Topic: '77 CB750F Running Hot? (now with Temperatures!)

[HellFishTat]

I searched for a little while.  Now I'm going to post, then go back and search more.

I'm trying to locate the air mixture screw on my '77 CB750F.  It has the carbs where the fuel bowls screw on rather than held in place with a bail.

My bike is bad once it gets hot (and it feels hotter than normal too).  I just verified the valves were adjusted properly, and I have electronic ignition.  I wanted to "rich it up" some to see if that helped.

The bike rode/ran fine this spring but now not so good.

The only other thing I have changed is the oil and the spark plugs.  I verified those were identical to the ones I took out (can't remember the number) and they are adjusted .025".  


Finally got around to testing

After 5 minutes of warm up and taken at a constant 3,000rpm 1/2" away from the head

#1 = 760f
#2 = 780f
#3 = 760f
#4 = 750f

Temp on top cooling fin above exhaust

#1 = 270
#2 = 250
#3 = 290
#4 = 260

[TwoTired]

You don't have an air mixture screw on the PD style carbs.
There is an idle mixture screw (IMS; meters total fuel and air from the pilot system), located in front of the fuel bowl toward the engine facing down.  You'll need a right angle screw driver or a really short one with insulated hands.  Turn out for richer in for leaner.  But, it has very minimal effect at throttle positions over 1/8 turn.

I'm guessing you have pods and an exhaust change from stock.  If so, don't look to be saved by the IMS.  You're going to need bigger jets and a slide needle position change, and the skill to read spark plug deposits.

Alternately, you may have gummed up pilot jets.  Are all the head pipes evenly hot at idle?

Cheers,

[HellFishTat]

All of that is correct (pods, Mac 4-1).  But it wall all done by the previous owner and I have ridden it 3,400 miles since I got it last August.

The plugs that I took out were cardboard color, and according to my brother (35yr auto/motorcycle mechanic), the Honda dealer, and what I've found on this forum, that's what I want.

I have now triple check the gap...good

The IMS was 1/2 turn out on all of them.  I turned another 1/4 out. 

The only changes I have made is the new spark plugs.  This is bothering the #$%* out of me.  I just want to ride.

I'll get it running again and get my Infrared Themometer  and get back to you on the exhaust temps.

You don't have an air mixture screw on the PD style carbs.
There is an idle mixture screw (IMS; meters total fuel and air from the pilot system), located in front of the fuel bowl toward the engine facing down.  You'll need a right angle screw driver or a really short one with insulated hands.  Turn out for richer in for leaner.  But, it has very minimal effect at throttle positions over 1/8 turn.

I'm guessing you have pods and an exhaust change from stock.  If so, don't look to be saved by the IMS.  You're going to need bigger jets and a slide needle position change, and the skill to read spark plug deposits.

Alternately, you may have gummed up pilot jets.  Are all the head pipes evenly hot at idle?

Cheers,

[vern401]

Yes check the pipe temps. It is very possible you have an ignition problem.
Bad plug even though new you can get a bad one.
Bad or faulty plug wire or plug cap.
Make sure your points are in good shape too and set correctly.
If one set of the points is arched over then you may need to replace it and the condensor for it.

[HellFishTat]

temps posted.  let me know what you think

[TwoTired]

I think you didn't read the directions with the temperature unit.

FYI, Oil turns to tar over 320-ish degrees.  (If memory serves)

[OldSchool_IsCool]

Dang!

Was ur thermometer set on the Kelvin scale?? 

[HellFishTat]

I thought the same thing, but I read the direction and then check based on the concrete floor and my white Karmann Ghia that was sitting in the yard.  Concrete floor was 78 and the Ghia was 86.  The temperature at the time I did my check was 89.  I really think it is running that hot which is why I posted the numbers.  Do you think the engine cooling fin temperature were exceedingly high or just the header temperature?

[TwoTired]

Since you didn't tell us, I have to guess that you are using one of those inexpensive temp guns.
You do know that the laser is just a pointer, not that actual test spot.  The spot size is far greater, about 7/8 in diameter at the source and fans out bigger with distance away.   Further,  the laser spot is only centered in sampling spot a fixed distance away from the gun.  At other distances it is above or below the spot sample area.

Combustion gasses are about 1200 -1500 F.  The exhaust gas is far hotter than the temp of the cylinder. and will change temperature with the power output of the engine.
The headers are a heat sink/exchanger and the surface temp varies with the air movement over them, the surface texture and color, and the heat volume and extremes put into it.

The engine cooling fins are hotter near the base and the outer tips run cooler.  They are a heat exchanger and move heat into the air at a rate depending on the temperature differential between source and recipient. IE, if the air is the same temp as the fin, no heat is exchanged.  This is why there must be air moving over the fin, as the heated air moves (along with the absorbed heat), and cooler air  takes its place to receive more heat.

What you want to know is the heat at the spark plug base, which gives you an indication of cylinder heat before the cooling fins begin to remove heat in a gradient from base to tip.

Alternately, you want to know the oil temp as it leaves the heated surfaces of the engine components.  The engine crankcase is the primary cooler of this oil.
If you have an oil tank, check its outer surface temperature or the inlet fitting.  Also check the oil pan temp in an area where there are no cooling fins.  So, you can measure the base metal and not the tip of a fin.

Ideally, the engine will keep the oil at about 180F.  But this will vary with the outside air temperature and the volume of air flowing past the engine cooling fins.

 Hope this helps.






 

[HellFishTat]

I don't know the exact cost of  the gauge.  It's a craftsman that we use at work.  I imagine it would fall on the lower  end of the price range (I work for a publicly traded Engineering/Construction Firm) and they don't  often buy the "best" of anything.

Let's just say I'm a complete idiot and know nothing about heat transfer and have never taken a thermodymanic's class.  Aside from an excessive lean condition (and a crappy IR temperature gauge), what else could cause readings this high?

[TwoTired]

What kind of air flow was over the engine and what temperature was the air?

But wait, it doesn't matter, the points you measured aren't really indicative of engine core or oil temp.

Sorry, but the readings you've taken don't have an accurate reference base upon which to make an assessment.

I'm not even sure the readings you taken are "high".   Maybe someone else can give you the answers you seek.

Cheers,
(Sorry I can't help.)

[scunny]

to do the poor mans test to see if your running lean, get the bike warm then as you ride along try different increments of choke at various throttle positions. if you are indeed lean this will improve things. as an aside are your pods oiled(which restricts air)