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Discussion Starter #1
OK, I'm doing this as a favour to catboy, who has no internet at home.

His new cb750f (1976) seems to be having a rich condition. Had trouble starting it, and when we changed the plugs, they were very dark & sooty, or dark & wet, on the 2 cylinders that weren't firing. New plugs solved the problem, but of course, that's only treating the symptom. So, he checked the air filter, and it was apparently completely plugged up & black & oily. However, the same richness issue is still occurring, running without any filter at all. Jetting issue, I assume. Since they're not cb350 cv carbs carbs, I know very little about them, but I assume we want to adjust the needle jet? He has a Clymer manual, which came with the bike, but it apparently doesn't tell him much about tuning. I know I'm breaking the rules by not searching, but as I said he has no internet & I'll be damned if I'm going to spend my day off looking for info about his bike.

Any quick help would be appreciated, since we were planning to get in a ride tomorrow (unless we can fix this, it looks like he will be borrowing on Baby Sister - again).
 

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Discussion Starter #2
Let me add, before the scolding...

Stock airbox, stock exhaust (with many rust holes in muffler), stock ignition.
Choke seems to be working properly. With new plugs, bike starts & runs strong. Starts no trouble, even without choke.
 

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well cough up something useful to go on.........

this surely doesn't help one bit

"With new plugs, bike starts & runs strong. Starts no trouble, even without choke"

sounds to me like you don't need any help.....

have you any idea how to test for 'weak ignition'?
 

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I would do a compression test of the motor - it's looking like either bad rings or bad valves or both. The sooty plugs are coming from oil being burned in the cylinder. Its an easy enough fix, pull the engine, re-ring the pistons and redo the valves. Sounds like this motor was beat before he got it - maybe it had been sitting for a few years before it was sold.. What ever the case, start with a motor compression test...
 

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Discussion Starter #6
Bike shows 15,000 miles on the odometer. And it's in such good shape, I'm inclined to believe it. Doesn't seem like it should need rings. I was under the impression black & sooty meant rich carburetion.

Hack - weak ignition was suggested by a friend, and no, I don't know how to test for weak ignition (then again, it's not my bike - mine are running with no problems). Same friend said hotter plugs & premium gas would cure it, but I'm skeptical. It starts & runs well with new plugs, but fouls them out in 15 - 20 miles, so I'm told.
 

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shiny black fouled is oil

dull black is fuel

what do you have?
 

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Catboy,
I believe the crankcase vent went to the bottom of the airbox on those. If so, then they will trash out the airfilter with oily gunk and leave a layer in the bottom of the box. So make sure the whole air box is clean. Have you checked the carbs? I would go for float levels being too high. Have you checked the points on cyl 2-3? It could be an ignition issue. Finally, have you checked the valve clearances?

Ken
 

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the tell tale thing I am able to extract is that the plugs are likely dull black (fuel fouled) and the only thing I am going on is the fact you mention it starts easily (I'll presume from cold) without the choke

no mention is made of any symptoms that manifest themselves as the engine warms up or even right before it fouls plugs

no mention is even made of which plugs the bike has

too much missing info to do anything but indulge you in a game of blind folded dart throwing
 

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Discussion Starter #10
Can't remember the plug #, but they are the correct plugs recommended in the Clymer manual.

Dull, sooty black fouled plugs. I can read a plug, and it's fuel fouled. Fouling even when run without an air filter.

I am not aware of any symptoms the bike exhibits as it warms up, as I haven't ridden it, except around the block once, right after we put new plugs in. I'm told it just loses power, due, I assume, to the fouling plugs.

Ken, my vote has been carbs from the get-go. Matt says ignition. He doesn't want to fuck around with carbs yet. Points on cyl 2-3... It's fouling all 4 plugs. Perhaps all points are bad? That's something we haven't checked.
 

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any overflow from any of the bowl vent lines?

floats all good and levels correct?

trash in tank or float bowls?

it seems quite common on a rack of four that has been sitting to find a bad float or two, a bad float level or two, and trash and goo in maybe all of them

but maybe you have something silly like all four main jets missing or a needle jet and holder gone


it would make sense for a rich condition to worsen as the engine comes up to operating temp and if it's rich enough to foul plugs

there certainly should be some throttle position/engine speed related symptoms which are quite obvious
 

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quote:Originally posted by kenessex

Catboy,
I believe the crankcase vent went to the bottom of the airbox on those. If so, then they will trash out the airfilter with oily gunk and leave a layer in the bottom of the box. So make sure the whole air box is clean. Have you checked the carbs? I would go for float levels being too high. Have you checked the points on cyl 2-3? It could be an ignition issue. Finally, have you checked the valve clearances?

Ken

Ken is right on with all of this.

about the only thing I would add is that occasionally, and for no reason whatsoever, cb750s carbs sometimes drop a jet into the float bowl. It has happened to me twice in my life (out of 10 cb750s total owned that is not such a bad ratio), pull the float bowl and there is the main jet rolling around in the bottom.
 

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Catboy,
You didn't specify exactly how many and which plugs are fouling. As you know the 750 has a paired ignition setup, so 1 &4 are paired as are 2&3. So if it is 2 cylinders and they are one of the pairs then I would lean toward ignition problems. What is the condition of the points? Any sign of severe arcing? could be a condenser. If it is not a pair of cylinders I would lean toward carburation. If it is all 4 cylinders look to the airbox. If the bike sat on the sidestand for a long time before you bought it, then I would look at the downhill carbs for gunk and such in the float bowls.

Ken

Ken
 

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Discussion Starter #14
All four cylinders are fouling. Although, before we replaced the plugs, it was starting on only two of them, and Unless I'm mistaken, not paired cylinders, either. It's carburetors, just like I told him. Float bowls, eh? Think that's the first thing we'll look at.
 

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I still recommend doing a compression test - it's quick and easy and will give you a quick assessment the health of the engine. Other than the engine being over jetted, I can not see how the float level being slightly off will cause a fuel foul of the plugs??? My thinking on this problem is the fuel is not being burned completely and hence the fuel fouling on the plugs. I know there are different plug heat ranges that can be run in the 750's. And I realize the ignition setup can affect the fuel burn. My take on solving this problem is to start at one end of the spectrum and take a system approach to solving the problem. With a stock bike - I have to believe the stock setups should work.

So I'm all ears on this one and bow to the masters.
 

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I don't want to be mean, but is this a choke problem? That would affect all 4 cylinders in such a fashion.

Ken
 

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Discussion Starter #17
Don't shoot the messenger. It's not my bike. I helped remove & replace wheels for new tires, and helped replace the stock bars for superbike bars. Rode it once after we put in new plugs, and it ran like a champ. I'm doing a favour for a friend who's impatient to ride after 2+ years of no bike. It doesn't really matter today, because his girl took him on an overnight for his birthday, and the dick left me on my own, on a perfect day for a ride. I'm going anyway.

I really do appreciate the help. It gives us more to go on. I still think it's a carburetion problem, and Ken, the choke seems to operate fine.

Please remember I'm only an assistant. My job is to observe & drink Matt's cheap beer. Thanks for the advice & I will be sure to keep you all posted. I'm off for a ride, fellas... all by myself.
 

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Judey,
It is bad enough that Catboy drinks that cheap stuff, DO NOT DRINK IT. If you ave some of it too, it just reinforces the notion that it is ok. Remember, I wouldn't even cook my bratwurst in that stuff. If he can afford a bike and gas, he can afford a slightly higher minimum standard beer.

Ken
 

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Discussion Starter #19
Quantity Ken, not quality. Not my rule, but his beer costs me nothing.

Hoping for bike time in the next few days. I'll keep all posted. Thanks to all for the advice.
 

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...i'm back online. god chicago is a cool city!

there's actually little i can add that grant hasn't been able to throw out there. at this point, starting with easiest first, new filter, (ken: indeed it was an oily, gas fowled filthy abortion of a mess, but as grant said, the problem persisted even without a filter in the box) hotter plugs (all four evenly flat black: not oil fowled), and higher octane fuel. i thought about checking the points last night and will try that too.

hack; i'm familiar with a multimeter but don't have one at the moment. by the end of the week.

advice from the likes of ken and geeto on carburation is very appreciated and duely noted. in fact, and history speaks for itself, you're both likely right. however, i'm still elimintating the easy, cheap shit first. at this point, carburation is further down the road as far as i'm concerned. in that regard, and please excuse my ignorance, i'm struggling to understand how a jetting issue could be so consistent across all four cylinders? one of my first thoughts was perhaps the choke assembly was causing it, but it doesn't seem to be a problem unless the SLIGHTEST, almost invisible hangup in the linkage would cause an overly rich condition and it seems to move freely open to close. i'm basically thinking of anything that could affect all four cylinders at once.

and no, though i've pulled and looked at the plugs a lot, i've not done a proper, controled test. i've yet to proceed to checking valve clearence yet either. getting time to really dig into it has been tough over the last several days.

i humbly thank you guys for the advice.

the beer thing: ken, quantity is king. when you drink as much beer as i do, you simply have to drink low cal. either that, or be happy with carrying a gut like santa claus and what hottie wants to knock boots with father christmas? (safety disclaimer: i don't drink and ride. i have two beautiful little girls that need their daddy.)
 
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