Topic: CB750K4 won't maintain engine speed/rev past 6k

[Beergineer]

Ok, bike has the stock airbox, a 4-2 exhaust, and stock jets. Static timing is set, carbs are bench synced and float heights are set to 26mm. It will start and idle well.

The bike will hold an engine speed (2K-4K) for a few seconds then slowly die. It also won't rev past 6K unless you really snap the throttle open.

I'm thinking this is a fuel starvation issue? The problem exists with both my auxiliary tank (1/4" main line to 1/4" Tee to the carbs) and the actual tank (which currently has 3/16" line on it). I appreciate the help!

[flybox1]

Timing - recheck your timing and advance to be sure you are getting full advance.
Clear tube test your float bowls to ensure proper fuel level.
Pull your main jet/emulsion tube bits and make sure they are adequately clean and retest.

[Beergineer]

I set my static timing pretty well, but I don't have a strobe to check the advance. With a little luck tomorrow I'll have some adapters made up to clear tube test the bowls. The jets and tubes are squeaky clean. I went up to 1/4" fuel line just to see, but nothing changed.

Here's a video of what's going on:

[Jarrett_Honda]

What about compression?

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[Beergineer]

Compression was checked before the engine went back in the frame. All 4 cylinders were up around 140psi.

[ekpent]

 A bad condenser or two if you still have points can sour the performance.

[seanbarney41]

I'd be double checking the float bowl vents are clear.

[Jarrett_Honda]

Isn't 140psi below the minimum spec of 150-180psi? I could be wrong. I don't know how much that would affect anything though.

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[Beergineer]

Thanks for the suggestions folks! Condensers are new and the old ones checked fine (changed them because I had them) and for compression 140 across the board is fine for a number of reasons detailed in some previous heated threads.

I'm pretty convinced this is fuel related.
SeanBarney: by Bowl vents do you mean over flow tubes? I'm pretty sure they are, but I'll run some guitar string through them anyways to check.

Lucky for me its a rainy day, which means yard work gets swapped for bike work.

[flybox1]

Isn't 140psi below the minimum spec of 150-180psi? I could be wrong. I don't know how much that would affect anything though.

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The # measured doesnt matter so much as having all cylinders similar in pressure. 
Also, his comp tester is not the HONDA tester used to set book numbers, so there will always be a difference.

Twist in some PVC vacuum line connectors to make your clear tube test adapters.  No need to drill out any bowl drain screws

[seanbarney41]

No not the overflows ... 2 nipples coming out the top of bowl and angling rearwards.  Should have short hose that drapes over the air box left open to atmospheric pressure

[Beergineer]

Alright, so I adjusted the float levels. They were definitely low, ~15mm below the gasket. They're now in the 6-8mm range. I also pulled out the spark advancer and cleaned/lubed it. Also checked the bowl vents, they're clear. After making the level adjustment the bike will hold a steady rpm at 4K or lower. It would still only rev higher if you snap the throttle open, and then it would billow bluish smoke. So I decided that maybe it just needed to be driven a bit (engine hasn't run in about 7 years) and cleaned out.

That's when I found out that it really doesn't want to accelerate under load (I live on a steep hill: down goes to big road, up goes to residential neighborhood). So I lugged it in first at 2K up the hill to flat land. There it wouldn't smoothly rev past 3K, but if you snapped the throttle open it would open right up.

I'm now thinking these may be symptoms of very poorly synced carbs? The cylinders start to fight each other around 3-4K and unless you race to full throttle it starts to shut things down?

[BomberMann650]

Yep, time for a full tune up.
If you're running points, make certain the 1/4 and 2/3 hit the mark at the same degree of crank rotation.  Hate points?  Buy a pamco.

[ekpent]

Alright, so I adjusted the float levels. They were definitely low, ~15mm below the gasket. They're now in the 6-8mm range. I also pulled out the spark advancer and cleaned/lubed it. Also checked the bowl vents, they're clear. After making the level adjustment the bike will hold a steady rpm at 4K or lower. It would still only rev higher if you snap the throttle open, and then it would billow bluish smoke. So I decided that maybe it just needed to be driven a bit (engine hasn't run in about 7 years) and cleaned out.

That's when I found out that it really doesn't want to accelerate under load (I live on a steep hill: down goes to big road, up goes to residential neighborhood). So I lugged it in first at 2K up the hill to flat land. There it wouldn't smoothly rev past 3K, but if you snapped the throttle open it would open right up.

I'm now thinking these may be symptoms of very poorly synced carbs? The cylinders start to fight each other around 3-4K and unless you race to full throttle it starts to shut things down?

Synching the carbs mostly helps out for making a smooth idle. A proper bench synch on the carbs should allow it to rev to redline barring other problems.

[Beergineer]

I'm still running points, and they're (as far as I can tell) well adjusted.

ekpent-That's just the thing, it will rev to redline. It just seems to get 'stuck' around 5k when parked, 3k when driving.

I just ordered a sync tool, that way I have it for when my brother finally gets around to putting my dad's 76F back on the road.

[ekpent]

 The one story I can offer is a friend bought a 75 cb750F model from me. It would idle and rev fine parked but fall on its face at 4-5 grand on the road under load. He tried changing pilot and main jets and chased the carbs for quite awhile.I told him they were clean and fine. Saw him again later and he happily reported when he changed the condensers all was good and it ran great. Not an answer to your problem but a real life experience with one. Maybe you could borrow and condenser or two from your brothers bike just to try out and check that box off the list.

[flybox1]

Alright, so I adjusted the float levels. They were definitely low, ~15mm below the gasket. They're now in the 6-8mm range. Still too low. I also pulled out the spark advancer and cleaned/lubed it. Also checked the bowl vents, they're clear. After making the level adjustment the bike will hold a steady rpm at 4K or lower. It would still only rev higher if you snap the throttle open, and then it would billow bluish smoke. So I decided that maybe it just needed to be driven a bit (engine hasn't run in about 7 years) and cleaned out.

That's when I found out that it really doesn't want to accelerate under load (I live on a steep hill: down goes to big road, up goes to residential neighborhood). So I lugged it in first at 2K up the hill to flat land. There it wouldn't smoothly rev past 3K, but if you snapped the throttle open it would open right up. 

I'm now thinking these may be symptoms of very poorly synced carbs? The cylinders start to fight each other around 3-4K and unless you race to full throttle it starts to shut things down?

Once fuel level is closer to 3-4mm, timing/advance is spot on, and carbs are vacuum sync'd, move on to an idle plug chop.  You need to work through the throttle range, starting with idle, and verify/adjust the A/F mixture until it is correct.  Cant do this properly if carbs and ignition are out of sorts.