Allow me to introduce myself

SquidHunter

Been Around the Block
My name is Brad, and I’m not afraid to admit: I don’t know squat about motorcycles. I’ve always loved them though. A few years ago the opportunity to scoop up a 1980 Cb750f came up, and I jumped on it. It was cheap. It only had 10,000 miles on it, it ran, and it was all there.

The exhaust was rotted out, so I swapped for 4 into 1. The bike was running lean, so I rejet the carbs and tuned it up to the point I thought it was good or at least the idle was good, and it pulled through the rpms. Still has the stock airbox, so it wasn’t a big deal. I went from 68/98 to 75/102, and swapped the worn rubber bits: the air cutoffs etc. It ran good, so I rode it ALOT. I’ve put 20,000 more miles on it over the past few years.

Since then I’ve learned a lot. I’m not even close to an expert, but I’ve been fortunate enough to meet a couple people who would actually hand down some real knowledge. Not much but some.

I’ve kept the bike stock the whole time, but I’ve been through it. Especially in preparation for my last trip. All this time I’ve kept the jets the same on the carbs, even though I’ve cleaned them a few times. The bike has been through some charging issues (typical). I’ve had to clean and seal the tank. I’ve changed the tires and shocks, and fork seals. All the typical maintenance items I’ve learned: checking the valves and compression, and using a multimeter to check the charging system.

On my trip the electrical system finally showed its true colors. I got caught in heavy rain and broke down about 30 miles from home.

Since then I’ve learned more. I’ve decided on doing a bit of a resto-mod, or more of a sleeper. Since the bike was down, and money had to be spent anyway, I went with upgrades instead of replacements. I started with the charging system, then the ignition system, and with some help I wired the bike up myself (until I can afford a m-unit replacement).

I’ve now rebuilt the carbs on a level that I haven’t before. I used a heated ultra sonic and the whole deal. Put the carbs on and the bike fired right up. When dialing them in, I realized they weren’t right. I pulled them and cleaned them again (this time adjusted everything right). When I put them back on the bike I can’t get it to run (I know).

Everything checks out, and I’ve narrowed it down to two possibilities (in my mind): either the plugs magically fowled or now the bike is way to rich due to my previous tuning attempt. Maybe now that the carbs have been properly cleaned 75/102 is too big? I’ve been told that simply adding 4 into 1 exhaust doesn’t justify a rejet by someone extremely knowledgeable about cb750s. But I also know each bike has its own personality. Either way, it ran when the carbs were wrong and it won’t when they are right (excluding the jetting).

The whole purpose, for this excruciating long post, is I’m hoping to meet some guys or girls who are willing to help me out. I know newbies may annoy some, and some are here just to show off. I get that, and most have earned it. But the internet and shop manual will only take me so far. I’ve posted some phots of my bike, and I’m absolutely in love with it and want to make the most out of it. Any help and information would be greatly appreciated


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Welcome to DTT, Brad.
Sounds like you know your bike pretty well, and not afraid to learn more. Thats a great thing.

Im sorry but I don't know a darn thing about the 750s. But I do know a thing or two about making a bike run.

Check the easiest things first.

Pull the plugs one at a time and ensure you have spark on all of them. (if no spark, then start hunting for wires that may have been accidentally unplugged, or poorly connected, Check fuses, Or faulty switch - Kill, kickstand, neutral switches, etc ).

If you got good spark at plugs...

I would pull the fuel tap and ensure its clean and flowing fuel well. (as you mentioned tank has been sealed)

Crack open the carb bowl drain screws to ensure you got gas in the carbs. As the float needle valves can stick closed. Check float height.

Since it reads like thats all you've messed w chances are the problems there. It ran before.

Go through carb adjustments. Air/Fuel mixture screws and idle control adjuster may very well be the issue.

IDK enough about your bike. If it has ECI or points, A vacuum fuel pump or or anything about all that. ::)

Keep us posted.
 
What ignition system did you change to? As Trek mentioned, that's where I would start. Do you have a spark on both pairs of coils and is teh spark at the right time?
 
Welcome. you'll find most folks here are the kind to help Noobs without criticism and only get on you for safety issues. As for the CB750 I do not know much but I have a 79 GL1000 that is exhibiting bad behavior ignition wise in the rain so know your pain there. If it's an electronic ignition and you've rewired and now won't run, doubel check every connection, If it's like my CM450 from the 80's they can have the Black Box (CDI) just quit and you need to replace it. Check for spark like Trek said, then check it's at the right time, it that is good, look for fuel issues.
 
After I broke down, on my way back from my trip, that’s when I changed the ignition. Before I cleaned up the carbs real good the first time. So, the bike started the first time with the new ignition. It’s a power arc ignition from cycle x. It’s a one coil set up.

I pulled the carbs a second time to clean them again because I couldn’t get a good sync. That’s when I realized I hadn’t made the adjustments on the mixture screws. They were still all the way in. But, the bike ran and idled ok. After the second cleaning the bike wouldn’t run.

Here’s what I’ve checked: the plugs out, they were wet with gas. Gapped them, and cleaned them on a wire wheel. Then blew out the gas. Checked the spark with a checker, and it’s getting damn good spark from the wires. When I reinstalled, nothing. Did not check if I was getting spark through the plugs though. I’m going to reset the timing, because that’s easy. I’ve checked the compression (155 on all cylinders), and the valves are in tolerance (not perfect but passable per manual). The bike has been totally rewired by me. There’s only one main fuse, and only one switch (ignition/kill and start).

It turns over fine and more than strong enough charge.

I’m on the fence as to weather the plugs may have mysteriously gone bad (not sure that even happens), or maybe the timing was a touch off. On the timing theory: maybe since it wasn’t getting enough gas the timing being off made it run ok? Now that it’s getting more than enough it won’t run? I don’t know if that’s even possible.

What do you guys think on the jetting? Like I mentioned before, I’m running 75 and 102. Only air/fuel mod is the 4 into 1 exhaust. Ken at cycle x says it shouldn’t need a rejet. Stock is 68/98. My theory is: I never got the carbs proper clean before, so now I’m flooding the cylinders. Should I go back try going back to stock if the timing adjustment doesn’t work?

I’m 99.9 percent certain it’s not electrical. I double checked all my connections before I loomed up the wires. It did run before the second carb clean, and the bike hasn’t moved. If all else fails, it’s easy enough to go through all my connections but I am getting strong spark through the plug wires (maybe not the plugs)

I’m leaving to go out of town for the rest of the week, so I won’t be able to work on it. It will give me some time to think on it though.

The only thing that should have changed is the carbs when I went through them the second time. So that leads me back them


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The cdi boxes have been eliminated. The charging system is new and charges strong (usually my source of problems, that’s why I changed it all. New charging system is triumph charging system ((similar to a Harley)) machined to fit my bike. It’s 4 lbs lighter and charges twice as much)

Another theory for the ignition timing: when it started the first time, I warmed the bike and kept it warm trying to sync the carbs, so when it cooled off it may have been just enough so it won’t start again?


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If it was running w air/fuel adjusters all the way in. Then it was running far from properly. I don’t know the particulars about those carbs nor how the air/fuel adjusters operate on them. But there’s a pretty good chance it would be rich w them all the way in.
If you have a Honda workshop manual it will explain how a/f adjusters operate on those carbs.
Yes, you need to check you got spark on the spark plugs your fitting. It’s possible they are fouled.
 
You're in good hands mate and in the right place. Great looking ride, welcome from Montreal. Is that your son wrenching on his ride too? Kickass!
 
Manual....

http://www.dotheton.com/forum/index.php?topic=44399.msg484864#msg484864
 
trek97 said:
If it was running w air/fuel adjusters all the way in. Then it was running far from properly. I don’t know the particulars about those carbs nor how the air/fuel adjusters operate on them. But there’s a pretty good chance it would be rich w them all the way in.
If you have a Honda workshop manual it will explain how a/f adjusters operate on those carbs.
Yes, you need to check you got spark on the spark plugs your fitting. It’s possible they are fouled.

I’ve always thought the air/fuel screws introduced air, like your describing. My shop pro, who went to MMI, and has been a motorcycle mechanic for nearly a decade, stomps up and down swearing they introduce fuel. His theory may hold some weight due to the fact that with screws all the way in the plugs were dry, and now they are soaked. But, the manual, in the Cb750f addendum, it says to start with a 1.5 turn on the mixture screws then adjust out according. I even tried turning the mix screws out some more, and still no crank. ‍♂️
Could it be way too rich with my jetting, now the carbs are actually clean? Maybe I just got lucky before now? I would be interested to know what others are running for just a 4 into 1 exhaust, stock filter, and airbox. Also, my slider pucks had some light scarring so I polished those up. Would that screw up the carbs?


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The Jimbonaut said:
You're in good hands mate and in the right place. Great looking ride, welcome from Montreal. Is that your son wrenching on his ride too? Kickass!

Hey! Thanks! Yep! Any time I’m working under the shade tree, he has to join me


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trek97 said:
Manual....

http://www.dotheton.com/forum/index.php?topic=44399.msg484864#msg484864

Fortunately I have a hard copy. It’s worth it’s weight in gold!

I can’t find anything on weather or not the air/fuel screws introduce air, or fuel. Since it’s on the engine side I’m thinking fuel?


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Squidhunter, Both you and your MMI friend are correct, that is sometimes. Some carbs depending on brand and model, the mixture screws increases or decreases fuel, others they increase or decrease air. You need to know which kind you have to know what they are doing and generally there is a section in the manual that show which type they are. You could have richened it so much you fouled the plugs and can't get it to start anymore for sure. What is the stock jet size and stock needle, mixture settings? Go back to that, put fresh plugs in and start from there.
 
trek97 said:
If it was running w air/fuel adjusters all the way in. Then it was running far from properly. I don’t know the particulars about those carbs nor how the air/fuel adjusters operate on them. But there’s a pretty good chance it would be rich w them all the way in.
If you have a Honda workshop manual it will explain how a/f adjusters operate on those carbs.
Yes, you need to check you got spark on the spark plugs your fitting. It’s possible they are fouled.

It was definitely far from right. It sounded ok (not happy tho), but when I started trying to sync the carbs I knew something was wrong. That’s why I pulled them the second time


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Allow me to introduce myself

SquidHunter said:
I’ve always thought the air/fuel screws introduced air, like your describing. My shop pro, who went to MMI, and has been a motorcycle mechanic for nearly a decade, stomps up and down swearing they introduce fuel. His theory may hold some weight due to the fact that with screws all the way in the plugs were dry, and now they are soaked. But, the manual, in the Cb750f addendum, it says to start with a 1.5 turn on the mixture screws then adjust out according. I even tried turning the mix screws out some more, and still no crank. ‍♂️
Could it be way too rich with my jetting, now the carbs are actually clean? Maybe I just got lucky before now? I would be interested to know what others are running for just a 4 into 1 exhaust, stock filter, and airbox. Also, my slider pucks had some light scarring so I polished those up. Would that screw up the carbs?


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Depending on carb type, the screws could be either air or fuel adjusters

Example, mikuni VM carb has a fuel screw.
Example 2, gs750 carbs have an air screw.

Look and see if you can follow the passage leading to the adjustment screw, if it goes into the bowl it’s fuel, if it goes to the carb throat (at the entrance) it’s air.

Your manual should tell you what it controls in the carburetor section.

Haha maritime beat me to it
You have an air screw on those carbs,
Note how the passage goes up to the air inlet.
 
Maritime said:
Squidhunter, Both you and your MMI friend are correct, that is sometimes. Some carbs depending on brand and model, the mixture screws increases or decreases fuel, others they increase or decrease air. You need to know which kind you have to know what they are doing and generally there is a section in the manual that show which type they are. You could have richened it so much you fouled the plugs and can't get it to start anymore for sure. What is the stock jet size and stock needle, mixture settings? Go back to that, put fresh plugs in and start from there.

That’s what I’m thinking. Go back to the stock jets and start over. The second go around I did originally set the mixture screws by the book. If anyone is interested: my carbs are the VBAB or “B” set. The setting are listed under the 1980 F model addendum.

2nd go around: bike was cold, I choked the carbs turned ignition, gas it some gas.... nothing. I then unchoked the carbs, hit the ignition and it fired and died. After that I couldn’t get anything.

At this point it’s sounding more and more like the plugs. They are iridium plugs. I did take them out, and clean them on the wire wheel. Still got nothing. Could they be fouled without anything being visible to indicate they were? Like an internal foul ‍♂️


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farmer92 said:
Depending on carb type, the screws could be either air or fuel adjusters

Example, mikuni VM carb has a fuel screw.
Example 2, gs750 carbs have an air screw.

Look and see if you can follow the passage leading to the adjustment screw, if it goes into the bowl it’s fuel, if it goes to the carb throat (at the entrance) it’s air.

Your manual should tell you what it controls in the carburetor section.

Haha maritime beat me to it
You have an air screw on those carbs,
Note how the passage goes up to the air inlet.

I’ve always thought they were air too. When I blow through them, it comes out in the float bowl


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Depending on the type of irdium a wire wheel can wreck them and they won't work, easy enough to test. put on in a wire, ground it to the engine and turn over bike, fat blue spark they're fine. yellow weak or no spark they're toast. Get the old school NGK standard plugs until you get it right, they are easier to clean and cheaper too.

manual says D8EA just like a billion other Hondas.
 
You can never clean a spark plug
It will work for a little while, but in the end it will foul so much easier.
Just drop the 5$ a piece and replace them to eliminate that possibility.

What do you mean when you blow through them?
You removed the screws and blew into them?
It would come out the pilot jet and air inlet if they are air screws.
 
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