Express SR cuts out and dies at full throttle under load

Jack Rutherford /

I guess this would be repair. Could be performance.

Anyhow, I've been helping a friend with his Express SR. In the quest for performance, things went south. I got it running and it ran pretty well. We added a somewhat better than stock exhaust - a tecno boss pipe. Ran well with that. We decided to raise the exhaust port. So we did so 1.6mm. Went ahead and replaced the piston and rings and honed the cylinder. I had to go with 2 base gaskets to seal up an air leak so I guess that pushed the cylinder up a tiny bit. Compression is 130 with those two base gaskets.

I heat cycled it 5 times and had trouble keeping it running because previous owner had removed the auto choke system. After I got sick of screwing with keeping it running for the heat cycles, I put the auto choke system back on. It still required too much monkeying around with carb settings to get it to idle right so I popped on another stock carb which seemed to help.

took it out on several break in rides and finally got it to run ok on the last couple. Now it wants to die under load and at full throttle. We've tried 65, 70 and 75 main jets (65 is stock on this bike) without any real changes. Could the crankshaft seals have just taken a complete and sudden dump? Damn thing!Edited 1 time(s). Last edit at 12/23/18 02:14PM by churioz.

Re: Express SR cuts out and dies at full throttle under load

Jack Rutherford /

85 jet seems to have fixed it

Re: Express SR cuts out and dies at full throttle under load

By the way...when you raised the exhaust port you also lowered your trapped compression ratio. http://www.torqsoft.net/compression-ratio.html

Re: Express SR cuts out and dies at full throttle under load

85 should have been stock on the SR unless it was an Iowa or something. With more port timing you'll need to go higher still.

Re: Express SR cuts out and dies at full throttle under load

Jack Rutherford /

> DPC Ryan Wrote:

> -------------------------------------------------------

> 85 should have been stock on the SR unless it was an Iowa or something.

> With more port timing you'll need to go higher still.

that would explain a lot. for some reason I thought I read somewhere that 65 was stock on it. babbittsonline lists stock main as a 65.

Re: Express SR cuts out and dies at full throttle under load

Jack Rutherford /

> Den 23 Wrote:

> -------------------------------------------------------

> By the way...when you raised the exhaust port you also lowered your

> trapped compression ratio.

> http://www.torqsoft.net/compression-ratio.html

maybe i'm looking at this wrong but i think i would have increased it. what the formula

Re: Express SR cuts out and dies at full throttle under load

> Jack Rutherford Wrote:

> -------------------------------------------------------

> > Den 23 Wrote:

>

> > -------------------------------------------------------

>

> > By the way...when you raised the exhaust port you also lowered your

>

> > trapped compression ratio.

>

> > http://www.torqsoft.net/compression-ratio.html

>

> maybe i'm looking at this wrong but i think i would have increased it.

> what the formula

You're kidding, right? You didn't even click on the link?

Re: Express SR cuts out and dies at full throttle under load

Jack Rutherford /

I clicked on the link but they don't provide you with the formula just places to plug in values. Whatever the formula, I guess the distance from top of exhaust port to end of cylinder is not in the denominator as it appears that I did reduce trapped compression ratio - slightly. thanks

Re: Express SR cuts out and dies at full throttle under load

Jack Rutherford /

anyhow, it ran with the 85 jet but not like it should - according to my friend. He tried a 90 jet and he couldn't tell a difference but I think it ran worse. So I tried an 80 jet thinking maybe it was too rich and it would just cut out again.

I went ahead and replaced the crankshaft bearing seals. Switched the carb again and it still runs like shit. When you first start it, you can't get it to idle unless you turn the throttle stop screw all the way to the right so that the slide is fairly well open. It acts like it wants more air as it spews out smoke at idle and barely maintains an idle.

Thinking we may have opened the exhaust port too much, I even switched to an unadulterated stock cylinder and no change.

At this point, I'm thinking maybe the reeds are hosed. At idle, it seems way too rich but it would rev like crazy when you coaxed it enough and almost seems to be too lean at full throttle. Or perhaps the exhaust is clogged? any thoughts? timing messed up - but this is fixed timing?Edited 1 time(s). Last edit at 01/14/19 09:42PM by churioz.

Re: Express SR cuts out and dies at full throttle under load

Jack Rutherford /

could the cdi have gone bad?

Re: Express SR cuts out and dies at full throttle under load

After all this rejetting have you put in a NEW spark plug of the correct heat range? May be all that's wrong.

Re: Express SR cuts out and dies at full throttle under load

Go to the dollar store and buy a can of oven cleaner for the exhaust .

Even if it isn't totally clogged , the clean out is cheap insurance and eliminates the possibility of an exhaust 'choke' .

I just fill the muffler as mach as possible and let it set for a couple cups of coffee . Then simply wash it out with a garden hose , a couple of rinses seems to do the trick .

Re: Express SR cuts out and dies at full throttle under load

Jack Rutherford /

I'm going to check spark and try new plug. If spark is poor and new plug doesn't help, i have another coil, stator, and cdi i can try. if spark is fine I'll look into the exhaust cleaning.

Re: Express SR cuts out and dies at full throttle under load

> Jack Rutherford Wrote:

> -------------------------------------------------------

> I'm going to check spark and try new plug. If spark is poor and new plug

> doesn't help, i have another coil, stator, and cdi i can try. if spark

> is fine I'll look into the exhaust cleaning.

That's the ticket,Jack!

Re: Express SR cuts out and dies at full throttle under load

Jack Rutherford /

Ok spark was poor. A new plug and different boot fixed that. Now it starts much easier but still runs poorly. I put on a different exhaust. No change. I check the reeds - they were fine. I pulled out the timing gun - if I did it right - timing is off. I think I did do it right because I checked the timing on an nu50 and it was right on. Different CDI did nothing. So I assume it's the pulse coil on the stator. I'll switch that out next. I guess "fixed timing" is relative on these bikes if some component goes bad.

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