It runs!!! ...but just barely

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May 11, 2007
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Well she finally fired up after 8 months of rebuilding and 50 kicks or so while I tried to figure out which jets to use and idle/air screw settings. However, she don't exactly run that well and I could use some input... First off, the setup: 73 750 running single 34mm Mk2 just cleaned and rebuilt with all new jets and needle (270 main, 106 needle, and 30 idle), big bore 1.5" exhaust system, Pazon statically timed to 31 degrees, new battery, new plugs and leads, Lucas 12V coils (i.e. not new).
It still takes a couple kicks and screwing with the carb screws and throttle before it stays running, and even then it won't idle below 1500 or 2000 without conking out at some point. When it's warm it's marginally easier to start but idle is still inconsistent.
All signs point to lean running: popping even at idle, stumbling off idle when running, lurching and general unsteadiness at low throttle openings with popping. A quick goose of the throttle at idle is guaranteed to backfire if not completely stall it out. Plus my new pipes are already deep yellow. Grrr.... This is all after swapping my idle jet up to 30 from 20, and the biggest idle they make as far as i know is 35. Air screw position seems to make little difference with the 30, and actually when the 20 was in it wouldn't even start unless the screw was fully closed.
But wait - there's more! After all this, even after running for a couple minutes my plugs are completely black, though at least they're dry.
My personal theory is that the Pazon needs 6V coils. They recommend two 6's or a single 12, but say that dual 12's will work on "low" compression engines. Resistance readings across both coils fall within standard specs.
I thought, not being a Combat, the compression on mine was low, but maybe it's borderline and it really needs two 6's instead? Or am I way off thinking it's the coils?
 
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she runs

You say -- statically timed at 31 deg.,-- why no strobe timing?, I'm not familiar with Pazon units but this does not sound right,I think that you need to get your timing corrected before anymore screwing around with the carb.,check that the graduated scale in the primary case is correct before doing anything else, use a degree wheel on the crankshaft or cam shaft to get the right timing mark then set your static timing timing just to get the motor running, then strobe time it, 31deg. 5000 rpm ??.
Good luck. James.
 

L.A.B.

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Scheffy.G said:
My personal theory is that the Pazon needs 6V coils. They recommend two 6's or a single 12, but say that dual 12's will work on "low" compression engines. Resistance readings across both coils fall within standard specs.
I thought, not being a Combat, the compression on mine was low, but maybe it's borderline and it really needs two 6's instead? Or am I way off thinking it's the coils?

I'm not sure that the 12V coils will actually be the cause of the problem or not?
But maybe they are not helping matters? 6V coils (and the necessary ballast resistor) would have been the standard setup on a '73 model.

The Pazon instructions recommend that twin 12V coils can be used on engines with less than 9:1 compression ratio, and a standard 1973 750 compression ratio would be 9:1, that is assuming that your pistons/Cyl. head are at the standard compression ratio? The Combat being 10:1 ratio, (but that was achieved by skimming 40 thou from the Combat heads).


james said:
You say -- statically timed at 31 deg.,-- why no strobe timing?, I'm not familiar with Pazon units but this does not sound right,I think that you need to get your timing corrected before anymore screwing around with the carb.

The Pazon is initially set similar to the Boyer unit (31 degrees BTDC) static, but does still need to be strobed, -only this has to be done at 4000 RPM with the Pazon (or can be done at 3000 RPM minus 2 degrees on a newly rebuilt engine).

Pazon Sure-Fire instructions: http://www.pazon.com/files/PDF/PA2.pdf
 
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Static timing's the best I can do at the moment since my strobe is currently out of commission, and I did it with a degree wheel (indicator on the inside of the primary is 4 degrees off). Twice actually - has to be done during the initial install, and I did it again today after I couldn't get it running right just to make sure I did it right the first time.
Was hoping I could just get it running somewhat reasonably until the new strobe gets here but not sure if that's the case...
 
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I'm pretty sure your pilot jet should be a 15 with the air screw initially one and a half turns out. I have found the choke seal missing on one problem Mk2 carb, (it would freeze up on wet days) and on another the fuel / air drillings were blocked with what looked like plastic packing. It might be worth while to have a close look at the carb and check the inlet manifold to head joint isn't leaking.

Having said that; I reckon you want to get your hands on a pair of 6 volt coils or a dual output 12 volter, hell, you need the best spark you can get. Is there no one near you can borrow coils from? A second hand Jap points coil would do for a trial.

Cash
 
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A read though this thread from 2004 has it been that long? Might be of help.
I believe you might want to check and see if your still running the alum. bushing in the throat. Or one of those fine new bright white floats they like to sell. It is sometimes challenging to get the enricher to stay all the way off on these pieces as well . Read on : http://accessnorton.com/norton_commando ... t=mark+two
 
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Where exactly is that bushing located? Is it the little pressed-in one at the 6 o-clock pos. at the mouth of the carb? And by removing it, do you mean just removing it and running it wide open, or blocking it off with something? So many questions, so little comprehension (on my part).
 
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Air jet is an alum bushing behind the air filter adapter it just sits in a hole .
A UK OR GENERAL EXPORT MODEL OR SOMEONE IN THE US MIGHT HAVE PUT A EUROPEAN KIT IN THE BIKE: .106 NEEDLE JET, 3 1/2 SLIDE, 2A1 NEEDLE, 2A1 NEEDLE,15 PILOT JET, 35 CHOKE JET, (AIR PLUG REMOVED).
Mark two's are sensitive about needle jet wear as little as >0005 can cause problems.
 
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Note that with this set up they were using a 15 PILOT JET I have used a 25 to get mine running right with the bushing removed and fresh needle jets. Smaller hole =faster air =more fuel
 
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Amal say "The air jet controls the amount of air which pre-atomises the fuel before it enters the mixing chamber body". It influences the depression on the main jet and will have little is any effect on the pilot circuit. Basically the 2.5 used on two strokes increases the depression above that of the 3.5 used on four strokes and therefore it sucks more fuel out of the main jet. If you remove it you'll run lean on full throttle.

If I were you, I'd put the carb back to the standard setting 'cept for a 15 or 20 pilot jet, cos that's what most fit, set the air screw one and a half turns out, fit a brand new pair of plugs. Then start doing one thing at a time. And I'd first start with the coils. If two 12v coils will only work on a low compression engine the spark has just got to be weak, so that's the first thing to put right.


If you can't get the loan of any coils I'll post you a dual one to try.

Cash
 
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Depends on how the carb started life you can't mix and match the two set ups. From another board: Mr. Healy was good enough to post this.
-------------------------
The MKII carbs came with two basic set-ups for T140s:
European (non-EPA)
124/026 #15 pilot jet
124/012 #35 choke jet
2928/122 .106 needle jet
2928/030 #3 1/2 slide (leaner than US equivelent)
2622/124 2A1 needle
No air jet fitted

US EPA
124/026 #25 pilot jet
124/026 #50 choke jet
2928/031 .105 (special stainless steel)
2926/060 #3 slide
2928/030 2C3 needle
An air jet was fitted to the central air inlet on US EPA models.
This, of course, assumes a factory outfitted bike.
--------------------------
And yes there may be a coil problem. But if you are running the 106 jet than you are going to need to check all the other parts in the carb to see if they are a set. You are running it as a single carb your going to need to adjust it in of course.
 
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The PO must have been swapping stuff out before so I'm not sure what it originated as. Here's what was in it when I got it:
#20 pilot
#50 choke
.106 needle jet
3.5 slide
2A1 needle
300 main (seemed ridiculously big to me)
Also fitted with air jet, though not sure of the size since i haven't pulled it out yet.

New coils are already on order. I figured with the rest of the electrics replaced I might as well get those in there as well and I can use all the spark I can get anyway. Plus, while they gave decent readings, the one body was visibly pinched by overtightening the clamp so I figured it was only a matter of time before vibrations made it start shorting out. The offer to ship a tester 12V is much appreciated though - thanks cash.
 
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Well you need the coils but if you remove the bushing I think you will get to the carb problem. I would not run with the USA EPA settings. And someone has gone and changed it all to the euro settings but not this bushing?That would do it.
 
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My apologies for beating this topic to death. I took the carb off to remove the bushing (I'm assuming just heat around it and pull it out? After cleaning out all the gas of course... :shock: ) and I noticed the opening immediately to the left of it at about 7 o'clock has a plug in it. Is this plug to remain in place, or should it also be removed while I'm in the process of removing the air jet?
Note this is still on the face of the carb; I'm not talking about the blanking plug which appears to screw into the passage leading from the plugged hole, the one that can hold the pilot jet.
 
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Ok, here's where I'm at now: timing, #25 pilot and new coils seemed to cure 95% of the problem and it's actually running fairly well now. My one remaining "serious" problem is a stumble immediately after opening the throttle. If I keep the idle at 1K or lower I run the risk of it stalling when I first open the throttle. I consider it a serious problem since the traffic lights here seem to stay red for 10 minutes before turning green, which puts me in a particularly uncomfortable position when the line of traffic that's been sitting behind me for 9.5 minutes realizes the only thing now stopping them from moving is something much much smaller than their Ford F95000 SuperDutyDoubleCrewMountainMover dually tri-axle monstrosity... and it doesn't even have a gun rack! But I digress...
Once the RPMs are up even a little everything's fine, it's just that initial opening that's the trick. To be honest I was excited enough to have it not stalling below a 2K idle so the time I spent adjusting the air screw position was minimal - is this where the problem lies? Or am I looking at another $40 getting a new slide? Running a 3.5 slide now.
 
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Now it's time to strobe light the timing. You have to get it right before going any deeper. I have found that a stumble off idle is normally a lean spot you might find the highest idle with the air screw adjustment and then turn the screws in to the richest setting the idle will allow it will be about 1/4 to 1/2 a turn. You might try different needle setting by lowering the clip by one which raises the needle, just to see how it effects all the mid-range running. after it has run a while out on the road re tighten the carb spigot clamps a bit.
 
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