81 CB750k spark on 1&4 but not running

Clemmons04

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Hey all first post because I'm kinda stumped. 81 750k overhauled 5k miles ago. I also have Murray's carbs set up on here... that's a whole nother issue😔. They're a pain to keep from fouling plugs because he tunes them so rich. Just did a service about 50mi ago because it was breaking up on acceleration. I believe I got some bad gas as I got 93 from a station that probably doesn't get many 93 customers. Changed oil and filter. Dumped the gas and changed the filter as well. Pulled the carbs down and cleaned. Had a good run last night about 20mi and the carb slides were sticking a bit. Tore them down again and rerouted the cables as Murray wants. Came out this morning for a ride and now I'm only running on 2&3... WTF

So... I double checked everything. No issues. I got spark on 1&4 but the pipes are cold enough to hold. I figured if it was a carb issue at least 2&3 would be acting up since 1&2 share the left carb and 3&4 share the right. I e got a couple good coils so I swapped them. I swapped the spark units. I swapped the carbs around. I swapped the plugs around. Idk where to look. If I have spark it should run. Is it possible the pulsers are an issue here even though it's sending spark?

165 psi on all cylinders. Valves have been shimmed OMG what a process. Genuine Honda cam chains and tensioners. New coils. New wires. New plugs. Confirmed spark units working. Only mods are the Murray's carbs and they're running about as good as they can.

Got any thoughts?
 

brettp

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I have a lot of questions about this.
It sounds like a coil problem. Number 1 coil feeding cylinders 1 and 4. Check the connections on the back of the coils.
If you get spark while the spark plug is outside of the cylinder, make sure the porcelain is not cracked and the spark goes from the electrode to the metal tip. You definitely need to tune those carbs because running rich will foul plugs to the point the spark won't ignite the fuel. You also don't need to waste your money on 93 octane. Seriously, don't do that.
 

Clemmons04

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I've checked the wires and swapped to a new coil. I've also just put brand new plugs in. It doesn't make sense to me because it's firing the plugs out of the cylinder b
 

Clemmons04

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What would cause this condition? Where would you look if everything is working like it should but still not running? And the gas was supposed to be ethanol free. It's the only place I've seen 93 ethanol free it's usually 90
 

brettp

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I would tune my carbs first. If I knew I had spark, compression, and air, fuel is the last thing. It is ok to richen the pilot a bit (because they are lean from the factory) but the needle and maindon't need a lot. I know Murray sets the carbs bar on your info but you need to fix that first (in my opinion).
Pilot jets are clean?
 

Clemmons04

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I'm actually looking at getting an Africa Twin and was going out to take pics to put her up for sale.... This is what happens when you discuss things in front of your machines.

Timing verified... I'm going to pull the carbs down and see if I missed something. Since it was running just fine and I pulled the carbs because of a hanging slide maybe I missed/messed something up. No more fuel injection talk in front of her lol
 

mrtwowheel

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Was the compression test before or after doing the valve adjustment? Maybe you should reinvestigate the valve adjustment and compression? I learned that checking the valve clearance with the lobes in the air DOES NOT give the same readings as following the Honda manual. Honda KNOWS how they ground these cams and where to place them when measuring valve clearance.
 

Clemmons04

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The valve lash was done back in December with the engine rebuild. She got all new genuine Honda valves and Honda shims. Rod and main bearings as well. I'm confident in the lash as the top end has only 5k miles on it. Compression today is 165 across all 4. I think what I've got going on here is carbs. I'm pretty sure now after all day with this BS that 1&4 are getting soaked with fuel and 2&3 are just barely firing. #3 is hot AF and #2 is about 80* cooler... All plugs are new and look wet now.

Apparently the carbs were cleaned in an ultrasonic cleaner with Zep purple degreaser. I think it etched the carbs as they came out darker than they went in. And when I say apparently I mean I'm pretty sure I fudged up my carbs. Never had an issue with ultrasonic cleaning before but they didn't have the solution I normally use so I got that....... So lesson learned for me😮‍💨💸. It was running fine before and now it's not. It's firing on all 4, timing is good, compression is good. Super simple system.... It should be running unless it's not atomizing.... Which I don't think it is.🏍️⚰️🪦

Oh well... New carbs are in my future 😁
 

Clemmons04

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So.... I figured out what had happened 🫣😨.
I got my new carbs in and it started right up no problems... Well accept that my slides are still sticking around 4k. Pretty sure that is a cable issue. So I was starting to sync the carbs and I was using the old set as a gauge of what to use to fit in there. I started with some metric drill bits at about 6mm. When I went to put them on the carbs on the bike they were extremely to small so I got to wondering.... 😞😞🫣🫣😮‍💨😮‍💨

Because I had the carbs apart multiple times I must have put the slides in the wrong carb. So now my cutout is facing the engine and at idle I was pulling vacuum through the whole carb instead of the pilot circuit. With so much fuel pouring into the motor it was drowning the plugs causing my problem.

I think I'm going to send the old carbs to Murray to rebuild and sync anyways that way I'll have a backup set or he will tell me the chemical dip did in fact damage the bodies of the carbs. I'm all about learning lessons here lately... Why not one more lol 😂
 

brettp

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That sounds like a lot of money and effort to me. If you want to learn, do the carbs yourself, put the carbs in, sync yourself. But I'm not in your shoes.
 
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