Missing out at 4000 Rpm

[redliner1973]

My GK has suddenly started running bad, and I am at a loss. When driving, say, 60-70 MPH I can feel a 'surging' in the engine. I give it more throttle and it hesitates, tries to accelerate, and starts to miss. Below that RPM, it runs just fine, idles good, etc. Removed plugs, and ALL are white centre, and black around the 'ring'. Ground electrode is grey. This indicates to me it is lean, but don't know why, it ran great last season. I have replaced the boots and o-rings and adjusted valves last year, coils last month, and this is the second tank of gas I ran in it so far this year ( I just got her back on the road after some major 'shock therapy' along with new brake lines, some tire action and new((used)) forks.) The first tank of gas was ran out pretty quick, since refueled (at good quality station) and about half empty is when I noticed it. I can understand maybe a cyl or two, but all 4 plugs look exactly the same. pure white in the middle. Air filter was serviced with no change in running. Any suggestions? :twisted:

[lhanscom]

This sounds a lot like an ignition problem. Check your wires, and check the gaps on the pickup, not sure if you have points, but if you do, reset them. If that isn't it, take the carbs off and clean them up.

[redliner1973]

No Points, it's an '83 1100. Why ignition? :twisted:

[redliner1973]

Anyone to help? I checked the plug wires, ohmed the coils, removed and inspected the petcock and lines, put 12 ounces of toluene in an almost empty tank and ran it, no change, refilled with full tank of gas, pulled all the plugs again, still same colour, above 3000, or when accelerating heavy, feels one cylinder ( maybe two, but not sure ) missing out. If I barely open the throttle is seems to run much better, yet still spits out after 3000 rpm. Also hear mild popping in the right side exhaust pipe not alot, but about 2-5 seconds apart. I pulled the cover on the right side and didn't see anything obvious on the trigger area, Both coils ohm out the same, 3.7 ohms. Caps are relatively new, about 1-1/2 to 2 years old. Plugs themselves were changed last summer. does anyone have any ideas???

[frosty5011]

Tim,

I'm just wildly guessing here, but are you running a fuel filter, one that might be partially clogged?

My first guess would've been fuel starvation.

[redliner1973]

No, no filter, straight line to the tank, and tank itself is rust-free. when I pulled out the petcock, it was very clean. Just strange, as it idles, starts perfect, just misses out when you load the engine or drive normally at or above 3k RPM I had just got teh bike back on the raod, and was running good for the first tank, and most of the second tank of gas. It came on slowly and is getting worse, if that makes any sense. I also ran it without a gas cap, thinking that maybe it wasn't venting...nope, wasn't that...

[frosty5011]

I see.

The reason I asked, is last year, I had almost identical symptons.......the cause of which was my fuel line developed a "kink" during the hottest part of the summer. This restricted the fuel flow.

It would start up great, and would also run fine at slow speeds/RPM's. But when you wanted to accelerate, or ride constant at a faster speed, it would also start the miss/run game, and got consistantly worse, the longer I tried to maintain a higher speed.

Wish you good luck in finding the problem, I know how aggravating it can be,

[Anonymous]

I would try running it on prime.

Steve

[kz]

Have no idea if this applys here, but:

I ones had a car with Stromberg carb, same basic design as our Mikuni CVS carbs.

The car behaived the same way as you discriebe that your bike is behaiving.

I troubleshoted a lot on the car engine and it turned out to be a leak in the carb membran.

If you don't come up with a better idea, I would check the membrans in the carbs.

[earlfor]

I think you are running out of gas. Either the screen on your petcock is partially blocked or the fuel line is pinched, or you have a faulty diaphram in the petcock which is not allowing it to open fully. Most probable is another vacuum diaphram petcock has taken a dump.

Earl

[Anonymous]

I am having a very similar problem with my 1980 gs1000g had the carbs cleaned and synced up and the petcock rebuilt and it is running much better but it still goes pretty flat around 3500 rpm's it idles fine and you can rev it up fine but once it is under load it has a nasty flat spot, it does come back strong after you get past it . I read one of the model reviews on this site from aguy whith the same bike as mine and he mentioned a similar problem with his because the carbs had too big of a throat on them does this sound possible I am also running a k&n filter could it be letting too much air in ?

[redliner1973]

I took the ptcock out, it was really clean. hooked up a line and sucked on it, and fuel came out ok. I had run on prime earlier when I had the problem, and, as with the choke, didn't seem to help. Checked for routing problems orr kinks, couln't see any, I am throwing my arms up over this. I consider myself a fair mechanic, but this has me stumped bigtime.

[redliner1973]

Also, took the bike to work this AM, had to use full choke to keep all cylinders firing, and even then was not really smooth...... :twisted:

[nabrams]

It sounds like you've checked a lot of different things, but you didn't mention whether you've checked for air leaks. If the leak is getting progressively worse that would explain your slowly deteriorating performance. When I had the problem you describe it turned out to be a combination of a bad air filter and several air leaks. A leak between the air box lid and the air box, minor leaks at the boots connecting the carbs to the engine, and a leak at the boots connecting the carbs to the air box.
Your bike is an '83 which I assume has CV carbs - especially sensitive to air leaks.
Do you get a bog-down while revving the bike in nuetral to high RPMs (like 7000 RPM)? If yes, then I suggest the following test: restrict the air flow by putting varying amounts of cotton t-shirt in the air box and rev the engine to 7000 RPM. If you can find a combination that removes the engine hesitation and/or backfiring then you've just proved that you've got some kind of air leak issue. That's how I knew. Take all those t-shirts out, find the leaks and fix them. Once I fixed the air leaks my problem disappeared.

[redliner1973]

I have checked around for those, like I stated earlier, I replaced both boots and O-rings last year, and the airbox is sealed tight. This problem just started happening over about a hundred miles, No problem revving from a neutral-gear, just moreso when under load or above 3000+ RPM.