Carbs, Spark plugs, coils, points - good. What's wrong???

[rafanomenon]

Hey everyone,

I checked the FAQ and did the comprehensive search and skim (i'm a search master, btw) and couldn't find any answers to my specific problem.

I have a 1977 Honda CB750F (SuperSport). I cleaned the carbs, new air filter, oil change, new oil filter, new spark plugs.

I found that the float needles for carb #1 and carb #4 were a little 'off' because the spring mechanism in the float needles weren't quite holding the floats as taught as the middle two were. This really wouldn't affect what's going on as you read ahead... After cleaning we reassembled, made sure the idle screws were to factory specs (1.5 turns) and started it up. 2 of the 4 cylinders were firing. No good. We disassembled the carbs and cleaned once again.

We reassembled and still, the same problem. I gave Chris a call to get a second opinion. Chris is one of the super cool vintage fanatics that emailed me and told me to keep my bike when I tried to sell it. Way to go, Chris. Living in West Seattle, he came right on over. We diagnosed common problem areas, and found out the spark plugs for #3 and #4 weren’t screwed in all the way. Great. A no brainer.

After screwing in all the plugs, we tried starting it back up. Still, nothing firing from #3 and #4. We took the wooden handle of a hammer and tapped the carbs to get any air bubbles that might be hanging out. We also tipped the bike off the center stand, and rocked it far left and far right to increase chances of fuel getting into each of the four carbs. Fingers crossed, we fired up again.

Progress. Using the fingers-on-the-headpipe method, we checked to see if there was combustion/heat coming from all 4 headpipes. #1 and #2 were super hot, #3 was warmer, and #4 is still running a bit cold-to-touch. We pulled the spark plugs out, grounded against the engine and checked for spark. Good. Bright and blue. We disassembled and checked the carbs once more (this is the 4th time doing this), and all the valves, floats, needles - everything - were clean and operational. We even tried switching the carb components from #1 and #4 to troubleshoot if it was actually a carb problem. Still, cylinder #4 didn’t seem to be getting enough spark/gas/combustion/we don’t know. We then did more troubleshooting and looked to the coils, timing possibility, combustion, and even the points. All of which seemed to be a-okay.

In a nutshell, we checked and re-cleaned the carbs 4 different times. Spark plugs are good. Coils seem to be good. Points good. Cylinder #1 and #2 are firing well, and the headpipes are hot. #3 is warmer, and #4 is lagging and doesn’t seem to be running like the others. Could it be a lean/rich setting on the carb? A spark plug problem? Combustion? timing? Points? New coils? Something majorly wrong within the engine?

Somebody. Anybody. Help.

check my blog for in-depth details and progress: www.rjrcreative.com/bike

Thank you all in advance!

 

[HerrDeacon]

Have you checked both the compression and the valve clearances? If compression is low, there is a possibility that this cylinder is just not drawing in enough fuel. Valve clearances can also affect this. Worth a check.

Also, have you sync'd your carbs? Although you'd need it running on all four before this.

I feel yor pain though, I had the same issue with my bike before putting it away this fall. Still not figured out, so I'll be tackling it this spring. My problem is only on startup. Once off enrichener it is fine, so I'm thinking there is an issue with either the enrichener or pilot circuit on mine.

Keep us updated.

 

[j.c.motorsports]

Hey everyone,

I checked the FAQ and did the comprehensive search and skim (i'm a search master, btw) and couldn't find any answers to my specific problem.

I have a 1977 Honda CB750F (SuperSport). I cleaned the carbs, new air filter, oil change, new oil filter, new spark plugs.

I found that the float needles for carb #1 and carb #4 were a little 'off' because the spring mechanism in the float needles weren't quite holding the floats as taught as the middle two were. This really wouldn't affect what's going on as you read ahead... After cleaning we reassembled, made sure the idle screws were to factory specs (1.5 turns) and started it up. 2 of the 4 cylinders were firing. No good. We disassembled the carbs and cleaned once again.

We reassembled and still, the same problem. I gave Chris a call to get a second opinion. Chris is one of the super cool vintage fanatics that emailed me and told me to keep my bike when I tried to sell it. Way to go, Chris. Living in West Seattle, he came right on over. We diagnosed common problem areas, and found out the spark plugs for #3 and #4 weren’t screwed in all the way. Great. A no brainer.

After screwing in all the plugs, we tried starting it back up. Still, nothing firing from #3 and #4. We took the wooden handle of a hammer and tapped the carbs to get any air bubbles that might be hanging out. We also tipped the bike off the center stand, and rocked it far left and far right to increase chances of fuel getting into each of the four carbs. Fingers crossed, we fired up again.

Progress. Using the fingers-on-the-headpipe method, we checked to see if there was combustion/heat coming from all 4 headpipes. #1 and #2 were super hot, #3 was warmer, and #4 is still running a bit cold-to-touch. We pulled the spark plugs out, grounded against the engine and checked for spark. Good. Bright and blue. We disassembled and checked the carbs once more (this is the 4th time doing this), and all the valves, floats, needles - everything - were clean and operational. We even tried switching the carb components from #1 and #4 to troubleshoot if it was actually a carb problem. Still, cylinder #4 didn’t seem to be getting enough spark/gas/combustion/we don’t know. We then did more troubleshooting and looked to the coils, timing possibility, combustion, and even the points. All of which seemed to be a-okay.

In a nutshell, we checked and re-cleaned the carbs 4 different times. Spark plugs are good. Coils seem to be good. Points good. Cylinder #1 and #2 are firing well, and the headpipes are hot. #3 is warmer, and #4 is lagging and doesn’t seem to be running like the others. Could it be a lean/rich setting on the carb? A spark plug problem? Combustion? timing? Points? New coils? Something majorly wrong within the engine?

Somebody. Anybody. Help.

check my blog for in-depth details and progress: www.rjrcreative.com/bike

Thank you all in advance!

hey ,
first of all you have to do a compression and leak down test
then try 4 brand new plugs . also make sure the valves are set correctly. you can check the plug caps for proper resistance as well. try this first and the call me at the shop if needed at 519-458-2453 --Chris --J.C.Motorsports

 

[zixxerboy]

I agree that a compresion test and valve clearance should be done to rule out the compression side. Since the coils fire criss crossed & #1 & #2 are good that should eliminate the coils. If I had good blue spark I wouldn't replace the plugs. The blue part is key - orange spark is weak. Doulbe check point gap/timing and that will bring you back to gas.

You've already cleaned the carbs so I'm stuck.

 

[rafanomenon]

I also posted on the SOHC4 Forums and received a ton of possible solutions.

http://forums.sohc4.net/index.php?topic=47403.0

Compression and valves first. Then, recleaning the carbs i.e. removing all the press in valves and all the other pain-in-the-ass stuff to clean in the post-'77 model Kei Hin carbs. Ugh.