Diff'rent Strokers

[youngjohnny]

Hi everyone,
Just signed up to get a taste of the biker camaradie and maybe find some info for my mvx250f. I noticed at least two other members have them.

This V3 motor has a bad reputation for ruining it's upright mid rear cylinder and piston. Could the overweight middle conrod be the cause. For engine balance I imagine it has to be the same weight as the combined weight of the other two rods. Would this side-loading of the piston as the conrod swings its arc be causing the damage.

Looking at pics this rod has a huge small-end bearing but this only travels up and down in line with the piston, and the body of the rod doesn't seem much thicker than the other rods. Therefore the swinging bit may not be too heavy and I could be wrong about the piston side-loading.

I have searched the 'net for possible titanium rods to fit the horizontal cylinders so that a standard alloy steel rod could be used for the rear cylinder and engine balancing. (and revability). They couldn't be found but they can be manufactured. It's only a thought as I would not be able to afford these expensive one-offs.

The other thing possibly causing this damage may be the oil injection lines. When starting up, the rear cylinder doesn't emit the smoke the others emit, until its warmed up. Maybe the longer rear oil line and higher located (oil draining back?) oil injection point have something to do with this.

I would love to keep this bike but it needs to be durable and reliable since spares are rare items. I love the short wheelbase, lack of weight and the smoothness of the motor. At present it's unrego'd under a cover outside as it's rattling quite a bit and I don't want to destroy the motor.

Any thoughts or suggestions appreciated. Does anyone know of a Japanese bike site which has intelligible English translations on it?

youngjohnny (not quite as old as young Mr.Grace)
_________________
Its not how big it is, its how hard you use it.

[Mr FFF]

Hi Johnny

I have two of the little darlings and probably the largest spares stock in the world now.

I have also heard about the reliability probs of the rear cylinder. Honda must of too, as I have about 6 different rear pistons. The large rear conrod has a 20mm small end. There are pistons that use an 18mm pin and some with a 14mm pin and a collar. Then there are teflon coated ones in both types and other part numbers where I cant see any differences. So obviously Honda were doing something with rear piston revisions.

Now, I would love someone to confirm the NS400 layout. Did they use the same piston and rods on all three cylinders? and if so did they use a balance shaft of some type?

With two brand new cranks, and a complete set of spare rods, I have been toying with the idea of building a crank with three front rods, just to see what happens. Thoughts anyone?

However, the front pistons are the ones i'm short of, so would need to look at using CR85 pistons for all three pots.

I have also heard that the problem was finally traced to the rear cylinder ingition system causing fluctuating timing. Anyone else heard of this? Modified RGV system !!!!!!! maybe

If you need spares, I may be able to help, and I may still be able to get brand new cranks. I did sent one to trev45 down your way earlier this year. Have a chat with him if you want some feedback.

Regards from the UK

Ian
_________________
MVX250 1983 MVX250 1984 GT380J 1972

Valves are for toilets unless they are power valves.
Claim to fame. Have beeten Casey Stoner over a championship season.

[trev45]

Cool one more for the road in OZ and like Ian said any help no problem

Trev 0415 884 838
_________________
Racing is for the track, riding is for the road
Both are FUN

[kiwi mvx]

Hi everyone,
Just to add my two cents worth to the topic of reliability, I put these comments on the Wikipedia page :

"In 1984 the MVX 250 was sold new for $3299 New Zealand dollars.
Initially many bikes suffered engine failures, often within the warranty period. Honda released a number of engine upgrades including a larger rear conrod to address engine balance issues in an effort to improve engine reliability. After prolonged periods of full throttle many engines suffered piston failure on the rear cylinder. There is much anecdotal evidence to suggest that incorrect ignition timing was the cause of piston failures and subsequent engine seizures"

I have often thought about the longer oil feed line and proximity to the rear cylinder, however its a tough one to prove. I personally don't think it is a problem as many MVX's over here ran up 40 -60 000 km no problem, however some seized within 100 km from the shop when sold new. If it was aproblem I think it would have been a problem with all bikes.

I can confirm that many of the MVX's over here seized or melted a hole through the rear piston on prolonged periods of full throttle in the higher gears which leads me to believe it is a timing issue. I retarded the ignition timing when I rebuilt my motor (lost some top end performance) but the bike has been ridden hard, probably too hard for approx 8000 km with no problems.

I can't guarantee that is the answer however it has worked well for me. You can't overlook the importance of good crankseals, inlet manifolds etc at the same time. Bloody legend of a bike in my opinion, all the more appealling beacause of it's archilies heel.


Dave.

[Mr FFF]

I haven't found any revisions on the crank. Only one part number and with the RSJ rear rod. The only revisions I can find are to do with the size and weight of the rear piston pin.

Do you have any details and part numbers of the different cranks?
_________________
MVX250 1983 MVX250 1984 GT380J 1972

Valves are for toilets unless they are power valves.
Claim to fame. Have beeten Casey Stoner over a championship season.

[Mr FFF]

Been doing a bit of digging through the parts book.

Up to Engine # 1017533 the rear piston used an 18mm pin and an 18x20x12 bearing

From engine # 1017536 a 14mm pin was used with a collar to make it possible to use the same bearing

Hope that helps

Ian
_________________
MVX250 1983 MVX250 1984 GT380J 1972

Valves are for toilets unless they are power valves.
Claim to fame. Have beeten Casey Stoner over a championship season.

[Alastair]

[Mr FFF]

On my GT380 all the oil lines are different lengths, so that won't be the reason.

It just means that when you bleed from new, the shorter one bleaeds first and the longest last
_________________
MVX250 1983 MVX250 1984 GT380J 1972

Valves are for toilets unless they are power valves.
Claim to fame. Have beeten Casey Stoner over a championship season.

[Alastair]

[Mr FFF]

I think the oil line length is a red herring.

As I've never ridden an NS400, what is the engine like vibration wise. Interested as Im am really looking at building an MVX crank with three front rods.
_________________
MVX250 1983 MVX250 1984 GT380J 1972

Valves are for toilets unless they are power valves.
Claim to fame. Have beeten Casey Stoner over a championship season.

[Alastair]

[RSC]

Major gravedigging here,but here goes:

Has anyone compared the weight of the solid wrist pin+bushing with the hollow one? If they weigh the same,there should be no difference in side loads so why the change?

[Kev93]

Hi RSC, good to see a recent post on the mighty MVX, I too have recently added a couple of derelict MVX's to my list of projects. One is running one is not, albeit the running bike has what appears to be piston knock [3 -4 thou clearance, piston to bore. Reliability - one has done 58,000km the other 45,000.

Sourcing parts is my biggest issue, I need some new front pistons. CR 80/85 are supposedly the answer, but can't find these either. I also cannot find new front piston pins [gudgeon pins] or a rear small end bearing. Did you make any progress of a source of parts supply?

As for you question re the rear piston weight, my understanding is the heavier rear cylinder configuration counteracts the two front pistons combined weight. Honda made this modification [larger rear piston/weight] due to excessive vibration. If the rear pin weights are the same, as you suggest below, there would be no difference. However, a hollow one made from the same material will be lighter and at a piston changing direction 12,000 times each minute [6,000 rpm] my guess would be a significant balance issue.

Any comments from others on the above would be welcomed.

[DSBBadmin]

Practical Sportsbikes magazine will be running an MVX buyers/owners guide in the next issue, and are looking for useful info, experiences, hints/tips etc. if anyone can help.

[Kev93]

Thanks, appreciated. UK mag by the looks of things , I will keep a lookout.