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General Category >> Rubber Side Down! >> vm36 , backfires and cuts out at 1/4 throttle
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Message started by Finney138 on 08/22/12 at 09:54:17

Title: vm36 , backfires and cuts out at 1/4 throttle
Post by Finney138 on 08/22/12 at 09:54:17

ill start off with some background, stage 1 cam /ported exhaust /mostly straight through slip on exhaust does have some baffling tho/ running a "U" intake tube from air filter to carb, the VM has a UFO and dial a jet.

it runs great at idle except the odd stumble when its hot, pulls strong as long as im giving it more throttle all the the way to wide open ,if i try to hold it at 1/4 throttle it stumbles alot and pops and cuts out but keeps on truckin. pretty much the same thing happens at any throttle position if i hold it in one position for too long (sucks for trying to keep a steady speed) another thing that happens when i come to a stop i have to give it a little throttle to keep it from dying completely. seem to be burning more gas than average too.

any ideas would be helpful. thanks in advance

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by Dave on 08/22/12 at 10:18:04

Have all these changes (cam, porting, exhaust, carb) been made at the same time?

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by bill67 on 08/22/12 at 10:24:36

Sounds like slide maybe sticking,or jet needle needs adjusting. Did it run right at one time.

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by Serowbot on 08/22/12 at 10:29:07

Sounds to me like you just built yer'self a drag bike... :-?...

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by Finney138 on 08/22/12 at 10:52:18

dave everything except the exhaust was done over the winter, when i took my exhaust off to take my right side cover i broke the mounting bolt off the slip on so i ditched it for the new more straight through one.

bill67 it ran perfect before the winter haha, it turned over first shot when i put everything back together but ran terrible, runs pretty good now except for this issue, ive been running it the way it is for a while now just to be on two wheels, i would like to get this fixed before i have to put it away for this upcoming winter

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by Finney138 on 08/22/12 at 10:53:36

serowbot thats what it feels like to me too haha, poppin wheelies and everything just cant run at normal people speeds.

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by Dave on 08/22/12 at 10:56:02

Do you still have the stock carb in the corner of your garage somewhere? Am I asking too many questions?

Have you reead the thread oh the UFO?
http://suzukisavage.com/cgi-bin/YaBB.pl?num=1149364740/0

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by Finney138 on 08/22/12 at 11:23:21

used my old carb as part trade for the cam, wish i didnt so i could compare but this is the boat im in now, i read that thread a while ago i made my slide seat properly with instructions from lancer and it was a big improvement. my pilot is soldered shut may still be a little rich on idle but no smoke

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by teabowl13 on 08/22/12 at 11:46:41

If the exhaust is the only thing that changed, I'd start there. Sounds like a more straight through exhaust with your set-up is going to make your bike more of a race rocket, and also much less road-friendly.
Maybe there's a way to add more baffling and/or more backpressure to your new exhaust to bring it back closer to your original setup?
I assume you re-jetted the carb when you installed the new exhaust... maybe go back to your first jet settings and try to tune the exhaust to fit your jetting rather than the other way around (I know that seems totally backwards, but you might get a more user friendly experience from dialing back your set up a little bit...)

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by verslagen1 on 08/22/12 at 11:53:19

6D4245454E521A18132B0 wrote:

Could be you dial a jet, could be your ram air intake.

As far as I've seen, dial a jet takes a complex situation and kicks it up a notch. I think you'd be better off w/out it.

And ram air, don't know anybody with it, so you're gonna have to work out some issues yourself.

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by teabowl13 on 08/22/12 at 12:03:31

Yeah... you've got a lot going on... ;D

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by Finney138 on 08/22/12 at 13:07:04

i was thinking about baffling my pipe a bit more.

ill try running it with the dial a jet unhooked tn.

also gonna have to try er with out the intake tube, really wanna keep that tho.

yea its alot goin on but only one problem ,hopin someone may have ran into this one in the past, if not my troubleshooting might open up some doors for someone else.

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by teabowl13 on 08/22/12 at 21:54:45

Actually, I'd like to know what you mean by a "U" shaped intake tube; do you have a picture of that?

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by Finney138 on 08/23/12 at 05:29:08

heres what the intake looks like

http://i.imgur.com/is7n3l.jpg

heres what the whole thing looks like

http://i.imgur.com/LC775l.jpg

http://i.imgur.com/Qph1Bl.jpg

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by teabowl13 on 08/23/12 at 06:31:48

THAT looks the business for sure!
I don't want to hijack your original topic, but I'd love to know more about how you came up with this thing, how you jetted the carb for it, how it works etc.... I might just have to copy you when the time comes. Got a lot of other projects ion the bench right now...

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by Finney138 on 08/24/12 at 07:42:17

Tried it without the dial a jet ,getting the same symptoms but the bike doesn't run as strong so I can rule that out and say for sure that the dial a jet is worth every penny!

Teabowl I actually used 2.5 inch black plumbing peices to try out the concept ,had to heat up one opening in order to slide it over tge mouth o the carb for a perfect fit and used black rtv to seal all the joints. Also had to make my own battery box so the battery would sit lower so the tubing would fit between the batt and the frame. will Probly get one made from steel tubing if I find out its not the culprit for the backfires. I used the baseline jetting lancer recommended with pilot soldered closed. Bit of a headache but not as bad as I expected. All worth it for how fast this thing is now.

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by teabowl13 on 08/24/12 at 20:04:19

I am NOOO carb tuning expert at all, but from what you've described, and what I've read, I can't imagine how the intake would be causing your problem.
Certainly would be easy enough to test though: take it off and mount the filter directly to the carb and see what happens.
All the other intakes I have seen like yours are either straight or made from a single 90 degree piece. The extra length is supposed to help smooth out the flow of the air before it goes through the carb.
I suppose it's possible that your set up, with the joint in the middle, might be creating turbulence in the air stream? Still, I would think it should be better than just attaching the filter directly...

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by rfw2003 on 08/24/12 at 20:07:03

477672717C647F2220130 wrote:

A ram air setup is gonna need a richer cruise throttle range while the MC is in motion, otherwise it will act like your running lean, because it is while your moving. When your sitting still it will be on the rich side but as you get upto speed it will lean it out to the proper mixture assuming you have your carb jetted properly.

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by Boofer on 08/24/12 at 22:26:25

This is really out of left field, but it seems your intake is sucking hot air directly off the engine into the carb. Could that be a problem?

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by Smokestack on 08/25/12 at 09:38:31

Watching this thread with interest; I've been having similar symptoms...

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by LANCER on 08/26/12 at 03:51:33

At this point are you still using the soldered pilot jet ?
Pilot screw setting ?
What notch is the clip on the needle ?
What main jet is in it ?

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by verslagen1 on 08/26/12 at 09:48:23

That might be a true ram air, but here's why ram air don't work...

79747C707273797774701D0 wrote:

If you have a filtered opening pointed into or somehow catching the wind and it's fed to an airbox, the airbox will do a lot to handle the increased air pressures. But fed directly into a carb the pressure would increase with speed. It wouldn't draw much fuel through the opening. I know you know this already but allow me to explain it for other member's benefits. With that setup, air is being pushed through the carb instead of being sucked through. It creates air pressure in the carb that is greater than the air pressure outside of the carb. The carb needs pressure that is lower than outside air. It is that low pressure that creates a vacuum effect and pulls the fuel up through the jets. Pressurized air may in fact push into the jets and make it harder for the gas to come up from the float bowl. That type of setup works best when combined with an airbox and FI.

Title: Re: vm36 , backfires and cuts out at 1/4 throttle
Post by Finney138 on 08/27/12 at 17:29:08

still using the soldered pilot
pilot screw is 2 1/2 turns out
second needle clip position
running the 180 main

out of nowhere the symptoms let off ,able to run a cruising speed ,when i let off the throttle it doesnt hicup like it used to , still happens the odd time , thinking maybe dirty gas ? rust in my tank perhaps

still running rich ,my back tire is full of carbon soot, and my idle goes wayyyy down when i come to a stop, if i dont compensate with a little throttle it will die most times ,i can barely see smoke in the exhaust but its deffinately there