Backfire

[bkuhr]

83 venture, finally have all cylinders firing. Runs very good. As in previous thread, I am concerned I may have reamed out 'pilot air jet' in carbs 2 and 4. They were plugged and I run .020" stainless lock wire thru jets. When compared to good jets, .020 was too large to pass thru.

 

Now I have serious "M2 .50 cal gun blast" out right side, only during engine braking/down shifting, as tach comes down below 3000rpm. Scared the s**t out of me, every time!

If I pull clutch then brake to slow, no backfire to matter how hard I close throttle.

 

I have read most backfire air leak in exhaust, but I can't detect one.

What is the effect of my 'reamed out' pilot air jets?

 

I also have what sounds like air leak on intake side. Kind of 'chugging sound' at idle, hardly noticable at normal cruise, pretty loud during loading like hill climbing.

I suspect maybe aircleaner boot loose and air bypassing aircleaner, but could be below carbs I guess. Haven't had time to check yet.

 

Any idea's before I get started?

 

I'm thinking I have a carb too rich, and engine braking with throttle partially closed causes to much fuel to be pulled thru, unburnt, into exhaust. Maybe due to pilot air jet?(not sure what this REALLY does)

Also thinking only idle circuit too rich as backfire only on closed throttle when main fuel circuit is closed.----Any of this make sense, or am I way off base trying to over analyzing it!

[Marcarl]

Air leak is definitely not good, so should be fixed.

Back firing is really a loud bang in the carbs, but what I think you mean is that you have an explosion in the exhaust, also commonly referred to as 'back fire', this is caused by too much fuel in the exhaust system, and so in effect you have an 'after burn' situation, very easily caused by too rich a mixture on the downswing, yep, can be caused by too large a pilot jet.

[dingy]

Air leak is definitely not good, so should be fixed.

Back firing is really a loud bang in the carbs, but what I think you mean is that you have an explosion in the exhaust, also commonly referred to as 'back fire', this is caused by too much fuel in the exhaust system, and so in effect you have an 'after burn' situation, very easily caused by too rich a mixture on the downswing, yep, can be caused by too large a pilot jet.

 

 

I would disagree with the gentleman from up north.

 

The air jets in our Mikuni carbs are there to induce atmospheric clean air into the fuel circuits.

 

Enlarging these jets will have the effect to lean out the final A/F ratio.

 

See attached crosssection view of very similar Vmax carb.

 

Items #2 & #4 are the changeable air jets.

 

Gary

 

http://i1007.photobucket.com/albums/af193/gdingy101/vmaxcarb.jpg

[MikesBike]

Is it possible that the carbs are not set all the way into their sockets? After having mine out, I had similar symptoms. Turns out I didn't have the carb rack pushed deep enough to keep them from sucking air. They should seat with the socket near the bottom of the carb, very little if any of the nipple (for lack of a better word) should be showing.

[bkuhr]

In Gary's diagram, the jet I reamed out was #15 under the rubber plug.

 

What does this jet really do?

 

Edited August 24, 2010 by bkuhr
add pic

[Marcarl]

Seeing as I am the most northern replier, I'll stand corrected if necessary.

[bkuhr]

I appreaciate all replys, and in Marcarl defense, this jet does have direct path to the main fuel jet.

 

And per Dingy, it has a direct path to the air jet above it.

 

In between is the main needle jet passage.

 

I am really not sure how this jet is designed to work in the scheme of things.

 

I am thinking during engine braking (throttle closed, or nearly so, and high RPM) there is higher than normal vacum in this port, and oversized jet on fuel side allows to much fuel to be drawn in bypassing idle only circuit. (now I am just talking out loud and not really sure I know what I am talking about)

 

Any carb experts out there? All other replies also helpful

Edited August 24, 2010 by bkuhr
typo

[Marcarl]

I appreaciate all replys, and in Marcarl defense, this jet does have direct path to the main fuel jet.

 

And per Dingy, it has a direct path to the air jet above it.

 

In between is the main needle jet passage.

 

I am really not sure how this jet is designed to work in the scheme of things.

 

I am thinking during engine braking (throttle closed, or nearly so, and high RPM) there is higher than normal vacum in this port, and oversized jet on fuel side allows to much fuel to be drawn in bypassing idle only circuit. (now I am just talking out loud and not really sure I know what I am talking about)

 

Any carb experts out there? All other replies also helpful

Now I'm all confused, in my email it says MiCarl, and so I gets all upset and get over hear to set things straight, because I wouldn't want him to have to take blame for something I did,,, and then it says: Marcarl.

I tell you, these computers are funny things, can't trust them no how.

[bkuhr]

Now I'm all confused, in my email it says MiCarl, and so I gets all upset and get over hear to set things straight, because I wouldn't want him to have to take blame for something I did,,, and then it says: Marcarl.

I tell you, these computers are funny things, can't trust them no how.

 

Thought I caught it before anyone noticed...oops:doh:

[Dano]

Check for a cracked or split intake boot right at the base of the carburetors, Big Dude just found one on his MK2. It took him taking a stethiscope, breaking off the rod so he had an empty tube at the end of it and listening all around the carbs till he found the leak. Have you synched the carbs? Does it change over temperature range? Cold versus hot? Inquiring minds need to know!

 

Dano

[bkuhr]

Inquiring minds need to know

 

Air leak is definitely not good, so should be fixed.

Back firing is really a loud bang in the carbs, but what I think you mean is that you have an explosion in the exhaust, also commonly referred to as 'back fire', this is caused by too much fuel in the exhaust system, and so in effect you have an 'after burn' situation, very easily caused by too rich a mixture on the downswing, yep, can be caused by too large a pilot jet.

 

My problem is a 'back fire' as opposed to Back firing, out the exhaust pipes.

When troubleshooting non working cylinders, sprayed starter fluid all along all boots and vac plugs/hoses, was completely free of vac leaks of any kind. Finally got all cylinders working with carb removal, and reamed out plugged jets above in carbs #2 and #4. Reinstalled carbs and resynced. Runs great, only problem is current 'back fire' only during engine braking. When cold engine, just very light pop, After 5 minutes warmed up, serious gun shot. Possible during reassemble something not tight (boots). Haven't had time to recheck vac leaks yet.

[bkuhr]

Parts manuel lists

29 4G0-14142-42-A0 . . JET, PILOT (#42.5)

 

Assuming #42.5 means .425mm, this converts to .0167inches.

I reamed clogged jet with .020inches wire.

This means, if above is correct, that my jets are 0.0033 too big.

Should I be ordering 2 new jets and pull carbs again to install them, or is the reaming likely a non issue and need to concentrate on other possiblities?

[bobcat]

Just for sh*** and grins check those rubber caps that go over the ports you use to sync the carbs..I had a similiar problem and it wound up being one of those rubber caps had fallen off.

[dingy]

Parts manuel lists

29 4G0-14142-42-A0 . . JET, PILOT (#42.5)

 

Assuming #42.5 means .425mm, this converts to .0167inches.

I reamed clogged jet with .020inches wire.

This means, if above is correct, that my jets are 0.0033 too big.

 

Should I be ordering 2 new jets and pull carbs again to install them, or is the reaming likely a non issue and need to concentrate on other possiblities?

 

 

Just a little more info to further confuse this.

 

Mikuni jets are not sized by hole dia. They are sized by flow rate.

 

I do not have a chart that shows the smaller sizes, but below is one that lists the differences between Mikuni & Dynojet sizes.

 

Gary

http://i1007.photobucket.com/albums/af193/gdingy101/mikuni-dynojetsize.jpg

[mbrood]

Per the drawing... from 1k rpm to about 2.5 k, the idle mixture jet (#10) is a primary fuel meter. As the slide retracts, your #16 takes over... so probably 2k - 4k? When the throttle is very open, then #15 is uncovered and this is the MAIN fuel jet... so I would think you just richened the midrange, probably dumping unburnt fuel out into the exhaust. If you eased it up and eased it down, the transiotion would probably allow the fuel to be more efficiently burned and not notice backfire... aggressive throttle and when you back off... pow... but that's just my thinking !

[bkuhr]

mbrood, great theory of operation. Does this mean you DO believe I need to replace my jets?

 

I went out and preformed quick vac leak check. Sprayed carb cleaner under and over carbs, idle increased when under carbs ON ALL 4!, and appeared to increase when sprayed at #3 boot plug. not missing any boot plugs per bobcat. No leaks above carbs.

Don't understand what I have wrong for all 4 carbs boots to be leaking, when they were fine before last carb removal. They are seated all the way down, and clamps tight.

 

Only thing different is last time I installed carbs rubbed a little vasoline inside boots to help set carbs. This time did not clean vasoline out, but did not install any more either. Maybe vasoline helped create airtight seal before on worn boots last time???

 

So, whats the plan?

1. order 2 new jets,

2. pull carbs again to install jets,

3. add more vasoline and reinstall carbs again?

[mbrood]

If you reamed the pilot air jets (#5). I would definitely replace but those are behind the diaphragms and could be done with the carb rack mounted. Since these see only air, I can't imagine them clogged, though. If you had the carb fuel bowls open, it would critical to clean the corresponding pilot fuel jets (#15) as these are super fine holes and prone to buildup and clogging. If it's THOSE you reamed... yep, pull the carb bank. Remember that you can split the bank with left and right pairs and work on the back of them like that... I've seen some that say an oversized hole can be soldered shut and redrilled but that fuel flow is also a tiny bit abrasive, SLOWLY trying to wear at the brass... so soft solder is not a long term fix in my opinion... especially with what's involved in getting back in there again.

 

I've mounted my rack and it "looked" fully seated but she ran like junk... I pulled them back up and "really" cleaned the lower carb mount throat and the inner boots at the seal... I too used a bit of grease to ease mount... this time they seemed to "pop" into place rather than "ease down" like earlier and she was ever so much happier! If you can spray around the boots an affect idle, you have an air leak and vacuum proved it.

[bkuhr]

Yep, it was #15, guess I'll order 2.

Thanks

[bkuhr]

Got new jets today, pulled carbs again, sigh, and look what I found.

On last install I tore up #1 boot.

 

Now I need to order more parts.

 

FYI, I mic'd .020 wire I used to ream out jets, it mic'd at .017"

I stuck in piece of copper wire strand mic'd at .012" it went in easily

So jet is between .013" and .016". not much of a difference, and likely not cause of my backfire. More likely is my FUBAR to the #1 boot.