TOPIC:

Hello All,

Here are the specs
-4to1 exhuast with baffle
-39mm Airpod filter
-GPZ head
-GPZ cams
-GPZ Spark Advancer
-Valves all in spec
-Bottom end is stock 82 KZ750 motor
-3.0 dnya coils with stock pick up coils
-VM26 with only fuel mixture screws
-17.5 pilot jet, 110 main jets, 5DL31-4 ( tried 3 postilion too), Air corrector jet kit (zed1015)

With about 1 turn out on the fuel screw, engine idles fine. However, when I give a little throttle (1/10) it back fires through the carbs. When I lift the throttle high enough (1/8) or higher it revs nicely. I tried carb syncing and ended up completely out of tune. The fuel level is constant on all bowls and looks to be in specs. The throttle cable isn't installed, using my hand to rev the engine up. I also noticed the engine is slightly slow to return to idle, Only slightly. Sounds like I am running lean. However, I tried turning the fuel mixture screw out 1/4 of a turn and it make everything worse. I needed to adjust idle screw more to make sure the engine didn't die.

Back to the original fuel mixture setting, I installed new spark plugs and let the engine idle for 5-10 mins and removed the plugs and they still looked brand new, the engine was previously warmed up already. So, the pilot circuit looks to be in specs and operating normally. I have a feeling it has to do with my 5DL31 jet needle since the air corrector are limiting the air flow. I haven't tried removing the air corrector yet. Just remember it only backfires at 1/10 of the throttle. I have some different jet needle I can try or should I go to 5DL31-5?

Just maybe Since I have the top covers off of the carbs, that when I give the throttle the air is coming from the top? I honestly think this might be the issue. Does anyone know if that plays affect when giving throttle. Because the covers only need to be off when tuning the idle circuit.

Also, the temperature here is cold, about 20 degree F. Don't think this is causing my issue though. I know I will need slight tuning when the weather becomes warmer.

What do you guys think is causing the back fire?

Back firing off idle does indicate a lean condition on pilot and you should start with the pilot fuel mix screw around 1 and 1/2 turns out.
The hanging revs also indicates this and also possible air leaks on the boots or vac ports.
The correctors richen up the off idle to compensate for the increased venturi pressure created by losing the airbox and does not affect the pilot so removing them will only add to your lean issue.
If your carbs have an unused fuel tap vac port on #3 carb make sure it is capped off and not open to atmosphere.

Pilot fuel screw is on bottom, pilot air screw on the side. From the description here it sounds like you have the fuel screw, in which case about 1 turn is correct.

Did you install new carb boots?

Yes, I installed new carburetor boots to work with the head and the VM26, I Applied silicone around the boots too. I can’t find any air leaks though.

About one turn out on the fuel screw gave me the best idle results.

Does anyone think it might have to do with the carburetor top covers?

I stand corrected, the carburetors currently have 5CN8 jet needle I tried position 3 and 4.

I have extra set of 5DL36 and a knock off set of “Y63”

By a visual inspection it’s seems the 5CN8 is the most aggressive

I going to tried the 5DL36-3 and see how it works

What model 1000 are the 26m carbs off.
You need to start by specing the carbs to a particular model that uses those carbs as a base line and go from there.
Doesn't matter which model but the pilot jet, jet needle, needle jet , emulsion tube ,main jet are a balanced combination designed to work with a particular airbox and exhaust system depending on model and once the airbox has been removed and corrector jets fitted you should have a set up that will deliver the correct fuel mix to any engine with only main jetting changes needed.
Overall jetting is a stack up of each fuel circuit and just throwing random needles in will only confuse the issue.
You say you are weak on 1/10 throttle which is idle and pilot but the needle controls 1/8 to 3/4 throttle which is not your problem area.
At the most you should be looking at pilot jet restriction and weak pilot fuel/air mix or air leaks.

This may help
www.iwt.com.au/mikunicarb.htm

Having the tops off the carbs does not effect the mixture.

Good news, the carburetor don’t backfire at 1/10 of the throttle. I changed the needle jet like I said however at the same time I also turned the pilot screws 1.5 turns out.

I ended up with 1.75 turns out and no more back firing and no more sluggish acceleration on 1/10 throttle

You were right Zed it was the pilot circuit that whole time. Which I also suspected.

I going to put the other needle jets back on because they seemed more aggressive. Plus those are the original needles for those carburetors. And this carburetor set doesn’t come with air adjustments for the pilot circuit so just maybe that’s why the ‘5CN8‘ are so aggressive compared to the other ones I have.

Link:

Glad you got it sorted.
Not surprised it was the pilot
Bike sounds good and best to put the correct needles back in.
I see you bought the air jets through my ebay ad.
Should have Pm,d me on here as I sell them cheaper with no Ebay fees etc to cover.
Would be nice if you can leave + feedback for the jets.
Rob.

Sounds good. Cold there?

Ya, it’s cold here. Yesterday was pretty warm about high 30s.


I’ll leave a positive feedback too.