Distributor springs for modified CX100

[cliffrod]

My 81 CX100 engine was reportedly fully rebuilt over 20 yrs ago for the previous owner and then barely ridden before being parked.  He was not mechanically fluent and could provide few specific details of the rebuild beyond significantly lightened flywheel, “bigger” camshaft with timing gear drive conversion and was built by a “BMW mechanic”.   It’s no longer a stock-spec SP equivalent engine.  It has gilardoni cylinders, so if purchased as a kit for a late round fin 1000 engine probably has stock crown pistons vs higher compression performance pistons.  I’ve looked but found no info about such cylinder kits for 1000 engines.

The engine runs great but has serious detonation issues under hard throttle from start that are not resolved with distributor adjustment.  For comparison, when ridden in same way, the same gas in my V7 Sport produces no detonation.   Under slower throttle on the CX, acceleration is detonation-free as is applying light or heavy throttle at higher rpm & road speed. 

Per Guzziology, the lower performance engines like SP have distributor with lighter springs allowing faster advance while higher performance engines like v7 Sport & Lemans will have heavier springs that produce slower advance.  Given the performance upgrades my engine apparently has, having springs to slow advance to limit detonation seems to make sense.

Whether my distributor has original SP-spec springs that are good or fatigued or maybe even heavier Lemans-spec springs, I don’t know yet.   I have both sets of springs to install.  Before I take it apart and lose track of what goes where, Are there specific installation locations for each spring in a respective pair?  If there is a specific location for each spring, what about changing from light to heavy springs?   My plan is to try the heavier springs to slow advance to address detonation issue.

[Antietam Classic Cycle]

Either spring can go in either position. The heavier "Le Mans" springs may help. On Loopframe Guzzis that have ping issues, I've used two of the "normal" springs vs. one normal and one with an elongated loop. Doing the same on a Tonti would really slow the advance down.

[cliffrod]

Thanks, Charlie.  I tried to make sense of the mix & match discussion from the Guzziology write-up and figured I would get enough springs to try it if the matched set of heavier springs or lighter springs didn’t solve this issue.

[Frenchfrog]

I'm running virtually the same set up as you and experience the same pinging as you describe ...but only when the ambient temperature is above top twenties.
Backing off the ignition helps, the T 3 curve helps  but I'm pretty convinced that a complete solution is to go for twin plugging.That's not a very cheap answer though !

[cliffrod]

I'm running virtually the same set up as you and experience the same pinging as you describe ...but only when the ambient temperature is above top twenties.
Backing off the ignition helps, the T 3 curve helps  but I'm pretty convinced that a complete solution is to go for twin plugging.That's not a very cheap answer though !

What springs do you have in your distributor?   

The newly stamped timing mark on the lightened flywheel is basically for static timing setting for engine to run. I have not marked any newer timing marks with paint or punch because the best operation has not yet been achieved.  I have varied the distributor position to address the detonation with only nominal results- distributor position alone isn’t the issue.

Not interested in twin plugging.  Swapping back to a stock camshaft and flywheel will happen first.

[blackcat]

My 81-CX has these modifications: Milich lightened flywheel, 36mm carbs, B-10 cam, timing gears, 1000CC Gilardoni cylinders/pistons and a Dyna ignition. Quite frankly I don’t remember if I ever changed the advance springs but I’ve never had any detonation issues since I made these changes.

[czakky82]

What about setting squish?
Some do sealant instead of base gaskets. Some do thinner copper head gaskets.

By my mind you’ve got a longer stroke, bigger bore, less advance than your V7sport. Yet you’re detonating. Spark is stretched too thin across the bore. Tighter squish, better swirl, less area to ignite.

[Frenchfrog]

What springs do you have in your distributor?   

The newly stamped timing mark on the lightened flywheel is basically for static timing setting for engine to run. I have not marked any newer timing marks with paint or punch because the best operation has not yet been achieved.  I have varied the distributor position to address the detonation with only nominal results- distributor position alone isn’t the issue.

Not interested in twin plugging.  Swapping back to a stock camshaft and flywheel will happen first.

I have a sachse  crank mounted ignition.You can change the curves  in seconds.
I can't see how the flywheel or camshaft would effect detonation ....it's an ignition issue.

[blackcat]

"...higher compression performance pistons"

Does your engine have the high dome pistons?

[cliffrod]

I have a sachse  crank mounted ignition.You can change the curves  in seconds.
I can't see how the flywheel or camshaft would effect detonation ....it's an ignition issue.

Compared to my V7 Sport, this engine accelerates very differently which I believe to be a result of the modified flywheel.  Same with downshifts, it’s much easier to blip throttle & downshift without typical matching engine speed to road speed & chosen gear to avoid rear wheel issues.  It feels more like a later Ducati twin or 4 cylinder ujm than the heavy flywheel Guzzi Vtwin that I know.  When my two bikes are ridden back to back, it’s very noticeable to me.

This engine has a dyna ignition (like my V7 Sport), stock PHF30’s with blended intake manifolds on non-ported heads.

What about setting squish?
Some do sealant instead of base gaskets. Some do thinner copper head gaskets.

By my mind you’ve got a longer stroke, bigger bore, less advance than your V7sport. Yet you’re detonating. Spark is stretched too thin across the bore. Tighter squish, better swirl, less area to ignite.

I have considered it may be related to gasket issues.  The top gasket appears to be a regular composite head gasket.  Base gasket looks like an aluminum shim gasket that’s probably 50% thinner than the regular composite base gasket I have on my V7 Sport.  That may the problem.  With the detonation issues at this point, I’m reluctant to tighten it up even more.  Changing distributor springs is easier, cheaper and most easily reversed as an initial fix.  No matter, if compression was too high based upon cylinder assembly, I would expect  detonation more consistently vs only when I really grab the throttle.

"...higher compression performance pistons"

Does your engine have the high dome pistons?

The pistons are flat top, not domed like those in my V7 Sport.  I would assume if there were higher compression, they would have either  a dome or noticeable valve reliefs.


The discussion in Guzziology about fast vs slow distributor springs vs tune of engine is what has led me to considering spring changes.  He talks about how counterintuitive it seems to have a faster advance configuration on a low performance engine.  He also talks of how most performance-minded people would assume that a performance engine would benefit from a faster advance when the opposite is true for these engines.  The original fast springs (if still installed) may simply be fatigued and allowing advance to happen even more quickly.  That problem/remedy is also discussed in Guzziology.

Edit- my first assessment would normally be to blame bad/poor quality gas.  But this ethanol-free gas comes from the same can as the gas in my V7 Sport.  I know it has domed pistons and it doesn’t ping at all on this gas.

[blackcat]

"Base gasket looks like an aluminum shim gasket that’s probably 50% thinner than the regular composite base gasket"

I'd change that and see what happens.

Does this detonation happen in hotter months or does it consistently happen no matter what the temperature?

If you want to spend some money you should consider a pair of 36mm Delortos.

[cliffrod]

Can’t say much about hotter vs colder temps yet.  It’s been a lean year sculpture-wise, which relegates bike stuff to not as important as other stuff status.  After a busy year+, right after I got this bike, the stone business hit the wall industry-wide.  It’s getting better but still not back to normal yet.   gaskets will probably get changed to see if that helps if springs don’t make a difference..

Carbs have been on the goodie list.  I missed couple of new pairs of 36’s last year, one pair at the meltdown right after I got the CX and another pair later on fbmp.  Both sets were in $250-$300 range….  Oh well.

[blackcat]

"I missed couple of new pairs of 36’s last year, one pair at the meltdown right after I got the CX and another pair later on fbmp.  Both sets were in $250-$300 range….”

Careful, new 36’s for that price just might be fakes. Normal price for 36mm and 40mm are around $300 each.

[cliffrod]

That’s exactly why I skipped the first pair- seemed to good to be true because I know they’re much more $$ than that. I understand some are not worth having and didn’t know exactly how to id them on the spot.  The second pair had a much better history and seemed legit but I was the second responder so missed the deal.

[John A]

You can measure the squish without disassembling the engine by using a length of soft solder in the plug hole, being mindful of the valves, and turning the engine over to squish the solder .

[spmoto]

I haven't seen anyone mention this yet, so, have you made sure the distributor is sufficiently lubed and everything is moving freely? It's an easy thing to skip over. Simple enough to pull the distributor and give it a good check-up and cleaning etc. If, as you say, the bike hasn't been used much I'd certainly start with the basic maintenance stuff first. Cheap too.
Steve M

[80CX100]

Hey Cliffrod; I'm sure my CX has the stock advance springs,set to static idle;but as I shared a while back in a pm I went overboard doing the electrical,new everything,dedicated relay feeding Blue Bosches,etc;never a detonation issue.

Fwiw I know from installing fuel maps, that were originally developed for fuel down in your area,that didn't seem to run quite right in my bike;not all ethanol free gas is or burns the same.

It sounds like your bike was hot rodded within budget restraints.

From talking to some VRRA racers one of the first cheap tricks they like to do is mill a little material from the base/top? idk off the cylinder iirc & fitting a thinner gasket like yours has, raising the compression ratio,they consider it free hp.

Just for shits & giggles some day, before you chase all the possible gremlins;drain the fuel & run a can of higher octane aviation fuel though it if you have an airport close by.

Maybe all you need is thicker base gaskets to lower the compression.

fwiw idk good luck

[cliffrod]

It was hot rodded.  I just don’t think it was done very conscientiously.  The owner didn’t know what to do or even what had been done.  He just paid for it.  What the man who built it for him knew, who knows?

Some quality high octane gas is on the list of things to try, but I want to fix the issue instead of being stuck needing AV-level gas.   Like I said, I would be suspect of gas being at least part of the issue but know the same gas is fine in my V7 Sport. 

Knowing some here have hot rodded their engines without changing distributor springs is good info.  The different base gasket may be a big part of the problem.  Looks like I’ll finally have some free time this week & weekend to work on it.

[Scout63]

Is the flywheel set up right Clint?  I wonder if it’s possible to check static timing with a degree wheel on the crank nose. I haven’t thought this through…

[chuck peterson]

Hi test or Regular octane?

[cliffrod]

Flywheel position/installation is fine, Ben.  The new prick marks for timing are not perfectly placed like factory marks.  Not a big deal. They work well enough for allow intial timing with minimal adjustment for instant engine starting.  When things are right, I'll do better timing marks.

Gas is not hi test but if inadequate, it should be producing comparable detonation in my V7 Sport.  It's not.

Planning to work on it over the next few nights and Sat.  Backordered parts are finally here and i have some rare solo time.  If distributor springs don't resolve issue, I'll probably install regular base gaskets after i do some measuring.  Will measure camshaft as well to see if its anything noteworthy or possibly even stock-spec, regardless of what po said.

[guzzisteve]

I would raise needle in carbs & see if it has an effect, if you haven't done that yet.

[cliffrod]

Update..

When I removed the distributor, what appeared to be original fast springs (correct for bike) were in place.   Both were noticeably stretched compared to the new fast springs I had. 

One had a nearly coil-wide gap between two coils-





the other was simply gapped wider between every coil vs the new one-





Still, based upon the engine upgrades and excessive spark knock, I went ahead and fitted the slow V7 Sport/Lemans springs.  It ran great in the shop. 

Yesterday, Diaphragms in both carbs were replaced during strip & clean double-checking.  Nearly brand new K&N pods from a WG member (sorry I can’t remember his name right now) replaced the old hard & hammered ones.  It started and ran good until I went for a short ride.  It promptly died…. Would not start on key, bump start, nothing.  After 15 min, it did start but was now breaking up badly- seemingly more electrical.  Couldn’t find anything obvious and quit to make dinner.

This afternoon, I started chasing the issue.  When I removed side cover, the single green wire lead to spade terminal on the dyna coil had been neatly pulled from the original heat shrink & crimp female connector.  Looking closer, it appears that the terminal had heat-shrink tubing addressed but was never been crimped from new from Dyna.  Whether this could have been a basis for ongoing untraceable gremlins  ever since the full rebuild by PO that led to subsequent limited use, ??  It seems likely. 

Wire was easily replaced back into terminal and properly crimped (and others checked as well) and all is good.  Detonation issue is resolved.  I’m running the same gas from same can as before.  Bike runs fantastic, to say the least.  Even if it won’t carry it forever, I have no doubt that it will lift the front wheel.   I still need to swap out the rear wheel, but it’s here and waiting.

Aside from how it sounds when cranking before it starts, it’s a lot different than my V7 Sport.   Even if a CX100 isn’t a “real” Lemans, I’m pretty happy.  Pretty sure this isn’t how a plain CX100 runs





And thanks again to Ben Scout63 for the one-stop-shop box of parts.  You and your generosity really helped make this bike come together for me.  I won’t forget it.

[guzzisteve]

Hey Nice you found the issues. I'm sure it runs like a normal LM with the mods, maybe better!

[chuck peterson]

Pre-ethanol gas?

What would happen w a few gallons of race gas and a VR46 tuneup for a few days …

Crudded up piston crowns?

[cliffrod]

Thanks for your advice, Steve.    Nice knowing that you’re just a call away.

Pre-ethanol gas?

What would happen w a few gallons of race gas and a VR46 tuneup for a few days …

Crudded up piston crowns?

No crud in this engine, Chuck.  I don’t believe engine is even well broken-in yet, with maybe 500-1k miles since the full top end rebuild.  These results are all Guzzi-Content using the same 87(?) octane ethanol-free gas that feeds my lawn mower.  If I spend up to the good stuff, I’ll probably be even happier.

[czakky82]

A worthy owner. Thanks for the update