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Topic-icon '76 R90/6: Backfiring at idle on left cylinder. Have tried everything.

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2 weeks 1 day ago - 1 week 6 days ago #6844 by raphelps
New to the forum. New to a 76 R90/6 that had been sitting in a garage for 7 years. Have previously restored 5 Honda twins.

Stumped. Will list all I have done first then a few quirky items.

Carbs meticulously cleaned and rebuilt with new jets/o rings/floats/needle (everything checked 3 times on left side) Everything installed correctly.
Diaphragms good
Butterfly installed correctly
Float heights set
Enrichers installed correctly
New cylinder to carb rubbers
New points and condenser
new plugs
Timing set
Valves adjusted
Advance unit cleaned and inspected
145 compression
I do not hear any timing chain noise
Fuel coming from a remote tank

The bike seems to run fine on right cylinder. Left cylinder will backfire at idle if mixture screw is more than 1/4 turn out. The mixture screw does not effect the engine idle one way or the other like the right side. The engine seems to run fine over 2-3k rpm but hesitates a fraction when turning throttle. I have reversed the plug wires (and plugs) and problem remains on left side. Opening the enricher slightly just on the left side seems to make the hesitation when opening throttle go away. The right exhaust pipe is hotter by 100 degrees than left side at idle.
One really weird item. I adjusted the left side intake valve incorrectly (wrong position of crank) and had about 1/8 gap. When I started the bike other than the valve noise, the issue with the 1/4 turn at mixture screw and the hesitation when opening throttle went away. The exhaust header temps were relatively close. I re adjusted the intake valve and now back to square 1. Any help appreciated. I have searched the forums and not seen anything quite like this. HELP!

Last edit: 1 week 6 days ago by Wobbly. Reason: Mods altered title

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1 week 6 days ago - 1 week 6 days ago #6852 by Wobbly
Welcome Aboard !

• If any out-of-date fuel was in the tank, then it immediately fouled your spark plug, regardless of age or previous performance.

• Above naturally leads to the question, "How old is the fuel in the remote tank ??" Modern fuels are worthless after about 8 weeks without stabilizers. Best to simly dump it into your car and buy fresh fuel.

• It would be more correct to say that you have checked to make sure the ignition timing is close. It cannot be "set" without a strobe and the engine running at high RPM.

• I sounds like the low speed circuit and/or the slow speed jet is clogged.

• Idle Mixture screws should start at about 1-1/2 turns out.

More tips and tricks HERE .

Don't hide 'em, Ride 'em !!
Last edit: 1 week 6 days ago by Wobbly.

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1 week 4 days ago - 1 week 3 days ago #6867 by arni
if turning the idle air screw (the idle mix is fixed by a jet, the idle air screw determines how much mix is fed to the head---some semantics.) does not change idle speed then the o-ring on it is bad. Install new o-ring. It does not matter if the o-ring on there is brand new, take it off and look for a cut from when it was installed.

Edit: I buy those o-rings in 10 packs. I have screwed up new installations slicing a ring a bit.. On a tech day inevitably;ly there will be someone (or more) with a bad one. I have seen rings that looked great but when I took them off I saw the cracks. It is quite a pronounced effect. It leads to leaning of the mix and that causes the backfiring.

If you remove the idle air screw ( don't lose the spring) and the tip is mashed, replace the screw. In any event when it is out spray through with carb cleaner.
Last edit: 1 week 3 days ago by arni. Reason: mo'

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