75 XS won't REV! - Help me troubleshoot!

Enginerd

OK gents.

I have a 1975 XS 650, all stock except for some aftermarket slip on megaphones and a Boyer ignition.

I bought the bike in about 1995 for the princely sum of $150. I put a new chain and tires on and have ridden it for the past 15 years with nothing but routine maintenance.

However.......there is a problem. Confused

When I first bought it, the bike would max out at about 5500rpm, which was 100mph, although I had only turned the throttle 3/4 of the way. There was no more speed left in her.

Every year since that first year, the bike has gotten incrementally slower. The top speed is now 85mph at about 3500rpm. That is only about 1/2 throttlew to reach 85mph. The engine refuses to run faster than that, whether its under load in 5th gear, or revving freely in 3rd gear. It just wont REV beyond 3500. Mad

I removed the points and installed a Boyer Brandsen ignition, and it made no change. I have checked the exhaust to ensure it is not plugged. I tried a complete different set of 34mm carbs from a 78 XS with no change.

The bike starts and runs PERFECTLY. It will kick start with 2-3 kicks after having sat over the entire winter. Its great for around town, and is super tourqey, but it just wont rev.

What do you think Question

kingwj

Is it breaking up or just won't pull?

I think I'd look at advance of the ignition. Hook up a timing light and rev the bike looking at the mark.

xs1961

Actually sounds like a duff rev-counter(tacho) ..100MPH at 5500 ?! 85MPH at 3500 !!!
I only get 87MPH at 5000 with 18/30 sprocket set-up..

nudude53

xs1961
I'm with you on this one, I'm running a 17/32 and pulling 3100 at 55 mph

Enginerd

nudude53 wrote:

xs1961
I'm with you on this one, I'm running a 17/32 and pulling 3100 at 55 mph

My memory must be playing tricks on me. In actuality, my bike must be geared similarly to yours, as I am really at 3000rpm at 55mph. Rolling Eyes

Sorry.

Enginerd

kingwj wrote:

Is it breaking up or just won't pull?

I think I'd look at advance of the ignition. Hook up a timing light and rev the bike looking at the mark.

Its not breaking up. It just won't rev. When I installed the Boyer ignition, it seems like the advance "looked" about right. Doesn't the Boyer ignition advance the spark timing electronically? It should do what the springs and weight did with the points setup, right?

gordo

It sound's like a flow restriction, Easy test, unbolt the header's & go for a very short ride,testing the limit of the rev's then.These bike's stock exhaust is a two pipe deal, one inside the other, If the inside has collapsed or rotted alot, enough to partially block the exhaust, that kind of problem happen's.When I worked at Ford back in the '80s, we got alot of call's from the dealer tech's about this problem, come to find out, that was when we started to farm out the exhaust pipe's, & the inner pipe was ,shall I say, less than desireable quality,& just the heat of the engine exhaust would 'melt the pipe, slowly closing the pipe.Our simple method test was to roll a golf ball through the pipe,if it came through, all was well with the pipe,and were then telling the tech's to check ign. advance, fuel flow,ect. try that, & let us know, Gordo Ps. Iread you used another set of carb's, but what kind of shape are the fuel line's in?Should be at least 1/4",. Smile

jimmythetrucker · Full Member Joined: Mar 12, 2008 Posts: 1283

I'm with XS1961: It's a tach/speedo problem. 5500 revs @ 100 MPH is ridiculous. An XS650 Special with stock (17x34) sprockets and a 16-inch rear wheel is tacho-redlined at about 100-105 mph. The little twin is literally screaming at that speed, and the vibration is so bad you think the machine is coming apart.

Enginerd

gordo wrote:

It sound's like a flow restriction. Easy test, unbolt the header's & go for a very short ride,testing the limit of the rev's then.These bike's stock exhaust is a two pipe deal, one inside the other, If the inside has collapsed or rotted alot, enough to partially block the exhaust, that kind of problem happen's.

Ps. I read you used another set of carb's, but what kind of shape are the fuel line's in?Should be at least 1/4",. Smile

Thanks for the help Gordo! Very Happy

I have stripped the exhaust down to the headpipes and tried it before, but never removed the headpipes. I didn't realize they were dual walled. It must be this sort of thing as the problem has been slowly getting worse every year for about 15 years.

By the way, I have cleaned the tank, the in-tank filters, and made sure the 1/4" fuel line is clear and flows a strong and continuous stream.

dpmphoto

check for leaks or cracks at the carb manifolds. Run it up to the problem then shut it down coast into garage and check plugs.

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