Dellorto PHBL 22 AS Jetting?

[Samurai]

Hi Guys,

After trying for ages to get consistent running from the original MB22B carb, I've finally caved and fitted a modern Dellorto PHBL22AS for my 175TSs. Eurocarb here in the UK were very helpful and provided a slight change to the standard jetting, based upon the original settings for the MB22B, to get me in the right ball park.

However, although I noted down the original settings as supplied and methodically worked my way through adjustment, I can't get the bike to fire and idle for more than about 5 secs tops and it dies immediately if you try to give it any choke or open the throttle. When it does fire, the plug is black an sooty and the engine often stops with it spitting back through the carb and a strong smell of fuel.

The best results (if that's what you can call them), are when the idle mixture screw is turned all the way in (thus weakening the mixture) and the idle speed screw turned out about 5 turns (just lifting the slide slightly), so I'm guessing that the 45 Idle jet is way too big.

My question is, what is a sensible amount to drop in idle jet size? I obviously don't want to order a whole bunch in single steps just for the hell of it, it'd be way too expensive and wasteful, but neither do I want to jump wildly about and go to the other end of the problem scale and make it too weak! So, is a drop down by steps of '5' too much? The idle jet fitted at the moment is a '45', so I'd be looking at a '40' and maybe a '35' to maybe get me in the right ball park.

By the way, I've checked that there is slack in the throttle and choke cable and that the timing and spark seem Ok. The jets currently fitted as supplied are:
Idle Jet = 45
Choke Jet = 70
Needle Jet = 268k
Main Jet 98
Slide = 40
Float Height = 24
Needle = D24

Any advice is greatly appreciated, this is the bike that just keeps on giving . . me grief that is

Cheers,

Jase.

[ranton_rambler]

I'm feeling a bit guilty as I suggested you go to Eurocarb.......

I had a look in my books and depending which one you take as correct, the idle jet should be either 40 or 45 for a 175TS, so I seems unlikely it's too big. Normally when I've been unable to get the idle mixture lean enough on my Guzzis with Dellorto carbs, I've found it's either the needle valve not shutting off properly (dirt on the seat) or the cold start plunger not sealing properly. Unlikely to be either as the carb is new.
Are you running with or without an air cleaner?
Another thought 1- if they have swapped the idle jet, is it screwed in tight?
Another thought 2 - if you have the idle speed screw holding the slide up, is the needle/needle jet influencing the idle mixture?
Another thought 3 - if it's spitting back through the carb, are you absolutely sure the ignition timing is right? Can you put the old carb back on check timing with a strobe?

I have all of this to come, although was going to go for a PHBL 24 for mine.

Ian

[Samurai]

Hi Ian,

All good questions! Going to speak to Rob at Eurocarb today to see what he suggests as well, but I will double-check all your points in the meantime and update you on my findings.

The bike is just running without an air filter, just a trumpet with gauze filter to stop small rodents from nesting in there

I started this project knowing nothing about bevel gear shimming or electrics, beyond changing a fuse and have now rebuilt the entire engine and made a brand new functioning wiring loom. Looks like the bike wants to teach me about carbs now

[Samurai]

Just an update to your suggestions:

1. Double-checked timing and static timing is spot on. It was also firing up and. Idling ok with the old carb, although that was flooding easily too.

2. Double checked the position of the slide and I did try to start it at the very bottom of it's travel, as I had during the initial tests as well, but to no avail.

3. Raised the float height to 24.5mm (max spec), as well as checking for leaks, but no change.

I've ordered up pilot jets in steps of 5 from 30 up to 40, which will hopefully give me a 'starting' point, so to speak! I've also ordered a 50 to go slightly richer than the current 45, just because that is what is standard for the PHBL22 and I'm barking up the wrong tree for some reason.

I'll let you all know how I get on when they arrive

[Samurai]

Ok, I fitted each of the jets in turn, started out by resetting everything to the supplied settings each time e.g. mixture screw 1 1/4 turns out, idle speed screw 4 turns out (lifting slide a tad) and still no joy

Result of methodical experimentation was the same as before. Screw the mixture screw in nearly all the way and screw in the idle speed screw so it was two turns from full home. This resulted in the bike trying to run for a split second nearly every time with a clean plug and a .30 idle jet fitted (gave the best result of all of them).

I've slackened off the 'choke' cable so much that is actually trying to disconnect itself from the lever when off, so I'm pretty sure that isn't the cause of the problem.

The floats themselves show no sign of leaking and the float level is set to 24.5mm, the maximum end of the spec.

After a number of attempts at starting, some of which involve spitting back through the carb, fuel is just and just weeping out of the two small holes at the front of the venturie. There also seems to be a lack of compression if I let the slide bottom out all the way? I've done a compression test and am getting a consistent 150 psi with the throttle fully open, so there doesn't seem to be anything wrong in that area?

The needle is in the second to highest slot, si is probably lifting slightly when the piston is raised by the speed screw, but I didn't think that jet had any affect on starting or idling?

Anyone with more experience able to throw me some suggestions?

[double diamond]

I’ll share a few guidelinesthat were recently reinforced as I addressed jetting issues on two of my own motorcycles. Assuming your cam timing is correct and you have an appropriate exhaust system installed, it is imperative that you have no air leaks in the intake system. This includes the mating surface of the intake manifold to the cylinder head (flange is flat, air tight seal at the gasket) and that the carb has a snug fit to the intake manifold pipe. It appears that the PHBL has a plastic sleeve inserted in the body that slides on to the intake manifold. This should be a snug fit on the manifold; if the fit is loose, you may have an air leak. Your comment “dies immediately if you try to give it any choke or open the throttle” suggests that you are trying to start the engine without the enrichening circuit (choke) deployed. When cold, the engine requires a rich mixture in order to start. If you are able to start the engine without the enrichening circuit deployed, this would indicate that the jetting is too rich. Ideally, you would start the engine with the enrichening circuit deployed, then, when the engine is up to normal operating temperature, you would close the enrichening circuit and transition to operating on the normal fuel supply circuits (mixture screw, pilot jet, etc.). So you should be aiming to get the engine to start with the enrichening circuit open, run the engine on the richened mixture until it’s up to operating temperature, close the enrichening circuit and have it idle steadily on the idle circuit. The enrichment circuit can be “jetted” the same as other circuits, so if it seems your mixture is too rich or too lean during starting/warmup, you can adjust the mixture accordingly. The idle mixture screw on the PHBL is downstream of the main fuel circuit, which typically means this is a fuel metering screw (rather than air metering, which is typically upstream of the main fuel circuit). So turning the screw in leans the idle mixture, out is richer. The idle mixture ideally is set independently of the pilot circuit at idle. If the pilot jet is too rich or lean, the idle mixture screw can be used to compensate off-idle response but this is not ideal. The mixture screw should be adjusted in its normal range (usually 1.5 - 3 turns) at normal tickover. If it is out of this range, the pilot jet is probably too rich (less than 1.5 turns) or lean (more than 3 turns). At idle, you should be able to adjust the mixture screw within its range to achieve the highest possible idle speed. Above and below this high idle, idle speed should drop off. If idle speed continues to increase out of range, the pilot jet is too rich/lean (depending on which way the mixture screw is being adjusted). You can also try measuring the cutaway on the slide and compare with the MB22B. I would expect they should be the same. Cutaway is going to have significant effect on off-idle transition. The taper on the needle doesn’t clear the needle jet until the slide has been raised at least quarter throttle, so the needle position doesn’t come into play until the taper clears the needle jet. Needle jet size will come into play before this point as vacuum in the intake will begin to draw fuel up through the main needle circuit before the taper clears the needle jet. This is where needle jet size comes into play. A 268 needle jet seems a size too big; most Ducati carbs use a 265 but I’m referencing the UBF series carbs here. Hope this gives you some direction.

Matt

[Samurai]

Hi Matt,
Fantastic advice, many thanks!

Some of this I knew, or had recently learned, but some of the other things you mentioned, like the air leaks and needle jet size, are things which were either just hovering on the outside of my mind, or which I was completely unaware of. Many thanks for the explanations and troubleshooting tips, I'll try and run these down at the weekend and report back.

There do seem to be some contradictory things happening here, so it maybe as you suggest a combination of things. What I do know is that with the MB22B, I have been able to get the bike idling well and ridable (sort of), at least enough to get me 25 miles to the MOT and back with a lot of caughing, spluttering and sooty plugs. Although it was always a bit of a pain to get it fired up to start with, even with the choke fully down in the Venturi. The bike is trying to fire up with the new carb too, but only for a second or two at best.

I'm therefore presuming that the basic engine setup is not a million miles away but am going back through the basics once more to triple-check!

Many thanks once again for the comprehensive help, it's very much appreciated, as I'm at the limits of my mechanical knowledge when it comes to setting up carbs!

[double diamond]

Another thing to consider is the heat range of the spark plug you're running. NGK is the manufacturer I'm most familiar with. The heat range recommended by NGK is a B6HS, which is a pretty hot heat range. If you're running a colder heat range, this will exacerbate your starting/jetting problems.

Matt

[Samurai]

Hi Chaps,

A more positive update on the jetting issues today

Using Double-Diamond's (Matt's) excellent troubleshooting guide, I checked for air leaks around the carb/manifold and found that in my haste & excitement when fitting the engine to the frame for the first time, I had used one of the old manifold gaskets that came with the bike and it had seen better days. So I replaced this, checked the manifold itself to ensure it was level and clean and put a smear of high-temperature sealant on each of faces of the two gaskets and 'insulator' that make up the manifold sandwich. I also used PTFE tape on the manifold stub to help ensure a good carb seal.

This did seem to help produce slightly better compression and I duly set about methodically working my way down from the .45 jet supplied and at each stage starting with 1 1/4 turns out for the mixture and four turns out for the idle speed screw. I experimented by carefully adjusting the mixture each way and seeing if the engine appeared to try and fire better/worse. At the larger sizes, ANY use of the choke just seemed to kill any enthusiasm on the part of the engine to even try and fire. Plug inspection seemed to agree that the mixture was too rich.

Ultimately, I managed to get the bike fired and idling well using a 0.30 jet, 1 1/2 turns out on the mixture screw, the idle speed set to about 4 turns out and the needle itself on the highest notch e.g. leanest position. Incidentally, the position of the latter had almost as big effect on the bike's attempts to start as playing with the mixture. If I left the slide at the very bottom of it's travel, which logic told me should result in the highest speed of airflow through the venturi, I got next to no compression. As I screwed the speed screw in, there came a definite point where compression was re-established and the bike made some attempt at firing.

So I went with my gut and what the engine appeared to be telling me and slowly arrived at the settings above. It actually transpires that I also need to give the bike at least 1/4 throttle when kicking it over, but with these settings it now fires pretty much first kick every time and eventually settles down to an even idle. However, it starts without ANY choke, so I think that I'm going to have to reduce the choke jet size significantly from the current .70.

The throttle response was good at rest e.g. not hanging when released and seeming to pick up Ok, so I went for a test ride. However, I seem to be back at pretty much the same point as I was with the old MB22B now e.g. the bike will pull away on the level at 1/4 throttle fine, but if there is an incline and you give it any more, it starts juddering and surging. Wacking open the throttle wide seems to produce a slightly better result temporarily, but the surging soon comes back, almost as if the bike is running out of fuel and only reduces if you close the throttle to 1/4 or below again.

From the information that Double-Diamond supplied, together with other symptoms found elsewhere on the internet, am I correct in thinking that I'm going to have to reduce the needle jet size down a size or two, from the 268k to something like a 266k or 264k? If I'm understanding things correctly, then I believe this will help lean out the starting circuit further and together with a smaller choke jet, will give a more correct setting for this region of the setup and allow me to use the choke from cold.?

To address the surging and juddering from 1/4 throttle onwards, am I right in thinking that with the smaller needle jet to aid the starting, that I will then be able to raise the needle position to provide more fuel for the mid-range?

Any further advice from those more experienced than this tuning novice, would be much appreciated to give me some more confidence in my diagnosis. I don't mind buying replacement jets, but as you can appreciate although the idle jets are relatively cheap, the cost of the rest of them is significantly more and I don't want to waste money for the sake of it, especially as I've just shelled out for a new carb

Still, we're getting there!

Thanks,

Jason.

[Bevel bob]

My thinking is that when on the move the needle and needle jet play the biggest part and you are seeing a rich plug,that would say to me that you are too rich. Richness also makes for jerky running which can make you think the fuel supply is intermittent, but its the firing thats intermitent due to the plug fouling.