Topic: CB750 ignition timing question

[timtune]

1976 CB750F - Trying to set timing statically with a light and 2 alligator clips. Light should come on at "F" mark but I havn't quite enough travel in the base plate slots. Pretty sure my cam timing is good because the bike fires up great. Idles fine and spins up to redline on the side. (would it run that nice being off a tooth on the cam chain??)
Problem is going down the road under load I can't get much past 5000rpms. Feels like the ignition is breaking down. Any thoughts appreciated.

Important background: Rode this bike hard for about a decade parked it leaking oil but running good 18 years ago. Last year and half I spent on a total rebuild. Disassembled and cleaned the carbs but no changes in jetting. Motor got fresh rings and a hone.

It's very short but here's a clip:

[69cb750]

Remove advancer, check you can easily advance by hand and springs pull back.

[timtune]

Have had the advancer out. Seems to work fine.
Managed to get it set close with my timing light (Mac tools TL87)
Took it out today 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 some and climbed to near redline.

Beginning maybe it is fuel issue....but an advance that's hanging up and needs high rpm to get moving also fits right now.

[MauiK3]

Advancers are known to stick.
Also look at springs

[RAFster122s]

Have you pulled and tested your spark plug caps? Coils sometimes start failing, but generally hold up well. Parts N More I think is who carries a pair of replacement 5 ohm coils for points bikes for around $80 for the pair.

[Don R]

  Some newer brand X points plates are a problem to get the timing right. The point gap may be adjusted to help get the points plate where it needs to be. 
  I've seen points with dry cams and the rubbing blocks on the points worn down recently, there isn't much discussion of points cam lube but it's also important.

[Bigmant]

Had a very similar issue where I couldn’t get up past 4k.

It’s hard to diagnose if it’s fuel or timing with timing as finicky as these old points are. In my case, even with a dwell meter and timing light, NOTHING I could do would get the timing spot on. Even bought a new timing shaft. Bought the dyna coil and have almost never spent money for a better cause.

My mains turned out to be too small for pods and 4 into 1 exhaust. But I never would have gotten to that realization if I hadn’t made the timing spot on first. Bigger main jets and I was finally able to push through 4 and 5k smoothly.

I will say, removing the advancer and thoroughly cleaning and lunricating the pivoting parts is good advice and a good place to start.

[scottly]

1976 CB750F - Trying to set timing statically with a light and 2 alligator clips. Light should come on at "F" mark but I havn't quite enough travel in the base plate slots.

Where does the light turn on? Have you checked both 1-4 and 2-3 timing?

[ekpent]

I sold a 1975 750 F to a friend once that had the same problem where the bike would fall on its face under load in that rev range. After much fiddling around with carbs etc. he replaced the ignition condensers and all was well again.

[ekpent]

1976 CB750F - Trying to set timing statically with a light and 2 alligator clips. Light should come on at "F" mark but I havn't quite enough travel in the base plate slots.

Where does the light turn on? Have you checked both 1-4 and 2-3 timing?

F for fire and T for tappets on the plate.

[scottly]

Eric, I know where it's supposed to be, I'm asking Tim where it actually is.

[ekpent]

Eric, I know where it's supposed to be, I'm asking Tim where it actually is.

I was just reminding him Scott in case he was confused about that a little. Your one of our electrical experts !!

[timtune]

Yes I was confused about the "T" "F" marks but went back to the manual to get it straight. Again I have the timing set pretty good with a timing light. New points & condensers. I hesitate to want to blame jetting since it's the same jets (jetting) that worked so well decades ago. 
I've had the advancer off and apart. Looks fine. Maybe just for grins I'll swap it with another.
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?
Thanks for the help. Please keep it coming.

[RAFster122s]

Tim if you are referring to a Gunson Colortune, don’t rev the bike with it installed…you will melt the glass causing a distortion of the view and color as it will darken from soot melted into the glass.

[ekpent]

Yes I was confused about the "T" "F" marks but went back to the manual to get it straight. Again I have the timing set pretty good with a timing light. New points & condensers. I hesitate to want to blame jetting since it's the same jets (jetting) that worked so well decades ago. 
I've had the advancer off and apart. Looks fine. Maybe just for grins I'll swap it with another.
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?
Thanks for the help. Please keep it coming.

If all 4 header pipes sizzle well when you touch a them with a wet rag  then you know all 4 are getting a spark. Spark plug color can also tell a little bit  of a story.

[timtune]

Yes all pipes are too hot to handle. Havn't pulled plugs yet but was also thinking they may hold a clue.

[BenelliSEI]

Before you go too much further, where did you get your plugs?

There are a ton of totally fake NGK spark plugs out on the market. I had one box of 10 where three or four plugs only worked for a few seconds when cool, then start missing. They work well laying on the cam cover, firing as a grounded plug, but fail when in real use. Visit NGK Factory website for a video on how to identify fake plugs.

I’ve been caught this way a few times, even buying plugs from reputable vendors! They just don’t have any idea where half their stuff comes from. I now only buy plugs from <clubplug.ca>. The fellow is in Montreal, Canada, and only distributes NGK plugs and ignition components he gets directly from the manufacturer. Be suspicious of anything less.

[david 750f]

Stop with the ignition
Just read your posts on GTA motorcycle.

Bike hasn't run in 18 years? Correct?

[david 750f]

I have the same bike, more than happy to walk you through it

[HondaMan]

I sold a 1975 750 F to a friend once that had the same problem where the bike would fall on its face under load in that rev range. After much fiddling around with carbs etc. he replaced the ignition condensers and all was well again.

Timtune:
if your 'new' points and condensors are marked "Daiichi", the condensors are causing the problems, bet on it. Get your old ones and put them in, instead: they do not "go bad", but last over 50 years. They are changed altogether too often (even in old cars).

I can explain in more detail, but I've done it so many times on these forums that I'm sure you can find it by searching for "Daiichi" and follow my comments.

[RAFster122s]

The Daiichi points and condensors have a 3 pointed blade logo, like a propellor.

[70CB750]

I ditched points and never looked back.  I had enough trouble with them with my old CZ 250 overseas.

I got Pamco but Hondaman makes electronic ignitiion that works with points. 

Just saying