CRF Stroked and bored

[PAULRY]

I have just nicely assembled the bike with an Athena 100mm cylinder kit and a Falicon 4.3mm stroked crank.

I have been trying to get the motor to start, very unsuccessfully. I have checked the TDC marks and both cam and crank are aligned. The motor is running approx. 12.6 to 1 compression ratio up from 12 to 1, so I reduced the clearance on the auto decompressor from the 450 setting of 0.63mm to 0.55mm. The motor fired to life for about three revs of the crank and stalled.

I am not sure whet the problem. I think it stems from increased compression and incorrect jetting. Do I need to jet richer or leaner? Will this have help with the starting. Is reducing the clearance on the auto decompressor a step in the right direction. I would appreciate any help.

In anycase I will keep you posted once I have ridden it and as to the reliability issues.

 

[nephron]

This could be totally wrong (OK, thanks nephron...then what the hell are you responding for? ;) ), but I think based on statistics alone....well, then add personal experience, I'd bet on valvetrain. If it was mine, I'd go back through it this way:

1) Lash (or 'equivalent')
2) Chain sprockets lined up
3) Piston/valve clearance

I doubt your stock jetting would make your bike unstartable.

Consider racegas--BigLou's 450 had big probs with fuel sensitivity and starting.

Got a plug in it? :laugh:

 

[MrLuckey]

Consider racegas--BigLou's 450 had big probs with fuel sensitivity and starting.

If I remember correctly Lou never got the 426 to start with race gas.

 

[biglou]

It's a long story, thread's in here somewhere, but bottom line was I could not get the 426 to run with oxygenated race gas, and believe me, I tried my little heart out... It did "ok" with the non-oxy stuff, but I couldn't get enough out of it to justify the cost.

I'd say if you're trying to fire it on the good stuff, go back to pump swill till you get it running steady, then make changes from there.

 

[PAULRY]

Yesterday I got the motor running, not by kick starting but by push start. Runs fine once started. Ran it for 15 mins at small throttle openings, seems to have good torque.

Nephron, I have been through everything I can think of and the only thing left has to be the valve train. Is it possible that the cam could need degreeing for the different stroke/displacement.

Thanks for the replies.

And NO, there is no plug in it. I checked both inlet and outlet, since this has happened to me before.

 

[DEANSFASTWAY]

Maybe IGN timing needs to be advanced a bit . Cant really do it easily on the 450 but maybe you could use a offset key . I feel the TRX 450 seems to need more timing as its not really all that snappy down low with less comp than the CRF. Good Luck

 

[evenslower]

Maybe I'm over-simplifying but if you got it running for a bit and it "seems to have good torque", how can it be a valve train issue. If you can start it by push starting and not kickstarting I would tend to go with fuel/air/jetting/carb issues. If that seemed kosher maybe ignition issues. Checked all wires and connections?

But then again what do I know.

 

[bclapham]

if you have increased the bore and stroke then the signal should be bumped quite a bit and maybe its running too rich on the pilot circuit to start up easily- i know, rich pilots are usually good starters, LOL.

you might want to call Big Gun about the carb setup- i know they built a crazy CRF big bore/stroker since ive been trying to get my friend that tests for them to get them to bring it out for us to have a go on!!! I bet they made a custom pipe for it also......:)

 

[PAULRY]

Since yesterday, I have rechecked the valve train, this is all in spec. The head has recently been done, the valves sank into the head so I have replaced the valves and springs with the Kibble White setup. I am now back to fueling as pointed out by 'evenslower' and 'bclapham'.

I spoke with a local tuner this side yesterday and he pointed out that restroking the crank would put the cam approx. 2 to 3 degrees out, which can be corrected by degreeing the cam. This he however feels will not account for the 'not starting' issue. From his experience when stroking and increasing the bore size, the jetting on the pilot or start circuit becomes critical. With the increased displacement the motor is drawing far more air into the cylinder and consequently more fuel due to the venturi effect, so I need to lean out the pilot circuit. I am currently at my leanest pilot jet (40) and waiting for leaner jets.

Will keep you posted on my progress should any of you wish to go down the same road and thanks for the advice.

 

[bclapham]

that sounds logical- you might want to search some older threads here- a guy made a KTM520 into a 600 or something, they run a similar carb so he might have seen a similar trend.

 

[JYD]

I have just nicely assembled the bike with an Athena 100mm cylinder kit and a Falicon 4.3mm stroked crank.

I have been trying to get the motor to start, very unsuccessfully. I have checked the TDC marks and both cam and crank are aligned. The motor is running approx. 12.6 to 1 compression ratio up from 12 to 1, so I reduced the clearance on the auto decompressor from the 450 setting of 0.63mm to 0.55mm. The motor fired to life for about three revs of the crank and stalled.

I am not sure whet the problem. I think it stems from increased compression and incorrect jetting. Do I need to jet richer or leaner? Will this have help with the starting. Is reducing the clearance on the auto decompressor a step in the right direction. I would appreciate any help.

In anycase I will keep you posted once I have ridden it and as to the reliability issues.

what is an athena 100mm cylinder kit? do they make them for yamaha?

 

[PAULRY]

Well this past weekend, I finally got to ride the bike and what an experience. The bike has unbelievable amounts of torque. It pulled third/fourth gear around most of what I would call a tight track with phenominal acceleration. It has definitely made the bike easier to ride.

As for the stats, 55.3HP at 8600RPM and 52.4NM. I have been informed that the bike is not running optimally. The exhaust is restrictive, the spark is weak, the CDI could do with re-programming and finally a higher lift cam for improved flow.

As for reliability issues, it is to early to tell. On this front I will keep you informed.

Reliabilty issues aside, I would recommend anyone to try this setup, provided you have the knowledge to get the bike setup or know someone who can get it running. I have lost alot of hair trying to get the bike to start with the kick start and eventually had to call on people with more knowledge than myself to get it running. The major problem I am told was ignition timing, poor spark (not yet rectified) and jetting.

 

[PAULRY]

JYD,

Athena do make a kit fot the Yamaha. It consists of a top end kit (100mm diameter piston), a cast cylinder to suit and a gasket kit. The cylinder casting and piston are well made and on a par with the stock Honda product.