Topic: 1970 CB750. Sudden Change in Performance....

[BenelliSEI]

My 1970 CB750K0 was running fine. Took it to a show, and have only ridden it a few kilometres. Carbs were rebuilt by Mike Nixon, year’s ago, but fresh out of the bags. Valve clearances set, new points plate, points, condensers. Dwell and timing set, spot on.

Yesterday, I fired it up and it stumbled along, even when warm. Pipe #2 stayed cool. Only ran it 3-4 minutes, but observed gas on the floor. Pulled the 4 X over flow pipes out to the side. No fuel from there. On close inspection, while idling, raw fuel spraying out #2 exhaust pipe vent (the one at the welded connection between the downpipe and muffler).

A quick check confirmed the plug is firing. Float height is good (lots of fuel!) and no overflow. I wonder if a jet fell out or a choKe “flap” is stuck? Can’t get into it for a few days. Anyone else experienced this sudden change/symptoms? Thanks. 

[emlupi]

Usually if I have one cylinder acting that way I find a fouled plug. I have a K2 with the original motor and I believe the valve seals are beginning to leak, especially #4. If it sits for awhile I find the plug oil fouled in that cylinder. ,
That’s a beautiful bike, good luck with it.

[ekpent]

Old brass floats which a K0 had can get a pinhole in them and fill with gas which causes them to sink. Other then that a stuck float will flood.

[BenelliSEI]

Guys...... thanks for the suggestions. It will be interesting to see what I find. As the over flow pipes were not leaking, I don’t think it’s floats. Will post findings....... any more thoughts??

[Jerry Rxman Griffin aka MuthaF'er]

If you don't find a carb problem run a compression check for the valves and/or rings.

[Gene]

I had that problem with a 550, jet was sitting in the bottom of the bowl. Totally my fault and I don't know why yours would do that if you hadn't fiddled with it lately, but yep.

[BenelliSEI]

“Shop Day” planned for Friday....... will check all that’s been suggested. Thanks!

[DickL]

Check the boots for leak on No 2. Use WD40 when its idling

[PeWe]

Are the overflow pipes open and free to flow fuel out?
I had some blocked not letting the fuel out that way. Out to the air box.

[ekpent]

  Overflow tubes are part of the venting system and have seen the same thing as PeWe when one or more are blocked.

[BenelliSEI]

So far, here’s what I found.......
1. Brass float in #2 carb has a pin hole and needs repair.
2. Dropped all the bits from one and two carb (bowls, floats, jets etc.). While staring at the two carb undersides, realized that the #2 needle was not visible (with main jet and tube removed)! Pulled the top off that carb and discovered the needle clip was ABOVE the black retainer, just floating in the slide! Impossible to just “happen” so can only assume it was like that to start?

Repairing the float and then starting over...... will have to do the full carb sync again. Total mystery why the bike ran OK to start with. More news to follow. Totally weird!

[ekpent]

 At least in the K1 bikes and I would assume the K0 a plastic/foam float from a later model can be swapped but am sure in your case to keep it 100% stock you want the brass. I have swapped them out before and put the brass in a bag for the next owner someday. So far looks like a pretty easy fix and may run even better then before.

[BenelliSEI]

Repaired the float, reset the needle and no more issues. Next job will be to get out the carb sync tools and do it all again, but 200% improvement already! Back to a nice “bark” from the HM300’s. Very satisfying day! Thanks for all the advice!

[PeWe]

My airbox got holes drilled after overflow episode. One small hole in each corner, 1;5-2mm to drain ev fuel next time. I cleaned the vents and have not seen any fuel leak. Floats were OK before that.
 I think some overpressure from tank helped to overflow the fuel.

[BenelliSEI]

There was some fuel in there. I’ll add some small holes. Thanks.

[BenelliSEI]

Today I got the carbs sync’d. This bike is finally running perfectly, now all I have to do is change the oil and filter, dry out the carbs, drain the tank and add some two-stroke oil, and pack it away! This one is too nice to ride and all my other cb750’s (K1,3,4 and ‘75F) are running perfectly!

How rare is that! This winter I will finally tear down the K4 “rat bike”. Bought it several years ago and my son and I use it the most. The front and back end are done, but the engine has never been out of the frame and the carbs have never been off. It always ran too well to touch. Hope we don’t mess is up!

[seanbarney41]

Today I got the carbs sync’d. This bike is finally running perfectly, now all I have to do is change the oil and filter, dry out the carbs, drain the tank and add some two-stroke oil, and pack it away! This one is too nice to ride and all my other cb750’s (K1,3,4 and ‘75F) are running perfectly!

How rare is that! This winter I will finally tear down the K4 “rat bike”. Bought it several years ago and my son and I use it the most. The front and back end are done, but the engine has never been out of the frame and the carbs have never been off. It always ran too well to touch. Hope we don’t mess is up!

if it runs good, don't mess with it.  Even Honda's part quality is not as good as what is on an original bike.  You should get some new pictures though,  getting pretty tired of that one in every one of your threads.  ha ha

[BenelliSEI]

Sean..... you might be right, that darn bike runs so well, I should just leave it alone! It’s the only picture I have of my son on it! 

[scottly]

2. Dropped all the bits from one and two carb (bowls, floats, jets etc.). While staring at the two carb undersides, realized that the #2 needle was not visible (with main jet and tube removed)! Pulled the top off that carb and discovered the needle clip was ABOVE the black retainer, just floating in the slide! Impossible to just “happen” so can only assume it was like that to start?

Better check the top of carbs 1,3, and 4 and make sure Mr Nixon didn't make the same mistake on all four carbs.

[BenelliSEI]

“Scottly” ........... I did, all is good. In fairness to Mike, these have been sitting around for 5 years, waiting for me to finish the bike. Weird.