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Popping back fire on cylinder #1, #1 exhaust cooler than the others.

Jrushtx

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1982 CB750c all stock. Purchased this bike in not running condition, compression 160 lbs on all 4 cylinders. Serviced carbs (I did remove that stupid vacuum controlled petcock) I used factory sized jets I used sizing chart from the shop manual and they were the same size that were installed, new plugs, and the bike started right up, seems to idle ok but could be better. Once the engine is warmed up I get a popping backfire that appears to be coming from the #1, can smell raw fuel on the left exhaust, the #1 exhaust pipe is cooler than the others. Pulled plugs, they all look good. I checked the valve lash, all 16 fell within specs .003 +-.001, checked timing, performed a leakdown test on all 4 cylinders all are less than 15%. Im guessing the #1 is missing intermittently, causing unburned fuel to ignite in the exhaust. The only thing I have not done is new coils and wires, ordered them today, so I can eliminate that, my only reservation is these coils each run 2 cylinders, and I only have one missing cylinder. The bike seems to smooth out at 4k and up, but I dont know if it quits backfiring or I just cannot hear it at that speed, but it does seem worse at lower rpm. The backfire is not super loud, just pops loud enough to make CB feel embarrassed at stop lights, it does not bother me, but it is affecting the self confidence of the bike.

Im hoping one of you DOHC Ninja's reply to this post with, "you forgot to check xyz, dumbass" and I will not take offense as I feel like Im missing something pretty simple. Appreciate any input I can get on this failure.
 
You say 3 plus or minus 1 on valves, any valve at 2 can be held open (0) at running, .003" is too close and I already know what the manual says. You always assume that .002" of your obtained clearance is fake, it does not show up in actual running. A DOHC quirk, the engines run far longer set at .005".

Check the #1 cylinder valves if a valve is at .002", your problem.
 
Your right amc I did the ignition, wires, plugs, no change. The only thing that makes sense is raw fuel getting pushed out the exhaust valve during the compression stroke, thus pushing raw gas into the exhaust, and the fuel ignites in the exhaust after the it gets hot enough. Next will shim the tight exhaust valve, I have a motion pro valve stem tool, I need to get some shims, do you have any advise on the shims, should I purchase a kit, or should I go through all of them, do the math and get only the ones I need. Ive seen a couple of generic kits but do not know if they are worth the cost. Thanks amc49, and I wont ask what the hell your doing up at 4am...lol.
 
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I get only the ones I need and sometimes ones coming out of one spot will go into another. You can use .004"-.006" but I prefer the looser if a choice to be made.
 
Got the valves shimmed to the spec, they are now all at .005, I was able to dig through my local honda dealers junk and found some good shims, its nice to have friends in high places...lol. Yet this did not solve the issue, maybe even a little worse, so I kinda got feeling the valve is leaking. I pulled the exhaust pipes, spark plugs, and the valve cover, put the #1 at tdc making sure the cam lobes were not engaged, squirted some fuel into the #1, applied 10psi of shop air via spark plug port, and with my snake camera was able to see the valve seats are leaking quite a lot, blowing fuel right past the valves into the exhaust port on starboard valve fuel just blows past it. One thing I have learned while troubleshooting this, is, Im going to throw the leak down test set away, it is useless, these valves are leaking severely and the leakdown test should have proved that.

I guess my next step is pulling the head, Im going to search the forums and see if it is possible, to pull the head while not pulling the engine, it does not look like it but I might get lucky. I appreciate your help with this fault, and you definitely nailed the symptom, unfortunately it wouldn't be a simple adjustment.

Thanks again AMC, your input put me on the correct track.
 
Leakdown testers are more for extremely small leaks like on drag racers looking at 5% and less leaking, or nearly perfect. Regular street engines can easily leak more than that making them pretty much useless, a regular compression test works fine for the latter as long as you run more than one test to verify and use a good quality gauge.
 
Update: I first shimmed the valves like we discussed earlier. That did not help, so I pulled the engine out, removed the top end sent the head over to Wes's machine shop in Haltom City and had him cut new seats and grind the valves, I put new rings and honed the cylinders. When the head was done I put everything back together (I had to reshim all the valves because of the valve job), got good compression all the way across 165, I dont trust my cheap compression tester, but they were all the same. Reinstalled the engine, When I started it, she purred like a kitten, I balanced the carbs, and checked the timing which was off a little, so I adjusted the timing to the mfg. specs. The engine runs much better with more power, like I said earlier alot smoother. But.....I still have popping out left exhaust, #1 is still running cooler. Pulled the plugs #1 looks like it way to clean, and #2 looks way to rich. I think I have carburetor problems on #1 and #2.

Before I tear the carbs apart again, I was thinking about swapping the #1 with #4 and see if the popping moves to right side of the exhaust, what do you think ? Carburetor right road to be on at this point?
 
Swap the coils. Check the ignition wire including the plug cap for excess resistance, the resistor in cap can be bad and I generally remove them. I look for 5K ohm max per 12 inches of plug wire length.

Hope you did NOT actually grind the VALVES, you cannot as they are surface hardened only and it is super thin. Removing that exposes super soft steel to have the valves burn in as little as 10K miles or less although engine will run fine at first. Why Honda says replace valve only as a fix. The seats can cut.
 
I dont know about the valve grind, Like i stated I had Wes's machine shop do the valve job, he cut the seats and I dont know if he ground or just lapped the valves, I guess Ill find out shortly. As far as the ignition goes, I replaced coils, wires and plugs previously. Looked at the carb bank, and there is no swapping anyway. I just noticed you live in FW too, I live in midcities, Richland Hills my shop is right off 121, small world.
 
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