CB750 FAQ
The CB750 FAQ contains a compendium of CB750-specific technical information from the SOHC/4 forums.
Oil Change Tips & Tricks
The CB750 is the only SOHC/4 model with a dry sump. As a result, the oil change process is more involved than just removing the sump bolt since you need to drain the oil tank as well.
Excellent set of instructions posted by Jonesy:
Here’s how I do it (FWIW…)
- Start up bike and let it run for about 10 minutes. This warms up the oil and gets things roiled up inside, but not so hot you’ll burn yourself. This helps lets the oil pick up crud and let it drain away with the oil, rather than having it settled out and stay in the engine.
- After the bike is warmed up a bit, shut it off and put it on the center stand.
- I start with the oil tank first. I remove the sidecover and temporarily undo the rear brake light switch to get it out of the way. I also crack the filler cap. Have a drain pan ready and remove the plug. I have a piece of aluminum that I bent into a small trough to keep the oil from running all over the frame.
- After the tank is drained, replace and tighten the plug. Proceed on to the oil pan plug and drain the oil from there. Again, replace and tighten the plug.
- With the oil pan plug out, kick the engine over a few times to pump out the remaining oil.
- Lastly, remove the oil filter housing. If all the parts are in the housing, there is a thin washer between the oil filter element and the spring. This likes to stick to the filter and before you know it, you’ve thrown it away. (This is probably why 90% of them are missing) In other words, when you remove the innards, don’t lose it!.
- Wash out any sludge or particulates that have collected in the oil filter housing. Take a good look at what’s in there, as any sizable bits of metal might be a warning that something’s on it’s way out. In some cases it’s worthwhile to drop the oil pan to give it a good cleaning and look for any signs of trouble.
- If you bought an oil filter kit that includes new O-rings, fit the small O-ring on the oil filter bolt and seat the big O-ring in the groove of the filter housing. Apply a bit of clean oil to the oil filter bolt O-ring to make it easier to reinsert into the housing.
- With the bolt back in place, reassemble the oil filter housing with the spring first, then the washer and finally the new filter. Reinstall the filter on the engine, being careful not to overtighten the bolt.
- Reinstall the sidecover over the oil tank and put the brake switch back in place.
- Fill the oil tank with 3 quarts of oil. According to the owner’s manual, the oil level will settle into the correct range when the engine is started. So far, I’ve found this to be true.
- I like to hit the starter a few times with the emergency stop switch off to circulate the oil into the empty filter housing before running the engine. After doing this, fire up the bike and make sure the oil light goes out in a few seconds.
Again, this is how I like to do it. It doesn’t HAVE to be done this way, but I hope it’s helpful.
From jaknight:
If you happen to want or need to drop your oil pan (I obviously don’t know how familar you are with these bikes), just a heads up to keep track of what holes the different bolts come from; they do vary in their length.
If you happen to need the torque specs for the different bolts:
| OIL TANK DRAIN PLUG | 24 FT LBS (ideal median) |
| CRANKCASE DRAIN PLUG | 24 FT LBS (ideal median) |
| OIL FILTER HOUSING BOLT | 20 FT LBS (ideal median) |
| OIL PAN BOLTS | 7 to 10 FT LBS (pick your median) |
From toycollector10:
On an older bike you will probably have a lot of gunk sitting in the bottom of the oil tank.
After you have drained the oil put a screwdriver down there and see what you bring up stuck to the tip, and use a flashlight to have a look inside the tank. If it is a mess down there I recommend you pull the tank off the bike and clean it out properly.
and Hondaman:
Although the torque on the oil bolt is listed at 20 ft-lbs: for many years, I’ve only tightened them to (snug + 1/8 turn). This works out around 7-8 ft-lbs on a T-wrench. I’ve always done this because the oil filter housing distorts and locks that $%#*! bolt in something awful, often causing its destruction the next time around.
…just some experience. It won’t leak if your large o-ring is new…
CB750 Engine Changes
This table of CB750 engine modifications was compiled by Axl Griessman at satanicmechanic.org, and is based on Reinhard Hopp’s book, “Honda CB 750 - Die Geschichte einer Legende”.
| Engine # | Changes |
|---|---|
| CB750E-1000001 | K0 |
| CB750E-1000140 | Bolt at the final drive changed from M8x80 to M10x82 |
| CB750E-1000220 | new lower case, engine case only available as set |
| CB750E-1000425 | Transmission changed for better shifting |
| CB750E-1001081 | new crankshaft |
| CB750E-1001760 | 7 instead of 8 clutch disks |
| CB750E-1003528 | new final drive shaft and sprocket |
| CB750E-1005307 | fixed right bearing of main tranny shaft |
| CB750E-1007220 | new lower case, deeper oil pan |
| CB750E-1007415 | introduced circular opening in lower case |
| CB750E-1007500 | End of sandcast engines |
| CB750E-1009554 | finned oil filter casing |
| CB750E-1010336 | cam holders with additional oil passages, new rocker shafts and valve cover |
| CB750E-1014996 | new exhaust valve guides, new valve guide seals |
| CB750E-1026144 | new cylinder head, cylinder block: new central M6 bolt, cyl. block w/8 rubbers i/o 12. Case with guard to prevent damages from broken drive chains. New sprocket cover, new shift mechanism w/11 parts i/o 7 |
| CB750E-1042806 | weaker clutch springs |
| CB750E-1044813 | K1 |
| CB750E-1044805 | new case, final drive 18/48 i/o 16/45 or 17/45, chain oiler |
| CB750E-1044812 | carbs w/links i/o individual cables |
| CB750E-1044968 | improved neutral position |
| CB750E-1056080 | new clutch basket |
| CB750E-1064903 | introduced special washerless bolts |
| CB750E-1068376 | cramp for scavange hoses |
| CB750E-1071336 | fuel hoses 5.5mm dia. i/o 5.0mm |
| CB750E-1113723 | grooves in gear shift drum changed from 0.5mm to 1mm for 4th and 5th gear |
| CB750E-1114461 | new head gasket |
| CB750E-2000001 | K2 |
| CB750E-2061311 | shorter oil hose w/white mark. Main jets 110, smaller cutouts in air filter case |
| CB750E-2200001 | K3 |
| CB750E-2200001 | new cylinder head, new valve guide seals, improved oil passages to camshaft, new piston rings (three part oil scavenge rings), new cam chain guide, larger piston pin circlips, new final drive shaft, new sprocket cover |
| CB750E-2228679 | new cylinder head, new valve guides, new valve guide seals, no more rubbers between fins |
| CB750E-2300001 | K4 |
| CB750E-2304501 | new cylinder block, smaller cam chain tunnel |
| CB750E-2352923 | new cylinder block |
| CB750E-2348093 | crankcase mission cover now has gear indicator |
| CB750E-2372115 | K5 |
| CB750E-2428762 | K6 |
| CB750E-2470427 | clutch almost indentical to F1: new clutch basket, new clutch cover and chrome cover |
| CB750E-2434657 to CB750E-2439607 new gear shift drum, new center shift fork for improved shifting. New gears on countershaft w/bronce bushings, new case | |
| CB750E-2700001 | K7 - carbs w/accelerator pump, shorter intake rubbers, new final drive shaft, sprocket w/center bolt, wider chain line (as F2), final drive 15/41, new clutch, new camshaft (as F1), new pistons (as F), compression ratio 9.2/1. New transmission: 4th gear 1.133 i/o 1.087. Final drive gear 50 teeth i/o 56 (as F1), primary drive 1.985 i/o 1.708 |
| CB750E-2719530 | new valve guides |
| CB750E-2719997 | new cam carriers (as F2) |
| CB750E-3000001 | K8 - new intake and exhaust valves, new valve spring retainers and cotters |
| CB750E-3021913 | new cylinder block w/larger cam chain tunnel |
| CB750FE-2500004 | F1 - new camshaft, carbs w/additional idle air. New pistons, compression ratio 9.2/1. New case w/o primary chain oiler. Final drive gear ratio 43/50 i/o 48/50. 4th gear on main shaft 31 i/o 30, 5th gear on countershaft 31 i/o 32 |
| CB750E-2600014 & CB750GE-1000014 | F2 - new cylinder head with bigger valves (34/31mm i/o 32/28mm), larger combustion chamber, larger carb mount rubbers, stronger cam chain, new camshaft, stronger valve springs, new retainers, cotters, new pistons compression ratio 9/1. New rod bigend bolts and bearings, stronger clutch springs, additional fins on crankcase, larger fins on oil pan, “oil cooler” - finned plate between oil filter case and engine. Final drive 15/43 or 14/43. |
| CB750E-3100001 | F3 |
CB750 Engine Removal and Replacement
Quote from: jdigga on June 08, 2007, 11:19:07 PM
So the weeping from my fins is at the point where stuffing half a Shop Towel in between isn’t enough to stem the spray. I’m a bit tired of having to wear a specific pair of oil-stained riding jeans, so it’s time to make the necessary repairs.
I started the tear-down around 5:00pm. Most everything came off without any problems. I did have to use an impact driver on the front sprocket cover and found to my dismay that the sprocket bolt was loose! Luckily it appears that the cover has a little shaft on the inside to keep the bolt in check. I also discovered that the rear brake splined shaft is slightly bent, but it functions just fine.
I knew the actual removal of the engine would be challenging, but I had no idea what I was in for. I tried putting the bike on its ride side but I could get the right angle to pull the frame off. The engine was getting hung up everywhere, and as soon as I’d release one spot, it would hang up somewhere else. I must have put the bike upright and back down again half a dozen times.
By now it’s around 8:30. Took a breather for dinner and browsing this forum for some tips.
Went back out for one last attempt before calling it a night. The bike was on its side and while staring at it I realized the engine wasn’t going to come out that way.
I put it back upright on the centerstand and broke out my set of lady fingers. Shoved one in the lower right rear mount hole, one underneath the rear of the engine, and one in the upper right rear mount hole to act as a handle. Pried the rear up to clear the lower mount. Pulled up the front of the engine to move it towards the right a bit. Basically I walked it out of the side, front-back-front-back.
Turned off the garage lights and closed the door at 10:30pm.
In the process of putting the bike down so many times, my points cover took a beating (even though I put it down on those rubber puzzle mats). The right bar on my clubmans is pointing slightly more downward than the left. I forgot that I left my key in the seat lock and it broke off–I was able to retrieve the bit inside the lock, but I only have one key.
Honda made a huge access improvement in 1979 with the DOHC bikes. The lower right frame rail is detachable and the engine pretty much falls right out. I pulled my other engine the other week, and other than it being a heavy SOB it was hardly a challenge.
And to think I’m only 1/4 of the way there. I still have to take the engine apart–I have no doubt I’ll run into problems there. Plus all the cleanup of old sludge buildup and repair the busted up bits. Then put it all back together again. Don’t get me wrong–I love tinkering with my bikes and learning about them. But sometime it’s just frustrating as hell. I’ll feel better when I’m back on the road again.
Just had to get that off my chest…
Before you put it back in….there is a flange on the motor mount on the lower right side. Grind or file the top of the flange off to the height of the rest of the mount, then shoot a bit of paint on it. This makes the whole assemble-disassemble job MUCH easier. This little flange has probably left several notches in the rib on the bottom of the engine where the bolt goes thru, which can actually jam the engine so badly on the way out as to require a frame cutout to unjam the whole thing (don’t ask me how I know that….). But, removing this tiny little flange top will prevent all of this…
Henry Abe 900cc Piston Rings (CB750)
Quite often the guys that have the 900 Henry Abe kits for the 750′s are looking for ring sets. I just got off the phone with Ed at Total Seal (www.totalseal.com) I shipped my 4 NOS Henry Abe pistons to him to fit or cut & fit for 3 piece oil rings. The good news in that Total Seal can supply all 3 rings (top, 2nd, and 3 piece oil) without cutting the piston. Henry Abe made 2 different 900 pistons however and my qualifier here is that I have the smaller/lighter of the 2. Axel’s site (www.satanicmechanic.org) lists my pistons as the slipper type.
Here are the Total Seal part numbers:
| Top Ring | 203778 |
| 2nd ring | 201336 |
| Oil Ring, 3 piece | 001549 |
Hope this can help others.
Jerry
Gearing Chart
This spreadsheet can be used to calculate gear ratios for your CB750.
Service Bulletin Index
| Bulletin Number | Issue Date | Subject |
|---|---|---|
| Recall Letter | 08/13/1971 | Safety Recall of pre-K1 CB750’s to modify the drive train to reduce drive chain shock loading and extend drive chain life. |
| 750 #4 | 09/22/1969 | Drive Sprocket Factory Recall |
| 750 #6 | 11/3/1969 | Tachometer Gear Stopper Bolt Security to Prevent Cylinder Head Damage |
| 750 #7 | 10/17/1971 | Wing Bolt Retaining Nut Security - Early CB750 Air Cleaner Covers |
| 750 #8 | 11/6/1969 | Ignition Timing With Bent Advancer Shaft |
| 750 #9 | 05/07/1973 | Vacuum Guages: Model Differences and Calibration Instructions |
| 750 #10 | 12/10/1969 | Dead Batteries - Loose Alternator Rotor Set Bolt |
| 750 #11 | 11/01/1971 | Brake Lever Free Play Adjustment |
| 750 #13 | 01/26/1970 | Modification to Prevent Oil Filter Case Damage |
| 750 #14 | 10/06/1972 | Crankshaft Bearing Insert Selection |
| 750 #15 | 01/31/1975 | Modified Throttle Valve and Cable Kit |
| 750 #16 | 05/20/1970 | Camshaft Holders and Caps |
| 750 #18 | 06/13/1975 | Drive Chain and Master Link Selection |
| 750 #19 | 07/02/1970 | Adjustment of Cam Chain Tensioner |
| 750 #21 | 09/03/1971 | Drive Chain Maintenance |
| 750 #22 | 08/28/1970 | After Sales Service: Modified Chain Cover and Chain Oiler Parts |
| 750 #23 | 09/17/1971 | Brake Pad Wear |
| 750 #25 | 09/21/1971 | Carburetor Adjustment, CB750K1 |
| 750 #26 | 12/23/1970 | Front Brake Disc Mounting Bolts |
| 750 #28 | 02/09/1971 | Drive Chain Link Removal and Installation Tool for CB750K1 |
| 750 #29 | Crankcase and Final Drive Bearing Replacement | |
| 750 #30 | 05/21/1971 | Drive Chain Oiler Adjustment and Replacement |
| 750 #31 | 08/28/1974 Connecting Rod Weight Selection and Size | |
| 750 #32 | 08/09/1971 | Drive Chain Recall Notice |
| 750 #33 | 11/03/1972 Wiring Harness and Breather Tube Routing, CB750K1 & K2 | |
| 750 #34 | 05/04/1972 Disc Brake Caliper Noise: CB750 & CB750K1 | |
| 750 #35 | 11/17/1972 New Molded Countour Oil Hose | |
| 750 #36 | 04/06/1973 Rear Brake Wear Indicator, CB750K3 | |
| 750 #37 | 11/19/1973 Wheel Sprocket Diameter and Chain Pitch Compatibility | |
| 750 #38 | 11/26/1973 Muffler Warranty Extension | |
| 750 #39 | 02/01/1974 Modified Head Gasket Removal | |
| 750 #40 | 04/01/1974 Modified Shift Drums | |
| 750 #41 | 04/05/1974 Front Fork Oil Seal Change, CB750K1 & K2 | |
| 750 #42 | 07/10/1975 Cylinder Head Gasket Oil Sealing | |
| 750 #43 | 01/10/1975 Battery Cable Extra Lead | |
| 750 #44 | 04/18/1975 CB750F License Plate Bracket Plug | |
| 750 #45 | 03/24/1976 Soft Mounting Cushions Improve Instrument Accuracy |