Topic: Degree Cam, How to Get +3 Degrees of Ignition Timing?

[Finnigan]

Finished putting together my high-ish performance motor and degree'd my Yoshi Daytona cam according to the specifications in the literature.  Lobe centers are set exactly as they should (105 degrees).  According to the parts catalogue this cam should have 3 degrees of advanced ignition timing.  Since I can't go off the standard plate markings where 'F' is marked, how do I dial in the additional 3 degrees?

Thanks in advance

[scottly]

Assuming you are talking about ignition timing, which has nothing to do with cam timing, the full advance ' ' marks are 2 degrees apart, at 34* and 36*, with the nominal 35* in the center of the two marks. You can estimate 3 degrees by 1 1/2 times the distance between the hash marks.

[Finnigan]

Thanks!

[Tracksnblades1]

Good question..

Advanced timing rarely comes up here..

I’ve ran an advanced static (initial) timing and limited centrifugal advancer for years on a 76F1..

What compression did you end up with..?

What fuel are you planning on running..?

[willbird]

There are plenty of timing lights that will let you dial in advance ??

[MRieck]

There are plenty of timing lights that will let you dial in advance ??

Ya think? 

[willbird]

There are plenty of timing lights that will let you dial in advance ??

Ya think? 

Luddite that I am I have not owned one yet :-). Not ruling it out tho LOL

[Finnigan]

Since top dead center was found a few degrees off of the plate markings I wonder if the ‘F’ mark can be used. 

On a stock cb750 does the spark happen at the top of the stroke?  It f I know that I can use the degree wheel again to get dead on.

[Old Scrambler]

No............Assuming oem ignition components..........the CB750 sohc fires approximately 6 to 8 degrees BTC at idle of 900-rpms or so.

The oem advancer unit is all-in at or before 2500 rpms, depending on spring condition..............so a timing light will then flash at approximately 35 degrees. You want 38 to 39 degrees so rotate the plate counter-clockwise by 3 degrees. I use a fine-tipped marker over a scribe-scratch to mark the original and new plate locations.

Now you should be ready to go to a dyno-test and get a base-line for your motor. While on the dyno carefully rotate the plate a little at a time to see if you can improve the power output. I found my peak power at approximately 42-degrees with about 12:1 compression on 110 gas.

[Tracksnblades1]

No............Assuming oem ignition components..........the CB750 sohc fires approximately 6 to 8 degrees BTC at idle of 900-rpms or so.

The oem advancer unit is all-in at or before 2500 rpms, depending on spring condition..............so a timing light will then flash at approximately 35 degrees. You want 38 to 39 degrees so rotate the plate counter-clockwise by 3 degrees. I use a fine-tipped marker over a scribe-scratch to mark the original and new plate locations.

Now you should be ready to go to a dyno-test and get a base-line for your motor. While on the dyno carefully rotate the plate a little at a time to see if you can improve the power output. I found my peak power at approximately 42-degrees with about 12:1 compression on 110 gas.

+1 depending on fuel and if your chamber is OEM..
Honda specs timing at 40* @ 2500rpm..Yoshimura +3 would be 43*..
It’s more dependent on your fuel and chamber efficiency.

+1 on the dyno verification…

Good question..

Advanced timing rarely comes up here..

I’ve ran an advanced static (initial) timing and limited centrifugal advancer for years on a 76F1..

What compression did you end up with..?

What fuel are you planning on running..?

[Tracksnblades1]

Since top dead center was found a few degrees off of the plate markings I wonder if the ‘F’ mark can be used. 

On a stock cb750 does the spark happen at the top of the stroke?  It f I know that I can use the degree wheel again to get dead on.

As PeWe has posted several times:

The OEM centrifugal advancer can have some inherent play (several degrees) in its locating pin and crankshaft index hole..
He writes how to he takes up this slack in his post..

I read it as the same procedure used to compensate for the stacking up of back lash in multiple gear driven idle gear driven combinations when timing cams and injection pumps..

[willbird]

There are plenty of timing lights that will let you dial in advance ??

Ya think? 

of course I meant the timing lights with the advance dial ON the timing light :-)

[scottly]

Honda specs timing at 40* @ 2500rpm.

Where did you come up with that spec? I've measured the marks on the advancer using a rotary table, and the F mark is at 10* BTDC, the first full-advance hash mark is at 34* BTDC, and the second hash mark is at 36*. The nominal full advance is 35* BTDC.

[scottly]

On a stock cb750 does the spark happen at the top of the stroke?  It f I know that I can use the degree wheel again to get dead on.

The spark happens at the F (for fire) mark, which is 10 degrees before the top of the stroke, when the engine is at idle speeds.

[MRieck]

On a stock cb750 does the spark happen at the top of the stroke?  It f I know that I can use the degree wheel again to get dead on.

The spark happens at the F (for fire) mark, which is 10 degrees before the top of the stroke, when the engine is at idle speeds.

Maybe 10 degrees....if you are lucky. Scotty does know what he is talking about this.......AND OTHER $HIT.   I'm just looking around.

[scottly]

Hey Mike, I should have said the F mark was 10* before the T (for top dead center) mark on the advancer. Whether or not the T mark is actually indicating TDC may be a different thing.

[Tracksnblades1]

Honda specs timing at 40* @ 2500rpm.

Where did you come up with that spec? I've measured the marks on the advancer using a rotary table, and the F mark is at 10* BTDC, the first full-advance hash mark is at 34* BTDC, and the second hash mark is at 36*. The nominal full advance is 35* BTDC.

Oh just out of the service manual that Honda published..

Perhaps it’s just another misprint…🤔

I’ll copy and paste it again for your review….

And like I inferred with the indexing pin and hole, unless one verifies the advancer your just guessing at 10* +/- a degree or 2..

[Finnigan]

Hey Mike, I should have said the F mark was 10* before the T (for top dead center) mark on the advancer. Whether or not the T mark is actually indicating TDC may be a different thing.

When finding true TDC I found it was about 2 degrees difference from the plate marking, so I plan to add 3 degrees from this true mark.

Maybe going into too much detail but if I don't plan to dyno right away is plus or minus 1 degree going to make that big of a difference?

[Tracksnblades1]

Hey Mike, I should have said the F mark was 10* before the T (for top dead center) mark on the advancer. Whether or not the T mark is actually indicating TDC may be a different thing.

When finding true TDC I found it was about 2 degrees difference from the plate marking, so I plan to add 3 degrees from this true mark.

Maybe going into too much detail but if I don't plan to dyno right away is plus or minus 1 degree going to make that big of a difference?

The saying  “Timing is Everything” is still true today as it was when it was coined.

Was your timing early or late compared to the degree wheel’s TDC..?

[scottly]

The index hole in the crankshaft is 4mm, but some advancers have a 3mm index pin, which allows 1mm worth of error; just how many degrees that translates to I don't know, but maybe Finnigan could check it for us? BTW, some advancers did have a 4mm pin, which didn't have the possible error.
In any case, you probably won't notice 1 or 2 degrees either way.

[Finnigan]

Hey Mike, I should have said the F mark was 10* before the T (for top dead center) mark on the advancer. Whether or not the T mark is actually indicating TDC may be a different thing.

When finding true TDC I found it was about 2 degrees difference from the plate marking, so I plan to add 3 degrees from this true mark.

Maybe going into too much detail but if I don't plan to dyno right away is plus or minus 1 degree going to make that big of a difference?

The saying  “Timing is Everything” is still true today as it was when it was coined.

Was your timing early or late compared to the degree wheel’s TDC..?

I found TDC with the wheel and saw the mark on the plate was clockwise by a very small amount (1mm?)

[Tracksnblades1]

Finished putting together my high-ish performance motor and degree'd my Yoshi Daytona cam according to the specifications in the literature.  Lobe centers are set exactly as they should (105 degrees).  According to the parts catalogue this cam should have 3 degrees of advanced ignition timing.  Since I can't go off the standard plate markings where 'F' is marked, how do I dial in the additional 3 degrees?

Thanks in advance

What is your compression ratio in your “High-ish Performance engine”..?

With higher than stock compression and 87 octane,  1 -2 degrees of ignition advance can be the difference from destructive detonation or not when on the ragged edge.. Especially with an air cooled engine..

Your compression ratio and fuel will determine your best advance or the maximum allowable ignition..It is good you know where your cam and TDC is…

Timing is everything in high performance.

Like Scambler found what was best in his application. I’ve seen the same best ignition lead results with the old RC 836 12:1 kits using the old 95/96 octane leaded gasoline.. I like the new unleaded all gasoline better but may error too by using higher than actual needed octane….I’ve tried VP leaded gasoline but never really felt “butt dyno” a difference other than a much lighter wallet..

[Finnigan]

I plan to use pump gas, we have 91 octane here in CA.  I never found my compression ratio as I have a slightly modified head and Yoshi 823 pistons.  Don't suppose there's a way to find out based on compression test?

Engine:
Line bored cases and coated bearings
Balanced stock weight crank
Yoshi SuperRods
Coated rod bearings
Yoshi 823 NOS kit
Daytona Cam
HD cam chain and MessnerMoto tensioner
Ported 71 head (Jerry Branch)
Valve job by Kurt and HD APE springs

[scottly]

I found TDC with the wheel and saw the mark on the plate was clockwise by a very small amount (1mm?)

With the crank set at TDC, loosen the 10mm nut on the end of the advancer shaft, then see how far the advancer T mark moves when when turning the big washer/nut thing back and forth. Does it vary from 1mm clockwise to 1mm counter-clockwise? 

[MRieck]

Finished putting together my high-ish performance motor and degree'd my Yoshi Daytona cam according to the specifications in the literature.  Lobe centers are set exactly as they should (105 degrees).  According to the parts catalogue this cam should have 3 degrees of advanced ignition timing.  Since I can't go off the standard plate markings where 'F' is marked, how do I dial in the additional 3 degrees?

Thanks in advance

What is your compression ratio in your “High-ish Performance engine”..?

With higher than stock compression and 87 octane,  1 -2 degrees of ignition advance can be the difference from destructive detonation or not when on the ragged edge.. Especially with an air cooled engine..

Your compression ratio and fuel will determine your best advance or the maximum allowable ignition..It is good you know where your cam and TDC is…

Timing is everything in high performance.

Like Scambler found what was best in his application. I’ve seen the same best ignition lead results with the old RC 836 12:1 kits using the old 95/96 octane leaded gasoline.. I like the new unleaded all gasoline better but may error too by using higher than actual needed octane….I’ve tried VP leaded gasoline but never really felt “butt dyno” a difference other than a much lighter wallet..

             If you get the chance try the VP non-ethanol small engine fuel with 94 octane....my bikes love it and it smells like the old Testor's model cement from the 60's. unfortunately the price for a 5 gallon pail has gone through the roof. Lucky for me there is a gas station 3 miles down the road from my new house in NH that sells non ethanol out of the pump.