Common oversights?

[Moooools]

I just put new barrels and pistons on my CB125T, it was running beforehand, and now it will not start. :(

It has spark.
The HT leads are on the correct sides.
I can hear air being sucked through the filters.
There is fuel in the bowls and tank.
The jetting is the same as when it was running.

The only difference is that there is no huge oil leak past the pistons as there was previously. :yes:

So what I am thinking is that this is just an oversight of some sort.
Any thoughts?

Cheers
Max

[Kickaha]

The HT leads are on the correct sides.

But do you have spark?

[Moooools]

That was the first thing I said.:yes:

But you can feel free to ignore me now. Decided to take a filter off and put my hand over the carb. Hmmmmmm. Carb not sucking, Carb blowing! Timing out 180 degrees.:facepalm::facepalm::facepalm:

Feel like a dumbass.

[Kickaha]

That was the first thing I said.:yes:

Shows how much attention I was paying:facepalm:


Timing out 180 degrees.:facepalm::facepalm::facepalm:

Common mistake on those

[Arronduke]

Great thread... the best bit is you posted it....:shutup:

I did a simular thing when rebuilding my RG500... being a square four twin crank i had the second crank out of timing and you got it... I had a two cyclinder bike...

I think we have all done a simular thing at least once.

[Moooools]

It is interesting how similar the noise was from the engine. The pipes made the same noise, however come to think of it there was a lot of induction noise. Well it wasn't really induction. More like exhaust out the wrong end.:blink:

I have loved the learning curve of doing the rebuild. There is something nice about being able to diagnose problems with the engine and know how to fix them. And you can only really learn by actually doing it. I still give my brother shit for when I asked him to point out the exhaust ports, and he went hunting under the sump for them; ignoring the big holes in the front.:yes:

I can't really be stuffed fixing it tonight. I will wait until my mate is here tomorrow and when I am fresh. Hopefully it all works.

[fi5hy]

So we should see you at Kaitoke on the 17th????

[Shorty_925]

If the weather report is for :sunny: two of us will be down.

[Moooools]

So we should see you at Kaitoke on the 17th????

I don't know about that. :(

I had it running pretty well today, slightly rich, switched the jets and then had terrible carb leaking issues. It is like the float valve isn't working but when I take the carb apart and test it is seems to work just fine. I really can't work it out.

If I leave it with fuel in the bowl for a while it stops leaking??!!

The idle is also all over the place. Sometimes it is normal sometimes it is ridiculously high. I just can't get my head around what's going wrong.

Then once I get that all fixed I need to nylon it up and tidy up some bits and pieces.

So the 17th would be optimistic.

What I need is someone who knows their stuff to help me with the set up of the carbs. I just get lost too quickly. Offers? Anyone? Work for booze?:yes:

Anyway I will persevere for now. Trying different jets now.

[bucketracer]

.
These are problems I have seen here at work........


......... switched the jets and then had terrible carb leaking issues. It is like the float valve isn't working but when I take the carb apart and test it is seems to work just fine.............

The float can be set to high and touches the top before closing the needle valve or the float could be a little bent and catching on the sides of the float chamber.

If the brass needle seat has an "O" ring sealing it into the carb body, we have found that when they get old they can shrink and let petrol leak around the outside of the needle jet assemble and the carb floods even though the float valve is working properly.


...........The idle is also all over the place. Sometimes it is normal sometimes it is ridiculously high. I just can't get my head around what's going wrong...........

If the ignition is advanced from standard the engine will rev, this can happen if a mechanical auto advancer is sticky and needs a clean.

Or the carb cable is caught on something or not sitting down into the carb adjusters on the carb cap or to short and pulling the carb open when you turn the handle bars.

Then the obvious but hard to see manifold air leak.

[TZ350]

A mistake I have seen is carb slides swapped side to side or turned around and hanging up on the idle adjuster screws.

[Moooools]

I think the float is catching on something. If I give the carb a shake it gurgles a little and then stops leaking...

I haven't tackled the funny idle yet. I am picking there is a leak where the old hard rubber grommet meats the metal manifold. I don't think it is well sealed at all.

What would be good to seal that join with. Keeping in mind I will probably not be able to get the rubber grommet off without breaking it.

I went up in jet sizes from 85 to a 92 because the standard 88 was better than the 85. The 92 is better again so I will try one more above, i they are 98. It should give me a good indication of whats happening.

[Henk]

Changing the mains should have liitle to no effect on the idle. I'd get the idle sorted first or you may end up muddying the picture later on.
If the inlet manifolds are leaking the idle should come down if you spray CRC on them when the the bike is runing. If anybody has a non flamable alternative I'd love to know what it is.
If they are shot relplacement is the best bet but I'm not sure how available they wil be.

[Moooools]

The leaky carbs are being problematic. I have a pair off of a TZR that I assume won't work but I thought I would ask here just incase.

Firstly: are 2 stroke carbs the same as 4 stroke carbs?
Secondly: will going from 2x 24mm carbs to 2x 28mm carbs still work? Or will they be too big to get proper tuning and throttle response?

[F5 Dave]

they will work, but probably not very well & likely the jetting will be miles out. 4 strokes & 2 stroke carbs have different emulsion tubes (with exception of rotary valve 2 strokes). I have seen people bang on 2 stroke carbs & go ok, but I suspect you will have more problems than you solve & you will be entering a big unknown. Also on a stock engine they will be too big.


ok so the stock carbs leak. Are they brass floats? Have they leaked & are full of petrol? What does the float valves look like? grooved (& with the pointy bit doing the sealing, I have seen them put in upside down, how we laughed).

can you take the brass seats out? is there an o-ring? you can sometimes fix the seat with a small ball bearing in the hole & a whack with a punch.

Have you tried adjusting the float height so the fuel level is lower?

[Moooools]

Yeah i thought that would be the case.

The floats are all plastic.
I had a thought about leaky floats but they never have petrol in them.
The valve is in the right way.
There is no O-ring the valve, the tip is rubber.

When I have just reassembled them I put a piece of tube on the fuel inlet and blow into it, then turn it up side down so the float valve closes and then try and blow in, but can't. SO I think it is sealing fine.

They only leak after they are rebuilt (after changing jets etc.) but if I leave them for a bit with fuel in them (but fuel turned off) they sort themselves out. They also sort themselves out sometimes if I shake them.

I think that the floats are hitting something and catching, but have no idea what.

My other theory is air being trapped at the top of the carb bowl preventing the float from rising. I am not sure about the merits of that though.

[all4A50s]

How are you getting on?

[Moooools]

Pretty god really.

I have exams coming up so the parents haven't been happy with me spending too much time in the garage, but because of the NCEA system the exams mean almost nothing to me. S I will have study leave to work on it all day.

It was pretty rideable. I managed to go for a few laps around the streets but the rocker gear cover is leaking oil under the seal. The previous owner of the engine rooted one of the cover screws and It won't stay down. I will take the engine out and tap those out to M8 bolts.

It could have still gone up a few jet sizes but the leaky carbs were pissing me off so I am replacing them with two FZR 250 carbs. They won't be quite on the right angle compared to the massive downdraft of the FZR, but they should be on a good enough angle to work.

I have done some of the nyloning using the stuff they making chopping boards out of (can't remember what it is called), and have some chopping boards to do the forks and mufflers. I will try and laser cut them at school this week.

I hope to set the 28th as my first outing with it. All going to plan that is.
The little things take the longest.

[TZ350]

There is no O-ring the valve, the tip is rubber.

They only leak after they are rebuilt (after changing jets etc.) but if I leave them for a bit with fuel in them (but fuel turned off) they sort themselves out. They also sort themselves out sometimes if I shake them.

I think that the floats are hitting something and catching, but have no idea what.


The floats hitting something has sometimes been a problem for us too......

222430

Also this is the "O" ring, around the needle valve seat, (top left) that Bucket was talking about. It shrinks over (a long) time and leaks causing flooding, we always replace these now as a matter of course .......

[all4A50s]

Just out of interest (and I know this is off topic a bit) has anyone tried to fuel injection to a CB125T before?

[Yow Ling]

Just out of interest (and I know this is off topic a bit) has anyone tried to fuel injection to a CB125T before?

Cant imagine anyone has.

Do you think there would be some advantage to it?

Maybe easier to rejet, but how often do you do that?

[all4A50s]

Cant imagine anyone has.

Do you think there would be some advantage to it?

Maybe easier to rejet, but how often do you do that?

It was more of a ponderance question. The benefit would be the loss of time you get for the vacuum in the carb to happen. From memory it's about 0.2 seconds every time you twist the throttle, which at the Kaitoke track would be about 2 seconds a lap (10 corners). So it might be small but it is an advantage.

Then you would also be able to potentially control the fuel/air mix through out the throttle movement. Which could give abit more as well.