Topic: bike boggs at 4500-6500 RPM

[Mingo]

Okay, so this is sort of a re-post, but I have some more information.

So, a few days ago, I got my new-to-me '78 cb 750 running and took it out for its first real ride. The bike had several issues: locking brakes, a flikering-on-and-of-in-5th-gear neutral light, blowing fuses. But those seem to have been resolved. I've done a fair amount of wiring cleanup. I've adjusted the valves, the bike has new sparkplugs, but they're already dirty looking. The bike has new points, I just adjusted the timing. The bike seems to charge okay. I've used a wet rag to check that all cylinders are firing. It has a kirker 4-into-exhaust.

The problem is that it hesitates and bogs at around 5000 rpm. Otherwise it pulls nicely.

Here's what I wrote on another post a few days ago:

In first and second, it bogs at about 5,500.
In third, it bogs at around 5000.
In fourth and fifth, it bogs at 4500.

So, today, I took it for a long ride, including some freeway time, and I learned a bit more about how it responds:

The bike pulls normally until around, depending on the gear, 4500-6000 rpm.

Today, however, I discovered on the freeway, that when I get to 4500 in 5th, and it starts to bog, if i pull back a bit on the throttle, and let the bike settle i can ease back on the throttle and get a bit higher in the rpms before it bogs again.

For example, on the freeway, I initially bogged in 5th at about 4500 and had a top speed of 53 mph. But I eased back and slowly increased throttle and worked up to 6000, or so, rpm, hitting about 70 mph.

I went on to experiment with the other gears and found that I could hit the bog point in any gear, ease back, and slowly sneak by it to go maybe 1000 rpms higher than the initial bog point. Eventually, it would bog in any gear, though.

Other info:

A friend was following me and said when my rpms go high, the bike smells rich, like unburnt fuel. He also said that at some point while pushing my rpms past the bog point, a big puff of smoke came out of my bike.

Also, earlier I didn't reveal that one of the bike's POs cut out the lower front of the air box. I instinctively covered this opening with duct tape when I got the bike, but maybe I shouldn't have.

Posters responding to my previous questions have suggested I use the transparent tube method to check float bowl levels, which I will do tomorrow. Also, someone told me to check my advance mechanism, behind the timing plate. I don't understand this advice. Can someone tell me how to do this. I did the timing with my timing gun, and did the advance timing, too, at about 4000 rpms. Is this something different?

I think I'll try taking off the duct tape too, if I can find time tomorrow.

Any other advice, or suspicions about what is causing this would be great.

Thanks.

[killersoundz]

Sounds lean to me. Try doing what I have to do with my bike right now until I get bigger main jets. Turn the choke on about half way and try to hit those RPM's. If it doesn't bog out with the choke on half way or a little more than you're lean. A big hole in the air box will definitely cause it to be lean.

[trueblue]

Also, someone told me to check my advance mechanism, behind the timing plate. I don't understand this advice. Can someone tell me how to do this. I did the timing with my timing gun, and did the advance timing, too, at about 4000 rpms. Is this something different?

If you have checked with a strobe timing light that the ignition is reaching full advance, then your advance mechanism is working.  Another thing that could cause breaking down at higher revs is your condensors.  Most bogging issues on these bikes are blamed on the carbs, but after tearing into the carbs numerous times most people realise that there is an ignition problem and they have been chasing rainbows in their carbs.

[scottly]

Also, earlier I didn't reveal that one of the bike's POs cut out the lower front of the air box. I instinctively covered this opening with duct tape when I got the bike, but maybe I shouldn't have.

Remove the duct tape and report back.

[Mingo]

Also, earlier I didn't reveal that one of the bike's POs cut out the lower front of the air box. I instinctively covered this opening with duct tape when I got the bike, but maybe I shouldn't have.

Remove the duct tape and report back.

Amazingly, this did the trick.

The bike now pulls strongly through redline.

So, what does this mean? Does this indicate how the bike was jetted? Or some other carb related issue?

I presume that now if i buy a stock airbox, the bike will run badly.

I've read a bunch of posts about how getting pods means many more adjustments and problems in the future. Is this cut airbox going to have a similar effect?

I guess I'm not going to check the fluid levels of the bowls with transparent tubing now.

[Mingo]

Picture.

[scottly]

Also, earlier I didn't reveal that one of the bike's POs cut out the lower front of the air box. I instinctively covered this opening with duct tape when I got the bike, but maybe I shouldn't have.

Remove the duct tape and report back.

Amazingly, this did the trick.

The bike now pulls strongly through redline.

So, what does this mean? Does this indicate how the bike was jetted? Or some other carb related issue?

Sorry for the late reply: I missed this post.
What happened was you not only blocked off the window cut into the airbox by the PO, but you also blocked off the stock intake opening at the bottom of the airbox, choking off the air supply. No wonder the motor couldn't rev, and your spark plugs were getting sooty.
If it makes you feel any better, a good friend of mine did roughly the same thing on his 400 Maico. He fought it for a couple of months, then one day I was looking at his bike, and I asked him "Ronnie, what is with the duct tape on the airbox? How is the air supposed to get into the carb?" I still tease him about it.

[HondaMan]

That airbox breathes in at the back, normally: has that vent back there been modified, too?

[scottly]

That airbox breathes in at the back, normally: has that vent back there been modified, too?

I wondered about that, too. Can the airbox bottom be reversed?

[HondaMan]

That airbox breathes in at the back, normally: has that vent back there been modified, too?

I wondered about that, too. Can the airbox bottom be reversed?

No, it has a "D" shape to it. And, it's a bugger to get apart, sometimes.

[scottly]

That airbox breathes in at the back, normally: has that vent back there been modified, too?

I wondered about that, too. Can the airbox bottom be reversed?

No, it has a "D" shape to it. And, it's a bugger to get apart, sometimes.

That's what I thought. Still, the duct tape was somehow blocking the air intake, as removing it solved the problem..?

[trueblue]

Blocked air filter?