Topic: Puffs of air / backfiring and fuel leak from carb boot

[gmoneymagna]

The bike
1982 CB650 Nighthawk with 12k miles. SOHC I4. I purchased it with ~7k miles 4 years ago. I regret I have previously been a poor motorcycle owner and have not done proper maintenance aside from regular oil changes, although I have taken steps to perform schedule maintenance on my motorcycles.

The problem

• It has recently been running a bit rough at idle and when I take off from a stop too fast it sounds like it tries to die. It also stumbles and tries to die if I give it a lot of throttle <4K rpm. I'm not sure if this was just the nature of the I4 engine, not warmed up engine or carbs out of sync.
• The larger problem started after performing valve adjustment. I carefully followed the owners manual. This was my first time adjusting valves. ~3 valves were too tight. I also replaced the spark plugs, the fuel petcock with a new one, new fuel lines and fuel filter from tank
• Now after adjustment the bike has rough idle and dies without choke. Occasionally (once every 5-10 seconds) the 4th cylinder puffs out air backwards and the carb bounces backwards. I also noticed fuel leaking from where the carb meets the carb boot.

The workup

• All four exhaust bikes are hot.
• All valves are within spec - assuming I performed this correctly

Current hypothesis

• I'm awaiting delivery of a tool to sync the carbs. It has never been done during my ownership. I'm hoping that this could be caused by out of sync carbs. Although it ran rough at idle before, the carb backfire issue and dying at idle is new

Future plans

• sync carbs when adjusting tool arrives - I might make this the first thing I do in case syncing them up complete fixes the problem
• Compression test if needed
• Remove and replace spark plugs - this was my first time putting in new plugs and I may have done it wrong
  
• Re-adjust valves if needed

UPDATE

I loosened all of the carb boots to the engine and pushed the carb bank up towards the engine until the lip of carbs 3/4 were inside the boot and then tightened it up. Then I replaced the new spark plug (NGK DR8ES-L) with the old one NGK "R" DPR8EA "9" only on cylinder 4 which was the one puffing air from the carb boot. Now the first three cylinders have the new plugs recommended by the manual and the 4th has the old old one. Started right up and was able to idle in short time without abnormal noises or puffs of air from the carb boots. Yes!

I'm on my way to get a spark plug gap in-case that was the problem. However, what do you all think about using spark plugs that are not in the manual? Does it make that much of a difference? I could put all the old plugs in, gap and reinstall all my new plugs, or buy new plugs of the old type. Which do you think is best? Thanks!

Once the plug issue is sorted out I will continue with a full tune up including cam chain tensioner adjustment and carb sync.

[scottly]

The larger problem started after performing valve adjustment. I carefully followed the owners manual. This was my first time adjusting valves.

Re-adjust the valves. Set both intake and exhaust valves for each cylinder, one cylinder at a time.

[gmoneymagna]

The manual says to set cylinder 1 to TDC on compression stroke and then adjust a number of valves in that setting. The valves in cylinder 1 were tight so I had a hard time ensuring that there was freeplay in both intake and exhaust rocker arms (thus ensuring that that it was truly TDC on compression stroke), but in the end I think I got it.

I imagine if I botched the job as badly as setting the valves in the wrong timing I would have more symptoms? Also since this was my first time adjust valves I went with the manual's description of "slight resistance" on the appropriate feeler gauge, perhaps I set them too loose?

However, I completely agree with you and I will check the valves again as my next step. Thank you!

[scottly]

The manual says to set cylinder 1 to TDC on compression stroke and then adjust a number of valves in that setting. The valves in cylinder 1 were tight

Sounds like you may have been at TDC on the compression stroke for cylinder 4? Did you have to loosen the adjuster screws more than a fraction of a turn?

[gmoneymagna]

I don't remember having to make radical adjustments to cylinder 4. After I set 1 to TDC on compression stroke and adjusted the appropriate valves. I then followed the manual and turned the crank 360 deg until I came back to the 1.4 T 1,4 mark and adjusted the appropriate valves (including cyl 4 intake and exhaust)



Edit: When I go back to double check the valves, how do I ensure the desired cylinder is TDC on compression stroke especially now that I may have botched it up. I spent considerable time checking for this the first go around.

[Tracksnblades1]

That puffing is compression coming back through the intake valve.
+1 on adjusting them 1 cylinder at a time. At least until your a pro at it.
Don't toast #4's intake valve then it will puff until you reseat it.

[gmoneymagna]

That puffing is compression coming back through the intake valve.
+1 on adjusting them 1 cylinder at a time. At least until your a pro at it.
Don't toast #4's intake valve then it will puff until you reseat it.

If this were the case would the puffing occur so infrequently? I assume the cylinder is firing many more times than I see this occurring. What is the method for 1 cylinder at a time? I have just been following the manual so far. Thanks!

I have only tried to idle the bike for < 5 min so far. Occasionally after the puff of air I would here a quiet "clunk". Hopefully there is not substantial damage.

[scottly]

I don't remember having to make radical adjustments to cylinder 4.

How about cylinder 1?

[gmoneymagna]

As I remember both the valves were tight on cylinder 1 and adjusted them both. I tested for freeplay many times before adjusting by spinning the crank 360 deg, checking and then doing it again. The freeplay that I think I felt was minimal. Perhaps I could have got it wrong. Would this cause the backfire to occur in only 1 cylinder though?

[gmoneymagna]

Okay hold on. I was trying to find free play in the rocker arm to identify when cylindar 1 was TDC. Do you find this by feeling for free play at where the the lock nut and screw adjuster are located, or where the the rocker arm pad meets the camshaft lobe? I was trying to find free play where the rocker arm pad meets the camshaft lobe (where I slide the feeler gauges?.

[scottly]

The feeler gauges are placed between the adjuster screw and the end of the valve stem.

[gmoneymagna]

The feeler gauges are placed between the adjuster screw and the end of the valve stem.

Yes I remember seeing a youtube video so I think I might be off. However the manual appears to me at least to say otherwise so I'm confused.

Edit: This video shows what I mean.

[Tracksnblades1]

Gmoney, I think what is being recommended is the simplest and surest method to ensure all valves have been adjusted wether the cam is stock or other. The single cylinder method ensures both valve are set a TDC compression where intake and exhaust lobes are to far away from the rocker arms to influence your measurement.

Provided Cylinder #1 valve lash is correct or greater, only then will you have play (lash) at TDC compression. If you can't find your play at TDC you may have to back the lash off. You should be able to watch the rocker arms and follow the cycles to ensure your at Compression TDC and not exhaust and intake overlap TDC.

A tight intake valve (leaking some (small) compression in the intake tract) is more prevalent when the throttle is in transition. The cylinder pops more when it actually has a higher volumetric charge to burn rather than just an idle mixture.

[gmoneymagna]

Gmoney, I think what is being recommended is the simplest and surest method to ensure all valves have been adjusted wether the cam is stock or other. The single cylinder method ensures both valve are set a TDC compression where intake and exhaust lobes are to far away from the rocker arms to influence your measurement.

Provided Cylinder #1 valve lash is correct or greater, only then will you have play (lash) at TDC compression. If you can't find your play at TDC you may have to back the lash off. You should be able to watch the rocker arms and follow the cycles to ensure your at Compression TDC and not exhaust and intake overlap TDC.

A tight intake valve (leaking some (small) compression in the intake tract) is more prevalent when the throttle is in transition. The cylinder pops more when it actually has a higher volumetric charge to burn rather than just an idle mixture.

Yes I completely agree, I'm just trying to wrap my head around the single cylinder method. So I watch the valves and then when the intake valve closes (the rocker arm moves up) I advance crankshaft to the next TDC mark corresponding to that cylinder and then adjust both intake and exhaust valves?

[Tracksnblades1]

Your headed there now.
 I watch the rockers for valve overlap TDC first, backwards for most probably.  So try this if your not going to take the spark plug out and hold your finger over the hole easiest on 1+4.  Without spark plug out, exhaust rocker closing exhaust valve  and intake rocker opening intake valve. Continue turning crank slowly ( must be in normal directions as engine runs) until intake valve closes. Continue turning crank until appropriate cylinder firing mark aligns. You should be on that cylinders compression TDC.  You should have some lash now or like already stated with minor adjustment.

[gmoneymagna]

Thanks I think I get the gist of if it now. It seems that if I set to the next TDC mark of that cylinder following the closing of that cylinder's intake valve, then it must be at TDC compression. Thanks everyone for your help! I will do this tomorrow and report back. Hopefully I have not caused any lasting damage.

[Tracksnblades1]

Good Luck.
Your video shows lash being measured at cam to rocker. This is how I would do it if this is what your manual indicates.

The 750's sohc don't afford one such luxury unless the entire valve cover is off. And then their manual specifies between the lash cap and adjusting screw.

[ekpent]

 The rocker that you are going to check/set should be able to wiggle around a little when you grab it with your fingers. I always just do one cylinder at a time to keep it simple and work my way around the plate. Just recently did a 550 and took about 45 minutes. Pretty easy stuff.

[gmoneymagna]

The rocker that you are going to check/set should be able to wiggle around a little when you grab it with your fingers. I always just do one cylinder at a time to keep it simple and work my way around the plate. Just recently did a 550 and took about 45 minutes. Pretty easy stuff.

By the rocker do you mean the feeling play by grabbing the lock but and adjuster screw area or where the rocker pad meats the cam lobe?

What do you mean by working around the plate?



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[ekpent]

 Yes the arm with the adjuster nut. By working around the plate I mean I start with #1 cylinder and as I turn the crank and look for the t marks I do what pair of rockers are next as I rotate it looking at the 'plate'. Nothing too meaningful 

[gmoneymagna]

Very frustrated.

I re-checked the valves. Some might have been a tad tight so I readjusted a few. I double and triple checked my work both with the manual directions and cylinder by cylinder at TDC on the compression stroke (after the intake valve closes/the intake rocker arm moves up). Each valve can fit the appropriate feeler with "slight resistance". I started it up and is still puffing out the carb on cylinder 4 and some puffs (not backfires) out of the right exhaust. It does not happen with each stroke of that cylinder and gets worse with the engine warms up. I can see fuel spurting out from where the manifold meats the carb between the carb and the engine. There is also some water dripping from the right exhaust although it could be condensation, however none is dropping from the left exhaust. There is no smoke and the liquid does not smell like oil or gas.

I'm not sure what to do next. I can't imagine the valves not being correct now. The carb bank looks like it might be skewed with 3/4 being further back than 1/2. At the carb side of the manifold there is also a metal raised ring showing that I think should be inside the manifold. I don't know if this vacuum leak would cause these issues.

I also installed a new fuel petcock, fuel filter and fuel hosing to the fuel accelerator. However I believe the issue was apparent before this, after the first valve adjustment.

I look forward to hearing back. Thanks!

https://www.thoughtco.com/how-to-diagnose-carburetor-problems-743503

According to this website out of sync carbs may cause "backfiring". Is it possible the carbs and the valves aged and went out of alignment together and now that the valves are adjusted the carbs aren't happy? I'm grasping at straws.

[gmoneymagna]

Ok here is my plan going forward.

I don't think it is a stuck intake valve  as this backfire is not happening on every engine stroke and I trippled check the clearance. My leading theory is that the carbs were already a little out of sync before the adjustment, and now that the valves have been adjusted they are even more out of syn as it relates to the rest of the engine and now it's causing weird stuff.

• reseat the 3/4 carbs by pushing them further into the intake boot
• sync carbs
• compression check (is it okay to do this with the engine not completely warmed up? I'm afraid of causing damage).
• check spark plug gab. I installed brand new new spark plugs (recommended ones from manual) before this started and have not checked the gap
• adjust cam chain tensioner
• check static and advanced timing if still persists

I'm open to suggetions!

[TwoTired]

Puffs out the carbs can only mean the intake valve isn't closed when the piston is rising.

A bent valve can make the valve "sticky" as could carbon deposits making the valve stem draggy in the valve guide.  It is possible for this to be intermittant And the symptoms change with heat, causing dimension changes.  Sticking valves can also make clearance adjustments tricky, but unlikely they could be adjusted too tight using the correct procedure.  Suspected valves can be watched as the engine is turned over for any jerky motion during operation, preferably by hand turning.

 A sticking valve can be bent if it sticks open and the piston smacks it.  Bent valves never seal properly, and can be identified with a leak down tester.  Supply compressed air with piston at TDC between compression and firing stroke and read the pressure that the cylinder will hold, compare to what is supplied.  If it leaks you can hear where, out exhaust, out intake or into the crankcase.

A compression test gives limited data. Pass/fail.  And is prone to errant diagnosis due to apparatus selection and misunderstanding of what indications mean. See engine FAQ.

I think you should verify engine machanical integrity before mucking with other parts about the motor.

My 2 cents worth.

Cheers,

[gmoneymagna]

Puffs out the carbs can only mean the intake valve isn't closed when the piston is rising.

A bent valve can make the valve "sticky" as could carbon deposits making the valve stem draggy in the valve guide.  It is possible for this to be intermittant And the symptoms change with heat, causing dimension changes.  Sticking valves can also make clearance adjustments tricky, but unlikely they could be adjusted too tight using the correct procedure.  Suspected valves can be watched as the engine is turned over for any jerky motion during operation, preferably by hand turning.

 A sticking valve can be bent if it sticks open and the piston smacks it.  Bent valves never seal properly, and can be identified with a leak down tester.  Supply compressed air with piston at TDC between compression and firing stroke and read the pressure that the cylinder will hold, compare to what is supplied.  If it leaks you can hear where, out exhaust, out intake or into the crankcase.

A compression test gives limited data. Pass/fail.  And is prone to errant diagnosis due to apparatus selection and misunderstanding of what indications mean. See engine FAQ.

I think you should verify engine machanical integrity before mucking with other parts about the motor.

My 2 cents worth.

Cheers,

Thank you very much for your reply. I'm awaiting leakdown tester

I'm not sure how this bent valve could have occured as it was not having these symptoms before the valve adjustment which I did correctly to the best of my knowledge.  There was slight upper engine noise before adjustment. Do you think it could be caused by something else causing the mixutre to ignite at an innapropriate time while the intake or exhaust valves are open?

[TwoTired]

I'm not sure how this bent valve could have occured as it was not having these symptoms before the valve adjustment which I did correctly to the best of my knowledge.  There was slight upper engine noise before adjustment. Do you think it could be caused by something else causing the mixutre to ignite at an innapropriate time while the intake or exhaust valves are open?

As to the last question, no.  The valve bends with some mechanical interference.  In a wasted spark system, it is normal for the spark plugs to fire between the exhaust and intake stroke. Maybe some valve open overlap at that point.  No biggie.

If the valve guides/valve stems get a carbon build up on them, they can't slide smoothly and can hang open.  This would allow the piston to come up and smack the valves.   Improper cam timing can also cause this.

On the 550, reinstalling the cam cover/cylinder cover improperly can also bend valves.  I don't know if the 650 shares this procedural danger.   Has the cam cover been off the engine since new?
They don't have to bend much to cause sticking.  Some have reported hearing the valves snap shut while turning the engine over slowly by hand.

Back in the 50's and 60's OHV engines using the non-detergent oils available at the time, frequently had valve sticking issues. Over the counter additives were offered to "clean" the valves and loosen them up.  I think all the detergent oils already have such an additive since then.

If you have been using the proper oil in this engine, I think carbon on the valve guides is unlikely.  But, what of historical treatment/maintenance of this motor?

Anyway,the leakdown test should reveal if you have an issue there.


Good luck!

[gmoneymagna]

I'm not sure how this bent valve could have occured as it was not having these symptoms before the valve adjustment which I did correctly to the best of my knowledge.  There was slight upper engine noise before adjustment. Do you think it could be caused by something else causing the mixutre to ignite at an innapropriate time while the intake or exhaust valves are open?

As to the last question, no.  The valve bends with some mechanical interference.  In a wasted spark system, it is normal for the spark plugs to fire between the exhaust and intake stroke. Maybe some valve open overlap at that point.  No biggie.

If the valve guides/valve stems get a carbon build up on them, they can't slide smoothly and can hang open.  This would allow the piston to come up and smack the valves.   Improper cam timing can also cause this.

On the 550, reinstalling the cam cover/cylinder cover improperly can also bend valves.  I don't know if the 650 shares this procedural danger.   Has the cam cover been off the engine since new?
They don't have to bend much to cause sticking.  Some have reported hearing the valves snap shut while turning the engine over slowly by hand.

Back in the 50's and 60's OHV engines using the non-detergent oils available at the time, frequently had valve sticking issues. Over the counter additives were offered to "clean" the valves and loosen them up.  I think all the detergent oils already have such an additive since then.

If you have been using the proper oil in this engine, I think carbon on the valve guides is unlikely.  But, what of historical treatment/maintenance of this motor?

Anyway,the leakdown test should reveal if you have an issue there.


Good luck!

No signs of hearing valves snap shut while turning engine over by hand. I have always used standard (without modern friction additive) valvaline 10w-40 oil yearly or every 3,000 miles. When remove the breather tube cover and valve covers I can see the cam lobe and measure the feelers ontop of it. In the manual there is no specific instructions for replacing these it just states to replace them back onto the motor. The rubber gaskets appear to be in good shape. I have never done any upper motor work or removed that cam shaft.

Historically I have had this bike for 4-5 yrs for 5k miles. I bought it from a nice gentlman who restores them. I never checked or did any initial maintenance. I have also never done any maintenace aside from oil changes and brake fluid changes. I have been a pretty poor motorcycle owner but I'm trying to learn maintenance tasks so I can have well performing bikes in the future. However this situation has been discoraging as the bike was running better than before I did the maintenance. I'll keep chugging along.

[Tracksnblades1]

Gmoney,

Does this popping puffing intensify when loading the engine as slipping the clutch to take off....?

Have you checked #4 spark plug for proper gap and condition (color) compared to another properly running cylinder...?

Have you tried moving spark plug #4 to cylinder #1. To see if the popping moves to #1......?

And you have seated all your carbs And rubber intakes completely on the spigots...?
Your previous post indicates carbs on right side may have slipped back.

Have you corrected fuel leaking out of #4 cylinder intake..? Sometimes a dead cylinder (non-firing) cylinder will intermittently hit  and ignite a poor air/fuel mixture from a malfunctioning carburetor. But this would not pop (reversion) back through the carb.

Try compartmentalizing your tests.

 Verify Ignition is functioning normally on cylinder 4.

#4 carb float is not the source of your indicated fuel leak.

Leak down test cylinder #4 to ensure intake and exhaust valves are seating.

A good compression tester could hint of a problem when caompared to a good cylinder. Not as conclusive as leak down tester but you'll still have to compare to a good cylinder.

You'll find it, just keep after it.

[gmoneymagna]

Gmoney,

Does this popping puffing intensify when loading the engine as slipping the clutch to take off....?

Have you checked #4 spark plug for proper gap and condition (color) compared to another properly running cylinder...?

Have you tried moving spark plug #4 to cylinder #1. To see if the popping moves to #1......?

And you have seated all your carbs And rubber intakes completely on the spigots...?
Your previous post indicates carbs on right side may have slipped back.

Have you corrected fuel leaking out of #4 cylinder intake..? Sometimes a dead cylinder (non-firing) cylinder will intermittently hit  and ignite a poor air/fuel mixture from a malfunctioning carburetor. But this would not pop (reversion) back through the carb.

Try compartmentalizing your tests.

 Verify Ignition is functioning normally on cylinder 4.

#4 carb float is not the source of your indicated fuel leak.

Leak down test cylinder #4 to ensure intake and exhaust valves are seating.

A good compression tester could hint of a problem when compared to a good cylinder. Not as conclusive as leak down tester but you'll still have to compare to a good cylinder.

You'll find it, just keep after it.

I have not driven the bike yet since this started but the puffing is worse when the engine warms up and when the revs are higher with choke. The bike also dies without choke. All exhaust pipes are hot. I will check the gap of the plug in number 4 and try to switch it to 1 and see what happens. I assume this won't alter timing?

I have to adjust the carb exit further into the rubber boot in 3/4. The fuel is not really leaking from carb 4 intake boot as much as it is shooting out with the puffing of air. The carbs have never given me trouble on this bike as I use only non-ethanol fuel and it was not leaking before the initial valve adjustment. I will complete the tests laid out in your posts and others this week. Thanks for the support!

[Tracksnblades1]

Can you post a video of it running?

[Don R]

 I skipped ahead, but if you haven't seated the carbs into the manifolds properly yet do that next. A vacuum leak can cause all sorts of odd things to happen. Be sure to loosen the clamps on all four carbs so you don't bend or break the carb plate, get them seated then proceed with anything else you feel is needed. Plugged jets will also create a lean condition that can cause popping. Good luck,

[Tracksnblades1]

Gmoney,

After rereading all your post, I would try to get the bike running as well as "before lash adjustment" prior to synchronizing the carburetors. The bike will have to run properly with out the choke on before synchronizing with any kind of success.

Stay at it a post a video with sound of it running and taking the choke off slowly.

[gmoneymagna]

Gmoney,

After rereading all your post, I would try to get the bike running as well as "before lash adjustment" prior to synchronizing the carburetors. The bike will have to run properly with out the choke on before synchronizing with any kind of success.

Stay at it a post a video with sound of it running and taking the choke off slowly.

That makes sense. However it really only started running badly after the adjustment. I will post a video tomorrow. Thanks!

[gmoneymagna]

UPDATE

I loosened all of the carb boots to the engine and pushed the carb bank up towards the engine until the lip of carbs 3/4 were inside the boot and then tightened it up. Then I replaced the new spark plug (NGK DR8ES-L) with the old one NGK "R" DPR8EA "9" only on cylinder 4 which was the one puffing air from the carb boot. Now the first three cylinders have the new plugs recommended by the manual and the 4th has the old old one. Started right up and was able to idle in short time without abnormal noises or puffs of air from the carb boots. Yes!

I'm on my way to get a spark plug gap in-case that was the problem. However, what do you all think about using spark plugs that are not in the manual? Does it make that much of a difference? I could put all the old plugs in, gap and reinstall all my new plugs, or buy new plugs of the old type. Which do you think is best? Thanks!

Once the plug issue is sorted out I will continue with a full tune up including cam chain tensioner adjustment and carb sync.

[Don R]

 I'm glad you had some success G. Thanks for the report.

[Tracksnblades1]

GMoney,

Great news. Spark Plugs are like engine oil. They've both came along way in the last 30 years and they both have the same job they had before. I like the NGK and ND for the old Honda. You'll have lots of options just with those two. More if you include more brands.

Well Done, persistence pays off but costs along the way.

[gmoneymagna]

Update for everyone.

I replaced and properly gapped all 4 spark plugs and and found the carbs were already in sync. The bike absolutely soars to the redline now and low rpm dying is basically gone.

The tick is still present on the top right of the engine. I double checked vavles of 3 and 4 again and they are in spec.

I plan to replace the exhaust with a MAC 4 into 1 with a cone engineering muffler so I may just see if that fixes a tick that may be due and exhaust gasket leak.

Thanks for the help everyone!

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