Author Topic: Dellorto question (Read 3468 times)

Canuck750 · « **on:** August 30, 2020, 08:26:38 PM »

I have a pair of Dellorto PHF 36mm carbs as used on the Le Mans 850 and many other Italian motorcycles. I have been chasing an erratic idle on a Laverda with these PHF 36's
I gave up on the original carbs as the slides were shot and got a new pair of Dellorto 36mm carb bodies and assembled the components suited to the bike. New slides, atomizers, needles, pilot and main jets but the return valves were original. Pulling the plug leads off one at a time I can get the right carb to run OK on its own but not the left.

After repeated stripping of the carbs I found one of the return valves (on the left carb) was missing the check valve, just an open tube.

This is the return valve I am talking about, it sits beside the float and points down into the float bowl. The tip of the valve will typically be immersed in fuel.

I am guessing that because this tube is missing the check valve this may be the root cause of the poor idle, the idle will suddenly climb way up and after a minute drop down, then back up. I checked for manifold air leaks and there are no leaks.

Anyone know just what this tube / valve does and how it will effect the dunning if there is no check valve in the passage?

fotoguzzi · « **Reply #1 on:** August 30, 2020, 10:19:05 PM »

SED · « **Reply #2 on:** August 30, 2020, 10:58:31 PM »

I think it may be the inlet for the accelerator pump. Missing check ball = smaller jet of fuel when lifting the slide. I'll send you the complete file.

FWIW I sorted uneven idle and poor off idle transition by getting the LM to idle and then turning in the throttle stop screw on one side, then the other. It would stumble just as soon as the slide started to lift and richer idle jets solved it. It was worse on the left and the intake valve guide is badly worn on that side...

Canuck750 · « **Reply #3 on:** August 30, 2020, 11:08:16 PM »

Quote from: SED on August 30, 2020, 10:58:31 PM

I think it may be the inlet for the accelerator pump. Missing check ball = smaller jet of fuel when lifting the slide. I'll send you the complete file.

FWIW I sorted uneven idle and poor off idle transition by getting the LM to idle and then turning in the throttle stop screw on one side, then the other. It would stumble just as soon as the slide started to lift and richer idle jets solved it. It was worse on the left and the intake valve guide is badly worn on that side...

Thanks Shawn, I will take a read of the file,

Jim

1down5up · « **Reply #4 on:** August 31, 2020, 03:23:35 PM »

Missing check ball on the non return valve (which is for the accelerator pump system) shouldn't affect the idle. If its missing the accelerator pump wont function properly as you will not be able to keep any head of fuel in the galley up to the pump - unless the vacuum in the intake of the carb is drawing fuel through the tiny pump jet hole and up through the pump and from the fuel bowl - i doubt it though.

I normally have the other problem - a ball check valve that doesn't seat properly (prob due to crud), so the pump circuit doesn't seal properly and any fuel that gets up into the pump system dribbles out (like taking your finger off a straw full of water)

moto-uno · « **Reply #5 on:** August 31, 2020, 05:11:41 PM »

So why not exchange the 2 and see if the problem goes to the other cylinder ? Then you'll know for sure . Peter

Canuck750 · « **Reply #6 on:** August 31, 2020, 06:21:46 PM »

Today I once again ultrasonic cleaned the carbs and carefully checked that the idle adjuster screws were properly lifting the slides, good O rings and springs. Then I checked the throttle cables. It is a single cable to a splitter and it is all greased and moving smoothly. The slide return springs are new and strong. Bike fires right up, it has electronic ignition and electric start. Just a bit of choke on one carb and it fires up. Problem is after a while the idle goes from 1500 to 4000 rpm for no reason and I have to back the idle screws out to slow it down then idle drops but will stall unless I wind the idle screws back in. Then after a could minutes then idle goes back up. I checked for intake manifold air leaks by spraying WD40
around the rubber intake manifold boots but no change. The o rings on the carb top caps are new so I just can’t figure out this idle speed increase. The

What gives???

Thanks

Jim

SED · « **Reply #7 on:** August 31, 2020, 11:54:28 PM »

My WAG is the idle jets are too lean. How many turns out are the idle mix screws?

Another WAG is that the there are emulsion tubes under the idle jets that are clogged. (don't know why they would be in new carb bodies) Guzziology suggests you can remove them.

Another is, do you have a shrouded needle (lean burn carb bodies)?

Finally, are the atomizers (needle jets) correct? The different designs are denoted by the letters AB or T or ???

(its probably electrical

)

Does it make a difference which carb you choke to start from cold?

When I start the LM3 on the choke it will be too rich in about 20 seconds and start spluttering. Taking the choke off and holding the idle at 1500 with a throttle lock keeps it going, but will eventually (1-2 minutes) run clean and surges up to 4000. The surge is sudden. At this point it still won't idle all the way down to the stops without stalling until it warms a little more.

Apologies if you've tried this or have a better idea, but I would warm it up with a short ride so it is warm, but not hot. Get it to idle on the slide stops and then try richening the idle mix. (my PHFs idle screws limit fuel, not air) If richening makes no difference, try screwing in the idle stop screw on one side - if it starts to stumble idle jet is too lean. Switch sides to check the other carb.

If none of this works it's probably electrical...

Groover · « **Reply #8 on:** September 01, 2020, 08:19:56 AM »

Sounds to me also that is goes lean as the float bowl empties, then once air flow gets stronger when idle goes up, things balance out in the bowl out and idle drops back down? I'd double-check the air inlets (9) in diagram, and also make sure the float isn't sticking in full-bowl state then as the RPM's go up, then the float maybe breaks loose and fills the bowl again dropping the idle. Just thinking this through as everyone here. I'm sure you've checked all those things, but I guess maybe quadrupole-check?

As someone suggested above, I would also borrow the check valve from the right carb to test and be sure that's not the issue.

Good luck

Canuck750 · « **Reply #9 on:** September 01, 2020, 09:43:59 AM »

Good advice guys, more exploring required.

I had the gas tank off an a clear test bottle hung from above and I fitted new clear lines. When the engine was surging to 4000 rpm I noticed that air bubble were travelling back up the clear fuel lines from the carbs to the hanging fuel bottle.

Today I am going to fit my clear plastic carb bowls and observe the floats.

wirespokes · « **Reply #10 on:** September 01, 2020, 12:33:48 PM »

I've never had clear fuel lines so don't know if bubbles are normal, but they give me the idea the vents are partly blocked.

Canuck750 · « **Reply #11 on:** September 01, 2020, 03:11:34 PM »

I cleaned and checked very darn passage again, especially the idle passages in the carb body, air passes through all the tine idle passages and still it will not hardly idle without stalling and then it revs up again, throwing in the towel, loading it into the back of my pickup truck and driving to see the Laverda guru; Wolfgang Haerter in Nakusp B.C. , just a short 1000km drive.

wirespokes · « **Reply #12 on:** September 01, 2020, 04:03:12 PM »

Did you check the vents? The float bowl vents to atmosphere and if that's plugged you'll have problems. Guaranteed!

I was having trouble with the LM3, gas overflowing and wouldn't run right. Turned out a vent line was kinked.

Canuck750 · « **Reply #13 on:** September 01, 2020, 04:47:27 PM »

Quote from: wirespokes on September 01, 2020, 04:03:12 PM

Did you check the vents? The float bowl vents to atmosphere and if that's plugged you'll have problems. Guaranteed!

I was having trouble with the LM3, gas overflowing and wouldn't run right. Turned out a vent line was kinked.

I did check the vents, even pulled the little 90 degree cast fittings out and cleaned them too.

moto-uno · « **Reply #14 on:** September 01, 2020, 07:52:37 PM »

" Canuck750" , are you in B.C, Canada ? I live in Burnaby B.C and have quite a lot of Dellorto parts ( and experience ) .
If I can be of assistance , don't hesitate to PM me . Peter

Canuck750 · « **Reply #15 on:** September 01, 2020, 09:22:30 PM »

Quote from: moto-uno on September 01, 2020, 07:52:37 PM

" Canuck750" , are you in B.C, Canada ? I live in Burnaby B.C and have quite a lot of Dellorto parts ( and experience ) .
If I can be of assistance , don't hesitate to PM me . Peter

Hi Peter, I am in Edmonton, Thank you for your kind offer, leaving early tomorrow morning to go visit Wolfgang, SF1 in the back of the truck.

Canuck750 · « **Reply #16 on:** September 05, 2020, 07:41:21 PM »

On a related Dellorto note I fitted a new pair of 36mm Dellorto pumpers to my 1975 Ducati 860 GT on Friday. I spoke to Steve Alan at Bevel Heaven about the performance of my Ducati and he had me take a close look at the carb bodies and slides, determine the slide fit, and pay attention to the extent of scoring on the slides and carbs. My carbs were just too far gone to waste money fitting new internals and slides, I bit the bullet and went for new carbs with rubber boots to attach to the manifolds. The new carbs have transformed the bike, so smoother, no flat spots, more power, I put on over 200 miles today and I did not expect such an improvement!

The stock carbs are 32mm but Dellorto does not offer new ones, Bevel Heaven sets up the 36mm carbs to suit the 860.

From now on I am going to quit trying to get 40 year old+ carbs to work and just replace with new when possible.

Groover · « **Reply #17 on:** September 06, 2020, 09:42:21 AM »

Thanks for the follow up.

moto-uno · « **Reply #18 on:** September 06, 2020, 05:39:49 PM »

^^ Well where's the fun in that

. No endless stench of gas and bleeding knuckles or stories of woe , my oh my , what has this generation come to ! Peter

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Author Topic: Dellorto question (Read 3468 times)