Topic: Engine pops at idle

[mkeefe]

(1977 CB550F) My engine pops and sometimes backfires when I'm idling. I've been having problems with it generally running rich (carbon fouled plugs), solved only by unscrewing the air pilot screws much further than I think I should (3-4 turns out from gently seated). I have not checked the jet sizes (unfortunately), although the carbs were taken off and cleaned twice this season. No gaskets or o-rings were replaced either time, but they seemed to be in decent shape. Maybe now is my chance to throw some pod filters on there and avoid the headache of carburetor removal? (I know, changing multiple variables=bad mechanics). Carbs were synchronized recently, and engine compression seems to be on the low end of the usable spectrum (112 or so, if i recall). Also, the RPMs tend to spike from 2000 RPMs to 3000 RPMs, then rise evenly again after that (i.e. a jolt in  acceleration at slow speeds). Not sure if all this is relevant, but wanted to paint a whole picture here. Thanks!

However, the bike seems to run fine otherwise.

[OldSchool_IsCool]

choke operating properly?
float hight set correctly?
does she pop only when cold or when hot too?
fuel pump pressure to spec?

[mkeefe]

Float heights were measured & adjusted when I cleaned the carbs recently. Choke seems to be operating correctly based on a visual inspection when the unit was off the bike. Fuel is gravity-fed. Popping tends to happen when its cold.

[OldSchool_IsCool]

yea, I know its gravity fed, but I figured if you can put a red herring in the question, I can put one in the answer! 

[martino1972]

when was the last time you set your valves??

[mkeefe]

Valves clearances were done a couple months ago. I'm gonna pull the plugs and see if they're fouled. If that's the case, then maybe they need to be cleaned and then I start looking into why they fouled in the first place?

Can anyone affirm that 3-4 turns away from "gently seated" of the air screw is farther than recommended? I'm afraid they'll fall out if I unscrew them any more. What would cause it to run rich? Filter is clean & new, intake rubbers are in good condition. Furthermore, can anyone explain why the engine revs & jumps from 2000 Rpm to 3000 Rpm, and then rises steadily in the typical fashion?

[martino1972]

you mean she's reving up while idling???
then i would look for an air leak between the carbs and the engine,spray some wd-40 on those areas while idling and see if rpms change...

[mkeefe]

To clarify: when I roll on the throttle, Rpms rise smoothly, then when it hits the 2000 mark it jumps up to 3000 rpms very quickly. It's as if it were getting an extra jolt of fuel or something. After the 2000-3000 maek, throttle operation is normal and smooth.

[martino1972]

needles worn out?

[Tower]

Good call on the needles, martino1972.  Its a clear possibility.

Although the idle air screws can be left 3-4 turns out, it indicates you have low and mid range rpm setting problems, and the likelihood of over rich settings.   Are your plugs carbon fouled?

The settings as they are now are probably masking the real problem.  Hence, if put back to factory spec would reveal that problem, ie.e to be either burned valves, clogged filters, clogged jets, etc.

[mkeefe]

Plugs were getting carbon fouled earlier in the season, so I kept turning the air screws out a little more until it stopped happening. Anyone know a source for replacement needles? How do I check for burned valves? As I said earlier, air filter is brand new, jets were cleaned a month ago, tank has been recently cleaned & resealed with POR-15, so I doubt the jets are getting clogged; inline fuel filter is in place.

I guess I'll have to pull the carbs off AGAIN, and check those needles.

[Sporkfly]

Compression test might reveal something in regards to valves or rings.

+1 on the needles.

[Pinhead]

Sounds like an ignition problem to me...  Reminds me of some of the stuff that I went through on my '79 650 before I got the alternator working properly........