Topic: WOT to start. How do I fix this?

[Patrick]

I have a K2 750 engine in my F3 750 that is equipped with the PD carbs from the original engine. These are my first PD carbs and I do not yet quite understand how to dial them in exactly. The bike starts hard. If I pull the choke all the way open and twist the throttle once to engage the accelerator pump the bike will act like it wants to start, but it won't. The bike will only start if I twist the throttle to WOT, hold it there and then hit the start button. It will then start immediately. I then have to feather the throttle for a short time before the bike will idle on choke. The behavior is the same whether I have the air filter on or not. Once the bike warms up it runs very well. I don't really like having my stone cold engine revving high on start up. Can anyone tell me why this is happening? How do I fix this?

Patrick

[tango911]

assuming the carbs have been gone through and cleaned right?

[Patrick]

Yeah, I completely disassembled them and cleaned them up real nice. Like I said, it runs great once it is warmed up, so I have discounted (for now) the idea of clogged jets. I know opening up the throttle allows the greatest amount of air to pass through and should lean out the mixture. But when I try to start it without choke to lean out the mixture it is a no go.

[Kong]

Well, I'm pretty sure you have contradictory things going on there.  The problem is that you are delivering too much fuel.  That is evidenced by the fact that it will start when held wide open.   As you may know it is the classic way to start a 'flooded' engine, to hold the throttle wide open and crank it.  Well, that is what you have, essentially a flooded engine.  You are exacerbating that condition by using the choke but it is not your use of choke that is causing the problem, that is deeper in the carbs.  You'll have to go in there and start cleaning to see what it is, but the problem more than likely is too much fuel, not too little.

[Patrick]

That's sorta what I was thinking, Kong, but it is the reason for that I am having a hard time grasping. The jets are stock size. I set the floats as per the manual. Perhaps I need to lower the floats a little. Maybe that's my problem. I set the floats with the carbs sitting at about a 40 degree angle, just like they were older style carbs. Is that the right thing to do with PD carbs? Or should they be sitting straight upside down, like the carbs on my Yamahas?

I want to just pull the carbs once more. Anything else you can think of that would cause the carbs to deliver too much fuel at startup and then run correctly when warm?

[Patrick]

OK, carbs on the bench. I reset floats to 15mm. The jets are clear. Anything else I ought to check before I put the carbs back on? Anyone?

[tango911]

OK, carbs on the bench. I reset floats to 15mm. The jets are clear. Anything else I ought to check before I put the carbs back on? Anyone?

i dont think 15 mm is correct.  Should the floats be set at 26mm??  

got the below from this link.  you might want to re check all your settings. Unless the PD carbs are suppose to be 15mm on the floats.
http://oldmanhonda.com/MC/DataTable.html

CB750 F
   
Floats
mm/inch
26/1.02
   
Air screw turns out
1.00
   
Spark plug
D8ESL
   

[TwoTired]

Make certain the choke closes fully on all carbs.
If you have the feather the throttle to keep the R's up, the accel pump is adding richness.

Do you have an open exhaust?
Stock air filter?

Why aren't you using 14.5mm float height as Honda recommends?
Set the float with carbs on their side, float pivot pin vertical.

Have you read the spark plug deposits just after start up?

D8 or D7?

What about the rest of the tune up item checklist?

[mrrch]

Verify the float levels by mounting the carbs in a vice upright, run some clear hose from each drain fitting up along side the carb bodies, when the drains are open it will show the actual fuel height in each carb.
I did this on my carbs and it worked great.

[Patrick]

I went with 15mm instead of 14.5mm because I had previously set them at 14.5mm and I was getting this condition. I set the floats with the carbs almost, but not quite, on their sides. Since the fuel needles have those little wires from the float to the needle putting them right on their side caused the float to pull on the needle and mess up my measurements. I believe the gap is right, though, as I used a magnifying glass to confirm that the float tang was just touching and not depressing the needle when at rest. Also, the 15mm measurement is consistent on all four corners of each float.

I have done a complete tuneup, TT. I set the dwell and the timing previously. It runs really well when warm.

I did not, though, check the color of the plugs (D8EA) after I started it. I will do that after I remount them since I have to check the air screws anyway.

I am going to do that vise thing, though. I have a vise and clear tubing. I also need to check the choke, as long as I have that abomination of a stock airbox (UNI foam filter, freshly oiled) off anyway. I can have the carbs remounted and hooked up within about 15 minutes. The airbox will take about an hour more.

I also have a Mac 4-2 exhaust that is fully baffled.

[Patrick]

For archival purposes, since I hate finding a thread right on point with my problem but not solving my problem because the original poster failed to say whether any suggested solutions worked or not...

I am surprised that a small adjustment to float height can make such a difference, but when I raised my float height from 14.5mm to 15mm my problem went away. It now starts with choke just like it is supposed to.

Patrick