1978 Jota carburettor problems.

[Jota-Brian]

Hello everyone. I would like some help, please, in sorting out what I believe to be carburettor problems on my bike.

It is a 1978 UK Jota. It has done 26,000 miles from new, and is completely unmodified. Standard airbox (no cracks) and filter, original exhaust system, never been messed about with. Standard ignition system (I get mixed up which type is which, but it's the one with a silver Bosch box under the tank with two HT leads coming from one end and one from the other) . New correct spark plugs. Compression checked and is correct, valve clearances correct, ignition timing set by strobe light, good recent battery. I always use Shell Optimate with Millers VSP lead replacement and octane booster. The bike uses no oil to speak of, none comes out of the breather and it does not smoke.

When I bought the bike it had not been run for fourteen years but had been dry stored for that time. I rebuilt the carburettors with proper Dell'Orto bits including the diaphragms. All jets are correct and clean, checked with a magnifying glass to make sure. Float levels are correct. The bike has the balancing tube between the three carbs, this pipework and the T-piece are new.

The problem is this: the bike will not run smoothly at low rpm on small throttle openings. The tickover especially when cold is not reliable or consistent.

When I first ran the bike after recommisioning it, I could not get it to run anything like right, and Stevio correctly diagnosed a blocked pilot gallery on the one carb. There was muck blocking the small hole where the carb fits onto the inlet manifold. I tried carb cleaner and compressed air but it wouldn?t shift it, so I cleaned this out manually (poked it out carefully with an offcut of guitar string), then used carb cleaner and air again. This made it loads better. It blocked up again shortly afterwards, I checked with a magnifying glass- more muck, so I repeated the process again.

The carbs have been balanced with mercury gauges, as per the instructions in the Green Book, and the columns are flickering all within half an inch of each other at the absolute most, and at about 1000 rpm. It will tick over fairly reliably when hot, but not when it is cold. The choke plungers are in good condition (I turned them over to expose the fresh sides when I rebuilt the carbs), and are seating correctly, based on the fact that there is a little free play in the choke cables when it is not in use. Petrol filters on the carbs are clean. Air filter is clean and as new.

But it has never been quite right. I still use it, thinking it might get better with use, but it doesn't. Specifically, when riding on a small throttle opening at low speeds, say in a built up area, it is jerky and sounds like it is hesitating on one cylinder. On larger throttle openings it is a different bike. Open the throttle and it pulls like a train; never a hesitation, doesn't miss a beat. Sounds totally on song. Slow down again, small throttle opening, not running right.

"Many carburettor problems are electrical".... I have been wondering. Many on this forum condemn the standard ignition system, but because the bike pulls strongly from low revs when the throttle is open, no hesitation or misfiring at all, I do not believe it to be causing this particular problem.

Thankyou for taking the time to read all this!

I've tried everything I can think of. Any ideas anyone please? I really would welcome any help whatsoever.

Brian.

 

[lavmv]

<DIV>I would continue to look for blockage...check the filters on the carbs. Is the inside of the tank rust free or are there deposits gravitating thru the system? Are you using a filter before the carbs - some don't like them, but until a few tanks of gas go thru it may be worthwhile to catch any deposits being loosened by your "vibratory cleaner on wheels"<IMG height=19 src="/forum/msnemoticons/emwink.gif" width=19>. If you find any deposits (usually they will appear in the float bowls) - you would need to check the jets for blockage. Where are your air mixture screws set? ...how many turns out? 1 /3/4 is the suggested start but try backing them all out anther 1/2 turn if this is where they are. </DIV> <DIV>Check the choke plungers - were they replaced? It is important to measure the depth of the cavity they sit in and make the cable is longer than that distance...from the bottom of the cast flange to the end of the cable. I use a caliper because I can lock the position but using a small screw driver and marking the depth would work too.</DIV> <DIV>What do the spark plugs look like? Actually its an indicator but I would recommend replacing them even if you only have gone 100 miles. Cut a 1/4 off eachignition wire and reseat the resistors.</DIV> <DIV>After a closer reading your comments on in town riding your problem could surely be ignition related. Carefully set the gap between the pickups and rotor using a brass (non-magnetic) shim ).010" is what I used when I had that system and check the timing. ).010' is a bit less than the standard recommendation but the smaller the gap the lower the RPM at which the advance kicks in. The comment about one cylinder "hesitating" could indicate that the gaps in the two pickups are different and thus the cylinders are advancing at different RPM.</DIV>

 

[Zim]

<DIV>One simple test is try a differant fuel,I have had problems with plug fowling on Optimax in OZ,its great in cars with knock detectors but unless you jet for it my bike does not like it,the only differance with mine is when the plug fowls it does not come good on its own,you need to pull it and hit it with a wire brush.Easy test to do.</DIV>

 

[thethreedellortos]

<DIV>Don't know if this will help...</DIV> <DIV> </DIV> <DIV> have you tried swapping the outer plug leads on their two cylinders to see if the problem can replicated on another pot ??</DIV> <DIV> </DIV> <DIV> When I rebuilt the original Dellortos on my bike, after I ultrasonically cleaned them (RECOMMENDED !!). I discovered that the mixture screw, which should have a small spring, tiny washer and a O ring was missing the steel washer on two fuel mixture screws....This caused all sorts of rough tickover related problems....When I replaced these the transformation was remarkable...</DIV> <DIV> </DIV> <DIV>  Also check the carbs for cracks around the inlet stub clamps and carefully look around the choke circuit casting.....Afew months ago I was lucky enough to buy a large box of Dellorto carbs and sundries....When getting the best carb bodies and bits together I was supprised to find two bodies in a box of ten, had small perferations in the choke circuit casting.....I'll try to explain...</DIV> <DIV> </DIV> <DIV> When you view the choke gallery from the airbox side of the carb and look down at the plunger, follow that passage to the back of the main throttle slide....As the choke journal is cast there is a lead plug inserted to clean out the casting out at manufacture....Its this choke passage that had teeny weeny pin pricks in it......This only became apparent after ultsasonic cleaning as all the crud had been removed......</DIV> <DIV> </DIV> <DIV>                       Good Luck</DIV> <DIV> </DIV> <DIV>                              Tim   </DIV>

 

[Stevio-Laverda]

The easy way to check the progression galleries with carbs built and on the bike is -----remove pilot jets and mixture screws-----open throttle----use a can of carb cleaner wth extension tube fitted -------put the tube up the hole where the pilot jet goes cover the mixture screw hole with finger and squirt cleaner while looking for a stream coming up ino the venturi. just in front of the back edge of te slide.
Then put the tube in the mxture screw hole cover the pilot jet hole with finger and squirt cleaner whilst looking for a stream coming out between the slide and the engine.
Then put the tube into the little air hole under the carb intake (not the "middle" one----the other drilling)----squirt and look for cleaner in venturi.

 

[Scrumpy]

<DIV>Hi Brian</DIV> <DIV> </DIV> <DIV>This isn't necessarily going to fix your problem, but this issue is very similar to one I had with my 1979 Jota some years ago, after "modern" fuel was introduced down here.  It had previously run very well on Champion N2C plugs, but when this struck I put in a set of N3Cs and it went away.  The fuel then improved a bit and I have used NGK B8ES ever since.  I have just fitted a Witt, everything is set up right, and I now have a very mild case of the same thing.  Next weekend I will be trying my trusty N3s to see if that fixes it again.</DIV> <DIV> </DIV> <DIV>Cheers</DIV> <DIV>NickNZ</DIV>