I own a 1977 Sportster 1000 that I bought new. It's still showroom stock except for some chrome engine mounts and the staggered dual exhaust. The bike has 25,000 miles on it and the oil and filter has been changed every 1000 miles. Shortly after I got the bike it was suggested that I put a larger low speed jet in it for the staggard dual pipes. Well that worked fine at sea level but since I have retired now I live at 4,000 feet. The bike has sat for awhile and this year it's been getting a work out for a change. The bike never really liked to run well below 2000 rpm and especially at this altitude it seemed worse. So I rejetted back to the stock setting of #76 for the low speed and #175 for the high speed. Well this seemed to make it run much better but the idle never was right. I resealed the intake manifold and when I spray carb cleaner on it the engine never sees it so the manifold is tight. But the bike won't idle cold and then when it warms up it has a fast idle of 1400. I try to turn it down to 1000 and it suddenly drops to about 600. Snap the throttle and it idles high again and then it idles down to about 1000 and seems okay. But listening to the bike it just doesn't sound right. I put my hand at the front pipe outlet and the air coming out is actually cold but the rear cylinder never misses a beat. I put a infra red temp sensing gun on the front pipe and it's at 250 degrees while that rear pipe is almost 500 degrees at idle.

Now riding the bike if you grab a handful of throttle it takes off like it should with lots of power but come back to cruise at 2200 - 2400 and the bike kind of bucks along and this is what I thought was an overly rich mixture so that is why I have been playing with the carb mixture. But upon really checking this front cylinder is just firing occasionally it seems.

Going back to trying to adjust the idle speed for a moment after a five minute run at 50 mph or better and then pull to a stop the bike has a high idle about 1400 but then it will slowly back down to about 1100 and the front cylinder is not firing. I pulled the plug after having it idle for awhile and it came out black so the high speed run will clean it off but is soots up again from not firing at idle or RPM's below 2400. I pulled the plug from the rear cylinder and it's perfectly clean, I swap plugs and the bike runs exactly the same. I tested the spark plug wire and it's just fine and if fact I swaped it out with a new one and no change. So now I pull both plugs and the take a compression check and the front cylinder is 90/130 and the rear cylinder is 80/110. The bike has decent compression. As I said open up the throttle and the bike will take off and pull hard with out a miss at all! But low speed or partial throttle and the bike bucks as the front cylinder is coming in and out power.

Any thoughts gentlemen? I've run out of idea's and I'd love to have this bike idle and run well at cruise power of 2000 rpm and have a consistant idle again.

Thanks for your help.

 

Nice 77. Ya'll got a really long story to try to follow. The thing is, a 77 came with a dual fire ignition system so both plugs fire at the same time. An easy (maybe) fix is a new set of plugs. Might wanna look at the centrifugal advance, lube it and check that the weight return easily. Might wanna check for a vacuum leak at the intake manifold seals. How is the accelerator pump shot?

Got a real soft spot for the 1977's

 

Sounds like the beginning of an intake leak, erratic idle and mid range surging at a steady speed are signs.

 

Nice looking bike there TexasScooterTrash and thanks for taking your time to respond to my long winded description there. I was trying to cover all the bases on what I've done to the bike. Great idea about the advance unit! About 25 years ago I put an electronic ignition system in it and I haven't had that cover off since. That advance unit could be stuck as you say as the bike has a tendency to ping easily if I get on it too much. That front cylinder could be just out of time enough to not let it fire at the lower RPM's. Now on the intake manifold I very carefully resealed that with new O-rings and when I spray carb cleaner around those joints or the carb base gasket I get no response out of the engine so I believe I'm good there. The plugs are new with less then 200 miles on them so that's out. The accelerator pump is brand new as I put a kit in the carb when I rejetted it. But you can bet I'll get into that timing advance unit and check it out. I think you may have nailed it there. If not it needs to be looked at and serviced anyway.

Thank you again so much for your time to respond and wish me luck.

 

Thanks TwiZted Biker for your thoughts here. BTW Niles Canyon is my old stomping ground. Beautiful place and my Sportster has been up and down that road between Hayward and Pleasanton many times over the years. I have to agree with you that a vacuum leak is what it sounds like but I have sprayed carb cleaner around those joints and the carb base and the engine doesn't respond to it. Besides I did reseal those first about a month ago. I've been though the carb too and kited it and it's all clean, blown out with a new kit.

Maybe the bike just misses the SF Bay Area and that specific area where you live. It doesn't like the high country of southern Oregon now. :-)

Thank you for taking your time to respond to my issue.

 

What ignition you running? Points or electronic ? Points I'd be looking that the advance unit. Could also check the screen on the petcock, next time she acts up flip it to reserve, see if it helps.

 

The bike has aftermarket Electronic Ignition in it. Been there along time. But the advance unit hasn't been checked for ages either and I'm betting TexasScooterTrash nailed that suggestion. I can't wait for tomorrow to come and pull that ignition cover adn give that came a twist. I'll be it's frozen after all those years.

 

I have nothing to offer, other than to say that’s a great looking bike.

oh yeah, and why did you say “single cylinder” ?

That’s all...

 

Thanks TriGeezer for the compliment on the old Sportster. It's the only vehicle I ever bought new in my life. I was living at home and had two jobs and that red color nailed it for me.

On the "single cylinder" - well because the front cylinder is a hit and miss most of the time. Have to rev the bike above 2500 to get it to fire and pull consistantly. The gentlemen in this tread I think nailed it with a stuck timing advance unit. Can't wait to get out to the shop tomorrow afternoon and pull that ignition cover and check it out. I'd be really lucky if that's all it is and the more I think about it TexasScooterTrash nailed it. I'll certainly report back to the forum tomorrow when I check it out!

Best regards, Lisa

 

I would venture to suggest that 76 is way too big for a pilot jet . Most Stage One later models run a 180 main / 45 pilot set up .