Left side not firing

[caricamento]

Having experienced an issue with the left side missing on a throttle pull around the 3-5,000 RPM range, I read that the carb might have a bad float level - but the float level is right on spec.

Carb cleaned down to the individual parts and confirmed jets are clean and right size, but engine is totally unresponsive to adjustment of idle screw or mixture screw. If I remove the air filter and spray some carb cleaner into the chamber, then it will respond with an increase in RPM so I know the plug is firing.

Timing is correct. Points gap (new points) is correct. Valve clearance, spark plug gap is correct, paired BPR6ES plugs and LB05F 5k ohm caps almost check out -- I'm seeing a lot of caution against using "R" plugs and resistor caps, but that's what MG Cycle told me to use and the same setup is working fine on the right side. I thought it would be the plug, but the problem persists if I swap the L & R plugs. So we can discount that. The problem would appear to be with fuel delivery since I get a spirited response from the blast of carb cleaner into the chamber. But I cleaned the daylights out the carb...

Left side gets warm but not hot, apparently from friction but I'm not sure. Exhaust fumes are as cold as the air outside. I can pretty much put my hand on the header pipe and feel the warmth but not get burned. The cylinder head - same thing. Warm but not hot.

The right cylinder is running like a champ even with a known minor vacuum leak. The left side is sealed completely. I am totally at a loss. The carb isn't missing any pieces and it's clean as a whistle.

 

[Kevin.NZ]

Hi Caricamento,
What is the compression like on the LHS and also check the ignition coil, could be breaking down under load
Cheers Kevin

 

[caricamento]

Kevin - thanks for the ideas.

Just when I finished these tests I noticed something really funny with the points visible - the left side points (bottom, green wire) were creating a spark of their own whereas the right side points (top, red wire) weren't. That should be my clue right off the bat (bad capacitor??) It looks like the points are "stealing" the spark??

Checked the coils - they were cold (about 12 degrees C and returning 3.9 ohms for the primary (was unable to get a reading on the secondary for some reason). This is out of spec, so I should replace them soon. In any case the T3 runs identically to before after switching the coils.

Left side: Avg 90 psi running & max 140 at engine shutoff
Right side Avg 120 psi, unable to get running on L cylinder alone

I appreciate your advice.

Alex

 

[john zibell]

Compression test, both plugs out. Hold throttle wide open and spin on the starter only, do not start engine. Let us know what you get. You should be in the 150 psi range for both sides.

 

[caricamento]

OK - I've made progress. First thing - I went out and bought a pair of Champion plugs. That did something for me. The left side is running hot now.

After reading other message boards about motorcycles and lawn mowers it sounds like the problem is caused by either a gunked Pilot Jet (unlikely---this was carb-cleaned twice) or an air leak. The air leak looks like the prime culprit - on inspection, the carb's top gasket isn't up to spec so this would explain why it runs only at or above 1/4 choke AND why the idle screw adjustment made no difference.

Compression test Left(S, not running properly) @ 125 psi and the Right(D, running) @ 120. I fixed the vacuum leak on the Right(D) side. Not sure if that was even an issue in the first place, being so minor.

 

[caricamento]

Guys, thanks for your help earlier with this issue. Almost solved, but not quite. She runs, but the idle mix is clearly out of spec...

I discovered something very interesting. The choke cable lifter has to be extremely high, almost unscrewed out of the hole, but the left side will fire perfectly fine when that happens. It should be 1-2 mm high. It looks about 5mm high to me, and in any case it's almost unseated. I don't think the choke cable needs replacing, it's pretty smooth and the two ends look identical when I compare them (same sizes, same lengths of cable poking out of housing, and same springs). Here's another part of the puzzle - when I try to balance the carbs by getting the engine up to normal operating temperature, removing the opposite plug cap, setting my idle mix and then my idle speed according to the Haynes manual procedure, the left side runs fine on its own. When the right side is also plugged in for normal riding, the left side doesn't activate until I enrich it by raising the aforementioned choke height adjuster. The plug on the right side looks normal, the plug on the left side is carbon fouled. (Yes, I cleaned them then reinstalled for a fresh read).

If my math is correct, there is too much air for too little fuel so it will only run in an "enriched" state. This sucks because of the plug reading matte black. I am certain that there are no vacuum leaks because A. I've sealed the living daylights out of it and B. spraying starter fluid around the carb housing does not generate any engine speed increase whatsoever.

Should I step up the idle jet size by five and pray that I'm not about to waste 15 bucks? Or should I shorten my L side choke and call it a day?

 

[contractor20]

Boy o Boy these things can be extremely frustrating especially when there are a lot of small things combining to give you greif. My round barrel has conspired to set out on several events and then be taken home in disgrace with a mix of ignition and fuelling "Issues" Some, like yours, seem to be resolved only to shift their little quirk to a different rev point or riding condition.
Wish I could offer some significant and magicial solution but no :whistle: Cept another set of eyes can often uncover an oversight.
(Mine has gone from cutting out on one cylinder every time I get level with the driver of the car being overtaken, to just doing it on light throttle doodlin' around town and occasionally at other random times. My spannering reputation has been shredded. After six or seven claims of success I now just get a pittying look from my Ducati riding wife. Strewth whats a bloke to do)

 

[Series4]

Choke plungers can leak air like crazy. Remove the plunger and block the port and see if it makes a difference. Replacing the plungers and O rings made a difference for me. Another thing to check is whether the needle and seat are working correctly. Also check for air leaks where the manifold bolts to the head.

 

[Mc Tool]

My 850t ,turns out has a different weight float in each carb , requiring two significantly different float height settings..... could be worth a look ,( actually its bugging the shit outa me and I am going to get 2 new floats ) and while Im on the floats , I have previously solved fueling problem by polishing the float valve seat and inner bore with a cotton bud and brasso , and the same for the sides of the needle if its not new

 

[caricamento]

Gentlemen, I have seen the writing on the wall... it does not bode well. I didn't think I'd end up dragging the ethanol canard into this, but I have no choice!

http://www.nytimes.com/2011/03/20/autom ... SOHOL.html

"As E10 has worked its way into the nation’s fuel supply, old-car restorers have taken to rebuilding carburetors, whenever possible, with larger jets to let more gasoline into the engine.

With standard jets, the usual problem is drivability, Larry Claypool, a mechanic and restorer in Frankfort, Ill., said. “The cars have hesitation or surging — symptoms of running lean.”

That is not a problem on later models. "In the newer cars that have electronic fuel injection and oxygen sensors, a sensor reads the exhaust and tells the computer to change the mixture,” said Rod Dahlgren, a classic car collector and restorer in Napa, Calif. “A carburetor can’t do that.” "

 

[john zibell]

Gentlemen, I have seen the writing on the wall... it does not bode well. I didn't think I'd end up dragging the ethanol canard into this, but I have no choice!

http://www.nytimes.com/2011/03/20/autom ... SOHOL.html

"As E10 has worked its way into the nation’s fuel supply, old-car restorers have taken to rebuilding carburetors, whenever possible, with larger jets to let more gasoline into the engine.

With standard jets, the usual problem is drivability, Larry Claypool, a mechanic and restorer in Frankfort, Ill., said. “The cars have hesitation or surging — symptoms of running lean.”

That is not a problem on later models. "In the newer cars that have electronic fuel injection and oxygen sensors, a sensor reads the exhaust and tells the computer to change the mixture,” said Rod Dahlgren, a classic car collector and restorer in Napa, Calif. “A carburetor can’t do that.” "

It isn't the ethanol. My MkV runs fine on it.

 

[caricamento]

I will grant that it's not the ethanol per se, but if my gasoline content is being reduced by 10% (e.g. 90% gas, 10% ethanol) then wouldn't it stand to reason that I need to increase gasoline delivery volume by 11% for the idle circuit in order to match the original content? I mean, if I tried to run the whole engine on ethanol it would go nowhere. So I don't particularly buy the argument that ethanol is *just fine* for vintage machines. To say nothing of the rumor I heard the other day that gas in the 70s was routinely 96 octane... that would make today's 87 octane // 90% gasoline pale in comparison. Obviously it wouldn't matter at higher intake levels - we're comparing idle (whisper) to full throttle (scream). But when I'm tuning my carbs find the lowest possible idle point, I would definitely be suspect of emasculated gasoline.

 

[john zibell]

Ethanol is a form of Alcohol and it does burn, just not with the same energy release as gasoline. So, you don't get a 10% reduction on power with 10 % ethanol, I believe it is more in the order of 5% reduction. However, you can run an engine on alcohol, just ask model airplane folks. Also 1970s fuel wasn't 96 octane unless you went to a Sunoco station and paid a premium for it, I was there. So yes you may need to open the idle adjustment screws just a little more with ethanol, but I don't see a need for jetting changes unless you have other modifications. If the right side is firing on the fuel, fuel by being 10% ethanol isn't your problem with the left cylinder.

 

[Cleatusj]

How much slack is in the timing chain?

 

[T Bill]

Sorry but my 850T and SP1000, both carbureted, both run fine on ethanol mix gas. Suppose I've just hexed myself by uttering this. Sounds to me like you have multiple problems. If it was me, I'd put on fresh sparkplug wires and make darn sure it isn't the ignition system first. I just went out and checked and measuring the resistance of the coil secondaries from the plug cap to the engine block, I see 10.5kohm. If you are seeing an open circuit here, you have an open coil secondary, wire, or plug cap. Or maybe a defective ohmeter, also a possibility. The resistors in plug caps have definitely been known to open up. Secondary problems are confusing because the opens will arc over in some conditions and appear to work.

IMHO the worry about ethanol is a red herring. Good luck!
-Bill

 

[bkt]

I have the same problem on my 850 T3 Cali, however in mine right cylinder is the one failing.

Caricamento, did you solve your problem?

Cleatusj, I know I have a little chain slack because on idle it sounds a little. But I do not know how this can be related to this problem.

Any help, suggestions and ideas will be appreciated.

 

[DanPez]

I had my LM3 right side with a similar problem last year.
- Made sure the compression on both cylinders were the same at approx. 150psi
- Made sure the jets were per specs. (Got help here)
- Ordered a gasket/o-ring kit for the carbs …. changed that!
- Adjusted the height on the floats as per manual. (Was confused …. Didn’t know if I was to hold the carburetor upside down or on its side to adjust the height) Upside down was much better.
- Adjusted the lenght for the choke seating at the “on and off” position with a caliper.

The symptoms was on the right side leaner than the left cylinder ….. adjustment of the mixing screw didn’t change a damn thing.
Ended up being the wrong length mixture screws I ordered. :whistle:

This year I had the same problem (On the right cylinder again) ….. but this time the pump accelerator diaphragm wasn’t working properly.

I'll get it eventually :blink: ....... Good luck!

 

[bkt]

Hello.

Thank you Dan for your answer.
Finally I have located the source of my problem, left point was too close.
I have adjusted it and now works fine again.