The Reincarnation Of The Infamous Mamba.

[BSWIFT]

Having been building my business for the last several years, riding got harder and harder to work into my schedule. I've decided I have to live a bit more so I went for a ride with some of the Spodes and couldn't believe how hard my YZ was to ride, twitchy and rough. The motor was strong but just hard to manage.
So I decided to do the top end (looked new) and spring for a Forward Motion big bore, 390 Kit with Mobetta porting. The suspenders went to Wisconsin for Jer's guys to service and update(2006 was the last upgrade). The chain guard was bent so ordered a new one along with a chain slider, new grips, clutch cable, and a Rekluse EVO Core clutch. A few other bling goodies and I might even do a face lift with the "restyled" plastics. Amazingly enough, the Tag handle bars are from the Mamba and have been on 5 bikes!
The big bore arrived this evening along with chain and sprockets, tires will arrive any day so I'll get back to the deep cleaning. Bearings are all good so I hope I remember how to put it all back together. I really miss riding so I'm just going to make it part of my schedule instead of trying to work in between my schedule.
I'll get some pics this weekend and a few when it is done BEFORE it gets some new RED dirt on it.

 

[Rich Rohrich]

SWEETTT!!!!!!! A project worthy of the Mamba name. I can't wait to see the end result.

 

[jaguar]

The powerband characteristics are controlled more by the pipe design than the porting.
The 250 was peaky and now the 390 will be also. That is my prediction.

 

[BSWIFT]

Basing my opinion from my experience on the KX, the Mobetta porting makes the power come on sooner and smoother. Having FM tell me there was a broken tab on one of the power valve springs kind of explains the twitchy feel I was experiencing. The YZ has always had awesome low end with a massive hit in the mid range, hoping the smoothing of the mid range hit will tame the wicked beast.
I had approximately 150 hours on the original top end and I had used the Jetting Guide from DRN to set up the YZ. I had only changed the plug once thinking a had fouled out but it turned out to be a clogged main jet and it was starving for fuel. Anyway, life is too short to work all the time. Looking forward to some seat time (the knees don't hold up like they used to). Just saw the typo, it will be a 295 kit though a 390 would be really cool!:)

 

[Rich Rohrich]

The powerband characteristics are controlled more by the pipe design than the porting.
The 250 was peaky and now the 390 will be also. That is my prediction.

It will always be a combination of the pipe and the porting that ultimately determines the spread of power. Mismatches in either can leave big holes in the torque curve and tend to make for odd peaks in the power.

Having ridden a number of Eric's big bore 250s (290cc) I know Swifty will be thrilled with the power distribution.

 

[jaguar]

true, the pipe needs to be designed to work in harmony with the porting.
With porting the transfer port height determines the powerband, whether it favors high rpm power or not.
the same holds true for the pipe. It can be designed to favor peaky power or spread the power out over the total powerband more evenly.

 

[Rich Rohrich]

The available pipe options are fairly well understood at this point so Eric is able to easily tailor the power to the riders requirement with the porting and head mods. The bigger piston increases the transfer port velocities and gives some leeway in terms of what you can change without hurting the area under the torque curve.

 

[MXSparx]

Happy for ya Swifty, shifting daily items and re-prioritizing is something I need to do also. Pics, Pics, Pics

 

[BSWIFT]

Got the top end reassembled and put back in the frame. Remounted the swing arm with a new slider. I'll mount the new Michelin tires tomorrow, clean and mount the carb and clutch cable. Clean up the air box and put in a new No-toil filter. I'll do some other cleaning while I wait for the suspension and Rekluse.
Had to call Forward Motion today, my piston did not have an arrow to install(Friday the 13th). Got the cut away on the skirt towards the intake.. All of that and got some more important chores done.

 

[BSWIFT]

Sparks, I have never stopped thinking about riding, still win world SX championships every night in my dreams. I wouldn't change owning my own business just to have a 8-5 job but I really have to set more time aside for my toys.

 

[mx547]

Cool Brian, good to see you riding again.

I still have my CR but since I got old, we've pretty much switched to dual-sporting. It's so much easier on my old, fat, tired, aching body. We ride nearly every weekend that the weather permits.

 

[BSWIFT]

Jay, we need to get together sometime an bench race.

 

[BSWIFT]

This is where I've gotten to. Still waiting for the suspension and Rekluse(any day now). I soda blasted the lower cases. Painted the ignition cover, replaced all of the vent lines, soda blasted the carb, replaced the clutch cable, put on new grips, and mounted the controls. I was bored and painted the rear break guard and rear rotor guard as well. The new Michelins are mounted and sprockets to boot. After years of riding in the Stillwater red dirt, everything has a red hue to it. The soda blaster does wonders without destroying the finish. I used it on the wheel hubs and the radiator guards. They look new now. I also used the soda blaster on the ignition cover before painting with a bar top laquer black. Not sure if it will hold up but if it doesn't, I'll use the soda blaster to remove the paint.
I did the dirtiest of the two stroke maintenance as well, I repacked my silencer. OMG that thing was nasty. I may have to switch to an 80:1 mix instead of 40:1 mix. I could of lubed my truck with the amount of oil in the old packing. I used some packing material that I won at Dirtweek and also found the signed front number plate from DW03, Doug Dubach. It was dusty but in pristine condition. I also found the name badges the BigLou made, brought back a bunch of good times!

 

[BSWIFT]

Just prior to receiving the Rekluse.

 

[Rich Rohrich]

I did the dirtiest of the two stroke maintenance as well, I repacked my silencer. OMG that thing was nasty. I may have to switch to an 80:1 mix instead of 40:1 mix.

Swifty - If you put decent fuel in it and get the jetting right you can the bike at 16:1 without spooge, let alone 40:1. I don't even run my trials bike at 80:1 LOL

 

[BSWIFT]

Rich, I don't think I can get the jetting any better. Throttle response is super clean all the way through. Getting better fuel is whole different story. I ordered the top end to run on pump fuel so I don't know if I can get a benefit from race fuel consistency.:MADGUY:
BTW, I may have to de-tune this bike. It is an absolute monster to ride. I think my brain way surpassed my abilities as a rider.

 

[Rich Rohrich]

BTW, I may have to de-tune this bike. It is an absolute monster to ride. I think my brain way surpassed my abilities as a rider.

Welcome to my hell Brian :-)

 

[motometal]

Have you considered a flywheel weight or do you have one already? I love mine. Seems to be no downside to it.

And yes I'm still riding the same old CR...it just works so well, hard to move on. Rode a couple new bikes and I'd be lying if I didn't say there were noticed improvements but...considering mine is 13 model years old, sure works good for what I do with it (woods).

 

[Rich Rohrich]

Rich, I don't think I can get the jetting any better. Throttle response is super clean all the way through. Getting better fuel is whole different story. I ordered the top end to run on pump fuel so I don't know if I can get a benefit from race fuel consistency.:MADGUY:

Brian even with the compression and squish velocity set for pump fuel there are some benefits to better fuel. Pump fuel has a bunch of high distillation temp components (upwards of 400f) that never vaporize and lead to the exhaust deposits that destroy the exhaust valves. Premium pump can be even worse in this regard because some of the heavier hydrocarbons that are used to boost the octane can have fairly high distillation temps.

Automotive engines turn so slow and have such long hot intake tracts that it's not a problem. But in the super short and relatively cool intakes in the current crop of MX engines, anything with a distillation temp above about 260f is unlikely to vaporize once the engine sees the far side of about 7000 rpm. These higher temp end components never vaporize properly and are a major contributor to spooge. Even spot on jetting will be subject to these issues. So even if the additional octane isn't required by an engine lack of spooge and better throttle response are additional reasons to think about more 2t specific fuel. Good synthetic oil can aid in this as well. Whether or not the cost difference is worth it for the advantages gained is another question entirely.


Here's an example of a good oxygenated unleaded race fuel and Illinois pump gas that sort of shows it in graphic form.

Graph courtesy of Dave Redszus at Precision AutoResearch
Precision AutoResearch

 

[BSWIFT]

Rich, the cost difference for my abilities of a rider just makes my money "vaporize".:EEK: I'm running an 11 oz. FWW and can't even imagine what this bike would be like without it. Been considering additional weight for the flywheel as well.
I've been toying with the idea to get the bike tagged, putting 17's on, and joining up with the local hooligans for some street/dirt action. The problem with that is I've been quite vocal about the county sheriff over the last year so I don't really want to put myself into ANY interaction with his deputies.:KING:.

 

[a454elk]

I've heard about you all the way out here pal, we're on to you!! Nice job Brian!

 

[Rich Rohrich]

... and Elk sets down his rake as he is tapped to join the hit squad heading to OKC. :YIKES: