GT500 Jumpy off idle

[Ellerbrock]

Hello all,

This is my first post on this forum, but I have been on here a few times over the last 2 years of working on my '76 GT500. I've got it cleaned up and rideable, but I just can't seem to get it to idle right. I can get it running and adjust the carbs to get it to settle to a decent enough 1000 rpm idle, but even a slight pull of the throttle winds the engine to 4000-5000 rpm and it takes several seconds for the idle to work itself back down to normal levels. It also seems very touchy as sometimes it will stall and others it will idle too fast with no adjustment of the set-screw. I initially considered that this might be a normal thing, but in youtube videos of other people's bikes, they always settle right back down to a nice angry idle. I have had the carbs apart a few times making sure that everything seems ok in there (plugged or incorrect pilot jet seemed like a possible culprit, but it all checked out), so I don't think that the issue lies in the carbs and I have the carbs tuned to the point that I get just a little smoke, so I don't think it's a too lean or too rich issue. What could be causing this crappy idle? Anyone else have a similar experience?

Thanks,
Ellerbrock

[wvc]

Usually a hanging idle is a symptom of running to lean (fuel/air mixture, not oil), a lot of times due to an air leak. Make sure the air filter is installed, in good condition, and properly oiled. Make sure the lid is sealing good, and the rubber boot is in good condition with no cracks. Also check the forward rubber boots for cracks. If everything there looks good, double check the carbs again, make sure all the passages are clear, jets cleaned, and float levels properly set. I've also seen carb slides not moving freely, that will hang up, causing a high idle.
Good luck.

[Ellerbrock]

Thank you for the quick reply. I am away from the bike right now, but I know that the boot from the carbs to the air cleaner doesn't seal properly. It seems the boot has actually shrunk and will not reach the carb inlets well enough to tighten a hose clamp around it. I guess I just didn't think that could cause such an issue.

The attached picture from before I started on the bike shows the condition of the rubber boot as it seems to have shrunk away from the carbs.

When I get back home, I'll have to see if I can rig something up, cause I really don't want to spend $43 at bikebandit.com on a little piece of rubber. Anyways, thank you for the quick reply. I'll be sure to update on the progress once I get it worked out or run into another wall.

Thanks again,
Ellerbrock

[wvc]

I'd say its time to spring for a new airbox boot. Also, those carbs are filthy on the outside, hopefully they're much cleaner inside. Take a little carb/choke cleaner and a toothbrush and scrub the outsides down too.

[Ellerbrock]

This was just a picture I took when I got the bike. Don't worry, it's much cleaner (inside and out) now.

Nathan

[TLRam1]

Still may have part of the fuel circuit blocked, soak the carbs in cleaner also.

[Coyote]

I must admit, I'm stumped. How can an air leak on the 'intake' side affect anything?? My 550 runs the same with factory intake system, pods or no air filters at all. Of course an air leak between the engine and carbs is a different story. But the way I see it, it should not have any effect at all on the intake side. Do check for air leaks on the engine side of the carbs!
Check that you have the proper free play in the throttle cable and definately check the cable for binding. Pull the air boot off and with the bike off, open the throttle and let go. Do the slides slap home? If not, you have a binding cable. The slight twist of the throttle reving to 4 or 5K indicates not enough free play in the cable.

[wvc]

A properly sealed airbox, and oiled air filter adds a little restriction to the intake, and the carbs are jetted for this amount of airflow. With no airbox, air filter, or if pod filters are installed, etc, it increases the airflow into the carbs, resulting in a leaner mixture. Not as noticeable on the 2 strokes, but very noticeable on the 4 strokes, and will cause a very slow return to idle rpm after being revved. It will also cause the engine to bog down at higher rpms.

[tz375]

At idle, it should make very little difference what is on that side of the carbs. There should be little or no effective restriction at that level of demand for air.

As revs and load rise, it's a different story and it will have an effect on Air:Fuel mixture strength.

It is either a mechanical issue like sticky cables, insufficient free play etc or it's a leak in the rubber intake boots between the carbs and the barrels. A leak in the base gasket might also create the same symptoms.

[Ellerbrock]

The throttle cable was a little sticky at first, which at one point resulted in it winding uncontrollably to somewhere around 9K until I finally killed it by holding the brake and letting out the clutch (kill switch did nothing to stop it), but it moves freely now and I can hear and feel the slides hit the idle set-screw when I release the throttle.

The boots between the carbs and the engine are in perfect shape and seal just fine.

Also, the gaskets in the carbs are brand new and aren't leaking.

The only other problem you have listed is no slack in the throttle cable. I have actually tightened the throttle cable to remove all slack. The engine reacts so quickly as soon as those slides move, that I adjusted it so I know right where that point is... as soon as I twist the grip. It has made it a little more easy to deal with, and it didn't behave any better with a sloppy cable.

Any other thoughts? I'm about ready to break down and order a new boot at bikebandit so I can try it out when I get back in the states and hope for the best.

Thanks,
Ellerbrock

[tz375]

There should always be slack in the throttle cable - the only issue is how much. On a race bike, I like them fairly tight, so 1mm or so is fine but on the street a little more is safer.

Are the pilot jets OEM/Mikuni and are they the right size? Where are the mixture screws set and how sensitive are they to change? Jets in Carb overhaul kits are not good.

Are they right type of jets? There are two designs that look almost identical, but they flow very differently

Is float height set correctly with carbs tilted - not upside down?

Have you tested the front edge of the rubber intakes where they are bolted to the barrels to be sure there are no leaks there?

Are the gaskets between the manifold and the barrels sound?

Have those bolt-on intakes ever been overtightened causing them to bow in the middle? Remove them and test against a straight edge to be sure.

Have you checked that the Choke plungers are sealing properly or are the old and hard or from a jet kit?

[Ellerbrock]

Let me first say that I won't be able to check anything on the bike now, as I am currently in Germany for a couple of months; why I waited until now to finally post my troubles, I don't really know. This would have been much easier the last couple summers when I was actually home and working on the bike.

I'll be sure to let a little slack in the throttle. Tightening it was just a bit of a band-aid that made it a little easier to work with while trying to figure things out.

The pilot jets are the ones that were in the bike when I bought it, and they are clean. I can only assume that they are correct. The ones in the rebuild kits I bought looked much different, so I didn't try them.

I have tried the stock (I believe 97.5) and new 102.5 main jets with no discernible difference between the two.

The mixture screws, as I recall, are set about halfway between tight and falling out. I tinkered with adjusting them, but it had no real effect on how the bike ran, so I just left them in the middle.

I was a little over my head working with the floats. They appeared to be in good shape, unbent, and moving freely, but I can't really say about the level of the floats. How would I even check this?

I have checked the engine side rubber boots for proper sealing and everything checked out just fine. The rubber boots on this side are in great shape.

I didn't think to check the gaskets between the bolted-on metal intakes and the actual block, or the condition of the intakes. They appear to have never been removed, but they'll be something to look at when I get home.

Those rubber pieces of the choke are also original, but are at least still pliable. I'll also have to check them for proper sealing when I get home.

I think that's everything. Again, I apologize for posting this when I'm not with my bike. I realize now that that wasn't the smartest decision.

Thanks for all the comments and suggestions,
Ellerbrock

[Buffalo-guy]

By your description, it sounds like part of the problem is with your air screw adjustment. They are designed to be set approximately 1 1/4 turns out from fully seated, not half way out. the further out they are, the leaner it gets. In regards to cable adjustments, be sure you have noticeable free play, no matter what the handlebar position. They tend to tighten as you turn the bars. Do yourself a favour, and buy the new rubber intake boot. Until that part is fully connected, your faithful steed is injesting un-filtered air. There's my contribution for today. Cheers.
Fred

[rbond]

I fully agree with Buffalo-guy. You need the slack in the cables. I had nearly the same idle problems you are having. I replaced the carb to airfilter boot, made a major improvement in starting, idle and overall running. Also check timing with a light. I had to advance the mark by a 1/4 of an inch to line up the marks. Well worth the money. Also check fuel flow right at the carbs, as far as the floats, Google for ozebooks to get a manual to set the float levels.