Topic: 400F won't rev past 5000 rpm

[76400F]

Bodi: Thanks for the feedback on the slide needles. I figured as much seeing how there really isn't any way to stop them from moving. Wanted to make sure.

Hondaman: I originally decided to use the old boots, did the wintergreen thing which softened them up nice, and replaced the o-rings. Once I started having all these issues I decided to just buy new ones. Sirius had them in stock. No difference. The coils are new and I have not tested them. Definitely something to look at. 

So with the needle in the 4th groove, Keihin 80 mains and 38 pilots, floats set at 23mm and everything perfectly synched, the bike ran like an absolute bag of crap. From idle to 1/4 was a lot of hesitation and sputtering, 1/4 back down to idle in gear it would just quit. 1/4 to 1/2 was sluggish and more hesitation, 1/2 to 3/4 was a little better, but 3/4 to WOT seemed to be a lot better. Actually hit 9500rpm in 4th and pulled decently.

So this is the perfect example of why you only change one thing at a time, because now I'm going to have to go back and figure out which of the 3 changes I made has had a negative effect. At least I'm now fairly certain that the 80 main is right. Won't know for sure until the plug chop.

nvr2old: I've decided to stop annoying my neighbours (and myself) and replace the megaphone exhaust with original. It's arriving tonight. I'm hoping it also alleviates some of these tuning issues.

So the weekend plan (with the wife and kids out of town until Monday  ) is to install the stock exhaust and start doing plug chops. Beer intake levels will be directly proportional to success levels. Success>Beer.

[MoMo]

I had a somewhat similar problem with a MAC exhaust, original header and new muffler from DSS cured the ailment

[76400F]

Well, my new pipe didn't arrive yesterday so I'm still tweaking with the megaphone.

But there's some good news!! It seems like my problem the whole time has been my floats and needle sets. This morning I went back to my bin with all my original carb parts and pulled out the floats. They looked to be in fantastic shape other than being very dirty. I soda blasted them and straightened them out a bit and installed them with my new float needles and seats. The ones I was using had huge divots where the pins were contacting the tab. Bench set them back to 21mm. Filled them on the bench and not a drop of overflow. OK, getting excited. Reinstall the carbs and start it up. Getting more excited. Sounds great and seems to have new life. Slap the tank and seat back on and take it out. It's a totally different bike. Sounds great and is pulling decently. Other than a feeling of being a bit under-powered it's revving nice and running well. Stop and pull a plug and it's super rich. Take it back home still excited because I'm thinking the final tweaking should be easy peasy. Get home and realize that I forgot to put the pods back on! Put the pods on and take it back out for a good run. Other than a little hesitation transitioning from 1/4 throttle up, it's running great. Ride it for a while and take it home and pull the plugs. So NOW I'm in a good position to start plug chops and recheck timing, points, etc. Thanks to everyone who helped me get to this stage.

[76400F]

Sorry, that should have read 'running super lean'.

[76400F]

I still don't understand how float levels can play such a critical role in how the bike runs. I can see if the level is really low or really high but 1mm off and the bike runs like crap?

Need someone to explain that to me.

[BomberMann650]

I still don't understand how float levels can play such a critical role in how the bike runs. I can see if the level is really low or really high but 1mm off and the bike runs like crap?

Need someone to explain that to me.

A local honda legend reminded me once how everything about these motors is relying on a Vacuum to operate.
There is nothing about this fuel delivery system to aid the cylinder in receiving fuel to burn.  So as the cylinder goes through its intake stroke, it's literally sucking fuel from float bowl.  Thru the jets and down to the combustion chamber.
If the float levels are out of spec, the vaccum effect is negatively altered.

This is how we have fun with physics!

[76400F]

When you put it like that, it should be so simple to tune these. Make sure there's fuel and provide delivery at the right rate!

Here are the pics from the WOT plug chop I did this morning. Looks like it's still very lean. HIT a top speed of about 65-70 mph. It was still climbing slowly but far from it's potential.

80 mains
38 pilots
needle clip in the 4th position

Going to do the idle chop now.

[BomberMann650]

When you put it like that, it should be so simple to tune these. Make sure there's fuel and provide delivery at the right rate!

There will always be that missing link between science and the mystery of life!


Definitely need to richen that fuel mix.

[76400F]

So here's the idle chop results. Just synched the carbs so either they're not staying synched or the mixture screws need to be adjusted individually. I always thought that you set the mixture adjustment screws the same.

[BomberMann650]

I don't believe there is a rule against using the AF mix screw for fine tuning.  That is what it's there for!

Double check the sync if you're worried about it.

1/2 got sooty

[76400F]

So I'm thinking 1/4 turn out on 1 and 2. 1/2 turn in on 3. 1/4 turn in on 4.

[76400F]

By the way, I broke a new record on my last WOT plug chop: 2 cars, 3 motorcyclists and a guy on a lawn tractor all stopped to make sure I was OK while changing my plugs on the side of the road. Gotta love rural Ontario. Still good people left in the world.

[BomberMann650]

By the way, I broke a new record on my last WOT plug chop: 2 cars, 3 motorcyclists and a guy on a lawn tractor all stopped to make sure I was OK while changing my plugs on the side of the road. Gotta love rural Ontario. Still good people left in the world.

Just the lawn tractor alone is impressive enough.

[HondaMan]

So here's the idle chop results. Just synched the carbs so either they're not staying synched or the mixture screws need to be adjusted individually. I always thought that you set the mixture adjustment screws the same.

Yes. If you keep trying to adjust the color/mixture with varying air screw settings, you will chase your tail!
Set them at the factory setting, all the same. If the plug colors vary, look for the reasons elsewhere. After 40+ years with these bikes, I find this is the only sure way to track down the issues involved.

I have also found that the O-rings in the carb flanges, the clamps, the tiny (aftermarket, too stiff) float valves that were used in these bowls, and leaking exhaust valves with bad guides all have variously caused plugs to show different colors. It had nothing to do with the air screw settings.