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

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Old 03-04-2012, 03:35 PM   #1 (permalink)
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76 T140 Backfiring

apologies if there's a post on here that might resolve this but i can't find it through lots of search words...

I've worked through a series of problems and here's where I'm at:

Boyer EI & Boyer PB newly installed by a pro
new 6V coils
Electrical wiring gone through meticously by a pro
(above was done prior to discovering a battery with 1 bad cell - just noting in case relevant)

New battery
new spark plugs
tore apart carbs and sprayed with/dipped in carb cleaner to clean these buggers thoroughly, cleaned pilot hole with guitar string, blew air through everything re-installed with new needles and gaskets.
new external fuel filters (both old ones were chrome plated in/out and 1 had the inner chrome peeling away under the filter - news ones are anodized)
new petcocks with filters
fuel tank relined with Por 15
checked valve clearances - in at .002 and ex at .004

she starts perfectly, idles nicely and even ran up and down the block through 5th gear very nicely yesterday am before weekend plans - a very light backfire or two - but had full throttle control and reponse so felt my problems were finally gone.

took her out this am for a longer run after about a 1.5 miles started getting backfires and bad throttle response in gears 1-4 in the 2500-4500 rpm range (1/4 to 3/4 throttle range) but post 4500 rpm she'd run great, put in 5th and she backfires through throttle but if I just went into cruise mode with up to 1/4 throttle she purred like a kitten.

Read somewhere I may need to check the timing on my EI - but this was just installed and also read that once Boyers are set they really don't need to be tampered with.

could this be the valves? should I open the head and see if they need to be replaced or get some grinding compound?

would love advice on what to do next. I've watched way too many warm winter days go by in Cali and am officially frustrated.

Last edited by furious furball; 03-04-2012 at 10:59 PM.
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Old 03-04-2012, 09:32 PM   #2 (permalink)
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Hi FF,

It still sounds like you have an electrical problem. At a guess I would say the battery. The symptoms you describe are the same I as I had when I blew a fuse; well the bike not me . Basically, without the battery there is not enough power from the alternator to run the EI unless the bike is running at or above 4.5K RPM.
It could be that you have another electrical component draining the battery.
I would suggest try charging the battery and giving the bike a run, if it runs OK disconnect the battery next time you plan on leaving the bike for a day or two, if everything is OK once you have reconnected the battery, you know you have something draining it., if not you must have a dud battery.

Good Luck

Webby
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Old 03-04-2012, 10:47 PM   #3 (permalink)
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Originally Posted by furious furball View Post
could this be the valves? should I open the head and see if they need to be replaced or get some grinding compound?

would love advice on what to do next.
It could have something to do with valves.Unless you've changed the camshafts in this engine,you should be setting valve clearances by the book:0.008" INTAKE , 0.006" EXHAUST.

Adjust the valves to that,then run the engine with no exhaust pipes for about 30 seconds.Try to make a lot of noise,without excessive revs.Opon the throttle,then fully close itn/off,on off,on off.

When the engine cools down check the valve clearances again,and fit the pipes back on.You might have more compression by then.


There could be other things to look at,but that would be a good start.ALL E.I systems need strobe timing.Do that too.

Last edited by Mr.Pete; 03-04-2012 at 10:50 PM.
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Old 03-04-2012, 10:56 PM   #4 (permalink)
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Sorry Webby - my bad, the new battery came after discovering the prior battery had a bad cell. The new battery holds a perfect charge even after sitting for a week. Old battery wouldn't hold a charge for an hour or two. I'm going to fix my post as I see the confusion there.

Mr Pete - you say going by the book - my Haynes manual says IN is .002 and EX is .004 - I wouldn't normally challenge you here as I respect your advice but just want to make 100% certain as this is a really big difference in a setting like this.

In my downtime trying to figure things out I did put the valves to .002 / .004 as noted above and since then have been having this trouble with back fires and throttle response so perhaps you are right but please do confirm this much of a change in valve clearances is ok.
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Old 03-05-2012, 04:07 AM   #5 (permalink)
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It's time to look at a better manual:http://classicbike.biz/
650 units used 0.002" intake,0.004" exhaust.
T140 has different cams,0.008" INTAKE and 0.006" EXHAUST is right.
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Old 03-06-2012, 08:43 AM   #6 (permalink)
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Mr Pete - that classicbike.biz link is brilliant - thanks for posting it!
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Old 03-08-2012, 10:28 PM   #7 (permalink)
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Mr Furball -
You need to find the correct workshop book for the model you own, and not trust the Haynes. Valve clearances changed for 750 twins in the mid-70's. Haynes is noted for all sorts of mistakes as they don't do a good job of distinguishing between years.

As a note... Although the factory recommended .002 & .004" up to 1972, Triumph Baltimore recommended .004 & .006", so I never use .002" on anything. Valve lash is not the super critical adjustment some people make it out to be... unless it's too tight!

If your bike is still missing, then check the timing at full advance (5000 RPM) with a strobe lamp. If you are running B8ES, then swap to the correct B7ES.

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Old 03-09-2012, 03:37 PM   #8 (permalink)
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Quote:
Originally Posted by GABMA View Post
Mr Furball -
You need to find the correct workshop book for the model you own, and not trust the Haynes. Valve clearances changed for 750 twins in the mid-70's. Haynes is noted for all sorts of mistakes as they don't do a good job of distinguishing between years.

As a note... Although the factory recommended .002 & .004" up to 1972, Triumph Baltimore recommended .004 & .006", so I never use .002" on anything. Valve lash is not the super critical adjustment some people make it out to be... unless it's too tight!

If your bike is still missing, then check the timing at full advance (5000 RPM) with a strobe lamp. If you are running B8ES, then swap to the correct B7ES.

Manual issue is my fault, would help if I was reading the 75-83 year info at the back...

I've put a strobe on it and the timing mark at any RPM up to 5K doesn't show up neatly like I've seen on demo videos a la Lunmad on YouTube. The mark keeps spinning, won't sit stationary in that optical illusion sort of way, so I can't get an accurate reading of the timing. I had previously removed the stator/rotor and clutch basket to fix the infamous leaking oil seal behind the sprocket so please let me know if there's something I did wrong in re-assembly to be causing this or if its something else. I did properly torque the nut on the rotor and the key is installed on the shaft.

I'm running B8ES so will swap those out as you recommend.
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Old 03-10-2012, 12:44 AM   #9 (permalink)
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Originally Posted by furious furball View Post
The mark keeps spinning, won't sit stationary in that optical illusion sort of way, so I can't get an accurate reading of the timing.
It could just be that you need a brighter strobe or a darker work area.
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Old 03-10-2012, 04:13 PM   #10 (permalink)
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Quote:
Originally Posted by Mr.Pete View Post
It could just be that you need a brighter strobe or a darker work area.
I've got a brand new digital timing gun by Actron with a Xenon bulb...straight out of the box, powered to my Silverado truck battery as the Triumph manual says not to power it to the bike's battery.

Very sporadically will it line up to the needle.
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