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Help! Poorly running 750k

Stah1one

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Biddeford, ME
Bought a 78 cb750k last year and didn't get to ride it (beginning winter). Went to fire it up this year and found it only running on 1st and 2nd cylinder. After much tinkering found it was bad coils and replaced them. Now it runs on all cylinders, but idles poorly, sputters, and dies as soon as you even slightly turn the throttle. I've completely cleaned the carbs, my jets are all correct (at least according to my Clymer manual), I set the float heights, and made sure I'm using new gas that hasn't been sitting. Compression is good (115-120) in all cylinders. I'm at a bit of a loss as to what to do now. Check timing? Check points? Check valves? Any help would be greatly appreciated! I'm dying to go for a ride!!
 
The 3 checks mentioned. Do them. Your compression not that great, normal is closer to 150+. You get dead cylinders at 100 that won't run for spit. What 37 year old bikes do.

If they run ethanol in your area fuel then leaving that in during the winter did not help you. Idling poorly says your carb work not as good as you think or engine is leaking compression. It will idle like crap if not synced at carbs.

Many do not get at all that the valves are simple low dog steel with nitride coating that once worn off leak compression to never run right. The valves were fine for low lead fuel but true 100% unleaded gives fits on them and especially the exhausts. Honda pretty much never used premium materials in their valves and it shows over the long term.
 
Do the full 3000 mile tune up. Make sure you don't have any vacuum leaks at the carb to engine boots as well. The old boots can get so hard that they can become difficult to seal well.
 
X2, any diagnosis before having ALL tuneup specs in place is a waste of time. And will most likely be in serious error too.
 
-Update_
So today I drained my carbs and put all new gas in. Warmed up the bike and checked my compression; 120 across the board, plugs are brand new but do look like they are starting to get some carbon on them, guess the bike is running rich? I adjusted my valve clearances, my cam chain, breaker points, and ignition timing. Put everything back together and now the bike idles like a dream..unfortunately it still dies as soon as you give it throttle. I did find vacuum leaks at all four insulators between my carbs and head, so I'm going to look at those and might as well look into the air filter as well. Also figure I'll tear down the carbs completely again and go over them with a fine toothed comb to see if there's anything I missed. I'm open for any other ideas you guys might have.
 
You have to fix the leaks at the carb boots before doing anything else. Assuming you have the stock pd carbs you need to look at the accelerator pump system. These carbs are jetted lean at idle and use an accel pump to keep them from stumbling offf idle. The pump and check valves are very picky and often do not work right away and take some time to get fuel through the system. keep opening the throttle until you get fuel squirting out of the squirters on the air filter side of the carbs. If you have no fuel squirt then you have issues with the accelerator pump.
 
Alright I pulled the carbs off and took a look at all the insulator boots. They're not cracked and are stiff but still squish some when squeezed. I put everything back together as carefully as possible and really tightened everything down and the bike no longer revs when sprayed with carb spray at any of those points. I also twisted the throttle a bunch with the bike off and I do have fuel spraying into all four carbs from the accelerator pump system. Still no change though. Bike still dies as soon as you give it any throttle. I bought a new air filter because the old one was pretty grubby and I'll throw it in when it gets here next Monday. Thanks for all the help so far!
 
It entailed completely stripping the carbs down and spraying carb cleaner and compressed air through every hole I could find on the carbs to make sure they were clear. Also very carefully went over them again because the first time I took them apart I found whoever cleaned them before I bought the bike put one of the slides in backwards. Idle mixture screws are turned out 1 and 1/2 turns right now because that's what the Clymber manual advised. Was planning on adjusting further once I got the bike running correctly.
 
It entailed completely stripping the carbs down

There are different interpretations of "completely".

>Did you separate the individual carb bodies from the rack, and from each other?
>Did you remove the idle set screws and replace the o-rings? If so, was the little spring and washer in place?
>Did you remove and inspect at least one of the idle jets?
>Did you remove and inspect at least one of the emulsifier tubes that holds the main jet?
>How do you know someone did not drill out the main jets to a larger size than what is stamped on them?
>How do you know someone did not drill out the idle jets to a larger size than what is stamped on them?

It sounds like something in the transition from the idle circuit to the main circuit. Seems like one or the other is too rich or too lean.
 
'Still no change though.'

Anybody surprised at that? We're back again at thinking carb work alone fixes all. Gonna laugh if that timing ends up being the issue. Slow timing can utterly kill off-idle transitions and affect idle quality too. Valves not set can do the same if too tight.

Do ALL of it before pronouncing judgement..................as said, your compression is way down, it will require all other things to be dead on to run, low compression makes all other tune aspects super sensitive. Slow timing and low compression are a death double-combo on idle and off-idle. Dwell off on point gap is killer to idle too.

ALL OF IT.
 
I bet a '78 is EPA lean like the '79 and later were, why they added the accel pump. That means you may well get better off-idle by cracking the mixture screws open a bit more. PROVIDED OF COURSE they are fuel/air and not just air alone. I do not know but can tell by taking carb apart myself.
 
I bet a '78 is EPA lean like the '79 and later were, why they added the accel pump. That means you may well get better off-idle by cracking the mixture screws open a bit more. PROVIDED OF COURSE they are fuel/air and not just air alone. I do not know but can tell by taking carb apart myself.


The mixture screws do indeed richen when opened (backed out, CCW).

Stah1one has indicated that "I adjusted my valve clearances, my cam chain, breaker points, and ignition timing" so I'm giving him the benefit of the doubt...
 
Well................I went looking specifically for it............and jumped right over it. What a f--king idiot.

My apologies to all and most especially the OP.
 
Thank you so much for all the feedback everyone. In medical school and boards are coming up so motorcycle work has slowed down a lot for me. As for the carbs all four were separated so I had four individual carb bodies, I removed the main jets and imulsifier rubes from each, the pump from my 2nd carb, and the idle screws for all of them. Took the tops off and removed the slide/needle mechanism from each (little slot on the side of the slide, but they will go in either way. Nothing stops them). The jets didn't appear drilled, but other than that I have no idea if they're modified. Bought an air filter and new insulator boots, if that doesn't help I may look into a carb rebuild kit next.
 
There should be a pin in the carb body that locates the slides in the slot. If not then someone broke them out, they are needed to keep the slides from binding and to keep the slides correctly situated. Are you sure that you have the slides in the correct way? I would start looking for a different set of carbs, they seem to have been hacked together and you dont know what else has been "modified"
 
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