Topic: Tuned a 71 CB750 today and have some advice

[MRieck]

 I rebuilt the top end so all the ring sealing, valve sealing has been addressed. Deck height 0 as both the bottom (pulled the sleeves) and top of the cylinder were absolute crap after a "rebuild". Fresh guides , valves etc etc.....rest of the engine left along.
 Replaced the points plate. First off....you can only use a dwell meter to get them perfect. F feeler gauges, static lights etc etc etc. A dwell gauge. Set them to 23 degrees on an 8 cylinder dwell unit.
 Synch the crabs after you are sure there are no vacuum leaks. The manifolds were new....I insisted when the engine was built. Get the carbs synched...the large nut controls slide height. If the synch meter reads to high INCREASE the slide height to LOWER the vacuum (turn CLOCKWISE). Secure the setting with the small nut.
 Adjusted the#2 & 3 idle settings. Counterclockwise LEANS the mixture....clockwise richens it,
 Use a strobe light for the timing...forget about any other method.
 This engine runs so sweet with a rev authority...(with the 0 deck and some porting)... that it was a pleasure to have worked on it. I'm not saying you'll get a stock 71 to perform as well BUT is was a pleasure getting the points right ( use a dwell meter!!!!) and hearing it rev with 4 300 series pipes.
PS...I haven't touched points in 20 years.

[kmb69]

What you talking 'bout Willis?

You start talking that "dwell meter" and "strobe light" stuff and you gonna have some Old Timers arguing with ya!!!!

I have read many posts on this forum about feeler gauges and static timing being good enough, but........

You are absolutely CORRECT! 

And if you want the SWEETEST carb sync on the planet, you gotta use a mercury filled, 4 tube manometer.

Unbelievable how quiet (engine noise wise) but crisp one of these old girls runs when precision tuned. 

[Jerry Rxman Griffin aka MuthaF'er]

Sync your crabs pal!

[PeWe]

Thanks for the good hints. I'll check with a dwell meter now when I know that I need to set to 8 cyl. I checked my points very careful with 0.3mm vs 0.4mm blades and rotated the crank a lot to be sure about max opening point
I'll see if I got it right with a dwell meter.

- No Hondaman ign module for the points yet?
Mike, you need an own old CB750 with 4x300 pipes and points... Find one and start a restore/improvement project thread

[MRieck]

Sync your crabs pal!

Figures you'd notice that Jerry. Feeler gauges are OK for the initial set up of the points. The problem with the feller gauges is you are measuring the gap of a spring loaded device. the spring is always going to give a bit and give a skewed reading. The dwell meter measures under running conditions and allows you to get both points exactly the same.

[Johnie]

OK...I have a dwell meter I used on my 69 GTO back in the day of points. Connected the contacts to the coil and ground I believe? Where do you connect the + and - to on the CB750K? I will give it a try...

[MRieck]

Connect to points themselves ( where the yellow or blue wire are connected to the points). Attach ground to engine case

[Duke McDukiedook]

Sync your crabs pal!

Sync yo mutha effin' crabs.
Get down with your bad self! 

[Oldtech]

Old School points static timing. Like we used to do on Brit bikes with magnetos.

1. Set the correct gap with a feeler guage. I go for the small end of the range given in the manual to get more dwell.
2. Get a cigarette rolling paper. You can probably borrow one from the teenager next door.
3. Roll the engine around to the 'F' timing mark for the cylinder that you're timing and insert the paper between the points.
4. Advance or retard the points (don't change the gap) until you can just pull the paper out.

Done properly this will get you within a few degees of spot on. You can now start the engine and use the dwell meter and timing light to get it perfect.

[greenjeans]

Always used feller gauges and a test light.  Always checked them repeatedly.  Always had  pretty good, smooth running machines, but I always
knew in my heart that it should be just a bit quieter and smoother.    MRieck has inspired me to get me a dwell meter.

Any suggestions for a decent one ?    Don't want to break the bank, but I also tend to steer clear of the cheap crap. 

[chewbacca5000]

Hey Mike,

Can you post us up a pic of your dwell meter and show us where to look for the 23 degrees.  Reading that you have to double the 8 cylinder reading for a four cylinder?

[MRieck]

Hey Mike,

Can you post us up a pic of your dwell meter and show us where to look for the 23 degrees.  Reading that you have to double the 8 cylinder reading for a four cylinder?

Here you go Chewy. Very simple. I have an big,older Craftsman unit that is analytical too but this unit is small and simple. Reasonable price too.
http://www.amazon.com/Actron-CP7605-Tachometer-Voltmeter-Analyzer/dp/B00062YUUS

[kmb69]

......
Can you post us up a pic of your dwell meter and show us where to look for the 23 degrees.  Reading that you have to double the 8 cylinder reading for a four cylinder?

Chewy, You should read 23 degrees on the 8 cylinder scale. That "double" business is for 4 cylinders with distributors. The SOHC's are running at crank speed.

[mrbreeze]

Yeah Mike, I learned that lesson back in the 80's. My 77 LTD 1000 Kawi was the last year before they went electronic ignition. I set my points a few times with feeler gauges but one time I hooked up a dwell meter that I had purchased to do automotive tune-ups and the difference was like night and day. I still own that dwell meter but it lives in the under section of my tool box along with other tools from the past. I like using that meter to set points on old Chevys.....just raise the little door in the distributor cap and set it while it's running.....easy peezy!!!

[Johnie]

Breeze...yah, those were the days. Distributor doors and easy to set points. Most those early cars had the distributor in the back, but you could get to it. Ford had that distributor right in the front which was really easy. I think I will have to pull out my old dwell meter and give it a go...

[chewbacca5000]

......
Can you post us up a pic of your dwell meter and show us where to look for the 23 degrees.  Reading that you have to double the 8 cylinder reading for a four cylinder?

Chewy, You should read 23 degrees on the 8 cylinder scale. That "double" business is for 4 cylinders with distributors. The SOHC's are running at crank speed.

Thanks this answers my question.  For $25 it is not a bad deal.  Maybe I'll get one along with a Hondaman ignition and be done with it.

[ekpent]

 Think maybe I will crawl out of the cave also and check to see how far off my set-up is with one of those fancy electronical gizmo's. May have to blow some dust off but never too old to learn and try new and better ways. We all can get a little set in our ways sometimes.

[chewbacca5000]

Think maybe I will crawl out of the cave also and check to see how far off my set-up is with one of those fancy electronical gizmo's. May have to blow some dust off but never too old to learn and try new and better ways. We all can get a little set in our ways sometimes.

Erik,

You must have a good with wife to withstand that mighty Uni-brow you are rocking!  Love to see a pic of you ridding one of your 750s. ha ha ha

[PeWe]

I checked my points today with a meter with dwell  + 8cyl functionality.
Engine running on idle. Hondaman ignition connected, no condensors as usual. It was easier than expected when I did not need to disconnect anything.
1-4 and 2-3 were different. I really spent time when I adjusted them (NOS TEC) the old way with feeler gauges about 1500km's ago.
Still irritating when I have got correct value, tighten the screw the last bit and it changes.... after some tries I got them both close to 23 degrees.
Much easier to adjust with a dwell meter. The result that affect the engine is visible. I guess that equal values 1-4 vs 2-3 is very important.

EDIT: I just found this thread: http://forums.sohc4.net/index.php?topic=61607.0
I might have too low with 22.9?  Closer to 24 might be better?
I'll try tomorrow and hear what how my engine will say about that.

[PeWe]

More about the ignition. I hope others will have use of the information.
I adjusted the points again to enter the spoken area of dwell, "23-24.5 degrees on a 8 cylinder dwell meter is correct"

I confirmed that smaller gap give higher value. 23 degrees must be ideal for long time use without adjusting the points again when the dwell will increase by the wear of the points bakelite part pressing against the rotor (point gap become smaller). I guess it will take a while to reach 24.5 degrees. Especially with Hondaman ign module that is said to have no contact wear (pitting) when the current over the points is very low when the are triggers for the ign module that handling the coils.

Both points to have same angle, today about 23,4. I had to readjust the ignition timing again when point gaps were adjusted.
2-3 was irritating. Ok dwell, adjusted timing, check dwell again that had moved when 2-3 plate slide around when adj timing. I had to adjust several times to get both OK, dwell and timing of 2-3. 1-4 possible to adj timing without affecting dwell.

The base plate (1-4) and the 2-3 plate are both not close to an end point of the oval holes as it usally has been in the past. This might indicate a good adjusted ignition.
Photo below after adjustment of points plus a re-check of the timing with timing lamp. The red marks show the setting before when I had adjusted the points with feeler gauge around 0.35mm + timing lamp 'F' at idle.  0.3mm to feel light, 0.4mm not possible.
(point gap setting 0.3-0.4mm)
Timing must be better now!

[dusterdude]

Remember,dwell effects timing but timing does not effect dwell

[Johnie]

So can you adjust the dwell while the motor is running like we did back in the day with cars? Move the points slightly while watching meter? Get it spot on and tighten?

[mrbreeze]

You can on our bikes and you can on a Chevy. Fords and others have to be done with the dist. cap off so you have to play with it till you get to the right number.

[PeWe]

Remember,dwell effects timing but timing does not effect dwell

1-4 no problem but 2-3 plate moved radial too when releasing the screws when the slots is wider than the screws. I had to adjust again after timing.

So can you adjust the dwell while the motor is running like we did back in the day with cars? Move the points slightly while watching meter? Get it spot on and tighten?

Yes, much easier than using feeler gauges.

[MRieck]

Timing under actual running conditions rules....period.