Topic: 81 CB 650 throttle problems

[Noah100]

I just put a set of Murrays carbs on my 81 CB650. Bike starts up awesome! Starts easy, warms up well, idles great with enriches off. My problem is as follows.

I can give the throttle a quick wack and the idle returns back to normal rpms. However if I roll on the throttle slowly the rpms stay and sometimes slowly increase and won't come back down at all. However, if I kill the bike with the kill switch and switch it to on again before the engine dies, the idle returns back to normal rpms. It will also return to normal if I actually shut the bike fully off and restart.

I have new rubber boots on the intake and have confirmed no leaks there. New gaskets on the exhaust header as well. I am also running an OPEN 4 into 1 header until I get a new muffler as this is a project bike

I have done this test with the throttle cable hooked up to the throttle tube and just the cable free itself. No change. The throttle cable is also not sticky or bound up

I've yet to actually call Murray and have not tinkered with anything on the carbs

Any ideas or help?

Thankyou

[fizzlebottom]

I've read about Murray's carb setup, and watched a few videos of installs. Two things I've noticed:

1. If you specified your exact setup when ordering from him, he should have prepped the carbs according to your exhaust and any other engine modifications. If you did not do that, then you need to contact them and see what needs changing.
2. Did you get the quick turn throttle tube? I've read a few accounts of folks installing carbs without it and having troubles. They caved and installed the quick turn throttle tube and all problems went away.

[Noah100]

I've read about Murray's carb setup, and watched a few videos of installs. Two things I've noticed:

1. If you specified your exact setup when ordering from him, he should have prepped the carbs according to your exhaust and any other engine modifications. If you did not do that, then you need to contact them and see what needs changing.
2. Did you get the quick turn throttle tube? I've read a few accounts of folks installing carbs without it and having troubles. They caved and installed the quick turn throttle tube and all problems went away.

1.Yes I specified 4 into 1 exhaust setup

2. Yes also bought the quick throttle tube from Murray as well


It's just peculiar to me because it returns to normal idle instantly with a blip of the kill switch or a complete shut off. Could it be something with the timing? Doesn't make sense to me but I'm puzzled

[fizzlebottom]

Could it be something with the timing? Doesn't make sense to me but I'm puzzled

I don't think so, but it very well might be possible. In fact, I've heard of instances in which the advance mechanism was binding up and the weights were not freely moving outward or returning to their normal position. So I don't think the timing itself needs adjusting, but the mechanical advance mechanism probably has 40 year old grease.

Aside from that, I suggest calling Murray's. He'd hopefully have some good insight on what steps to take if those steps involve the carbs. But before that, I might definitely consider cleaning and regreasing the advance. Take note that they are assembled different for every bike and very very specifically aligned. No manual will tell you about it, but Common Motor has a great video going over all of the details (not bike-specific):

[Noah100]

Alright I have cleaned and lubed the advance. And still no luck. Can't get ahold of Murray but am still trying. Any other ideas or thoughts while I wait?

I do remember when I first installed the carbs, after opening the fuel petcock, one of the float bowls did leak fuel out the tube coming out of the float bowl for a minute or 2. I didn't think anything of it at the time as it stopped shortly after...

[fizzlebottom]

Just got #$%*s and giggles, make sure your valves are nicely adjusted and compression is healthy. Just want to make sure we can rule non-carb bits out.

I do think something is going on with the carbs and you'll need Murray to help with that. Key trying to get hold of them.

[Noah100]

Alright got ahold of Marty and we are trying to isolate some things. So far I've visually verified that the slides are not hanging up when the engine is racing. They are in fact in the idle position. As well that the cable is not binding and is routed correctly. Which I'm guessing still points to a vacuum leak then?

[fizzlebottom]

Vacuum leak would be at the top of my list for sure. Folks typically go with the "spritzing carb/brake cleaner" method around the isolators to see if idle surges. I don't like such volatile chemical, so WD-40 is often seen as a better alternative. Short bursts in strategic areas to see if any specific spot gets your idle to briefly change.

[Noah100]

Alright so new info. And I'm surprised I didn't catch this before.  So I've pulled the filters off the carbs and noticed when the bike does stay revved high and won't come down, when i turn the bike off, the slides drop slightly by themselves down into the seated position. So apparently they are hanging open...

However this test was done with the cable lubed, not pinches, and not even hooked up to the throttle tube. So I'm fairly certain it's not the cable binding. Can the engine pull so much vacuum that it's not allowing the slides to return? Or would this be maybe the cause of weak slide return springs?

[Noah100]

Just to wrap this up for anyone else having this problem. I ended up putting another set of vm34 slide springs in the carbs so each has 2 springs instead of 1. And that fixed my problem. I guess the engine was just pulling too much vacuum on the carbs, holding the slides up and keeping them from seating.

Thankyou for all the help!

[fizzlebottom]

That's pretty nuts. Was this a solution that folks @ Murray's came up with? If not, how did you come to that idea?