Author Topic: Puff backwards from #1 carb (Read 1691 times)

jakec · « **on:** December 10, 2020, 02:38:16 PM »

What does it mean if the carb puffs back at you? This happens occasionally on the 750 I'm working on. The bike is running and going down the road but will occasionally puff back in my direction out of the #1 carb. I saw it in real time when syncing the carbs, the "puff" was white like unburnt fuel/ air mixture. Not really a backfire.

Bodi · « **Reply #1 on:** December 10, 2020, 03:27:32 PM »

Either the intake valve is sticking open occasionally, the tappet is too tight, or the cam timing is off. But the last two usually are more constant, not occasionally.

jakec · « **Reply #2 on:** December 10, 2020, 03:49:56 PM »

If it was the intake valve sticking open, does that mean the compression stroke pushes the fuel/air back out of the cylinder, then that cylinder won't combust on that revolution?

What would cause the sticking valve and what's the fix? New valve and seat? It is occasional. Not constant.

HondaMan · « **Reply #3 on:** December 10, 2020, 06:34:36 PM »

Also, try turning the air screw inward a little bit on that carb. If it is running richer than the others at low speed it builds up unburned fuel in the intake tract, and during overlap cycle the wet sparkplug can fire it just enough to make it sneeze back at you. The air screws should be between 7/8 and 1 turn out from (gently) fully closed.

jakec · « **Reply #4 on:** December 11, 2020, 12:37:43 PM »

I did not do a valve job on this bike. Just got it running recently.

I actually have the screws all out 1-1/4 turns right now. I am running round tops on a later engine, with stock jetting, straight pipes, and a breadbox filter

not exactly stock. I think it is running rich. What would you suggest I do?

HondaMan · « **Reply #5 on:** December 11, 2020, 01:00:29 PM »

Quote from: jakec on December 11, 2020, 12:37:43 PM

I did not do a valve job on this bike. Just got it running recently.

I actually have the screws all out 1-1/4 turns right now. I am running round tops on a later engine, with stock jetting, straight pipes, and a breadbox filter not exactly stock. I think it is running rich. What would you suggest I do?

Start by turning those airscrews into 7/8 turn. Then check th eplugs: if they are black (probably are) then get new ones. For tuning and messing with it, buy the D7EA (hotter) heatrange until you get it sorted out: then you can switch to ethe DR8ES-L or X24ES-U for a heatrange in between the "7" and "8". The modern D8EA sparkplug is too cold for anything but high-speed interstate riding with today's gas. The original plug for this engine is the D8ES-L, which was an effectively D7.6ES (imaginary) heatrange sparkplug, created by Nippon Denso at Honda's request in 1971 (ND owned NGK then). The modern D8EA sparkplug came from the Clinton Administration when the overly-self-important DOT decided to tell NGK that they could not sell a super-wide heatrange sparkplug in America, because they [reportedly] 'did not believe' it could handle that wide of a range. It did. It wasn't until 2004 that the close relationship appeared between the head of Clinton's DOT and Champion's Sparkplug division, who were golfing buddies...and the DOT head's kids had bought a WHOLE lot of Champion stock...sound familiar?

ekpent · « **Reply #6 on:** December 11, 2020, 01:11:46 PM »

HA ha- that's funny. I just bought 8 D8EA for a couple of 750's I am reviving from a very long slumber. Damn those politicians for messing with my plugs and ruining my bikes

Randakk has some interesting spark plug stuff also https://www.randakksblog.com/spark-plug-tricks/

HondaMan · « **Reply #7 on:** December 11, 2020, 07:07:11 PM »

Quote from: ekpent on December 11, 2020, 01:11:46 PM

HA ha- that's funny. I just bought 8 D8EA for a couple of 750's I am reviving from a very long slumber. Damn those politicians for messing with my plugs and ruining my bikes Randakk has some interesting spark plug stuff also https://www.randakksblog.com/spark-plug-tricks/

One 'trick' that Randakk didn't mention involves especially the water-cooled bikes: you can jet them leaner for increased heat of combustion, then use a colder plug. This increase horsepower at the expense of low-end torque, an old roadracer's trick. I use it in my own 750, but then I ride it a certain way (always have) where the engine is seldom below 3500 RPM.

Water-cooled bikes are more forgiving of increasing the combustion heat, like cars: this is why today's 5.8L V-8 engines can get 20 MPG when with carbs they were pretty much stuck in the mid-teens MPG. EFI can monitor exhaust temps and adjust as needed, running toward the lean side for better MPG. If their throttle load is high (evidenced by the mass air flow intake sensors) then they can adjust richer to keep temps under control.

But, EFI is susceptible to EMP...

Deltarider · « **Reply #8 on:** December 12, 2020, 06:25:48 AM »

Before we all drift into the realms of phantasy, has any of you had a close look at possible differences in build between the D8ES-L and D8EA plug? Differences that can substantiate popular claims? Any close-ups of insulator noses and/or centre electrodes maybe?
Lovers of conspiracy plots, read The Plot Against America, by Philip Roth, one of the best American writers. It's a novel (2004), so it is fiction, but after finishing the read, you may have a clear understanding of whom you were conned by, say the last 4-5 years. And - if that isn't enough - it features another American aviation hero: Charles Lindbergh. Therefore, recommended!

PeWe · « **Reply #9 on:** December 12, 2020, 07:18:01 AM »

My CB750 K6 had got some carb sneezing at low throttle lifts during slow accelerations when running lean at low lift.
(Not stock carbs)

Too advanced ignition or too lean were my thoughts.
I have fixed it by retarding the ignition but loss of power made me to fix the jetting and set ignition back to correct setting.

This is very irritating since I noticed carbon outside the affected carbs. It more or less exploded passing carb boots clamps on carbs.

Who knows how much dirt that sneezing backwards can infect the carbs internal fuel/air channels/pilot circuit?

Now solved for good I hope.

Little_Phil · « **Reply #10 on:** December 12, 2020, 07:58:42 AM »

PeWe, good to see you getting this thread back on track. What with Hondaman purporting to know about deals over a round of golf and delta biting back. Delta, agree , but not here, stick to the subject. Noticed to much creeping in here from all sides. This is an international site and one thing we all have in common.

PeWe · « **Reply #11 on:** December 12, 2020, 01:09:52 PM »

Air screws as Hondaman wrote is probably the right step.
Make sure that carb boots do not leak.

jakec · « **Reply #12 on:** December 14, 2020, 04:07:04 PM »

I have turned the screws in to 1 turn (I was on the road so less exact than 7/8). That got rid of 90% of the puffing. I will try out the D7 plugs soon as well. Need to figure out an electrical issue first..

jakec · « **Reply #13 on:** May 21, 2021, 06:49:39 PM »

After riding the bike for a while I decided to take the carbs off today to take a look. Even though the puffing mostly stopped, it would still sometimes happen a couple times when the bike was warming up, or if you blipped the throttle under load when it was still not warmed up.

What I have found is this. All plugs are a grey color except #4 which is beige.

#1 carb had fuel on the rubber boots on both sides. The breadbox boot has fuel on it. The engine boot had fuel on it as well.

I would think this is a super rich condition at #1, but the plug is showing me grey. So right now my hypothesis is that all carbs are a little lean, maybe due to float height since #4 is beige and all other settings are the same. I think I will raise the needle one notch and check float heights again. Not sure what’s wrong with #1 still causing it to apparently puff backwards.

jakec · « **Reply #14 on:** May 21, 2021, 06:56:18 PM »

Photo of plugs

maxheadflow · « **Reply #15 on:** May 21, 2021, 07:15:57 PM »

Quote from: jakec on May 21, 2021, 06:49:39 PM

After riding the bike for a while I decided to take the carbs off today to take a look. Even though the puffing mostly stopped, it would still sometimes happen a couple times when the bike was warming up, or if you blipped the throttle under load when it was still not warmed up.

What I have found is this. All plugs are a grey color except #4 which is beige.

#1 carb had fuel on the rubber boots on both sides. The breadbox boot has fuel on it. The engine boot had fuel on it as well.

I would think this is a super rich condition at #1, but the plug is showing me grey. So right now my hypothesis is that all carbs are a little lean, maybe due to float height since #4 is beige and all other settings are the same. I think I will raise the needle one notch and check float heights again. Not sure what’s wrong with #1 still causing it to apparently puff backwards.

1. It's more than likely too lean not too rich. A cough is common issue with not enough fuel or timing. The fact that it does it more cold than rich is another indicator. Cold motors like a rich mixture.

2. The fuel is from the cough. When it coughs, it creates a reverse flow. Carb acts like a carb, mixes gas with the flow.

3. Doubt you hypothesis is correct. I would check for a manifold leak #1, make sure the timing is correct and motor not funky.

jakec · « **Reply #16 on:** May 21, 2021, 07:24:03 PM »

Yes I will be checking for manifold leak but that requires the bike to be running so in the meantime I will try other things since I have the carbs off. After more reading I was beginning to think that the problem causing the puff could also possibly be the pilot jet being clogged? Or pilot circuit. I will check that as well as all float heights. I still plan to move the needle as all plugs appear lean.

TwoTired · « **Reply #17 on:** May 22, 2021, 11:30:53 AM »

I was wondering if the carbs were vacuum synced.

Cheers,

jakec · « **Reply #18 on:** May 22, 2021, 12:48:11 PM »

They were previously but not now since I just moved the needles.

So I have moved all needles to middle groove (previously 2nd from bottom). Also small adjustment about 2mm on #2 float. Checked idle jet of#1 and totally clear. Set idle screws to 3/4 out. Removed #1 manifold and inspected, looks good.

I just started the bike up and headed idling. It is still puffing a little bit out of number one. And there is visible white in the exhaust from the left pipe (4 into 2). So it seems I’m passing through unburnt fuel at #1.

I grab the carb cleaner and gave each Kirby squirt about 30 seconds apart at both sides of the manifold. No change in idol. So I suppose that means no air leaks.

TwoTired · « **Reply #19 on:** May 22, 2021, 12:55:07 PM »

I wouldn't troubleshoot any further without verifying balanced carbs. Had a 550 that spit until the carbs were balanced. If #1 has the slide more open than others...

Cheers,

jakec · « **Reply #20 on:** May 22, 2021, 01:04:31 PM »

Ok, I will do the carb balance, probably another day. They were vacuum synced before but now they are just bench synced.

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Author Topic: Puff backwards from #1 carb (Read 1691 times)