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

[Chuck78]

Alright, 3rd time's a charm... not really. Kickstarting my GS750-920cc for the first time this year, an all too familiar thing happened AGAIN... replaced my stolen VM29 smoothbores finally, with a freshly rebuilt set of VM26SS's. Installed on bike. I kicked engine over previous, felt normal. Put carbs on, petcock in prime, ignition off, I kick it over a few times to get fuel into the pilot circuit. Ignition on, kick it and notice more pressure than without carbs earlier. Kick it 8 or so times, no firing, much more resistance. Then once more, smack... #4 piston stops as it bumps into an open valve....

after the first two times this happened, I made sure to use more than adequate force when kick starting, as it had kicked back on me fairly hard a couple times prior, which I suspected just caused extreme slack in the opposite side of the cam chain from the tensioner, causing it to jump off of the crank sprocket. This time, I never allowed it to kick backwards on me (but I noticed a lot of extra pressure).This the 3rd time this has happened to me since going kickstart-only and installing MTC 10:1 920cc pistons...

I am beginning to wonder if it is pretty much absolutely necessary to convert to a manual cam chain tensioner if running high compression pistons and big bore, or else maybe something is wrong with my cam chain tensioner. This only happens when kick starting, never when running. I never had this happen before the engine rebuild on any of my kick start engines, but this is my first high compression piston set. Did I do something wrong with the cam chain tensioner reassembly? Or is this just a regular side effect of high compression big bore upgrades?

 

[Chuck78]

The only positive of this situation is that it made me realize the massive potential gains of degreeing your cams, as when it jumps one tooth, the bike instantly feels like I just gained 30 horsepower in the mid range, and was doing endless burnouts and pulling acceleration wheelies like I had never seen. Then a few or few dozen kickstart later, it would jump another tooth (trying to kickstart cold at home always is when this happens), then game over, pull tank, top cover, cams, re-time, curse & swear, then ride happily for a while, then repeat... 1.5 seasons on engine as is, 1500-1700 miles maybe on this engine.

 

[Nessism]

Something must be wrong with the tensioner. Don't think jumping time has anything to do with having a big bore kit and high compression.

 

[tkent02]

Don't think jumping time has anything to do with having a big bore kit and high compression.

But if there is, it would explain why the racer types are always advocating manual tensioners, while the stock ones have always worked fine for the rest of us... Interesting.

 

[Chuck78]

I Google searched the forum, reading bunch of old posts.

I'm pretty darn sure I followed these instructions, but noted Steve's advice here on why other members' cam chains got loose and caused issues like jumped time. I will double check this tomorrow:

It is rather common to install the tensioner incorrectly.

Almost everybody understands compressing the plunger, locking it in place, install the tensioner, release the plunger. What they seem to NOT understand is what happens next. After releasing the plunger, turn the setscrew in until it stops, turn it back out 1/4 turn and lock it in that position with the locknut.

Too many times I have seen tensioners that were locked in place, not allowing them to move and do their job.

Also Rusty Bronco and one other member had small "misplaced" parts jammed into their crankshaft cam chain sprocket, such as missing half moon cam bore end plugs from the ends of the cyl head, and a wire spring clip type hose retainer clamp from the breather cover hose. it seems as if this occurrence would cause the cam to jump time very quickly. this is probably not my issue, but doesn't hurt to look down into the tunnel and watch a full rotation of the crank while looking for problems with the crank sprocket.

 

[Chuck78]

it would make sense if my tensioner were stuck and allowing extra chain slack, but as Tom pointed out, it does make you wonder why most race bikes always convert to manual cam chain tensioners despite the fact that the Suzuki automatic cam chain tensioners are known to be far better than the unreliable ones made by other manufacturers of the the same era.

the last two times it happened, I did compression tests afterwards to find I still had around 190psi on all 4 cylinders. I did not do a leak down test, but I don't think that would be necessary. it seems as if it would jump two teeth, because the compression that the engine would produce would be boosted substantially all of the sudden, trapping more charge in the cylinders at mid rpm range, and giving it tremendously more power on the street, as well as noticeably more effort required on the Kickstarter.

The first time this happened, I thought that maybe I had not done a good enough break in on the piston rings, and that after 1000 miles, they finally sealed up really well and were giving me much more compression. That was not the case! then it would jump another tooth in the distant future, ALWAYS WHEN kick starting it from cold. luckily I was just using the kickstarter (deleted my starter motor and all gears, downsized my battery) and so I would fortunately immediately feel the piston to valve contact through my leg kick near the beginning/top of the kick stroke, and stop, as to minimize further damage. If this happened with an electric start with a strong battery, bent valves would be pretty certain... crossing my fingers again here.

 

[Nessism]

If you have never taken your tensioner apart I recommend it. learn how it works. I wound the spring up as much as possible before it coil bound. Push in the plunger and tighten the set screw. Later, when you release the set screw, you should hear the plunger jump out. That's how you know the thing is working as it should.

I think the reason the racers lock the tensioner manually is to keep it from backing out at high rpm. Not during starting.

 

[Buffalo Bill]

Well, frustrating problems, bummer.
The only outside thing you can look at is the tensioner, Ed's advice would be my first step.
The final thing would be taking the cover off, checking the chain tension and watching the action as your assistant kicks it over until it happens.

 

[trippivot]

it is not high compression in the piston area that creates cam chain resistance. It is valve spring pressure that makes the camchain not want to turn when the crank does. but high compression does make the movement more violent/ sharper.

ok so a stock tensioner has a plunger that gets held in place with a lock bolt pushing against it at a 90 degree simple friction lock. or do you let yours bounce in and out? how do I see it?

hold a nail with a pair of pliers steady it against a vice - now hit the nail with a hammer - don't let it slip in the pliers grip - in theory this happens hundreds of times

-- yeah when your camchain curls like the end of a whip it hits your tensioner like a hammer - and the slack is like a tidal wave that slings around your engine -- end result ? chain jumps off of your sprockets - get a ape positive stop - manual tensioner - and don't set it too tight just enough to keep the chain engaged on your sprockets.

 

[sharpy]

Does anyone ever read the manual. Is the cam chain new/right length or one that been in there for 60,000 miles. Yes i know the advantage of a manual tensioner but never needed one, maybe im lucky. Not many people are true experts so always handy to have a look in a manual if not sure....

 

[Chuck78]

Went to bed early, but I couldn't sleep last night after a phone call at 1015pm, so I went out to the garage again to see where the tensioner was at. Broke the locknut loose, turned in setscrew and it stopped at less than 1/8th turn in... I went by bwringer's writeup based on the manual, but I question now how careful of attention I was paying when I clamped the locknut down. From memory I thought it was 1/4 turn. I see now it says 1/4 to 1/2 turn out.
I hope this is why the thing wasn't doing its job properly, hopefully all user error. I barely got any travel on the kick stroke this time it happened, so I'm pretty confident that if the valves weren't bent before, they aren't now, but I should check again I suppose. I was previously getting almost 190psi on all 4 cylinders. That's on a cold engine with no starter motor, kicker only. I was pretty ecstatic at both that cold kicking figure on its own, and not having any noticeable compression loss after smacking the piston into at least 2 valves!

I am still considering a manual tensioner, but less seriously now. We'll see how this does

 

[Big T]

Since you may have locked your tensioner in place for several years, I'd recommend a rebuild to be sure it's in top working condition, rather than just readjust it

 

[Chuck78]

I fully rebuilt it at the time of the full engine rebuild
EDIT: 1.5 seasons ago, less than 1,700 miles.

 

[Guest]

OK, I am a bit confused now. Do you have a stock tensioner or a manual one installed?

If it is stock, the 1/8th turn of looseness isn't bad, you just don't ever want to go more than 1/2 turn.

If it is manual, I would think that you don't want ANY slack on it, but I have never had one, have not 'needed' one (yet).

 

[Chuck78]

it is a stock tensioner. When I said less than 1/8 turn on the set screw, it was really closer to 1/16 of a turn, a hair more. Manual says 1/4-1/2 turn. I am hoping this is why I had the trouble I am having. Cleaning the top cover off from lack of front fender (110/90-18 tire barely fits in 175mm fork width without a fender). getting ready to pull the cams & retime. I can't tell how far they are off because I can't rotate the engine to the proper position, but if I recall, it definitely jumped off of the crank sprocket previously, as the cams still had 20 pins between the marks

 

[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.

 

[Guest]

..., 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.

 

[Guest]

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...