Lean pop when secondaries start to open - marine Q-jet

[Tom-in-KY]

I have a 1988 Q-jet on a warmed over 454 Chevy - oval port heads, mild hydraulic cam, stock marine exhaust, etc.  The carb was jetted a little richer on the primary and secondary sides when the motor was built ten years ago (I don't have the numbers off the jets and metering rods in front of me, but they were a couple steps larger than the stock ones).  The boat ran great for the last ten years, then right at the end of last season, it developed a pop/backfire through the carb when I would hit the throttle to open the secondaries from a 3000 RPM cruise.  Engine definitely seems to be leaning out and sputtering.  It cruises fine at part throttle on the primary side, although it does seem to idle rich now.  I put a standard rebuild kit in it a few weeks ago, which did not cure the problem.  Thinking the fuel pump was the issue, I switched from a mechanical to an electric pump, and added a fuel pressure guage, which now seems to read 6-8 lbs of pressure.

I had a friend observe the carb when the lean backfire was occurring, and he said that it looked like the secondary air valve was not opening all the way.  I suspect that this is preventing the secondary metering rods from being lifted off the jets, so when the big secondary butterflies open up, there is not enough fuel being delivered.  Funny thing is, the boat seems to take off fine if I mash the throttle from a dead stop, and if I ease into the secondaries, I can achieve about 4000 RPM before it once again seems to run out of fuel.

I thought about trying to adjust the secondary air valve spring, but this doesn't really make sense - nothing changed on the boat when this started happening.

Any ideas?  I have also noticed that it seems that I can turn the idle mixture screws all the way in on both sides without much effect on the idle quality.

I am thinking about trying a stock Merc 330 hp Q-jet that I have had sitting on a bench, but I am suspicious that this carb will need a kit too since it has been sitting so long.

Looking forward to any advice, especially from Cliff.     Thanks in advance!

[Cliff Ruggles]

Right to start with, I would recomend using our HP rebuild kits.  They contain upgraded components for this new fuel, and all of the parts needed for a complete/correct rebuild. 

We include the larger .135" Viton needle/seat assembly, and the upgraded accl pump assembly with custom springs for improved fuel delivery.

The secondary plastic cam and spring should be replaced as well, as they have a tremendous impact of secondary performance, and transition going to full throttle.

We also include new idle tubes in our kits.  The idle tubes need to be removed to clean in and under them, and can get damaged in the process.

Also keep in mind that this new fuel contains less BTU's, and another slight recalibration may be required if your particular engine combination doesn't like it?......Cliff

[Tom-in-KY]

I will order a kit.  Do I need to specify the numbers stamped on the side of the carb?  The carb I am using now was originally set up for a 260HP 350 small block that came in the boat.  When I dropped in the 454, I changed out the primary jets and rods, as well as the secondary rods to richen up the mixture a little bit.  I can give you the exact numbers on those parts if you need them.

Alternately, I can give you the numbers off of a stock 454 330HP Q-Jet that I also have on hand, if you think this carb would be a better starting point.  It is basically identical to the small block version, but I think it has a richer mix of rods and jets than the 350 configuration.

I also ordered your book - hoping to have it by Saturday.  Is there a particular "recipe" in the book that would apply to a marine engine?

Thanks for all the help!

[Tom-in-KY]

I pulled the numbers off of one of the 330 HP carbs - 17080563 - marine

It came with the following jets and metering rods:

Primary: jets - 70; rods - 37
Secondary: DP rods on a "G" hanger.

I noticed that the HP kits come with a selection of power piston springs.  I have the original carb off of my 325 HP Chevy 396 car - 7029215 (date code 7037931).  This is the original carb off of the car, and it is missing the power piston spring.  Will one of the springs in the kit work for this carb, or do I need to order a specific part?

Thanks for all the help, Cliff!

[Cliff Ruggles]

All of our HP kits come with 4 different power piston springs.  They were chosen to cover a broad range of applications.  There is an orange spring for low vacuum/big cammed engines and a light blue spring for engines with moderate size cams in them.  Also included are a longer green spring with a slightly higher opening point and faster rate, and a dark blue spring for stock or near stock engines that have a high vacuum signal at idle speed.

There is one in the kit that will work in the factory big block carburetor.  If the engine is completely stock I'd use the strongest dark blue spring.  It will get the metering rods up quickly.  This new fuel contains less energy, and from what we've seen, increasing fuel delivery to the engine improves throttle response, felt power, and helps eliminate hesitation, stumble, bog, etc......Cliff

[Tom-in-KY]

Back to the boat issue - I tried a completely different Q-jet that came off of a stock 454 boat engine - MerCruiser version of the Q-jet.  I was getting the same issues as with my other carb.  Oddly enough, the lean pop seemed to ease greatly, and almost go away, when I pulled the flame arrester off and was running the boat with the hatch up slightly for observation.  I also ran it with the flame arrester off and the hatch down, and it seemed to act up again.  At this point, I am starting to think that I have some type of ignition problem.  I am suspecting that something is breaking down with the heat, and the cooler air helped to alleviate the issue.  I suppose it is possible that it is an air delivery issue with the carb too, but it ran fine for years with the hatch down and the arrester installed.

Nevertheless, I got your book on Friday, and have already burned through it twice over.  I have come to the realization that I was way off base when I rejetted my stock Q-jet originally.  I was probably on the rich side - I had a "B" hanger on the secondary with "CC" rods.  It looks like I have the smaller CFM (750) carb with the smaller primaries.  I am getting a case of booster drip at idle with either carb (neither has an idle air bypass circuit).

Your book mentions the benefit of having as much CFM as possible on the primary side for marine engines.  Do you think it would be worthwhile to find a carb with the larger primaries for this application?  Of course, I would have to close off the venting for the float bowl.  Alternately, I was thinking about removing the outer ring from the primary boosters on one of my marine carbs as described in the book.  Would you advise doing this?  I have an extra carb body, so it wouldn't be a major sacrifice if it didn't work.

I am going to order one of your HP kits after I figure out if I have an ignition issue.  Also, if I am going to remove the booster rings or go with a completely different carb, I probably will need different rods and jets.

Thanks for the help, Cliff!

[Cliff Ruggles]

Havne't tried removing the outer booster rings for a Marine application to date.

We have built later model large cfm castings for Marine applications, with great success.  I always use the linkage found on later carburetors that allows more throttle angle from the primaries before engaging the secondaries.  These were common on mid to late 70's models, Oldsmobiles in particular if memory serves me correctly.

I've set up plenty of factory Marine carburetors for big block applications as well.  The need to be calibrated exactly for the application just like anything else.  You can add some bypass air by drilling the casting and baseplate as shown in our book.  Drilling the throttle plates will work, but not as well as actually having bypass air installed......Cliff

[Tom-in-KY]

Thanks for the answer, Cliff.  Do you think I would be better served to try removing the primary outer booster ring from one of my existing carbs, or would I be better off trying to find one of the large bore Olds Q-jets like you described?

[Cliff Ruggles]

Tough question.  When you loose the booster area(s) the carburetor becomes less effective/efficient at fine metering.  This results in increased fuel consumption.  You are basically making it work like a small bored Holley carburetor. 

For Marine applications with "warmed over" engines, and all of our race only carburetors,  I like to set them up to run straight off the jets and not use primary metering rods in them......Cliff