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Author Topic: Allow me to introduce myself  (Read 3354 times)

Offline SquidHunter

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Allow me to introduce myself
« on: Jun 18, 2018, 01:09:26 »
My name is Brad, and Iím not afraid to admit: I donít know squat about motorcycles. Iíve 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 wasnít 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. Iíve put 20,000 more miles on it over the past few years.

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

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

Iíve now rebuilt the carbs on a level that I havenít 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 werenít right. I pulled them and cleaned them again (this time adjusted everything right). When I put them back on the bike I canít get it to run (I know).

Everything checks out, and Iíve 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? Iíve been told that simply adding 4 into 1 exhaust doesnít 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 wonít when they are right (excluding the jetting).

The whole purpose, for this excruciating long post, is Iím 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. Iíve posted some phots of my bike, and Iím 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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Offline trek97

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Re: Allow me to introduce myself
« Reply #1 on: Jun 18, 2018, 07:26:27 »
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.


« Last Edit: Jun 18, 2018, 07:44:19 by trek97 »

Offline haircuttergirl

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Re: Allow me to introduce myself
« Reply #2 on: Jun 18, 2018, 08:37:22 »
Welcome to DTT. Looking forward to seeing more from your bike project.

Offline teazer

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Re: Allow me to introduce myself
« Reply #3 on: Jun 18, 2018, 09:23:12 »
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?

Offline Maritime

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Re: Allow me to introduce myself
« Reply #4 on: Jun 18, 2018, 11:05:29 »
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.
The GL Rebirth: http://www.dotheton.com/forum/index.php?topic=68337.0
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Offline SquidHunter

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Re: Allow me to introduce myself
« Reply #5 on: Jun 18, 2018, 13:59:42 »
After I broke down, on my way back from my trip, thatís 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. Itís a power arc ignition from cycle x. Itís a one coil set up.

I pulled the carbs a second time to clean them again because I couldnít get a good sync. Thatís when I realized I hadnít 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 wouldnít run.

Hereís what Iíve 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 itís getting damn good spark from the wires. When I reinstalled, nothing. Did not check if I was getting spark through the plugs though. Iím going to reset the timing, because thatís easy. Iíve 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. Thereís only one main fuse, and only one switch (ignition/kill and start).

It turns over fine and more than strong enough charge.

Iím 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 wasnít getting enough gas the timing being off made it run ok? Now that itís getting more than enough it wonít run? I donít know if thatís even possible.

What do you guys think on the jetting? Like I mentioned before, Iím running 75 and 102. Only air/fuel mod is the 4 into 1 exhaust. Ken at cycle x says it shouldnít need a rejet. Stock is 68/98. My theory is: I never got the carbs proper clean before, so now Iím flooding the cylinders. Should I go back try going back to stock if the timing adjustment doesnít work?

Iím 99.9 percent certain itís 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 hasnít moved. If all else fails, itís easy enough to go through all my connections but I am getting strong spark through the plug wires (maybe not the plugs)

Iím leaving to go out of town for the rest of the week, so I wonít 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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Offline SquidHunter

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Re: Allow me to introduce myself
« Reply #6 on: Jun 18, 2018, 14:05:48 »
The cdi boxes have been eliminated. The charging system is new and charges strong (usually my source of problems, thatís why I changed it all. New charging system is triumph charging system ((similar to a Harley)) machined to fit my bike. Itís 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 wonít start again?


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Offline trek97

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Re: Allow me to introduce myself
« Reply #7 on: Jun 18, 2018, 14:15:09 »
If it was running w air/fuel adjusters all the way in.  Then it was running far from properly.  I donít know the particulars about those carbs nor how the air/fuel adjusters operate on them.  But thereís 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.  Itís possible they are fouled.

Online Jimbonaut

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Re: Allow me to introduce myself
« Reply #8 on: Jun 18, 2018, 14:21:35 »
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!
"I'm telling you Donnie, nuthin' but nuthin' but right"

Offline trek97

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