Compression numbers

"The backfire is fairly loud a

"The backfire is fairly loud almost as loud as a fire cracker, or the crack of a .22 rifle when it goes. It also has frequent muffled pops that sound to be more on the exhaust side. The muffled pops are concern enough but the backfire up the carb is very troubling."

"John:

If you are leaking d

"John:

If you are leaking droplets of water from the choke into the carb throat at idle, it vaporizes and actually makes the air charge denser which improves combustion. The problem arises when your engine is running at 3,000+ RPMs and all of a sudden injests more water than it can vaporize and the backfire occurs as the now vaporized water/air/fuel charge enters the exhaust stream and is ignited.

I suspect that is why you have an exhaust leak--from the explosions in the exhaust manifold which are sometimes violent enough to cause gasket and other damage.

My suggestion is to fix the choke/water leak or change to an electric choke and replace the exhaust manifold gasket. Soak the nuts and bolts when cold overnight with a rust disolving penetrating oil. Then give each one a small 1/8" turn with a wrench or socket. If it moves easily, rock it back and forth a few times and repeat the pentrating oil while rocking back and forth. Let it sit a few hours or overnight. Repeat as needed untill they begin to move more freely. You can apply heat from a torch to the the nut on a stud if it is rusted to it--REMEMBER THE FUEL IN THE CARB...BOOM. I use a fan to blow any fumes away from the flame.

Side note: Small amounts of vaporized water will knock carbon off of the valves and clean out the combustion chamber.

An old school mechanic used to decarbonize engines that had carbon knock with a cup of very, very hot water that he s-l-o-w-l-y poured into the PCV intake hose of a HOT engine running wide open. Soot and crap would fly out the exhaust pipe."

"Guy

""An old schoo

"Guy

""An old school mechanic used to decarbonize engines that had carbon knock with a cup of very, very hot water that he s-l-o-w-l-y poured into the PCV intake hose of a HOT engine running wide open. Soot and crap would fly out the exhaust pipe.""

That is an old school one, I havent heard that one for years, but it does work.

John

Can you get your engine warm and then dissconect the water supply to the choke and see what happens ?? (you will have to back off the choke mechanism as well).

If that doesnt help, after hearing your last description of the noise I realy think you have a sticky valve or lifter as Rod mentioned back in an earlier post.

I have thought about the gasket problem at the manifold and from memory the job I had the problem with had a fairly lumpy cam (big valve overlap) which you wouldnt have, so although the leak needs to be repaired I now doubt it is the root of your problem, but still may be contributing and needs to be fixed.

Cheers
Peter C"

"Removing the choke is what I&

"Removing the choke is what I'll try next time I get it out. I'll let my help know what I find, thanks.

John"

"The latest observations: I to

"The latest observations: I took off the tube that I thought controlled the choke opening. With the flare and compression fittings it was attached with, it seemed that hot water flowed through it to control the butterfly. I then ran the engine until warm, no water came out of the opening. Plugged manifold opening? ('72 era 2 barrel Rochester carb.) While looking down the throat of the carb with the engine running about 1500 rpm I see gas running in with large drops on the port side barrel of the carb, while nothing appears to be metering in on the starboard side of the carb. Below this carb side by side are the adjustment screws(2). To see the effect I screwed in the port side adjuster until it seated with no affect on the r's. This was the one that I could see the drops of gas dribbling in. It looks like each screw governs its respective carb barrel. Is this correct? Is the carb in need of an R&R?"

John:

Your carb needs an ov

John:

Your carb needs an overhaul. The tube you removed is called a stove pipe if it is connected to the exhaust manifold. It is a conduit thru which hot exhaust gases flow thru to warm the choke coil via a vent hole in the exhaust manifold.

My 1987 4.3L had a 2 tube exhaust heat source to the choke but was changed to an electric choke.

"Yes to your question, mixture

"Yes to your question, mixture screw controls each "bore". These are idle mixture screws. If you close one or both, the engine will stall.
You closed one screw with no effect which means there is a vacuum leak somewhere. Most common is in the base gasget. You said earlier that you do not have a pcv valve. If you have a vacuum hose connected the the back of the carb base plate and the other end is in usually the left (port) valve cover and there is no pcv valve attached to this hose and looking at the hose, all you see is a black hole, than you have a serious uncontroled vacuum leak caused by the missing pcv valve. Although the pcv valve sucks air, it has internal restrictions to control amount of suck. To compensate of this added air being sucked out of the crankcase, the carb is rejeted from previous years of no pcv's. You should put a pcv valve in this hose and then check the idle again. The usual starting position of both idle mixture screws is 2 turns from lightly seated. Final adjustment is backing out mixture screw until rpm does not increase then 1/4 turn c.w.
If there is still no response to rpms with this adjustment then look for vacuum leak in base gasget. The backfire can blow out this gasget. Worse case is vacuum leak in intake manifold gasget. Do not forget to check any vacuum hoses to anything else for leak. If you pull the carb, remove cast iron base plate from the carb, make sure there are no cracks on the base plate. If there is a vacuum fitting on base plate, a crack is common from the bore thru the treaded hole were the pcv hose is threaded to the base plate. If there is heat ports, clean them and check for cracks along the passages also. Install new gasget between base plate and carb and tighten. Install new base gasget between base plate and maniforld and tighten.
Don't try to r&r the carb yet, just check the carb assembly screws for tightness and replace the base gasget. Set idle to 500 and adjust mixture screws, if there is no vacuum leak, there should be a response in rpm's to mixture screw adjustments. If not, the only other place a vacuum leak would be is the intake manifold gasget/s"

"Bill:

The rubber tube com

"Bill:

The rubber tube coming from the valve cover attaches to a nipple that enters the carb in the center, it feeds air in above the jets, before the venturi, just under the ring the flame arrestor sets on. It really looks like it was never set up with a pcv valve, the oil cap is a breather/filter style.

It has the "stovepipe" choke control. Is it normal for one side jet to feed more fuel than the other? The port side bore dribbles in alot more than the starboard. In fact I can't see any coming in on that side at 1500r's."

"This AM tinkering: I removed

"This AM tinkering: I removed and plugged the rubber tube from the valve cover to the carb top so no air can enter this way. I started the engine; no change in performance. I throttled up to 2000rpm, then turned both adjuster screws in to lightly seat simultaneously , still no change in r's. The port bore functions the same, visually metering alot of fuel in while in the other bore I see none. Thoughts?"

"Hold the thought on pcv issue

"Hold the thought on pcv issue for later.
Both bores must meter fuel the same.
The setting of the mixture screws MUST be done at slow idle of 500-700 rpm. These mixture screws are for setting carb for smooth idle. If closing mixture screw at this idle speed does not stall engine than vacume leak is probable.
Now,
increasing rpms causes butterfly's to open causing vacuum to draw fuel from the main metering system via the venturi at the top of the carb throat. This cone shaped venturi is centered in each bore, the vacuum draws fuel from the main jets up to the venturi where the cone causes a cyclone effect and atomizes the fuel with air. If the fuel passages through the main jets and up to the venturi are restricted in any degree, than this will cause lean condition. This restriction can be somewhat intermentent and when this happens, it is VERY possible to get this spitting/backfire you experience at cruise speed.
Also when rpm's are above 1000, the adjustment of idle mixture screws/system have no effect on rpm.
1. Set idle mixture screws at 2 turns from seated.
2. Set idle at 500-700
3. readjust mexture screws until idle increases then adjust to maintain 500-700. You should be able to easily maintain 500 and then closing the screw should stall engine if not there is vacuum leak. If you can get it down to reasonable idle speed, use small squirts of carb cleaner around base gasget which will increase rpm briefly if there is a leak. If so and considering the dribbling venturi, you should R&R the carb.
I know this is a litle long winded but it is important to find exact problems so you can verify repairs is actually fixing problems."

"I'll try changing the mix

"I'll try changing the mix screws at idle, but from what I can tell the port side dribbles in, at idle (350 rpm) nearly as much as at 2000. On the starboard I see nowhere near this quantity of fuel entering. "Both bores must meter fuel the same." On this carb I can visually see this is not so. I did wet down the exterior carb base with water to look if I could see any of it got drawn in, I saw none. I'll try with the carb cleaner though. How does it run so well (except for the bfire) with this much fuel metering difference?"

"John
Bill is on the money ab

"John
Bill is on the money about a vac leak and its affect on carby adjustment.
You have already decided that you have a manifold leak in an earlier posting so my advice is to replace the manifold gasket, then try your carburetta adjustments again. Once that is done you can move forward.
Peter C"

"What brand of carb do you hav

"What brand of carb do you have?
Are you able to send photo/s of top of engine. View carb, top of valve covers, hoses from valve cover to carb? A picture is worth thousands of words.
Email diagnose is tough at best.
If you see droplets of ??? and you can reach those droplets with a Q-tip, get a drop on the Q-rip, and take a whiff. Does it smell like gas? Being safe, put a match to the Q-tip, does it ignite like gas? Sounds wierd but lets make sure it is gas and not caughing up water or unmixed additives floating around the fuel tank or from the float bowl."

2 barrel Rochester 2 GC is the

2 barrel Rochester 2 GC is the carb. It's a new tank and fresh gas. I'll tinker with it again in the AM.

"Here is a thought about dribb

"Here is a thought about dribbling carbs.
First, when you open the throttle for more speed no matter how slight or at what speed, the accelerator pump will squirt a shot of fuel into BOTH bores. If the accelerator pump is weak or the fuel passages for the accelerator pump are plugged, you will either get nothing or just a dribble in one or both or a dribble in one and nothing in the other. Anyway, in your Rochester carb, there is a accelerator port next to each venturi. Disconect the throttle cable so that you can jog the throttle by hand. Without starting the engine, look down the bores and open the throttle to wide open. You should see 2 jets of fuel about the size of a pencil lead. If not, you will definately get backfire because when engine is running and opening the throttle requires a shot of fuel to keep engine from going lean. No dribble, no phisttttt, just a solid stream of fuel. This is a easy carb to work on, more on that if needed.
Continue with the base gasget test also.
I am sure you are getting to the origin of backfire and pharting.
Put the pcv topic on the back burner, those engine guys at the factory change systems like I change my shorts!
Also, look at the Logo on your carb, there should be a GM cast there also, probably what keeps that old FORD and mine, running this long!
Going to take my old girl 100mile run to sea for a Halibut run and my only concern is if my picknick basket will hold enough beer."

"I do have the "2 jets of

"I do have the "2 jets of fuel about the size of a pencil lead". I squirted carb cleaner along the base several times from all angles, but get no change in r's. I turned in the idle screws: starboard causes the engine to run very rough, and after some moments die. Port side: if I turn in this side nothing seems to change, still idles OK. This engine idles almost as low as I have even made one run, sounding just a little ragged. I also have the GM stamping, and Rochester 2 barrel on the float cover plate. Without obvious sign of a leak near the carb base is the best next step replacing the manifold gasket?"

"The post just above is not qu

"The post just above is not quite correct, further screwing around shows: turning in to seat port carb idle screw has no effect on idle speed as before, but starboard will run too with the idle screw seated though more ragged than before. In fact it will run when warm with both screws against the seats. I sprayed carb cleaner around the intake joints (on top) and the carb base with no change in performance. Can the manifold gasket cause this? I can't get at the under side of the manifold with the carb spray to see."

"Refering back to a previous p

"Refering back to a previous post..
You said vigorous puffs from manifold? That was intake manifold? If so any puffs from there is a vacume leak.
Easy R&R.
If you are not going to R&R the carb than leave it on the intake manifold. After pulling the intake, cover the valley with paper towels to keep any crud from getting in. Examine the casget for signs of leak but dont be supprised if you dont see obvious sign of leak. Clean intake and head surfaces totally eat off it clean. Look for any cracks or defects especially in area you seen the puffs.
When ready to assemble...
Coat lightly around water passages of head and intake with non hardening permetex. If there are flaws (small gouges) going from vacume port to either inside or outside of head or manifold, coat these also.
There should be a rubber gasget that lays on the flat horz surface, front and back of the valley. Small tabs on each end of these gasgets must rest against block and head.
Look at the gasget surfaces for identifying label that may say "RIGHT SIDE" this is the starbord gasget. If there are no identifying labels than the gasgets can be mounted on any side. If there is a metal plate on the gasget that is covering the carb heat port, remove it. If there is no opening in the gasget for the carb heat passage, you must cut a hole to match this port. If you dont do this, the auto choke may not function correctly.
Lay the manifold to head gasget in place and make sure the little tabs rest in the grooves of the rubber gasget.
Coat these two tab surfaces with permetex also.
Now lay the intake manifold into position. Do this by holding the manifold directly over the area and slowly lower is straight down ( extra hands and eyes help alot). Do not move the manifold when it is sitting on gasgets otherwise you can move the gasgets out of positon.
Using clean bolts, coat the threads of all the bolts with permetex and start all the bolts with only a few turns. After all the bolts are inserted with a few turns, check that all hardware being used with thes bolts are in place and finish turning all the bolts until they are lightly seated.
Now you must torque the bolts evenly starting from center of manifold and side to side working your way to both ends of the manifold. The first torque should be 5 to 10 foot pounds. Then reapeat with 20 foot pounds and finish with 35 foot pounds. Finally re-torque again at 35."

"How was the Halibut fishin&#3

"How was the Halibut fishin', Bill? Thanks for the detailed input regarding the manifold. My engine is a little 4 cyl. 153 cid. In your post it seems like you describe a V6 or V8 configuration. The intake and exhaust are integral, the exhaust side leaks. I had planned to blow the intake-exhaust manifold off with air first and the vacuum the same area with a shop vac next. Use engine cleaner if I still see crud."

"Good grief, where did I get t

"Good grief, where did I get the idea we were dealing with a v-8? Not keeping up with original posting.
Senior/stroke moment!........never mind
Halibut run this weekend.
Go ahead with R&R the manifold but dont be supprised if you have to do some scraping to get it eat off of it clean.
2 gc carb on 4 cylinders...hmmmm.
Some 4 cyl engines use 2 barrel carb but use one side/bore as primary and other bore as secondary, similar principal to 4 barrel carb on v-8
Do both butterflys open together with throttle movement?
Choke plate cover the entire carb or just one side/bore? Just checking.
Sorry for the missinfo confusion."

"It looks like the port side b

"It looks like the port side bore feeds 1 & 4 the starboard feeds 2, & 3. Both B-flys open together the choke plate covers both. That primary/secondary arangement sounds like it'd be more fuel stingy, no I don't think I have this."

"Good, it is still a true 2gc.

"Good, it is still a true 2gc. The same principal of mixture adjustment screw still applies.
Just a side note.. the fewer the cylinders the more effect vacuum leak and carb adjustment will effect performance."

"My questions to you all I gue

"My questions to you all I guess by now don't seem to end. I think I'll start a new post, focus on the carb. I took out the idle needles this AM, found them quite pitted, and not very smooth and round. I put them in the drill chuck and spun them in some steel wool to clean off the rust. I put them back in after spraying carb cleaner into the seats. Result: only marginally better. Can the seats be replaced? They look permanent. I haven't removed the carb yet put I'm beginning to think it must be full of crud, and buying a rebuilt might be the best option."

John:

Good idea. It will g

John:

Good idea. It will get you back on the water faster. Your carb is ready for Davy Jones' Locker.

"Thanks Guy that'll be my

"Thanks Guy that'll be my starting point, I'm wondering though are the needle seats replacable?
I could keep this carb for a backup if the various orifices were replacable."

"The carb idle mixture seats a

"The carb idle mixture seats are nothing more than drilled holes in the casting. The needles are replaceable. The main jets are replacable but rarely need replacing, just cleaning. Accelerator pump, float bowl seat, spring and steel bb used for a check valve, set float level, 3 or 4 gasgets and blow out all the passages is all that is usually required.
The 2gc carb is used on all engines. It goes back to the 50's. Because this carb will run any engine, it is jetted for each type of engine. 4-6-8 engines all require different main jets and the main jet size is also influenced by the performance requirements of the engine. If there is a id tag on the carb, it may contain the numbers required for a exact match of replacement carb. $400-$450
Marine carbs may get dirty but since they are used only seasonly and not daily like auto, they rarely wear out, just get dirty.
Fixit option one
$40 carb kit, $10 carb cleaner, $10 mixture screws and clean it yourself.
Option 2
add $100ish for a labor at the carb shop.
Option 3
$450+ for new carb and hope choke fits, base plate fits, throttle linkage is compatable and contains the exact match of main jets as the old carb.
There are 3 risk factors.
1. you get the new carb and everything is fine.
2. You get a new carb that runs lean which will burn holes in the pistons.
3. Get a new bcarb the runs rich and fills the engine with soot and gas sucks through it like a Hoover."

"John
Wow, over 56 postings o

"John
Wow, over 56 postings on the original question, that has to be almost a record.

John you seem to be skirting around the fringe a bit here, as Guy has suggested, your carburetta, if suffering from substantial corrosion is only good for Dj's Locker. Your problem all along has ben one of a high speed backfire under load and no matter what you do with those 2 "Idle Mixture" adjustment screws it will make no difference to what your carburetta does at speed and under load.

Now we have already established that you have a manifold leak that may be leading to a vacum leak which is going to affect the performance of no matter what carburetta you mount on it.

You are trying to diagnose additional possible problems without fixing the break in the supply chain first, as suggested previously you need to replace the manifold gasket and get a good seal there before you move on to any problems that may remain.

"NOW DO IT AND THEN GET BACK TO US"

(sorry if I sound a little pushy but I think Bill may need a rest)

Frustratedly
Peter C"

All true. I've heard this

All true. I've heard this before.

"I'm back-
You guys have

"I'm back-
You guys have been a great resource for this dumb shadetree. I've posted before regarding backfire through the carb. Checked all the electrical, I'm now on the fuel side, I had an obvious manifold leak(exhaust) the advice was that it could be the source. I've got the manifold off, the gasket that was on it looks fairly new not original, hmmm. Then I went to cleaning up the mating surfaces and I see no. 1 intake has a thin layer of oil in the bottom of it. 1)Could anyone please tell me if I should go further and take the head off? The compression for this cylinder was 120/125 if I remember correctly. 2)Or does it just need lash adjustment? I'd like to kill this problem so I can concentrate on fishin. 3)Do you need some sealant on a new manifold gasket or does it go on dry?"

"Like PC suggests, I'm tak

"Like PC suggests, I'm taking a rest and do as suggested, keep buying new parts and throw money at it until it is fixed."