Author Topic: Backfiring / Timing / Carb issue CB750 - Help! (Read 26408 times)

Deltarider · « **Reply #25 on:** May 29, 2016, 01:57:53 PM »

If I would find those floatlevels (+/- 1 or 2 mms) on mine, I'd leave them alone and would not start messing with them.

andyvclifford · « **Reply #26 on:** May 29, 2016, 03:05:49 PM »

I fixed all of the fuel levels in the bowls, and warmed the bike up. Advance timing looks spot on with my timing light, I really just am at a loss, is there something wrong in the engine?

andyvclifford · « **Reply #27 on:** May 29, 2016, 04:21:49 PM »

Here is the results of post float bowl and timing adjustment plug chops with new plugs

andyvclifford · « **Reply #28 on:** May 29, 2016, 05:43:48 PM »

Issue still persists! Pretty badly actually

DaveBarbier · « **Reply #29 on:** May 29, 2016, 07:02:55 PM »

Just to answer a question you had. If the fuel level is 2mm below, it doesn't correlate to lowering the floats 2mm. At least not in my experience.

Nice 360

andyvclifford · « **Reply #30 on:** May 29, 2016, 08:59:28 PM »

Popping in exhaust is prety loud especially on the left side of the bike at idle. Throttle blubbers and wants to quit between idle and about 2.5k After that its rideable

andyvclifford · « **Reply #31 on:** May 30, 2016, 03:28:41 PM »

So if these plugs look normal for idling then my problem is definitely electrical ?

NoLuck · « **Reply #32 on:** May 30, 2016, 07:05:04 PM »

Dirty air filter? Removing the air filter would make the bike run better if rich and worse if lean. Also pilot screws can affect 1/4 throttle response. My bike when working would run bad till i turned the pilot screws out to 2.25-2 turns out to make it leaner. Throttle response was better for me and seemed to run smoother. This is all based off of what i experienced with my bike. Try turning pilot screws in (clock wise) to richen mix.

andyvclifford · « **Reply #33 on:** May 30, 2016, 08:00:49 PM »

Brand new air filter. Air screws currently at factory setting of 1

NoLuck · « **Reply #34 on:** May 30, 2016, 08:16:23 PM »

Yes it would. Did you try turning them clockwise a half turn? Did that make any difference in throttle response?

Mitchell · « **Reply #35 on:** May 31, 2016, 10:18:24 AM »

I would suggest setting the valve lash then go from there, if you havent already.

flybox1 · « **Reply #36 on:** May 31, 2016, 11:55:07 AM »

ALL of those plugs are lean still. Lets see some color.
Keep going richer on those screws. #2 is the only one showing some promise.

flybox1 · « **Reply #37 on:** May 31, 2016, 12:38:44 PM »

Quote from: calj737 on May 31, 2016, 12:33:42 PM

Quote from: flybox1 on May 31, 2016, 11:55:07 AM
ALL of those plugs are lean still. Lets see some color.
Keep going richer on those screws. #2 is the only one showing some promise.
#4 or #2 plug, Fly? Picture 2 is of plug #4. Or at least thats how I deciphered the writing on the napkin in the background.

Second picture from the top. I understood it as the #2 plug

I have a 6-sided standard bit I use to make adjustments with my mixture screws.
IIWM, id go 1/3 turn richer on the plug in the second picture ^^^, and 2/3 turn richer on all the rest....and retest idle.

flybox1 · « **Reply #38 on:** May 31, 2016, 12:40:09 PM »

For ease of reading, put them all in 1 picture....1-2-3-4 (L to R)

andyvclifford · « **Reply #39 on:** May 31, 2016, 07:39:34 PM »

Once I get home tomorrow evening I'll do a run with new settings and post better pictures ! Thank you! Silly question but on idle the spark should be going off at the F mark not the T mark correct ?

flybox1 · « **Reply #40 on:** May 31, 2016, 08:53:54 PM »

correct. timing light flashes with the F at the TDC mark

andyvclifford · « **Reply #41 on:** June 04, 2016, 01:53:40 PM »

Hi all,

So i was able to do some more tests.

Timing

I went back and reset the points gap and static timing. Bought a new strobe light and checked the running timing and got it to fire right on the F mark for both 1-4 and 2-3. When i checked the advance timing it was just on the threshold for 1-4 of being too far out but i think its ok. For 2-3 when i put the clamp on wire 2 the light stopped functioning when testing advance. When i put it on 3 the light functioned just fine, with the same results as 1-4. What does this mean? Is the plug not working when under load? That could definitely explain why when i rev the engine it slows way down?

Thanks!
Andy

andyvclifford · « **Reply #42 on:** June 04, 2016, 06:23:12 PM »

Also - Spent about 4 hours tuning the carbs again and trying various different idle plug chops. I was able to get all 4 plugs to be somewhat tan on idle - except for plug #1 which was consistantly white. I found a small vacuum leak in the intake from the airbox into the carb and tightened that clamp.

The bike idles like a champ with basically no spitback or popping. The second i give it any throttle it almost dies however. I am really lost at this point and feeling hopeless. I dont know where to go from here

. Mechanic?

Deltarider · « **Reply #43 on:** June 05, 2016, 12:54:59 AM »

Quote

For 2-3 when i put the clamp on wire 2 the light stopped functioning when testing advance. When i put it on 3 the light functioned just fine, with the same results as 1-4. What does this mean?

Nothing serious. When using a timinglight there is polarity to be observed. On the pick-up clamp it probably says THIS SIDE TOWARDS PLUG. Well, with our bikes this is only true for one of the two as respectively 1& 4 and 2&3 form a circuit. So if the strobe doesn't flash or intermittently, flip the clamp and you'll find it works fine. Or you could move the clamp to the other HTlead of the two, from 1 to 4 or from 2 to 3 respectively. And close the clamp well.
Furthermore the HTleads should be clean (no grease, no dirt) or the timinglight will malfunction. If needed you can use a little alcohol to clean.
BTW, be careful with that clamp. It is fragile. On many of them you will read DO NOT DROP. Not only that, when dangling avoid it shocks against the frame or engine.

Quote

The bike idles like a champ with basically no spitback or popping. The second i give it any throttle it almost dies however. I am really lost at this point and feeling hopeless. I dont know where to go from here . Mechanic?

I'm not a fan of setting idle by plug chops. What I know of the CB500/550 is that it needs a relatively rich idle to enable good pick up and driveability whilst accelerating. But yours is a 750.

andyvclifford · « **Reply #44 on:** June 05, 2016, 11:27:48 PM »

the bike tends to run worse as it warms up. Is this a sign of bad valves ? How can i check for this ?

jonda500 · « **Reply #45 on:** June 06, 2016, 04:51:07 AM »

sign of a rich mixture

Are you 100% sure the petcock is the only thing you changed from when it was working fine?
John

flybox1 · « **Reply #46 on:** June 06, 2016, 08:17:35 AM »

Andy,
Give us an updated carb jetting size list, mixture screw setting, and a picture of your latest idle plug chop pls.

andyvclifford · « **Reply #47 on:** June 07, 2016, 08:01:43 PM »

Carb Settings - 3/4 turn out on 2, 3, 4 and 1/2 turn on 1. Pilots are 40, Mains 110.

I dont have a current picture of the idle plug chop i did over the weekend but with those settings i was able to get a "tanish" color to the plugs.

The bike runs well for about 10 minutes until its warmed up and then it just stumbles and falls flat.

I will buy some new plugs to do another plug chop soon, maybe tomorrow?

Deltarider · « **Reply #48 on:** June 07, 2016, 10:50:36 PM »

Quote

The bike runs well for about 10 minutes until its warmed up and then it just stumbles and falls flat.

Sometimes condensors show they're bad after the warming up period. Coils too, btw, but they rarely fail. Are points clean? Have you measured possible voltage drop? When > 0,2 Volts with points closed, they're bad. When in doubt run a contactbreaker file between them one or two strokes.
Is fuel flow OK and consistent? No kinks or inline fuel filters? Tank ventilation?
Maybe a shot of Seafoam or similar. These are all easy tests.

andyvclifford · « **Reply #49 on:** June 08, 2016, 01:33:23 PM »

Planning to do some more tests tonight. Shot of Seafoam into the gas tank I assume? There are no kinks in the fuel line and it should be routed just fine. When I was doing clear tube tests the fuel flowed just fine.

I will check the tappet clearance one more time, and do another idle plug chop with new plugs. Im also going to change the oil once more and make sure theres nothing going on there. I'm really at a loss and wondering if there is something wrong inside the engine, perhaps the valve train? Should i do another compression test? I assume its best to do a compression test after the bike is warmed up and the plugs are all pulled?

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Author Topic: Backfiring / Timing / Carb issue CB750 - Help! (Read 26408 times)