Hesitation / lack of power around 5k rpm

Old Man Rock wrote:

Based on your post symptoms and reporting...

"Upsized main jets (from standard 102.5) to 107.5"...
Why was this done for this is a fairly large jump in mains jets...

The symptom you're describing seems the term "Falling on it's face" when describing carb issues. Based on the Jett charting graph below... running Lean! @ 5Krpm, this would be ~ 3/4 range right between needle and main operations.

As mentioned, go back to 102 jet and take for a test ride, hit the 5Krpm range and make note of performance. If all good, then tackle mid rpm range needle clip position/jet.

Good luck...

OMR

That's kind of cool...

Motor Head wrote:

You don't say if it ran right before the 4 into 1, these require a bigger step up in main jetting, but fist you must make sure your not having another issue. First as suggested make sure your floats are all set to the correct level, as if low this will lean your carb's and high rich.
As far as the washer at the needle position, you can check your manual to be sure of it's inclusion or position. When tuning carb's try to get a plug reading when operated under the same condition, bring you plug wrench and pull over and pull them all out after running at the problem area. Motors are Air Pumps, 4 into 1 means it will pump more air, so more fuel needed to keep the correct air/ fuel ratio. Sounds simple right? It may take 1 adjustment of jets or it may take several, be patient.

Bought the bike with the 4-1. The washer is not shown on the manual. Interestingly when I got the bike it did not have an air filter fitted. It was my first bike after passing my test a couple of years ago so lack of power at the higher speeds was not an issue. Now with more experiance its getting more and more frustrating. Its especially annoying on the dual carrigeway as its worse at about 70mph.

Old Man Rock wrote:

Based on your post symptoms and reporting...

"Upsized main jets (from standard 102.5) to 107.5"...
Why was this done for this is a fairly large jump in mains jets...

The symptom you're describing seems the term "Falling on it's face" when describing carb issues. Based on the Jett charting graph below... running Lean! @ 5Krpm, this would be ~ 3/4 range right between needle and main operations.

As mentioned, go back to 102 jet and take for a test ride, hit the 5Krpm range and make note of performance. If all good, then tackle mid rpm range needle clip position/jet.

Good luck...



OMR

Upsized jets on info on the internet. ie 4-1 may need bigger jets. Did not seem to make much difference to be honest.

Will try fitting the originals and go back as far as possible to factory settings and try again from there.

The diagram is great. as I said before it is at about 5000rpm but it is worse on full throttle. Ease off a little and it improves but still lack of power.

Thanks. Will report back

Keep in mind, all the jets overlap each other...

Insure no vacuum leaks on the intakes (I use vacuum grease, works wonders), no cable binding etc...

You're close, keep at it...

OMR

Okay, just had the carbs off again.

Here they are on the bench.



Checked the float levels again. The Clymer manual I have says that the float level should be 18mm. They measure 18mm to line 2 but should this be to line 1?? See below.



Tried everything else so set the level of 18mm to 1 and refitted the carbs. Left the upsized main jets in at this stage (107.5).

Fired her up and tried a quick run. At small throttle turn it runs okay but a bit more throttle and she just bogs down and starts to splutter and die.

Will try going back to original main jet tomorrow (why I originally removed the carbs this time) and see what happens.

Stewart

Check the needles and make sure they are in stock position.

Fergie wrote:

...I have had the carbs off (at least 3 times) and checked the float levels seem okay. 18.6mm in the Clymer manual?

Fuel heights. Do you need a special tool for that or can I measure it with a tube from the drain pipe.

Can anyone also confirm the spark plug gap. On searching the internet there seems to be a bigger gap recommended for 3ohm coils??....

The physical float height measurement with carbs upside down on the workbench is notoriously inaccurate. Getting the fuel level anywhere close to spec with this method is imo pretty lucky.

The clear tube test is very accurate, and reveals the true fuel level with carbs fully assembled in usual position under normal operating conditions.

The standard spark plug gap should be just fine. In my experience, most new NGK plugs come properly gapped right out of the box.

Good Fortune!

Patton wrote:

Fergie wrote:

...I have had the carbs off (at least 3 times) and checked the float levels seem okay. 18.6mm in the Clymer manual?

Fuel heights. Do you need a special tool for that or can I measure it with a tube from the drain pipe.

Can anyone also confirm the spark plug gap. On searching the internet there seems to be a bigger gap recommended for 3ohm coils??....

The physical float height measurement with carbs upside down on the workbench is notoriously inaccurate. Getting the fuel level anywhere close to spec with this method is imo pretty lucky.

The clear tube test is very accurate, and reveals the true fuel level with carbs fully assembled in usual position under normal operating conditions.

The standard spark plug gap should be just fine. In my experience, most new NGK plugs come properly gapped right out of the box.

Good Fortune!

Yep.... that's the best method for checking the fuel level.

If everything, including timing advancer is working correctly, the problem is most likely the needle.

When working with CV carbs, the fastest route to success is to buy a jet kit. They come with new needles that are tapered correctly for operation without the airbox installed.

Generally OEM CV carb needles have extremely gradual first tapers on the needles. Lifting them with washers doesn't have much affect.

If all the mechanical elements are correct for the bike, then buy the jet kit and put it in with baseline settings. You'll save yourself a lot of frustration and time.

Larry C

follow Pattons advice. a clear tube from the drain pipe.
it sounds like a lean condition. 102.5 to 107.5 is not a big jump at all(onlt 2 sizes), when u have a major fuel issue.
what is the stock main jet size for this bike?
what was the dif in operation befor and after adding air filters? u said it had no air filter at all before, right?

Okay, went back to the 102.5. This is the stock main jet size. Clip in central position. No washer. Checked the fuel levels with a clear tube. 1-3 seemed fine but 4 was a bit out.

Took off the bowl and had a closer look at the float. On first viewing it seemed identical to the others but took it off had a closer look and refitted it. During this I noticed that when pushing it down it fouled on the inside of the carb. Took it off and noticed that it was very, very slightly different to the other three. I think it has been replaced at some time in its history. Gave it a tweak to avoid it fouling. This was only by no more than 1/2 a mm or so but enough. Refitted and tried.

Much better. Feels good. Slight loss of power at about 5000rpm but easy to "manage" past this point.

Not far off now I think. Bit more work tomorrow and I think I may have cracked it. Might also try the 107.5 back in as I think this might be okay for a 4-1 exhaust.

Thanks

Stewart

i completly missed Larrys post. Larry, has a lot of experiance with carbs, and valid points.
what happens after 5000rpm?
does it happen at the same throttle opening in dif gears?
carb problems are more to slide position (throttle position)than rpm.

Like I say it is better now but I think it is worst at full throttle. Ease off slightly and it pulls better again. Thats why I am going to try the bigger main jets again today.

Thanks

Stewart