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Classic, Vintage & Veteran For Coventry and Meriden Models. Anything pre-Hinckley goes.

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Old 11-22-2012, 07:49 PM   #1 (permalink)
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T140 Cold Misfire

Can't seem to nail this one down.
Bike has developed a misfire on all cold starts, in all types of weather, that lasts for a couple of miles then it runs fine. Restarting a warm engine shows no issues.
It seems like it might be from just the left cylinder.

There are new coils, points, condensers, voltage regulator and main wiring harness (but that's another story). Carbs looked clean but were shot with cleaner anyways, set the floats (repeatedly), new O rings on the manifolds, etc, as well as a fresh dose of high octane fuel.

There is ample compression, perhaps a bit much and there is possibly some carbon buildup on the pistons. From what I can wee in the ports, there is a bit of buildup on the backs of the valves, but as far as I can tell not on the seating edges. The engine has never been open, but there are only about 12,000 miles on the clock.

I might add that it fires right up, cold or warm and has never needed choking, but shots from the tickler while running seems to help.

So, any ideas what could cause a cold miss?

Last edited by Beemie; 11-22-2012 at 07:58 PM.
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Old 11-22-2012, 11:11 PM   #2 (permalink)
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Do you ever use your choke for the first few miles to eliminate this? Is it spitting back when accelerating? What is your plug gap and heat range on your plugs? Bob
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Old 11-23-2012, 01:54 AM   #3 (permalink)
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I go along with Colorado Bob on this one & would agree it sounds like useage of choke.
My T140E has similar symptoms I tend to drive with the choke partially on for a mile or so , then turn it off and every thing settles down just fine - I have a handlebar mounted choke and run a pair of Mikunis Crabs .
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Old 11-23-2012, 02:39 PM   #4 (permalink)
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I've owned it since new and it never needed choking.
This is a relatively new symptom that started a couple seasons ago and it happens in both warm and cold seasons.

It does fire right up and idles well, but cracking the throttle brings the misfire.

That being said, choking doesn't cure the problem and I suspect that it has the opposite effect on the cylinder that I think is acting normally.

The plugs are standard issue gapped to spec. I haven't pulled them while it's acting up, but after a "normal" short ride they are tan to light cocoa as always.

I didn't mention it before, but the wires are also relatively new and are solid copper core.
Carbs are not original but replacements about 5 years old with probably a two to three thousand miles and the skip didn't begin when they were installed, but about 3 years later.
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Old 11-23-2012, 08:04 PM   #5 (permalink)
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cold misfire

I usually try new plugs first, even when they're fairly new already. Are the throttle slides starting to wear, allowing too much air in? Usually this causes a poor idle. When the misfire persists after using the choke, I'd think a worn needle is less likely the culprit as this would richen the mixture. I suspect gasoline this time of year when they switch over to the winter mix but your problem is not seasonal. I'd check the little o-ring on the air screw. This could seal poorly until the carb heats up. I'd also try a bubble mix at the carb-manifold junction to test for leaks. I assume you used the thick O-rings but the Amal ones cause more issues when the engine is hot. Being the problem seems one-sided, does the exhaust on that side smell different or is colder when you hold your hand nearby? Is the spark on that side more yellowish? I'd synch the carb slides (cable) if it's been a while. Checking the valve lash can't hurt. My twins spit back thru the carb when cold and given significant throttle. The Amal is better than the Mikuni. Good luck. Bob

Last edited by coloradobob; 11-23-2012 at 08:25 PM.
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Old 11-23-2012, 08:34 PM   #6 (permalink)
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Spitting back thru the carb normally indicates a lean mixture. The mentioon of tickling while running backs this up because tickling tends to flood the engine. Surprised to hear that tickling works, but choking does not. It could be because only the offending carb needs tickling and when you apply the choke, you choke both carbs. I tend to agree with others that the carbs might need synching. The offending carb might have the slide a bit more closed than the other and be right on the ragged edge when the engine is cold.

I would not expect a worn slide if you have already replaced the carbs within 12K of life. You could check by swapping the slides and see if it makes the problem side change.

O-ring leakage has already been mentioned. What about the balance tube between the intakes? Any deterioration or poor connections?

If I am not mistaken, the o-rings on the adjustments screws are not for sealing, but rather to provide friction to maintain the setting.

regards,
Rob
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Old 11-23-2012, 09:23 PM   #7 (permalink)
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Definitely check the valve clearances.When one intake valve is approximately fully open,you should have 0.008" clearance at the other intake.
Do the similar at the exhaust valves,only 0.006" clearance.

It sounds like an air leak at one carb.What are the mixture screw settings for best running with a hot engine?Should be somewhere about 1-1/2 turns from bottomed,with no air leaks or blocked pilot jet.If it's much less,suspect a problem.

Carb cleaner won't fix a blocked pilot jet.Did you test them with a small wire or #78 drill?
If the jets are clear,check the float bowl surface is flat enough to seal the gasket without creating an air leak at the pilot fuel channel.
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Old 11-23-2012, 09:50 PM   #8 (permalink)
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Bob,
I'll throw in another set of plugs to check.

Gasoline is not the issue as it's fresh Sunoco racing fuel from the distributor that is just down the road. The problem has occurred through many tanks of gas.

I do have some needle O rings that I could try too. The carbs are mounted using new thick O rings. I had previously checked the manifold sealing but will do it again.

I should note that there is no spitting back, it's a misfire.

I haven't checked the exhaust temperature, but that's a good idea too.

Cables are sync'd and valves are set.

Basically last winter afforded time to run through just about all the maintenance tweaks that are generally required by the overall service schedule.

Thanks for the ideas. There are a couple that you mentioned that will get done.

Last edited by Beemie; 11-23-2012 at 10:26 PM.
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Old 11-23-2012, 09:57 PM   #9 (permalink)
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Rob,

There's no spit-back. It's a misfire, the feel of an ignition miss, but I'm thinking it's not due to the tickling cure and just about all ignition parts are new.

As I mentioned to Bob, carbs were sync'd very recently but the issue occured both before and after that. But all things are worth rechecking. That one is not so hard to get right though.

The balance tube between the intakes is fairly new and has hose clamps to aid in sealing them. I could test that item by removing the hose and temporarily plug the inlets.

Thanks for the input.

Last edited by Beemie; 11-23-2012 at 10:09 PM.
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Old 11-23-2012, 10:08 PM   #10 (permalink)
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Mr. Pete,
Valve clearances were set as part of the recent overall check.

Carb mixture screws were also just set but I can't say that I paid attention to the number of turns, only the quality and speed of idle. I'll double back and check the number of turns on the settings.

Jets were not checked with a wire or drill; only sprayed, so that's another place to double back.

Float bowl surfaces are flat. They were trued up with wet sanding atop a piece of thick glass.

So there are a couple of ideas there to recheck.


The one that makes sense to me is a manifold air leak that seals when heated, and I seem to recall checking that a while back when this issue started, but that can be run through again too.

Thanks.



Followup: Checking the mixture screw turns shows something odd. The carb that benefits from tickling while warming up is set to about 1 1/2 turns out, but the other non-suspect side is less than 1 turn out. These were set by warm running conditions.
So I guess that means first check for air leaks again and then dive into the pilot jets.
The issue is primarily at cracked to part throttle settings.

Last edited by Beemie; 11-23-2012 at 10:41 PM. Reason: Followup on mixture screw settings
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