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VM26 snap crackle bog at 3/4 to full throttle

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hitaburmedusty

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Discussion starter · #1 ·
This is the never ending headache. Ive narrowed it down to thinking it's either fuel starvation or coil arking/ignition problems. Engine is a 165 yx ported, high comp, inner rotor with Reeg weight, and I am at about sea level.



Prognosis: 22.5 pilot 155-165 main and the bike was on fire fast with no probs the other day. I can still smack the throttle from idle with no bog. Plug chop read a lite tan color.



Sat in the garage for a week (did not wash) as i was just tuning on road. Pulled it out for a ride this past sunday (kinda damp and rainy day). Warmed it up getting ready to ride, took it for a quick spin and it falls on it's face at 3/4 to full throttle only when the rpm is in mid-top then I twist to full. It'll rev up nice if i don't pass 3/4 and will even go way high in rpm if i keep it there. As soon as i give it that extra twist to full it well bog, backfire, and sputter like a bucking bronco! I thought it might be the rain but i took it out again today (sunny and dry) and snap, crackle, bog we go again.



I am leaning more towards fuel starvation only because when i keep it at 3/4 it will still reach way high and rev out but as soon as i peg it is when it happens.



But then again it was running super good. Today I checked valve clearance and timing and they are spot on. Also took the entire carb apart and cleaned everything with carb cleaner.
 
#6 ·
Run a killswitch?

Had the same trouble as you discribe last year when the internals of my killswitch (one with a wristcord on it) came loose.

When riding the bike on the (Go-kart) track the vibration in the higher revs region made it to short-circuit on the handle-bars and sputter and bog. :rolleyes:



I run a 190 main in my VM26 on my 150cc Yx engine. This is a bit to rich. Live at sealevel. ;)
 
#9 ·
ibmike13 said:
As it gets warmer, your jets go down and down, 230 on a 146 seems way,way to big.
Click to expand...


Thats what i thought. I tune my main by running WFO in fourth gear and monitor how the revs build and with plug chops. I would go higher and notice if it gurgles up top then step down as needed. i guess a nice flowing head and a free flowing exhaust really make a difference too. i adjust the needle clip according to temperature. so far the plugs have been nice and tan.
 
#10 ·
something other than jetting seems wrong here. Sticking valve, bad irk or cdi box or bad ground. Jetting won't go from feeling spot on to backfiring, bogging and bucking with just a minor weather change.



I have a 1700 VTwin with an intermittent sticky valve and when it sticks, it acts just like that. Yamaha had a known problem with undersized valve guides. Who did your port work?
 
Discussion starter · #11 ·
took the bike out today(very dry day) after working on it last night which included: checking the entire wire harness and all connections. Re-sanded and connected everything. Only problem I found was that the kill switch top where the metal clamps on the bars was pinching the black wire to the bars and looked like it was shorting. fixed it. sanded all surface rust off of rotor and pickup. sprayed it all down with silicone. I also tried spraying the silicone spray on the IRK while the motor was idling and it would kill it if it got on the magneto output wire area (is this normal or is this a sign of a bad connection?).



Today the bike ran a lot better and would almost rev-out good when it was from low RPM all the way up to high but if I was in the mid range then pin it to win it, it would snap crackle bog again for a bit then be back and rev out pretty good.



So basically I still don't know whats going on. Moist weather messing with the electronics, Valves irratically sticking, carb fuel starvation,(I did make sure gas tank was venting and fuel is getting to the carb fuel bowl I know for sure.



Things that made it run better since yesterday: Kill switch may have been shorting, dryer weather, Electronics all cleaned.



Maybe you guys can help me narrow this down a bit more, i'm no pro.
 
Discussion starter · #13 ·
chiro972b said:
My valve example was on a totally different kind of bike, so probably doesn't apply. I still think it sounds like an electrical problem or a fuel flow into the carb issue. Is your tank cap vent working?
Click to expand...


Yah Chiro I tested the entire fuel circut ie: blew in and out through the vent cap, turned on the petcock and fuel flow was strong with out the carb on, plugged the gas line in to the carb without the fuel bowl on and seemed like flow was good.



i'm thinking about changing the entire ignition system to a higher quality but thats alot of mulah.
 
Discussion starter · #14 ·
76 VETTE said:
Run a killswitch?

Had the same trouble as you discribe last year when the internals of my killswitch (one with a wristcord on it) came loose.

When riding the bike on the (Go-kart) track the vibration in the higher revs region made it to short-circuit on the handle-bars and sputter and bog. :rolleyes:



I run a 190 main in my VM26 on my 150cc Yx engine. This is a bit to rich. Live at sealevel. ;)
Click to expand...


I started with the 190 main and worked my way down 185,175, 165, 155 .... 190 had it hesitating on accel. 165 was nice but had a little stumble 1/4 to half 155 semmed to take care of that, and all this was done when the bike was running tops last week.
 
#15 ·
I think you are lean. At sea level on a VM26 Boonaka is right...he is jetted 22.5/230 and it is spot on a 146. I am jetted 25/240 on my Lifan 155cc on the same carb.



A 165 with carb, cam, port, exhaust is going to have to go bigger. Did you do the work yourself on the motor upgrade?



Here are some things that can cause your problems.



1. Ground or short in the system

2. Change your sparkplug. Just try it.

3. Your china inner rotor took a dump

4. You are Lean and generally speaking, not jetted correctly

5. You have a slightly bent valve

6. Your cam timing is off

7. Your motor vent is pinched shut

8. Your CDI pickup is not adjusted correctly
 
Discussion starter · #16 ·
fatcaaat said:
I think you are lean. At sea level on a VM26 Boonaka is right...he is jetted 22.5/230 and it is spot on a 146. I am jetted 25/240 on my Lifan 155cc on the same carb.



A 165 with carb, cam, port, exhaust is going to have to go bigger. Did you do the work yourself on the motor upgrade?



Here are some things that can cause your problems.



1. Ground or short in the system

2. Change your sparkplug. Just try it.

3. Your china inner rotor took a dump

4. You are Lean and generally speaking, not jetted correctly

5. You have a slightly bent valve

6. Your cam timing is off

7. Your motor vent is pinched shut

8. Your CDI pickup is not adjusted correctly
Click to expand...


Fatcaaat, Thank you for gracing us with your knowlege.



I guess i'll be getting some massive Jets for the vm26 then. Might as well spend the least amount to see if it works then go from there.



This motor was done by Cat-5 and rips hard when it does.
 
#17 ·
hitaburmedusty said:
Fatcaaat, Thank you for gracing us with your knowlege.



I guess i'll be getting some massive Jets for the vm26 then. Might as well spend the least amount to see if it works then go from there.



This motor was done by Cat-5 and rips hard when it does.
Click to expand...


Why not just ask Cat-5 what jetting they recommend for this combination? The standard vm26 kits come with 155, 165, 175 jets.
 
Discussion starter · #18 ·
fatcaaat said:
Why not just ask Cat-5 what jetting they recommend for this combination? The standard vm26 kits come with 155, 165, 175 jets.
Click to expand...


I was recommended to start with 17.5/150 when I first got the motor. Sounds like thats is much to lean for my conditions though. Plug was white with those jets and it didn't run well.
 
#20 ·
hitaburmedusty said:
I was recommended to start with 17.5/150 when I first got the motor. Sounds like thats is much to lean for my conditions though. Plug was white with those jets and it didn't run well.
Click to expand...


go by this and you cant go wrong:



1) GET A PLUG READING: You need to run the bike in as tall of a gear as you can. and then kill the engine with the throttle wide open. Dont idle the engine, now remove the spark plug and look at the INSULATOR, the white porcelain part. as a rule it should look like coffee and cream( TAN ) If its Dark brown or sooty black your motor is running RICH. A gray ashy or white reading means its running LEAN. A plug reading only gives you an overall view of how the motor is running.

Do this with a new plug and make sure your bike is warm first.



2) THE MAIN JET: The carbs main jet affects how the carb works from half to full throttle After you have made your test run with the stock jetting. Install a main jet that is two sizes richer it will have a higher number than you removed. do your test run again, pay attention to how the bike feels. If your bike is reluctant to REV out like before, then you are too rich. Now go back and install the next smaller main jet keep doing this until you feel a noticable improvement in power once you have the main jet set go on to your pilot jet.



3) PILOT JET: The pilot jet has the greatest influence from idle to 1/4 throttle to test the pilot jet that is two sizes richer (just like the main jet) now go out and test it pay attention to how its feels when you pull out of the slow turns. If your bike is too rich it will feel sluggish and unresponcive untill you are past 1/4 throttle A perfectly tuned pilot jet will help your bike accelerate off the line and out of corners. Repete this test until you get one that feels the best.



4) THE AIR SCREW: How to adjust it, with a warm engine bring the bike up to a steady throttle setting (about 1800 rpms) get a friend to hold the throttle steady......Once the engine is running at fast idle. Turn the air screw in clockwise until the engine rpm drops. Then slowly turn the air screw out until engine rpms pickup.....Stop turning the air screw once the RPMs reaches its peak. Peak rpms is when the engine runs its cleanest and fastest. The most common air screw adjustment is from 1/2 a turn to 2 turns out. If your air screw is more than 2 turns out it is a warning that you should switch to the next LEANEST PILOT JET....If the engine has peaked before turning a 1/2 turn out. Then switch to the next largest Pilot jet.....



5) NEEDLE JET: Your carbs needle affects throttle settings from 1/4 to 3/4 turns lucky you dont have to replace the needle to adjust. Start out with the clip in the middle setting make a couple of runs, then change the clip down two positions. This is the quickest and fastest improvement you can make to the bikes jetting. Dont be afraid to move the clip up..LEANER or down..RICHER. You spend most of your time riding in this range.

Try all clip positions on the needle see what runs the best.... If you find your clip position at the very top or bottom. then you need to go back to your main jet and pilot jet and make a different setting...

When all is done your clip should be in one of the three middle notches.
 
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