Topic: 74 CB750 guzzling gas and 2 personalities

[piefairy]

Ok, so I got a chance to go out in between the rain storms. I had the velocity stacks on and tried using some aquarium filter foam 30-PPI to help filter a bit. It wouldn't get over 7500 RPM. Pulled out the foam and it ran like a champ... for about 10 minutes. The engine note in the bike changed a bit, went from a smooth sweet sound to a bit of a mechanical, harsh sound. The RPMs would still hold at around 8500, but it didn't want to jump up to that range like earlier. I am assuming that the engine wasn't up to temperature until about that 10 minute mark, but not sure if there is something I should look into. I am doing a plug chop and clear tube test in the next few days. It is still draining way to much gas. Went through around 1.5 gallons going less than 30 miles. I was keeping it in what should be the power band, 6.5 to 8k the whole trip. Also, after it warmed up, it did not want to idle anymore. It would die unless I payed attention to the throttle. I am running a supertrapp exhaust with all the plates on. I dont know if this would create to much backpressure at temp?

I did add some ZDDP to the oil, and it shifts a hell of a lot better now.

[BobR]

You are going to need to look into the jetting. On my 74 all I have to do is unclip the bowls and access to the jets is pretty good. Fairly simple.
  fwiw- The reason I never went to pods (I do like them) is that I dont have a dyno near by anymore. used to but they closed shop. Cant be bothered/dont trust all the plug chop stuff- but thats me. Bob

[piefairy]

My bike came with pods, and I would like to get them to work because they look cleaner IMO. Plus, a new air box isn't that cheap.

[BobR]

My bike came with pods, and I would like to get them to work because they look cleaner IMO. Plus, a new air box isn't that cheap.

 I hear ya. Like I said I like them too. If it was me and I was committed to using them I'd check my main and pilot jet sizes. If they needed changing (sounds like the mains are too big if it runs better with no pods) then take I'd make my best guestimate (gathered from other pod users set ups) then do plug chop tests. Bob

[piefairy]

Alright, I did a clear tube test today, fuel seems to be at the correct height, 3mm below gasket. I ran the bike for a minute and noticed a lot of white smoke, it looked like a bug bomb going off, which prompted me to check my oil level. It seems low, just barely making a mark on the bottom of the dipstick (I waited until after the bike cooled down and the tank wasn't under pressure. I know I topped it off on Sunday to mid way up the etched markings on the dipstick. I assume its the same as a car engine and thats the needed level. I did not check it post ride. Also, the bike is on a side stand, so I stood it up when I checked the level, but I did not wait long so I'm not sure if alot was still in the sump. Next I pulled the plugs and saw a couple of issues. First, the plugs were only finger tight. I know this will effect just about everything on how the bike runs. Second, it had the wrong plugs. They were Champion something or another. Third, 1,2,3 are black but not oily feeling, 4 was tan, but was also the loosest of the 3 plugs. I have photos.
 Number 1


Number 2


Number 3


Number 4


After putting in new NGK plugs and tightening them with a ratchet (I have not torqued them, not sure if there is a spec, ill have to look it up), the bike instantly sounded smoother. The idle was stable, I wasn't getting the 100-200rpm variance I was getting. I doubt this could be a possible cause for the oil loss, but I am hoping 

I did my best to get a picture of the exhaust with the new plugs in. It is 100 times better than it was prior to changing them.


One other thing I did notice was number 3 cylinder exhaust was heating slower than the others.

Hopefully I will be able to get it out for a bit tomorrow, the rains are moving in for today, and see what it runs like. I will be topping off the oil prior to going though.

[kpier883]

Looks like blue smoke to me. Which is oil related .  Could just be that it was not firing on all cylinders and got a bit of oil into the non firing cylinder(s) and now it is burning it out (a good thing).

The plugs looke realky sooty but i am viewing the pics on a phone so i could be wrong.

But the bottom line is you seem to be fouling a plug or plugs.  When you put new plugs in it runs good.  You have to figure out why you are fouling plugs.

And you have to figure out why you are drinking too much gas. 

It could be that you are fouling plugs because the engine is leaking oil past the rings or valve guides. It could be that you are fouling plugs because the carbs are giving the engine too much gas.  It could be  that you are fouling plugs because it is using oil AND getting too much gas.

You could be using too much gas because you are fouling plus from too much oil.  Then the engine runs poorly causing you to open the throttle much wider to achieve the speed you want to run, leading to terrible mpg.  It could be that you are getting too much gas into the cylinders which causes you to foul plugs which causes your already high gas consumption to go even higher. 

I’d check the main jet size and if it is way over stock size (or not genuine Keihin).  If it is way over stock i think i’d Order jets half way between the stock size and your current size jet.  Then test with that combo and see if it is better, worse or about the same.    Then adjust further according to how it responds. 

[piefairy]

I appreciate it. I will pull the carbs tomorrow and check the jets as compared to stock. I am glad to hear that I may not be poring oil into the engine and it could be misfire related. I would agree the plugs look sooty.

[piefairy]

I took it for one last run with the new plugs to see what may have changed. It ran well, still a little smokey. I did notice that if I got the bike to between 8-9k rpm and kept it there for a bit, it would lose power and ride crappy for a bit. Almost like I'm losing a cylinder (there is a tick or ping sound right as it happens, seemingly from the right side of the bike). If I cruise at around 4500-5500 for a bit the bike starts to get surgy and then the power comes back. I seemingly can replicate it, I repeated the same thing 2 more times and same result. Overheating?

I think I have found the oil leak. After getting back I was checking for leaks and noticed the cover over the oil tank has oil all over it. I'm guessing the cap failed? The only other thing I have noticed is a bit of exhaust comeing from the front of the bike, I'm assuming the exhaust gaskets, and I am also getting smoke for a few minutes from the velocity stacks when the bike is off.

Thoughts?

Thanks again

I was doing some digging into the oil issue, it seems it may be the oil leaking past the check valve? I was reading that the tank will appear empty, you add oil, and end up overfilling it and it blows out the vent. This correct? This may be exactly what I have done. The tank is building pressure and will hiss for a bit after a ride, which sound exactly like the post I was reading.  The suggestion to check oil level was to check it with the engine idling, but if the tank is under pressure how can I do that?

Thanks

[ekpent]

 You should be able to remove the oil tank cap anytime whether the bike is running or not and there will not be a problem. If your getting pressure in your tank then check to make sure the vent lines are routed correctly or none are plugged including the line from the valve cover to air. Bad piston rings can cause some pressure issues also.

[piefairy]

You should be able to remove the oil tank cap anytime whether the bike is running or not and there will not be a problem. If your getting pressure in your tank then check to make sure the vent lines are routed correctly or none are plugged including the line from the valve cover to air. Bad piston rings can cause some pressure issues also.

Ok. On the post I mentioned earlier, they indicated that the pressure in the tank may be due to the excess oil? Does that sound correct?

[ekpent]

 Not really in my opinion. If the tank had too much oil it would simply go out the tank vent or around the cap. Back in my younger days I would just dump all 4 qts of oil in the bike and run it. It would measure above the full line but never overflowed. I go by the book a little better now that I have grown older and wiser