Sluggish T250

[Tripleking]

I had the T250 out for the first time this spring. I just installed new Uni-Filter sock-style filters (original air box was long gone before I rescued the bike). I had small Uni-filters which I crammed on the bike just to break in the engine last fall. I jetted the carb until it ran well. I installed the four inch long 1-7/8 inch filters on it. It was definitely more sluggish on low rpms, but after 3,000 rpm it runs well. After 4,000 it begins to come on the pipe. Could the new bigger filter somehow be causing a flow problem?

P.S. It gets worse when the engine is warm.

[Craig380]

Sounds like the different filters have changed the airflow through the carbs, but it's hard to say whether it's now running too rich or too lean.

With the engine warmed up, try turning the pilot screws IN by a quarter turn. If it runs better around the 3000rpm mark, it was too lean. If worse, it was too rich. Take it easy on the road until you're confident the jetting is safe for the motor!

[Tripleking]

Understood. On a more positive note, I started the GT380 for the first time since last November (had the tank and side covers painted) It ran like a champ!

[Vintageman]

Tripleking

below 3000 rpm it is sluggish. That's pretty low rpm. Was this under light, medium, or heavy load?

But, possibly more importantly is how is it related to throttle position. Choose a throttle position and see if you can climb up from low to high rpm smoothly while not changing throttle position under test. You have to be on a flat road for example, just a little loading.

If the issue just lower throttle position. How is the condition of your Needle jet inside for example?
If all is smooth and shinny change the jetting circuit that applies mostly to the throttle position with the problem.

Sometimes we think it is rpm related for we give a little more throttle, that causes the carb to use other jet circuits and at the same time engine revs up, confusing which is the root problem.

Usually pods have greater affect 1/4 throttle and up and as more air flow comes into play or as rpms increase. Not so much low rpm small throttle in my experience

[titan performance]

A look at the plugs may reveal some indication.
If I had to guess, I would say you are flowing more air, and it's running lean. The pilot jet is now too small at low revs, making it sluggish, and when hot and the throttle open, the main is also too small. If possible, once the engine is hot, try and open the choke a tiny bit and see if it improves, if it does, then you know it's lean, if it worsens, it's too rich.
As stated above, take care until you know, as a melted piston is a pain in the butt.

[Vintageman]

Triplking,


I don't want to you to skip over Titan Performance with this post, So read his points as well.

My last post was a test for static carb position.

Another important test is dynamic throttle position.

-> If you are running between 3000-4000 OK and at small throttle, then you open throttle wide real fast it may #1 be OK, #2 go quite or hesitate for a moment, #3 stubble / gurgling until you get above 4000 RPM.

If #2 Pilot jet could be too lean. This will should up in the static test I mention too for wont rev up far.

If # 3 due to short pods you may have lost the momentum from of column of air you would have had with the air box and Y boot. Here the intake port is blowing back fuel as piston comes down causing air to pass through carb twice. One solution to this is to add a bit of hose length between carb and pod.


So as far as carb goes you have static test that verify a jetting circuit in you carb for right size or condition and dynamic tests.

Of course there is the electric side of things.... stick some fine tip plugs in case coil is week, verify battery up to charge, and points are gapped OK and not pitted whatsoever and timing OK

Mike S

[Tripleking]

Tripleking

below 3000 rpm it is sluggish. That's pretty low rpm. Was this under light, medium, or heavy load?

But, possibly more importantly is how is it related to throttle position. Choose a throttle position and see if you can climb up from low to high rpm smoothly while not changing throttle position under test. You have to be on a flat road for example, just a little loading.

If the issue just lower throttle position. How is the condition of your Needle jet inside for example?
If all is smooth and shinny change the jetting circuit that applies mostly to the throttle position with the problem.

Sometimes we think it is rpm related for we give a little more throttle, that causes the carb to use other jet circuits and at the same time engine revs up, confusing which is the root problem.

Usually pods have greater affect 1/4 throttle and up and as more air flow comes into play or as rpms increase. Not so much low rpm small throttle in my experience

It barely moves at any throttle position below 3,000 RPM. Once 3,000 is reached, it starts to accelerate. Upper ranges pull nicely. I removed the filters and ran it about a total of 1/2 of a mile and in ran nicely without filters. However, that is not something I wish to do.

With these filters in place and the bike under load (me at 250 lbs onboard. It barely responds to the throttle. It gets worse when it is warm. I forget which primary jet I have in it now (but I think 78 sounds right). The carbs are Mikuni VM26 two-stroke carbs and are brand new. I jetted them last fall. The bike ran very nicely with the old filters. The only change I made was the filters. The new filters are Uni 4 inch long with the 1-7/8 opening. They have been oiled as per instructions. The old filters were 3/4 opening 4 inch filters which were shoe-horned on and run without oil. I am thinking that the filter oil is blocking enough air flow when the RPMs are low to make it run sluggishly. At higher RPMs, especially with the throttle open 1/2 to full, there is enough velocity to pull enough air through.

Plugs are still normal because there are only 20 miles on the engine, 17 of them were when the engine was running fine on the old filters.

[Vintageman]

I installed the four inch long 1-7/8 inch filters on it.

That only (1.875" * 4") + ( pi * 0.9375"^2) = 10.26"^2

That is too small (didn't catch only 4" last time). UNI pods are usually heaver density foam and when oiled are very good at blocking fine dust and dirt. I think K&N type flow a little easier (thinner filter matter and folded for more area). That is much less area than the factory air filter provides.

If you like that induction sound see if the Y boot single large K&N offering for RD will work. I like Y boot for you get better ramming affect for both carbs pull from same source but at different times in these engines of course.

Or look for T250 stock setup. T350 has large box opening to atmosphere... you can modify factory box innards to be less restrictive until your ears hurt as well.

Or longer UNI or K&N

[Tripleking]

Cleaned the filters and it now runs wonderfully. I had too much oil. Pulls nice almost from idle.

[Vintageman]

. Pulls nice almost from idle.

Does it pull from idle perfect if no pods? Or same now you have more air flow through pods?

I assume when you pull from idle test your are not going WOT, just easing throttle a little?

I think you said you tried all throttle positions, just want to be sure you ease throttle tested too.

This interests me for I obsess over ensuring low RPM and small throttle running is perfect. Getting WOT right is almost trivial in comparison. Unless perfect street riding is never enjoyable as it can be

Please keep sharing you thoughts on this if more work is needed low speed tuning

[Tripleking]

It runs pretty much the same or without the filters. It pulls smoothly at partial throttle. The only criticism is that it feels like it is geared too high. From a dead stop in first, much clutch slipping is needed. Starting up hill is fairly difficult.

[Vintageman]

Thanks for reply

Maybe geared too high is another good thought?

Did you verify sprocket teeth front and rear? T250 I think had larger rear sprocket (41?) than T350 (38?). I ran the 38 on my 74 gt250 for a while until I found a new one (help from guys on this board).

It was OK, but not enough power to pull redline top gears which I wanted touse all gears fully. I run the 41 on my T305 and will T350 as well.

Other ideas

Maybe suffering from blubbers to do blow back intake at low rpm since you have no intake length anymore.

And, if it also occurs when running say 30 mph 3rd gear (whatever you need to make it in the 3xxx RPM), and it just doesn't run well until you open just a tiny bit more ... needle jet pitted, jet needle worn, needle air jet plugged a little. Pilot holes plugged. I know you said it didn’t have this issue but now that you have better air flow just be sure that fact still holds true. I have seen this many times (more time Needle Jet).The needle jet pitted inside is one often missed for need to look into it.

Of course the other thing, when that low rpm, the batt. is providing most of the energy for ignition so we'll assume all is well here. I assume mechanically sound engine too and timing OK. Iridium or other fine tip plug can help here too.

When a bike runs poor low RPM it is not enjoyable as it can be. For when running low rpm you are consciously or subconsciously making your wrist move trying to manually govern a steady slow speed when there are subtle problems: That takes work

[Tripleking]

It is actually running pretty well now. I know what you mean about intake length. Think the problem is gearing. I will count the rear teeth this evening. The engine is actually out of a GT250. Did the GT250 have a different front sprocket than the T250?

I can say this: The T250 can do 25 MPH in first gear at about 5000 RPM. That seems a little highly geared.

[Vintageman]

I am not sure if T250 same as gt250. Can you look it up here?

buymotorcycleparts.com/

I don't think its gearing even if front is one tooth more (that may be about 8% more?), but may be wrong.

You would make me very happy if you looked at your Needle Jets and made sure they are not worn/damaged at exit and then really look inside (bright light, good eyes or a few times magnifier) to ensure the first part of metering barrel inside not pitted. You will have to take them out to see.

My experience this sounds very possible the issue too. I have seen many like this. These carbs have set a long time with jet needle in needle jet very small space which was collecting dust/moisture/in our Oxygen corrosive planet for decades

Check jet needle too for wear, etc

make sure no crap in carb body air passage from Bell Mouth to Needle Jet air hole inlet. And nothing in jet needle air inlet hole area buggering up air/fuel atomizing. Very important stuff to run well where you are telling me. If not it we can focus elsewhere.

[Tripleking]

Curbs are brand new VM26s