Topic: big engine trouble

[phi]

Hi guys.
Recently I bought a new bike, a K5. The head gasket was leaking, so I pulled the engine to fix it. It turned out that I have a bore on 67 mm., which adds up to somewhere around 900 cc. plus the head cc.
 However happy I am about this, it has after reassembling given me a new problem: at around 5000 RPMs the engine backs out and start to stumble. I suspect it`s not getting enough air, as the carbs are original with stock jetting. The question is: Do I need bigger carbs to get this monster running decent, or is it possible for me to re-jet the originals to a usable degree.
The cam appear to be normal, the head has been polished, the valves are stock size.
I'm looking for torque and reliability, not neccesarily top end power.

[MoMo]

Did you have this problem before replacing the head gasket?....Larry

[phi]

I never rode the bike before I bought it because of the blown gasket.

[MoMo]

If the bike hadn't been ridden for a while, the carbs most probably need a complete cleaning. Timing is spot on? What clearance did you set the valves to?  Stock exhaust?  Stock air filters? Make sure the routine things are good before you go fiddling with the jetting. 

[MCRider]

Lots of other issues per Photolar, but basically, you should get the stock carbs to work, rejetting definitley necessary.  FYI:  CCs in the head are not counted as they are constant. The measurement is for "displacement" so its the number of cc's displaced that marks the engine size.

Yours is 888.8, (as is mine, but i am running VM29 Mikunis) which is often rounded to 900 for marketing purposes. That size with stock cam and valves, and a ported head and small carbs, would favor torque mid-range over higher RPM performance, I think. Should be a sweet ride.

http://www.nightrider.com/biketech/calc_displacement.htm

[phi]

The valve clerances are set to stock meassurements, 0,5 mm for intake and 0,8 mm for exhaust, the timing and point gaps are spot on, 0.35 mm. It has a 4-1 exhaust and stock airfilter. every thing else is stock.

[MCRider]

The valve clerances are set to stock meassurements, 0,5 mm for intake and 0,8 mm for exhaust, the timing and point gaps are spot on, 0.35 mm. It has a 4-1 exhaust and stock airfilter. every thing else is stock.

You may be lucky. If the faltering begins at 5000 rpms, that's about where the main jet kicks in. They are easier to change. Guessing you'll need to go up a step or two. If 130s, go to 135 etc. Main jets are in increments of 5.

Trial and error or more "professionally" , read the plugs.

But strictly speaking, the slow jets, (low rpms), the needle postion (mid rpms) and the main jets (higher rpms) may all need tweaking.

[mlinder]

Check ignition timing.
Check ignition advance.
Check cam timing.
Check for carb boot leaks.
What are you running for a filter and airbox? Anything?

[phi]

the main jets are 120 as stock. Where should I start? 140? 150?
Would I gain anything by replacing with cb 900 carbs?

[mlinder]

the main jets are 120 as stock. Where should I start? 140? 150?
Would I gain anything by replacing with cb 900 carbs?

Check and answer these questions:

Check ignition timing.
Check ignition advance.
Check cam timing.
Check for carb boot leaks.
What are you running for a filter and airbox? Anything?

If you are running stock intake, stock jetting from the k0's (120's, which you have) is fine. Could even be rich, with an otherwise stock k5.

[MCRider]

the main jets are 120 as stock. Where should I start? 140? 150?
Would I gain anything by replacing with cb 900 carbs?

No I'd go a step at a time. 125s are next.

Don't know if the CB900 carbs are a bolt up switch or not, doubt it. You want torque, I'd stick with the smaller carbs, generally speaking.

[mlinder]

the main jets are 120 as stock. Where should I start? 140? 150?
Would I gain anything by replacing with cb 900 carbs?

No I'd go a step at a time. 125s are next.

Don't know if the CB900 carbs are a bolt up switch or not, doubt it. You want torque, I'd stick with the smaller carbs, generally speaking.

If his intake system is stock and he has 120's on a k5, it's probably slightly rich, depending on his altitude.

He needs to check the things I told him to, and comment on his intake setup, before rejetting.

[MCRider]

the main jets are 120 as stock. Where should I start? 140? 150?
Would I gain anything by replacing with cb 900 carbs?

No I'd go a step at a time. 125s are next.

Don't know if the CB900 carbs are a bolt up switch or not, doubt it. You want torque, I'd stick with the smaller carbs, generally speaking.

If his intake system is stock and he has 120's on a k5, it's probably slightly rich, depending on his altitude.

He needs to check the things I told him to, and comment on his intake setup, before rejetting.

Well i agree with checking those things out. Assuming they're OK... he's got a stock air filter, ported head, 4-1 pipe, with 888cc.  Never seen a big bore with a pipe that didn't want more fuel.   I vote he needs to go richer, but hey its not the hill I want to die on. Easy to try both.

FYI: Avg altitude of Denmark is about 150ft. Highest point about 500+ ft.

[phi]

I may not be expressing myself accurately, for that I beg for forgiveness. As you may have noticed, English is not my native language.
I have checked and adjusted the engine, then rechecked. It starts by the slightest bump of the starter. it idles smooth and takes the gas without hesitating or stumbling. It runs perfect, the torque is strong and accurate.
But when it under load reaches 4500-5000 RPM, it starts coughing and is no longer responding. When I lower the RPMs it will again run smoothly. Maybe the advance, but when checking, it seemes to respond as it should. Maybe the ignition, but it is timed as it should be. The intake is as mentioned stock, even the filter. The boots are brand new, and checked with etherspray.

[mlinder]

Isn't stock jetting for a k5 a 105? He's at 120, which aint stock. Bigger bore with stock cam and intake can actually cause the mixture to run richer with stock jetting. Higher vacuum with the same intake restrictions will often pull more fuel through the jets.

[mlinder]

I may not be expressing myself accurately, for that I beg for forgiveness. As you may have noticed, English is not my native language.
I have checked and adjusted the engine, then rechecked. It starts by the slightest bump of the starter. it idles smooth and takes the gas without hesitating or stumbling. It runs perfect, the torque is strong and accurate.
But when it under load reaches 4500-5000 RPM, it starts coughing and is no longer responding. When I lower the RPMs it will again run smoothly. Maybe the advance, but when checking, it seemes to respond as it should. Maybe the ignition, but it is timed as it should be. The intake is as mentioned stock, even the filter. The boots are brand new, and checked with etherspray.

You've checked the engine.

Does this mean you've checked that the advance is working?

[mlinder]

Moved here for more help for this fellow.

[Danno]

i would move the jets back to 110 and did you say 4 into 1 exhaust most 4 into 1 are restrictive (tight) you may need to drop it back even further to 105 but i doubt it but the problem you are having sounds like a combination of too big a main jet and too restrictive exhaust on the plus side it sounds like the slow jet is right on but i would lean out the idle screws a half turn this may help make up for the restrictive exhaust

[dave500]

i think the slow jet is too rich,try the next size down,,its easy to try without resync etc,,the stumble of richness can feel like a lean out,especially if you open the throttle and it goes good once again,the rpm at those revs easily equal an only partly open throttle with no load,under load itll go ok.,id go for the stock main jet and one lower slow jet.,jets dont "kick in" at a certain rpm,,they kick in at throttle position,do you have the stock air box and filter or separate pod type?

[Gearheadgreg]

What condition is your Air filter Old or New ? stock or Pods?  Edit you didn't comment if your Air filter was New just it was stock ? Mine was stock also when i purchased my F3,.......Can't imagine why the PO was having tuning issues,,,,

[Tintop]

Gee guys, try reading the thread, the OP has answered all the setup questions multiple times.

My .02 - try running it with the filter removed from the stock air box.  An older stock paper one can look great, and still cause excessive restriction because the element has absorbed water.

[bryanj]

If you have really set the valve clearances at .5 and .8 thats 10 times what they should be

[MCRider]

If you have really set the valve clearances at .5 and .8 thats 10 times what they should be

He stated .5MM, I'm assuming he dropped a 0 as .002" = .05MM.  He's from Denmark = metric.

[sick_seven_fifty]

If you have really set the valve clearances at .5 and .8 thats 10 times what they should be

He stated .5MM, I'm assuming he dropped a 0 as .002" = .05MM.  He's from Denmark = metric.

I think he understood that it was in metric...that's why he said it was ten times more than it should be. (.05x10=.5)

[MCRider]

If you have really set the valve clearances at .5 and .8 thats 10 times what they should be

He stated .5MM, I'm assuming he dropped a 0 as .002" = .05MM.  He's from Denmark = metric.

I think he understood that it was in metric...that's why he said it was ten times more than it should be. (.05x10=.5)

Oops you're right.  Apologies.

So I'm back to assuming he said .5 and meant .05mm and as we all know we shouldn't assume.

[mlinder]

Phi:

Is your advance working properly?
Are we assuming you left out a 0 after the decimal point correctly?
Are you running on all 4?

Lets get confirmation on these things before you start rejetting..

[phi]

I'm sorry, I have left a zero out, the valve clearance is of course 0.05 mm (0.002 in) and 0.08 mm (0.003 in)
You may have a point with the airfilter, I have just cleaned it with pressurized air and inspected it. A new stock airfilter is on its way.
In the meantime I have read the plugs by killing the engine on the troublespot, and they are white. I need more gas, not more air as I assumed. I have, based on this knowledge, dropped the 900 carb idea and start by jetting the original. It is definitely not running too rich, so I will try to bump the slow jet one step up, and the main jet 2 steps up.
Just interesting info: I wondered why the bikes pistons and valves was fine with so lean a mixture, so i contacted the seller. It turned out that the bike had been running some big CR carbs in the past, but those had been used for something else, and had, without further concern, been replaced with the stock ones and never run again. No wonder it is lean.
Thank you for help and inputs.

[willbird]

Can you raise the needles ? My bike when I put the header on would act sort of like yours does, the plugs were white, raising the needles one notch cured the lean condition.

In my case easing in some choke made the problem go away at cruise speed (55 mph) so I suspected a lean condition, checking the plugs confirmed it.

Bill

[phi]

Update:
This is magic: I raised the needles one step from the original position, and the problem spot raised to around 6500 RPM. A 130 main jet took care of the rest. I am one happy guy! Now I just need to adjust the embarrasing high idle when hot. Adjusting the airscrews will probably take care of that.
I feel like a evil engine genious right now, so once again, thank you all for inputs and ideas.

[MCRider]

Update:
This is magic: I raised the needles one step from the original position, and the problem spot raised to around 6500 RPM. A 130 main jet took care of the rest. I am one happy guy! Now I just need to adjust the embarrasing high idle when hot. Adjusting the airscrews will probably take care of that.
I feel like a evil engine genious right now, so once again, thank you all for inputs and ideas.

You ARE the evil genius (positive reinforcement). 

Those big cylinders just needed a little more go juice.  Have fun.

There is an idle jet in the carb that can be changed IIRC. May need to go there.

Of course all the usual suspects, air leak, hanging cables, etc, should be checked.