I used to build race engines for a living. you are getting fuel, air compression and spark. It has to be timing. If the timing is far enough out, you could have pushed the fuel out of the cylinder before the plug fires. Or if in the middle of the stroke its not going to ignite the fuel. Spark plugs really aren't good at Igniting an airfuel mixture without compression. Far enough off they won't Ignite anything. Turn the cylinder to around top dead center on the compression stroke, and see if the spark triggers. It could be possible the spark is extremely weak. I have noticed with very weak spark if you get the engine to temp. it will usually make it start to fire. The temperature of the engine will Make it easier for the spark plug to ignite the fuel. So if it seems the cylinders come alive when the engine is hot, its weak spark. But truly i believe its timing.
Thanks so much for the comment!checked the timing and it did seem to be a little off but once it was corrected it was still having the same issue, also checked the gap on the pick up coils and tightened that up. Now I’m positive I have good spark and good timing but still not running right. Gonna try to get my hands on a different set of carbs so I can eliminate that from the equation
@Biggerthanbikes it's been a while since i watched the video.something i forgot about is points. Looks like the cb750 uses points ignition to control the spark. There should be 2 points on your bike. try cleaning them with sand paper and make sure they are set to the proper dwell time. Points will 100% do this. It says the points control 2 cylinders each. Verify the wires from the points go to the proper coils, and the coils and wires are going to the proper cylinder. Many times, i have seen the wiring all wrong, giving problems like you are having.... Its very easy to tell if its the carb problem just unhook the carb and shoot a small amount of fuel into the cylinder. It will run for a second or so. If it does not run on that cylinder, it's not fuel. If it starts to run, then yes, it's fuel, and it's a carb problem. Please check the points and wiring one last time. i would hate to see you buy a set of carbs and not fix the problem. Best of luck.
@@Biggerthanbikes I was having the same issue as you with my 1982 cb900f and it was the carburetors! I have one old man that knows a hell of alot and he told me it's all caused by the carburetors not having the airbox he did manage to get mine right tho so it's possible but he told me that if you want to never have these type of issues look into the mukuni racing carbs
I agree with snappers and mx5. The points ignition on them can be sensitive. Like a bad condenser or just not a great contact for power or ground. The other thing is your bike runs on the keihin CV carbs. The older SOHC bikes had the mechanic PD carbs that are a lot less sensitive to the removal of the airbox. My old 900 with those same CV's ran terrible on pod filters and if your trying to start it with the openings raw, they might not like it. You could always throw the airbox back on it and see if it makes a difference to keep the fuel mixtures consistent.
1: Check timing signal on 1 and 4. 2: Measure primary and secondary side of the coils :3 Looks like you're running CV carbs, they need a filter with some resistance to run properly due to the need of creating a vacuum. It's not your problem right now. 4: Check pickup and CDI. I am willing to bet that this is an electrical issue 100%. Since it's both cylinders I doubt it is coils, of course they could both be bad, but I doubt it. So this leaves us with a timing light. Compressions sounds OK, no galloping as far as I can hear.
I don't have any idea, but just wanted to say how cool it is to read the comments. I love the moto community, they're always eager to help and share knowledge. Good luck fixing it, you got this!
I struggled with this same problem for months just to realize the intake boots from the carb to the engine were not sealed completely causing it to run on two cylinders the carb boots on your bike appear to be the only thing you haven't replaced
I'm sorry to hear you are having issues. I was excited to see a full fairing CB750, especially your progress on the fairing is very interessting to me. I did some work on my RC42 CB750 so here are my best guesses because I think it's a air-fuel problem: - Have you checked the rubbers from the intake between carb and engine? I've heard them going bad in these old 750s. - maybe stick some air filters on so you don't run as lean? - I guess you replaced the float needle? - Maybe try a different carburetor? I think this option has the most quick-fix potential. - are the spark cables the original ones? I've also had bad aftermarket ones once. I wish you best of luck. I think you are very close to having a great running 4 cylinder. 4 cylinders can be quite a pain to setup right.
I'm definitely no expert but I would check the CDI. I had a KLR600 that showed spark but wouldn't fire consistently. I later found out it was a faulty CDI. I would just like to say Ive followed your build with much interest Im sure you will sort out the issue and complete the build. I think its a beautiful cafe racer worthy of a place on Racer TV in my humble opinion! Good luck SR
Bro check if your coil wires are plugged in correctly also try some new wires sometimes the wires get short due to excessive heat. Also check your choke settings and your CDI unit. To me it seems more like a coil issue, also try to check your intake valve and exhaust valve thoroughly maybe a bent or broken valve
Hey man. I saw your video on having problems with your bike not running correctly. I had a couple of suggestions. One check that your coils are paired correctly. 1 and 4 fire together with blue wire coil and 2 and 3 fire together with the yellow wire coil. Check those are paired together. Cause I ran into a similar issue on my Honda
I had a similar problem in my Vfr 750 rc36. I installed new spark plugs and the engine only ran on two cylinders. I checked everything again and replaced the spark plugs with new ones and now the bike runs beautifully.
I also had that problem, but only on cylinder 4. And after doing the same as you, I tightened the pilot screw further and then it ran on 4 cylinders. Make sure you warm up your spark plug first.Because with that unburned gasoline. Hope it works
I had a similar issue with a bike I’m rebuilding. Try swapping the plug wires for cylinder 1 & 4. Even though it’s sparking, it will be so out of time that it would be trying to ignite the fuel mixture on the complete opposite stroke instead of tdc on compression.
Welcome to the world of cafe racers!Prepare to ride less and spend time like in this video more 🤷🏻♂️ Can’t solve some issues on mine even after almost two years.Good luck! 🙏
On the spark plug, you have an ideal distance between the anode and the cathode (like 0.7mm on an xs650). Did you check? You can have spark but bad spark and you cylinder will not run.
I have similar issues.. clean and check carbs so many times everything seems fine but it won run on 4 ... So i took another set of carbs and replaced the bodys that wasn't running and the 4 was running then
Its look like its not coils, plugs, leads,electrical.. i would think. Valves sticking or valve gap, with that spark and fuel it should fire... see if some one can lend a good set off carbs... but look at rocker cover removal at least see iff valves are moving while cranking
maybe is the spark plug shorting when in somehow? maybe that way you'd get spark when testing but not when they're in? honestly very weird. I can't think of any reason other than what you tried already.
Curious how you would change it? It matches what the Ducati manual says and it only fits on one way. I’m no expert but I’m pretty positive it’s mounted correctly
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I used to build race engines for a living. you are getting fuel, air compression and spark. It has to be timing. If the timing is far enough out, you could have pushed the fuel out of the cylinder before the plug fires. Or if in the middle of the stroke its not going to ignite the fuel. Spark plugs really aren't good at Igniting an airfuel mixture without compression. Far enough off they won't Ignite anything. Turn the cylinder to around top dead center on the compression stroke, and see if the spark triggers. It could be possible the spark is extremely weak. I have noticed with very weak spark if you get the engine to temp. it will usually make it start to fire. The temperature of the engine will Make it easier for the spark plug to ignite the fuel. So if it seems the cylinders come alive when the engine is hot, its weak spark. But truly i believe its timing.
100% agree. Just because the coils are good and it has spark, still too early to rule that department good. Need to triple check the timing.
Thanks so much for the comment!checked the timing and it did seem to be a little off but once it was corrected it was still having the same issue, also checked the gap on the pick up coils and tightened that up. Now I’m positive I have good spark and good timing but still not running right. Gonna try to get my hands on a different set of carbs so I can eliminate that from the equation
@Biggerthanbikes it's been a while since i watched the video.something i forgot about is points. Looks like the cb750 uses points ignition to control the spark. There should be 2 points on your bike. try cleaning them with sand paper and make sure they are set to the proper dwell time. Points will 100% do this. It says the points control 2 cylinders each. Verify the wires from the points go to the proper coils, and the coils and wires are going to the proper cylinder. Many times, i have seen the wiring all wrong, giving problems like you are having.... Its very easy to tell if its the carb problem just unhook the carb and shoot a small amount of fuel into the cylinder. It will run for a second or so. If it does not run on that cylinder, it's not fuel. If it starts to run, then yes, it's fuel, and it's a carb problem. Please check the points and wiring one last time. i would hate to see you buy a set of carbs and not fix the problem. Best of luck.
@@Biggerthanbikes I was having the same issue as you with my 1982 cb900f and it was the carburetors! I have one old man that knows a hell of alot and he told me it's all caused by the carburetors not having the airbox he did manage to get mine right tho so it's possible but he told me that if you want to never have these type of issues look into the mukuni racing carbs
I agree with snappers and mx5. The points ignition on them can be sensitive. Like a bad condenser or just not a great contact for power or ground. The other thing is your bike runs on the keihin CV carbs. The older SOHC bikes had the mechanic PD carbs that are a lot less sensitive to the removal of the airbox. My old 900 with those same CV's ran terrible on pod filters and if your trying to start it with the openings raw, they might not like it. You could always throw the airbox back on it and see if it makes a difference to keep the fuel mixtures consistent.
1: Check timing signal on 1 and 4. 2: Measure primary and secondary side of the coils :3 Looks like you're running CV carbs, they need a filter with some resistance to run properly due to the need of creating a vacuum. It's not your problem right now. 4: Check pickup and CDI. I am willing to bet that this is an electrical issue 100%. Since it's both cylinders I doubt it is coils, of course they could both be bad, but I doubt it. So this leaves us with a timing light. Compressions sounds OK, no galloping as far as I can hear.
I fully agree with that
I don't have any idea, but just wanted to say how cool it is to read the comments. I love the moto community, they're always eager to help and share knowledge. Good luck fixing it, you got this!
100% agree!! definitely checked a few things I other wise would have overlooked, I appreciate everyone’s input!
Check your timing. Look. No 1 and no 4 piston going up and down at the same time. So start with your timing.
I struggled with this same problem for months just to realize the intake boots from the carb to the engine were not sealed completely causing it to run on two cylinders the carb boots on your bike appear to be the only thing you haven't replaced
I'm sorry to hear you are having issues. I was excited to see a full fairing CB750, especially your progress on the fairing is very interessting to me. I did some work on my RC42 CB750 so here are my best guesses because I think it's a air-fuel problem:
- Have you checked the rubbers from the intake between carb and engine? I've heard them going bad in these old 750s.
- maybe stick some air filters on so you don't run as lean?
- I guess you replaced the float needle?
- Maybe try a different carburetor? I think this option has the most quick-fix potential.
- are the spark cables the original ones? I've also had bad aftermarket ones once.
I wish you best of luck. I think you are very close to having a great running 4 cylinder. 4 cylinders can be quite a pain to setup right.
I'm definitely no expert but I would check the CDI. I had a KLR600 that showed spark but wouldn't fire consistently. I later found out it was a faulty CDI.
I would just like to say Ive followed your build with much interest Im sure you will sort out the issue and complete the build. I think its a beautiful cafe racer worthy of a place on Racer TV in my humble opinion!
Good luck SR
Bro check if your coil wires are plugged in correctly also try some new wires sometimes the wires get short due to excessive heat. Also check your choke settings and your CDI unit. To me it seems more like a coil issue, also try to check your intake valve and exhaust valve thoroughly maybe a bent or broken valve
You might could try the reversing the wirs that fire the coils and try that maybe cdi box but fuel and compression seem fine
Dang hopefully it works out , wanna see the fairing kit
Hey man. I saw your video on having problems with your bike not running correctly.
I had a couple of suggestions. One check that your coils are paired correctly. 1 and 4 fire together with blue wire coil and 2 and 3 fire together with the yellow wire coil. Check those are paired together. Cause I ran into a similar issue on my Honda
I had a similar problem in my Vfr 750 rc36. I installed new spark plugs and the engine only ran on two cylinders. I checked everything again and replaced the spark plugs with new ones and now the bike runs beautifully.
U gotta make sure that your getting spark always and if the has points u need to clean them
I also had that problem, but only on cylinder 4. And after doing the same as you, I tightened the pilot screw further and then it ran on 4 cylinders. Make sure you warm up your spark plug first.Because with that unburned gasoline. Hope it works
And u should make sure that your getting good airflow in the carb
I had a similar issue with a bike I’m rebuilding. Try swapping the plug wires for cylinder 1 & 4. Even though it’s sparking, it will be so out of time that it would be trying to ignite the fuel mixture on the complete opposite stroke instead of tdc on compression.
Iv noticed your rear suspension linkage is not put together correctly..look at pics online..
Welcome to the world of cafe racers!Prepare to ride less and spend time like in this video more 🤷🏻♂️ Can’t solve some issues on mine even after almost two years.Good luck! 🙏
e corrente nas bobines ,as bobines est5ão boas mas precisam de corrente.
Get one of those inline spark checkers,and verify that each is sparking. Hell,switch out the plugs,maybe you have some bad new plugs!
On the spark plug, you have an ideal distance between the anode and the cathode (like 0.7mm on an xs650). Did you check?
You can have spark but bad spark and you cylinder will not run.
Man my cb750 seized beforenineben got it started so I'm getting an xj650 😂
I have similar issues.. clean and check carbs so many times everything seems fine but it won run on 4 ... So i took another set of carbs and replaced the bodys that wasn't running and the 4 was running then
then these carburettors are vacuum controlled. they don't work as they should without an air filter
Its look like its not coils, plugs, leads,electrical.. i would think. Valves sticking or valve gap, with that spark and fuel it should fire... see if some one can lend a good set off carbs... but look at rocker cover removal at least see iff valves are moving while cranking
With theese twin cams if the carbs are out off balance, it sound as the primary chain is stuffed. But not your issue yet
maybe is the spark plug shorting when in somehow? maybe that way you'd get spark when testing but not when they're in? honestly very weird. I can't think of any reason other than what you tried already.
Rear suspension isn't correct, bro. Linkage is wrong. You get a major bump, and you'll get an ass full of coils!
Curious how you would change it? It matches what the Ducati manual says and it only fits on one way. I’m no expert but I’m pretty positive it’s mounted correctly
sounds electrical ,did it run when you 1st got it ? are both pick ups working ? , is it the correct cdi/ecu for the bike ?
Revisa la distribución parece ser que está fuera de punto
Ojalá y puedas con ello
Check valve lash
Never mind, I just heard you mention valve's. Changing the springs may be the ticket though, so I'll wait and see how the new ones work out