Running Rich

D

dave48

Junior member
Location
Missouri USA
The center cylinder on my 78 Jarama is running extremely rich. Its always ran a little richer than the outside two but all of a sudden has gotten really bad. A new spark plug will turn very black within 10 miles, dry black not wet.
I?ve check everything I can think of checking. Any suggestions? Can fuel leak through the accelerator pump?
Thanks
 
Check the choke isn't hanging on in the spliter cable holder or in the carb top, also the rubber tip on the chock plunger can develop divots that leak. there must be free play on the choke cable. Have I mentioned The Choke enough? Another option is high or overflowing float bowl.
 
Have you set the float heights correctly?
All jets the same size (and which ones are in there)?
Which needles are in there and at which notch is the clip?
How many turns out are the mixture screws on each carb?

A blockage in the carb would normally lead to the mixture leaning out so I would look to all the settings first.
The choke has already been suggested so I won't repeat it again.
 
The float height was correct. I don?t know how that could change but I haven?t rechecked it.
All jets are the same size. Main 122, pilot 55, K1 needle.
Needle clip on center notch.
All 1/34 turns out on mix screw.
Like I said, I?ve checked everything I could think of.
 
Or plug cap.
Try cutting a few mm off the end of the HT lead and screwing the cap back on.
Or try a new set of plugs and caps.
 
its a 180, so, a fault on the centre cylinder could well be ignition related,  as it stands alone, the outer two run together, electrics wise, standard Bosch ignition still fitted? if so swap the boxes over and see if the problem moves to the outer two cylinders, don't do anything else or you will pickle the vision. Next check the pickup wires for rubbing on the rotor, cracking insulation, fraying etc. Do one thing at a time.
CLEM
 
He mentions in his second sentance the plug is dry black so that tell me rich. What is your sync like and what happens if you bring the middle mixture screw in  does the engine speed up ? How did you determine 1 and 3/4 turns esp for all carbs.. 

When did you last check the float levels anyways and what did you do them at I see a lot of carbs where the owner tells me the float levels are fine and yet when I check them they are anything but.

Have you checked also your needle valves as they deteriate over time and are a servicable item one of them may be leaking and its usually the middle one for some reason... .. also the carb bleeds are they blocked being a Jarama means its a US bike so tell me does it have the breather tubes ?? or just the little flat over the vent....
as if that is blocked your float level will become high and make run rich, seen that before.
Doubt it would be out of the pump circuit...

 
pretty-sure-your-compression-is-running-rich.jpg
 
I also have been leaning toward a misfire ignition related problem. Like Clem said the center cylinder is standalone. DMC system was installed in 2013. The pick-up wires had cracked insulation and I repaired them with shrink tubing. I don?t think it?s a wiring problem but could be an intermittent pick-up.

I was not aware that the plug caps were resistor type. Has new coils/plug wires (2013) but it does have the original 40-year-old caps.

Brett ? The carbs were ultrasonic cleaned and all new jets and needle valve about 2500-3000 miles ago. That?s when the floats were last checked. The carbs are the same as a standard 3CL, just a flap over the breather/ overflow. It would sometimes slowly lose RMP?s and die at idle. Changing the mix screw from 1 and ? to 1 and ? turns seemed to help that.

I did check the compression. All three cylinders are about the same. Center cylinder is a couple PSI higher than other two.

I have ordered new float bowl O-rings. When I get them, I?ll pull the carbs and recheck the float height and replace the float needles.

I?ll drag out my oscilloscope and check for intermittent pick-up. I don?t have a high voltage probe so can?t check that.

I?ll try what Grant suggested and cut back the HT lead. but it sounds like I should invest in new plug caps.

Thanks everyone.
 
My brother Tim's 3C had a similar problem a few years back. It suddenly started running as rich as buggery on the centre cylinder, as anyone who rode behind him on the way to the last Laverda rally at Bellingen could attest. They were choking on the black smoke coming out of his pipes! When he got back home we threw another set of carbs on the bike (they were off my Jota after I'd fitted a set of Mikunis). That fixed the problem, so we left those carbs on the bike. His old carbs were chucked in a box and haven't been looked at to this day, so we never did get any closer to a proper diagnosis other than confirming it was a carby issue.  Not much help eh?  :D

As both yours and Tim's rich running problem had a sudden onset, I'd be looking for something that could fail suddenly. There are a couple of things that come to mind ...

Check that the rubber tip on the float needle hasn't fallen off, and that the float valve is still screwed into the carb body with an good gasket.
Also check that the float still does what the name says (floats) and the tab that closes the valve hasn't broken off (seen that once - must have been adjusted once too often) .

I see that you've eliminated the choke plungers as a problem by swapping them around (not actually a choke, but we all call it that). But have a look at the choke valve seat on the middle carb. It's unlikely to be a problem (maybe a chunk fallen out of the edge of the seat or a piece of crud in there stopping the thing from closing) but worth casting your eye on it just to be sure.

BTW, if anyone has a problem with the choke valves not sealing very well, check that the rubber seal in the end of the brass piston isn't worn out. If the circular groove where it sits on top of the valve opening gets too deep, sometimes the plunger can hang on the edge of the groove and not seal. Rather than shelling out a handful of money for new choke plungers, the first thing to try is to pick the rubber seal out of the end and flip it over to expose a fresh surface. If you've already done that and worn out both sides, you can't buy replacement rubbers but you can make them. Get hold of some 3mm fuel resistant rubber sheet and punch them out with a hole punch - 6mm dia I think (memory is a bit hazy on that, so check the punch diameter against the old rubbers). 3mm (1/8") thick x 100mm square Viton sheet is available on eBay for about $8. That's about the cost of a single choke plunger, so if you have a bunch to do it's worth doing. If you regularly work on Dellorto carbs, it'll save quite a bit in the long run.

 
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