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Cant get my CX500 to go more than 70mph. Carb issue?

3K views 39 replies 11 participants last post by  krharris 
#1 ·
I got the bike last fall and just started riding it this summer. First thing I did when I got it was change the oil, adjust the valves (they were pretty close), changed the radiator fluid, and replace the air filter.

For the most part the bike runs pretty good. It's my first bike and I have been riding it around a lot of country roads and having a blast. Well today I was on a nice straight road with no car in sight and got brave enough to see what it could do. I was disappointed that I couldn't get it past about 75mph. In lower gears I've taken the RPMs up to 8-9000 without any problems, but in 5th gear with the throttle all the way open it stops around 7000 RPM.

The other things I've noticed about the bike is that when downshifting the engine pops quite a bit. Also if I leave the petcock on when I park one of the carbs drips gas from the overflow (must be a bad float).

Could this be a carb issue? They must be at least somewhat gunked up based on the gas leak... I bought larrys guide and one of these days I am going to give the carbs a thorough clean.
 
#4 ·
Hmm thanks. I am 170 lbs and all I have is one of those spitfire windsheilds so I don't think I'm weighing it down too badly.

Maybe I will try to rebuild the carbs soon. I have had my eye on an ultrasonic cleaner and harbor freight has a 25% off coupon for the next 3 days. Good excuse as any to spend some money :p
 
#8 ·
Hey sir,

I'm wondering if you had any luck with the carb rebuild. I'm having a similar problem with my 81 GL500i. It starts, idles and runs smoothly until I want to climb a hill or throttle up for highway driving, then it boggs down. I've replaced all of the tune up parts and even replaced the ignition coils and wires. I've cleaned the carb twice, but still no luck. Any suggestions?

Phil
 
#10 ·
No vacuum petcock on the '79.

Electrodude, if you're anywhere near Minneapolis, I can give you a hand. First time disassembly of the carbs can be a handful.


R
 
#12 · (Edited)
I did take my petcock out once to see how screen looked, and it turns out I am missing one. The fuel flow seems fine but I guess I am not really sure how fast is normal. Maybe I'll throw my petcock in the ultrasonic when I do my carbs just in case. I also have an inline filter I haven't had a chance to put in yet.

EDIT: Oh I also just remembered I have some pretty gnarly holes rusted in the ends of my mufflers. Could that be the cause?
 
#13 ·
Quickest thing to check would be carb ballance. If both are not "synced" then you will have a limit to the bikes performance. Since you can get the tach to 9K in lower gears but not 5th then it may simply be wind resistance putting a huge load on the engine, and out of sync carbs will make the matter worse.
 
#15 ·
Quickest thing to check would be carb ballance..
I can help with that, too. I didn't mentioned it because, as Murray said, that usually manifests at the lower end.


R
 
#14 ·
you cant get the carbs that out of sync its a myth above 3 k all that matters of the whole sync issue is butterfly position and we dont tune for that or

even check but should

you guys put a lot more importance on carb sync than it actually has on a twin

its biggest effect on a properly working bike is idle and off idle after that the sensing system controls the slide lift and the sync does not matter as sensing is

done by vacuum and pressure in the ports not by throttle position

what could be happening is a slide sticking that may be worth checking
 
#16 ·
Electrodude,

I just looked through some old posts, and see that you're in Buffalo. Not all that far away. Let me know if you want to come play in the garage.

Based on a comment Murray made last month, I'm considering conducting a carb clinic some weekend. This might be a good start to that.


R
 
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#19 ·
I've got the gauges, too. I finally fixed the stem that my then-toddler broke years ago, and recently figured out how to calibrate them. (Cheap dial gauges were reading very differently.) I synced the Grub on Saturday, but haven't ridden it yet. Won't get a chance until Tuesday morning.


R
 
#18 · (Edited)
The carb clinic idea sounds fun but I don't know if I can wait. As soon as I get the parts in the mail I'm going to be itching to start tearing into them.

I repair manufacturing equipment for a living so I am pretty experienced with the tearing things apart and back together. I should be fine doing it on my own.

EDIT: Don't get me wrong though, I am all for the carb clinic idea. If it's on a weekend I don't work I'm in.
 
#20 ·
I got the carbs apart without too much difficulty, even the fabled idle jet wasn't that bad to get out. Currently waiting as the ultrasonic does it's thing. It's one of those stupid harbor freight ultrasonics that can only run for 8 minutes at a time.

I double checked and found out I did actually have a fuel strainer stuck inside my tank but I didn't realize it before. The fuel strainer wasn't very dirty but I cleaned it up anyways and am just going to put it back in.

I also installed sportster mufflers a few days ago.

Can't wait to get it all back together. I'm hoping it will feel like a brand new motorcycle.

Auto part Bicycle part Engine
 
#21 ·
When you reassemble, be careful to align the carb bodies accurately. Otherwise, you could get some binding the the throttle linkage.

Larry covers that in his book, if you have it.


R
 
#22 ·
The carb rebuild went well. And yeah I did everything by Larry's book. I found it hard to tell when the alignment was good enough but I did the best I could.

I went out for a test ride a couple of hours ago and it runs great. Especially noticed better performance at lower RPMs. Along with the new mufflers I am loving it.

The only problem is my right carb still leaks gas out the overflow. I think it's actually even worse than before. I guess I'll have to tinker with the float some more.

I didn't try to see if I could go over 70 but the way it was running I sure felt like it would have been able to.
 
#23 ·
Check that bowl for a cracked overflow tube. It's not uncommon.


R
 
#25 ·
Well I'll be damned. The gas leak was driving me crazy until I couldn't take it anymore and went back out into the garage. Turns out the overflow tube was cracked afterall. I soldered it up but I am not that confident on my soldering job. All I have is a little iron for electronics. The leak is gone for now though.

Thanks for the suggestion.
 
#26 ·
That's good news on the carb cleaning.

As far as the crakced over flow tube, I learned a little trick on another forum board is to take electrical wire shrink wrap and cut it to length and shrink it over the over flow tube. Gasoline deosn't seem to affect the plastic and it makes a good tight fit.
 
#27 ·
Well I guess I still am having the same issue in 5th gear once it gets to around 7k RPMs. It actually does kind of seem like a gas starvation issue, when my bike has hit the reserve it does kind of the same thing. I'm going to explore that route.

From 0-70 though it runs like a champ with the new carbs.
 
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