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Rochester Quardrajet issue

transcorpmerc

New member
Hi, I'm new to this forum and have a question about the Rochester Quadrajet 4BBL carb on my 1982 Mercruiser 260 I/O

I recently replaced the OEM carb with a rebuilt one from National Carburetor. My mechanic has tuned it to the point where it is performing very well, except for WOT. When attempting WOT from idle or mid-range, I get hesitation and an intermittent loud "knocking" sound that seems to be coming back through the carb. I've posted this video so this knocking (popping) can be heard (towards the end of the video):

https://youtu.be/9qSTvK6KxnM

I suspect the rebuilt carb isn't quite right. I know there could be 100 things causing this (i.e. lean condition, improper jetting, secondaries out of adjustment, anti-siphon fuel line blocked, etc.) but my main concern is that just running it to the place where it begins to knock 3-4 times might have caused further damage to the engine. I've always backed off of it whenever I've heard it. The engine runs almost perfectly otherwise.

Any thoughts?




 
if you replaced it, I hope you still have the old one. You need to swap jets and needles, the one on engine is terribly lean on top end.
Could be a lower volume accelerator pump or lever arm in wrong position contributing, another reason to have the old one for reference.
 
if you replaced it, I hope you still have the old one. You need to swap jets and needles, the one on engine is terribly lean on top end.
Could be a lower volume accelerator pump or lever arm in wrong position contributing, another reason to have the old one for reference.

Thanks. You would think a rebuilt marine carb designed to replace the OEM would come with the right jets and needles...
 
Yes...just spent $650 for a complete tune-up (coil, dist,wires,plugs,filters,etc.)
This WOT hesitation/knocking issue was there with this rebuilt carb and before the tune-up
 
the module would not normally be a tune up part. I've seen a couple fail and run, usually they cut off after getting warm.

Tuning for boat weight, prop, drag is all something the rebuilder would not know, so expecting the jetting to remain the same from carb to carb and application to application is a bit if a stretch.

We can eliminate a lean condition by removing a plug as quickly as possible after it pops and look at the color. If it's white or looks real clean, she's lean. If it has color or is brownish, trouble shoot the module.
 
Would a bad module affect the low to mid-range performance as well? I'm not having any trouble below 1500.

If your dist pick up still has screw terminals and does not have three wires on it, CHANGE IT! When the pick up degrades over time it will cause all types of timing issues.

If you just had the engines ignition parts replaced I would be checking the firing order and make sure you do not have a crossed/loose ignition wire.

I change distributor pick ups with a tune up if the pick up looks questionable.

In your video I hear your engine dropping and regaining cylinders, (Ignition related).
 
Also, If the timing and plugs are good, could it could also be a stuck valve?
If the engine pops through the carburetor, that means the valve opened before the exhaust forcing the air up through the carb., right?
 
I could not tell that from the video the sound is not very good but,

What I did hear was the secondary's opening...not sure if they were opening a bit too early? Sounds like they are.

As suggested if you have the original carb, you need to see if the needles and jets are the same.

Also as suggested pull the plugs and take pictures of them, confirm firing order is ok.

If it was a stuck valve it would have popped as soon as you accelerated right back thru the carb. and it would be a constant popping.

also it would be good if you could determine if the issue is lean or rich condition
 
I know...my camera skills are about the same as my mechanical!
I will get the plugs pulled and see what they look like. Pretty sure they will be white(clean). If the secondaries are opening, how do I correct the lean condition?
Btw, the carb shop put in "76" jets. Also it is a 650 CFM
I could not tell that from the video the sound is not very good but,

What I did hear was the secondary's opening...not sure if they were opening a bit too early? Sounds like they are.

As suggested if you have the original carb, you need to see if the needles and jets are the same.

Also as suggested pull the plugs and take pictures of them, confirm firing order is ok.

If it was a stuck valve it would have popped as soon as you accelerated right back thru the carb. and it would be a constant popping.

also it would be good if you could determine if the issue is lean or rich condition
 
No

If the secondary's are opening early it will run rich, That may be the "bogging" sound when you throttle up in the video.

Again I am only saying it sounds like it I am not saying it is happening.
If you suspect a lean condition, remove the flame arrestor and when this issue occurs close the choke butterfly some and see if it improves. This will richen the fuel supply to the engine. If it runs worse then it is running to rich. (be careful when doing this and make sure you have someone else with you who understands what you are doing!!!!!!!!!!!!)

All Rochester carbs are 750 cfm, in the automotive world sometime in the 80's they had some 850 cfm also.

Because of its design the secondary's are a variable cfm design. There is also a hard stop and if used that will not allow greater than 625-650 cfm.

Below is the Mercruiser manuals specs. Main jet 68, main metering rods # (inside carb body) and hanger ID # which you can see from the top of carb. The hanger is lifted when the secondary butterfly opens and lifts two secondary metering rods out of the jets in bowl area inside of carb.

All V8's of that vintage and all the way thru 1989 which use Rochester 4 bbl carbs all have main jet sizes between 67 and 69....So anything larger is too large. (76)

So in the end I would suggest getting someone with carb knowledge and disassemble your old original one, get the main jets, primary metering rods and hanger and secondary metering rods and switch them out with the new carb. Or send it to me..........I will rebuild it...but I will want one of the carbs for me for payment.
 
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I used to have hundreds of Q jets for various parts, went to EFI and got rid of em all.
Not all Q jets are equal, baseplates, main bodies and upper covers can be swapped to fine tune one to a application; provided you have the reference books and a flow bench.
I've run on pump gas, race gas, and methanol with above expected success.
Unfortunately, most of us old farts that could make one sing are retired or have lost knowledge to lack of practice.

Going back to the original carburetor, do you still have it or no?
I'd bet that the carb already had previous work done, or the manufacturer had done on a test hull and specced the engine with proper jets from the manufacturer.

If the carb is gone, you may be needing to find a gray haired guy at the local NAPA or hot rod shop to source you the jets and procedures.
 
Most tuning parts are available and many others.
There are several on line carb shops that carry lots of parts.
 
No

If the secondary's are opening early it will run rich, That may be the "bogging" sound when you throttle up in the video.

Again I am only saying it sounds like it I am not saying it is happening.
If you suspect a lean condition, remove the flame arrestor and when this issue occurs close the choke butterfly some and see if it improves. This will richen the fuel supply to the engine. If it runs worse then it is running to rich. (be careful when doing this and make sure you have someone else with you who understands what you are doing!!!!!!!!!!!!)

All Rochester carbs are 750 cfm, in the automotive world sometime in the 80's they had some 850 cfm also.

Because of its design the secondary's are a variable cfm design. There is also a hard stop and if used that will not allow greater than 625-650 cfm.

Below is the Mercruiser manuals specs. Main jet 68, main metering rods # (inside carb body) and hanger ID # which you can see from the top of carb. The hanger is lifted when the secondary butterfly opens and lifts two secondary metering rods out of the jets in bowl area inside of carb.

All V8's of that vintage and all the way thru 1989 which use Rochester 4 bbl carbs all have main jet sizes between 67 and 69....So anything larger is too large. (76)

So in the end I would suggest getting someone with carb knowledge and disassemble your old original one, get the main jets, primary metering rods and hanger and secondary metering rods and switch them out with the new carb. Or send it to me..........I will rebuild it...but I will want one of the carbs for me for payment.

Thanks. I'm not there and may not be able to take it back out and run it until spring.
So if it is running too rich ratrher than too lean, would this also cause the popping noise at WOT?
As a footnote, there seems to be more black material coming out the exhaust than before. Also National told me they put 76 jets in it. Would this be an indication of running too rich?
 
Yes and Yes

If you are running very rich the motor will stumble badly at higher rpms.........and if enough fuel is there it could cause a symptom such as yours.

Very hard to tell from a "lousy" video so I cannot say exactly............
 
Interesting...when I pull the plugs I guess that will tell the tale if we are rich or lean here!

My gut says it's too rich. When I first installed this carb it was VERY rich, black smoke at startup and weird choke problems. Sent it back to National and all this went away except for the WOT bog, hesitation and intermittent popping noise.

If the secondaries are coming in too soon, would that also contribute to being too rich under load?
 
Reminds me... re:"find a gray haired guy at the local NAPA or hot rod shop". Had a chat with one last month when I tried to buy a "shop tach". He said 10 years ago they stocked "3 or 4 different ones"... Last few years...none. ( Lots of aftermarket ones (at other chain stores) for "tuners" though! Eventually found a used Sears/Penske Analyzer on line. I actually have a Heathkit one I built from a kit a lifetime ago, but its not been powered up even once in the last 40 years, so I figure the electrolytics are shot and the calibration can't be trusted. Maybe this winter if I get bored, I'll rebuild it.
 
Interesting...when I pull the plugs I guess that will tell the tale if we are rich or lean here!

My gut says it's too rich. When I first installed this carb it was VERY rich, black smoke at startup and weird choke problems. Sent it back to National and all this went away except for the WOT bog, hesitation and intermittent popping noise.

If the secondaries are coming in too soon, would that also contribute to being too rich under load?

It would bog down but depending on the rest of the set up once on plane it would eventually work out OK, But getting there may not be the best ride.

Unless they used the Mercruiser factory settings then your secondary opening timing is a crap shoot.

If and when you get to that point ask again and the adjustment can be explained with some graphics.

I have posted this info in other posts but don't remember which ones.......
 
As one of those 'old farts' I recall how the black plastic floats in Q-Jets used to get 'water-logged' and cause a flooding condition (as mine was--it would not take an idle setting). When I discovered the 86 carb in my 230 hp/ 305 Mercruiser had one of them, I tossed it out and installed a brass float. That might have been your problem as well.

Jeff
 
I would not add that to the list.
Purchasing a new non brass float is easy and they are readily available if you even need one.

The brass floats are notorious for leaking as the soldered joint leaks............that is why they went to the plastic type material. All one piece!!

Get the calibration correct first, if you suspect the float may be bad then when you have it apart remove the float and put some gas in a class jar and drop the float into it over night...see if it sinks..........If not then no issues!!
 
The float will still float, Jack, it just doesn't float as it should, leading to minor flooding and lousy idle issues. We replaced those plastic floats by the dozen back then (with brass) and eliminated the problem.

Personally, I've only had one brass float go bad. It was one of three on my Pontiac (with triples). Managed to drive it several miles home, burning gas like crazy, leaving a smoke trail and running like crap. My Mom was with me at the time s I had to clamp my mouth shut!


Jeff
 
Actually I dont believe you can get a brass float for the 4 bbl. I think they are only for the 2 bbl.
 
Just installed one in mine. So well built I didn't have to alter it for float level. Bought it on this site.

Jeff
 
I see, NON OEM though made for and sold for Sierra...


All the Carb online stires sell the OEM style. that I have looked at.

Still don't rust them seen to many brass ones leak
 
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