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CB750 1980 unburnt fuel

Oil consumption is probably whatever you and the surrounding traffic can bear. ALL engines use a slight bit but I consider it to be a problem once you have to add before the next change at say 3K miles. Most of the time on these it's the valve seals doing it, there's a guy who sells Viton seals that are supposedly longer lasting than the Honda ones. Most aftermarket ones suck and die really fast.

Your adding a quart every 1200 miles by above means you are burning some at least. That's about the amount most cars say is enough to do repair over, but then they run cats and oil literally poisons those.
 
The front cam chain guide thing...........

http://s728.photobucket.com/user/WBHYDE/media/DSC02863Large.jpg.html

Dunno if you are familiar with that guy but possibly the top early Honda DOHC racer in the world (New Zealand) right now. Lots to be learned looking through those pics and plenty of one off race pieces available nowhere else.

Oh yes, I saw a stunning rebuild of a 900 in Australia, using that guide (again, thought it was just a 900 problem before you told me). But the installation of these mods require the engine to be torn apart top and bottom (for that modified oiler), and I prefer not going down that path at least long as the engine's compression is within normal.
The valve guide oil seals I used at previous rebuild were genuine Honda. They may be past their best, but if they were truly shot, I suppose the bike would use much more oil?
I will install the cooler with pressed fittings and start collecting these 900 sump and oil pump parts. Hope it helps the oil consumption by keeping the oil cooler.
BTW, any idea where I could get the correct oil line fittings for the 900/1100 sump (they are rectangular shaped)? Of course I could get a used set of Honda lines and cut them at the top to push on the Derale cooler, but I'd prefer not to install oil lines that are 30+ years old to start with.
 
I would have to agree that the plug does not look lean. Maybe a little oil burning deposits or more then likely fuel additive deposits. I am sure the additives they put in the fuel is different from what we get over here, different additives do make different types of deposits and colors on plugs. As AMC said valve guide seals are killed from heat, just for future reference, Dynoman Performance has valve guide seals that are made to take the heat better then stock. I have used the in my sohc big bore motors with success.
 
We got AMC and DD interested now ;) These guys know more about these bikes then I do.

I am however stuck on the 7000 rpm thing?

Are you driving around in 3rd gear or cruising at 150mph (240KPH) because my bike would be going about that at 7000 rpm in top gear.

Sounds to me like you have the wrong sprockets on it. You should be seeing somewhere closer to 4000 rpm cruising in top gear.

The plugs look hot with a little oil to me, which they would be running at 7000 rpm all day.

PS Don't use car oil, it's not a car, your clutch is in there. Just use good quality regular oil or find some made specifically for wet clutch bikes.
 
[emoji4] Dave, are we talking about the same bike?)))
Mine has 18/46 which is the stock ratio of 2.55. Its roughly 65mph @ 5000rpm at top gear, with a top speed of 200kph (125mph).

7000 rpm in top gear is only about 140kph, no big deal of a speed. This bike is pretty slow overall, to be honest, but it's a shortcoming I've been able to live with for many years.

I never used car oil, only motorcycle specific 15w50 semi-synthetic.



 
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Anyways, I'm about to take the Saturday to clean the carbs. When I think of it, last time I cleaned them, was about 3.5 years ago.

 
Oil cooler will NOT help bad oil consumption that is already there. Just slow down any increase. You can get the lines and then carefully remove the rubber to change it. Normal hose clamps will seal since no pressure there once the crimps are gone.

Watch those oil seal materials, even formerly best of the best APE ones have gone to cheap crap material now and Honda claimed to be the same now and crap as well. Same with official Honda timing chains for these DOHC, they are Chinese smaller pin crap that break much easier. The people who frag the engines hard are breaking them left and right when they used to stand up to it.

I went to 18/42 (OEM 1100) gearing for the wide open spaces around here........I too wondered at the speed there, you should be around 85+ mph. Of course it may be acceptable there. I preferred to fast cruise at 2/3 max or around 6000. It was just past the Kerker header tune point of around 5500-5600 where the individual exhaust pulses smooth out to turn into the howl headers do as they come into powerband.
 
Oil cooler will NOT help bad oil consumption that is already there. Just slow down any increase. You can get the lines and then carefully remove the rubber to change it. Normal hose clamps will seal since no pressure there once the crimps are gone.

Slowing down increase in oil consumption is good enough for me at this point, as is anything else that would postpone any major head/cylinder job that would be a lot of hassle in my circumstances.

I went to 18/42 (OEM 1100) gearing for the wide open spaces around here........I too wondered at the speed there, you should be around 85+ mph. Of course it may be acceptable there. I preferred to fast cruise at 2/3 max or around 6000. It was just past the Kerker header tune point of around 5500-5600 where the individual exhaust pulses smooth out to turn into the howl headers do as they come into powerband.

That means lowering the final ratio by about 10% (from 2.55 to 2.33). It is tempting to have the bike cruising at lower RPM at given speed, but provided that 750 doesn't have the torque and the power of 1100, wouldn't that have a significant adverse effect on acceleration? (BTW, according to Sprocket Specialists, stock CB1100F ratio is 17/42, which gives 2.47, so did you actually use 18/42?)
 
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I honestly don't know what sprockets are on my 74. At one point I took it to the shop and told them to rebuild the carbs with all new jets and replace the chain and sprockets. I told them to put on an O-Ring chain and keep the current sprocket sizes. The back tire is not stock, instead of the 4.00-18 I have a 130/90-16 which I think is actually a bit smaller in circumference.

According to the stock specs, your bike makes 5hp more then mine and @ 5000 rpm I'm doing 80 mph and I still have plenty of power to pass. I also shift to third at about 55 mph when accelerating at full throttle.

From what you guys are saying my sprockets must not be stock sizes. My old bike will go more then 125 mph in 5th well before red lining if I hold the throttle open long enough.

I've ridden newer sports bikes and my CB750 is an old pig in comparison but it's still pretty damn fast.
 
I honestly don't know what sprockets are on my 74. At one point I took it to the shop and told them to rebuild the carbs with all new jets and replace the chain and sprockets. I told them to put on an O-Ring chain and keep the current sprocket sizes. The back tire is not stock, instead of the 4.00-18 I have a 130/90-16 which I think is actually a bit smaller in circumference.

According to the stock specs, your bike makes 5hp more then mine and @ 5000 rpm I'm doing 80 mph and I still have plenty of power to pass. I also shift to third at about 55 mph when accelerating at full throttle.

From what you guys are saying my sprockets must not be stock sizes. My old bike will go more then 125 mph in 5th well before red lining if I hold the throttle open long enough.

I've ridden newer sports bikes and my CB750 is an old pig in comparison but it's still pretty damn fast.

And what is your front wheel size? Did you change it too?
Did you measure speed with the bike's speedo or GPS?
How did the bike accelerate?

 
You are right and making me dig around in cobwebs. The front was a 750-18 tooth. I made a 17 at the time (very early '80s) for drag racing and also had a 49 tooth rear IIRC. By '1100 gearing' I meant a 42 on the back only and 18 up front. My bad. Had to make the 17 sprocket because the 1100 not out yet. I had no acceleration issues at all, bike would run up to 115+ easy as spit. Cars certainly couldn't stay up with it. Header, pods, carb work. The header alone I modded the baffle to make a whopping amount more midrange, the full silencer quietened greatly but killed torque. The Kerker header had a unique baffle design that allowed that to work. I figured close to 7.5+ hp there if not more. It sure ran like it. It ran pretty close to dead even with CB900Fs and I beat a few Kaw KZ900s streetracing (bad word kids!) with it.

The absolute top end unknown as I had pods on it. Even running at their best which was pretty good, they tended to not let the carb slides open all the way up at absolute load in 5th gear, hence the 115 mph thing. I was around 8000-8200 then and the OEM engine will run higher rpm pullout than that but the pods pretty much put a damper on it, the engine wanted to quit revving around that. I was satisfied with the lower speed envelope performance and no real need for 125+ so quit tuning at that time. Later on I found a couple other things to affect that, it could have been better.

750 is no 1100 but if you're talking fast enough to kill you irrelevant.

If the 130 is a wide one with low profile then yes, not as tall as a normal 4.00 tire. I had a 130-18 on back on mine at one time and it was visibly smaller OD from the side than the 120-19 up front. Sidewall height is everything there. It was way shorter in back than the 4.50-18 I had on at one time.
 
And what is your front wheel size? Did you change it too?
Did you measure speed with the bike's speedo or GPS?
How did the bike accelerate?

Front is a 100/90-19 which I believe is pretty close to stock.

Measured with my daughters 95 Honda civic cruising next to me.

It accelerates fine for me but that's all perspective I guess. My clutch is thin and if I push it too hard off the line it'll slip so I tend to roll on the throttle instead of dumping it open.

The only bike I ever went up against was my friends 98 soft-tail springer, 80CI I believe. I left him off the line then at about 1/8 mile he thumped on by and kept on pulling away.
 
Dave, 100/90-19 IS stock. Could your clutch go thin BECAUSE of the gearing?
I like having enough torque on tap in any gear.
My clutch plates are about 40000mi old and spinning the rear wheel is the only "slipping" it does :)
The trade off is optimal cruising speed of just 80mph (@~6500rpm), but it's fine for a bike which has no wind protection of any kind. Also today most of my commuting is within a 20mi radius from home, so cruising speed is not such an issue. To be honest, I don't even remember when was the last time I rode more than 65mi one end. Years, probably.

 
Actually, a 3.25-19 is stock. Mine uses inner tubes too, 74 remember? :D (being a smart ass)

I literally got this bike out of a snow bank in MI. in the middle of winter, only the bars were showing. The owner left it with someone else, they left it outside. I offered to take it away, the owner was contacted and a week later he mailed me the keys and title for free.

Other then the seat cover and bars it looks pretty much like it did when I got it just with a lot less rust.

I've replaced - Jets, fuel lines, ignition system, blinkers, tail lens, chain/sprockets, mirrors, grips, bars, rear shocks, and tires. I had the seat cover redone in velure (a fabric resembling velvet) and the rear wheel re-laced (It's a harley rim laced to the stock hub). The local Honda shop (at the time Big Valley Honda in Reno, NV) rebuilt the carbs and the local Harley shop corrected their mistakes so the bike would run right again. I converted the lighting to LED except the headlamp, it's a Cibie RH Sealed beam that they don't even make any more.

I just haven't gotten around to replacing the clutch. ;)
 
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These are the plugs 1-->4 (left to right).
Does no. 3 look a bit lean?
BTW, I think there was a way to tell, how the engine performs on high or low rpm (or throttle positions) by looking at different areas of the plug, or was it an urban legend?

 
Fine. Splitting atoms there. Not so much lean possibly as hotter, the inside cylinders can run ever so slightly hotter as they only have front/rear cooling and none on the sides like the outside ones do. Two ways to control that, rejet for center or replug for center. I used to pick a one stage colder plug to run in the center of Kawasaki threes I used to race, that way the jetting stayed the same. There was more difference than what you show there though.

You're probably warping the old adage that when plugs are brand new and you can't see any color on them yet, they first begin to color up way up inside the shell where porcelain meets the steel shell. Why racers commonly use a flashlight to look up in there. The only way you can tell high or low is by holding that position a long time then cutting engine and looking right then. How you look at high load jetting, hotrod the hell out of it and kill motor at high rpm peak. Meaning you clutch it, kill it and roll down with motor not turning at all.
 
1-3 have a little build up, oil as you've said. 4 looks like it was leaking as the threads are entirely black but looks perfect otherwise. 1 looks lighter in color. Hot / lean / lighting in photo? Overall not bad.


Lets recap:

#1 You only smell it at idle. Well of course, any other time your moving and leaving the odor behind you.
#2 It has a bazillion hard miles on it
#3 You know it burns oil.
#4 The spark plugs look good.

I'm going with exhaust valve guides leaking oil into the exhaust ports.
 
1-3 have a little build up, oil as you've said. 4 looks like it was leaking as the threads are entirely black but looks perfect otherwise. 1 looks lighter in color. Hot / lean / lighting in photo? Overall not bad.
No flash on photo, the colors look true. So, would the board recommend, say, 1/4 turn out on the idle mixture screws? My main jets are stock keihin #68.

 
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