84 Honda CR 250 Modifications

[Billet YZ]

Hi Guys,
            i have been reading this thread for a while now but have stayed out of it. There have been a lot of good replies which should have nailed it by now, my 2 cents worth is from a very similar problem that i had with my Saloon car i was racing back in 2005/2006.

My main problem was trying to keep the engine at 90 deg c during all race conditions because as you know the hotter the engine gets the more power drops off, this i didn`t need as it was a controlled engine combo and you needed all the power you could get. A lot of hours were spent on the dyno and i started with a temperature probe in the bottom radiator hose and also another in the top hose, this immediately showed me 2 very different water temps and pointed me straight to the water pump having an issue.

A brand new water pump was purchased with a cast iron impeller not the pressed steel type, every second blade was removed and any casting marks and high spots were blended to give a smoother finish but not a polished finish. Took the car back to the dyno and ran it up 3 to 4 times with the probes in to get the temp readings again which were still a lot different to each other, left the car set up on the rollers and changed the water pump on the spot. Bled the coolant system as per normal and then ran the car until it was back at 90 deg c operating temperature, then gave the car 3 to 4 hard runs and each time i had 90 deg c coming out of top and bottom hoses.

The problem was always the water pump not pushing solid water thru the whole cooling system, at higher revs it would not push solid water and this would leave steam pockets in the corners of the heads and create the boiling problems and inconsistent water temps. I believe this is what is happening to your Honda and i wouldn`t go past spending a lot of time in getting your impellor and housing back to what it was when new and then looking to see if you can improve it even further than new.

Just because you can see the coolant circulating thru the radiators on idle may not mean it is pushing solid water at higher revs. Good luck with it i`m sure you will nut it out.  Regards Peter.

[Kenneth S (222)]

Hi Guys,

Mick, all checks out,

-you have new radiators, with hoses correctly routed?
Yes and replaced.

-you have a top end with gasket correctly positioned and that it contains and isolates combustion gases from the cooling jacket?
Yes, all 5 or 6 of them were positioned correctly. They have to be as it is a 7 stud pattern and will only go on one way.

-you are not producing excessive heat from a combination of incorrect fuel, timing or a lean fuel/air mix.
Its running rich at the moment. It hasn't run long enough to give me time to jet it.

I have used 4 different radiator caps. The original Honda one, an 85 250 one I borrowed from Hardo, a 1.4 new replacement and a 1.1 new one which came with the new radiators. It happened on all 4.

Mick, I'll do the squish test as you say next time I'm working on it. Not sure how I am going to find out what the clearance should be but I'll cross that bridge when I come to it.

Thanks Peter, when I put the clear tubes on it in place of the hoses leading from the top of the radiators to the head, it confirmed good flow both sides but there was definitely small air bubbles through the coolant as well.

I think Lozza is right. It might be time to book some time on the dyno where I can observe what happens and narrow it down systematically. I don't want to go through another race season like the last.

If it gets to that is there a place in Sydney anyone would recommend?

[mick25]

I hope you get it sorted ken This has tested every bodys throughts, and its the most talked about CR250 eva

[PCMAX]

Not sure where you live in SYD Ken but RB Racing have a dyno at Taren Point it's only about 3km from Greg Ball  Engineering.  Not sure if you can run a rear knobby tire on the rig though, he mainly does road & race bikes.
The owner there is also into kart racing and is very knowlegeable, it  would be worth giving him a call anyhow.

 
 http://www.rbimports.com.au/


Peter

[crs-and-rms]

after seeing the clutch cover part of the water pump im sure that is the problem  the repair is quite poor and has none of the shape that it should have the clearance between impelar and cover would be all wrong i will try and get a pic of my nos cover and post so you can see the differance

[Stewart Allen]

One last and most obvious thing I forgot to mention.

Make sure there are no Kawasaki's caught in your radiator cooling fins.


Cheers
Stewart

[Montynut]

Hi Guys,
            i have been reading this thread for a while now but have stayed out of it. There have been a lot of good replies which should have nailed it by now, my 2 cents worth is from a very similar problem that i had with my Saloon car i was racing back in 2005/2006.

My main problem was trying to keep the engine at 90 deg c during all race conditions because as you know the hotter the engine gets the more power drops off, this i didn`t need as it was a controlled engine combo and you needed all the power you could get. A lot of hours were spent on the dyno and i started with a temperature probe in the bottom radiator hose and also another in the top hose, this immediately showed me 2 very different water temps and pointed me straight to the water pump having an issue.

A brand new water pump was purchased with a cast iron impeller not the pressed steel type, every second blade was removed and any casting marks and high spots were blended to give a smoother finish but not a polished finish. Took the car back to the dyno and ran it up 3 to 4 times with the probes in to get the temp readings again which were still a lot different to each other, left the car set up on the rollers and changed the water pump on the spot. Bled the coolant system as per normal and then ran the car until it was back at 90 deg c operating temperature, then gave the car 3 to 4 hard runs and each time i had 90 deg c coming out of top and bottom hoses.

The problem was always the water pump not pushing solid water thru the whole cooling system, at higher revs it would not push solid water and this would leave steam pockets in the corners of the heads and create the boiling problems and inconsistent water temps. I believe this is what is happening to your Honda and i wouldn`t go past spending a lot of time in getting your impellor and housing back to what it was when new and then looking to see if you can improve it even further than new.

Just because you can see the coolant circulating thru the radiators on idle may not mean it is pushing solid water at higher revs. Good luck with it i`m sure you will nut it out.  Regards Peter.

Off topic

Stewart Peter I am not following this are you saying the inlet and outlet of your radiator were the same temperature Wouldn't you want a temperature drop across your radiator ie the radiator is getting rid of heat ot of the coolant? I wouls have thought that coolant temperature would be high when exiting the engine would cool as it passes through the radiator and then returned to the engine ready to absorb heat again. Basically a cooling system is a heat pump. I am not trying to be a Smart ar.. or an I miss reading your post

[Stewart Allen]

Hey Scrivo,
               Not my post but I also agree with your question, I would expect coolant leaving the radiator to cool the engine would be colder than coolant exiting the engine after it has cooled the engine.

Peter (Billet YZ) can you enlighten a couple of inquisitive old bar studs.

Cheers
Stewart

[Kenneth S (222)]

How much is the radiator going to disperse heat if there is no wind rushing through it while it sits on the dyno? I can't imagine a lot. Not trying to be a smart a... either but I legitimately can't imagine it doing much to cool it off without the air flow through the fins.

[PCMAX]

How much is the radiator going to disperse heat if there is no wind rushing through it while it sits on the dyno? I can't imagine a lot. Not trying to be a smart a... either but I legitimately can't imagine it doing much to cool it off without the air flow through the fins.

Pretty sure they set up big fans blowing air at the radiators.

[Stewart Allen]

All the rolling road dyno's I,ve been involved with have had the fans, as Peter says.

[Kenneth S (222)]

How much is the radiator going to disperse heat if there is no wind rushing through it while it sits on the dyno? I can't imagine a lot. Not trying to be a smart a... either but I legitimately can't imagine it doing much to cool it off without the air flow through the fins.

Pretty sure they set up big fans blowing air at the radiators.

That would make sense then. Perhaps Peter had the fans turned off for his test

[Stewart Allen]

There would still be a significant variation between top & bottom (unless he's running a crossflow radiator not common). What you find with out fans is that the temp would continuously increase, the rate would generally be dependent of the load applied.

[Billet YZ]

Sorry for the slow reply guys just got home from work.
                                                                             The dyno i was using did have the normal large fan as without this you would have overheating problems very quickly, the aim was always to have consistent temps out of both hoses. With the race car if you had cold water coming out of the top hose this pointed to the water staying in the radiator for too long and the fan doing too much of a good job, this then points to the water not being pumped thru the system consistently.

In my case this proved the water pump was creating air and not pumping solid water thru the system, once the new water pump was fitted my method of achieving the desired water temp of 90 deg c was a disc that i replaced the thermostat with and it had a 13 mm hole in the middle. If this hole was to big the flow would have been faster and the coolant may not have had enough time in the radiator to cool, if the hole was to small the coolant would have stayed in the engine for to long and got to a higher temp and again this would introduce different temps from the top and bottom hoses.

In the end the water temp maintained 90 deg c and the retarders on the dyno had the problem of overheating and needing a rest, this is a problem we never had before the water pump was sorted. I believe the fans for most dyno`s simulate approx. 100 kmh of road speed. 

My main point is that you need to have solid water pumping thru the whole system and pushing out any steam pockets that may be in corners of the coolant passages before you start to worry about achieving the correct running temps. With no provision for a thermostat in the bikes you would need to look at a few other methods of controlling water flow thru the system.   Peter.

[crs-and-rms]

just found the nos clutch cover and taken the pics just have to work out how to post them in here