Popping on de-accel

[Shadow24v]

Just got my 78 special on the road recently and i have noticed that on deaccel from around 4000rpms and up, i get some fairly good pops out of the exhaust. From what i can tell, the mufflers were replaced at some time as they don't look stock. Do i need to add a little more fuel to compensate for the extra flow?

Also, the right muffler may not have a good seal to the header pipe (air leak) which may cause the issue. What sort of clamp can i use here and not ruin/rust out the pipes?

 

[jayel]

first off a tight seal at the heads and pipe joints are a must, measure pipe joint diameter JC Whitney carrys clamps MikesXS should also have them if it still pops after that adjust (open air/fuel mix screw 1/8 turn) on the side that is popping, I smear a Hi-temp silicon gasket (copper based) stuff into the pipe/muffler joint to help seal it

 

[gggGary]

Are the carbs synched? That will cause backfire also. Have you done a compression test?

 

[Shadow24v]

Jayel - thanks for the info on the clamps and sealer

gggGary - carbs have been synched with dead cyl method, compression seems good, runs quite well. ill see if my compression tester is around and see what i get tonight

 

[gggGary]

Do a sync, there is a cheap DIY sync gauge you can build for a buck or two. You can set your idle mixtures "dead cylinder" but I wouldn't count on it for an accurate sync. (AFAIK)

Down and dirty DIY trick. the local auto store has metric zirc fittings, you can easily drill the ball and spring out to make sync tube adapters (if you don't have vacuum spigots) and float gauges.

 

[5twins]

Where are your mix screws set? Spec for your '78 is 2 1/4 turns out. With your mods and no rejetting, you may need up near 2 3/4 turns out to alleviate the popping. You shouldn't need to use the dead cylinder method for your model. You should be able to hear the effects your mix screw adjusting is having just by kneeling next to the cylinder you're tuning.

If you get the exhaust sealed and mix screw adjustments don't help, you probably need to go up a size on your pilots.

Also, your original mufflers were welded onto the headpipe and must be sawed off to replace them. The headpipes are double wall. When adding aftermarket mufflers, they should be clamped to the inner pipe not the outer. If you clamp them on the outer, exhaust gases flow up between the inner and outer pipe and sputter out a breather hole up near the head.

 

[Shadow24v]

Hmm, ill examine the exhaust more. the PO was the one who did the exhaust. The mufflers are clamped on the outside of the header pipes. No idea if he compensated with the carbs correctly or not.

 

[retiredgentleman]

On my 78SE I use "dead cylinder" method to adjust the mixture screws. I use a manometer to adjust the "sync" screw between the carbs. Both are very simple to do.

 

[BigBoreSwede]

On my 78SE I use "dead cylinder" method to adjust the mixture screws. I use a manometer to adjust the "sync" screw between the carbs. Both are very simple to do.

Hello.

I do like simple methods but here I have huge troubles with the language! Can anyone explain the "dead cylinder " method to an idiot from Sweden I don´t understand sh-t

Thanks!
/BigBoreSwede

 

[gggGary]

Pull one spark plug lead so it is running on only one cylinder, one cylinder is "dead" you set the idle mixture on the running cylinder, Replace the wire, pull the other plug wire and repeat. Be sure to have a grounded spark plug on the loose lead.
I think you understand the meaning of sh-t perfectly.

 

[jayel]

Pull one spark plug lead so it is running on only one cylinder, one cylinder is "dead" you set the idle mixture on the running cylinder, Replace the wire, pull the other plug wire and repeat. Be sure to have a grounded spark plug on the loose lead.
I think you understand the meaning of sh-t perfectly.

set RPM to highest point, adjust throttle stop screw to bring idle down if needed, both throttle stops later after both cylinders are set, usually idle will be high after the carb sync

 

[BigBoreSwede]

Pull one spark plug lead so it is running on only one cylinder, one cylinder is "dead" you set the idle mixture on the running cylinder, Replace the wire, pull the other plug wire and repeat. Be sure to have a grounded spark plug on the loose lead.
I think you understand the meaning of sh-t perfectly.

Yeah.....the sh-t word is one of the easy ones

Grounded plug.....means I´ll put it on the cooling fins or something like that?? Otherwise, will the coil break?


So when I set the idle mixture screw, what will I look for? Highest RPM possible as long it runs fine or do I get Jayel wrong here?


/BigBoreSwede

 

[jayel]

jumper wire plug base to engine ground (loose plugs tend to fall off vibrating engines) yes ungrounded plugs can knock out coils on electronic ignitions, adjusting for highest rpm= to lean on the air mix tends to die, run rough, to rich starts to hunt (blubber) there will be a sweet spot of about a 1/4 turn adjust to center

 

[retiredgentleman]

Here's a picture of what I use for "dead cylinder" to set mixture screw.

I bet your English is a whole lot better that my Swedish

 

[Gordon]

Swede, Jayel is saying that the highest speed with the mixture screw, not the idle speed screw. Hope that help's you. When the engine reach's it's highest speed with the mixture screw, it is running lean. Then turn it out about 1/4 to 1/2 turn for the right mix. Do that to both carb's then reset your idle speed and do it again. The first thing is to do a cam chain, valve adjustment. Timing is next. The last part is the carb's. I know your chasing a lean pop on decell, but all the basic's have to be correct before carb adjustment's are done. Just FYI, some do not uderstand that. From one dumb Swede to another. Vooshigue.

 

[BigBoreSwede]

Wow Thanks!


I´ll be right on it.

Very very soon all the snow is gone here up north and I guess I´ll have to wear screwdrivers and monkeywrench in my backpocket while I´m out riding for some time ahead.


/BigBoreSwede

 

[BigBoreSwede]

Hey.

I hear you, Gordon and I totally agree there.
But anyhow, I got back to the garage today and did the "dead cylinder" method. I had NO!! response at all while turning the mixture screw in or out. While the bike was at idle and ran fine on one cyl I had the screw turned out about 3-4 turns and also screwed in to the bottom.......no diffirence.

I got both sparks in and fired it up with both mixture screw down to the bottom......no diffirence and it runs fine with good response....only issue is that it won´t go back to idle at once when trottle is twisted and sparks are a little wet.

I go with 750, boyer ignition, and stock 81 carbs, main jets 140 something if I remember right


What´s going on??
/BigBoreSwede

 

[jayel]

plugged low speed jet or passage way also check air jet passage way to idle circuit

 

[retiredgentleman]

BigBoreSwede;

There is a little trick to doing the "dead cylinder" method. Its NOT done at 1200 rpm. Warm the engine up and use a fan to blow air on the engine. Use the idle speed screw to lower the rpm down to about 300 or 400 rpm. Adjust the mixture screw for the highest smooth speed, while going back and forth to the idle speed screw to lower the rpm.

Because of the 3 bypass holes that allow fuel/air mixture to go around the mixture screw, the mixture screw will have no effect at 1200 rpm. By using the idle speed screw to lower down to 300 or 400 rpm, you are allowing the butterfly to close off the 3 bypass holes in the carb throat. The mixture screw will then be effective, and you will find the sweet spot