Ignition timing??

T

timtune

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I'm struggling with my recently completed 76' CB750F. Early test rides showed it unwilling to run well into the higher rpms under load. Sitting still it winds up to redline just fine. I figured it's ignition timing and have been pulling my hair out since. Setting the timing static with a light bulb and two roach clips has me running out of slot in the baseplate before I can get to the desired setting. I've mucked with it both advanced and retarded and only made things worse. Barely get it over 5K down the road - still winds up fine in the driveway.

Am I wrong in thinking that with the problem only under load it's probably not a fuel issue??

The bike was assembled from a bunch of doners and I havn't dug through the piles to find out if there are differences in the baseplate or timing advance mechanism between K and Fs. A dynatek electronic ignition is only 200$.........

Thoughts?
 
May not be the best plan but to make my life simpler, I'd probably put it on the centrestand with the front wheel against a wall and use the rear brake to load the engine while troubleshooting. Easier to do that than fiddle on the road.

Not a bike but the mini had a timing quirk when I put in a new engine using donor parts from original engine. Distributor was off by 90 degrees. It would run but obviously very poorly. Move plug wires around from where they "should' be to match the proper orientation and we were off to the races.
 
TK4 said:
ALL CB750s from 1969 to 1978 use the same points plate. Cam timing ?
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Was thinking that too but it fires up great. Idles just fine & spools up real nice on the stand. To be off a tooth would would surely show up in some of that I would think.
Same points plates for K and Fs?
 
timtune said:
Was thinking that too but it fires up great. Idles just fine & spools up real nice on the stand. To be off a tooth would surely show up in some of that I would think.
Same points plates for K and Fs?
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ALL CB750s (Ks and Fs) through those years.
Something's making it not advance properly - I know it's a hassle (pull the motor) but better to find out now than later.
 
TK4 said:
ALL CB750s (Ks and Fs) through those years.
Something's making it not advance properly - I know it's a hassle (pull the motor) but better to find out now than later.
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Do they have vacuum advance? Is it accessible? I can pull the hose off on the mini and suck to change advance.
 
No vacuum, it has centrifugal advance, and the RPM of the advance is controlled by the two springs.
Set the static timing to get the thing running, then use a timing light to set the timing at full advance. Check the static timing again, and if it isn't too whack, send it.
Timing at full advance should be 32-34 BTDC and as long as the static is 10-15 degrees, you're golden.
Being able to reach redline in neutral and not being able to reach redline under load is either lean on the main, or the motor is pooched.
IIRC there are two CB750 ignition setups, Hitachi and Toyo
 
Been a while since I worked on one... I'd start by making sure the timing is advancing af 2500 rpm with a strobe ($40ish on Amazon).

Then I'd verify jetting, I think i had 105s for a standard setup.

I remember mine being really picky about the airbox seals, I could never get pods working, and my juvenile airbox mods messed with the carbs.

Finally I'd check the valves.
 
@timtune , if you want to borrow a timing strobe, you are welcome to anytime. Gas to pick it up may cost more than buying one closer. Leaves are nice now though.
 
I have a good timing light - I was doing static to try and get close before pulling it out.
 
Jayell said:
That's where I'd start.
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This bike ran fine before I parked it 18 years ago. I had the carb apart to clean it and changed nothing so I'm leery of mucking with the carb yet.
 
timtune said:
This bike ran fine before I parked it 18 years ago. I had the carb apart to clean it and changed nothing so I'm leery of mucking with the carb yet.
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I'd say look for vacuum leak but that should make a bigger mess at idle than full throttle.
 
I managed to get the timing set close with my timing light (Mac tools TL87) but it meant going to the end of the adjustment for both the base plate and the 2-3 points plates. Took it out and just kept it pinned. It managed to hit and miss it's way up to about 6 1/2 - 7K where it cleared up marginally and managed to climbed to near redline. Can't see poor ignition clearing up with more RPM like that...unless it's an advance that's hanging up and needs high rpm to get the weights moving. That said I've had the advancer off and apart. Looks fine. Maybe just for grins I'll swap it with another.

I'm still struggling with whether it's fuel or ignition. I have one of those tools that goes between the plug and HT lead to let you "see" the spark. I've never used it. I'm wondering if that will shed light. Anyone used one of these?
 
timtune said:
I managed to get the timing set close with my timing light (Mac tools TL87) but it meant going to the end of the adjustment for both the base plate and the 2-3 points plates. Took it out and just kept it pinned. It managed to hit and miss it's way up to about 6 1/2 - 7K where it cleared up marginally and managed to climbed to near redline. Can't see poor ignition clearing up with more RPM like that...unless it's an advance that's hanging up and needs high rpm to get the weights moving. That said I've had the advancer off and apart. Looks fine. Maybe just for grins I'll swap it with another.

I'm still struggling with whether it's fuel or ignition. I have one of those tools that goes between the plug and HT lead to let you "see" the spark. I've never used it. I'm wondering if that will shed light. Anyone used one of these?
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I have some. Meh. You dont expect your problem to be lack of spark so they won't tell you much. If it wouldnt rev up in neutral, they could show you a dead coil.
 
Pulled the plugs. Spark was rather anemic across all four. 1-4 were were wet and sooty looking. 2-3 had a nice tan colour on the ceramic insulator. Start to think 1-4 coil is off. Fire it up and do a touch test on the pipes. 1-4 heat up noticeably quicker than 2-3. WTF?
I still think it's ignition. 56 year old coils and HT leads.......
Monday I'm dropping 400$ for the DynaTek upgrade of new coils, leads and electronic ignition.
 
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