Can’t get on plane

[Boatbuyingloser]

I have a 1993 Maxum 2300 SCR with a Jasper rebuild 5.7
I recently had a lot of work done to it, bellows, plugs, plug wires, distributor cap + rotor, compression checked, and a brand new carb but I’m still having issues getting on plane. It’s a little hard to start, and once started the RPMs seem to be low for the first 10 seconds or so, then come up to regular idle. I say “seem to” because my tachometer is usually off by +1,000 RPMs but will sometimes correct itself (could be part of my issue)
This was an issue before the work was done and continues to rear it’s ugly head most days I bring the boat out.
The boat usually runs fine (maybe a little rough, hard to tell as I’m used to it now) and can rev all the way to the top just fine in neutral (accidentally did this one day when it was particularly hard to start) however once I try bringing it on plane, it feels sluggish and at about 4k RPMs it will drop down to about 2,000 rpms before trying to jump back up. (Surging?) I’ve had to limp back to the boat dock at just above idle the last couple weekends.
Before spending more money at a shop, I’d just like to hear some input on what you guys think could be the culprit.

 

[Scott Danforth]

Reving in neutral means nothing as the motor will go to redline with only two cylinders firing and the carb gummed up

What are the compression numbers?

Is the "new" carb a Mercruiser carb or Amazon purchase?

 

[Boatbuyingloser]

Reving in neutral means nothing as the motor will go to redline with only two cylinders firing and the carb gummed up

What are the compression numbers?

Is the "new" carb a Mercruiser carb or Amazon purchase?

Ahhh ok didn’t know that, not sure the exact numbers but my mechanic told me they were all good (he checked before doing any work)
The carb is a rebuilt Rochester quadrajet from a trusted supplier, however it is one that “doesn’t need a tune”. I did bring the idle speed up (~750rpms) as it seemed to stumble/surge when it was around 500 RPMs

 

[Scott Danforth]

All carbs need to be properly set up.

You would have been better off rebuilding your old carb than spending money on a used 35 year old carb with a questionable rebuild.

Pull your plugs and get compression numbers. A fresh rebuild should be 150 psi +/- 5 across all 8 cylinders

 

[airshot]

The mechanic didn't do his job, so why did he get paid ?? Take it back and tell him to fix it right !!

 

[Boatbuyingloser]

All carbs need to be properly set up.

You would have been better off rebuilding your old carb than spending money on a used 35 year old carb with a questionable rebuild.

Pull your plugs and get compression numbers. A fresh rebuild should be 150 psi +/- 5 across all 8 cylinders

Ok, the exhaust smells a little rich, should I try to lean it out? I find it odd that after 2 carbs the issue is almost identical so I’m thinking it must be a different problem. Oh also my mechanic told me he refuses to rebuild Rochester quadrajets, as did another mechanic (not sure why). I’ll check compression and fuel pressure tomorrow.

 

[Boatbuyingloser]

The boat was in the shop for 6 weeks and I was eager to take it out. I’d rather not go back to them if possible, but from what I’ve heard they seem to be the only half decent shop besides marinas.

 

[Scott Danforth]

Ok, the exhaust smells a little rich, should I try to lean it out? I find it odd that after 2 carbs the issue is almost identical so I’m thinking it must be a different problem. Oh also my mechanic told me he refuses to rebuild Rochester quadrajets, as did another mechanic (not sure why). I’ll check compression and fuel pressure tomorrow.

Stop firing a parts cannon at your boat.

Start diagnosing what the problem may be

The mechanic probably doesn't want to work on the Rochester carb because unless he is 55 years old or older, probably has never seen one. The last ones came in boats back in 1989 and in cars/trucks about 1980 (not counting the electronic computer controlled ones )

Where did you get the carb? Only place worth while to get rebuild kits for old Rochester carbs is Mikes. Most of the "rebuild" companies send you junk zombie carbs or Frankenstein carbs. Any carb bought off Amazon or eBay will be highly suspect

If it smells rich, it is rich. Which calibration kit is in the carb? How is the carb set up? How is the idle mix screws set? How is the spring on the air flap set?

If these questions are Greek to you, start reading the articles on Mikes site or find an old grey-beard hotrodder that works on 60s and 70s GM vehicles

What is the compression numbers

 

[Scott Danforth]

here is the link to Mikes if your google fu isnt strong.

Mike's Carburetor Parts

www.carburetor-parts.com

 

[kenny nunez]

Aside from the questionable carburetor, have you ever tried using a separate fuel tank? It is possible that is the cause of your problem.
Follow SD’s advice on contacting Mikes. Look closely on the front of the carburetor’s main body, there is a set of numbers that identify which application the carburetor is jetted for. You may have to send it to Mikes to have it corrected.

 

[Grub54891]

A bad tachometer will cause running issues also. A quick check is to unhook it and give it a go. Other than that, A good rebuilder can tune that carb. I'm 66 years old and have done many of them. Thay aren't that hard to do. Any carb that has come across my desk from outside rebuilders usually needs a few touch ups to get it right.

 

[nola mike]

@Lou C wrote a good how to on a qjet rebuild. He loves qjet threads, surprised we haven't seen him yet.

 

[Lou C]

OK the problem with Quadrajets it that all the cores are very old now. I was able to get mine working right because it was an original un-modified carb, with all the correct parts and jets as built by GM. So all I had to do, was really and I mean REALLY clean it, especially the idle air tubes that made a big difference. I got a great quality rebuild kit from Cliff's High Performance Quadrajets. He does rebuild them as well, but that's more of a during the off season project. He sells better quality marine accel pumps that won't break down with the E-10 gas that we have to use.
If you have to replace a Q-Jet that you could not make work right, do not buy another "reman" one. Just buy a brand new Edelbrock or a Holley. You'll be better off. In the case of a Mercruiser engine the Edelbrock will be easier to rig since they used the very similar Weber 4bbl carb, both that Weber and the Edelbrock are simply updates of the old Carter 4bbl design used on many 60s and 70s Chrysler products.

 

[Lou C]

Ok, the exhaust smells a little rich, should I try to lean it out? I find it odd that after 2 carbs the issue is almost identical so I’m thinking it must be a different problem. Oh also my mechanic told me he refuses to rebuild Rochester quadrajets, as did another mechanic (not sure why). I’ll check compression and fuel pressure tomorrow.

Do check both compression and fuel pressure
Contrary to what some say rebuilding a Quadrajet is no harder than any other carb. The accel linkage and choke linkage is a little tricky to get off, that's all. The rest, is easy. Not like on a Holley where the bowl gaskets literally glue themselves to the main body!
They might refuse to rebuild them if they know that they are franken-carb reman units, in that case I don't blame them.

 

[Scott Danforth]

Not like on a Holley where the bowl gaskets literally glue themselves to the main body!

there are a few tricks to get the gaskets off and prevent them from sticking in the future.

 

[Lou C]

Do tell Scott!

 

[Scott Danforth]

Do tell Scott!

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[Lou C]

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[dubs283]

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[Scott Danforth]

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Was reading to see if it was safe to use on fiberglass and noticed the disclaimer. Wonder how easily any business could buy it

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