Using too much fuel

T

timtune

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1976 CB750F. Solved the ignition problem and now it runs good. It's a little "stumbly" below 4K but north of that it pulls strong towards redline. Fires up good and idles ok however I'm forced onto reserve at 60miles instead of 120. The fuel mileage is horribly bad. Plugs are sooty as expected.
Wondering what carb problem would let it run reasonably well but burn through so much gas. As part of the rebuild carbs were completely apart, cleaned ultrasonically and then assembled with the original brass and set to stock setting. I synched them cold on the bench with a drill bit. When last together (almost 20 years ago) it ran well. The float bowls don't piss gas.
I had the jugs honed and put in fresh rings. Got about 150 miles on it so far. Should it burn more gas (way more?) until the rings seat?
 
Give it some time to seat in.
Stock airbox ?
If not, (pod filters for example) you may need to do some major rejetting.
 
TK4 said:
Give it some time to seat in.
Click to expand...
So mileage does suffer until it's broken in?
Yeah stock airbox. Still running an old filter (new backordered) but it would have to be visibly and horibbly clogged to effect the mileage so bad I think.
 
timtune said:
So mileage does suffer until it's broken in?
Yeah stock airbox. Still running an old filter (new backordered) but it would have to be visibly and horribly clogged to effect the mileage so bad I think.
Click to expand...
Try taking the filter out of the box and see if anything changes ?
 
Pulled the filter and went for a short scoot. Ran a little smoother down low but it hampered performance over about 5-6k.
 
Sometimes WOT does that too.
 
So I think I found the problem. I pulled the carbs and found a main jet rolling around loose in the bottom of one float bowl.
Now all carbs were running a little rich (which I always prefer to running on the lean side) but that may have been how it was 20 odd years ago.
There is evidence of at least one float needle sticking so they are all getting replaced before they go back on.
I did read that the aftermarket float needles are much poorer quality than the OEM.
Anyone know about these things? Can I even get OEM's any more?
 
@bitzz I found the PN for the float valve (16011341004) and called WC. He said Keihin won't sell those. Their a Honda exclusive parts. He suggested Sudco in SoCal.
Any dealings with Sudco?
 
That's weird because Sudco is the de-facto North American distributor for Keihin and Winner's Circle is a reseller for Sudco.

Sure, try Sudco (I usually order directly from Sudco, as Winner's Circle has a bad habit of sitting on Sudco orders till they have a minimum order for free shipping... and sometimes that takes a while....)
 
1) "Chinesium junk" beats no parts EVERY time
2) MC Distributing is in the same family as Mike's XS, Z1 Enterprises, Dime City Cycles and Randakk's Cycle Shak (MC is owned by Mike's ex wife and brother IIRC) and have been at this for a while and stand behind what they sell.

It's not OEM, but it's probably better than Amazon
 
bitzz said:
It's not OEM, but it's probably better than Amazon
Click to expand...
I repeat - pay yer money and take yer chances.
MC Distributing has a lot of replacement stuff especially for obsolete machines, good on them for making a go of it.
Not slamming them in any way.
 
Ok so Sudco doesn't have that PN. Only has OEM float valves for 76-78Fs and 77/78 Ks. Pretty sure my franken motor isn't carbed with those but now I have to double check.
Hully Gully (just for grins) say they might be in stock on Jan 24 (might not too). Each one 54$ EFF ME.
CB750Vintage (MC I think) has a whole rebuild kit with valve 18$. At least his is Taiwanesium instead of chinesium. And as @bitzz says available.
Decided to putter with something else for a while.......
 
It's a 40+ year old bike. Others carbs will likely need work too.
And I really think I'm close....
 
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