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Losh's 1976 Yamaha RD 400

losh1971

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Bit about my bike. It has a blueprinted engine with machined down crank making it very smooth compared to stock and getting decent amount of weight off the normally quite heavy 400 crank. It has Wiseco pistons and TZ350 race big ends. It has an undercut gearbox and thicker clutch with CB750 clutch springs as it was slipping due to the increase in HP. Match ported with 34mm flatslide Mikuni's. A set of chambers made by Neville Lush and tapered headers off a RG350. Ignition is off a KX80 with modified black boxers to suit the RD. Reeds are off a KX 80 also. Reed blocks have been opened up to give plenty of flow and take the larger KX 80 pedals.
Last time I rode it was at Mallala on a ride day to test out the new front shockers and springs after getting Race Bike Services in Lonsdale SA, to fit emulators. I have stripped probably 25kgs of weight compared to stock. On the front I have RD400 1978 forks and spoke wheels. Front hub is off an early RD350 and the rear a 2.75" rim wheel off a TZ350 that I had the hub machined down to suit the RD gearbox and swing arm. It will rev out to 9.5k and makes decent HP anywhere from 6k -9k , with max at about 8.7/8.8k.
Hoping I can get it over in Oct this year but I want to get a pair of RD 250 heads reshaped to suit the 400 while at the same time get a better shaped than the old stock 400 heads I'm currently running with, I'm thinking I will get gains of around 5hp with the new heads and the power curve should hopefully then have no drop or virtually no drop from low revs right up to max power. Once I do that I will need it dyno tuned again and really need Neville to do that as he is the man and will save me melting a piston. Needs new tyres before I try it out at Symmons and I'm thinking I will go with Dunlop Gp's as I can not get Avons in the size I need in Australia.

Lunchtime today I spoke with Neville and he has booked me in to get the heads reshaped. When I see him I am going to ask about getting power valves made as that would give me more power in the lower rev range and could help to get me off the line a bit quicker.
 

losh1971

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So far the bike owes me about $8k, with around $4.5 of that amount spent on the engine and trans. If I get the heads reshaped and the bike dynoed this year as hoped I will be putting close to another $1k into it. I have an old DT 175 air cooled that I plan to sell for around $600 which will get me most of the way to paying for the work on the 400.
 

losh1971

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Bit more about the bike. When I built it we took everything off that could cause drag on the engine. This included removing the speedo drive, kick starter, oil pump. We also replaced the rear sprocket with an alloy one to reduce the amount of weight the engine had to turn over.
 

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A mate and I used to share an RD250LC in C grade Production races at Calder.
 

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Interesting thought about the powervalve - I'd be interested to know how you'd intend to go about it. Means you can go with MUCH more aggressive transfer and exhaust port timing. But basically involves a complete redesign of your cylinders - especially if you want to take full advantage of a powervalve system. How would they be activated? Servo operated or governor operated?

For your purpose, you'd probably be able to keep it singing pretty well so a powervalve system probably really isn't worth the time and effort overall tbh. Unless of course you're spending a lot of time in the lower rpm range.
 

losh1971

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I don't know a lot about the power valve, only that Neville mentioned he could make them. The main attraction would be getting off the line quicker. I have to ride the clutch at the which is quite normal on 2 stroke race bikes. The other place it would benefit me is coming out of hair pins. At the present I have to be in low gears, like 2nd but still in the power-band or you have trouble when you are racing against modern 750 4 strokes that just pull hard even in low revs. I was in a race with a girl on a 750 modern and on the corners I would catch her but pulling out she always had me at low speed. I eventually learnt to keep it in 2nd and power out soon as I hit half way. But the issue was I would get rear wheel spin which would be ok if you're a really good rider I'm fairly ok, well was. But the really good riders in the same race pull 1.14's at Malalla and I was more an 18-19.
having said that the reshaped heads will probably improve things in lower range. The really good riders expect to have wheel spin. I remember one bloke on his 750 triple 2 stroke used to wiggle his front wheel to get it to slip just a tad so he could get corner faster but he was an insane rider who won the nationals.
 
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losh1971

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When I eventually race again I want to have a really competitive bike because some guys on their moderns may not be the best riders but because everything in newer tech they have the edge on you before you get out. I'm pretty ok on the corners and I like to out-brake a lot of guys. Even though I'm on an old bike my front brake has a bigger disc off a 650 Yam of a similar year so I have some pretty decent stopping power.
 

losh1971

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Last time I rode my mate came out with me on his GSXR and he loved it and was surprised how well the old 76, RD 400 could be pushed around. Racing is an addiction though that you keep needing more power because what ever you have is never enough.... I imagine it is the same on dragway you always want more HP....
 

losh1971

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Interesting thought about the powervalve - I'd be interested to know how you'd intend to go about it. Means you can go with MUCH more aggressive transfer and exhaust port timing. But basically involves a complete redesign of your cylinders - especially if you want to take full advantage of a powervalve system. How would they be activated? Servo operated or governor operated?

For your purpose, you'd probably be able to keep it singing pretty well so a powervalve system probably really isn't worth the time and effort overall tbh. Unless of course you're spending a lot of time in the lower rpm range.
I will ask Neville what he thinks and a rough idea on cost. If it's 2k then the answer will be no. But if the price is right it could mean I could increase the porting on the exhaust side as I wouldn't always have to be in the power-band. There are a few factors that mean I am getting to the stage where I am going to have reliability issues if I try and squeeze too much more out of it, due to heat. As it is I have to watch I don't increase the compression too much or it could seize. If they allowed methanol in Period 5 (assume they still do not) it would be a no brainer I could achieve 65hp or even more and not be concerned with over heating.

In terms of activation I'd have to ask Neville as I don't run a battery so not sure how he would trigger it. When you say port timing what do you mean, increase the size?? Or do you mean enlarging the port on the bottom only and leaving the top?
 

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Port timing is the whole picture - raising the exhaust port decreases low end but increases top end and also decreases dynamic compression. Widening the port will have a pronounced effect throughout the midrange and bottom end all else remaining equal. You can play with base gasket thickness to get an idea of what works - but you need to be careful of squish and compression ratio.

Do you have an exhaust bridge? Sometimes a bridge is required to support the piston on the exhaust side if you’re widening the port considerably, but that requires resleeving.

Heat will always be an issue on the exhaust side on a 2 stroke at sustained high rpm - desert riding for example. On a track I’d say you’d be fine as long as it isn’t held wide open for minutes at a time. Only takes a few seconds of closed throttle to draw enough cool air past the exhaust side to stop things from melting.

2 strokes are inherently forced induction by design. That’s why crankcase volume, reed valves, port timing and pipe choice is so critical.

As a matter of interest, what oil do you run?
 
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