Will not rev past 5k

[Fishman]

Fresh from my success at returning my 83 Nighthawk 650 and my wife's '89 Kawasaki Eliminator 250 to good running condition, I attacked my 1981 GS550t with renewed vigor this weekend. Here is what I have done:

Carbs disassembled, dipped, etc. new orings. This was done over a weeks time a couple of months ago.

New carb to head boots and orings.

Replaced rusted out factory exhaust with ebay factory exhaust in so-so condition. Quiet enough so baffles must be mostly intact.

New stock foam on the air filter properly oiled.

Carbs synched using Motionpro tool.

New battery.

Probably not relevant, but other work included installing replacement used brake systems and all switches, as well as replacing some gauge parts for cosmetics.

The bike starts instantly and idles great. Runs smooth up to 4500 in first through 4th. I did not try higher gears because it still has the old tires on it. By 5k it hits a wall and will not rev past it. If it's in neutral it will rev to 8-9k right then with no hesitation.

I am open to suggestions. Here are some things that I think it might be:

Main jets clogged with something - seems unlikely given the fact that they look clean. I even pulled them before synching to check the float bowls and things looked fine.

I have not checked the state of the charging system. Some posts indicate that a R/R problem may cause these symptoms. I have not done the R/R mod and would appreciate a link to a good description of it. I probably should get a replacement R/R and I am torn between the Honda version and the Electrasport after reading plenty on it.

The bike still has the original coils, spark plug boots and spark plugs. Although a little dry looking, the wires look fine. I have pulled the plugs and they looked fine too. However, I have new plugs to put in, and I am not adverse to installing new plug boots and coils if someone would be kind enough to advise me which aftermarket one is appropriate and available, say at Z1.

I guess I'm looking for a little validation and some additional advice on where to go next. I am so close and I really want to get it back on the road.

Also still needed, but not the cause of my rpm issues, are a new chain and sprocket set. Any advice on that is welcome as well.

Sorry for asking so many questions.

 

[Steve]

Sorry for asking so many questions.

If you don't ask questions, how do you expect to get answers? :-k

Just to be sure we are talking about the right thing here, is it only when you approach 5k on full-throttle, or can you sneak up on it and go past? Either way, try it again, and when you get to 'the wall', pull out the choke knob to see if that helps. If it does help, you are running lean, and the extra fuel from the 'choke' circuit is helping.

Nice detailing on the work process, all of that is rather necessary to ensure good operation. One question remains, how far out are your idle mixture screws? Granted, running at 5k should have you off the mixture screws, but they do contribute at higher throttle openings, too.

You may have to end up doing plug chops. Rather than specify an engine speed, the carb circuits depend on throttle opening. As you have observed, you can rev the engine quite high, and you probably did not have to twist the throttle very far. You were still on the pilot circuit, maybe edging into the needles, while the bike was in neutral. With a load on it, you will be dipping into the main jets, but you won't know for sure until you do some plug chops. Info on plug chops can be found on BassCliffls 'little' website.

Another possibility is your spark plug caps. Unscrew them from the wires, trim about 1/4" of wire, screw the caps back on. Sometimes there is a little wear around the screw, and it just does not make good contact any more.

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[Fishman]

"If you don't ask questions, how do you expect to get answers?"


I re-read my longish post and you know . . . you're right! I don't know what I was thinking. Thankfully you were able to figure out what I was getting at in all that gibberish. My only excuse is that I posted it after a long day of work and I was in a hurry.

Anyway, to answer your questions. I can't remember where I set the idle mixture screws, but 1.5 turns sounds right. I think I pulled that information out of my Haynes or Clymer manual. Regardless, I set them at the factory defaults, which may not work too well.

I can't get past 5400 rpm no matter what I do, and that only in 1st gear. As I try it in higher gears, the rpm wall decreases. It's somewhere around 4800 rpm in 3rd gear.

I will read up on plug chops. Thank you for the heads up on where the info is. I will also check out the plug wire ends but, to be honest, I think I probably should get some new ones and eliminate that problem all together.

 

[Steve]

Nice idea about getting new plug wires, but be aware that stock plug wires are molded into the coil towers. Some have had success digging them out, but you might have to change coils. Try trimming the ends first.

Try setting the mixture screws at 2 or more turns out. Once the bike is warm, turn each one slowly, listening for highest engine rpm on each one. You may have to reset the master idle speed, but keep tweaking the screws. When I do this, I like to go back a second time, to make sure it is spot-on. I see that you have synchronized the carbs. That's good but it won't affect your 5,000 rpm 'wall'.

I did not see any mention of adjusting the valves. Valve clearance changes will affect valve timing, therefore how the engine will run. It would take a rather extreme case to make a 'wall' such as yours, but the valves should still be checked.

One other thing to check is the ignition advance mechanism. It is on the right side of the crank, just behind the ignition triggers. You should be able to move the advancer sleeve against some spring tension. If not, you need to free it up so it does move.

One last item to check is the air filter area. You say that you have "New stock foam on the air filter properly oiled." All well and good, but how about the sealing surfaces of the airbox and the filter-to-airbox? The slides on the carbs rely on vacuum to lift them, and if there are leaks in the intake system, there might not be enough vacuum to lift the slides.

Speaking of carbs and slides, are you sure that your diaphragms are in good shape and properly sealed under the covers? If you remove the airbox, you should be able to lift each of the slides and it will glide back down gently. If any of them feel different, you can remove the slides without removing the carbs from the bike.

Keep us posted.


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[Guest]

What are the chances you have two bikes with bad igniters? :-k

 

[Fishman]

" What are the chances you have two bikes with bad igniters? :-k"

Not very good since the Kaw 250 ignitor turned out to be just fine.:-k Of course I had to buy another one off ebay and find out that it tested "funky" too. Moral of the story, if the manual says use the Kaw multimeter to test their ignitor, better listen to them.

So I think I will exhaust most other options before I buy another ignitor. On the bright side, I now have a spare for the wife's bike and spare parts are good!

We're getting a lot of rain since yesterday which we really need here in Texas, but it makes it hard to mess around with the bike. Steve, I'm going to take your advice on the ignition advance, plug wires, and also look at the airbox. The slides were good this weekend when I reinstalled the carbs. I'll post back on what I find.

 

[Fishman]

I'm about ready to take this bike to the steel yard.

Well, not really, but my patience is starting to wear thin. I took the excellent advice I got here and checked the mechanical advance. It's fine, but I put a little graphite lube on it to be sure. Then I put in fresh plugs, trimmed the wires, and fired it up. It ran so smooth, but still had a wall around 5k that I couldn't get past. So I pulled the carbs and found that the slides were gummy and wouldn't slide freely. Eureka! A little cleanup and I would be back in business. After that I checked the airbox as best I could without removing it and it looked good. Filter sealing well and boots ok too. So I buttoned it all back up and got ready to take it on a test run, confident I'd found the problem. I hopped on the bike and pushed the starter button and . . .


Nothing! No click from the relay or anything. I pulled everything back off and checked that a wire hadn't been cut or damaged. Nothing. I pulled apart the on/off and starter switch to make sure it was functioning. Replaced switch with weathered original and still no start. So I bridged the relay and the bike will turn over, but there is no spark on the #4 plug. I was so frustrated I didn't check the others, but I suspect they are also dead, as the bike just cranks along with no attempt at catching.

This is very frustrating. The bike was starting so easily and I appeared to just be experiencing a fuel related problem. Now it's obviously something electrical.

Maybe Chef1366 can see into the future. I think the signs point to the ignitor, but I'm all ears. Please let me know what you all think.