Perfect Idle and rev up while standing. Stutters and jerks when driving

[Guest]

I guess my heading says it all - the bike (GS850G) idles awesome and when blipping the throttle it revs up quick, easy and smooth. When I start driving, the bike stutters and jerks as if some of the plugs are misfiring.

New plugs, new battery, new recitifier, stator is OK, ignition is good, coils good, plug leads and caps good. Valve clearnaces done to the recommended specs on this forum (all about 0.1 instead of 0.08mm), new petcock, new fuel filter, tank relined.

Carbs cleaned three times before I got smooth idle etc. - maybe have them ultrsound cleaned?? - guess I could mess around with the timing as well but do not think it is spark electrical ?

Any thoughts? - have not yet started to do diagnostics. This is a new project just completed, and this was the first drive with the bike.

 

[Rich82GS750TZ]

If you have a freshly lined tank, why the fuel filter? Take it out of the equation and see if it's been giving you fuel delivey problems. You could also check that your gas tank cap is venting properly. After running the bike a few minutes, shut it off and remove the gas cap. If you feel/hear a release of pressure, gas cap vent is blocked.

 

[Forden]

You say the carbs were cleaned three times but don't mention settings--are you confident you have the fuel/air mixture dialed in? What are your plugs telling you? Does throttle position affect what you are experiencing?

I don't know what year/carbs you have, so I'll let others with more experience counsel you on particulars. I know I experienced something similar at low throttle positions with my pilot (idle/low throttle) circuit out of whack.

 

[gustovh]

Knowing the year of bike is kinda important when asking for help.

V

 

[ajh]

you should really always check the timing and ignition bits before trying to tune the carbs.
As its easy to do ( and lot easier than doing carbs) may as well cross it off

 

[Guest]

Thanks for input so far. It is a 1981 model. I also replaced the rings. The carbs were very dirty so took a lot of work to get smooth idle and decent throttle response. Will start with electrics as suggested. Not fuel feed problem as fuel cap rubber needs replacement. Will try the fuel filter removal.

 

[sacruickshank]

To rule out a few obvious things ... is the bike fully warmed up when you notice this? Did you leave the choke on for a while to get it fully warmed up? If this is your first carb'd/vintage bike, they can take a lot longer to warm up than modern fuel injected bikes.

I've had this happen on past projects. In my experience, the idle and blipping performance is usually, but not always, a good indicator of fully cleaned and set carbs. It is possible to reach decent idle and blipping behavior while sitting in neutral, but have it act differently under load of actual riding.

Based on that experience and your comment "- maybe have them ultrasound cleaned? -" makes me think the idle circuits are not yet fully cleaned. I know it sucks to take carbs off and clean them again, but clogged passages are still a possibility if you have good compression and spark.

 

[timebombprod]

dipping the carbs works well, but ultrasonic cleaning works wonders, i would recommend doing a spray test to ensure the carb circuits are cleaned thoroughly and send fuel or air efficiently through the bike.

i also think removing the fuel filter is a good idea, after all the petcock does have one already.

make sure the mixture screws are set correctly!


spray test video
https://www.youtube.com/watch?v=gPvZ7S8WCH0&t=2s

 

[Guest]

Been at it since 6am this morning. Removed fuel filter. Done the electrics check with strobe light but at idle only. Timing seems to be OK. Spark on all 4 plugs nice and healthy so it must be Carbies. Will be taking them off and find someone with ultrasound unit big enough to clean the carbs.
Thanks for all the replies, much appreciated. Will report back on the outcome of the carbs after the ultrasound.

 

[Tom R]

Is the advance mechanism free?

 

[zed1015]

Will be taking them off and find someone with ultrasound unit big enough to clean the carbs..

You don't need a large ultrasonic tank for carb cleaning.
The carbs must be stripped right down to the individual bare bodies so even a fairly small tank will do the job.

 

[sacruickshank]

while we're on the topic of ultrasonic cleaning, there are lots of different views on the best cleaning solution. Pine sol, simple green, dawn, etc. My current favorite is 1/3 Berryman's B12 carb cleaner and 2/3's water. Works great and doesn't damage or discolor carb bodies as some cleaners like generic simple green will.

 

[timebombprod]

i just use gunk carb dip, i know the people on here like using berrymans but i believe i have some and i noticed some sort of white corrosion or something on my carb bodies when using it, that might be because i left them in for too long but i know the gunk brand did not do that.

honestly any carb dip will work great when in an ultrasonic cleaner though, those things seriously work wonders.

 

[Maddevill]

A lot of people remove the jets and clean them, then put them back in the carbs where the passage the jet sits in is still
plugged. Especially the pilot circuit. EVERYTHING must be clear.

Mad

 

[Guest]

As promised I said I will reprt back on the issue. On stripping the carbs and giving much more attention to the component parts as before, I noticed that the outer two sliders is slightly darker than the inner two. I checked the diaphragms and found no holes or tears. Something that stood out was that the needles on the two inner slides were longer than the outers. I then put them next to each other and found that the two inner slides wer slightly shorter than the outers. I then realised that someone had done this somewhere in the mystical past of the bike and to compensate, placed the needles in a dfferent position than the outers in an effort to emulate the same amount of fuel dispensed when giving throttle.
Anyways, the carbs are now really clean and the task of resetting everything awaits me. I have found two slides the same as the inners and have installed the same needles as well. All four now have needles at the same height, and they rest in the correct position in the carb body as per the Haynes Manual for 850 fours which is approximately 0.5mm. I am sure that the end result will be worth all this effort.

 

[Grimly]

The Dreaded Curse of the Previous Owner(s) strikes again!

 

[Guest]

Further Update - according to the Carb specs on BikeCliff website, the emulsion tube should be an X5 with 5D50 needle. I have now found that the carbs are fitted with X6 Emulsion Tubes. Can anyone please inform me what is the difference between these needle tubes
According to the chart the X6 comes out of Gs750 carbs So I must assume I have a bastardised set of Gs750 carbs on the Gs850.

 

[Guest]

Final update: It helps when one go back to basics. Hunt on this site gave the following specs - Main 115, Needle jet X6, Needle 5D57, Pilot 42,5, and Air jet 200 - Carb ID 45170 - took me some serious haggling with a few friiends but eventually got 4 x 45170 carbs and made sure that all passages are properly cleaned. Assembled, started bike and it purrs like a kitten.
Never again struggle with bastardised carbs I get with a project.

 

[KEITH KRAUSE]

One of the most common, classic symptom of fuel starvation, is the bike will idle and rev up well as long as there's NO LOAD on the engine. Under load, the motor needs more fuel to run well.
As others have said, it's easy to check the electrical system and be sure it's operating correctly. With any problem, you have to be sure the basic maintenance things are done before focusing on the carbs. A basic thing is correct fuel flow to the carbs. I think someone mentioned a fuel filter? Fuel filters are potentially more trouble than they're worth and if the tank is in good condition as it should be, fuel filters are unnecessary. Before taking the carbs apart, fix any tank issues, get rid of the fuel filter, be certain the fuel line is the correct diameter and not kinked. Service the petcock. Clean the gas tank cap venting. Be sure you have fresh, quality fuel. Once you're certain the fuel is flowing correctly and is clean, then any fuel starvation is obviously the carbs. Once in the carbs, check float levels first and float operation. Check the float needle valves and seats. Do NOT mix them up as they wear as a unit. Check that fuel flows through the main passages that feed into the valve seats. Once you're certain the fuel is entering the bowls correctly, you can check/clean the jetting and all carb passages and various o-rings.
If you're not certain about the bikes history, you need to verify all the jets are the correct size while you have things apart.

 

[KEITH KRAUSE]

I made the mistake of not reading ALL the recent replies. Obviously you were bitten by the previous owner(s) not knowing what they're doing. At least my last sentence of my previous reply might be helpful to someone.
Glad you persevered and got it right. It's all part of the fun of owning old bikes.