50 Yamaha 6H4

[ccaiken]

ok - here goes - not because I'm very knowledgeable, but because I would like to be more knowledgeable and believe we (at least us neophytes) can learn a lot from this forum. How about if we break it into stock block (ie bolt ons only) and modified, and see what we can document as good starting points for those of us who want to have some fun with their motors. The following is what I've gleaned, from other posts and personal email exchanges with people actually building these engines, and am planning for my 40:

Stock block/bolt ons:
* remove rev limiter - snip "gray wire" on some other rqts on other vintages.
* 50 tuner, with one inch cut off
* reeds - Boyesen or mentioned - I'll dig up recommendations sent to me/read here.
* mill head to increase cranking pressure to ~ 160 psi (input welcome) for use with pump gas (91 octane??)
* early heads w/ offset chamber seem to make more power
* 70 horse carbs - I've received an email saying the old 70 OMC carbs may bolt right on. Rejetting will be needed - anyone want to chime in with jetting recommendations? That would be a help to us dummies as starting points.
* lighten flywheel

my understanding (although never specifically noted by any of the guys who've done a lot of these mods so, guys, feel free to chime in here) is that this will allow the engine to make decent power to the 6500-7k range, and unless a bunch of jumping is done which loads and unloads the engine rapidly, will allow it run a long time without welding the crank. Guessing 70 horse output?

Based on my experience (modifying West Bends for karts in the 70's) and a lot of reading, I would guess the next steps would be:

stock block: combustion chamber mods to hemi and set up proper squish band. Pictures of this are on this site or S & F from a guy overseas who did one.

I am thinking this is about as far as bolts ons to stock unassembled block will go. Inputs from y'all w/ experience?

Modified:

* create port map to measure time areas of porting
* define desired full speed operating range - seems 8500 is pretty sporty for all but pure race guys
* define port shapes (reshape originals or add fingers) Any port maps out there?
* find a friend with a mill to cut the ports
* be ready to spend cubic $ testing and torching a few pistons.
* build custom exhaust tuner, shorter and better flowing than the 50 tuner. Any pics? I know there have been some made.

Full disclosure: mods I've done to my 40 to date: rebuild carbs after sitting with wonderful fuel for 3 years. Inherited recently from my brother w/ ~ 30 hrs on it - he didn't die, just upgraded.

[Phil McDaniel]

With all due respect, i don't think you are going to see a lot of "what-to-do-next" in regards to this motor. I, for one, won't elaborate on the things it took me some 10+ years to gather. I am not a river racer but have built and helped some of the guys in south&north carolina with ideas and parts.
I introduced this motor in the 80's to apba first and then aof as another motor for the formula/super "e" class. It is also legal in nbra.
Due to 2 kids in collage and a physical problem, i have had somewhat of a setback. Can't drive anymore, but the "run hard and take a left" has never gone away.
I have a good driver now, and am putting a lot of effort in the yamaha.
From what you have written in your message as well as other post that i have read, i'd say ya'll are playing in the correct ball park. There again, don't think you'll get a lot of info from the guys i know who have put a lot of time and money in this project(motor). Might be surprised. Look for the orange ropp boat and orange yama. Next race season. Phil mcdaniel-----myrtle beach south carolina

[Ballistic]

To start off, Note that doing certain stuff to a standart block won't improve performance.
Snipping the rev limiter won't give you extra power, specially if you where not hitting it in the first place.
Fastest speed me and my mates got out of 100% stock blocks was at 5800 to 6000rpm. Rev it harder and you'll run out of port timing. If you want to rev it harder you'll need to port it. Transfer and exhaust.

For reeds, go with the CCM ones. They are cheaper and supposibly better.

I'm not the expert on this but i think 160psi is too much for 91 octane. I already run 98 octane in the block i'm building which just has a milled head. Maybe someone knows more about this?

I believe the center plug head makes more power. Downside is you can't mill off as much as you can with the offcentre head. You'll have problems with your squishband.

Older style Yam 70 carbs bolt on but need to be rejetted.

I base my experience on a hobby project, i'm not a racer (yet) and deff not an expert on this. I am aiming for 6500 to 7000rpm to keep the engine in one piece for a longer time. I'm sure 8000 is possible with a lot of work but you'll do a few races and the block is done.

Pics of work in progress:

[88workcar]

The last few post have been very good, I will agree with them to a point. It's very common to me that the guys that have done great in the past will not share the info that they have learned or developed them sellves. I spoke to one guy and was told that he drugg his intire rig into the pasture and burned it. He said because I was the champ and worked to hard to get where I was and I will not give that to anyone (the info) he was 70+yrs old. In my opinion this just makes things sour. I don't mind paying for info, or buying parts in order to gain knowledge. And at the same time the few guys that have opened up to me and asked for me not to tell, I honor there request until I see it here. Like the offset head 40. But was also told that it is not all that on a properly designed motor (not sure either way). I will add that I do not usually do bolton motors. Porting is where it's at for me.

One thing that most folks never tell you is to listen to everyone but don't listen to everyone. Lets touch on fuel. One of you mentioned 91 octain and 160lbs of compression. Those are good numbers. What I tell everyone is this. TEST for yourself, TEST TEST TEST. I was as guilty as everyone else. 162lbs and 93 gas. One day the gas was high priced, I was mad, I put the 89 octain, I gained 200 rpm that day. The next tank I tried 87 octain, guess what, another 100 rpm, not just on occasion, all the time, every trip out. This was on a 90 turrning 7500rpm. I on ocasion have held this engine @ 7500 for 5 miles. With no problems. I have sence sold it and the guy with it runs 87 and sees 7400 all the time. What I am getting at here is nothing is written in stone. Compression and fuel also have timing and jetting figured in with them. It must all work together. So your package may be differant that the normal guide line build and still run strong and live a long span.

Another thing that is common is reeds and reed mods. I ran boysens for years. There are better ones out there. CCM TDR ect... But when you call for reeds, tell them what you are doing, other wise you will get a set of everyday reeds. Those are for the play around folks. They have diff thicknesses and diff materials for the guys who want more.

Reed cages. Some work better ported, some work better angle milled, some work better left alone. You have to test, TEST TEST.

Flywheels. For me the light ones work, this will get you in many arguments. Espescially with the big merc guys. Try it, again TEST TEST TEST. Simplest way on a rope start motor remove the ring gear. Otherwise you need to send it to a shop, I do them here with great results. Just on a 90 or 40 I spend 14 to 16 hours on one. It takes time and I am part time. I have a job that keeps me busy.

Bigger carbs seen to always work, BUT look at Proprider 8200! with stock 40 carbs! That is crazy. On a 40, as stated the old style 70 carbs work great. Thats what I use. You may want to get off the path (listening to everyone, even me) and try something else. You never know......

In the head dept. We all have great limitations. This gets expensive. Shaveing a stock piece is a good start. Welding and rechambering a stock piece is the next step, this leaves more cooling in the head. One can also buy the chambers from the 4 wheeler world (Yamaha banshee) and have a stock head milled out to accept them. And last a complete billet head. But here again TEST some times compression is good, too much can hurt at times. And I have see on a dyno (banshee) where compression stayed the same but we changed chamber design and gained 10hp... YOU MUST TEST.

A lost of disgussion goes into exhast. A 50 tuner in a 40 is a good start. 1" cut off the bottom is good too. Building your own is a great thing also, if it works you feel even better because you did it. I will say this on the tuner, build your own, stay very close to the size and shape of the 50 tuner just add some flare to the bottom. Long tuners make low power and short one give up low power. 1" from stock is far as most folks ever need to go. Just for your info I have never had any luck with the hydrotech tuners.

Porting..... Many arguments again. I agree a stock block has no business over 6000rpm. Porting done wrong can kill a block and make it worse than stock. A common set up is widen the intakes, add finger ports and raise the exhaust to 90 degrees BTC. This works on many engines. The 90 degrees you must be carefull, this gives up bottom end. On a heavy rig it may not work so well. One more thing that can be done is to not raise the exhast so high and put a set out plate on the exhast cover plate, (bolt on mod LOL) This will at time gain rpm. Again TEST. Just as opening the midsection can help or hurt, mine is better closed, and a lot better for the neighbors.

Timing, 22degrees is the normal for most folks. More gives bottom, less give top. too much burns pistons, not enough don't run. Again jetting and compression come into play with timeing. TEST!!!!

Where I don't see people talk is stuffing the crank. This gives accelleration, lots of it. Shaveing the intake at the reed surface is the best start up too .050 . A guy with time on his hands can epoxy the inside of the engine. Here again it has limits. The reeds will blow out with too much inside compression.

There is plenty more, I don't have time LOL got chores to do. Just remember, this is all my opinon. Not throwing rocks at anyone or tying to offend anyone. But there is much more that can be done to a 40 Yanmaha, a lot more. Hope this helps, Jason Johnson, Pierre Part, La. Later.....

[Ballistic]

@ima75man: I dont do anything with OMC engines. This is the Yamaha 40/50 thread so that is what i was talking about



"stuffing the crank"
Yeah i haven't heard about that before.

You shave off the plate where the carbs mount on? Can u tell me more i don't really understand

[ccaiken]

thanks for the replies folks - Phil - understand your position - I've got college kids too - just got one outa Carolina and got one in Aiken Tech on the way to Carolina, so anything i can learn to help build a strong runnin around motor without torchin pistons or worse is very valuable. That said, it would be nice to have a fraction of the "secrets of the OMC 50 mod" info for the Yamaho's. And I know it's hard to keep the "smack" out, but us dummies'll learn a lot more if we can do that.

Here's my summary so far with questions/comments. What makes sense to me is to provide data if we have it, rather than just a statement, because there seem to be conflicting positions on several points (as is the norm on these forums)

Stock block/bolt ons:
* remove rev limiter - snip "gray wire" on some other rqts on other vintages. Acknowledged that removing rev limiter wont make more power unless the engine continues to produce torque.
* 50 tuner, with one inch cut off
* reeds - Boyesen or mentioned - I'll dig up recommendations sent to me/read here. CCM reeds mentioned - any data on why they're "supposedly better?" Any back to back tests?
* mill head to increase cranking pressure to ~ 160 psi (input welcome) for use with pump gas (91 octane??)

yeah, CR could be a standalone thread, but ballistic says this: "i think 160psi is too much for 91 octane. I already run 98 octane in the block i'm building which just has a milled head. Maybe someone knows more about this?" while 88workcar says: A ride around motor only needs 160psi, regular ol pump gas will be fine. (88 - hope i didnt offend by publishing your private message) We need more data here - ballistic, what timing are you running, what carbs, and what jets? 22 degrees has been quoted by others, my understanding is this is a reasonable starting no. for stock block w/ all mods listed in my original post. I for one WILL run pump gas and there's at least one other post I can dig up saying the Yams can make good power on pump gas 87 octane even, which is fantastic.

* early heads w/ offset chamber seem to make more power. Any back to back testing? Another SC guy posted big gains from offset heads, (believe it was MHardee, check theS & F old threads) but unsure what other mods were done in conjuntion.
* 70 horse carbs - everyone (?) seems to be running the 70 yam carbs - only reason I suggested the OMC's was thinking they'd be more available in boneyards - which may be completely wrong. I've received an email saying the old 70 OMC carbs may bolt right on. Rejetting will be needed - anyone want to chime in with 70 yam carb jetting recommendations? That would be a help to us dummies as starting points.
* lighten flywheel - acknowledged no or little peak power gain, but better acceleration.

my understanding (although never specifically noted by any of the guys who've done a lot of these mods so, guys, feel free to chime in here) is that the bolt on, STOCK UNPORTED block mods above will allow the engine to make substantially more than 50 hp to the 6500-7k range but we have opinions of ~ 6k max without porting. 88workcar and yamahafastcraft, have I misunderstood your other posts? Yamahafastcraft, your ~7500 rpm fishing motor you've been running since 91 - is this with port mods but stock rotating assembly? If so, it seems the stock block is quite a bit more durable than some give it credit for.



and unless a bunch of jumping is done which loads and unloads the engine rapidly, will allow it run a long time without welding the crank. Guessing 70 horse?

Based on my experience (modifying West Bends for karts in the 70's) and a lot of reading, I would guess the next steps would be:

stock block: combustion chamber mods to hemi and set up proper squish band. Pictures of this are on this site or S & F from a guy overseas who did one.

I am thinking this is about as far as bolts ons to stock unassembled block will go. Inputs from y'all w/ experience?

Modified:

* create port map to measure time areas of porting
* define desired full speed operating range - seems 8500 is pretty sporty for all but pure race guys
* define port shapes (reshape originals or add fingers) Any port maps out there?
* find a friend with a mill to cut the ports
* be ready to spend cubic $ testing and torching a few pistons.
* build custom exhaust tuner, shorter and better flowing than the 50 tuner. Any pics? I know there have been some made.

Full disclosure: mods I've done to my 40 to date: rebuild carbs after sitting with wonderful fuel for 3 years. Inherited recently from my brother w/ ~ 30 hrs on it - he didn't die, just upgraded.

[ccaiken]

ha! lotsa guys on line drinkin coffee pecking on the keyboard! Thanks - esp Jason - very good info. Any others?

[88workcar]

Another thing that may be done as a bolt on is to remove the choke flap from the carb. Remove it all and plug the side holes. This may give enough extra air to cause a jet up situation. And one with time can try carb spacers. These things work for some folks.

[seapro]

i also have a yamaha,a yamaha is a upgraded omc...... some folks never learn to listen, they just to busy reading more than what posted.. good luck

them japs copyed omc so close they copyed all tere mistakes

[Ballistic]

ha! lotsa guys on line drinkin coffee pecking on the keyboard! Thanks - esp Jason - very good info. Any others?

Yeah, hopefully this thread will run a while.
To bad there are still people around who try to kill this thread by insulting other people who are just sharing thier opinion.

I'll just be the bigger man here and ignore it.

Lets stick to the Yamaha 6h4 and 6h5 blocks from 1984 to recent and share our improvement effords