Topic: Cb750k0 810cc overheating

[AdrenalineReaper]

my 70 cb gets real hot in about 20 minutes of sustained riding. My oil temp reads 255-260 and it get hard to find neutral . My carbs need balancing and I think a can of sea foam would do it good, due to a some missing under lower rpm. Other than that it pulls like my crotch rocket. How can I get the temps down? Oh and it does have a secondary oil cooler already. Engine is 810cc 1/4 stroked with a Daytona race cam. No air box just velocity stacks, and a large 4 into 1 non baffled straight pipe.

[BPellerine]

carbs might be lean or maybe the timing too advanced?bill

[cougar]

+ 1 ^^ on what Bill said.  We don't know the whole history about your bike. The Sea-Foam is a good cleaner but be sure to follow instructions and make sure to run it out. Some folks have had troubles with rubber products with it when sitting for a long period of time. Although it does sound initially like a lean condition or maybe timing if you notice any pinging.    ...cougar...

[AdrenalineReaper]

It has the timing locked out "no mechanical advancement" and I'm also not to sure how to properly check it. Is all aftermarket vintage stuff. The float bowls were also way out of adjustment not allowing fuel to enter the bowls on 2 carbs. "Corrected that" But I just took it for a ride and it left the line so hard it wheelied and hopped the rear tire. I than ran it up to 135mph no probs. The engine still gets hot real fast. So where should  my timing be when the engine is this radical, and I don't really now the cam specs, just that it is also radical?

[scottly]

Copy/pasted from the Satanic Mechanic site:

Yoshimura used to make different cams for the 750. Unfortunately, these have been discontinued years ago.

                Durat                                            In o/c   Run-
        Valve   ion   Lobe                                       Ex o/c   ning
Cam     Lift    @.04" center                                     btc/abc  clrnc
        inches  degr  degr    Description and Application        bbc/atc  inches
-------------------------------------------------------------------------------
Daytona .392    256           Must use Yoshimura valve springs   23/53     .004
Race    .360    256                                              53/23     .004
-------------------------------------------------------------------------------
Road &  .378    252                                              20/52     .004
Track   .341    244                                              46/18     .004
-------------------------------------------------------------------------------
Stage 1 .362    253   105                                        21.5/51.5 .006
        .330    244   105                                        47/17     .006
-------------------------------------------------------------------------------
Stage 2 .383    256   105                                        23/53     .006
        .339    256   105                                        53/23     .006
-------------------------------------------------------------------------------
Isle Of .354    264           Must use Yoshimura valve springs   27/57     .004
Man Race.337                                                     56/27     .004
-------------------------------------------------------------------------------
For racing applications, add .002" clearance

[AdrenalineReaper]

This is the ignition

[scottly]

Wow, I've never seen that ignition. It is set to maximum advance, looking at the adjustment slots.

[AdrenalineReaper]

And there is no mechanical advance. But the rotor isn't slotted, so you could adjust it by just loosening the rotor, turning it and retightening it. ? The same as rotating the whole plate, just reversed.

[scottly]

I honestly have no clue. Is the rotor mounted to the stock advancer plate? If so, the stock timing marks can be used to set the timing with a strobe. I'm not sure I would run a hi-po motor on the street with a locked advance, though...

[AdrenalineReaper]

Just looked into it closer, it's a Boyer Bransden ignition with control box. It automatically advances and retards the timing. And you set it with the engine over 4000 rpm, for full advance. From what I've read about it.

[AdrenalineReaper]

so from that chart it should be at 53 degrees, if im reading it right?

[scottly]

so from that chart it should be at 53 degrees, if im reading it right?

That's cam timing, not ignition. There are 2 hash marks on the stock advancer; full advance is set with the pointer on the case set between those marks. The pointer should be visible through the round opening near the top of the ignition plate.

[Retro Rocket]

Send Dave500 a PM mate, he used boyer ignitions and knows a bit about them....

http://forums.sohc4.net/index.php?action=profile;u=14739

[AdrenalineReaper]

Im following you on the timing now. but whats the harm in running a hi-po engine around town, and what defines hi-po vs hot street?

[01Thomas]

Hello A-R
That's a Boyer Bransen ignition. I have one fitted to my Seeley Honda.  I have the fitting instructions in front of me.   It is timed to the full advance position with the engine running at 4500 rpm. The advance/retard function is handled electronically by the control box.

I'll take the instructions to work with me tomorrow and scan them for posterity (and anybody here...)

regards
Thomas

Wow, I've never seen that ignition. It is set to maximum advance, looking at the adjustment slots.

[Tintop]

I run a BB ignition (blue box) on the sidecar (mechanical advance removed).  Mine is set to give 37 degrees full advance.  You need to use a stobe to set the advance timing with a BB.  The initial setup / install is done static using the engine timing marks.

Here is the install / setup instructions from BB site for the Honda 4 (black box).
http://www.boyerbransden.com/pdf/KIT00055__BOX00014_.pdf

[BPellerine]

a hi po engine is the same thing as hot street it all depends what amount of tuning you can live with.sounds like you need a total check.timing carbs maybe cam timing a friend of mine has the same problem with overheating after mods but refuses to think he has to make changes to jetting etc to make his bike run right almost bubbles the paint on head!bill

[AdrenalineReaper]

How hot is to hot, what is the oil temp red line

[scottly]

250* oil is about the red line; that's when dinosaur oil starts breaking down rapidly. What carbs are you running, and what jetting? The way I found that #40 idle jets were too small on my 826 w/Webers was after a 100 mile sprint at 70 MPH at 80-90* ambient temperature , then getting stuck at a traffic light. I shut the motor off to call for directions, and when I went to restart it, the motor was so hot I could just barely kick it over; not fast enough to get it started.

[AdrenalineReaper]

Well I'm not really sure what jets are in it. Pretty sure the carbs are stock, but I would imagine its been re-jetted. I just got this bike a couple of weeks ago. Guess I need to check into it.

[01Thomas]

That is the real question, the question we should all be asking: How hot is too hot and and how hot is the bike really?
Saying something like "my bike's running hot" is just too subjective.  Until we have a real-numbers temperature everything we offer is based on speculation. 
I suggest you go for a long hot 20 minute burn and then shove one of those long-probe meat thermometers into your oil tank - assuming your bike doesn't have an oil temperature gauge fitted.

cheers
Thomas

How hot is to hot, what is the oil temp red line

[AdrenalineReaper]

It has an oil temp gauge, and reads 260 after 20 mins of hard riding, but who's to say its correct. The oil pressure gauge was 40psi off, boy it scared the crap out of me when it got hot and red 0psi

[Tintop]

Be careful in assuming the pressure gauge is out.  The race car had 110+ on cold start / 50 @ idle warm / 70+ during race - 235 temp/ 15lb @ idle after race with hot oil.  Hot oil gets very thin & the pressure drops.

[scottly]

It has an oil temp gauge, and reads 260 after 20 mins of hard riding, but who's to say its correct. The oil pressure gauge was 40psi off, boy it scared the crap out of me when it got hot and red 0psi

Are these small VDO gauges? I've had bad experiences with them, such as the needle falling off a brand new oil pressure gauge within 150 miles..

[Tintop]

Easy check - boil some water and stick the sender in it, gauge should read 212F.  Although altitude can affect that somewhat.