Trials; 90 vs 100

[JPark]

Looking at the specs, it doesn't seem that there's all that much advantage to using a 100 motor over a 90 for trials. The torque is higher at 5500 rpm but the port timing would suggest that the low end would be more linear with the 90. Pistons and rings are still available for the 90, the bushing in the rod is not a disadvantage for trials usage and the four speed box is pretty much the same ratios in the first three.

Is there any other real downside to the 90 I've overlooked? I've never owned one, but one may be coming my way.

[thrownchain]

In trials you should never see 5500 rpm. You want torque at low rpm , geared accordingly.

[JPark]

Is there any history of lowering the exhaust port and raising the intake port for more low end torque and throttle response? I have access to full machining and welding. In theory, the cylinder wall should be cast iron, but 2mm or so of steel shouldn't be a problem especially when the main event is pretty much over. The intake could be built up with epoxy, but I'd never feel secure about having that come off and take a tour.

I know, a reed valve would fix the intake side, but I'm theologically a piston port guy, just 'cause.

[Al Harpster]

That's an important question.

I don't know, but I'd be inclined to say yes, lowering the exhaust port height by 2mm may give more HP at lower rpm.

The epoxy idea is intreaging, but the temperatures may be too high even for a test run.

That said, I'd try it if that's what your after.

My two cents.

[JPark]

I'll have a talk with my welding guy about what, if anything , he could come up with to lower the exhaust port. If there's nothing, then all I can do is build up the intake port floor with something and see how it goes. That, and a long header trials exhaust should flatten things out.

Apparently, epoxy has been used to reshape transfer ports, so the intake port should be plenty cool enough. I still wouldn't trust it unless it's well pegged, especially with the piston skirt flying by.

[olddogs]

I believe you could find better results and get the greatest impact exploring venturi dividers. They impact throttle response in the 1/4 to 1/2 throttle positions. They straighten and smooth out airflow at low rpms and increase torque. I use 2 vanes in my 125 Hodaka racer. Much easier than major cylinder modifications which could lead to unexpected changes across the board.

Another simple rule of thumb to increase torque and low end is to use a smaller carb. These changes are easier to reverse if you dont get the results you want.

[Al Harpster]

The carb modifications noted by old dogs make a lot of sense.

Only guy I know that'd do a good job on this would be Rich at Rich's Taylored Porting.

His web site has a lot of info on this.

A 20mm mikuni is pretty small but he might also be able to put in or make an Ultimate Flow Optimizer for the slide.

I think he's indicated that the UFO + the vanes would be a total package.

This is going to cost you some bucks. But unlike cylinder modifications you're not taking chances of permanently screwing up the cylinder.

Rich has been a regular participant on this forum, but not much lately.

I've used his services & I can recommend him.

I suggest you call him directly. His phone number is on his website. He may be able to help.

[JPark]

If you take a 250 Ossa Trials as an example, it uses a 24mm Mikuni carb. Proportionally that would equal 15mm on a 100 Hodaka. That's been my experience; the last 3/8 of throttle is redundant until you get on the pipe. That's why they don't use a 32 - which is proportional to the 20 on the 100 - on the Ossa other than for MX.

So I'm not seeing how a 20mm carb is small on a 100. Size goes by the square of the diameter. Because a small motor can inherently rev higher and is less catastrophic when peaky, you can use bigger carbs on them but at low rpm it's pretty proportional. I'd consider using a limiter of some kind to restrict the throttle to 3/4 and live with the carb being a bit large. Hard to get your head around a 20mm carb being too big....

[Al Harpster]

I see that mikuni has offered a 16mm, but I can't find a site that has one in stock.

Plenty of 18mm mikunis available. $80 give or take.

Seems like you'd need to work up jetting on these. Maybe from scratch.

Plus you'd have to figure out how to flange mount them.

It would be interesting work.

[dirty_rat]

As for carb's, you could also check with www.mid-atlantictrials.com they are selling a D slide carb (OKO Carb, Taiwan version, not the Chinese knock-off from ebay) for trials and they have them down to 19mm. I have heard of quite a few people that have switch to these carbs on their vintage trials bikes and like the results. Just something to think about.

[JPark]

The welder says he can build up the ports with silicon bronze. After looking at the port timing specs for the 90 vs the 100 the numbers for the 90 are already pretty reasonable by comparison. From what I know, you don't want the exhaust opening too close to the transfer opening in order to allow the cylinder pressures to drop so the exhaust doesn't head down towards the crankcase. I'm leaning towards just doing the intake side, which should be a lot easier to weld and not associated with the rings and such. The 100 is 131 duration and the 90 is 122; I'd guess that going closer to 110 degrees would still give plenty of time at TDC to fill the crankcase at trials rpm.

Anyone have some trials porting experience or thoughts on the matter?

[Bullfrog]

I have two thoughts to throw in. The first one relates to the very first post in this thread, and the second one relates to the post just before this one.

1. Speaking for myself - I have yet to ride a trials bike with less than 125cc displacement which would "loft" the front end virtually instantly on throttle command. Since I consider that an important trait . . . I'd recommend a 125cc engine over a 100 or a 90 (and a 100 over a 90). (My two cents.) All 100cc (or smaller) displacement trialers I have ever ridden require about a "two count" advance notice when lofting the front end.
2. If you are going to modify intake timing (only) on a piston port 90, I'd HIGHLY recommend using JB Weld - as opposed to actually welding on the cast iron cylinder - to achieve the necessary port timing change. I have been running a Combat Wombat cylinder on my Enduro/MX scooter which features the use of JB Weld to convert the (reed modified) cylinder from reed valve back to piston port for several seasons -- without problems. The JB Weld modification has proven to be reliable - and is completely reversible too.

Ed

[JPark]

To my surprise, I just discovered that the port timing of a Bultaco 250 Sherpa T is/was EX 156, TR 128, IN 122. The 90 is EX 160, TR 128, IN 122. The 100 is EX 164.5, TR 128, and IN 132. The Wombat is EX 162, TR 128, IN 139 or basically the same as the 100 with more intake duration. This is good news, as the port timing seems to have been lifted from a Sherpa T for the 90, which isn't surprising, given the era.

As 1mm is about 4 degrees, lowering the 90s EX port that much would put the timing identical to the Sherpa. Not at all worth bothering with. On a 100 you could probably raise the intake port floor by 3mm and drop the Ex port by 2mm for more low end. Hopefully.

But, as you say, starting with a 125 makes way more sense - if you have one.

[Joe Ormonde]

I can tell you that the number One thing you can do is to RAISE THE COMPRESSION. You can bore the ACE 90 Cylinder +.040 and install a Model 93 Super Rat head and gain more Power than messing with the Cylinder. The best way to find out is to keep trying different things. That`s where I would start: More Compression. Joe Ormonde.