1997 M750 Vertical Cylinder Rich

greencow · August 16, 2019, 07:47:30 PM

Hey guys, been a lurker for awhile now, but I'm kind of at a loss on this, so I thought I'd ask for your advice. Bike is running just a tad rich on the horizontal cylinder, but super rich on the vertical. Started a few months ago, and just got worse. To the point that I'm fouling out that plug every week or two.

Pertinent specs;

Jetting

Main - 140
Pilot - 40
Needle - Position 4 (second from the bottom)

Fuel enrichment screws - 3.5 turns out.

Drilled airbox

Arrow cans

NGK DCPR8E plugs

8,500 miles - all city driving.

Bike was starting to puke fuel out of the carb overflow, so I took them apart 2 months back. Cleaned everything, replaced all the gaskets and orings, synced carbs with a DIY manometer. Fitted a manual petcock while I was at it. Now I don't know if it's just coincidence or not, but this problem slowly started after this. I didn't change any jetting, the needle jets/emulsion tubes looked OK, but after 8,500 miles, all city, they could be shot and I just didn't realize it.

Vertical plug sooty black, sometimes slightly wet with fuel. I've been checking it after my commute home. Bike fires up right away, hot or cold, needs minimal choke to start, but it doesn't want to stay running for long, without a bit of throttle. Very rich exhaust smell, slightly smoky at start. After it warms up, it'll idle and run fine, still smells rich, no smoke though. Until the plug gets so fouled that it's popping and backfiring.

At first I thought weak spark, so I replaced the plugs/wires, problem went away for a week or two, then came back. Swapped the coils and ignition control units around, problem didn't move with that, so now I'm figuring its carb related, and not electrical.

Weird thing is, like I said, the bike ran decent-ish when I first got it. Not perfect, but OK. After I went through the carbs, and tuned everything, it was running like a champ. A month or two goes by, and this starts to happen. As of now, all I can figure is the needle jets are ovaling out, but I don't know for sure.

Speeddog · August 16, 2019, 10:29:32 PM

Did you replace the orings on the float valves?

koko64 · August 16, 2019, 10:44:57 PM

I wonder about the float height on that cylinders carb.

greencow · August 16, 2019, 11:07:39 PM

Hey Speeddog, yah, orings and gaskets for everything.

Koko, float was at 9 and 11 when I opened the carbs up, reset both to 14.

Speeddog · August 16, 2019, 11:20:22 PM

Quote from: greencow on August 16, 2019, 11:07:39 PM
Hey Speeddog, yah, orings and gaskets for everything.

Koko, float was at 9 and 11 when I opened the carbs up, reset both to 14.

Did you clamp the float valve down when you set them?

greencow · August 16, 2019, 11:46:48 PM

Yah, used the float bowl "trick". Unless I overlooked something, and the levels are set way outta whack. I'm not getting any fuel coming out of the overflow, if that would indicate anything.

I'll use some clear tubing to check the levels, just to verify they aren't set wrong.

Howie · August 17, 2019, 04:12:01 AM

Assuming the floats are set correctly and one needle jet is not worn more than the other it could be your enrichment plunger is leaking.

greencow · August 17, 2019, 06:43:47 AM

Is there any way to test for that? Short of just tearing it apart and replacing the seals on the plungers?

Howie · August 17, 2019, 10:29:20 AM

The only way I can think of is physical inspection. I don't remember if you can get to the plunger with the carbs still on the bike, but you probably want to go through the carbs again anyway.

Speeddog · August 17, 2019, 11:56:08 AM

You'll need to get free access to the carbs to pull the vert cyl enrichener.

Not much more needed to have the carbs off completely....

koko64 · August 17, 2019, 12:50:57 PM

Your bike have a vacuum or manual fuel tap? V cylinder manifold take the vacuum taps vacuum hose?

greencow · August 17, 2019, 01:17:46 PM

Fuel tap? Are you talking about the fuel petcock?

I replaced the vacuum petcock with a manual one, then capped off the vacuum line to the horizontal cylinder with a rubber nipple. The only other vacuum line running from the vert cylinder is going to the fuel pump.

ducpainter · August 17, 2019, 01:59:45 PM

Maybe pull the vacuum line to the pump first to see if there's any fuel in it? Perforated diaphragms aren't common, but it's possible.

Speeddog · August 17, 2019, 04:25:02 PM

What he said^.

greencow · August 17, 2019, 04:28:23 PM

Checked the quick/easy stuff first, gonna have to wait till the kiddos are sleeping before I can dig into the carbs.

Vacuum line to the fuel pump is bone dry, verified the floats are set correctly.

Gonna pull the carbs off tonight, if I can. Ordered a set of FP needle jets as well, figured at 8500 miles, might be worth it anyways.

News:

1997 M750 Vertical Cylinder Rich