Can you guys help me out with carb tuning a bit?
The weather has been pretty good over the past few weekends, and I have been able to put quite a few miles on my 750 Sport. Its running pretty good, but still a few niggling things I can't quite figure. I have a stock motor with Dellorto PFM 36 carbs, 2" rubber elbows and big UNI filters. I have some high output coils, and run hi test gas. I also have a 2 into 1 GiaCaMoto header and a Supertrapp silencer.

It starts right up, with a little bit of choke, and NO throttle. If I open the throttle, its really hard to start.
While rolling down the street with almost no load, I get a bit of burbling stacato sound out of the motor. It's just not even.

When I crack the throttle, everything gets better. it accelerates right up to redline, and the power feels very much like the curves I have seen posted for this motor. Even slight acceleration makes it run smoothly, but running at constant speeds with no load it isn't happy.

When it gets warm, it has a high idle around 2-2.5k that wavers a bit, and some times settles down to 1k. If I put it in gear and drag the motor down, then it will sit right at 1k forever.

It doesn't backfire at all on decelleration, or between shifts. It does sometimes backfire when I start it up and its cold, but that goes away quickly.

I am running the back cylinder one step richer on the main jets than the front .
I have done plug checks at full throttle, and the plugs look good. I haven't taken a plug check after a ride yet, but will do that tonight.

Any help as to which way I should try next would be of great help.
Leaner, or richer? Pilot, needle, or mains?

Thanks,

You could have a case of the 'dimples' on your throttle slides. Read this:

viewtopic.php?f=1&t=448

Yeah, I thought of that, but the symtptoms aren't the same. It idles fine when its cold, or cool, but has a high idle when its get hot.

I'm wondering if my floats are set too high. Should they be on the high side or the low side of the spec.? I think I set them to 10mm, but that was 3 years ago now.

Cheers,

I can't advise you about float height - I've never had any trouble with that. But the symptoms you describe sound like what I had (and also what Paul in South Africa had) before I found and got rid of the wear dimples on my slides, and set my idle using the 'Ducati Cuddle' technique.

I filed down the dimples on the slides on my Delortos and adjusted them the best I could with the "Ducati Cuddle", and my bike is acting similiarly. I have to use choke and no throttle to get it to start. While it is cold, it is rev'ing at about 1100-1200 rpm. Once the bike gets warm, it shoots up between 2500 and 3000 rpm. It runs fine other than this. I've had a few people suggest that it may be an air leak, but I have no idea, yet. I was hoping to mess with the bike some this last weekend, but I found other things to occupy my time. I'll be interested to hear what it is....if you get it straightened out.

Here's what my slides used to look like...

Obviously, for the motor to run properly, whether at idle or full throttle, the carburetors must be set so that each piston is doing the same work as the other at the same time.

At small throttle openings, very small movements of the slides (1 mm or less) can alter the engine speed by several hundred rpm, but only if the carburetors are already properly, or almost properly, adjusted, with each cylinder doing the same work.

The motor will run with the slides way out of adjustment, but in that situation, small adjustments to the slides or the mixture screws will have little effect. It is only when the carburetors are close to perfect setting that small adjustments have their real effect.

If your idle is racing when the motor is hot, the mixture screws could be set wrong.

It can take a long time to get your idle and off-idle settings right, but you should be able to do it using the links given under 'Del Orto Carbs' in the 750 Paso Technical FAQ Digest. But you have to follow every step, starting with dimple-free slides as low as possible in the carbs, and plenty of slack in the control cables.

It sounds like a lean condition caused by the carbs or an air leak. The "floaty" idle is usually lean at idle due to mixture settings. The float, unless it's flooding had no effect (or shouldn't) at idle. It doesn't say if you've balanced the carbs but to get a good base line you must do these tedious tasks.

How do the slides fit in the bodies?
Throttle shafts tight?
Compression? Leak Down?

Soooo many questions! No answers.

Where did you find the 2" rubber el's?? You've posted this info before but I can't find it.

John,
Thanks for the hint. I was thinking that it was lean also, just wanted another opinion.
The idle air mixture screw regulates how much air enters the pilot system, right? So that means if I turn it in, it will be richer. Turn it out, and it will be leaner. Right?

I'm trying to remember the old carb tuning rules. If it starts with no throttle its lean, if it starts with open throttle its rich? Is that right? Backfiring under deceleration is rich, right?

I followed the instructions on the Dellorto cuddle, and ran through the series of steps described, which was very helpful and got me to where it is now. I think now I will play with the mixture screws when it it hot and tune it from there. I think I might also go up one more on the pilot jet and see how it runs from there. I did all this last year some time, so its due to go through it again.

I got my rubber elbows from Airflow online.
http://www.airflo.com/rubberparts.html
or
http://store.airflo.com/rubber-hoses---clamps.html
part number 90HL2. They work just dandy and let you put nice big air cleaners up under the tank where the old airbox once lived. You will also need some short sections of 2" metal tubing to act as joiners between the rubber elbows and the air cleaners, as they will both have the same I.D. I found some 2" thinwall stainless exhaust pipe extensions at the autoparts store for cheap. You will need a total of 6 - 2" hose clamps. It also lets you pick up cooler air under the tank than the hotter air off the cylinders.

Cheers,

If the mixture screw is near the back end of the carburetor (ie near the inlet valve) then turning it in will reduce the fuel flowing through the idle system, making the mixture leaner.

If the mixture screw is at the other end of the carburetor, (ie near the mouth) turning it in will reduce the air, making it richer.

I found part of the problem with my Dellortos....I had the cables adjusted with almost no slack, so when I went to lower the idle, the slides were not dropping. Now I have the idle a little too low and it backfires at rpms under 4000. I rode it about 10 miles today while tweaking it. I may take it to a buddy of mine so that he can synchronize the carbs. I'm working ten-hour shifts at work and have gone back to school, so I'll have limited time to work on all the bikes and cars for a while.

Ok, mate. Good luck!

It sure pays to have a good bit of slack in the throttle cables. I reckon I've got anywhere up to a quarter of an inch movement before the slides start to lift.

Right. Having your cables in good condition is very important. For the plugs to keep running clean with both firing properly, those slides must move at exactly the same instant, which can be achieved using the 'Ducati Cuddle'.

They're fun to ride without the bodywork, though. I rode mine for a while once without even a petrol tank! I made myself a temporary one from one of those tennis ball cans. You could really flick the bike around.