Topic: Cycle X 849 kit major spark knock

[banzaibob]

Just recently put together a ground up CB750 with Cycle X's 849 kit.  I've had the kit for about a year and a half before I finally assembled it and got it running.  I'm going to call them and find out if they have records of the purchase and if the pistons are 10.5:1 high comp pistons (sorry, I can't remember that far back).

Everything went together easily.  It fired up no problem.  Set the Dyna II ignition, synced the carbs and prepared for its test ride.  Idled just fine.  It pulls smoothly and with great authority.  Problem:  under 4000rpm, anything more than modest acceleration or heavy throttle application gives some raspy, gnarly spark knock.  Above 4000 rpm you can make it knock but not so bad.  There is no surging or noticeably uneven acceleration, you simply have to back off the throttle or risk (what sounds like ) hand grenading the engine.

I went back, retarded the spark (best guess,15º), the knock isn't quite so bad but it is still bad enough to eventually destroy the engine.  Plus, it doesn't run nearly as smooth.  I checked the advance mechanism, full advance is spot on the hash marks at 2500-3000 rpm, within specs.  I even went out and tried timing it on the fly by pulling over and backing off the timing, still no dice. 

I would like to burn premium pump gas as I'm not going to race it but probably tour with it so reliability is what I am after.  Possible solutions?:  What does anybody think about tighter, stronger mechanical advance springs?  If I could move full advance up to 3000-4000rpm I think it would give me more wiggle room because the problem does not appear to be as bad above that.

Thank you in advance (no pun intended)!!   

[Don R]

More fuel might help. have you jetted for your new setup?

[Cqyqte]

What grade fuel are you running when this happens?  If you are running regular grade, try premium as it will retard your advance curve slightly more then regular.  Is the knocking only under extreme load? If so try a VOES that will retard the advance only under extreme load/low vacuum situations.

[banzaibob]

I am running 91 octane.  Looking at the plugs, they appear to be maybe slightly lean, not by much.  By their color they don't indicate that fuel is a problem, very light charcoal, chocolate color.  VOES?

[dave500]

the actual fuel wont retard your curve or alter the curve in any way,you could limit the curve action in the mechanism and pull a couple of degrees out of it and start with a slightly higher initial setting ending up with less total advance,,or use a cam with more overlap to reduce the dynamic compression if your using the stock cam?try good premium first though,if the compression is too high you might have to look at lowering it?what does a compression test show?

[scottly]

Are you sure it's really detonation, Bob? I'm running some pretty high compression 836 pistons, and I've never really noticed any spark knock with 91 octane. If retarding the timing by 15 degrees didn't stop it, then it doesn't sound like spark knock. Are you sure you don't have exhaust leaks where the pipes meet the head?

[Cqyqte]

the actual fuel wont retard your curve or alter the curve in any way,you could limit the curve action in the mechanism and pull a couple of degrees out of it and start with a slightly higher initial setting ending up with less total advance,,or use a cam with more overlap to reduce the dynamic compression if your using the stock cam?try good premium first though,if the compression is too high you might have to look at lowering it?what does a compression test show?

You are absolutely right Dave, but the flame wave propagation of regular fuel is much faster than higher octane grades.  In other words regular gas burns faster than hitest.  The mechanical timing doesn't change but the speed of the burn will which can help with issues of detonation, as would retarding the spark mechanically.  I am not am expert but an hour talking to Gary at PowerArc enlightened me on the benefits of playing with fuel grades and tuning the CB750 and Cycle X's twin Mikuni set up

[PeWe]

I had rather high CR with my RC836 many years ago 10.5:1. I heard knocks when driving on 5:th gear and twisted the throttle between 120-130km/h. Geared down 1 step to 4:th no problem, 2 steps down even better :-)
I always used 98 octane (Europe) and will continue to do that. If my CB750 should have low CR like OEM pistons, 95 (RON) could be tested

Too late ignition to avoid knocks might end up in other problems like overheated ex valves, right?

Important information about the octane levels

USA (AKI/PON) --- Europe (RON)
87 = 91
89 = 93
91 = 95
93 = 98
RON is always about 4 to 5 points higher than the equivalent PON (Pump Octane number = AKI

Next time I'll fire up my CB750 will be without led for the very first time..... 98 with the new (bad) additives.
We have Shell V-Power fuel here. I'll try that, next other 98 RON. Never 95 in my engine

[Cqyqte]

I am running 91 octane.  Looking at the plugs, they appear to be maybe slightly lean, not by much.  By their color they don't indicate that fuel is a problem, very light charcoal, chocolate color.  VOES?

VOES (Vacuum Operated Electric Switch) under high load vacuum levels will drop so if you incorporate a VOES into the ignition timing circuits to switch to a less advanced curve you can use it to retard the spark and eliminate knock or pings during heavy load periods.

I had an issue with my CB750 (849 piston) twin carb setup where at launch or  at loading up climbing a hill the the bike would fall on its face or ping.  I put a VOES in the timing curve selection circuit that would chose a OEM advance curve when vacuum fell below a given point and the bike was a totally different creature.  Didn't have to rev the #$%* out it to take off from a stop or downshift when pulling a hill...

[banzaibob]

First, I misspoke.  I have a Dyna S ignition that uses the stock centrifugal advance.  I don't see how a vacuum switch can offset the action of the stock advance.  Does anyone know of a "black box" that can be used in conjunction with the Dyna S to actuate via vacuum?  Should I opt for a programmable ignition? 

Tearing the engine down again to install cams, lower compression, etc. is simply not feasible.  I am not looking for race performance.  I am looking to pull that 3 mile hill at 80mph in 100º heat.  By the way, I have a Lockhart installed just below the headlight, not in front of the engine. 

The simple fix I have in mind is tightening the advance springs.  It appears to have relatively decent performance above 4000 rpm and will accept most throttle settings without a lot of complaint.  My problem is getting there.  At 2500 rpm, it is at full advance.  Shorter, tighter or stronger springs should move that advance curve "up" the rpm range therefore retarding the spark in the crucial range.  Does anyone know of any negatives to this? 

I'm not against using additives so long as they are readily available. 

[banzaibob]

Sorry I didn't put this in the previous post but:  I am sure that this is not an exhaust leak.  The fuel was donated by my 1976 BMW R90/6, 91 octane and purchased the day before.  The BMW shows no problems. 

[banzaibob]

Also, this is a photo of a previous big bore 836 build that I did about 3 years ago.  This bike runs flawlessly with absolutely no issue of any sort.  Dyna S, stock advance, pump gas of any kind.

[PeWe]

Also, this is a photo of a previous big bore 836 build that I did about 3 years ago.  This bike runs flawlessly with absolutely no issue of any sort.  Dyna S, stock advance, pump gas of any kind.

Big bore 836.... Is it higher compression pistons with higher domes or the flat ones that can be found cheap on Ebay?
The latter ones can probably run on lower octane without any problems. Otherwise very strange to invest money in high CR pistons and retard the ignition to avoid knocks. Better to continue with OEM piston CR then as I see it. Higher CR make a very nice engine sound, much harder, sound of "more hp".

[banzaibob]

The big bore 836 was with E-Bay specials.  So far they have performed very well.  I have had issues before with spark knock on bone stock CB750's.  On this recent build however it is dramatic enough to require a positive solution.  Even if I was to use this engine for racing applications, I would want this problem solved before I dropped the hammer.   

[scottly]

You shouldn't be having detonation with only 10.5:1 compression with premium fuel and 15* retarded timing. Was the head shaved, and/or the block decked? Something isn't right with this picture. Have you pulled the advancer to make sure the indexing pin is intact? We had a member recently who had one sheared off, throwing the indicated ignition timing way off...

[banzaibob]

No, the head has not been shaved and the block was purchased directly from Cycle X along with the pistons.  The advance unit indexing pin is intact and properly installed.  The engine was timed via a reliable and functioning timing light.  There appeared to be no "wobble" during the timing procedure.  The advance unit functions properly, advancing right off idle at approximately 1200 rpm and is at full advance at 2500 rpm. 

The carbs are clean, synced and adjusted.  I am not ruling out the notion that a re-jetting may be necessary especially increasing main jets one size up or needle jet one size up.  I do not think that it is that far off given plug color though.

I really do not relish the notion of having to purchase an all new programmable ignition.  The Dyna S has worked stone reliable.   

[Cqyqte]

There are several electronic ignitions with selectable advance curves, I went with CycleX's PowerArc. with 3 curves.  Ken sells a full kit coils, wires, caps, and ignition, I don't believe you would need your Dyna S.

[banzaibob]

I like the idea of a fully programmable ignition.  I had an '01 Road King TC88 that sounded like birdshot in a boxcar.  I put a Power Commander 3 on it, hooked it up to my serial port and presto!  Made me feel like a genius.  But before I go and spend $450, I would like to 1). see if I can solve the problem with $20 worth of jets and 50¢ worth of springs and 2). make sure that I don't have some other fatal flaw in the system. 

[Psychonaut]

I've done a few stand alone EFI set ups on high boost turbo cars and run wideband O2's on those and a few factory efi cars.  When the timing got over 40* at cruise one of them would knock when hot regardless of octane. This was a factory EFI set up and AFR was good (stoich), CR 8.8:1.  I would back the timing off from factory specs in the summer because it was just too much timing at cruise. 

Is there any way for you to check total timing at the rpm you have the problem? Do you have an advance timing light with the knock?

[banzaibob]

Talked to Ken at Cycle X today.  Good insight.  He said, "Hmmm, check your jetting."  I checked the jetting; carb 1 #112, carb 2 #115, carb 3 #115, carb 4 #115.  I then remembered that years ago we took this bike (same carbs, old engine) over the Rockies.  At high elevations we would foul the plug on #1 cylinder so in California we bought a bunch of different jets to try and offset the plug fouling.  Putting in 1 size leaner on #1 was our attempt at getting home.  It worked. 

Regardless, Ken seemed to suggest that at best, I should be running jets above #120 on an 849cc engine.  Although the spark knock still gives me pause for concern about the ignition system I think I'm going to see if I can whittle away at this problem with re-jetting.

However, tomorrow morning I am off to Wisconsin on my R1200 so this problem will have to wait until Sunday when I get back. 

[banzaibob]

Installed #120 main jets today.  The knocking is far less significant.  It is still there slightly but appears manageable.  I have a POS Virago up on the lift right now.  As soon as it comes off I'm going to put in some #125's.  I think that I am on the right track. 

[crazypj]

Did you check timing on both 'sides' of the Dyna?
I've found they can be off and need adjusting
 The Goldwing one I fitted to CB360 couldn't get to the correct timing  without modification, 750 one will probably be easier fix as there is a lot more space between trigger units.
It may be worthwhile fitting points plate to test?

[banzaibob]

Timing on 1-4 is spot on with 2-3.  It still pings somewhat under load but nowhere near as bad as the engine destroying sounds that it was making.  Almost makes me mad that I didn't think re-jetting first.  I'm getting close. 

[Xnavylfr]

You may need to change to this ignition!!

http://www.dynaonline.com/skins/products/sportbikes/dyna_2000/

With this you can set the timing curve advanced or retarded as needed!!

Xnavylfr(CHUCK)

[Vinhead1957]

Are you running 93 octane?