backfirin!

[IZZ]

Had her runnin good (my 750 with 800 pipes) with 2.5 turns - 45 pilots - 138.8 mains - 3rd clip -stock float height. (very clean carbs)

But after high speed runs the plugs would be a little whitish in color and playin around the yard and at idle on and off would flood out on start up almost everytime.

So - I tore her back down - 1 and 1/8th turn out - 45 pilot -142.5 main - 3rd clip - stock float height. But I also ran vent lines to airbox's when I put it back together.

Couldnt hardly get it started again after getting it together. I initially found it was very low on gas and added gas, then it seemed flooded I guess - kept pulling plugs, drying them and blowing out cylinders. I appear to be getting excellent spark. I finally got it going except for one cylinder and BANG! backfire. cleaned plugs again and got it running.

Now it would run great at WOT flat scream! and at midrange, but at idle it seemed iratic -up n down in rpm's and almost like it wasnt runnin on all four all the time. I continued runnin it around to experiment and realized that I had to be very slow and deliberate getting it to high rpm's, in other words I felt that if I smashed the throttle to hard or fast it wanted to fall on its face, so I tried it to see what would happen and BANG - BANG - BANG. Fall on its face it did with a whole series of backfires so loud it sent shivers thruogh my spine. Then I could hardly get it back in the garage - wouldnt run worth a crap and backfired constantly!

Choke cables adjusted proper, good spark. Its gotta be in these carbs I think or something to do with me venting to airbox?

I pulled carbs and plugs, plugs are wet. 1 plug had some carbon on it. Needles look good, float heights are good. slides are in sync. they still look very clean ( I just completly cleaned them prior to my 1st jet change)

Whats wrong with this 93 750 with 800 pipes? and whats up with the backfiring?

[Lewis]

Hey IZZ, sry I don't have any kind of useful info for you, but I was also wondering about the vent tubes, because of the article in the tech section. Not sure what my question is really, but is there a way to solve the pressure diff. etc...
Sry again, Lewis

[RDL 93 V-MAX4]

felt that if I smashed the throttle to hard or fast it wanted to fall on its face, so I tried it to see what would happen and BANG - BANG - BANG. Fall on its face it did with a whole series of backfires so loud it sent shivers thruogh my spine.

Thats sucks IZZ!!! I'm not shure what you shoud do at this point. But don't worry one of the guys will post some info for you soon.

Hey just wondering was it old gas from last winter you had in there? I had mine almost bone dry at the end of the season. Put new high octane in for the first run with some Isopropyl gasoline additive. Just a though I had.

[Ken]

Had her runnin good (my 750 with 800 pipes) with 2.5 turns - 45 pilots - 138.8 mains - 3rd clip -stock float height. (very clean carbs)

But after high speed runs the plugs would be a little whitish in color and playin around the yard and at idle on and off would flood out on start up almost everytime.

Where were you making the high speed runs, on a stand? I'm surprised that your plugs were white, lean.

It currently sounds like you are running rich with all the backfiring and wet plugs.

I have 800 pipes on my 92 and run 135 mains, 47.5 pilots, 2 turns, stock needle setting, and stock vent tube location. Plugs are brown after a WOT run.

I think you made too many changes at one time, my theory is one change at a time.

[IZZ]

Thanks Ken, I know Im kickin my self in the ass for being to gung ho on this.
I think Ill put the 138.8's back in and just turn the screws to 1.5 and see if that dont help my flooding problem. Ill keep the vent lines hangin and see what happens.
That way the only change Ill have made is the pilot screw position.

If it runs decent there, Ill reconsider the plug reading, and experiment with the vent lines.
This is my 1st year in dealing with trying to read plugs and such. So my thought is that maybe Im being to paranoid and should give it more time and consideration.

[RedWhiteandBlue]

Put 47.5 pilots in it and then play with the air screw starting at 1.5 turns and adjust from there if needed. IMHO...Blue

[IZZ]

OK, I put the 138.8's, left the 45 pilots (because I dont have any 47.5's), turned the airscrews out 1.5. I also hooked the vent lines up to see what would happen (figuring I can unhook them from the airboxes easy enough if I have problems)

Runs awesome! plugs arent wet anymore. I pull the plugs after about a 2/10ths of a mile WOT and there sort of a tannish / dull yellow color.

Now heres the only problem, and you would hardly even know there is one if your riding like normal. If you sit at a dead stop, and just smash the throttle,( which this isnt a racer just a trail machine so we never do that, I was just experimenting), it bogs right down and kinda backfires but its more of a cough than a bang. and then for a short bit (like a few seconds or under 50ft) it runs like its flooded or missing and then it will clean itself out and its good again unless you try that again.

Im at sealevel and the airtemp is like 15 - 20 degrees F right now. Im going back out right now to sea how it starts cold and to unplug the vents from the airboxes to see if it still does it.

Ill report back!

[racer7x]

IZZ, the last problem sounds like a lean bog, when you smash the throttle wide open. The pilot jet change should help that.

Try that same thing and blip the chok on when it happens and see if that helps. If it does, change those pilot jets..
JMO

[IZZ]

Hey racer 7x, I just came back in - heres the results from a dead stop, smash the throttle and stalls right out. You have to let off or it dies with a puff of smoke from the exhaust.

cruising around under 5000 rpm's it will kinda trip - stumble and hiccup, before it revs when you smash the throttle. The closer you get to 5000 grand the less hesitation until you get over 5000 grand then theres no hesitation at all, it just instantly revs through the roof.

The plugs look beautiful, good enough to eat, after playing around in the yard. Milk chocolate color

(and by the way it doesnt make a differance if the vents go to the airbox or not)

So does that still sound like lean pilots or airscrew or to rich?

[IZZ]

OK, I just tried what you said Racer, flipped the choke on 1 position and the problems gone!
So do I have to get the 47.5's then? or can I compensate with the air screws out farther?

[racer7x]

I'd rather see you get the pilots, I'm thinking you wont have enough adjustment with the screws.

[MadMax-4]

Hey IZZ,
I agree with Racer7x, the reason is the pilots run to 3/4 throtel after that the main jets kick in, what i am getting at is if you are to lean from the start (idel) and a littel rich in the end (wot) and you mash the throtel you woul intoduce a whole s%^& load of air with littel fuel at the biginning then the mains kick in henc the floding and back fireing, the back fireing is due when the exhost pips load up with unburnd fuel then the spark and POW.
As for the vent tubs routed to the air box dont wory about them, the reason for re-routing of the tubs is when you run at high speeds higher than 120Kph (70 Mph), the big max is known to creat under hood pressure inturn cosing a rich effect, so running the tubs to a nutral area it will eliminate the under hood pressure effect.
Did you check the carb float height, if the float is to low that can cose the boge at the start.
Just my .02

[Lewis]

Hey, sry again to hyjack, but could someone post a pic of the vent tubes re-routed to the airbox, just so I don't !*$% it up.
Much thanks, Lewis

[IZZ]

Hey Lewis, sorry to ignore ya, Ive just been a little preoccupied. If no one else posts a pic, I can tommorrow sometime, Although there is a thread in the site here already. Let me look........


Well this is the best I could find, Ill post pics of mine tommorrow for ya in the same thread below -
http://www.vmax4.com/vbulletin/showthread.php?t=16&highlight=vent+tubes

[RedWhiteandBlue]

Last time I'll tell ya, put bigger pilots in before you break it. The air screws will not compensate for a larger jet. They only fine tune around the jet size. Also, leave those 142.5's in there for now until you get good plug/wash readings that tell you to change or not.

The vent tubes do not even come into play until the sled in doing 80+ and the air pressure increases under the hood. The vent tube trick is so the pressure does not force extra gas through the mains making it richer. I would start with vent tubes stock and make sure you are not runing rich on WOT. It takes new plugs and a long WOT run to determine by the plugs. If you are not running rich then leave it alone. I personally do not recommend the vent tube trick for a trail machine. That little richness is your insurance policy when you decide to cross a lake WOT for a mile after the temps have dropped to -10/20! It makes no difference when trail riding unless you ride the trails that fast! Good luck...Blue

[IZZ]

Blue, I changed to the 47.5's put airscrews at 1.75 turns out (this is a change from the 45 pilots with airscrews at 1.5 turns)

I did leave the 138.8's in though for the time being.

I took it for a test run and it does the exact same thing! - stalls right out if you smash the throttle from an ideling stand still, but if you feather into it its just fine.

Tried flipping the choke on to see if it would fix the problem again and now it wont even idle at all with the choke flipped one position.

So now what? Could it be the mains? The thing is it did this with the 142.5's also, only way worse.

Or should I go more turns out with the airscrew?

[Ken]

You can set the air screw to 2 turns but I don't think that it has anything to do with your problem.

Did you do anything else to your sled, clutching?

[IZZ]

No Ken, I havent done anything else to it. But the deal is.....

This is the 93 750 I bought out in montana - long track with lots of goodies including psi quads. I bought it to put the quads on my 800.

So I took the quads off and put them on my 800, then I put my 800's pipes on the 750.

Then I took the carbs off to jet accordingly, since I realized this thing was set up for high elevation. What I found for jetting was 126.3's and 40 pilot's, stock needle clip (3rd position), stock float and I believe the airscrews were at 1.5 or maybe a little under.

So anyway I put my 138.8's and 45 pilots in, set the airscrew to 2.5 and decided to run it around the yard to check it out about a month ago. The thing ran great! And even though I assume the clutching was matched to the mountain elevation jetting and quads, it was quite pleasant to ride, and since this is to be my wifes sled, I decided to leave the cluth alone. It engages around 3500 - 3700 nice n smooth like.

Now the only problem I was having is very wet plugs, fouling alot, it just seemed like it was pretty rich ideling. And after some wot blasts the plugs looked whitish or greyish maybe? I dont know really, I couldnt run it far and I really wasnt sure if I could make an accurate determination or not, but I got to reading a post on here about jetting for 800 pipes on a 750 and people were saying to go with stock 800 jetting, so I thought wow I must be lean then with the 138.8's for sure. At the same time I figured my airscrews were probably out to far and thats probably why it was flooding so easy.

So anyway, the rest is history!

[RedWhiteandBlue]

Okay Izz. You really need to put it right to stock 800 jetting and use that for a start point! 47.5 pilots and 142.5's you have are close enough for mains. air screw @ 1.5 and never more than 2 or less than 1 turn. Now keep this in mind. there are different needles from 750 to 800 too as well as there are two more jets you need to look at really (two air pilot jets). Personally, with your 800 sitting there right next to it, I would take the 800 carbs off and try them on the 750! If they run good on the 800 you should use them to test. If the problem goes away for the most part then the 750 carbs need more attention. If it's the same problem then look for something else....Blue