Yet another M750 bogging down with throttle

[pneumadeux]

If I wanted to by-pass my fuel vacuum, what would you suggest I use? I found this one from MotionPro, but it says it isn't recommended for bikes over 200cc.

http://www.jcwhitney.com/motion-pro-universal-petcock-valve/p3074455.jcwx?filterid=c51113d59394y2000u0j1

[koko64]

I'm sure Motion Pro make other fuel taps with more capacity.

[ducpainter]

I believe Parts Unlimited offers some too. Easy to find at most bike shops.

5/16" in and out should do it.

[beethoven]

Make sure you are not using high octane fuel unless engine modified. This can cause fouled plugs on these tame motors.

[pneumadeux]

OK gang, the nature of my post now changes from "bogging down" to "how does this newbie set up this carb?"

With the sole exception of float needles (which were clean and springy) and breather tubes (which had slits near the ends but I trimmed and rejoined using plastic vacuum connectors) everything inside is new thanks to a rebuild with a Factory Pro kit.

- Pilot jet: #45
- Float height: 14mm (purchased and used Factory Pro's cool little black measuring tool)
- Needle clips set at 2nd position from top as per FP spec
- Main jet: 142.5
- Fuel screws: 2.75 turns as per spec

She started immediately without any priming, and with no choke, and hovered around 900-1000 RPMs

Turned on choke, to little if any change.

Cranked up idle screw 'til she hummed around 4000 for about 45 seconds, then turned off choke

Back to around 900 - 1000 despite a roaring choke

What do you suggest next?

I have the Factory Pro tuning guide, but of course, you have to have her up to a proper 1300 idle before you can take her out and go through those steps...

Also have a new Motion Pro carb balancer, but assume that is the final cherry-on-top step?

[ducpainter]

1300 is a little high for a Duc idle. The idle will likely hang if you set it at that number. I'd look for something around 1100, and then synch the carbs.

The rest of your info will come from riding the bike. It's hard to really tune a bike at idle.

[pneumadeux]

1300 is a little high for a Duc idle. The idle will likely hang if you set it at that number. I'd look for something around 1100, and then synch the carbs.

My question is â€" do I just keep on twisting that idle screw 'til it gets to 1100? Like I said, with choke, it was at 4000, which to my experience is really quite fast.

After turning it quite a lot to get it to where it is (900-1000) it would bog a little bit when I'd try to blip the throttle.

So I'm wondering if there's another adjustment to be made somewhere else to get the idle nailed before I try to ride it.

I've never ever tuned, much less rebuilt a carb before. This is all new territory.   Thanks for your patience, everyone.

[ducpainter]

Id back the fast idle, with choke, down to about 3K. 4K is unnecessary. Are you sure you're turning the correct screw?

Then, I'd synch the carbs. You turn the other screw up there to accomplish that, with the Motion Pro synch device attached.

Once you get them synched, you might need to play with the idle and fuel mixture screws a little, don't be afraid to open them to 3 turns.

Finally, you'll adjust the idle speed screw to obtain a hot idle around 1100...then ride it.

[pneumadeux]

That's super-helpful. Thank you.

I'll start with backing out the fuel / air screws to 3 turns, apply choke, start 'er up, and back down the idle to 3k.

Then we'll see if she'll settle to 1100 or so, and break out the MotionPro sync tool.

[the_Journeyman]

Have you checked the diaphragms for fine cracks?  My M750 was bogging a bit due to that. \\

JM

[ducpainter]

That's super-helpful. Thank you.

I'll start with backing out the fuel / air screws to 3 turns, apply choke, start 'er up, and back down the idle to 3k.

Then we'll see if she'll settle to 1100 or so, and break out the MotionPro sync tool.

You're jumping the gun on the fuel screws, but it won't do much harm.

[pneumadeux]

LATEST UPDATE

Cranked the idle screw down so the "hot idle" is right at 3000.

Slightly adjusted air / fuel screws so the non-choke idle is 1100 / 1200.

Used the Motion Pro sync tool and adjusted the carb sync screw to perfect balance.

IMPORTANT TO NOTE: Several times while these adjustments were being made, the bike would stutter / cough a little, and a few times, it backfired into the carbs, and once or twice, a small backfire out the exhaust. After getting everything synced, no stutters, tho.

Took the bike for a ride around the block â€" and blargh... it was like riding her through 3 feet of mud. No pull. At all. Struggled to get her RPMs up.

Thoughts?

I am still curious if my work with the carb breather hoses (cutting out the old splits and cuts, and joining the hoses with vacuum connectors) hasn't somehow restricted airflow to the detriment of the bike.

[ducpainter]

You joined the carb breather hoses? I must have missed that.

That won't work. They should have those little filters and be separate.

[pneumadeux]

 No, they are still separate.

When originally disassembling the carbs the two breather hoses displayed cracks down by the little filters. So, I cut the hoses so I could illuminate the cracks.  In order to keep the hoses at their original length, I used little plastic vacuum connectors to rejoin the cut pieces. That means the inner diameter of the hoses is smaller where the connectors are.  I have been concerned that reduction in diameter is impeding performance. To be honest, I'm not entirely certain what the function of those breather hoses are.

[Howie]

Those hoses vent the area above the diaphragms to atmospheric pressure.  The filters are to keep dirt out.  See if the diaphragms move as easily when throttle is applied hoses on and hoses off.  If not, the filters may be clogged Doubtful, but worth a try.