Topic: cb750 ignition timing

[kezz32]

hi guys
I have just finished building a race engine for classic sidecar. I ,m not sure were to set timing advance.
have used;
ported head with 3 angle valve seats
balanced crank with matched falicon rods balanced end to end
Daytona profile cam 0.040 lift, set at inlet opens 23% closes 53% exhaust opens 53% closes 23% , inlet full lift 9.95mm..exhaust full lift 9.15mm..squish clearance 0.042
polished rockers
dynaman DP62R 12.5.1 HIGH COMP PISTONS
I have 29mm smoothbore carbs
dynatek 2000 ignition
will be on dyno in 2 weeks , but need to get close to start.
thanks for any help
kezz

[bear]

G'day Kezz,

Sounds like a pretty nice combination for a sidecar.

I would start with the std ignition timing.
You will probably need to advance it a bit, but I would start it on the marks.
(Edit) Sorry Kezz haven't got any experience with electronic advace ignitions.

You've picked a good cam for a chair, good lift and no/little overlap.

Not sure how much dyno time you have but consider advancing the cam timing a little.
We usually add around 5deg it brings the top of the curves down the rev range a bit and gives you more grunt off the line and out of the turns.

Good luck with it and let us know how it goes.

Cheers,
Brian

[johno]

Hi Kezz,
Sounds like fun. I agree with Bear that start up on standard ignition timing is the go.

Also I have lots of spares new and used for the piston rings combo you have, if you ever need any just PM me ,  If you have the gear I would recommend a leakdown test after you finish, those oil rings are fiddly to install into the barrels, especially if you have a decent taper on the bore bottom it lets them roll over.  Normally the bore taper assists the assembly but with those oil rings it works against you.    Anyway if you need anything just let me know, they are just rusting away in the shed,      well not really 
johno

[bear]

Bugger it's those pistons.
Not sure a leakdown would pick that up Johno.
Any other suggestions?

Cheers,
Brian

[johno]

Kezz,
Had to set my ignition up last night and changed my mind about your question re timing 

So your ignition is like the CR750 motor, the Honda CR750 factory spec was 35 deg fixed no advance   11:1+ CR race fuel
The standard Honda cb 750 is static around 5 deg then 35 deg advance for total 40 deg    9:1, standard gas

Our engines are similiar so I have set my base advance at 10 deg static , all in at 36 deg by 2,500 rpm
With the power arc system you have 4 curves and I have set the other 3 curves up for totals 38 deg, 40 deg, 44deg. for some experimentation

Your comp ratio is a bit higher at 12:1  so what gas you using ?

Bear ; when I done a comp test after Melt down the other 3 cylinders were 160, 155,160.    The 155 psi had a folded oil ring rail , apparently it doesnt seem to effect things dramatically , other blokes have said to me they dont worry about the folded rails anymore as it has minimal effect with very slight oil usage and no comp loss , just stuffs the bore , but for one off racing everybody at the high end just do it ? 

[scottly]

The standard Honda cb 750 is static around 5 deg then 35 deg advance for total 40 deg    9:1, standard gas

Our engines are similiar so I have set my base advance at 10 deg static , all in at 36 deg by 2,500 rpm

The standard 750 is static @ 10*, with a total of 35* around 2400 RPM.
Kezz, the heavier weight of the chair, with riders, may call for a less aggressive timing curve than the same motor in a bike setup that weighs 1/2 as much.

[johno]

Ya sure capt webber ?   From memory  The spring advancer is 35 degrees plus the static set  = x degrees
Either way Kezz, seems like we all agree on with high load in mid range, high comp ratio around 12 :1, better have serious good race fuel and be conservative in the timing department .

[johno]

This page although blurred says the advancer is 35 deg not total.

Kezz I reckon be guided by the CR 750 specs, yours is similiar just more comp ratio so why not start with 35 total same as they did.

[bwaller]

Like has been said, be conservative on initial setup. Then the sparkplugs will help tell where the advance is. (among other things) You can then use that info to fine tune. 

[scottly]

Ya sure capt webber ?   From memory  The spring advancer is 35 degrees plus the static set  = x degrees
Either way Kezz, seems like we all agree on with high load in mid range, high comp ratio around 12 :1, better have serious good race fuel and be conservative in the timing department .

I've never seen that page, Johno; where is it in the manual? I actually measured the marks of a K7 advancer and it appeared to match the K1 advancer installed on the bike, as well as the specs quoted on page 263.

[turboguzzi]

a lot would depend if you managed to build into the CC/piston combo squish effect that speeds up flame propagation. It's not just a matter of squish distance but even more so, squish area. With good squish action you could get down to less the 30 degs max.

The least advance you can run without dropping peak torque will give the coolest running temperature/ less detontation potential

[kezz32]

thanks for the advice guys , im planning to fire up on sunday, will set advance at 35 .. im using a octane booster that should take gas to 101 ron. the plan so far.
johno
I would be interested in some spare rings, do you have any pistons at 10 5 1
thanks kezz

[johno]

Hi Kezz, sounds like a plan.

Re the rings I have plenty, 1 set of new and 1 set of slight used ie 2 minutes run time.   ( just one piston set missing on both sets cos I destroyed no 2.
Re pistons I have 3 goodies, just no 2 missing

Whats the story Kezz any racing or track days for this baby ?

cheers johno

[kezz32]

hi Johno
so I could make a decent set of rings from what you have , I would be interested in them for spares , whatsyour price inc shipping to LE13 0NQ .. UK.
Q. what happened to your piston set , and more to point why,? I get the feeling you are not to keen on them.

hi Bear, what ignition do you use on your outfit?
Guys
this year I plan to race in the IOM pre TT classic, we had a great ride there last year (managed a 3rd) won cash:)
we will race in Belgium at Chimay and Gedinne closed road races , and also Bob Mac meeting in Scotland.
then Cadwell and snetterton rounds of Camathias cup.
thanks kezz

[johno]

OOOOOO Bear, this looks nice, very nice

Chair looks sensational Kezz, a credit to you.

Re the pistons and rings I dont want any money but have you got something in the shed  that may interest me, besides the chair of course

The pistons are fine , I have one with a hole in but it wasnt the pistons fault, the only issue I had was with the oil control ring rails folding once they went into the taper, my theory it would be better with no taper,          But Ive gone all old school now back to Honda rings 
johno

[bear]

hi Bear, what ignition do you use on your outfit?
Guys
this year I plan to race in the IOM pre TT classic, we had a great ride there last year (managed a 3rd) won cash:)

After a few heart aches many years ago, we use the KISS principle with our ignition these days.
Dyna pickups and late model Honda coils.
Not very high tech I'll grant you but very reliable.

Cheers,
Brian 

[kezz32]

started eng on sunday ..35% and program 5 on dyna 2000 ..sounded sweet , so 5 starts and cool downs. I will take for a short run next w/end to see what it pulls like and how every thing runs then off to dyno on feb 7 .. will post dyno runs here.
thanks kezz

[bear]

Look forward to it.

Cheers,
Brian

[kezz32]

ok so hit the dyno on sat all went well, apart from no 4 kissing a plug, so doubled up on washers, gona fit a thicker gasket next strip down, hopefully dyno chart below.

[01Thomas]

That seems pretty decent!

[bear]

ok so hit the dyno on sat all went well, apart from no 4 kissing a plug, so doubled up on washers, gona fit a thicker gasket next strip down, hopefully dyno chart below.

Looks pretty good first up mate.
A little work on the carbs and those curves will fatten up nicely.
Don't know how much dyno access you have but if you get the chance try advancing your cam timing by 5deg over the card.

It should bring the top of your curves back down the Rev range a little and will give you a bit more grunt down low.
I find it makes them a bit easier to ride.

Cheers,
Brian