Next mods

[Relytt58]

I have a fairly stock bike and am looking for more out of it. I want to eventually do a 421 cub but that is down the road. I had a 4 mil crank with 115 rods with a 68 bore cub block that I was slowly getting all the parts together to put on until they got swiped out of my garage when I had roommates a few years back. I am not wanting to drop that much money into this thing right now seeing how I have twin 3 year old boys and work in Alaska for the summers. My sig has mostly everything I have done to it so far but like most I want more out of it. I know having a good port job is probably the next step for me but I just want to see what everyone on here has to say about it. I would like to keep getting things that will be compatible with the 421 for when I do end up getting it. I plan on ordering a cool head this summer when I get back from Alaska. Any input would be much appreciated.

[Relytt58]

Also I did a compression test alittle while back and I am sitting at between 115 and 120 psi on both cyl. I am not sure how much was shaved off the head I had that done about 5 years ago and dont remember. That to me seems low to me but I know that I never put a top end in it and I don't think the guy I got it from did either. This bike has been run for maybe 5 hours in the last 3 years. It has been sitting in the shop looking good for the past few years with the occasional 30 min ride around the property but the kids are getting to the age where we can start doing some small dune trips when we have time. I remember on the last trip I got smoked buy a guy on a big bore yfz on pump gas and I didn't enjoy that one to much so I went and borrowed my buddies cascade built 421 and walked by him just to make me want a 421 even more. The 421 will come with time. When I do go riding it is about 60% dune riding and 40% drags.

Edited April 1, 2015 by Relytt58

[baxterl7333]

A 421 with the right port work will make a big difference especially if you run alky. But for right now, port work to your stock cylinders, a timing plate, and pipes should satisfy you for the moment

[tfaith08]

If you plan to upgrade to an aftermarket set of cylinders, don't put the money into porting the stock ones IMO.  Buy things that you'll be able to use with the cub.  A good head that used PD domes is a nice addition, as well as a billet timing plate.  T5s will limit the cub though.

[baxterl7333]

That's true. Just didn't know how long it would be before he upgraded

[Relytt58]

What should the compression be at assuming brand new top end with the shaved head? I know that's hard to know not knowing how much was taken off the head but typically what are the numbers? I think for now I will get a cool head for it and leave it alone for now because I you guys are right. I have it in my head that I want a 421 and it doesn't make a whole lot of sense to put money into my stock cylinders. What are your guys suggestions on a cool head? Would I have a problem with putting some PWK 35's on there now or would I have problems of not having enough velocity to pick up the fuel? As far as the timing plate you guys were talking about I already have one in there with it set to around 4 degrees. What set of pipes would you guys suggest?

[tfaith08]

For every .010 (.25mm) that is shaved (up to about .050"), you can expect a reduction of .8cc in the dome.  A full .040" is equivalent to 3.2cc reduction.  That would put you around 19.5-20.5cc domes.  Those generally give about 165-170psi at sea level IIRC.  Somewhere between 125 and 170 would be my guess, but that's a mathematical conclusion.  I don't have any hands on experience with shaved heads.

 

For a head, I'd recommend a Pro Design, Chariot, or Noss Machine.  Don't use anything that requires proprietary domes.  For instance, I just heard that Trinity doesn't sell domes for their head anymore (idk why you'd ever use them anyway).

 

35s without port work on T5s won't make that much power.  35s on cubs would be great.

 

As for pipes, I'd go with Shearers.  They'll run fine on stock cylinders and a cub.

[RagunCajun]

If you are set on a cub, id just keep what you have running well. Then just save money a side for the complete cub set up. That way if family stuff comes up, you have a chunk of money just in case.

[tfaith08]

^ That's the best recommendation I can think of.

[Relytt58]

I already have a second account set up that I have a percentage of my paychecks go into just incase something happens but I will never touch that unless I absolutely have to for family reasons or for collage for my kids. Any of you guys do your own porting? I have port aluminum 4 stroke heads with some help from a buddy that used to build pro moto bike engines. I know that's a completely different set up from a 2 stroke twin but if I got the templates and took my time I personally think I could do it. I have a machine shop at work that I can use after hours if I don't already have everything tool wise my self. Worst case here I mess them up and have to do the 421 like I am planning on doing any way, it would just move getting the cub kit up a little sooner. I will get the Pro design cool head first and run that for awhile and as time goes I will slowly start getting the parts together for the 421. I would like to get the whole kit at once but I don't want to be dumping all that money in at once. Im sure others here have the same problem with the wife not understanding why so much money goes into a bike that is already fast enough to scare people.

[RagunCajun]

If you must mod the bike, I'll say do A-arms and suspension. Leave the engine alone and save for a cub and supporting mods.

[tfaith08]

Porting your own?  Unless you have a thorough grasp on fluid dynamics (both steady and unsteady), proper porting tools that you're familiar with, 2-stroke porting concepts, and can manage to find a way to justify potentially destroying a cylinder, I'd highly recommend you leave them along.  If you want some pointers, we can provide that.  However, I'd recommend that you let a professional do it if your goal is to make power.  Porting a 2-stroke is much more involved and has a much more dynamic thought process to it than 4-stroke porting.  A port map is only a part of the equation.  The port geometry (especially the transfers) are your largest concern.  I had an LRD 250R port map at one point, but that doesn't mean that I'll be able to port it to make as much power as Arlan, or even make more power at all. 

 

Most people don't fare very well when they try to port on their first go and make big power, all in an attempt to save money.  The only exception that I know of is some guy that got a Flotek port map from Denny a few years back and made like 70hp.  I'm sure the dyno chart looked like a church steeple though.

 

Long story short: If you want to learn, go for it.  If you want to make more power, send them off.

 

 

The other thing is that you may not benefit from that much power if you have things taking your focus off of actually riding.  I've found that I'm much faster when I fix the things that I notice most often.  My recent focus has been on a full YFZ brake swap.  Next, it'll be handlebars.  Next will be a shortened swingarm (maybe).  These things don't make you accelerate any faster, they allow you to utilize what power you have more effectively.