean the carburettor and fit the jets from my spare DT carburettor to it to stop any chance of the cylinder flooding again. Apart from that, do you guys have any other ideas why my bike won't start properly?

Thanks

Does the bike run at all?

Did the bike run before the carb was swapped?

How can you tell the bike is flooding? IS the plug soaked with fuel when you chop it?

First 3 things i would check. Fuel (air/petrol) - Spark - Compression.

I will say again, swap the float bowl (carb bottom) over and try it.

Have you checked:

The fuel for contamination (water, debris etc.) ?

The breather on the carb?

The reeds for cracks etc?

Ran a compression test?

Loose/corroded/damaged electrical connections?

I've not been working on the bike for quite a while due to school work and being too busy but on thursday I had some time to have a look at it. I replaced the spark plug with a brand new one (NGK BR9ES). Also, about a month ago, I bought a second hand carburettor off Ebay from a Yamaha TZR125 (Mikuni VM26SS). It's the same carburettor as the one fitted to the DT but has different jets. Since I've fitted the new spark plug, It hasn't solved the problem. It's still got good compression but it's not igniting the fuel. The only other thing I can think of is to thoroughly clean the TZR carburettor and fit the jets from the DT carburettor to it. Hopefully this will stop any chances of the cylinder flooding. But when I try to kick start the bike, I can smell the fuel strongly when I pull back on the throttle. This means that the fuel is definitely getting to the cylinder but there is no obvious reason why it's not igniting. I've fitted a new spark plug with a stronger spark than the previous plug so that should've made a difference when I try to kick start it, but it hasn't. Me and my mate have tried bump starting it and kick starting it loads of times but it still won't start properly. The last time it has started was when we fitted the new carburettor to it which was about 2 months ago. It started for about 2 seconds before cutting out. I waited until the next day before trying to kick start it again. It started on the first kick so I left it for a few seconds then gave it a rev. As I revved it more, it died down and cut out. It hasn't started since and me and Derik are out of ideas as to what could be wrong with it. The last thing we can try is to thoroughly clean the new carburettor and fit the DT jets to it. When I bought the bike, which was in august, it had a few problems. It has a broken kick start return spring so the previous owner personally found it easier to bump start but whenever I tried it, it didn't work so I just stuck to kick starting it. The old carburettor used to leak quite badly hence why I bought a replacement. I could only manage to get it started if the engine was warm and the cylinder had dried out. Everytime I started it, the engine eventually flooded and I wound have to wait a while until it had dried out to start it again. Since I've bought the bike, I've resprayed the frame black and resprayed the panels blue. It's a possibility that the wiring could be messed up. If I swap the jets and it doesn't make a difference then I'll have a look at the wiring.

 Sorry i can't help 

***Please someone else try to talk to this lad, he either isn't listening to me or i'm speaking the wrong language, i give up.  

Lol, sorry Vez. I'll try changing the float bowl and tell you how I get on.

Awrite guys,

Do youse think that a dirty carburettor could be the problem for my bike not starting?

Cheers

You need to do a compression test, it is quite possibly well down on compression which will make it difficult to start. Try squirting a bit of oil in the plug hole and try it again, if the compression is low this will increase it temporarily...then test the pressure properly

check to see it the magneto is generating electricity to the spark plug....

on my dt125 i had kinda the same issues... i would kick it over and over and over it would not start... a few times it started up but then bogged out and died.... turns out the PO grounded the magneto with a wire to the head fins and it got all dirty and corroded causing a grounding issue.....

anyways i took the magneto out and cleaned the contacts and re-installed it with a new bracket so it stayed in place and grounded properly to the frame.

1st kick after that it started right up.

also check that the tube on the top of the carb that goes to the fuel petcock is connected.

Awrite guys,

Do youse think that a dirty carburettor could be the problem for my bike not starting?

Cheers

Are you really that daft.

Vez knows these bikes better than his own todger, WHY are you ignoring him. I also completely agree with EVERYTHING he has so far suggested.

Did a compression test and the bike has very high compression so that's good. Because of the weather, I stripped it down and cleaned it all, etc, and the spark, the magneto, and all the electrics seem fine. My uncle's mechanic came over to have a look at it and he said that the problem is with the carb. There's a few hoses missing and the choke's not working properly. I bought the carb in september and it was from a TZR. The same carb more or less but with different jets. Even though it has different jets, it still should have at least kicked into life. The carb on it before was absolutely knackered.

Did a compression test and the bike has very high compression so that's good. Because of the weather, I stripped it down and cleaned it all, etc, and the spark, the magneto, and all the electrics seem fine. My uncle's mechanic came over to have a look at it and he said that the problem is with the carb. There's a few hoses missing and the choke's not working properly. I bought the carb in september and it was from a TZR. The same carb more or less but with different jets. Even though it has different jets, it still should have at least kicked into life. The carb on it before was absolutely knackered.

What was the results of the compression test? ( in PSI etc)

Yes a wrong size jet can stop a bike running, i have had a power jet pipe issue and the bike would just about start and tick over after 10-15 kicks, as soon as the throttle was touched it would stall.

Have you tried changing the float bowl yet? It is far easier than any of the other things you have tried.

The only thing i have found to knacker a carb on a DTR is a damaged mixture screw, so i am interested to see/hear exactly what it was that caused you to come to that conclusion. There will have been salvageable parts surely!

What was the results of the compression test? ( in PSI etc)

Yes a wrong size jet can stop a bike running, i have had a power jet pipe issue and the bike would just about start and tick over after 10-15 kicks, as soon as the throttle was touched it would stall.

Have you tried changing the float bowl yet? It is far easier than any of the other things you have tried.

The only thing i have found to knacker a carb on a DTR is a damaged mixture screw, so i am interested to see/hear exactly what it was that caused you to come to that conclusion. There will have been salvageable parts surely!

I don't know what the reading was in PSI but the mechanic assured me that the bike has good compression and that the problem lies with the carburettor. That's exactly what happens with my bike, 15-20 kicks to start then as soon as I touched the throttle it would stall.

The mechanic took the old carb away with him to try and fix it himself so I haven't been able to swap the float bowl yet.

When I bought the bike, the previous owner described the carb as being knackered (gushing fuel, dodgy jets, missing hoses, and a broken choke) so I decided to just buy another. However, even though the old carb was in bad condition, it still worked better than the current one so when I get the old one back I'll try swapping the float bowl over first. If that doesn't work, I'll buy a carb repair kit for the old carb that was on the bike before.

Hi I have a 2rk tzr with dt 3mb barrel and head. Should I use dt carb or tzr carb setting?