Trouble with Z1 original 28mm Mikuni

I have trouble with a Z1 28 mm Mikuni carb, the one's with the 17 mm drain plugs.
I have checked every "nook & cranny" of these carbs.
Symptom is they will not go up my steep driveway, can't tell whether they bog down or starve. Engine is always up to temp, before trying!
Pilot 20
Main 117.5
Needle std on 3rd noch.
Here is the strange part... Fitting a set of 26 mm Mikuni Z900-A4 I have no trouble powering up the slope.

>>> About to try # 22 pilots (only type available for these early carbs).
>>> Checking felt washers at top of carb body, as they may be missing and causing air leaksge (would only be small amount of air!)
>>> I'll change fuel to a lower octane (Say 95).
>>> Only other thing that I think it could be is worn needle jet and needles. I will be replacing needle jets.

Can anyone think of what else it could be?
RegardZ

IIRC the early vm28sc carbs have a rather large throttle valve (slide) cutaway which would decrease the velocity of the air pulled through at idle to low throttle. Less velocity would create less pressure differential and make it harder to pull fuel out of the bowl. So I would make sure the fuel level is correct in the bowls. Would also check for air / vacuum leaks on the carb holders.

The 26s being smaller would create more velocity of the air at idle / low throttle and may be one of the reasons they work better on your driveway.

I do have new manifold rubbers with sealant. I realise these carbs have a 2.5 cut-away, as opposed to the later Z1 series with a 1.5 cut-away, hence, I'm thinking about the chances of sucking air in thru where the felt-washers may or may not be.

I will check the manifold rubbers again. I have discovered, on other Z1's, that the retaining screws often bottom-out on the top "blind" holes of the manifold/ head joint.

Being a steep slope that stalls the bike, I have checked the fuel levels at 22, 24 & 26mm float levels with no appreciable difference.
Regardz

I just sold my all original 73 and I can tell you for a fact that it runs just fine with stock exhaust or a 4-1 with stock jetting. The pilot size is not your problem. Where are your mixture screws set at, are your points good & gapped right, did the bike run Ok before you got into the carbs, is your advance mechanism working, are your coils good ? If it ran good before you rebuilt the carbs then Id rule out the worn needle and main jets, but its running on the pilot circuit at that speed. The real Mikuni jets & needles are hardened but a lot of replacement ones arent. You might also check under the needle seats and see if the screens are clean and if gas is flowing out of the petcock OK. One last thing to check is the air bleeds in the front of the carbs. Are all the pipes hot when the motor if idling, if they are the pilot jets are Ok, if no they are clogged. I think youve conffused yourself into thinking the problem is the pilot size and its really something else. PM me if I can help you

These 28mm carbs have not been tried by myself before. I looked at timing (DynaS), timing advance unit, coils, compression, plus valve clearances on this newly reconditioned engine with standard air-box and 4 into 4's fitted.

When I fitted the later A4 carbs, it went well, without hesitation, up my steep drive. Now, as Gd4now has mentioned, it could be to do with the reduced vacuum of these carbs causing them to perform below standard, hence checking of manifold and any other leakage around carb.

All jets are clean, flow from petrol cock is certainly adequate.
Not quite sure what you mean "You might also check under the needle seats and see if the screens are clean" What screens? are you talking about the holes in the side of the Bleed pipe which holds the main jet.

The pilot jets are my last consideration. I'm really only considering them because I noticed that they did market a larger #22 as opposed to the standard #20, and did not mention a smaller one? Why would this be? Thanks for your interest.
RegardZ

Something I did not ask, do the 28sc carbs work well on a flat surface, and only have issue with your steep drive way? Have you done a bench sync and if so what did you set the slides to. IIRC it should be like .6 or .7 mm from the bottom of the notch on the slide to the bottom of the carb bore.

No, if the 26mm carbs worked Ok then the problem is definetly in the 28mm carbs. I had a set on my both of the 73s I sold that were jetted with 20s and thats plenty of jet & Miami is sea level. I had a brand new one in 73 that I raced every week and drove on the street in Miami Fl. The std jetting is not your problem. You have either a partially plugged pilot jet or the air passage going to it that vents it. I use guitar string and spray cleaner on them. For $10 you can buy a variety of sizes of it. In 75 they went to a 17.5 pilot even so I cant buy into needing 22 pilots. The best running stock carbs in my opinion on the street are a set of 76 900 26mm carbs. Not the fastest but the most trouble free. Are you sure you dont have a vacuum leak somewhere ? Have you tried spraying carb cleaner around the carbs with it running. A vacuum leak will show up real fast that way by speeding up the idle. You cant outjet a vacuum leak, you have to fix it. I never asked are you trying this with the airbox on or off

One thing I just thought of that you may not know of is that the VM28SC Z1 carbs use a specific type pilot jet VM28-213 thats different then later model pilot jets and they do not interchange. If you have the wrong ones in your carbs that could be your problem. Z1 Enterprises should have the correct ones. The carbs also came with 112.5 main jets in them not 117.5s

Thanks MtKawboy, I was aware of that (re the only sizes in Pilot jet is #20 & 22 that are available).

I have an easy way to recall which pilot jet for which model. I imagine the word "early" printed on the longer (15mm) shank of the early pilots and the word "late" on the shorter (11mm) shank.

My method of attack will be (while awaiting new pilot jets)
>> Re check new rubber manifold for tightness and leaks.
>> Change petrol to 95 octane (plus it's could be getting stale, by now).
>> Try Z1 carb on and test on slope.
>> If above fails, try a Z1B set I have (120 mains, 17.5 Pilot).
>> Try a Z1R carb set, I will have to put a tee piece in fuel line, to take single fuel inlet (112.5 mains, 15 Pilot).
Meanwhile, if anyone thinks of a basic problem that's is causing this dilemma, please let me know! I guess I'm learning all about these carbs anyway!
RegardZ.

Progress report, or rather non-progress report.

Removed, new rubber manifold rubbers, re-installed with red silicone,
Shortened retaining screws on blind holes upper-side of manifold/ head, just to make sure they were not bottoming out.
Replaced fuel, changed Irdium to normal new spark plugs.
Installed Z1 carb again (117.5 Main 20 pilot).
FAILED to drive up slope.. Three tries.
Will try Z1B carbs next.
RegardZ

Is that a slope or a hill climb ? Your missing something but without the carbs in my hands I cant help you. Somewhere there is a circuit blocked, either air or fuel, or a bad o-ring. Is the airbox off or on ? The B carbs are better anyway

Garn wrote: Progress report, or rather non-progress report.

Removed, new rubber manifold rubbers, re-installed with red silicone,
Shortened retaining screws on blind holes upper-side of manifold/ head, just to make sure they were not bottoming out.
Replaced fuel, changed Irdium to normal new spark plugs.
Installed Z1 carb again (117.5 Main 20 pilot).
FAILED to drive up slope.. Three tries.
Will try Z1B carbs next.
RegardZ

the red sillycone is not helping your problem. Use ThreeBond 1104, or Suzuki Bond on the manifold/head mating surface and let it sit 24 hours before you fire it up.
Don't let people tell you that these products are high temp sillycone sealer. It isn't.
Go with a set of '76 26s and don't look back.