Why does my high compression engine keep jumping cam time from kickstarting?

[Chuck78]

Oh, & the cam chain was a stock GS750 chain from my 3 spare engines. I picked the best / least stretched of all of them. Not a single one was anywhere near out of spec. Stock length of course.

I am really hoping it was just user error on my part, and that I can continue running GS cam chain tensioners on high compression builds. I have 2 or 4 more high compression GS engine builds planned for the next 2 years, 2 are kickstart only as well. Tiny battery to power the Dyna ignition & give me lights with the engine off.

[phydeauxmutt]

..., as the cams still had 20 pins between the marks

Hopefully you mean "20 pins INCLUDING the marks".

[Chuck78]

Hopefully you mean "20 pins INCLUDING the marks".

Of course..."20 pins between the marks INCLUDING the pins at the marks"

Not all GS's have this same measurement. The GS550 factory manual showed 23 pins I believe, until I realized I was looking at the page for the 83+ models! Looked at the correct page and I believe it listed 20 pins same as the gs750.

GS1000 is probably different as it lacks the idler gear in between the cams. Team Yoshimura kept having GS1000 engine failures in endurance races until they modified a GS1000 head to fit a gs750 type cam chain idler gear between the cams with a custom length cam chain. High rpm extreme race "useage" caused all kinds of violent undulating waves in the chain and caused total chain failure during endurance races.

[phydeauxmutt]

I only mentioned that because there are likely others that made the same mistake that I did. I can't (won't) tell you how many times I treated the link over the #2 mark on the cam as "pin 0" instead of properly calling it "pin 1". By doing that, the intake cam was retarded by one tooth. Still ran, but not quite right, and the pictures in the manuals never matched what I was seeing right in front of me.

Your description of "20 pins BETWEEN the marks" would be yet another tooth off, since you have to count the pins over the marks, making that actually 22 pins.

[GregT]

GS1000 is probably different as it lacks the idler gear in between the cams. Team Yoshimura kept having GS1000 engine failures in endurance races until they modified a GS1000 head to fit a gs750 type cam chain idler gear between the cams with a custom length cam chain. High rpm extreme race "useage" caused all kinds of violent undulating waves in the chain and caused total chain failure during endurance races.

tripivot is right that it's valve spring pressure which would cause snatch loading sufficient to make the tensioner move and the camchain jump.
The high compression has no part in that.
You don't say - as far as i can see anyway - whether the chain is jumping just on the cam(s) or on the crank...I'm curious to know.

The old Yosh thing of adding an idler sprocket to the head of the 1000's is not needed now IMO. Springs and cams have got better and the snatch loads are much lower. We're running our 1000 to 10,800 with no problems in the camchain or manual tensioner area. The stock camchain guides work very well.
I've always thought it was as much to get more wrapround on the cam sprockets as to try and damp resonances - more links on the sprocket lowers the load on a given link. Camchains weren't particularly good then...

[one_civic]

Personally I would just get a manual tensioner or fabricate your own like I did, could be a combination of a slightly stretched chain along with a sticking / faulty tensioner.
Here are some pic's of the one my machinist and I made.

[one_civic]

You didn't mention if your using the stocked degree'd cam's or aftermarket higher lift ones with stiffer springs

[Buffalo Bill]

GregT are you endurance racing 8, 12 or 24 hours?

[Brendan W]

Are you kicking on Prime ? What are the chances you are filling combustion chambers with fuel and locking up ?

[trippivot]

Nice piece you have there - very functional.

personally i would just get a manual tensioner or fabricate your own like i did, could be a combination of a slightly stretched chain along with a sticking / faulty tensioner.
Here are some pic's of the one my machinist and i made.

[GregT]

GregT are you endurance racing 8, 12 or 24 hours?

Short sprint races - 20 minutes max. But the camchain guides and tensioner blade get checked every teardown. If there were harmonics present it would show up pretty quickly at those revs, The camchain gets replaced now every two seasons racing...even though it's still within spec. DID heavy duty for those interested.

[Chuck78]

Pulled tensioner knurled nut off today as I felt the tension was inadequate, wound it 1.5 turns tighter preload, feels much better now.

Stock GS750 cams. Resurfaced head and cylinder block, thought I should degree the cams on engine rebuild, but didn't yet. I notice my arrows don't quite line up as they show in the manual, I suspect due to the resurfacing milling job I had done on both.

I thought I had a stripped thread on the tensioner spring winding shaft, felt as hard to loosen as tightening. It turns out I must've used some serious threebond semi-permanent hitemp thread locker on it as I recalled after the fact. I got it for internal engine parts. Hope I am able to get those apart again if I ever need to! That stuff was seriously stout...

I noticed the gs550's tensioner felt slightly firmer on the spring than mine was, but also noticed binding stiction on the wedged part. I recall reading bwringer's tutorial saying that often the wedge angled ends get worn down unevenly and need a light sanding or filing to clean up.

Should be checking valve clearances and firing it up tomorrow! My excuse to ride Saturday is so that I can get parts for the wife's bike at the vintage Japanese salvage shop... 66 degree Ohio winter day, can't wait!