T305 Ex port raised 2mm to 30.5mm not good

[Vintageman]

Suzuki T305. Has similar porting specs to T350 accept Ex is 33.3 mm published versus 32.0 mm (not sure where I got T350?)

I decided to raise my t305 Ex port 2 mm to 31.3 mm (my R5 at 31mm was my goal)

I used piston to take height measurements accurately with cylinder on bench. The Ex stock was 1.270” (32.26mm) and 1.280” (32.51) not 33.1mm? Also, the piston edge was very close to cylinder top maybe down only 0.010” (¼ mm) before I removed. The compression was 145 psi and 150 psi before porting (not sure which cylinder was which now)

When done porting both cylinders measured 1.200” ( 30.48mm). New rings while in there and very light hone with 320 ball hone stones. Cylinders were real decent for ~3xxx miles. I did not shave head any amount and figure I would try it first. Put back together and 145 psi even both sides.

Unfortunately, it has lost most of its power below 7000 rpm it seems to rev well and hold beyond this, but, only have 200 miles and did not want to go too high yet but did see +8500 for example OK.

But, if I now climb say a 3-4% grade at 6000 rpm it start to drop RPM. Give more throttle (> than ½) and loses power very quickly. Even on a flat if you give more throttle to accelerate same problem.

History: I ran the bike first year or so all stock, next added chambers and maybe last year or so with 2mm off piston skirt only. With 2mm off piston skirt my “Butt” Dyno told me the bike had more power. I justify that with my “Eye” Dyno for before the bike was done at 8000, and dead after 8500 rpm. With 2mm off skirt it pulled hard beyond 9000. Also it would hold the same grade above no worries at 3500 rpm if you did not want to shift and 4000 rpm it climbed like a goat. If you were at 3500 rpm and cracked the 32mm carbs quickly full open (silly to do that) it may not take it but, back off only a bit maybe to 7/8 throttle no problem (don’t recall all stock).

I wanted to stop at 31.0mm (like my torque R5), but oops went 30.5mm (well RD350 is 29.5mm and same with GT250A).
Any ideas?

Here are mine
1) I went too far and 30.5mm not wise for T305
2) The transfer are too low for that Ex height
3) Should raise compression some. Maybe ½ mm off head most I can get given piston deck height without reshaping head
4) The 2 mm off skirt previously done had placed the intake timing power pivot point at 6500 RPM. The pivot was slight tip / slope before but, the EX port change amplified this slope about the pivot.

Can’t fix 1) tough for me to do 2). I still have set off stock pistons and can put them back in to see if that helps with nothing off skirts.

Too old to “F”up but did again… or at least without spare parts to back out this time.

Be nice, I am already ashamed .

[Suzukidave]

Have you tried moving the timing .. advanced or retarded a bit ?

[jabcb]

Am no expert on this, but this might provide some insigt.

Suzuki hopped up the GT250 in 1976.
They raised the exhaust port 2.5mm & increased the number of transfer ports.
They also retarded the timing from 24 to 20 degrees.

See service bulletins GT-30 & GT-32:
http://www.ozebook.com/compendium/techb ... 027-33.pdf

[jeff kushner]

Without knowing the height of the transfers, it's really impossible to suggest a solution that's guaranteed to work however you have apparently overshot the mark for the blowdown or changed the roof angles of the transfers. Did you flatten the transfer roofs?
A good run followed by a close inspection would tell you what's going on inside the cylinder such as seeing if the primary compression charge being forced back down the transfers (streaks pointing downward inside the transfers. ) instead of fanning out above the piston but my guess is that if you raise the transfer roofs about 1.5mm, you'll regain your top end. The hard part will be raising the floors of the transfers and they will have to be raised. Some guys have used J&B weld or engine epoxy to do it. After reading many of your posts, I'm confident that you know what you're doing enough to find a epoxy or substitute to work for you. It will be very important to make both sides of the transfers equal and identical to keep them balanced.

Sorry, I was interrupted and wasn't done yet<LOL>. 90% of the time, a engine can be saved. It might not be the power-monster that you were aiming for but at worst, you end up with a torque monster!! I was going to add that TZ can run the numbers of what you presently have since he has the MOTA program and can at least point you in the right direction.

jeff

[Vintageman]

Good point. I did not change timing. I'll try 2.0mm

I have published the transfer height in other posts (even I can't find right now) but what I'll do is get all the data and put here. The T305 manual publishes all these dimensions. from what I have seen the T350 is the same but maybe a 1/2 mm more off Ex.

I'll take the numbers and run though them through the free timing software (get degrees) out there before and after

What I have now is a T305 with 2mm of intake (via Skirt) 2mm off Ex (Same shape as EOM), 24 -25 degrees. Stock Jetting/Air box and D&G chambers.

Considering I have a couple extra sets a Piston I could try notching piston at transfer to see if it helps first

I fear I may have went too far for a street engine. I need to put a couple more hours on engine and see how it goes top end

[tz375]

2mm on the inlet drops power below 6,000 a couple of HP and adds a similar amount at the peak. Raising the exhaust does the same thing but now the peak is at 8,000-8500 instead of at 7,000. When the motor is broken in, try letting it rev higher - more like an RD.

Before you modify the pistons or transfers, run it and see what it's like at higher revs. You should find it much stronger at the top and it should shut off with no over rev at around 8500-9000.

If you decide to change the transfer ports, the key is to make them all exactly the same height and angle and width.

I didn't follow Jeff's comments about raising the floor of the transfers. The floor of those ports flows very little and as long as they are not too far below the piston crown they should be fine.

[Vintageman]

TZ375

Thanks.

I'll do that and see what it does top end and where it like to run.

Sounds like it may be a good idea to move timing towards 2.0mm too just to keep heat off piston and into pipe for 8000-8500 rpm. Just like RDs and GT250A.

Also, I suppose if I do want some bottom back I could put in the stock pistons and still see a change in peak of 1000-1500 due to Ex mod alone. Make sense?

[tz375]

The changes should boost top end at some cost low down. So far you have experienced that drop low down but haven't had a chance to see what it's like at higher revs.

I'd run it up through the rev band and see how you like the changes. It will probably be happier running slightly lower overall gearing than before to let it run a few hundred revs higher at any given road speed. You may like the more revvy nature of the beast.

I wouldn't make any more changes (apart from ignition timing) until you get a better feeling for how it's running now.

[Vintageman]

The title of this post needs to be changed to " I'm an idiot".. Justification: I just finished my T350 engine rebuilt over the winter. That bike is all stock but D&G and 1/16" off skirt. The Ex port on that one was about 31.2mm. That bike runs very well.When I pulled out my bike with T305 engine It had no issue (I thought) but, just not as powerful as the T350. So believing the Ex port was 33.3mm as published just assumed that must be the diff.

When I measured 1.280" at start, I never checked if that was 33.3mm: it is 32.5 mm. Mistake #1 for I proceeded to take off the 2mm thus overshot the 31.2 by a bit more to 30.5mm. I need metric gauges if I am going to forget to convert to and fro. OK still 30.5 mm is not too far?

Tonight I adjusted the timing and found it was too far retarded. Mistake #2 and proved to be a big one.

I was going to set it to 2mm (0.079") direct ( RD specs). What I found was the points did not open until 0.033" BDC direct. I am not sure what that is degrees but, not many. In fact, the mark on the rotor was just barely visible as it past the static mark CCW in the allotted viewing window. Well both cyls wore the points about same amount too. I guess I rode that bike a lot last year.

I set the bike at ~22 degree. Decided I would split the difference between RD and T350. I really don't rev up long time when I do and more midrange law abiding street riding guy so why not 20 degrees to start (I can play with and more exactly later).

Now it runs nice! It was dark when I tried it and the head lamp on that one is real dim . but cruising around +/-5000 rpm I gave WOT no problem. It swallowed it just fine , ran up the tach and pulled harder as it increased rpms. I think I had hit over 8000 RPM.

Engines with more radical exhaust timing appear fussier than one that are not. I'll have to watch that more closely on this. And better check all my bikes

I did not want 30.5mm more like 31.2.mm but, it seems like it is going to be exciting too and different than the T350.

Thanks everyone I am glad I asked for your help

Fun stuff!

[tz375]

Glad to hear that it's running how you wanted it to and it just needed a few more revs and some adjustments.

Keep having fun

[Vintageman]

I still have the spare set of: GT550 cyls @ 0.5mm OS, new pistons to match with 2mm off skirts, 2mm off Ex port, and 3/4mm off head all ready to go on to my GT550K from last winter. You know motivated by the "RD400 Jugs on a GT550" post. (Bad influences )

I think the 2mm off a 60mm stroke engine (gt550) versus a 54 mm like (T305) should be less of a peak shift in power. I really just want to shift peak power 750 rpm. I'll run this T305 a little more and see if too much (can't ever drive relaxed) if not maybe I'll take another 1/2mm for 2.5mm on the Ex of GT550 (I'll never learn). I better determine what I have for height first and not assume the starting point was nominal. Or be more of a pro and get a degree wheel. ... me pro... nah.

Forget. Did we determine the top of Ex port on GT550 was 40mm (1.575")nominal?

[Suzukidave]

Here ya go print out a degree wheel for free https://www.google.com/search?q=Two+str ... 11&bih=587" onclick="window.open(this.href);return false;

[ja-moo]

A little late to the party, But glad you got it almost worked out. Street porting is rather mild, and "usually" 1 to 2mm raise in the EX port doesn't do too much damage to the bottom end. A bit more blow down with raise where the power comes in, and give you less back flow through the transfers. The intake timing has a decent effect with small changes, but your 2mm isn't that bad, as the intake timing is probably tame also. You really do need to know what your blow down is, if you do decide to work on transfer heights, and with rod angles, a minor change in transfer height can be a few degrees change. It's almost 2 to 1 compared to the exhaust. Not sure about the transfer port bottom mods either, not sure where that is coming from. There is almost no flow as the piston gets near the port bottom anyway. And it actually helps what little flow there to have the piston above the transfer port bottom.

Also, if you have a few miles on it, it is good to go ahead and run her up to redline, for a short one gear burst, a few times, with some cooling in between. i run to redline right off the bat with a new motor to seat the rings. you just only do it in spurts.

It is a good idea to degree wheel your port timing, you want your blowdown in the correct range for what you want out of the motor. And I wouldn't put a lot of comparatives with the RD, as it is a totally different motor with stroke and all. So it really doesn't cross over to the Suki......

[tz375]

RD400 64 x 62
GT550 61 x 62

That's where the attraction comes from. It's the old story of scope creep. I wonder what a 64mm might do to my motor. Oh. The 64mm pistons are all for reed motors. I wonder what a reed top end off a 400 might work like? It'll get you every time.

Stock GT550 port times are incredibly mild - and that's probably why it's such a pleasant (soft) bike to ride. They make 38HP at 6250-6500 when set up correctly, so there is a lot of scope to warm them up. I don't remember where this data comes from but according to my notes, Stock Inlet opens at 294 ATDC, Transfers at 124 and exhaust at 101 for 23 degrees of blowdown. RD400 has a lot more of everything - including transfer port area.

The 350 motor is not like an RD400 but at 61x54 it's comparable to an RD350 in terms of starting to understand the difference. Again the RD has more port time are and it has reeds to help the bottom end. T350 has around 26 degrees of blowdown with timing at 293/119/93 according to someone and those are all pretty mild too. Lots of scope to warm things up.

[ja-moo]

Wow, 26 degrees of blowdown, that's enduro numbers. I had a 550 for a short time, and it did kinda run like an enduro.....lol