yamaha 70 ces

[88workcar]

I think you will be quite happy with this rig just the way it is. I wonder how it ever made it sence all this stuff is wrong or going to break as soon as you try to use it. I am baffled.If you ever need 90 carbs which I don't think you do, I have about 30 sets, just let me know.

[Hounddog]

I thought I would post rather than e-mail several people and go back and forth.

I can only relate to my experience with the motor and how we would inspect any motor racing in T class in Ontario Canada.

First there were two UIM inspection sheets numbers 398 and 435 [I think there the numbers].
There are two different blocks. You can tell the later block because it has the mounting blocks for the newer style ignition. The one in this thread is the older block.
The original motor does not have 90 hp carbs. It has the older style 60 and 70 hp small carbs.
The crank, rods and pistons weight 1000 grams
Based on the original Specs from Japan the motor is 70 hp at 6400 RPM.
The compression is 150 psi.
The horsepower curve drops quickly after 6500 rpm. The motor will turn 8000 with very light load. The igniton is a weak link to run the motor a very high rpm over long periods of time.
The bell shaped short tuner helps to get higher rpm on a tunnel, but the stock tuner has more torgue and is equal or better on a V bottom.

The designers in Japan copied the OMC Stinger. The gear case is 1.71 ratio on the CES and 1.76 in the OMC. They both are 15 inch mids, off centre chamber heads, The horsepower curve is raised from 70 hp at 5500 to 70 in the yamaha at 6400 and 75 plus at 6000 in the OMC. Based in prop shaft HP the OMC is a little stronger. The big reason is the weight of the rotating assembly The yamaha is very heavy.
You pin or weld the crank on a CES or any modular crank in a performance application because they shift out of phrase. That changes the timing.

Racers in Europe motors pass inspection. That is their inspection and their business.
In T Class here we decided on a maximum compression rule of 160 psi. The motors are stock and run on pump gas, not super stock. Inspection is first fault, easiest first. Visual is first, next is compression. If it reads over 160 psi your out.

Can this motor run over 100 mph in the kilo on a F4 tunnel? Not in my opinion unless it was on a trailer pulled behind a car.

[ima75man]

if racers spent more time on props and setup, they would be faster than most racers modifying their moters.

[FrenchPhil]

Well, it did in at least once in 1992 - 101.93MPH - UIM record. Since the motor is no longer legal in European/World class races like Rouen, there's probably good deals to be made.

Can this motor run over 100 mph in the kilo on a F4 tunnel? Not in my opinion unless it was on a trailer pulled behind a car.

[Hounddog]

Well, it did in at least once in 1992 - 101.93MPH - UIM record. Since the motor is no longer legal in European/World class races like Rouen, there's probably good deals to be made.

Yes Sir, it's in the book and thats a 2 way average. Doing the math he turning a 26 pitch prop at over 8000 RPM from a 70 hp motor.

[FrenchPhil]

Wonder how much HP they actually had...

[Hounddog]

Phil, welcome to the How did they do it with a stock 70 CES Club. There are many of us in the club. No other F4 has ever tried to beat it.

[DoktorC]

The horsepower curve is raised from 70 hp at 5500 to 70 in the yamaha at 6400 and 75 plus at 6000 in the OMC.

Other than the head and tuner what else did they do to the CES to raise the peak power 900rpm? I have a tough time believing that by adding 60+psi of compression and a tuner that you still have 70hp IF you start with 70hp....and those two mods raising the power curve seems odd without raising the port timing.

[Fast Fred]

pull one washer out of the head bolt set, cut and drill a piece of alu' to cover the head, about an 1/8 thick, bolt that down on the head, it will keep the core plugs in the head. cuzz of the stock class they, the factory, can only use stock parts, with no mods, makes it only a matter of time for the power head. anyways, the flaws are, the crank press on the top pin sucks, it will come apart and cut the top of the cases off .the wrist pin clips need to have the tail cut off, they vibrate and jump out, then the pin drops in the port, boom. the squish band is negative, should be re-cut then you can lean on it. i remember my first one she was the bomb, stretched that crank out second ride, still had them 70hp carbs on it, put them on a 50hp. told ya about the tuna,

[Hounddog]

Other than the head and tuner what else did they do to the CES to raise the peak power 900rpm? I have a tough time believing that by adding 60+psi of compression and a tuner that you still have 70hp IF you start with 70hp....and those two mods raising the power curve seems odd without raising the port timing.

DoktorC they did basically the same as OMC. If you compare the normal 70 hp 49 motor with the longer tuner and head to the stinger style tuner, head and larger carbs in OMC's case. The shorter tuner will change the power curve of the motor. The shorter tuner will require more RPM to attain the same horsepower. The timing is the same. In many cases the yamaha engine builders will run less timing. say 17, especially if they have over 150 psi and a modified head. That is the reason they run the larger 90 carbs. The motor will run too lean with the smaller carbs.

Racers are always looking for an edge. The 70 CES in pure stock form is an excellent motor with very few problems. It is when the racer thinks they are smarter than the person or persons that designed it, they run into problems. Soon as you alter one part of it you end up altering another part because of a problem you created by altering the first part. It is from trial and error that mod engine builders learn. I bet except for a few, racers who have done their own mods do not have as good a motor as one totally stock.