Beginning to get frustrated with my 7000+ RPM problem.

[Chris Daniel]

I am at a loss.

I have done everything I can think of to solve this problem but I still cannot figure it out.

I have:
Changed petcock to a pingel with new fuel line (5/16).
Rebuilt/cleaned the carbs.
Reset the floats to SUZUKI factory settings (measuring to the top of the round part of the float instead of the flat part).
Put a dynojet stage 3 kit in the carbs.
Changed jets numerous times since then trying to solve it.
Put different coils on it.
Changed plugs.
Vacuum syncroed the carbs.
Taken the fuel tank vent apart and cleaned it.

The only thing that made any difference was syncing the carbs. Since I synced the carbs it will actually pull past 7000 but it is struggling severely (bucking and surging). Whereas before I synced the carbs it would hit a brick wall at 7000 RPM and refused to go any higher.

Before I rebuilt the topend it would pull to redline all day long. When I rebuilt the topend I put everything back together exactly the way it came off. Ever since then it has had this problem. I compression checked the engine the other day and everyone of them was perfect. When installing the cam chain/ cams, I followed my Clymers manual to the letter and double checked everything. So I honestly do not believe I have a problem there.

It is driving me crazy. Other than that problem it runs absolutly perfect. It runs a tiny bit on the rich side but it is not severe at all. It starts right up, after about a minute I can turn the choke completely off and it will idle perfectly at 1100 RPM and runs like a champ.......except for 7000+.

Anyway, if anyone can help me solve this I will owe them my sanity.

Thanks very much.

[chef1366]

What about your ignition advance? Check the assembly and spring action. Try spraying with contact cleaner.

[tfb]

Hmm, I would have thought that the one thing it can't be is the ignition advance... this is supposed to begin advancing at about 1500 rpm, and be fully advanced by 2300 rpm. So if there was a problem with the ATU then you'd expect it at the lower engine rpm, but not at 7000+.

(But look, check it anyway. Daft things happen.)

Mind you, saying what the problem isn't is always far easier than what the problem is...

So here's a suggestion off the top of my head: has the engine been rebuilt at any stage in the past? If so, I would pop off the cam cover and check the camshaft sprocket timing. It could be a tooth out on one of the sprockets -- although I'm not sure if this would bring about the lack of power over 7000 rpm.

Or.... maybe rust sediment/ water in the tank is fouling the nylon gauze filter in the tank when fuel demand is highest?

Mike.

[Chris Daniel]

I havent checked anything to do with the timing. Everything seemed to be working good when I rebuilt the engine so I left it alone.

Again, I checked and rechecked the cam timing when I put the topend back together so I do not believe it is realted to that. Also, when it comes to car engines, if you have the cam timing off one tooth either way they will not exhibit this type of behavior.

That is what keep pointing me towards something fuel related but I do not know what else to try related to the fuel. I have replaced the petcock with the "racing" pingel (no reserve) and added motion pro 5/16 fuel line.

[tfb]

OK, how about a filthy air filter element? If it's more than 10,000 km (=600 miles) since you replaced it, which is the service interval anyway I think, then it'll probably be chokkers with fine particles and robbing your engine of power, esp. at higher rpm.

Even when they look clean they can be chokkers... and I only mention it because I can't see it in your list of stuff that you've done. But it definitely sounds like something to do with fuel mixture.

[semerm]

will this help?

if it helps you when I bought my 81 gs850, it had pods on it and would only pull 5000rpms and did not run well at all. I found and installed a stock air box and it will redline and runs almost perfect. Does it have the stock airbox?

[Chris Daniel]

No, it does have pods, EMGO's to be exact. There again though, it ran just fine with them installed before all this started happening. When I bought the bike it had UNI pods on it. I replaced them after about a month of owning the bike because they were falling apart. I couldnt afford K&N filters so I went with the EMGO's. They are roughly 6-8 months old and just looking at them under the light they still look really clean.

The thing I cannot figure out is what is so magical about 7000 RPM? I mean any gear, any speed, does not matter....7000 RPM is where it starts this problem. Anywhere else it runs damn near perfect...tiny bit on the rich side, but it idles great, fires right up, cruises just fine (took it on a 100 mile round trip Sunday), I mean she runs better than she ever has.......untill I hit 7000 RPM then it acts like it is starving for fuel, but I cannot figure out why.

I appreciate the feedback, hopefully someone will hit on something that I havent tried that will solve it.

Thanks again.

[KEITH KRAUSE]

Run the bike (level or uphill) in 4th gear at SOLID 1/3 throttle, chop off and tell us what the plugs look like. Be sure the motor is completely warmed up. Find a safe place and run it a couple miles like this.
Also, remove the floatbowl vent lines if you haven't already. Leave the ports open.
You don't say anything about the jetting except installing a stage 3 kit. If you followed their "base settings", they are very often wrong.
If the bike is in otherwise good tune/basics checked (should be before any re-jet), then get those reads and let us know.

[Detman101]

I agree with Keith. When I bought the Dynojet Stage-3 kit for my ratbike and emgo pods....all the settings that Dynojet specified were dead WRONG!!

I wound up buying about 5 sets of main jets from advice on this board. The only thing that I used from the dynojet kit were the modified jet-needles that the dynojet kit supplied. Everything else was useless.

Every bike is different and I learned the hard way that carburetion is nothing but another variable in life. There is no solitary answer when it comes to carburetion and jetting.

You will more than likely have to experiment with different Main-jets to see which one is going to allow enough fuel to flow at 7000+ RPM.
And once you get the correct main-jet you will probably want to adjust your jet-needles for smoothness. But maybe not...depends.

[wiredgeorge]

I can't say much about the Dynajet kits for BS series carbs but the kits for the VM series carbs use the same jet needles for EVERY application. Seems to me that this will yeild differing results since the carburetors are fitted with different size needle jets (they and the main jets meter gas at various needle jet heights), different air jets and different diameter fuel pipes (different potentials for pulling gas through)... I have removed dozens of the Dynajet kits when owners have asked me to properly rejet. As far as your issues (not pulling over 7K rpm), this is probably done when the slides are open and you are running on main jet. Dynajet is famous for overjetting with its main jets but I feel you would benefit from larger main jets. Different BS series carbs react differently... for instance, yesterday, I was tuning a set of BS34s from a GPz750. They originally came with #110 Mikuni main jets. The bike has K&N pods and 4 into 1. I got it tuned to run perfectly at idle with the stock #37.5 pilot jets but it bogs (lean) a bit when transitioning to the jet needle, even with the jet needles clipped to the 5th slot. I need larger pilot jets... The MAIN problem I had is that I couldn't get the bike up over 5K rpm. I switched to #122.5 MAINS and couldn't get it over 7K rpm... then I switched to #132.5 mains and it was able to pull to redline. This is quite a bit larger jump than would be normal for main jets and has to do with the size of the needle jet I guess. Normally, the #122.5s would be a very healthy jump and probably all that would be needed. Get some genuine Mikuni main jets and do some plug chops to set your main jet size.

[Chris Daniel]

Alrighty, well unfortunatly mother nature has killed my plans due to rain and thunderstorms. As soon as I have a chance I will get some plug reads and let yall know.

On the main jet issue. When I got the dynojet kit it did not come with any instructions so I went to their website and found no pdf file for my bike.........so I just went to the GS750 stage three from the same year and installed it using those instructions. So I believe I put the needle clip in the 4th groove according to the instructions. Through deductive reasoning I figured the 155 jets were for pods and stock type exhaust and the 160 jets were for pods and free flowing aftermarket exhaust (my exhaust is a MAC 4-1 with the megaphone and all steel baffle, which according to them should require no rejetting......therfore I went with the "stock type" exhaust and pods setting).

I did try to go to the richest setting in the kit (dynojet 160) and it didnt seem to make any difference. I also tried the jets that were originally in the carbs when I bought the bike, which I believe were mikuni 150 and it didnt seem to make a difference. Then I tried the stock jets that came in the K&L which were 110, the bike would barely run with those jets, it surged and popped and did nasty things towards the lean side. So, I went back with the leaner setting in the dynojet kit which are dynojet 155 and runs really well everywhere except the 7000 thing. I have not messed with the needles at all since it runs so well in the midrange. Although judging by the amount of carbon that is around the end of my exhaust, it could stand to be a tad leaner.

On getting the plug reading, will it be alright to do this on a flat road?.....I live in Amarillo, Texas.....the flattest place on earth. Unfortunatly I have no hills or anything other than flat road to test on.

Thanks everyone for helping me out with this problem.

[chef1366]

Mikuni

155 dyno jets are between 145 and 147.5 in mikuni land. Maybe try a 145 mikuni main. Dial in your main first and work from there.

http://vmaxoutlaw.com/tech/dyno-mikuni.htm

[KEITH KRAUSE]

Level or uphill is fine for testing. Just be careful. I always feel funny suggesting plug /chop tests but it's all we can do without a Dyno.
Take a rag and a piece of snug fitting hose to help you remove and start the hot plugs. It's always good to take some reads after any re-jet even if the bike appears to be running perfect at all times.
I'm wondering if you'll be able to get a full throttle reading if you can't go beyond 7K. The main regulates the mixture starting at 3/4 throttle.
Be sure the carbs throttle plates are synched correctly before testing.
REMOVE any float bowl vent lines. This is necessary when running pods. Fuel starvation wil result if you leave them on.
If the bike runs a well as you say, except above 7K, and ALL OTHER basics have been checked, then it figures to be a small main jet causing this. But it's all about throttle position, not rpm. It's easy to get most bikes at or over 7K in the lower gears and still not be on the main yet.

[Chris Daniel]

Well, I got a chance to get the bike out on the highway for about 150 miles sunday. Cruising at 4500-5500 RPM is great. I tried a few seconds of full throttle runs and it bucks and surges pretty bad so I am pretty confident of the mains being lean.

Also if I snapped the throttle all the way open from cruising at around 5500+ it would have a split second dead spot/bog then pick up for a second then begin bucking and surging.

We were heading directly into about a 30-40 MPH head wind on the 75 mile trip back home and if I pushed it past 5500-6000 (roughly 1/2-3/4 throttle) it would start surging periodically, any farther than that (closer to full throttle) the surge would get more pronounced.

Needless to say I kept it at or below 5500 RPM so as to not hurt the engine. Unfortunatly I did not have my spark plug tools with me to take a plug reading, so I can only relay what the bike was doing.

SO, should I raise the needle one notch (lower the clip) to help with the 3/4 throttle and the dead spot when blipping the throttle?

Also, I am sitting at DJ-155 right now and I do have DJ-160 jets. Since the bike was still acting up when I have the 160's in it, what size jets should I try to get this problem solved, BTW I am going to use genuine mikuni jets, so I will need those numbers.

I just dont have enough cash to buy a stockpile of jets, so if I can get pretty close within a couple of sizes either way that would be great.

Thanks so much for the help, I really appreciate it.

[KEITH KRAUSE]

Your test suggests the main is too small, as your problem seems to get worse the closer you are to full throttle. However, you say you installed a stage 3 kit but don't mention what primary air jet you used, if you used them supplied in a stage 3 kit for CV carbs. ?? Need that info.
Also, plug reads are NEEDED at 1/3 throttle to determine if the jet needles are set too lean, even though they may (?) not be a part of the 7K problem. If the reads come back lean, then you know you have to raise the needles some, but your test suggests a full throttle problem too. Need those test results.
I have to assume the bike has been set up correctly and we just have to figure out the jetting. What were the floats set at and what was the procedure you followed? Basics must be correct before re-jetting.
If your problem, after jet needle tests, is a small main, I can only guess what size to try. I haven't jetted your model and (most importantly) I can't see what the plug colors are to determine HOW much larger to go.
If 160 showed no noticable improvement over 155, PLUS your "bucking/surging" suggests you're not that close yet, I'd probably try more of a jump, say 170? Very hard to guess without plug info.
Oh, just remembered before signing off... you want MIKUNI sizes?
I'm off to work, Late. I'll try to help later or someone else here? I think Mikuni 157.5 or 160 is about the same as DJ 170? Not sure without the chart. Gotta go.