• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

Fuel issue or spark issue, 81 Custom

Rozbabbo750c

CB750 Member
Messages
8
Reaction score
0
Points
1
Location
Buffalo,NY
Morning all, recently near completion of my 81 cb750c project. She runs l decent but I think there could either be a richness issue or weak spark issue.

When I pull a plug and check spark it's there but weak in my opinion. When I look at the plug its black which leads me to believe it's running rich.

The only mods are a K&N filter in the factory air box and a MAC 4into1 exhaust.

The carbs came off a stock configuration running bike and I have done nothing with them but install them onto my bike.

I took it for a quick ride and I feel like it wants to stall out once I come to a stop but it revs good. I dont notice any black smoke to signal too much fuel but the exhaust stinks bad.

Any feedback is greatly appreciated.
 
Those CVs do not like K&N even in the airbox, the slides open slower, as a result when opening slow they run richer due to increased vacuum across a smaller orifice (slide opening). Use an OEM paper filter to bring slides back in line, after that motor condition does the rest, see next.

If you haven't run the valves yet or compression test to see where they are you are going backwards. The draw that opens slides is based on engine compression.
 
Thanks for that tip. Time to hop online and find a filter. I did a full rebuild on the engine. Sat along side a barn for 15 years and 3 cylinders were full of water. Lapped the valves and did a leak test on the head. Everything was good to go. Cant believe these carbs are that sensitive.
 
They are labyrinth seal type, they have no rubber diaphragms at all, the seal there is a calibrated leak. Literally breathing on them messes the slides up.

A leak test is NOT a compression test, which will show issues the second you know the number. Even more not if you are referring to valves leaking only. If leak-DOWN test what was the result?

Lapping valves closes the gap up even more if anything, if too close already you may have compression leaks to run the motor rich, compression is heat to the plugs and pulls them away from being black. How most of these die or are found when bought, most never had the valve set because it required shims, commonly Honda wouldn't even set them dead on, just claim to and then collect money for work not done. You buy them with carbs messed up in a box commonly because today's guy auto-thinks it must be the carbs when the valves are leaking.

The carbs are well-known for being among the hardest to get 100% clean too, many have to do it 3-4 times before it's done right. There is a weird seesaw thing going one with pilot idle jet and primary jet, they both serve also as an air jet when the other is working, stopped up pilots then making for lean idle to die and immediately above that super rich primary jet just off idle because of the interlocking seesaw there, it drives people utterly nuts. You're dead lean to die and even breathe on throttle to open it and suddenly super rich. FUN. You need to screw in each mixture screw to see if it kills that cylinder, if no effect it's stopped up.
 
Last edited:
3 out of 4 cylinders held the fluid at full. Re lapped the valves on the 4th and it stayed with the others.

Now would this filter airbox issue cause the bike to be difficult to start? I have to turn it over quite a bit before it will stay running under it's own power. Should I mess with the fuel screw at all or see what it does after the filter change?

Thanks again.
 
The fuel screws ONLY affect idle and filter should not affect those, the flow being too low. Filter flow resistance comes in at a guess of 1500-1700 rpm to begin to have effect, you are not using mixture screws at that point, it's primary jet.

The starting issue is fouled plugs, the black plugs begin to short down the sides to short circuit to not make a hard spark through the gap as normal, it most definitely affects starting up. You observed the spark was 'weak', look close enough and you will likely see some of that jumping down the side of the porcelain in dim light. It lowers what happens at the gap to have little effect and why harder to start up. You'll likely need to change the plugs.

Lapping valves is fine but once they bed in to remove the highs and lows somewhat the valves generally end up closer, you can lose a solid thousandth there in 15 minutes running. Same as our amish speed test thing where somebody never set the valves and they bed in to be barely sealing, you go in and loosen them up then run the crap out of bike for 15 minutes or so and then CHECK THE VALVES AGAIN. Some will have settled in to be fine but often one or two or whatever settle in to be too tight AGAIN and you have to adjust those to be looser a second time. It happens because sometimes carbon on the valveseat is hard enough to stay in place, you drive the crap out of it to loosen and chip that away and then the valve is closer when it's done. Making you have to loosen it up again, at some point you get to the solid 4-6 thou number and things then stay the same after that. When you get down around to 2 there is really no way to tell how tight it is, all I know is a 2 is actually zero clearance at least, the engine will run like crap to show you so and the service manual saying you can get away with it is totally WRONG. And why Honda later changed the valve setting markings on cams, they knew they had a problem but they were arguing about what it was. Part of it was they failed completely to understand how much the cams were pushed around by the valvesprings to be nowhere where the lobes needed to be. The huge cam cap clearances made the problem even worse. As in, how do you adjust valves to a smaller number when the cams can move around in the caps almost twice that amount. It's a messed up design.

The engines will run like an utter bat out of hell but they are very picky about things to be able to do it. Attention to fine detail makes you or breaks you there. One running right will bury redline at 10K in every gear but high and they will pull out 9K in high. Very few of them do it though, too many things holding them back.
 
Very interesting. Funny you say the Honda factory made changes eventually to the manuals after arguing about it for a bit. I'm into the Yamaha XS650s as well. There are a lot of quarks with that bike where you question why the factory didnt just do it in the first place. Main one using rubber/plastics head stud washers instead of copper ones. After a while your torque setting on those head bolts dont mean anything because those rubber washers just collapse. Lots of head gasket leaks out there.

I'm definitely getting new plugs. I even contemplated getting another set of coils. Mine are used however the guy i got them from said he "tested" them and they were at spec.

Combination of fouled out plugs, bad air, possible bad gas I think it's all working together. Hopefully these changes will make a difference.

Thanks again
 
Plastic head stud washers............(shaking head sadly)

On these the cam cap clearances come OEM at tighter on the ends and looser in the middle toward cam drive gears. ???! It allows the springs to push the cams all over the place at hand rotation and one reason why about .002" of any perceived valve clearance is not real when running and the cams center up in the oil pressure and averaging.

OEM valve clearance checks are at 4K miles, set them at .005" instead of .003" they go easily twice that.
 
Ugh the cams...I had a hell of a time getting the shim gaps within spec. Contacted all the local Honda dealers who were happy to get rid of the shims for these things that have been sitting on their shelves for years. If I recall some of my gaps are towards the fat end but still within spec. Thanks again for the feedback.
 
You may also need to check the cdi boxes under your seat, if they are the black type they tend to crack and leak and cause a weak spark.
 
If the OEM part they leak because a crap tar compound that melts at 100 degrees was used to seal them. Typically it drips off to leak on things and typically the box stays OK because the tar even in a thin layer tends to still completely coat electronic parts to keep them weatherproof.

Even when brand new the spark is somewhat weak on these.
 
Back
Top