A little popping in the exhuast.

Hey all, so I'm having some running issues. I searched around and couldn't find anything exactly like what I am experiencing.

My bike, a 1982 Seca 750 with 8500 miles, has been running well. Recently I went to start it, and as usual it fired off nearly instantly on choke. This time though, instead of gaining RPM's as the engine warmed up, it would sort of surge between 2000-5000 RPM. If I took it off choke, it would run down to idle fine, but if I gave it some gas, it would pop and backfire a bit, and then it would die.

I took the carbs out and gave them a good cleaning. They weren't terribly dirty as I store the bike for the winter without gas in them and gas tank off the bike. But I cleaned all the passages just the same. Float height is set correctly, and there is no evidence of damage within them.

Thinking the bike was incurring a lean condition causing the popping/backfiring I took off all 4 carb-to-engine boots, resealed them and checked for cracks. I also put a new #3 vacuum to petcock line on, and replaced all the carb sync caps.

Then, not knowing when/if the last valve clearance adjustment was done, I accomplished that as well. The valves were all within tolerance except for #4 intake which was at .07mm. I have a new shim coming along with new valve cover gasket and donuts. Also, I don't think that this valve is being held open out of cycle since it's not at zero clearance.

Finally, I pulled the spark plug wires and cranked with them pulled and they had decent spark coming out of them. Then I pulled the spark plugs and they were quite black and sooty.

Could fouled up plugs be causing the problem? Knowing that this is a wasted spark system, if it's not burning off during the proper stroke could it be burning the remaining on the off stroke causing a backfire?

Thanks guys!

 

Random Q's - why are you running the bike in Feb, and, what is your "winter blend" like ?? Must be a really light gasoline, maybe?

 

I put the tank back on to run it before I was potentially going to sell it. The gas I put in was new from the pump 87 octane that was going to go in my snowblower. I think there's a proportional amount of B12 intake cleaner in it as well.

 

it might be the b12 cleaner.

it might be the plugs.

what you describe sounds like not enough gas. what did you set the floats at???? did you check for wet fuel height. the pilot and main jets are pefectly clean? emulsion tube that the main jet screws into. it pops out throught the inside of the carb.

 

My first guess would be you picked up some bad gas.

 

Alaska, try this, just for kicks....take a long screwdriver and while the bike is running lay the side of the shaft on the spark plug cap, then slide it down so you almost make contact with the head but do it in a way so you can watch the gap between the screwdriver shaft and the head....you are looking for spark leakage.....check the other cylinders as well. If you find any that are jumping, cut 1/4" from the end of the offending wire then check again.

Lastly, if your fuel is up to par, and this has come up suddenly chances are that you are running lean somewhere, somehow....check the timing to be sure it's on the money then I would go to your pilot air screws and have a lookie. I've had them get bits of crud in the system and exhibit exactly what you've described.
good luck,

jeff

 

he didn't say when he gased up last. might just be old gas.

 

Ok, well i purged the tank and put 2 gallons of fresh 90 octane in and a dash of HEET in case there was water in there. Not really any change.

I pulled the spark plug wires and noticed that #1 cylinder isn't really getting that "big fat spark". All 4 plugs look very sooty and fouled like a 2 stroke plug would. I think I am going to buy a set of plugs on the way home and see where that gets me.

 

if the wire isn't getting that BIG FAT SPARK. that's your problem.

i don't think any of them really get a big fat spark though.

the spark test would be to take a screwdriver and hold it 1/4 inch away from the motor. if you have to hold the screwdriver close to the engine then you got a weak spark. you should have spark up to 1/4 inch from ground.

 

All well & good, but if the gas was bad it would have got in the carbs.

 

did you do the spark LEAKAGE test yet? This is NOT the same as a spark test, it's to find out how bad your leads and caps are leaking. If, when the engine is running you can hear (and sometimes you can't hear it anyway)any spark ticking, you're leaking....then you need to find out which lead.....that's the whole reason for the test.

jeff

 

My spark leads aren't leaking. They are in very good shape actually. Except my number plug cap. It's broken at an angle exposing some inboard. I wrapped it with F4 (silicon) tape and it hasn't caused me any grief.

I did put 4 new NGK BP7ES plugs in properly gapped. Problem solved, it'll go down to the lowest rev's with no popping or sputtering.

Thanks for all the inputs guys!

 

You should change out the broken cap IMHO

 

How do you change these caps? Do they pull off and have a pin in them to contact the conductor in the wire?

 

They SCREW on and off; the pin you refer to is threaded (self-tapping) and screws into the core of the wire.

Quite often, removing the cap and lopping about 1/4"-3/8" off the wire and screwing it back on helps immensely.

The resistors within the caps themselves have been known to work loose as well; if you look in the spark plug hole in the cap you can see two screw slots on either side of where the plug goes. Be sure the resistor core isn't loose in the cap as well.

 

I was out today for a rip. The bike runs fine and has lots of power, but I noticed if I kept the rpms up around 4500, in 3rd gear with the engine not under any load, I was getting some backfiring. If I gave it more throttle I ran fine, but let off on the gas and it started popping again. No missing at idle and no missing when the engine is under strain. I've got a 4-1 exhaust on it. Any thoughts?

 

normally indicates a lean condition, what do the plugs look like ?

 

It's in the shop for fork seals. I'll pull them out when I get it back. I was just looking for some suggestions or comments. I've only owned the bike for a month and only filled it once since I've owned it. It might be something as simple as some water in the gas ( which I'm hoping) or the valves being in need of an adjustment.

 

backfiring is an indication that not enough gas is being pulled.

 

Did some searching today on the bike. #4 cylinder wasn't getting hot like the others at idle. Compression in all 4 was around 120, plugs looked good and there was good spark. If I cracked the choke a bit #4 got hot immediately, which leads me to suspect either a dirty jet, or water in the bowl. I filled it full of hi test, added some methyl hydrate and took it for a spin. It didn't seem as bad as before, but I can still get it to backfire some in 2nd gear at 4000 revs if I hold it at that speed for 10 -15 seconds. It won't do it at any thing less than that. If I labour the engine - like 30 kph in 5th gear, I can't feel any missing, which I should do if a cylinder isn't firing. So I'm thinking I'll just try and let it work it's way clean and if that doesn't fix it after a couple of hundred kms, then I'll take it in to the shop and let them figure it out. Any other suggestions or comments? - thanks, Rick

 

Honestly?

Get a manual and learn to work on it yourself, you're about to go broke...

You need a complete carb service, and probably a valve adjustment.

You can't get a good final sync after servicing the carbs if you've got a tight valve or three.

Trying to take shortcuts is not going to work; "running something through it" is NOT going to fix it, and if you're going to pay someone else to do everything that truly needs to be done to make it run 100% it's gonna cost a pretty penny (if they can even pull it off.)

You've got a plugged carb or two, possibly a few tight valves, and maybe even an air leak somewhere. Keep riding it hoping it will "clear out" and you will eventually damage the motor. Backfiring/popping is a lean condition (or a bad valve) and it will only get worse.

Sorry, but that's the bottom line, and you asked.

 

like fitz says.

pull the carbs off. check all 4 main jets inside the float bowl for plugging. take the caps off and pull all 4 emulsions tubes out to make sure they aren't plugged. take all 4 pilot jets out. inside the float bowl. and take all 4 pilot AIR jets out. located on the rear of the carb where air gets sucked into.

take the floats out so you can access the float seats. pull them out to make sure the screens aren't plugged. if they're still on. put back together and check the float heights. put bowls back on and do a wet fuel test for accuracy.

the last time i had plugged jets. i let boiling water do the trick. good as new.

 

Any idea what the last model year was that Yamaha used this type of carb? I was just wondering how hard it's going to be to order kits.

 

Mine was doing the samething. I did the propane test around the throttle shaft seals and found them leaking. Tore the carbs down again and installed the seals and the popping and backfiring stopped.

 

Once you've torn them apart a few times - how long does it take you now to take them off, clean them up and re install?

 

The shaft seal take alittle longer than the rest. You have to file or grind the screw off on the back side to remove them. the last time it took me about ten minutes to take them off. Then about two hours or so to grind the screws replace the seals and bench sync. ten minutes to put them back on. the first time took them off it took me about 45 minutes then I learned how to do it.

 

Not hard at all: http://www.xjbikes.com/Forums/viewforum/f=23.html XJ4Ever, logo link in site banner or PM member chacal.

 

2008 is still using THAT type of carb.

 

So, like you all said, I took the carb bank off, examined the bowls for dirt, cleaned the jets - which actually looked clean anyway, made sure the pistons 'plopped' down, even switched some jets with a carb that was working good ' just in case'. I redid the compression test with a good quality screw in tester, as opposed to my 40 year old 'push it in hard' type and found the compression was around 150 -160 on all four cylinders. Put it all back together only to have the same cool running cylinder. While I was down in that area, I thought I could hear some huffying coming from the intake manifold. I put my fingers around the connections and low and behold - air leaking like crazy. So the carbs came off again ( I'm getting really good at doing this now that I know you have to pull the rubber throats out of the air box), I pulled the offending manifold to find it only had half of a gasket on it, and the manifold itself had some age cracks on both the inside and outside of the body. I think I've found the problem. My local Yamaha dealer ordered the parts and I should be back on the road again in a couple of days. Thanks to Bigfitz for suggesting I tackle this problem with my manual and not waste any money on shortcuts.

 

My 81 XJ750 dose the same but I get flames, this may be because of the straight pipes its running just a tad rich (195 mains) but im not shore.

 

I did notice that mine was running a bit lean. The plugs were lighter than what I'm used to seeing. The PO put in a K&N filter and a 4-1 exhaust, but I don't know if they rejetted the carbs. I'm not much of a K&N fan, so I'll switch that out soon then see if it makes any difference to the colour the plugs are burning.

 

It will be definitely very lean with will do long trim damage you just need to experiment with jetting and getting it running right.

 

Problem fixed. I installed a new intake manifold gasket and the questionable cylinder heated up the same as the other three. Too bad the yamaha dealer ordered the wrong manifold - so now I'm going to have to take the carb bank off again and put the new manifold on - but I can do that on the next set of rainy days. Thanks guys for all your help