Carb synch problem - 1993 Nevada 750

[Nickpurser]

Hi, I have a 1993 Nevada that was stored in a barn for 8 years. I have cleaned the carbs (Ultrasound) and fitted all new O-rings and float needles, adjusted float heights. Checked Valve clearances, Air filter removed, checked and cleaned, new exhaust and inlet gaskets. Emptied petrol tank, cleaned taps and filters and fitted new pipes plus checked petrol cap breather. New plugs.
It idles and revs fine in the garage but, on the road it surges frequently above 4,000 revs and feels quite lumpy.
I bought a carb synchro tool, which was perfect at idle but, as soon as I increased the revs, the right column went up but the left column went down! I borrowed my mate's Carbtune and the same happened with that. Has anyone had the same problem or any idea what might be causing this anomaly.
I have not checked Compression.

 

[Kevin.NZ]

Do both slides operate simultaneously, are the main jets clear. I am not familiar with the Nevada, I did have a carburetor BSA years ago.

 

[V700Steve]

You need to shorten the outer sheath or give it more play on the side that is down.

 

[scottmastrocinque]

Hi.

I rebuild 100+ carburetors a year in my shop.

It is almost impossible to diagnose your problem over the internet.

Assuming you have properly adjusted the throttle cables that they are moving in unison to each other…(if you are not 100% positive of this, then you need to do this FIRST). The slack in each cable, is adjustable. I am providing you with the Service Manual for your motorcycle and it also is in FRENCH as a bonus for you!

On pages 127-137, are the procedures for syncing the carburetors.

If all of that that is all OK but you are still having a surge at cruising speed…

From what you are describing, you have either made a mistake in your cleaning and rebuild process, or you have a manifold leak.

(A lean surge condition at cruising speed is the likely culprit)

If you reused the rubber manifolds from 1993, then almost certainly (~99.9%) they will not properly reseal. They get old and stiff, and once you break the carburetor loose from the manifold, it will never seal properly ever no matter how tight you make the clamp.

I would get a can of aerosol starting fluid. Start the engine outside and let it idle. Spray the starting fluid all around the union of the carburetor and the manifold. If the engine speed changes at all, you have air leaks. If no leaks are detected at all, then you have made a mistake in your rebuilding.

Ça va?

Bon chance!

 

[john zibell]

Idle sync is not extremely important, but high speed is. There are cable adjusters that have to be used to vary the free lengh of the inner cable to achieve high speed sync. Have a friend hold the throttle open, and screw in the adjuster on the side with the low column. and if you have to lengthen the adjuster in the high column side.

 

[Nickpurser]

Hi.

I rebuild 100+ carburetors a year in my shop.

It is almost impossible to diagnose your problem over the internet.

I am not familiar with the Nevada but I assume there are 2 throttle cables, one going to each carb. Assuming you have properly adjusted the throttle cables that they are moving in unison to each other…(if you are not 100% positive if this, then you need to do this FIRST). The slack in each cable, is adjustable.

If that is all OK…

From what you are describing, you have either made a mistake in your cleaning and rebuild process, or you have a manifold leak.

(A lean surge condition at cruising speed is the likely culprit)

If you reused the rubber manifolds from 1993, then almost certainly (~99.9%) they will not properly reseal. They get old and stiff, and once you break the carburetor loose from the manifold, it will never seal properly ever no matter how tight you make the clamp.

I would get a can of aerosol starting fluid. Start the engine outside and let it idle. Spray the starting fluid all around the union of the carburetor and the manifold. If the engine speed changes at all, you have air leaks. If no leaks are detected at all, then you have made a mistake in your rebuilding.

Thanks for the advice, I presume they are the original manifolds so I'll try and source some new ones, I'm in France so there are plenty of Guzzi dealers available, fortunately. I was wondering that, as it was sat so long in a barn, perhaps the rings may have gummed up, do you think this could also give the same symptoms?

 

[Nickpurser]

Idle sync is not extremely important, but high speed is. There are cable adjusters that have to be used to vary the free lengh of the inner cable to achieve high speed sync. Have a friend hold the throttle open, and screw in the adjuster on the side with the low column. and if you have to lengthen the adjuster in the high column side.

I've checked that the slides move at the same time, I'm assuming there must be an air leak on the left cylinder but can't find it so I'll do as scotmastrcinque says. Thanks

 

[scottmastrocinque]

No, I do not believe so at all.

This is not a piston or ring or valves issue. It is a throttle cable synchronization, or air leaking, or improper rebuilding of the carburetor issue.

 

[scottmastrocinque]

I've checked that the slides move at the same time, I'm assuming there must be an air leak on the left cylinder but can't find it so I'll do as scotmastrcinque says. Thanks

You cannot check that the cables are balanced for their free-play by visual inspection only. You need a carb sync tool or rail to monitor the difference in manifold vacuum.

When you change the threaded adjuster on the top of the carb, it changes the slack in the cable, so that you can bring the two carbs into balance by watching the results on the carb syncing tool.

To understand the procedure, please read the pages in the service manual I sent to you and let me know if you have a question about anything you read.

Given that your idle circuit is working just fine on the carbs, then the problem is with the main circuit at cruising speed. It is one of those conditions I just listed above (throttle cable synchronization, or air leaking, or improper rebuilding of the carburetor issue).

 

[Nickpurser]

I have tried checking that the carbs are balanced with two different carb sync tools but with both tools, as soon as I increase the revs with the throttle, the level for the left hand carb actually descends whereas the level for the right hand carb ascend like it's supposed to.

 

[scottmastrocinque]

EDITED TO FIX MY TIRED BONEHEAD MISTAKE. APOLOGIES! THANK YOU -John Zibell.

Yes, that is because they are not in balance.

The side that the vacuum drops faster, is “working harder”. It is “working harder” because it is opening sooner and the vacuum drops faster than the other side.

So, you can make it work “less hard “ by lengthening the cable to the carburetor.

If you loosen the adjuster, and move it in on the side that is lower (thereby raising the slide later (I.e. working less hard), you will see the vacuum level raise on that side because it will open the slide inside the carburetor later than it currently is doing now, thereby increasing the vacuum on that side.

Idle speed balance is done by adjusting the idle set screw on each individual carb.

The high speed balance is done by changing the tension on the throttle cable adjuster.

This is how it is supposed to work!

 

[V700Steve]

Read my post above. If you don't have enough slack in adjuster to make them equal then you need to cut down the outer sheath. Razor blade, snips, needle nose pliers to cut back sheath and a file to smooth the cut.

 

[Nickpurser]

Yes, that is because they are not in balance. If you loosen the adjuster, and move it out on the side that is lower (thereby raising the slide earlier), you will see the vacuum level raise on that side because it will open the slide inside the carburetor earlier than it currently is doing now, thereby increasing the vacuum on that side.

Idle speed balance is done by adjusting the idle set screw on each individual carb. The high speed balance is done by changing the tension on the throttle cable adjuster.

This is how it is supposed to work!

Thanks again, I'll give that a go tomorrow, anticipated hangover allowing!

 

[Nickpurser]

Read my post above. If you don't have enough slack in adjuster to make them equal then you need to cut down the outer sheath. Razor blade, snips, needle nose pliers to cut back sheath and a file to smooth the cut.

I think i will have to do that, thanks for the tip.

 

[V700Steve]

It's a lost art, taking the chrome cap off, unraveling outer sheath wire, snip & file then screw cap back on.. I had to do this a few months ago on new cables to get them to work. Got to have lots of patience. Make sure your cables are seated correctly in a 2 into 1 splitter if you have one from throttle to carbs. Good Luck.

 

[john zibell]

Yes, that is because they are not in balance. If you loosen the adjuster, and move it out on the side that is lower (thereby raising the slide earlier), you will see the vacuum level raise on that side because it will open the slide inside the carburetor earlier than it currently is doing now, thereby increasing the vacuum on that side.

Idle speed balance is done by adjusting the idle set screw on each individual carb. The high speed balance is done by changing the tension on the throttle cable adjuster.

This is how it is supposed to work!

Scott, I think you typed too soon. If the vacuum is low, screw the adjuster in (toward the top of the carb) to lower the slide to increase vacuum on that side. If you raise the slide, vacuum will drop as the orifice is larger letting more air in.

 

[scottmastrocinque]

Scott, I think you typed too soon. If the vacuum is low, screw the adjuster in (toward the top of the carb) to lower the slide to increase vacuum on that side. If you raise the slide, vacuum will drop as the orifice is larger letting more air in.

WHOOPS!

See! See! I make mistakes just like every other person on the planet! I screwed up something I know very well… Guilty as charged!

Thank you John!

That will teach me to try and bang out a fast response on my busiest shop day! I’m so sorry.

Yes, the concept that I meant to use and wanted to explain it with but got interrupted about 10 times at work, is that you want to establish which side is “working harder” (the slide is opening sooner than the other side). as you slowly open the throttle.

The answer is the side that the vacuum drops faster in.

The slide that rises FIRST will open faster than the other carburetor because the cable is tighter. As the slide rises, the vacuum lowers and the balancing instrument will show the resulting lower vacuum.

To raise that vacuum to the level of the higher side, add more slack into the throttle cable to slow down the raising of the slide.

Grazie mille e’ sono desolato.

 

[Nickpurser]

Thank you Scott, Steve and John, I took 5mm off the end of the throttle cables, which was fun, and balanced the carbs with my synchronizer using the cable adjuster and they're now perfectly level all the way up to 7,000 revs. I haven't taken it out for a spin yet as, just as I was putting my tools away the heavens opened and it's now blowing a hooley but I'm confident it'll be fine thanks to your advice. If you fancy a holiday in, not so sunny France but near the Normandy beaches, you'd be most welcome!

 

[scottmastrocinque]

Outstanding!

I’m very happy that I could be of some help to you. Also the others who gave you and I help too I am sure.

Don’t forget… If you didn’t already grab it, please download the service manual I posted as an attachment in post # 4.

It’s specific for your carburetor version Nevada instead of the fuel injection manual. Enjoy!

Best wishes

Scott