Backfire through rear carburator

[SargeVT]

My 67 MKIII 3000 Healey back fires through the rear SU carburator when I start the engine. It is really bad when I rev the engine. After it warms up, it is much better. It has pushed the rod through the plastic cap where oil is added to the dash pot several times. I am tired of replacing them! I have checked, adjusted and or replaced everything I can think of. Have to be missing something. Any ideas?

 

[Keoke]

Hi Sarge, I was wondering the other day why I had not seen this problem in ages.Seems like years ago every SU Carb had the piston rod pushed through the cap.I am not dead sure as to what your problem may be but I would take a good look into the valve timing on the engine.---Fwiw---Keoke??

 

[SargeVT]

Keoke,
The thought of valve timing had crossed my mind but I didn't think it would affect only the rear carb. If it is valve timing, it must be off only one tooth. I am not sure that I know the simpliest way to verify that. Any suggestions?

 

[Keoke]

No, ButI had a similar problem some years ago and because it was not catastrophic I ignored it until time to replace the timing chain what I found was the valve timing rather than being off 180DEG so I could have timed off of #6 it was at 210. Engine ran ok after I found out how to time it but would not time per the book and did backfire through the carb initially. Have you looked at the valve lash recently also a little leanness on the back carb: shafts,carb to manafold gaskets,ballanced throttle openings check good for air leaks .What do the plugs say on the back three clyn, -"Lean" ??---Keoke

 

[vette]

Hi Sarge, I'd say Keoke covered all pretty good, But it goes back to the simpliest first theory. I'd say the rear carb is excessively lean, maybe both carbs. Unless you've burnt a valve. If you ran it real lean for long enough, it might be a lean carb and a burnt valve. Adjust the carbs first, read the plugs. Air leaks too, throttle shafts just like Keoke said. But read the plugs to see how lean the burn is. Remember, dirty plugs won't tell you much. You have to either clean them real good or put in new ones. /ubbthreads/images/graemlins/cheers.gif
Dave.

 

[vette]

Sarge, I was just thinking, (again), that is a dangerous concept, but could it be that your lean condition is only occurring on start-up and that the backfiring is only occurring on start-up of a COLD engine. Could the choke cable to the rear carb have slipped it's adjustment or come undone completely. Obviously no choke on the rear carb could easily cause lean start conditions and a backfire. Onced warmed up, voala runs fine.

 

[dklawson]

Obviously check the timing as others have suggested, but my first thought was that the back carb is way too lean. Make sure that the float valve is working properly allowing gas into the bowl, then move on to the standard carb adjustments.

 

[anthony7777]

when i bought my bj7 last may i couldnt get the carbs sinked no matter how i adjusted the timing ,i ran the car as best i could get it.opening the hood one day i found the plunger on the rear carb had popped out stripping the cap threads,i replaced both and found the new ones 3 eighths of an inch longer,i checked this out and determined the old ones to be that of an MG.what a differance the new ones made,no backfires,no smoke,greater exceleration,smother,and much less noise.not sure how this might relate to your problem but worth checking up on,and very interesting,good luck.anthony7777(1963 bj7 3000 mk2) /ubbthreads/images/graemlins/hammer.gifo.k.- o.k .keoke,about 3 eights of an inch! /ubbthreads/images/graemlins/rolleyes.gif

 

[Keoke]

/ubbthreads/images/graemlins/shocked.gif "About 3 eights of an inch!" /ubbthreads/images/graemlins/rolleyes.gif-Musn't of had no oil in em an jes wore the bottom end off.===Keoke /ubbthreads/images/graemlins/laugh.gif

 

[anthony7777]

my man vette,nice look a this,love the out of sink cable bit,{no such thing as complex problems,only simple solutions /ubbthreads/images/graemlins/cheers.gif}anthony7777 1963 bj7 3000 mk 2

 

[vette]

Hi Anthony, You know I like the K.I.S.S. principle. Keeping it simple. I have another principle, I'll just call it the 'Vette Principle'. It goes like this, 'a big problem is usually a long time coming, a little problem shows up almost overnight.' I hope Sarge lets us know what he discovers, carbs or something else.
Dave C. /ubbthreads/images/graemlins/patriot.gif /ubbthreads/images/graemlins/england.gif

 

[SargeVT]

Vette - I'm afraid that the answer to my backfire problem is going to be a long time coming! Checked gas level in carburator bowl and it seemed to be proper. Plugs had a slight bit of carbon on one side of the electrode - they all looked the same. I noticed some wear in the throttle rod into the carbs. May be air invasion there. Waiting for Keoke to explain the WD40/Propane test to me. Since my 3000 has a Servo on it and it is supplied vacuum from the intake between 5&6 cylinders that it is causing the backfire?

 

[Dave Russell]

[ QUOTE ]
Vette - I'm afraid that the answer to my backfire problem is going to be a long time coming! Checked gas level in carburator bowl and it seemed to be proper. Plugs had a slight bit of carbon on one side of the electrode - they all looked the same. I noticed some wear in the throttle rod into the carbs. May be air invasion there. Waiting for Keoke to explain the WD40/Propane test to me. Since my 3000 has a Servo on it and it is supplied vacuum from the intake between 5&6 cylinders that it is causing the backfire?

[/ QUOTE ]
There are only a few things that can cause the rear carb to run lean, which is what is happening according to the descriptions. Assuming that you have eliminated low float level, restricted fuel supply, plugged passage between float bowl & main jet, Jet needle loose (not lifting) or set at wrong height.

The only things left are:
carb slide damper not working, see if the slides present equal & noticeable resistance to being quickly raised by hand/finger & the slides are not binding. Return freely to closed with a click or klunk.

Serious carb flange or manifold leak. If the brake booster has a vacuum leak, this is a definite possibility. Check it by disconnecting & plugging the manifold hole or even flattening the hose with vice grips. Any other "mystery hoses" can be done the same way.

A plug missing from the manifold somewhere. Some manifolds have pressed in sheetmetal plugs that can blow out if there is an intake backfire. A missing plug on the back can be hard to see.

Throttle shaft leaks would have to be huge to cause the problem described, which is not likely.

If the engine is lean, an unlit propane torch can be directed to suspected leaks & speed changes observed. (The least messy method) Likewise, carb cleaner, penetrating oil, gasoline, or even water, can be squirted on suspect leaks to observe engine speed changes. Some of these will cause a faster idle, some, a slower idle. You might find more than one problem.

Good luck,
D

 

[Keoke]

HI Sarge.Sorry it was not a simple problem.The throttle shafts on these carbs have bearings and cork glandswich are often missing.The bearings if original are paper and simply wear thin, if you can wiggle the throttle shafts in the carb body then probably they are leaking air .A test method is to get some starting fluid and while the engine is running spray lightly around the shafts at both sides of the carb. If the engine speeds up you have found at least one souce for lean running.You mentioned the Vacuum line to the booster-do the assisted brakes operate correctly ?.In either event you can remove the line at the manifold and cap it off as a test.Using the stater fluid check that the intake manifold flanges and carb flanges are sealing correctly.Take the Tickover screw out and examine its rubber seal and metal compression washer too it may be that it is perished.Look at the intake manifold drain tube located at the rear and on the bottom of the manifold it should be long thin tube formed in the shape of a lazy "Z" where it then becomes captured in a simple retainer bracket attached to a sump bolt.Also look at the little rubber elbo on the brake booster they perish too. A final thought make certain that the air cleaners are "CLEAN, also remove the bell on the rear carb clear of the big spring,now without rotating the bell reinstall it on the carb and tighten the screws see what happens??-Keep us posted on your results and hopefully the problem can be identified.---Regards---Keoke

 

[vette]

Hello Dave, Those are excellent points for Sarge's trouble shooting process. I'm curious as to why he said all his spark plugs look the same. Unless they are all actually showing leaness, even though he said they had alittle soot on one side. SARGE, HOW WHITE OR BROWN WAS THE REST OF THE PLUG ELECTRODE. I also have my mind on the brake booster. It is a common problem. Shows up quickly, (not to hard to fix), and would be a major vaccum leak. SARGE, pull the hose off the booster and put a cork in it. See if things smooth out, but remember your carb settings may be way off now if this was the problem, if in you did carb adjustments while trying to find the problem. Also, if you drive it with the booster disconnected, you may have very poor brakes.
Dave C.

 

[SargeVT]

Hi vette, Spent time trying to find the problem yesterday. To answer your question, the electrodes showed more brown then white. With the carbon on the plugs, the appearence gave a feeling of a rich mixture rather than a lean mixture though the exaust does not indicate it. Blocked off vacuum hose to Servo. didn't seem to make any difference. Also sprayed a little quick start on the shafts of the carb. Engine seemed to slow on spraying the rear carb shaft. check timing and it is right on. When I connected the vacuum advance back up, I couldn't notice any difference in the movement of the line on the balance wheel. Has anyone had a faulty vacuum unit? Line to it is clear. What is the "tick over screw" as Keoke mentioned? Can't find it in my books!

 

[dklawson]

"tick over" is another term for idle-speed.

As far as vacuum units go, whether or not they greatly affect advance at idle depends on where they connect. Vacuum connections to the carb body are "ported" and won't "come on" until you've got a partially open throttle. Manifold connections will greatly advance timing at idle (with a stock cam).

You can test the operation of the vacuum unit by disconnecting and plugging the open tube. Temporarily put another tube on the vacuum unit and "suck" on the tube. You should see your timing advance.

 

[vette]

Well Sarge, your making this tough. But i think all of us will hang in there with you trying to figure this out. some more thoughts to check:
-most of the British cars I've worked on will show some sootyness in the exhaust unless they are really lean. Especially with unleaded gas.
-your spark plugs won't tell you the trueth unless you clean them really good or put in new ones. Then go for a cruising speed drive, push in the clutch, kill the engine, shift to nuetral, drift to the side of the road, pull the plugs.
-remember timing on a big 6 Healey is about 5deg, for each 1/4" on the damper. So somewhere between 1/2" and 3/4" is where you want it. Timing it fast should make it perform quite well, but it will probably have run-on when you shut it off and it might run hotter.
-vaccum advance has almost no noticeable affect at idle, but a leak in the advance mechanism of the distributor or a leak along the line is like any intake leak, meaning you would be sucking air from all the wrong places.
-if you want, plug the vaccum advanceport at the manifold, you don't need it for now.
-do check the very small tubes coming out of the bottom of the manifold as Keoke mentioned.
-if you are sure the engine is in the proper state of tune and you can't find any leaks, then maybe it is time for the HARD stuff. -- Do a compression test to determine valve and ring sealing. Hope you find it before you get this far. PS, After you make sure all tuning settings are right, also electrical wires are in good order, Then I would take the carbs apart to ensure cleanliness and proper set-up of jets and etc. /ubbthreads/images/graemlins/crazyeyes.gif Dave C.

 

[SargeVT]

Hi Dave,
I spent time on trying to find the problem today. I re-did several of the tests that I previously done. Everything checked out ok including the points and settings. My next step is to follow your suggestion and remove the carbs, clean and set. If i find nothing there , then I will pull the valve cover and check valves and timing. Tonight after its initial backfiring on startup, it ran beautifully.

I have got to get it straightened out as I want to drive it to the British Invaision in Stowe VT in September.