Popping like a shot gun (Louie)

[Anonymous]

Well know this folks. I have everytang hooked up good. I even rebuilt the carbs. Started her up yesterday and around 5500 going down the road she starts popping and backfiring. Now since I rebuilt the carbs that required me to replace that screw in front that controls airflow (i think). Since I changed exhaust to 4 into 1 and it was a MAC (does not require rejetting) should I change any settings on the carbs. There are 2 screws on my slide carbs (1979 750) that can control fuel and/or air.

1- Please verify these 2 screws and function.

2- Give me base number of turns on each 2 different screws to get things flowing correctly.

I will reverify my float height. I measured them according to manual and did this starting from the base where the float cover actually comes in contact and seals. Maximum height according to manual was .098 so I set at .94 where minimum was .90. Any other suggestions welcome. If after following all suggestions and directions she still pops like a shotgun I will have to assume that the bogging down at said RPM could be synchronization which will lead me to a shop.

Thanks.

Louie

PS- She burns so rich when running that I reak of fuel and fumes sting eyes. Will maintain an idle.

 

[Anonymous]

forgot to mention

forgot to mention

I do NOT have stock air box. I have Single filters.

Louie

 

[Anonymous]

Re: forgot to mention

Re: forgot to mention

I do NOT have stock air box. I have Single filters.

Louie

You also have a large pair doing all those things at one time.

 

[Billy Ricks]

Try raising the needles first. The rpm range the popping is occurring in would point to that. You may also need to go up a size or two on the mains.

 

[Anonymous]

okay

okay

Say I go up a notch on the needles. I am pretty sure I do not have any of the screws set right either and would still like a good determiner for those.

Louie

 

[earlfor]

On my 79 750E with stock jets, exhaust and airbox, the carb settings are fuel pilot screw (the one underneath that sticks out at the front of the floatbowl)
turn 1 5/8 to 1 3/4 turns out after VERY gently seating it all the way in.
Idle air screw (on the top aft right of the carb), gently turn all the way in and back it out 1 3/4 turns.

From your description of the gas smell, it sounds very much like the floats are not closing the fuel needles in the float bowl. I would remove the carb stack, turn the carbs upside down with the bowls removed and the floats weight resting on the fuel needles. The weight of the float is enough to shut off the flow, so when holding the carbs upside down, you should not be able to blow air through any of the fuel nipples on each carb. If you can, either the needles are not seating, or the floats are bottoming out against the top of the float chamber before they can activate the closing of the fuel needles.

Earl

 

[Anonymous]

Thanks

Thanks

Thanks Earl,

Whats your opinion with me having Single K&N filters on each carb in regards to screw placement. Should I decrease fuel flow or air a bit ?

I will pull carbs this coming week and redo the float height.

Louie

 

[earlfor]

Re: Thanks

Re: Thanks

I think the K&N filters are considerably less restrictive than a stock airbox.
I have a friend that runs a Mac header with stock airbox and he runs very slightly lean, but not enough to rejet, so my belief is the Mac header is very close to stock restriction.
Consequently, I will assume your problem lies primarily with the K&N filters and the decreased intake restriction. The idle air and fuel pilot screws do not change mixtures (to any large degree) in the rpm range your problem lies in. I would leave the idle air and pilot screws at the adjustments I gave you as those are already settings on the rich side. You will need those settings to get a decent idle and startup. I would do as Billy suggested and raise the slide needles one notch.

Earl

 

[saaz]

From memory, my 78 GS1000 had the same carbs originally. The mixture was very sensitive to changes in the combination of the air and fuel screws, but the idle circuit only worked with a narrow band of adjustment possible. Its main use was for pollution control, not easy tuning! If you play with them, only do one chnage at a time otherwise the results will not make sense.

I used one of those colortune devices, which is very handy for sorting out low speed jet circuits. I had to chnage the idle jet to get the mixture right as adjusting the screws was not enough.

If you can find someone with settings that work, try them. Good luck!

 

[nabrams]

Louie,

I was missing my air filter (so there was too much air coming in). No matter how I adjusted the needles or mixture srews, it would would not run right at mid-range RPM (hesitation, back-firing). Once I got the stock filter and sealed all air leaks, the bike ran well in all RPM ranges. In your case, the K&N filters are probably letting in too much air (as others have noted). You may be able to get away with raising the needles, but I suspect you may have to go to larger main jets. The reason I say this is because I've seen multiple posts from others (who've installed K&N singles) that said larger main jets were pretty much mandatory in this case. I'm pretty sure http://www.denniskirk.com/ has the jet kit.

Good Luck

 

[Don Lobacz]

When you replaced the fuel screw on the front/bottom of the carb, did you make sure that the tip of the old screw was not stuck in the orifice? I have seen this happen several times, and it would shut down any idle fuel flow to that cylinder (as well as ruin the next screw that is rammed up behind it). It is possible to push the tip back out of the hole where it sticks out into the carbs bore.

 

[Anonymous]

back fire problem

back fire problem

hi louie .i would say that don and earl on the right track,with my experiance if you change the original air box for indivigual filters and the pipes for a 4 into 1 you have to change the jetting. buy your self a k&n jet kit this will solve your problem, dont try playing with the original jets as this will only frustrate you. problem is the needles the stock ones are too lean and it will run very hot, the ones in the kit are very adjustable with 6 settings instead of 5 plus the taper is much richer espscialy in mid range right where you need it. while we are on carbs heres a little trick that has helped me greatly.think of your carbs as doing 3 things ,idle , mid range and mains.now think that your throttle has 3 positions the same as your carbs, if the carbs are running poorly in the 1st third it is the pilot circut,2nd third of your throttle you have a needle problem if it is the last third you have amain jet problem .agood thing to remember is that the main jet over laps all circuts except at an idle. hope this way of thinking about crbs will help good luck reg

 

[HiSPL]

I installed a mac 4-1, k&n pods, and accel coils and had to go up from 112.5(stock) to 147.5 main jets. so the header does flow a little better than stock. especially with other mods....

 

[Anonymous]

Update

Update

Played with bike tonight. I mastered how to pull floats and change clips for needle height without pulling the carbs. Very proud of myself. Even made a tool for adjusting the front fuel adjust screw.

First try = Cleaned plugs. fuel screws seated and turned out 1 and 1/2 turns. Air screws tuned 1 1/2 turns = One one run up street and back which equals fouled plugs on all four. Nice and black.

Second try = Cleaned plugs. fuel screws seated and turned out 1 and 1/2 turns (one twist from original and then half that). I thought 1 and 1/2 turns from first setting was 360 degrees instead of a single 180 degree turn equaling one. Air screws turned 1 and 1/4 turn = One run up street and back equals black fouled on all four.

Third Try = Cleaned plugs. fuel screws left alone at 1 and 1/2 turns but changed air screws to 1 and 3/4 turns. One run up street and back equals cylinder 1 and 4 lightly carbon (very light gray) and 2 and 3 black and fouled. Bike will only make one trip up road and back before fouling 2 and 3 plugs.

Ok your turn. Whatcha yall think now ?

Louie

PS- that first trip up the street initially is fast as all get out. I MEAN SCARY FAST !!

 

[Anonymous]

***update***

***update***

:lol: :lol: :lol: Okay I got things running. Here is final adjustments that kept me riding for about half hour with no major problems.

Following Settings are for a 1979 GS750 L model (same motor as everyone else) with the following mods:

Dyna 3 Ignition system.
MAC 4 into 1 exhaust.
Stock coils.
Stock spark plugs.
Stock carbs with stock needles.

Needle settings = Third Notch
1 and 4 fuel screw = 1 1/4 turns
2 and 3 fuel screw = 1 1/2 turns
1 and 4 air screw = 1 3/4 turns
2 and 3 air screw = 2 turns
Floats set to maximum height = .098
Spark plug gap = .027

This got the bike running really good and damn near perfect. I now feel comfortable enough to ride bike to shop to have carbs synchronized and while there check my tuning. Bike starts, idles, and runs like a champ. Pulls strong thru entire RPM range and attempts pickup of front wheel 1st thru 3rd gear with hard throttle twists. Barnett Racing Clutch really helped to grab with the torque. Power increase is very noticeable over stock.

Thanks for all your help. Another report will follow after visit to shop to see how close my settings are. I am requesting shop to note any changes made for future GSer's. I will post final after shop visit.

Louie

 

[Anonymous]

Very nice Louie, sounds like you are learning your stuff.

 

[Anonymous]

** UPDATE **

All 4 spark plugs get nice and carboned up when idling for long periods. Turns out a synchro will not fix this problem. Floats are set good. Looks like I am going to be resetting the needles to 2nd notch.

Another report to follow after change.

Louie

 

[earlfor]

Louie
Youre looking at two different ranges and two classes of settings.
Plugs carboning up after long periods of idling have to do with the adjustment of your fuel pilot and air screws. At idle, the needles and mains are not operating, so this condition has nothing at all to do with the adjustment of the slide needles.

For the correct mixtures relative to the main jets and slide needle settings, you must run the bike at the rpm range (on the road) that you are trying to set mixtures for. These settings cannot be determined in the shop with the bike idling.

For my mikuni slide carbs, changing the slide needle position requires that you re-synchroniize the carbs each time because the slides and their adjustment nuts must be removed to alter the needle settings.

I would first synchronize the carbs at idle. The slides need to operate uniformly and with the same intake velocity to enable you to accurately and equally set the fuel pilot and idle air screws. The reason I say this is because at idle, required engine air intake volume is a constant and changing initial slide height will not alter volume. But, it will alter velocity, which in turn alters fuel pick up capability and results in a mixture change.
Float level height is the 2nd factor which will alter mixture considerably.
Raising the slides in idle range synchronizing will reduce vacuum resulting in less fuel pickup capability and hence a leaner mixture.

Earl

** UPDATE **

All 4 spark plugs get nice and carboned up when idling for long periods. Turns out a synchro will not fix this problem. Floats are set good. Looks like I am going to be resetting the needles to 2nd notch.

Another report to follow after change.

Louie