Bog on take off, not every time.

[fastxl]

88 K75S, Here's this list of everything I just replaced

• Fuel Pump (Ebay not OEM)
• Fuel Filter (NAPA, not OEM)
• Fuel Injectors (Ebay)
• Intake Manifolds (OEM)
• Cleaned Throttle Bodies
• New Spark Plugs
• Air Filter
• New Vac Plugs on Throttle Bodies
• New Vac line to Fuel Pressure Regulator

Noticed the "Z" Pipe (the pipe that goes from the crankcase to the intake box) was VERY cracked and cracked through at one spot, so I "fixed" it by wrapping it in electrical tape until I can get a new one. Bike runs good, idles nice, took it for a ride and it runs good. However when stopped at a stop light or stop sign for a longer time when I take off it bogs bad and I almost have to pull over the get it to "clean" out by revving it.

Less than ideal "Z" pipe?
Or something else?

When I can make a quick stop at a stop sign it doesn't seem to have this issue.

[The Mighty Gryphon]

I'm going to go out on a limb here and say that if you had replaced the "z" pipe first, you had a pretty good chance of saving a bunch on money.

BTW, have you checked the valve clearance and balanced the throttle bodies yet.  Definitely do that after you install the new "z" pipe.

[fastxl]

I have not check valve clearance and balanced the Throttle Bodies yet. I inherited this bike from an uncle, it wasn't running. The fuel pump was dead and based on the amount of grit in the fuel tank it didn't hurt to do fuel injectors. Since I was in that area working why not throw leak prone intake manifolds on as well. I am going to order a new Z pipe today along with items to check the valve clearances.

[The Mighty Gryphon]

While you're waiting for the stuff to do the valves and "Z" tube, get a can of Deoxit and clean ALL of the electrical connections. 

[Martin]

Check the the mixture. With the bike warm and idling at 1000 RPM press the green starter button if the revs stay the same or rise very slightly the mixture is OK. If they rise significantly the mixture is lean. If the revs drop it is too rich. The mixture screw is accessed by removing a rubber plug on top of the air box a 5mm Allen key is required to do the adjustment. You can use a Exhaust Gas Analyser or the lean drop method to adjust the mixture.
Regards Martin.

[fastxl]

After doing a bit of searching around I kind of wonder if the Ebay Fuel Injectors are the problem. They are the 4 hole ones. This thread about the CO adjustment https://www.motobrick.com/index.php?topic=9925.25 has about the the same issues I have and the last post talks about swapping out 4 hole injectors back to OEM. I am going to send my OEM's to Mr. Injector. Won't hurt to have a set that are refurbed and OEM.

[Laitch]

After doing a bit of searching around I kind of wonder if the Ebay Fuel Injectors are the problem.

The most frequent comment I've read about four-hole injectors is that they give a marginal increase in performance at the cost of a decrease in miles per gallon.

To get a baseline idea of your engine's performance, a valve clearance check, a throttle body balance and a CO adjustment need to be performed. You've already got a good air filter, spark plugs and, except for the z-tube, a non-leaking air intake system if you installed the manifolds correctly.

[fastxl]

For sure plan to do the Valve, TB Balance and CO once I get my new Z-Tube. Will see what it's like after that. No reason not to send the original injectors out for refurb in the mean time. Will update after I install the Z-Tube.

[Martin]

For many years the OEM versus 4 hole debate has been going on. I have a mate who's son worked for a company that specialized in injectors. The OEM injectors are designed to spray onto the back of the inlet valves where the fuel further atomises. It also cools the head of the inlet valve. The OEM injector stream angle is designed to be very narrow to accomplish this. The cutaway drawing shows the injector aimed at the back of the valve. No company that sells 4 hole injectors has shown a dyno comparison between the two injector types. The improvement felt after fitting four hole injectors would also be felt if you fitted a clean set of one hole injectors.
Regards Martin.

Cutaway K Engine Injector angle .jpg.png (59.96 kB . 768x568 - viewed 466 times)

[frankenduck]

spray onto the back of the inlet valves where the fuel further atomises.

Important point. I think many people assume that the injectors fire directly into the cylinder.  They don't.

I've tried the four hole FIs on both a K75 and a K1100.  Didn't notice any difference in performance on either bike.

There's a reason there are no dyno charts. (wink)

[fastxl]

So here's an update. Z pipe has been replaced. Sent the OEM injectors off to Mr. Injector for refurb, got them back and installed yesterday. Started up, got it up to temp, could still tell it had the bog even in just the driveway. Took it for a spin, bog still existed, then it just stopped revving, couldn't get it home had to load it in the truck to bring it home. No power what so ever, can barley get it up to 2000rpm, even turning the throttle it just coughs and won't rev. I have a video that I will post later. Any suggestions? Plug wires? Took the plugs out and none of them seem fouled or wet. Electrical? I suppose the injectors could be bad but had this issue sort of with the other injectors. Thought maybe it was temperature related, but tried to fire it up this morning dead cold and still the same issue. Fuel pressure? Where to start?

[daveson]

Yep, I think you want to check fuel pressure. A couple of other quick checks, check that fuel is returning to the tank. Unlikely, there have been cases where the fuel hose kinks, restricting fuel, for example at the 90 degree bend near the tank of the return hose. If your lucky tilting the tank might make it better. Or maybe you can see a kink. Maybe the air flow meter wants a look.

Also reminds me of an air restriction, or a fuel restriction, like say a glove in the tank that sometimes blocks fuel from entering the pump but now wrapped around it. Also the pump might sound louder if it's restricted.

As a rough guide if you don't have a pressure tester, in the meantime, if you direct the fuel return hose into a bucket,  at idle I think you should get about two litres a minute.

[fastxl]

Well, now I feel like a "DA". Opened the fuel cap to see if I was getting return fuel and I heard the fuel pump starving for fuel. I didn't put much fuel in it for a test ride, but apparently I didn't put anywhere near enough at all. Essentially I ran out of gas    . I put 3 gallons in it and now it runs like it should, no bog, rips through all the RPM range. I'd say problem solved.

[Laitch]

I'd say problem solved.

I'd say the mechanical problem seems solved.    Congratulations!

[frankenduck]

If it bogs when you accelerate that's when what little fuel is in the tank sploshes to the rear and starves the fuel pump.

BTW: If you're ever getting near the very end of a tank then leaning the bike to the left (the side the fuel pump is on) and keeping your speed constant will get you a little bit farther and save you some pushing. I BARELY made it to a gas station once using this technique. Yeah, it looks stupid - but not as stupid as pushing.