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slight backfire popping
- bikerjim
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- 1986 T-type Regal
1959 Sears moped, 49cc
50's/60's Peugeot moped, 49cc
50's Mustang motorcycle
1959 Royal Enfield Indian Chief, 700cc
1979 KZ750B, owned since new
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- hocbj23
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- bountyhunter
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BTW: if you have any small leaks at where the headers bolt to the head, you will hear loud popping.
ALSO: the KZ750B4 comes with Mikuni BS-38 carbs with 45 pilots and even under best conditions, they will pop on decel. If you change to 50 pilots, the popping will stop and it will run better.
Guess how I learned that?:laugh: I run 50 at the pilots, start with pilot screws about two turns out from seated.
FORGOT: anytime you hear popping, you should check valve clearance in case you have one that isn't quite closing.
1979 KZ-750 Twin
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- bikerjim
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1959 Sears moped, 49cc
50's/60's Peugeot moped, 49cc
50's Mustang motorcycle
1959 Royal Enfield Indian Chief, 700cc
1979 KZ750B, owned since new
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- bountyhunter
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This is for "bountyhunter", re: slight backfire popping. Tnx for that reply. My stock setup was using 40 pilots. The right side is the popping culprit. I installed a new rebuild kit on the right carb, which came with 45 pilot. That seemed to help, as you indicated, but still has a slight propensity to pop. Where did you source your 50 pilot? And thanks.
I believe Z1 Enterprises has the BS30-96 pilots for these Mikuni carbs.
Another place is:
www.psep.biz/store/mikuni_pilot_jet.htm
scroll to the bottom and click on pilot jets, you want the BS30-96 type.
1979 KZ-750 Twin
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- bountyhunter
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1979 KZ-750 Twin
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- pusher
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The one way value about the heads regulates the fuel to air mixture in the exhaust gases, and if the rubber hosing is worn out (I've seen it on my kz550 and my friends kz650 now)then the mixture will be off and MIGHT cause it to pop. It's easier than jetting anyway, and a quick check. Good luck figuring it out.
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- bountyhunter
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1979 KZ-750 Twin
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- Patton
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The B twins do have a vacuum line between the intake manifolds. Good to replace it regularly in case it is leaking.
B twins do have a vacuum line between the intake manifolds.--- Wondering why they do that, instead of just capping off each vac nipple?
Good Fortune!
1973 Z1
KZ900 LTD
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- bountyhunter
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It's an equalization tube between the two intake manifolds. It forces the vacuum to be equal.bountyhunter wrote:
The B twins do have a vacuum line between the intake manifolds. Good to replace it regularly in case it is leaking.
B twins do have a vacuum line between the intake manifolds.--- Wondering why they do that, instead of just capping off each vac nipple?
Good Fortune!
1979 KZ-750 Twin
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- Patton
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Patton wrote:
It's an equalization tube between the two intake manifolds. It forces the vacuum to be equal.bountyhunter wrote:
The B twins do have a vacuum line between the intake manifolds. Good to replace it regularly in case it is leaking.
B twins do have a vacuum line between the intake manifolds.--- Wondering why they do that, instead of just capping off each vac nipple?
Good Fortune!
Thanks.
Now wondering why they don't connect all the vac nipples together on the 4 cylinder CV carb bikes? :lol:
Good Fortune!
1973 Z1
KZ900 LTD
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- TeK9iNe
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You think there was a problem with a little pop from high rpm on decel at the factory? With stock airbox, filter and exhaust, the stock jetting is fine. If you didn't want a little pop, all you had to do was add a little idle mixture with the screws.
Going richer on the pilot will make your cruise throttle position shrink, and make the bike seem jumpy at slow speeds.
There is an underlying issue and you will only make things worse by throwing off your carburetion.
Do a compression test.
Make sure that there is absolutely no air leaks from air filter to exhaust tip.
Make sure you have good consistant spark.
Set jets and mixtures to stock, maybe an extra 1/4 turn out.
If still popping, find where extra air is entering the cylinder with the WD40 method and watch the exhaust while you do, it will puff black after the engine sucks it in.
GL man.
No band-aiding!!! :laugh:
Motorcycle Shop Owner/Operator
79 Kawie Z1000 LTD
81 Kawie Z1000 CSR
83 Honda VT750C A
85 Kawie GPZ900 A2
86 Zukie GS1150 EG
93 Yamie XV1100 E
Lucky to have rolled many old bikes through my doors
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